Chest radiography
- Chest radiographs reveal some abnormality in approximately 98% of patients. The classic abnormal radiographic finding is a round or oval mass of uniform opacity. The mass is frequently lobulated and most commonly appears in the lower lobes. A chest radiograph can reveal features that may be undetectable on plain chest radiographs; examples include a feeding vessel, an artery radiating from the hilus, and the vein deviating toward the left atrium.
- In a patient who has clinical features suggestive of PAVM but normal chest radiographic findings, further evaluation with other modalities should be performed. Patients with microscopic PAVM may have normal chest radiographic findings. PAVM should also be considered in the differential diagnosis of a pulmonary nodule. A cautious approach to these patients is suggested before diagnostic needle biopsy is undertaken
Contrast-enhanced CT scanning
- The presence of PAVM and its vascular anatomy can also be evaluated by means of contrast-enhanced ultra-fast CT. CT allows for the detection of 90% of PAVMs, whereas, in one study, angiography allowed for the detection of only 60% of PAVMs. The superior sensitivity of CT is attributed to the absence of superimposition of lesions on CT views.
- Three-dimensional (3D) helical CT scanning produces images of vascular structures that are continuously reconstructed by a helical CT scanner. The accuracy of 3D helical CT scanning is reported to be 95%.
- Tests for confirmation of an intrapulmonary right-to-left shunt
- These tests should be performed initially. The shunt is best calculated by using the 100% oxygen method. Contrast echocardiography and radionuclide scanning have nearly 100% sensitivity and are used to confirm clinically significant PAVM. However, the 100% oxygen method for shunt calculation is the least expensive and readily available.
- In patients who have a shunt fraction of more than 5%, as determined with the 100% oxygen method, further assessment and management is recommended. In some patients with a shunt fraction of less than 5% but a high clinical suspicion for PAVM, additional evaluation with contrast echocardiography or radionuclide scanning is recommended.
Contrast echocardiography
- Contrast echocardiography is an excellent tool for evaluating cardiac or intrapulmonary shunts. This technique involves the injection of 5-10 mL of agitated saline into a peripheral vein while simultaneously imaging the right and left atria with 2-dimensional echocardiography. In patients without right-to-left shunting, contrast is rapidly visualized in the right atrium and then gradually dissipates. In patients with intracardiac shunts, contrast is visualized in the left heart chambers within 1 cardiac cycle, after its appearance in the right atrium. In patients with PAVM, contrast is visualized in the left atrium after a delay of 3-8 cardiac cycles. Contrast echocardiography is almost 100% sensitive in detecting clinically important PAVMs.
- In one case series, PAVM were visible in 11 of 14 patients with positive contrast echocardiographic findings who underwent pulmonary angiography. Six had abnormal chest radiographic results, and 8 had an increased A-a gradient. Contrast echocardiography had 100% sensitivity in this study. The finding of an intrapulmonary shunt by means of contrast echocardiography warrants further evaluation with standard pulmonary angiography or contrast-enhanced CT scanning.
Radionuclide perfusion lung scanning
- Radionuclide perfusion lung scanning is also useful in the diagnosis of PAVM, particularly if contrast echocardiography is not available.
- In patients without an intrapulmonary shunt, the peripheral intravenous injection of technetium 99m–labeled macroaggregated albumin results in the filtering of these particles by the lung capillaries. However, anatomic shunts with dilated pulmonary vascular channels allow these particles to pass through the lung, with subsequent filtering by the capillaries in the brain and kidneys.
- Pulmonary angiography
- Despite advances in noninvasive diagnostic techniques, contrast-enhanced pulmonary angiography remains the criterion standard in the diagnosis of PAVM. This test is usually necessary if embolotherapy is being considered. Perform pulmonary angiography in all lobes of the lungs to look for unsuspected PAVM.
- Currently, digital subtraction angiography appears to be replacing conventional angiography. Whether CT or MRI can replace standard pulmonary angiography in the diagnosis of PAVM requires further comparative studies. Presently, CT and MRI are appropriate noninvasive modalities for the follow-up evaluation of patients with proven PAVM.
Magnetic resonance (MR) imaging:
- MRI has been reported to be useful in the diagnosis of PAVM. Rapidly flowing blood results in an absent or minimal MR signal, a so-called flow void. However, a PAVM may be indistinguishable from an adjacent air-filled lung on MRI, a significant limitation in screening for small lesions. Therefore, spin-echo MRI has reduced sensitivity and specificity for detection of PAVM, compared with those of other techniques. Better results are obtained with phase-contrast cine sequences, and MR angiography can be used to define the vascular anatomy of a PAVM. A combination of MR techniques may be useful in differentiating PAVM from various other lesions, but more comparative data are required before the routine use of MRI is recommended.