Magnetic Resonance Mammography 

Introduction to Magnetic Resonance Mammography

The current use of contrast-enhanced magnetic resonance imaging (MRI) of the breast to detect and stage breast malignancy, with a dynamic, serial, high-temporal resolution or a slower, high-spatial resolution, is often known as magnetic resonance mammography (MRM). (Lesions delineated by MRI appear in the image below.)

Go to Breast Cancer for complete information on this topic.

Development of MRI-based breast examination

The breast was one of the first organs studied with magnetic resonance imaging (MRI) for the detection of cancer, albeit initially in vitro.^[1] The breast was also the first organ in which the detection of invasive tumor neovascularity was highlighted through the application of rapid serial imaging after an injection of contrast agent.

With the clinical application of nonenhanced breast MRI, the value of T1-weighted (T1W) and T2-weighted (T2W) spin-echo imaging rapidly became clear, through the analysis of characteristics such as lesion morphology, signal intensity, and tissue relaxation times.

In addition, some significant limitations of nonenhanced breast MRI also became clear. For example, the T2 relaxation rates of benign tissues and malignant tissues overlap,^[2] and in situ cancers could not be reliably detected at all. Initially, imaging was limited to 2-dimensional (2D) spin-echo acquisitions with intersection gaps and a significant limitation in the number of sections, which limited the overall sensitivity of the technique for small lesions.

It was the combination of rapid 2D gradient-echo (GRE) imaging with a dedicated breast coil, coupled with the bolus injection of gadolinium dimeglumine,^[3] that created the technique of dynamic contrast-enhanced breast MRI. This technique showed an extremely high sensitivity for breast malignancy, which in some cases exceeded that of conventional imaging. Although the technique was initially limited to a single section location, it was soon modified with newly developed multisection, spoiled GRE sequences, with no loss of sensitivity. This still forms the foundation of modern breast MRI.

Since then, numerous developments and refinements have improved the diagnostic performance of breast MRI. The technical developments that revolutionized breast MRI and propelled it into its current importance as an adjunct technique for the evaluation of breast disease include the following:

• The development of the first intravenous (IV) contrast medium for MRI, gadolinium dimeglumine
• The development of rapid GRE pulse sequences sensitive to contrast enhancement
• The development of high-field-strength magnets (>1 T) that enabled spectral fat-suppression techniques
• The development of dedicated breast coils for bilateral or independent breast imaging
• The exploitation of new methods of k-space filling to increase speed and resolution
• The development of computerized, automated techniques for contrast enhancement and architectural feature analysis of large image datasets
• The development of MRI-guided breast biopsy and localization methods

Prerequisites for the performance of MRI

MRM is a highly specialized diagnostic technique that complements clinical assessment and conventional imaging with mammography and US. It does not replace these techniques except in certain unusual situations. In general, MRM should not be performed without conventional imaging first.

MRM is best performed in a multidisciplinary setting with access to additional breast imaging, as well as close collaboration between the surgeon, radiologist, and pathologist.

Radiologists experienced in MRI but without a strong knowledge of breast disease and diagnosis often have major difficulties with the interpretation of breast MRI studies. The radiologist embarking on MRM must (1) have a thorough understanding of breast pathology and the management of breast diseases, (2) work closely with a breast surgeon and pathologist to ensure the diagnostic accuracy of MRM, (3) be experienced in the interpretation of mammograms and breast sonograms, and (4) be experienced with image-guided breast needle-biopsy techniques.

Role of MRM in mammographic examination

MRM is used as an adjunct to conventional mammographic assessment because, unless significant neoangiogenesis is present as well, it is inconsistent in the diagnosis of ductal carcinoma in situ (DCIS). This finding is typically seen in high-nuclear-grade DCIS. In some cases, DCIS has only weak enhancement that is indistinguishable from that of benign breast tissue.

Ongoing studies are addressing the benefits of using MRM in conjunction with conventional imaging for surveillance screening in women who are known to be at high risk, either because they have the gene for breast cancer (BRCA), because they have an extremely strong family history of breast and/or ovarian cancer in a first-degree relative, or because they have undergone prior radiation therapy in the chest wall (eg, those with Hodgkin disease).

To date, these studies have shown that MRM can depict small, invasive breast cancers before they become apparent on sonograms or mammograms. However, the cost of MRM is high, and the low specificity means that women with suspect findings usually have to undergo multiple scans and MRI-guided breast biopsies.^[4] It remains to be seen whether MRM has any impact on survival and mortality.

Diagnostic approaches

The increasing use of MRM has inevitably been accompanied by incidental enhancing abnormalities, which typically were not detected on earlier, conventional images. These apparent lesions may represent normal or dysplastic tissues, cyclic hormonal changes, benign tumors, or even unexpected malignant foci. If the enhancement rate, intensity, or pattern is suspicious, the nature of such foci must be clarified so that a cancer is not missed.

Three strategies are commonly used to diagnose these lesions: performing MRI-guided repeat US, repeating the MRM examination at another suitable time, or performing MRI-guided needle biopsy.

Indications for Performing MRM

The high cost and limited availability of MRM, as well as the difficulties inherent in performing and interpreting the studies, require careful recommendations for its use.^{[5, 6, 7, 8]} The following are commonly agreed-upon and useful indications for performing MRM:

• Detection of occult breast carcinoma in a patient with carcinoma in an axillary lymph node (as shown in the image below)
• Evaluation of suspected multifocal or bilateral tumor
• Evaluation of invasive lobular carcinoma (ILC), which has a high incidence of multifocality
• Evaluation of suspected, extensive, high-grade intraductal carcinoma
• Characterization of an indeterminate lesion after a full assessment with mammography, ultrasonography (US), and physical examination
• Detection of recurrent breast cancer
• Detection of occult primary breast carcinoma in the presence of metastatic adenocarcinoma of unknown origin
• Monitoring of the response to neoadjuvant chemotherapyOccult breast carcinoma occurring with axillary lymphadenopathy. A, Sonograms of the enlarged axillary lymph nodes. Sonograms of the breast did not reveal a lesion. Mammographic results were normal. B, Dynamic subtraction gadolinium-enhanced MRMs show a focal, ovoid, rapidly enhancing lesion in the lateral aspect of the left breast.

Contraindications to Performing MRM

In a number of situations, MRM is essentially contraindicated, usually because of physical constraints that prevent adequate patient positioning. These constraints include the following:

• Patient's inability to lie prone
• Marked kyphosis or kyphoscoliosis
• Marked obesity
• Extremely large breasts
• Severe claustrophobia

Another contraindication is the inability to use gadolinium-based contrast media in a patient (as in cases of allergy or pregnancy).

Relative contraindications to MRM also exist. These are essentially based on the high sensitivity, but limited specificity, of the technique. MRM may not be useful in cancer-phobic patients or for the assessment of mammographically detected microcalcifications.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see FDA Information on Gadolinium-Based Contrast Agents or Medscape.

Limitations of Conventional Breast Assessment

Conventional breast assessment includes the combination of screen-film radiographic mammography, high-resolution breast US, and clinical breast examination. This tried-and-true method forms the foundation of modern strategies for breast cancer detection. The results of this assessment also serve as the final arbiter for the use of breast biopsy in breast screening programs worldwide.

Despite the usefulness of this approach, almost every breast radiologist is familiar with cases in which conventional assessment fails to depict a breast malignancy accurately, as in 1 of the following scenarios:

• Palpable lesion without a focal imaging correlate
• So-called interval cancers that are missed or are not visible on initial images
• Understaging of the extent of the lesion or multifocality in the same or opposite breast
• Patient presenting with distant or axillary breast cancer metastasis, with no breast lesion found on mammograms or sonograms
• Chest wall invasion that is not detected
• Inaccurate clinical assessment of large tumors that are treated with neoadjuvant chemotherapy

In short, in a significant minority of patients, breast malignancy will not be adequately assessed with conventional imaging and physical examination. In each of the scenarios above, MRM has been shown to be a sensitive and effective method of detecting, diagnosing, and staging intramammary breast malignancy, even when conventional imaging results have been negative.

The use of MRM may change the clinical management in these situations if unexpected abnormalities are detected. Because of this ability to depict malignancies that are otherwise not visible, MRM has been the subject of active research and development around the world, and its use for certain specific indications has been accepted.

Advantages of Using MRM

The numerous advantages of MRM over conventional breast imaging for the detection of malignancy have become apparent with increasing clinical experience. These advantages include the following:

• No ionizing radiation
• All imaging planes possible
• Capability of imaging the entire breast volume and chest wall
• Superb 3-dimensional (3D) lesion mapping with techniques such as maximum intensity projection (MIP) slab 3D reconstruction, as shown in the first image below.
• Greater than 90% sensitivity to invasive carcinoma (see the second image below)
• Detection of occult, multifocal, or residual malignancy (see the third image below)
• Accurate size estimation for invasive carcinoma^[9]
• Good spatial resolution
• Ability to image regional lymph nodes (though accurate staging remains problematic)3D thick-slab maximum intensity projection (MIP) MRI shows 4 lesions with ductal enhancement extending to the retroareolar region. Lesions A, B, and C were in a line, with lesion D lying higher and more central. All lesions and the retroareolar ductal enhancement were confirmed to be malignant at mastectomy. Large breast carcinoma invading the pectoralis muscle. A, The mammographic depictions are misleading because they show an apparent plane of cleavage between the large stellate irregular tumor and the pectoralis muscle. B, MRI findings confirm that the tumor is attached to the pectoral fascia, and the images show some underlying muscle and fascial enhancement. Invasion of the pectoral muscle was confirmed surgically. Small carcinoma with unexpected multifocality detected during screening. A, Only 1 lesion—a focal, spiculated, small nodule in the medial right posterior aspect of the breast—was found at workup mammography and initial sonography. B, MRI shows 2 strongly enhancing irregular nodules, both of which were consistent with malignancy. These appear to be part of the same duct segment despite being several centimeters apart from each other. Ultrasonographic findings noted after MRI confirmed the second anterior lesion. Ultrasonography-guided needle biopsy confirmed a second carcinoma, which was removed at the time of primary surgery and confirmed histologically.

In a study by Wasif et al, MRI was found to be more accurate than ultrasonography or mammography in the determination of the size of breast cancer masses. Out of 61 lesions, MRI-based tumor size was within 1 cm of pathologic size in 44 (72%) tumors, more than 1 cm above pathologic size in 6 tumors, and more than 1 cm below pathologic size in 11 tumors.^[10]

In a study, Weinstein et al concluded that the addition of MRI to mammography in patients with a high risk of breast cancer has the greatest potential to detect additional, mammographically occult cancers. The investigators prospectively compared breast cancer detection of digital mammography (DM), whole-breast US (WBUS), and contrast-enhanced MRI in a high-risk screening population that previously screened negative when film screen mammography (FSM) was used.

In the study, the cancer yield by modality was evaluated, and 20 cancers were diagnosed in 18 patients (9 ductal carcinomas in situ and 11 invasive breast cancers). The overall cancer yield on a per-patient basis was 3.0% (18 of 609 patients). The cancer yield by modality was 1.0% for FSM (6 of 597 women), 1.2% for DM (7 of 569 women), 0.53% for WBUS (3 of 567 women), and 2.1% for MRI (12 of 571 women). Of the 20 cancers detected, some were detected only on 1 imaging modality (FSM, 1; DM, 3; WBUS, 1; MRI, 8).^[11]

Moy et al evaluated breast MRI in cases in which mammographic or ultrasonographic findings were inconclusive, and found that MRI had a sensitivity of 100% and a significantly higher specificity than mammography (91.7% vs 80.7%, respectively). MRI also had greater positive predictive value (40% vs 8.7%) and overall accuracy (92.2% vs 78.3%).^[12]

Disadvantages of Magnetic Resonance Mammography

The widespread use of MRM for the detection breast malignancy also has a number of disadvantages. These are as follows:

• High equipment costs
• High examination cost
• Limited scanner availability
• Need for the injection of a contrast agent
• No standard technique
• Poor throughput compared with that of US or mammography
• Large number of images
• Long learning curve for interpretation
• False positive enhancement in some benign tissues (limited specificity)
• Variable enhancement of in situ carcinoma
• An incidence of slowly or poorly enhancing invasive carcinomas of about 5%^[13]

MRM and Mastectomy

According to Dang et al, there has been concern that, because breast MRI has greater sensitivity in cancer detection than mammography and US do, the increased use of MRI for breast cancer screening will lead to an increased rate of mastectomy in women with early stage breast cancer. However, the authors found that from 2003 to 2007, although the annual number of breast MRI scans ordered by their institution rose from 68 to 358, the percentage of women who underwent mastectomy did not change over that period.^[14]

MRM Characteristics Associated With Differential Diagnosis

If early rapid enhancement due to neovascularity were unique to malignant tissues, MRM would be the standard in clinical practice today. Unfortunately, such enhancement is not specific, and several benign conditions may enhance in a fashion similar to cancer. Conversely, a small percentage of malignancies either enhance identically to benign breast parenchyma, or rarely, they do not enhance at all.

Common nonmalignant pathologies that may mimic malignant enhancement include the following:

• Cyclical parenchymal enhancement
• Fibroadenoma
• Sclerosing adenosis
• Florid epithelial hyperplasia
• Lobular carcinoma in situ
• Infection
• Fat necrosis
• Posttreatment scarring and/or granulation tissue

Uncommon nonmalignant pathologies that may mimic malignant enhancement include the following:

• Intraductal papilloma
• Juvenile papillomatosis
• Radial scar and/or complex sclerosing lesion
• Silicone granuloma

Low-grade DCIS is a common malignancy that may show benign-type enhancement.

Uncommon malignancies that may show benign-type enhancement include the following:

• High-grade DCIS
• Highly scirrhous invasive ductal carcinoma (IDC)
• Invasive lobular carcinoma
• Mucinous carcinoma
• Papillary carcinoma
• Tubular carcinoma

MRI-Guided Ultrasonography

High-resolution US of the suspect region of the breast should be the first method used, because it is rapid and can be performed immediately after the MR examination. MRM is used to guide the examination to determine the size, shape, and position of the suspect lesion.

Using this technique, the present authors and others have found malignancies previously missed with routine US, particularly in cases with multifocal or occult malignancies. Sometimes, such lesions are subtle. They may even appear benign on sonograms, being confidently detected only because the operator was aware of the existence of the lesion. See the image below.

If an MRI-guided sonogram depicts the same lesion that the MRM does, US-guided needle biopsy and/or localization of the lesion for excision can then be routinely performed. However, even when the examination is performed with care and by experienced operators, MRI-guided US can still fail to depict a tumor, possibly because of its size, location, or US characteristics.

Equipment Requirements for MRM Procedures

Field strength

Although MRM initially was successfully developed on the basis of magnets with field strengths of 0.35-0.5 T, it is now generally accepted that MRM should be performed at field strengths of 1 T or greater.

Commercial low-field-strength magnets lack the gradient subsystems required for high-speed, high-resolution volumetric (3D) images, which are now considered essential for the detection of small lesions and for architectural evaluation.

Narrow separation of fat and water peaks at mid- and low field strengths, precluding effective fat suppression. Although image subtraction techniques can overcome this to some extent, motion artifacts remain a major limitation of this postprocessing method for the removal of fat signal. (Motion artifacts are usually nonrigid and thus create complex subtraction artifacts.)

Sensitivity to gadolinium enhancement is reduced at lower field strengths because of the inherently shorter T1 of all tissues.

The signal-to-noise ratio (SNR) of all MRI methods is directly proportional to the field strength; at 1.5 T, the SNR is 3 times greater than the SNR at 0.5 T.

Breast coil design

A dedicated double breast surface coil is essential, because it permits simultaneous high-resolution, high-quality imaging of 1 or both breasts. An example of such a coil is shown in the image below.

The patient lies prone, with both breasts freely suspended in these coils. Such coils should have excellent homogeneity, with minimal image shading and hot spots; otherwise some regions show low signal intensity and poor enhancement, and fat suppression may be unpredictable.

A phased-array design is considered ideal today, but simpler coupled or switched coil designs are still commonplace. Newer designs are open on the sides, permitting access during imaging for interventional procedures, such as hookwire localization and needle biopsy.

A symmetrical coil platform design gives the patient the option of entering the magnet bore feet first or head first. Because of the need for IV access and because of claustrophobia concerns, the authors perform almost all studies with the patient entering feet first.

Contrast agents

In general, the group of extracellular fluid (ECF), paramagnetic, gadolinium-based, intravascular contrast agents has virtually identical pharmacokinetics and contrast-enhancement characteristics, and they are all equally suitable for breast MRI. All of these agents shorten T1 and increase tissue relaxation rates, increasing the signal intensity on T1W or spoiled GRE images.

Suitable agents include gadolinium dimeglumine (Magnevist, Schering), gadoteric acid (Gd-DOTA, Dotarem; Guerbet), gadoteridol (Gd-HPDO3A, ProHance; Squibb), gadodiamide (Gd-DTPA-BMA, Omniscan; Nycomed), and gadobenate dimeglumine (Gd-BOPTA, MultiHance; Bracco).

The administration of contrast material should be by means of a rapid bolus injection. The authors routinely use an MRI-compatible power injector at a rate of 2 mL/s, followed by a 20-mL isotonic sodium chloride flush. Although they are less convenient, hand injections are also successful.

For 2D GRE sequences, a dose of 0.16 mmol/kg appears to provide better sensitivity for lesion detection than does the standard dose of 0.1 mmol/kg at field strengths less than 1.0 T.^[15] However, good evidence suggests that, with 3D gradient echo sequences at higher field strengths, the standard dose of 0.1 mmol/kg does not reduce the sensitivity of the test,^[16] and the authors routinely use this dose successfully.

Injection timing is important for routine MRM, but it is not as critical as it is for MR angiography because of the inherent variation and unpredictability of the rate and intensity of lesion enhancement. Most investigators aim to acquire a complete post-contrast dataset within 1-2 minutes, preferably within 30-60 seconds.

The authors time the injection to commence about 15 seconds before starting the first dynamic imaging sequence so that the contrast agent bolus arrives in the arterial phase at about the time that the central lines of k-space are being acquired in the 50-second acquisition used. However, this time may vary with other factors, such as the patient's age and cardiovascular status. In general, these issues are not particularly critical if at least 1 dataset is acquired within 1-2 minutes after the injection.

The exception to this is the use of high-speed, dynamic, first-pass contrast susceptibility imaging.

Positioning of the Patient

As previously discussed, MRM can be contraindicated because of physical constraints that prevent adequate patient positioning. These constraints include the following:

• Patient's inability to lie prone
• Marked kyphosis or kyphoscoliosis
• Marked obesity
• Extremely large breasts
• Severe claustrophobia

Overview of the MRM Procedure

Patients undergoing breast MRI are often anxious either because they have a known diagnosis of breast cancer or because they are expecting one. Other forms of breast imaging are rapid and usually do not require either an IV injection or prolonged enclosure in a long tunnel in which the patient experiences loud tapping noises.

Anxious or claustrophobic patients can usually undergo imaging satisfactorily. In these patients, the physician and technologist can use a variety of strategies. For example, they can do the following:

• Provide a good explanation of the examination
• Use a calm, relaxed approach during the examination
• Allow the patient to enter the magnet bore feet first
• Intermittently move the patient out of the tunnel
• Appropriately shorten the sequences as appropriate

Experienced MRI technologists are crucial for successful completion of many studies. Failing these methods, extremely anxious patients may require IV sedation. The authors have found that 3-5 mg of midazolam via slow IV injection is effective.

Imaging Sequences

Numerous MRM protocols have been published, and the inexperienced reader can easily be confused as to which to follow. Interestingly, while some observers have criticized this wide variety, all of the published techniques are similar in the detection of breast malignancy. Therefore, it is more important for a clinician to be comfortable and familiar with a specific technique and protocol to ensure a high level of diagnostic accuracy than to try to always update to the latest sequence or technique.

The ideal contrast-enhanced MRM sequence has the following parameters:

• Temporal resolution of less than 30 seconds
• Volumetric acquisition with at least 28 sections and no intersection gaps
• Isotropic spatial resolution of less than 1 mm
• High-sensitivity with the contrast agent
• Perfect removal of the fat signal
• Capability to image both breasts entirely in 1 pass

Because of current hardware limitations, no current sequence has all of these characteristics. However, novel methods for k-space filling, such as spiral 3D imaging, may allow this ideal sequence to be achieved in the near future.

In general, all postcontrast imaging should be completed within 5-7 minutes, because the diffusion of contrast material into normal tissues limits diagnostic characterization after this time. To detect suspicious contrast enhancement, imaging times shorter than 10-60 seconds are generally unnecessary; as many as 5-10% of carcinomas enhance relatively slowly, reaching peak enhancement at 3 minutes or even longer.^[17]

Clinically proven approaches

Currently, 4 clinically proven approaches are widely used to obtain satisfactory diagnostic accuracy and sensitivity with MRM.

Rapid serial, double-breast scanning with 2D multisection GRE imaging may be performed after the injection of contrast material, with or without image subtraction. This method is almost unchanged from the initial technique described by Kaiser and Zeitler.^[3]

Slower, high-resolution, single-breast, fat-suppressed 3D imaging may be performed, with acquisitions in the first 3 minutes after the injection. High-resolution analysis of lesion architecture and enhancement is used.^{[16, 18]}

The use of 3–time-point, 3D, high-spatial-resolution acquisitions with these sequences has been described for additional low-temporal-resolution information.

A hybrid technique using a rapid serial 3D or 2D sequence for the first 2-3 minutes after the administration of contrast agent, followed by 2- to 3-minute high-resolution, fat-suppressed imaging in 1 plane yields similar results.^[19] Optionally, one may also add a rapid serial, dynamic, washout-phase acquisition after high-resolution imaging to increase diagnostic specificity for some invasive cancers.

In summary, crucial factors include the following:

• Temporal resolution of no longer than 3 minutes for high-resolution imaging and 1 minute for dynamic imaging
• A gadolinium-sensitive T1W sequence
• A section thickness of 5 mm or less
• Complete coverage of the breast
• Removal of fat signal by means of spectral suppression or subtraction
• A mammographer with experience in conventional breast imaging and assessment

Acquisition plane

The acquisition plane has a significant impact on the pulse sequence. Phase-direction motion artifacts due to breathing and heart motion are minimized by ensuring that the frequency-encoding direction is in the anteroposterior direction for axial and sagittal imaging. In the coronal plane, switching the phase direction craniocaudally allows the use of a rectangular field of view with reduced phase encoding, reducing the acquisition time.

However, intramammary mapping of lesion location can be difficult in the coronal plane, respiratory motion may produce unpredictable variations in signal across the images, and some coils and sequences generate unwanted moiré-like image artifacts at the edges of the breast due to variations in bulk tissue susceptibility.

For these reasons, the authors prefer the axial and sagittal planes. These are also generally easier to correlate with the mammographic projections. Despite the problems with coronal imaging, however, the International Multicentre Breast MRI Study used the coronal plane for dynamic acquisitions, with excellent results.^[20] Again, this testifies to the overall robustness of MRM as a technique, even with major differences in acquisition methods.

Removal of fat signal

In general, T1W sequences of any type that are sensitive to contrast enhancement are also highly sensitive to other intrinsic short-T1 substances, the most common of which in the breast is fat. While almost all large malignancies produce enhancement sufficiently strong to be detectable on routine T1W images, the signal intensity of fat is generally adequate to at least partly reduce the sensitivity of detection of small lesions or more subtle enhancement.

Early publications on dynamic MRM quoted enhancement thresholds for malignancy in raw units of signal-intensity change. However, these figures were too field-strength and machine-specific to be used widely.

A more robust and less instrumentation-dependent method is to use the percentage enhancement above baseline, with all values corrected to that of nonenhanced fat. The equation for this calculation is shown below.

Various groups have reported that threshold enhancement values 50-90% above baseline are highly suggestive of malignancy. However, such values vary with the field strength, pulse sequence, sequence timing, and dose of contrast agent. It is now accepted that no fixed enhancement threshold that invariably excludes invasive malignancy, though the absence of enhancement is rare in carcinomas.

Four major approaches have been used to reduce or remove fat signal to show enhancement more clearly: GRE technique, digital image subtraction, spectral fat saturation, and magnetization transfer suppression (MTS).^[21]

GRE imaging can be performed with a carefully chosen repetition time (TR), echo time (TE), and flip angle to minimize the (in-phase) fat signal while remaining sensitive to the presence of gadolinium enhancement. This approach was initially developed with field strengths of less than 1 T, and it tends to be less suitable with high field strengths, where the T1 of fat is longer. Because this method is usually a 2D technique with relatively thick sections, the risk of missing a small lesion is significant.

Pixel-by-pixel digital image subtraction with precontrast and postcontrast images is the only reliable method of fat suppression with low field strengths. This method permits the short imaging times required for MRM. Inversion recovery and Dixon techniques tend to be slow at low field strengths, largely because the poor SNR.

Subtraction is the best means of canceling signal inhomogeneity across the breast at any field strength. However, it is time-consuming and susceptible to motion artifacts. Worse, misregistration due to patient motion may cause a lesion to become less visible. Therefore, if subtraction is used, the source images must be carefully reviewed. Despite a clear need, no commercial, automated, nonrigid volumetric subtraction technique is available today.

Spectral fat saturation using frequency-selective pulses is another technique. At less than 1 T, the spectral separation of fat and water resonances is too narrow for this technique to be reliable. Successful application requires careful shimming; a good coil design; and, frequently, manual preimaging tuning. Even so, homogeneous fat suppression may not be possible with large breasts. Nevertheless, this technique remains the best method of obtaining high-spatial-resolution 3D scans without resorting to subtraction.

MTS method reduces the glandular tissue signal by using detuned saturation pulses prior to the imaging sequence; this shortens the water relaxation by coupling it to macromolecular motion. This technique, as either an adjunct to or a substitution for spectral fat suppression, achieves high sensitivity in terms of contrast enhancement sensitivity.^{[22, 23, 24]} However, the technique has not been formally studied to determine whether it is consistently superior to other methods.

Of all of these approaches, image subtraction and spectral fat suppression are the 2 most commonly used strategies for improving enhancement detection.

Quantitation of Enhancement

Temporal enhancement curves

The use of region-of-interest (ROI) temporal-enhancement curves has been strongly highlighted by proponents of dynamic MRM.

Three basic curve shapes have been described by Kuhl and co-investigators^[25] :

Type I is slowly enhancing in which gradual steady enhancement occurs over about 5 minutes. This has been further classified into type Ia, which is slow, linearly progressive enhancement, and Ib, which is slow enhancement with a late plateau that produces a bowlike curve.

Type II is a plateau with early strong enhancement (1-2 min) and a subsequent plateau phase. This is suspicious and often indicates malignancy.

Type III is washout with early strong enhancement (1-2 min) and a subsequent decline in enhancement. This produces a characteristic peak that some investigators have dubbed the cancer corner. This enhancement curve is strongly associated with malignancy.

In a study to determine the value of using such signal-time measurements with dynamic 2D MRM, Kuhl et al found that lesions with type I enhancement were more likely to be benign than malignant, whereas lesions with a type II or III enhancement curve were more likely to be malignant. The investigators studied 266 lesions.^[26] These were subsequently excised, and 101 were proven to be malignant.

The researchers showed that 9% of breast cancers had a type I enhancement curve; 33.6%, type II; and 57.4%, type III. Conversely, 83% of benign lesions had a type I curve; 11.5%, type II; and 5.5%, type III. In this analysis, the sensitivity, specificity, and diagnostic accuracy were 91%, 83%, and 86%, respectively. These results were significantly better than those obtained with the use of simple enhancement threshold measurements alone.

The development of neoangiogenesis in cancers results in early rim enhancement with centripetal slower internal enhancement; conversely, benign lesions tend to enhance centrifugally the temporal resolution is sufficient.^{[27, 28]} Such enhancement heterogeneity can be assessed by the use of small ROIs, with the user roaming around the image to detect the area of most suspicious enhancement. However, this approach is tedious and prone to operator error. Marked interoperator variation can occur in the ROI measurements in the same image datasets, particularly between experienced and inexperienced readers.^[29]

The use of automated, pixel-by-pixel, color parametric map analysis of the enhancement rate and intensity (shown below) is useful for improving reliability of assessment of the large number of images typically obtained with dynamic MRM.^[30] This technique can also be used to instantly graph the time-enhancement curve as the cursor is moved across the dataset; this information adds to the diagnostic confidence in assessing the nature of an enhancing area.

These maps are arbitrary, but typically, lesions that are strongly and/or rapidly enhancing appear red, whereas slowly or weakly enhancing lesions appear blue or green. Intermediate lesions usually are orange or yellow. These have been developed^[31] and commercialized to simplify the analysis of the large amount of image data. These maps are helpful for the rapid evaluation of multiple foci of enhancement.

Even if such a workstation is lacking, all MR consoles have ROI measurement capabilities. The detection of suspicious foci depends then on visual analysis, with ROIs placed over each suspect area and the mean signal intensity being read off the screen. This data can be entered into standard software programs, such as spreadsheet programs, for further analysis and charting.

High-resolution spatial analysis of lesion architecture

Nunes et al published a somewhat complex, but comprehensive and, most importantly, validated, image-analysis decision model to improve the classification of enhancing lesions depicted on low-temporal-resolution, high-spatial-resolution, 3D fat-suppressed (3D FSPGR) images.^[16] This detailed method yields high diagnostic accuracy and has formed the basis for subsequent studies of architectural-feature assessment.

This model was further validated, updated, and slightly modified. The resultant values for sensitivity, specificity, negative predictive value (NPV), positive predictive value (PPV), and accuracy were 96%, 80%, 96%, 78%, and 87%, respectively.^[32] These diagnostic performance indicators are slightly more sensitive than, but otherwise similar to, those obtained by enhancement–time course analysis using dynamic 2D MRM.

Combined Qualitative and Quantitative Analysis

Liu et al studied the diagnostic performance of combining enhancement quantitation with qualitative feature analysis and obtained sensitivity, specificity, and accuracy statistics of 93%, 74%, and 85%, respectively.^[33] These results are essentially identical to those obtained with time-enhancement–curve analysis and with high-resolution spatial analysis.

Hybrid MRM Protocol

A hybrid protocol using dynamic subtraction images and high-resolution fat-suppressed MRM is the authors' current protocol. This technique is performed at 1.5 T on a Signa MRI unit (GE Medical Systems). We use a GE Advantage Windows workstation with FuncTool software for image postprocessing, parametric map generation, and ROI enhancement measurement. A survey of a wide range of protocols has been published.^[17]

This approach combines dynamic 2D or 3D images without fat suppression after contrast enhancement with slightly delayed high-resolution, fat-suppressed 3D images. The authors have chosen the axial plane for the dynamic series, because bilateral breast imaging is possible, and it permits comparison with craniocaudal mammographic projections. The authors try to avoid oversampling (no phase wrap [NPW]) to suppress spatial aliasing in the phase-encoding direction, because this increases the imaging time. High-resolution images are acquired in the sagittal plane, because this permits faster acquisition than the axial plane if phase encoding is set to the head-foot direction and if NPW is turned off.

These images can also be readily compared with mediolateral mammographic projections, and these are useful for section-by-section correlation with mammograms of mastectomy specimens.

The authors find that the high-resolution, axial images are often not useful because the leakage of contrast material into normal tissues may obscure lesions by the time these images are obtained. Nevertheless, on occasion, this sequence provides excellent depiction of lesions.

The dynamic images are postprocessed by using batch subtraction, and then the location of each section is analyzed with parametric color maps to determine the lesion enhancement rate, peak, and time. Suspicious lesions are reviewed with a small ROI cursor with instantaneous, automated curve plotting, and the results are compared with the enhancement curves from fat, with the normal glandular parenchyma, and with the enhancement in the aorta or internal mammary arteries. (See the table below.)

Table 1. Protocol Parameters (Open Table in a new window)

Diagnostic Criteria for Malignant and Benign Processes

The following summary tables show the diagnostic criteria that permit characterization of an enhancing lesion as either malignant or benign. The data have been collated from a variety of publications and represent pooled approximations of published results.

Table 2. MRM Criteria for Malignant Processes (Open Table in a new window)

Table 3. MRM criteria for benign processes (Open Table in a new window)

Appearance of Normal Tissues

Appearance of fat

Normal fat appears moderately bright on most images obtained with gadolinium-sensitive, non–fat-saturated sequences used for MRM. For all practical purposes, normal fat does not enhance after the administration of gadolinium-based contrast material; thus, the signal intensity from the nearby fat may be used as an internal control for the standardization and correction of ROI enhancement calculations.

With fat suppression, normal fat typically appears dark gray, except in areas where either paradoxical water suppression or poor fat suppression occurs; both are usually due to poor local field homogeneity or incorrect preimaging tuning.

Appearance of glandular parenchyma

The enhancement of normal glandular parenchyma may have several different patterns:

• No enhancement
• Minimal or mild enhancement
• Diffuse, slow enhancement
• Slow or rapid, regional enhancement
• Slow or rapid, patchy enhancement
• Small focal areas of slow or rapid enhancement

These patterns and the intensity of enhancement depend on the patient's age and menstrual-cycle stage, as well as the patient's prior medical treatment. Normal parenchyma enhances more strongly in the 35- to 50-year age range and least in the second and third weeks of the menstrual cycle.^[34] Nonspecific, focal areas of enhancement may resolve or fluctuate in size from month to month.^{[35, 36]}

In practice, most dynamic MRMs show no enhancement, with variable amounts of parenchymal enhancement on delayed images. However, nonspecific regional enhancement may sometimes occur early; in these cases, the enhancement may be difficult to differentiate from malignancy.^[16] The use of MRI-guided US and/or repeat MRM at a different part of the menstrual cycle may be necessary to confirm benign enhancement.

Appearance of nipple and areola

Not infrequently, the normal nipple enhances intensely and rapidly. Retroareolar ducts may enhance normally, though usually not as intensely. In cases of extensive high-grade DCIS, these changes can make the interpretation of nipple enhancement problematic. (See image below.)

In contrast, the areola does not normally enhance, and it appears only slightly thicker than adjacent skin; this characteristic permits the detection of areolar infiltration by a malignancy. (See the image below.) Comparison with the contralateral side may be helpful if nipple disease is suspected, as shown in the image below. Asymmetrical hyperenhancement in the presence of DCIS may indicate the presence of nipple involvement, even in the absence of clinically obvious Paget disease.

Appearance of intramammary and axially lymph nodes

Normal intramammary and axillary nodes, as shown in the image below, may enhance moderately intensely, either slowly or rapidly. They may even show a washout time-enhancement curve in the absence of malignancy. Therefore, the enhancement pattern is generally not useful for distinguishing benign nodes from involved nodes.

A node may be diagnosed with confidence on high-resolution images by the presence of a fatty hilus and a sharply defined smooth contour. Careful comparison with the matching mammograms sometimes increases the diagnostic confidence if the typical mammographic appearance of the node can be demonstrated in the corresponding position.

Benign Abnormalities Seen on MRI

Fibrocystic disease

Perimenopausally, fibrocystic change is common and is best detected with T2W, fast spin-echo images, on which cysts appear uniformly hyperintense. Multiple hyperintense, sharply circumscribed, nonenhancing, fluid-filled cysts of variable size are the hallmarks of this condition. As a result of their thick, viscous, inspissated contents, older cysts are occasionally hyperintense on T1W images or hypointense on T2W images.

Mammary duct ectasia

Mammary duct ectasia may be visible on precontrast T1W images because of high protein inspissated secretions and hyperintense dilated retroareolar ducts. Such ducts are readily overlooked, particularly with subtraction techniques. With fat-suppressed 3D images, they can mimic prominent enhancing ducts, and they are potentially misleading; the use of precontrast images prevents this pitfall. Unlike DCIS, this condition always has sharply defined lesions on high-resolution images, and they remain unchanged in intensity throughout the time course of the study.

Sclerosing adenosis

Sclerosing adenosis is typically indistinguishable from glandular parenchyma. However, it may appear as an irregular area of slow, but strong, focal enhancement. If dynamic imaging has not been performed, this finding may appear suspicious.

Fibroadenoma imaging

Fibroadenomas (seen in the images below) may enhance rapidly and strongly when they are myxoid (usually in younger women). Such enhancement may be similar to that of invasive malignancy in terms of rate and intensity, although a type III (washout) curve is not seen. They usually have sharply defined, smooth or slightly lobulated borders with an ovoid shape. These features usually permit the distinction of a fibroadenoma from a malignancy, particularly mucinous or myxoid cancers. Although they are sharply defined, they are generally round. These features are analogous to the US criterion for the height-to-width ratio of a lesion (solid nodules with a ratio of 0.5 are almost always benign, whereas those with a ratio of about 1 are generally malignant).

Fibroadenomas may also have internal septa that do not enhance. This is a highly reliable sign of a benign lesion.

When fibromas are hyalinized (usually in older women), they generally enhance more slowly and weakly than do carcinomas. Larger fibroadenomas may appear moderately bright on T2W precontrast imaging, whereas almost 90% of larger carcinomas have a signal intensity lower than that of normal parenchyma.^[37] Densely hyalinized fibroadenomas show minimal or no enhancement.

Other benign tumors

Papillomas, an example of which is shown in image below, may appear indistinguishable from fibroadenomas in many cases. Before enhancement, they are typically hypointense, ovoid masses, and they may enhance strongly, moderately, or not at all. They usually have well-defined borders and can then often be classified as benign, although they must be distinguished from medullary and mucinous carcinomas. High-resolution US may be useful to determine whether the lesion is intraductal.

Because of their cellularity and papillary growth pattern, pathologists usually recommend their excision to confirm the diagnosis.

Lipomas and fibroadenolipomas (hamartomas) are benign, mesenchymal lesions, and they are usually readily diagnosed with conventional imaging. They are most often imaged with MRM only incidentally. Characteristically, the lesions have internal fat, and they are easily distinguishable from malignancies.

Breast infection

In acute, infective mastitis, conventional assessment is usually sufficient, and MRM has a limited role. The main differential diagnosis is inflammatory carcinoma, for which, again, MRM has few indications. If MRM is performed for the assessment of infections, strong, rapid enhancement may be present, usually with a poorly defined regional or diffuse pattern.

Granulomatous mastitis, shown in image below, is a rare and problematic inflammatory condition that may mimic acute, infective mastitis or even invasive breast cancer. It is usually idiopathic, but it may be caused by various mycobacteria or Actinomyces species, and it has also been described in association with sarcoidosis and Wegener granulomatosis. The patient experiences recurrent bouts of breast sepsis with sinus tracks and the discharge of purulent material. Cultures of the material are usually negative, and biopsy shows granulomatous inflammation.

MRM shows areas of strong, irregular enhancement around pockets of fluid-filled, infective material. MRM can be useful in mapping the full extent of intramammary disease in this condition. This mapping permits a more accurate evaluation of the extent of involvement, which helps in planning and monitoring therapy. This condition is notoriously difficult to define and treat otherwise. Surgery or corticosteroid therapy is the treatment of choice.

Borderline Abnormalities

Proliferative dysplasias

Proliferative dysplasias, including florid epithelial hyperplasia, usually appear as foci or regions of slow to moderate enhancement after the administration of contrast material. Generally, this enhancement is indistinguishable from that of normal parenchyma, and the diagnosis is made at pathologic examination.

Atypical ductal hyperplasia

Atypical ductal hyperplasia (ADH) is a recognized high-risk condition that may progress to DCIS and, eventually, to invasive carcinoma. Not infrequently, the presence of ADH indicates an adjacent malignancy, which usually justifies surgical excision biopsy when it is diagnosed. ADH may have associated neoangiogenesis, which may result in focal or regional, suspicious enhancement indistinguishable from that of a malignancy.^[17]

Radial scar or complex sclerosis lesion

These lesions, seen in the images below, are difficult to diagnose with all imaging modalities. Typically, they are detected as poorly defined, focal areas of stellate distortion on mammograms. They may have proliferative dysplasia, ADH, DCIS or even a focus of invasive carcinoma centrally.

With MRM, they may enhance fairly intensely and show an irregular, spiculated border. Sometimes, these may have an apparent central mass; in this case, they are indistinguishable from carcinomas. Surgical removal is routinely recommended for such lesions, because pathologists cannot confidently diagnose these lesions by using needle biopsy samples.

Juvenile papillomatosis

Juvenile papillomatosis, shown in the image below, is an uncommon condition seen in young women, and it may produce an appearance of multiple masses with marked distortion on mammograms. On MRMs, this may appear as a network of enhancing, beadlike nodules that are connected by enhancing ducts. Alternatively, they may appear as a lobulated, enhancing mass with small internal cysts.

Lobular carcinoma in situ

Lobular carcinoma in situ (LCIS) is a nonmalignant, proliferative condition that is a marker for an increased risk of breast malignancy. It is usually indistinguishable from benign parenchyma on MRMs. However, it has been described as occasionally showing intense, suspicious enhancement.

Malignancies on MRI Scans

Ductal carcinoma in situ

DCIS, seen in the images below, has a variety of enhancement patterns on MRMs, including minimal or no enhancement. Any combination of these patterns may be present, and commonly, 1 or more is seen in association with invasive carcinoma. The enhancement patterns include the following: ductal, focal nodular, regional, and benign.

The ductal pattern is a treelike, linear, branching or reticular pattern. It is usually seen with high nuclear grade DCIS,^[38] and it is caused by periductal angiogenesis. The enhancement may extend to the nipple; this suggests nipple involvement before Paget disease is clinically evident. Focal masses may be present at junctions of the branching pattern; these may be due to clumped DCIS, focal microinvasion, or small, invasive carcinomas.

In the focal nodular pattern, DCIS may appear masslike or stellate, and it may mimic an invasive carcinoma on conventional and MRM images. The diagnosis is usually made only by means of excision biopsy. This well-recognized, but atypical, type of DCIS is sometimes noncalcified.

With the regional pattern, ductal enhancement may appear as a region of strong enhancement on early or delayed images. This pattern may be bandlike or irregular, or it may appear as a geographic area of enhancement. DCIS may also appear as a poorly defined region of strong enhancement, usually around an invasive malignancy. It may even enlarge the apparent size of a small, invasive tumor; this is a cause of the overestimation of tumor size with MRM.

In the benign pattern, 15-40% of DCIS shows minimal to moderate enhancement that is indistinguishable from that of normal glandular tissues. This pattern tends to occur in low-grade DCIS,^[38] but it has also been described in comedocarcinoma.^[39]

The variability of DCIS enhancement is due to variations in angiogenesis, which in turn is somewhat related to the histologic grade.^[38] DCIS of a high nuclear grade tends to have stronger enhancement than that of a lower-grade DCIS. Note that 40% of DCIS is not calcified, even when it is high grade^[40] and present with concomitant angiogenesis.

Paradoxically, although MRM is unsuitable for an evaluation of microcalcifications, it sometimes shows the extent of DCIS (calcified or noncalcified) better than does mammography. Because this information has the potential to change the type and extent of surgical excision, MRM occasionally proves useful for the intramammary staging of DCIS, particularly when the noncalcified component is significant.

For the evaluation of resection margins, however, no evidence suggests that MRM is useful, either in the early postoperative period or in the surveillance period after resection. The ultimate role of MRM in the management of DCIS requires further investigation.

Invasive ductal carcinoma

On MRMs, IDC most often appears as irregular, spiculated, or multilobulated, nodular masses. These masses have strong, rapid contrast enhancement that is at least 60% above baseline. Rim or inhomogeneous, centripetal enhancement on dynamic scans may be present. Typically, either a type II or type III enhancement curve is observed. Surrounding architectural distortion may be noted. About 5% of IDCs enhance slowly and/or less strongly, particularly if they are highly scirrhous.^[17]

Surrounding enhancement of variable intensity may represent DCIS, florid dysplasia, or benign, parenchymal enhancement. In larger or multifocal lesions, MRM may show nipple or chest-wall involvement, which may not be otherwise evident. Multifocal IDC may show moderate, segmental, ductal enhancement connecting the masses; such masses are thus seen to be part of the same breast segment, even if they are not close to one another. Internal, enhancing septa are sometimes seen in invasive carcinomas; these must be distinguished from nonenhancing septations, which are typical in fibroadenomas.

Invasive lobular carcinoma

ILC accounts for 10-15% of breast carcinomas. ILC lesions can be mammographically occult or subtle in 20-40% of cases.^{[41, 42]} As many as 85% are isointense relative to glandular parenchyma, and a minority have malignant microcalcifications.^[43] The incidence of multifocal, multicentric, and bilateral, synchronous or metachronous involvement is much higher with ILCs than with IDCs; this involvement is found in as many as one half of all cases.

Mammography and US tend to cause marked underestimations of the extent of ILCs. MRM has been shown to be more accurate, correctly demonstrating the extent of disease in about 85% of cases.^[44] In the authors' center, MRM is routinely used in all women with a preoperative diagnosis of ILC.

In most cases, ILC shows focal, irregular, strong, rapid enhancement typical of a malignancy. Single or multiple masses in 1 or more quadrants are sometimes demonstrated. However, ILC occasionally has weak or moderate enhancement, and it may be difficult to distinguish from glandular parenchyma with this criterion alone.^[17] Recognizing this problem is easier if the diagnosis is already known from needle biopsy results. In these cases, a masslike contour or associated architectural distortion is usually present. Despite this limitation, MRM is better than conventional imaging for preoperative staging of breast ILCs.

Other breast malignancies

Mucinous or colloid carcinoma may be well defined, with a lobulated border and homogeneous enhancement. Superficially, these tumors may resemble a large fibroadenoma. However, these lesions are typically round rather than oval. Enhancing internal septa may be visible; if present, malignancy can be correctly diagnosed.^[16] If a great excess of mucin is present with relatively little malignant tissue, enhancement may be unremarkable or even absent in rare cases.

Papillary and tubular carcinomas may enhance strongly and rapidly. However, some of these tumors have weak angiogenesis, which reflects their relatively low biologic aggression. These lesions may then enhance relatively slowly and/or weakly.^[17]

Non-Hodgkin lymphoma of the breast is rare and is usually secondary to extensive involvement elsewhere in the body. It appears as a focal, well-defined mass with suggestive enhancement.^[17] Primary lymphoma may be synchronously or metachronously bilateral; in some cases, it grows rapidly to become large. An unwary pathologist may occasionally confuse this type of malignancy with an invasive carcinoma, particularly if needle-biopsy specimens are examined. MRM results are accurate in staging the intramammary extent of disease in such cases, and MRM may be useful for monitoring the response to subsequent chemotherapy, as with neoadjuvant therapy for large carcinomas.

In the breast, sarcomas are rare, malignant mesenchymal neoplasms. Metaplastic carcinomas are also rare, but they may undergo sarcomatous transformation, most typically to osteosarcoma. These generally appear as an otherwise nonspecific focal mass with suggestive enhancement. Sometimes, they are remarkably rounded with well-defined margins, but they may have markedly heterogeneous enhancement secondary to tumor necrosis.

Phyllodes tumors are usually diagnosed with mammography, US, and needle biopsy, and MRM adds little other than a true size measurement of large lesions. These lesions appear as large, well-circumscribed masses with rapid, strong enhancement; they often have internal lobulation and cystic spaces.^[17]

MRI-guided Biopsy or Localization

MRI-guided hookwire localization or some form of needle biopsy is an inevitable consequence of performing MRM, because MRM can depict lesions that are occult with all other forms of breast imaging. A number of research centers have been independently developed and reported various techniques, confirming that MRI-guided biopsy is at least practical and that it can be used to verify a malignant diagnosis of otherwise occult neoplasms.^{[45, 46]}

Early breast coils had a completely closed design that precluded biopsy or needle-based interventions in the breast. Breast coils that have an open design have also been developed for routine imaging. These permit free access to the breast from most directions. Unilateral, dedicated biopsy coils that have been developed allow complete access to the breast and much of the chest wall and axillae.^[47]

Several investigators have developed various techniques for MRI-guided fine-needle biopsy or localization.^{[17, 48, 49, 50, 51, 52, 53, 54]} These techniques range from simple freehand techniques, which remain useful for hookwire localization, to complex, robotic, automated systems, which are intended primarily for vacuum-assisted biopsy with large-bore needles.

Stereotactic needle guides and non-ferromagnetic biopsy needles and hookwires made specifically for breast MR interventions are commercially available. Typically, these devices are non-ferromagnetic, they compress the breast mediolaterally, they have a perforated needle-guide compression plate, they have MRI-visible markers, and they use mechanical needle positioning.

User preference and familiarity with conventional image-guided breast biopsy are critical factors for success, regardless of the device and choice of needle-biopsy technology.

Typically, imaging is performed with gentle, mediolateral compression by using compression plates to stabilize the breast. It is important not to apply too much pressure, because this may reduce lesion enhancement, sometimes markedly.^[17]

After contrast enhancement and appropriate targeting, the suspicious lesion is punctured with an MRI-compatible (non-ferromagnetic) needle by using repeated imaging to confirm the position of the needle. Needle biopsy can then be performed once the lesion is accurately localized, or a hookwire can be deployed for subsequent surgical excision. Although time-consuming, this procedure can confirm malignancy when the results of all other tests are negative.

Repeat MRM Examinations

In premenopausal patients, cyclic hormonal enhancement is a common cause of false-positive focal or multifocal enhancement. Although efforts to image premenopausal women in midcycle usually reduce the incidence of such foci, repeat midcycle MRM can show whether an apparent lesion changes from one cycle to the next. This is characteristic of hormonally influenced, benign tissue.

If a repeat examination shows a persistent abnormality with suspicious features that cannot be localized with US, the choice is to either continue to observe the lesion or to perform breast biopsy to achieve a definitive diagnosis. The decision should be based on the level of suspicion of the MRM findings; the options should be presented to and discussed with the patient.

Postbiopsy Changes on MRI Scans

Normal tissues do not significantly alter their enhancement after routine needle biopsy. Foci of punctate hemorrhage or a focal hypointense ferromagnetic metal artifact are occasionally seen at the biopsy site, particularly on GRE images.

Postoperative Changes on MRI Scans

Hematoma and seroma

Hematomas and seromas are usually seen shortly after surgery; they appear as fluid-filled nonenhancing, smoothly bounded cavities. Seromas are hyperintense on T2W images, and hematomas may be hyperintense or hypointense on T1W images, depending on the age and oxygenation state of the blood products.

Both usually show a rim of irregular and hypointense, but enhancing, granulation tissue. This enhancement may be intense, potentially masking residual malignancy at the boundary of the collection. This reactive postsurgical enhancement decreases over time. A delay of at least 28 days after surgery is needed to achieve a reasonable specificity (75%) and an NPV (86%) for the detection of residual tumor.^[55]

Fat necrosis

Fat necrosis at the site of surgery usually manifests as a small, nonenhancing hyperintense on T1W images. It appears as an ovoid focus with irregular stellate granulation tissue, which may be intensely enhancing after the administration of contrast material. This necrosis resembles a contracted hematoma or seroma. While this occasionally mimics a recurrent tumor, the typical lesion is usually readily diagnosable by its shape and enhancement pattern.

Postoperative scarring

Scar tissue, as shown in image below, generally appears as a low-signal-intensity, linear irregularity with variable enhancement, which largely depends on the interval since treatment. In the first few months after surgery, the borders of the surgical cavity may have strong enhancement, particularly if hemorrhage or fat necrosis has occurred.

This reactive enhancement gradually subsides. At 6 months after surgery without radiation therapy, most images show slow, minimal, or no enhancement. In such cases, the appearance of abnormal enhancement at the scar after 6 months should raise the suspicion of a recurrent malignancy.^[56] Again, tiny ferromagnetic, hypointense, sucker tip and instrument artifacts may be evident at the site of surgery, particularly if the native, unsubtracted GRE images are reviewed.

Post-Radiation Therapy Changes on MRI Scans

In the first 9-12 months after radiation therapy, a diffuse increase in capillary permeability occurs. This change initially causes marked parenchymal enhancement that later becomes patchy. In most women, this enhancement gradually declines after 18 months because of fibrosis.^[57] As a result, normal tissues have minimal enhancement, and any enhancing lesion on this background is suggestive of a recurrent tumor, which may appear nodular with carcinoma recurrence or may appear linear with DCIS.^[58]

Some patients may nevertheless benefit from MRM soon after surgery and radiation therapy if the presence of residual disease is strongly suspected. Although diffuse parenchymal enhancement is of little diagnostic value, the demonstration of typically malignant enhancement in a focal lesion should prompt repeat excision.

Postchemotherapy Changes on MRI Scans

MRM has been used successfully before, during, and after neoadjuvant chemotherapy to assess the preoperative tumor response in advanced local malignancy. Typically, such monitoring is performed by means of clinical palpation of the size of the tumor. However, this method can be highly inaccurate. For example, a good tumor response with necrosis and fibrosis may occur with only a slight reduction in the clinical size of a tumor. A clinical complete response may result in no enhancement or residual neovascularity visible on MRMs. In the latter case, this indicates residual viable tumor with a high degree of reliability.^{[59, 60]}

The correlation between MRM and pathologic findings of residual tumor is usually good.^[61] Chemotherapy does not produce the initial edema response seen with radiation therapy, and parenchymal enhancement becomes bland soon after the administration of contrast material; this characteristic also helps in the differentiation of normal tissues from malignant tissues.