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Milan Sheth, MD, and
Harold I. Litt, MD, PhD

CT AngiogrAphy of The pulmonAry
VAsCulATure

1. What are the primary methods for imaging for pulmonary embolus (PE)?

There are three main methods of pulmonary arterial imaging. One is pulmonary arteriography, which is accomplished by

inserting a catheter under fluoroscopic guidance into the pulmonary artery and injecting contrast agent to visualize the

pulmonary arteries directly. Nuclear ventilation/perfusion (

.

V/

.

Q) examinations compare the location of radiolabeled

ventilated particles with radiolabeled injected particles to help infer the presence of PE. The third method is computed

tomography (CT) pulmonary angiography, which involves injecting contrast agent into a vein and using CT to evaluate

the pulmonary arteries.

2. What plain film findings may suggest PE?

A chest radiograph is usually the first imaging modality used for a patient with suspected PE. The results of the chest

film may be negative or have nonspecific findings, such as pleural effusions and atelectasis, but occasionally the

film can have a wedge-shaped peripheral opacity, referred to as a Hampton hump. An area of lucency resulting from

decreased perfusion that is termed Westermark sign also may be seen.

3. What are the advantages of CT angiography over other methods in the evaluation

of PE?

One important advantage over catheter pulmonary arteriography and

.

V/

.

Q nuclear medicine scans is that CT

angiography may be used to diagnose other causes of the patient’s symptoms, such as a pleural effusion or atelectasis.

In contrast to arteriography, CT angiography is noninvasive and safer. CT angiography also takes less time to perform.

CT angiography has several advantages over nuclear

.

V/

.

Q imaging. It is less dependent on patient cooperation. In

addition, nuclear

.

V/

.

Q examinations may have a 60% to 70% indeterminate rate, especially in patients with underlying

lung disease or other comorbidities, which limit specificity.

4. How do you perform a CT angiography examination for PE?

The technique for performing CT angiography optimally involves the use of a multidetector row CT scanner. Contrast

agent is injected at a high rate into a peripheral vein. Scanning is timed to obtain optimal contrast opacification of the

pulmonary arteries. Thin-section axial CT images of 1- to 2-mm thickness are obtained throughout the chest. With CT

scanners containing 16, 64, or more slices, a CT scan for PE requires only a 5- to 10-second acquisition, and provides

high-quality images, even in very dyspneic patients.

5. What are the direct CT angiography findings of PE?

The most specific finding of a PE is a partial or complete intraluminal filling defect in a pulmonary artery (

Fig. 12-1

).

It should be present on at least two contiguous sections. Abrupt cutoff of the artery also indicates a PE.

6. What are the indirect findings of PE?

A pulmonary artery with an embolus may be slightly enlarged. Parenchymal findings, such as wedge-shaped opacities

denoting areas of infarction and atelectasis, sometimes can be seen as well. Areas of decreased perfusion can manifest

as wedge-shaped hyperlucency.

Key Points: CT Angiography

1. Multidetector row CT angiography has become the first-

line imaging examination for evaluation of suspected PE.

2. CT angiography has greater specificity than a

.

V/

.

Q scan.

3. CT angiography has greater availability and safety than

pulmonary angiography.

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CT AngiogrAphy of The pulmonAry VAsCulATure

7. How can acute PE be distinguished

from chronic PE on CT

angiography?

It may be difficult to differentiate acute from

chronic PE. Chronic PE may be complicated

by superimposed acute PE. Acute PE are often

seen as central filling defects, in contrast to

chronic emboli, which are more peripheral. This

is easy to remember because the peripheral

appearance of chronic PE is due to an acute

clot in a vessel that recanalizes over time.

Chronic emboli can also calcify, whereas acute

emboli do not. Long-standing PE are more

likely than acute emboli to cause pulmonary

hypertension, leading to right heart enlargement

and enlargement of the main and central

pulmonary arteries (

Fig. 12-2

). Lastly, increased

mediastinal collateral vessels can be seen in

some cases of chronic PE.

8. What are other uses for CT in PE?

CT also can be used to evaluate the lower

extremity venous system for thrombosis, using

the same contrast bolus, by imaging several

minutes after the PE study. Lower extremity

ultrasound (US) is generally used to evaluate

lower extremity veins, but CT has several

advantages. One is that it is not operator

dependent, in contrast to US. CT can be used to

evaluate for clot in areas that are inaccessible to

US or difficult to evaluate. This includes the deep

pelvic veins (a common source of thrombus

that may embolize to the lungs) and the region

around the adductor canal. US can be of limited

value in patients who are obese or have had

recent surgery. Within certain limitations,

CT may be a better choice than US in these

situations.

9. What is a pulmonary arteriovenous

malformation (AVM), and what

symptoms can it cause?

A pulmonary AVM is an abnormal communication

between the pulmonary artery and a draining vein, causing blood to bypass the pulmonary capillary bed before

returning to the left heart. AVMs may be asymptomatic, but they can manifest with a wide range of clinical symptoms.

Symptoms result from the loss of two essential physiologic functions of the lungs. Hypoxia may occur because of

shunting, and paradoxic emboli, stroke, or brain abscess may occur because of the loss of the “filter” effect of the

lung. Pulmonary AVMs may cause vague symptoms, such as chest pain and dyspnea on exertion, because of shunting

of deoxygenated blood. With severe shunting, high-output cardiac failure can result from right-to-left shunting.

10. What is orthodeoxia?

The term orthodeoxia is used to describe position-dependent oxygen desaturation. Most pulmonary AVMs occur in

the lower lobes. Shunting of blood and desaturation are maximum when blood flow to the lung bases is greatest.

Patients with large pulmonary AVMs in the lower lobes have larger shunts and lower oxygen saturation when

standing. When lying down, blood is redirected toward the lung apices, and the shunt fraction and desaturation may

decrease.

Figure 12-1.

Axial image from pulmonary CT angiography

examination shows bilateral PE, which are visualized as intraluminal

filling defects in the pulmonary arterial system (arrows).

Figure 12-2.

Axial CT angiography image shows bilateral chronic

PE (short arrows). Although these emboli have not calcified, the main

pulmonary artery (long arrow) is enlarged, indicating pulmonary

hypertension.

CT AngiogrAphy of The pulmonAry VAsCulATure

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CArDiAC AnD noninVAsiVe VAsCulAr imAging

11. What pulmonary AVMs are associated with what hereditary disorder?

Although pulmonary AVMs are often isolated, they can be associated with hereditary hemorrhagic telangiectasia, also

known as Osler-Weber-Rendu disease. Hereditary hemorrhagic telangiectasia is a genetic disorder that causes vascular

malformations that can lead to multiple pulmonary AVMs. It is characterized by telangiectasias of the skin and mucosal

linings, epistaxis, and AVMs in various internal organs such as the brain and liver. It is wise to screen family members of

patients who present with pulmonary AVMs so that they can receive treatment.

12. You are asked to start a peripheral intravenous line in a patient with a known

pulmonary AVM. What special precautions should you take?

Patients with pulmonary AVMs lack the “filter” function performed by the pulmonary capillary bed. This situation

predisposes patients to paradoxic emboli from endogenous and exogenous sources. Air or other material accidentally

introduced into the venous system could pass through the shunt and cause a stroke. Care must be taken that all venous

lines are free of air, and that filters are used to prevent paradoxic emboli.

13. What are the imaging characteristics of a pulmonary AVM on plain film and CT?

On plain film, pulmonary AVMs appear as serpiginous or nodular densities that can connect to the hilum. On CT, they

appear as a homogeneous noncalcified nodule that has a vascular connection with an enlarged feeding artery and

draining vein (

Fig. 12-3

). On dynamic images, the malformation enhances in a sequential manner from the feeding

artery to the malformation to the vein.

14. What is partial anomalous pulmonary venous return (PAPVR)?

PAPVR occurs when part of the pulmonary venous system drains directly into the systemic circulation. It can occur in

isolation or be associated with either atrial septal defect or a hypogenetic lung. When PAPVR is associated with atrial

septal defect (usually a sinus venosus defect), the right upper lobe drains into the superior vena cava. When PAPVR

occurs with a hypogenetic lung, it is a component of congenital pulmonary venolobar syndrome, also known as scimitar

syndrome.

15. What is scimitar syndrome, and what are its associated imaging findings?

Scimitar syndrome is a hypogenetic lung (almost exclusively on the right) that is drained by an anomalous vein.

This anomalous vein can drain into many structures, including the infradiaphragmatic inferior vena cava (IVC),

suprahepatic IVC, portal vein, or right atrium. The chest x-ray shows a small right lung with a tubular opacity paralleling

the right heart border (called the scimitar). Magnetic resonance imaging (MRI) and CT show the course and nature of the

abnormal vascular anatomy better (

Fig. 12-4

).

Figure 12-3.

Axial CT angiography image shows a pulmonary AVM,

with feeding artery and draining vein (arrows) associated with the lesion.

80

CT AngiogrAphy of The pulmonAry VAsCulATure

A

B

Figure 12-4.

Findings of scimitar syndrome.

A, Axial MR image shows small right lung and shift of the mediastinum to the right. B, Oblique

maximum intensity projection of magnetic resonance angiography (MRA) of the chest showing anomalous pulmonary venous drainage of the

right lower lobe to the IVC (arrow), the so-called scimitar vein. The upper lobe drains into the left atrium (arrowhead ).

16. What is the most common primary tumor of the pulmonary artery?

Tumors associated with the pulmonary vascular system are exceedingly rare. Angiosarcomas are most common.

These are rare tumors and sometimes can be difficult to distinguish from occlusive PE because both fill the lumen of the

pulmonary artery.