Pulmonary Vascular Imaging

1

What is the normal appearance of the pulmonary vessels on computed tomography (CT) and magnetic resonance imaging (MRI)?

The main pulmonary artery originates from the right ventricular outflow tract, anterior and to the left of the aortic root, and serves as a conduit for blood from the right ventricle to the pulmonary circulation. It bifurcates into the right and left pulmonary arteries. The right pulmonary artery passes horizontally through the mediastinum anterior to the major bronchi before bifurcating into the truncus arteriosus and right interlobar arteries, whereas the left pulmonary artery arches more superiorly to pass over the left mainstem bronchus before descending posterior to the major bronchi as the left interlobar artery. The pulmonary arteries then branch into lobar, segmental, and subsegmental pulmonary arterial branches.

The pulmonary veins, typically numbering four in total (right superior, right inferior, left superior, and left inferior), drain into the left atrium. However, common pulmonary venous trunks and accessory pulmonary veins are not uncommonly encountered.

On contrast-enhanced CT and MRI, the pulmonary arteries and veins will generally enhance to a degree similar to that of the right ventricle and left atrium, respectively, which will vary depending on the timing of image acquisition relative to the start of intravenous contrast administration. Other than the pulmonary valve leaflets at the origin of the main pulmonary artery, no filling defects are normally seen within the lumina of these vessels. The segmental pulmonary arteries are always located adjacent to bronchi, which are on average of similar caliber, whereas the pulmonary veins are located separately.

2

What are the indications for computed tomography angiography (CTA) versus magnetic resonance angiography (MRA) for pulmonary vascular imaging?

Both CTA and MRA demonstrate high sensitivity and specificity for diagnosis of pulmonary vascular diseases including acute and chronic pulmonary embolism (PE), pulmonary hypertension, pulmonary arterial aneurysm, pulmonary arteriovenous malformation (AVM), and partial or total anomalous pulmonary venous return.

Unlike catheter angiography or echocardiography, CTA is noninvasive and is not limited by body habitus, air, or bone in terms of obtaining an acoustic window. MRA does not utilize iodinated contrast material and does not involve ionizing radiation. Thus, MRA is especially suitable for imaging children, women of childbearing age, or patients with allergies or other contraindications to iodinated contrast material.

3

What are the typical CTA and MRA pulmonary vascular imaging protocols?

On modern multidetector computed tomography (MDCT) scanners, CTA for pulmonary arterial evaluation utilizes a high rate of iodinated contrast material injection (4 to 6 ml/sec) via a peripheral vein. Scanning is timed to obtain optimal contrast opacification of the pulmonary arteries. Thin-section axial and multiplanar CT images of 1 mm thickness are then reconstructed through the chest. With the latest generation CT scanners, image acquisition times can be less than 5 seconds, providing high-quality images even in patients who are unable to breath hold.

Bright blood MRA utilizing steady state free precession (SSFP) gradient echo sequences and 2D time of flight (TOF) techniques may be performed without gadolinium-based intravenous contrast material. 3D contrast-enhanced MRA is an additional technique used to image the pulmonary vasculature with greater detail. Similar to CTA, the image data can be reformatted on a computer workstation to obtain multiplanar views for optimal evaluation.

4

What is the role of chest radiography in the diagnosis of pulmonary embolism (PE)?

Chest radiography is usually the first imaging modality used on a patient with suspected PE. It is neither sensitive nor specific for PE but may demonstrate suggestive ancillary findings such as a pleural effusion, a dome-shaped peripheral opacity referred to as a Hampton hump that reflects a pulmonary infarct, a prominent central artery known as the Fleischner sign, or an area of apparent pulmonary lucency resulting from decreased lung perfusion termed the Westermark sign. It may alternatively demonstrate other findings that are diagnostic of other disease conditions.

5

What are advantages of CTA for assessment of PE?

CTA is the current reference standard for PE evaluation. One important advantage of CTA over catheter pulmonary arteriography and pulmonary scintigraphy is that it may be used to diagnose other causes of the patient's symptoms, such as pleural effusion, pneumonia, atelectasis, or malignancy. Moreover, in contrast to conventional pulmonary arteriography, CTA is noninvasive, safer, faster to perform, and more widely available. CTA has several advantages over pulmonary scintigraphy. It is less dependent on patient cooperation and is faster to perform. In addition, pulmonary scintigraphy may have a 60% to 70% indeterminate rate, especially in patients with underlying lung disease or other comorbidities, limiting diagnostic accuracy.

6

What are the direct CTA findings of PE?

The most specific finding of PE is a partial or complete hypoattenuating intraluminal filling defect within a pulmonary artery. It should be present on at least two contiguous images ( Figure 15-1, A ).