eMedicine Specialties > Radiology > Chest

Lung, Arteriovenous Malformation: Imaging

Author: Sat Sharma, MD, FRCP(C), FCCP, FACP, DABSM, Program Director, Associate Professor, Department of Internal Medicine, Divisions of Pulmonary and Critical Care Medicine, University of Manitoba; Site Director of Respiratory Medicine, St Boniface General Hospital
Coauthor(s): Krantikumar Rathod, MD, Lecturer, Section of Vascular and Interventional Radiology, Department of Radiology, KEM Hospital, Parel, Mumbai, India; Bruce Maycher, MD, Director of Pulmonary Radiology, St Boniface General Hospital; Associate Professor, Department of Radiology, University of Manitoba
Contributor Information and Disclosures

Updated: Oct 22, 2007

Radiography

Findings

Chest radiography is recommended for the initial evaluation of patients with HHT. PAVM may well be an incidental finding on a chest radiograph. A common radiographic finding is a round or oval mass of uniform opacity. The opacity may have sharply defined borders with occasional lobulation. The mass is usually 1-5 cm, and linear shadows are adjacent to the opacity; these are the feeding vessels. PAVMs are commonly present in the lower lobes, and approximately 50% of patients have 2-8 lesions.

Degree of Confidence

When the classic radiographic features are present on the chest radiograph in a patient with suspected PAVM, the diagnosis is certain. However, chest CT scanning is invariably required to confirm the finding.

False Positives/Negatives

Patients with microvascular telangiectases may have normal chest radiographic results. Of the 27 patients reported on in one series, the plain chest radiographic findings were strongly suggestive of PAVM in 6 patients, somewhat suggestive in 5 patients, and not suggestive in 6 patients. CT scanning may be more accurate than plain chest radiography in diagnosing PAVM. In a patient who has clinical features suggestive of PAVM but has a normal chest radiograph, further evaluation using other imaging modalities should be undertaken.

Computed Tomography

Findings

Contrast-enhanced CT scanning is the preferred imaging modality for confirming the diagnosis of PAVM. One study reported better sensitivity for PAVMs with ultrafast, contrast-enhanced CT scanning than with pulmonary angiography. CT scans depicted 98% of PAVMs in 20 patients, whereas angiography depicted only 60% of PAVMs.

Three-dimensional (3D), spiral CT scanning produces images of vascular structures that are continuously reconstructed. In one study, spiral CT scanning proved to be a better investigative tool than unilateral pulmonary angiography. The accuracy of 3D spiral CT scanning is reported to be more than 95%, and it may also be useful in identifying smaller PAVMs.⁴

Multidetector-row CT scanning

The development of multidetector-row CT (MDCT) scanning has resulted in a revolution in imaging capabilities. When compared with single–detector-row CT (SDCT) scanning, CT angiography with MDCT scanning provides superior imaging capabilities. The advantages include an increased per-second scanning area, the ability to retrospectively select the scanning width, the ability to routinely provide 1-mm collimation, and the possibility of higher resolution with no blooming of section thickness.^5,6,7

MDCT scanning also has several practical advantages in the evaluation of PAVM. These include volume imaging, with 3D displays of the volume data, and retrospective reconstruction of the CT scanning data in any plane with high resolution.⁶

Maximum-intensity projection (MIP) and surface rendering were the 3D techniques that were primarily applied to CT angiographic applications in the past. However, volume rendering is becoming the preferred 3D rendering technique for CT angiography.

Surface rendering

Surface rendering, the earliest method for 3D display, is commonly available and utilizes each voxel in the data set. The voxel intensity is compared with a threshold value, thereby defining the surface of the object, and the remaining data are discarded. Surface contours are typically modeled with surface shading; therefore, the resulting image is simplified and may provide a misleading representation of the structure. Conversion of data from a volume to a surface sacrifices a large portion of the data and limits the usefulness of surface-rendered medical images.

Maximum-intensity projection

MIP has been extensively utilized in creating angiographic images from CT scanning and MRI data. Each voxel is evaluated along a line through the image, and the value of the corresponding display pixel is selected based on the value the maximum voxel. The resulting images are typically not displayed with surface shading and increase the background mean of the image, thereby enhancing the structures. The problem with MIP is that volume averaging along with the algorithm may lead to artifacts. Despite its artifacts and deficiencies, MIP has been extensively evaluated and usually provides accuracy superior to that of surface rendering for CT angiography.

Volume rendering

This technique renders the entire volume of data rather than just the surfaces, with the total contributions of each voxel being utilized; therefore, more information is displayed than would be provided in a surface model. Volume technique is the most advanced form of 3D rendering currently available for creating accurate, clinically useful medical images and has revolutionized the field of vascular imaging.

Degree of Confidence

Contrast-enhanced CT scanning or spiral CT scanning is the standard of care for the diagnosis of PAVMs.

False Positives/Negatives

It may be difficult to differentiate a PAVM from a vascular tumor on a CT scan. In this situation, a false-positive diagnosis of PAVM is possible. Because prolonged breath holding is required, a large PAVM may be difficult to visualize on 3D spiral CT scans. Furthermore, CT scans may not depict microscopic or small PAVMs.

Magnetic Resonance Imaging

Findings

Conventional MRI may be useful in detecting PAVMs. However, compared with other techniques, it has reduced sensitivity and specificity. Because rapidly flowing blood results in an absent or minimal MRI signal, a PAVM may be indistinguishable from normal vascular structures or an air-filled lung.

Various techniques have been used to improve the sensitivity of MRI. These include a rotating, gated MRI technique and gradient recalled-echo MRI. Some investigators have suggested that phase-contrast cine sequences are most accurate in detecting PAVM. Magnetic resonance angiography (MRA) has been used to define the vascular anatomy of a PAVM.

Degree of Confidence

Although a combination of MRI techniques may be useful in differentiating PAVMs from other lesions, more studies are required before the routine use of MRI can be recommended.

False Positives/Negatives

In research investigating the role of MRI in the diagnosis of PAVM and examining the modality's false-positive rates, a relatively small number of patients were studied. The sensitivity and specificity of magnetic resonance images are lower than those of CT scans. Furthermore, MRI techniques often yield conflicting results.

Ultrasonography

Findings

Contrast-enhanced echocardiography is used to confirm a right-to-left intrapulmonary shunt. Cardiac imaging with 2-dimensional echocardiography is performed while 5-10 mL of agitated sodium chloride solution is injected into a peripheral vein. In a healthy person, microbubbles are rapidly visualized in the right atrium before they gradually dissipate. When an intracardiac shunt is present, the contrast material is visualized in the left heart chambers within 1 cardiac cycle after it appears in the right atrium. The visualization of contrast material in the left atrium after a delay of 3-8 cardiac cycles confirms the presence of an intrapulmonary shunt (ie, PAVM).

Degree of Confidence

Contrast-enhanced echocardiography has high sensitivity in detecting right-to-left intrapulmonary shunts, such as PAVMs. This finding should be confirmed by using other tests. The finding of an intrapulmonary shunt during contrast-enhanced echocardiography still warrants further anatomic evaluation with contrast-enhanced CT or pulmonary angiography.

Contrast-enhanced echocardiography is generally not used as a first-line screening test because of its cost, limited availability, and possible overdetection of clinically insignificant PAVMs.

False Positives/Negatives

The sensitivity of contrast-enhanced echocardiography approaches 100%; therefore, false-negative test results are unlikely. No specific data address false-positive results; however, contrast-enhanced echocardiography may depict microscopic or clinically insignificant PAVMs.

Nuclear Imaging

Findings

Radionuclide perfusion lung scanning also is used in the diagnosis of PAVM, and the findings usually confirm a right-to-left shunt. This test is particularly useful if contrast-enhanced echocardiography or the 100% oxygen method is not available. The test involves the peripheral, intravenous injection of macroaggregated albumin labeled with technetium-99m (^99m Tc). In healthy persons, these particles are filtered by pulmonary capillaries. However, in the presence of true right-to-left shunts, radiolabeled particles pass through the lungs and are trapped in the brain and kidneys. The shunt fraction may be calculated by quantifying the renal uptake as a percentage of the total dose given or lung uptake.

Degree of Confidence

Radionuclide scanning is accurate in detecting a right-to-left shunt, and it may have some advantages over the 100% oxygen method.

However, although radionuclide scanning is comparable to the 100% oxygen method in detecting PAVMs, its expense and limited availability prohibit its widespread use. Nonetheless, although the radionuclide method of shunt calculation is not routinely available at most hospitals, it has the following advantages over the 100% oxygen method:

• Radionuclide scanning does not require arterial blood gas sampling.
• The 100% oxygen method may cause overestimation of the intrapulmonary shunt.
• The radionuclide method is more suitable for shunt measurement during exercise.

False Positives/Negatives

Radionuclide perfusion scanning depicts only right-to-left shunts and is not useful in determining a shunt's intracardiac or intrapulmonary location.

Angiography

Findings

Contrast-enhanced pulmonary angiography should be used in the treatment of PAVM using embolization or prior to surgical treatment. This test is usually necessary to study the detailed anatomy of PAVM, particularly if the resection or obliterative therapy is planned. Furthermore, angiography may depict other unsuspected PAVMs, as well as unsuspected intrathoracic or extrathoracic vascular communications.

Digital subtraction angiography has replaced conventional angiography in the diagnostic imaging of PAVM. To the authors' knowledge, however, no systematic comparison of the 2 techniques has been reported in the literature. Because nonselective pulmonary angiograms may have difficulty depicting the anatomy of a PAVM, superselective pulmonary angiography is recommended.

Degree of Confidence

Pulmonary angiography has been the criterion standard for defining the anatomy of PAVMs. However, spiral CT scanning is now considered more appropriate for diagnosis.

False Positives/Negatives

Pulmonary angiography has high specificity but a sensitivity lower than that of contrast-enhanced ultrafast CT scanning or 3D spiral CT scanning.

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