Leiomyomas of the urterus (or uterine fibroids) are benign tumors that arise from the overgrowth of smooth muscle and connective tissue in the uterus. Histologically, a monoclonal proliferation of smooth muscle cells occurs. A genetic predisposition to leiomyoma growth exists. The radiologic characteristics of these neoplasms are shown in the images below.
Rarely, uterine leiomyomas may undergo malignant degeneration to become a sarcoma. The true incidence of malignant transformation is difficult to determine, because leiomyomas are common, whereas malignant leiomyosarcomas are rare and can arise de novo. The incidence of malignant degeneration is less than 1.0% and has been estimated to be as low as 0.2%.
The preferred imaging modality for the evaluation of uterine fibroids is ultrasonography (US), specifically, transabdominal and transvaginal US.
Calcified fibroids are often depicted on conventional radiographs of the pelvis. In some patients, magnetic resonance imaging (MRI) provides additional information. The role of computed tomography (CT) scanning is limited. Calcifications may be more visible on CT scans than on conventional radiographs because of the superior contrast differentiation achieved with CT scanning. [1, 2]
Limitations of techniques
In the detection of uterine fibroids, CT scanning is limited by the similar attenuation characteristics of fibroids and healthy myometrium, although some fibroids may be hypoattenuating. Fibroid calcifications can be depicted on CT scans.
Conventional radiographs have a limited role in the diagnosis of uterine fibroids, because only heavily calcified fibroids are depicted on these scans. Extreme enlargement of the uterus resulting from fibroids may be seen as a nonspecific soft-tissue mass of the pelvis that possibly displaces loops of bowel.
Like radiography, CT scanning also has a limited role in the diagnosis of uterine fibroids. On CT scans, fibroids are usually indistinguishable from healthy myometrium unless they are calcified or necrotic. Calcifications may be more visible on CT scans than on conventional radiographs because of the superior contrast differentiation in CT scanning. A fibroid has been identifed on the image below.
Magnetic Resonance Imaging
MRI has an important role in defining the anatomy of the uterus and ovaries, as well as in assessing disease in patients in whom US findings are confusing. MRI also may be helpful in planning myomectomy, or selective surgical removal of a fibroid.
Fibroids appear as sharply marginated areas of low to intermediate signal intensity on T1- and T2-weighted MRI scans. (A coronal, T2-weighted MRI fibroid scan appears below.)
One third of fibroids have a hyperintense rim on T2-weighted images (as demonstrated in the image below) as a result of dilated veins, lymphatics, or edema.
An inhomogeneous area of high signal intensity (seen in the image below) may be depicted on T2-weighted images; this results from hemorrhage, hyaline degeneration, edema, or highly cellular fibroids.
The intravenous administration of gadolinium-based contrast material usually is not required; however, if it is administered, fibroids usually enhance later than does the healthy myometrium. Fibroid enhancement can be hypointense (65%), isointense (23%), or hyperintense (12%) in relation to that of the myometrium.
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 Medscape Reference topic Nephrogenic Systemic Fibrosis. 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 magnetic resonance angiography (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 Medscape.
The primary limitation of MRI is the high cost. Its availability in terms of time and location is another factor that determines its usefulness. Additionally, patients with pacemakers or certain metallic foreign bodies cannot undergo MRI. Claustrophobia is a relative contraindication.
Degree of confidence
MRI has a sensitivity of 86-92%, a specificity of 100%, and an accuracy of 97% in the evaluation of probable fibroids.
In one study, MRI findings were assessed in helping predict early posttherapeutic response after uterine artery embolization (UAE) in 15 patients with 52 fibroids. The signal intensity ratios (SIRs) on T1-weighted images and gadolinium-enhanced images were useful for the prediction of the changes in size of fibroids responding to UAE. The sensitivity, specificity, and area under the ROC curve (AUC) in the prediction of the affected lesions were 92%, 50%, and 0.712 with SIR on T1-weighted images, and 85%, 62%, and 0.731 with SIR on gadolinium-enhanced images, respectively. 
Most fibroids are intramural, that is, located in the myometrium (as seen in the first image below); however, they can be submucosal or subserosal (as demonstrated in the second image below).
Uterine fibroids most often appear on ultrasonograms as concentric, solid, hypoechoic masses. This appearance results from the prevailing muscle, which is observed at histologic examination. These solid masses absorb sound waves and therefore cause a variable amount of acoustic shadowing. A hypoechoic fibroid is seen in the images below.
Fibroids may vary in their degree of echogenicity; they can be heterogeneous or hyperechoic, depending on the amount of fibrous tissue and/or calcification. Fibroids may have anechoic components resulting from necrosis.
If fibroids are small and isoechoic relative to the uterus, the only ultrasonographic sign may be a bulge in the uterine contour. Fibroids in the lower uterine segment may obstruct the uterine canal, causing fluid to accumulate in the endometrial canal.
The echogenic endometrial stripe may be displaced by a fibroid. Calcifications are hyperechoic, with sharp acoustic shadowing. Diffuse leiomyomatosis appears as an enlarged uterus with abnormal echogenicity.
Magnetic resonance-guided high-intensity focused ultrasound has been shown to be successful in reducing the size of fibroids. 
Vascular density, ischemic necrosis, and histological cellular activity score have been found to be statistically significantly associated with some 3D power Doppler ultrasound indices. A high histological cellular activity score, combining hypercellularity, a fibrosclerosis rate less than 25%, and positive Ki-67 staining, was found in one study to be statistically related in multivariate analyses to high 3D power Doppler VI in spherical samples and vascularization flow index (VFI). Positive CD31 staining was statistically related to high 3D power Doppler VI in spherical samples. In contrast, ischemic necrosis was statistically related to low 3D power Doppler VI in the total volume and VFI. 
Of 280 women who underwent magnetic resonance-guided focused ultrasound (MRgFUS), the rate of minor complications was 3.9%, and there were 3 serious complications (1.1%), including one skin burn, a fibroid expulsion, and one case of persistent neuropathy. According to the authors of the study, the nonperfused volume (NPV) achieved following MRgFUS have increased as the experience with this treatment has grown. In a 5-year follow-up study of 162 women, the overall reintervention rate was 58.64%, but in those treatments with greater than 50% NPV, the re-intervention rate was 50%. 
Degree of confidence
US has a sensitivity of 60%, a specificity of 99%, and an accuracy of 87%.
Although the ultrasonographic appearance of fibroids usually is diagnostic, in fewer than 5% of patients, fibroids (especially when necrotic) may mimic normal pelvic structures (particularly the ovaries) and pathologic pelvic conditions, including uterine variants and pregnancy-related conditions. MRI results often clarify confusing pelvic ultrasonographic findings.
Although angiography has no role in the diagnosis of uterine fibroids, it is used to guide uterine arterial embolization (UAE) of fibroids.
UAE is a minimally invasive procedure performed by an interventional radiologist. Typically, UAE is carried out via a percutaneous femoral arterial approach. Both of the uterine arteries are individually selected with angiographic guidance and are embolized with 300-500 µm polyvinyl alcohol (PVA) foam particles. [10, 11]