Thoracic Imaging 2003 - Society of Thoracic Radiology

ologies. Another approach involves a physiological division into
four categories: increased pulmonary flow, chronic hypoxia,
vessel obliteration and chronic pulmonary venous hypertension.
1. Increased (Hyperdynamic) flow: ASD, VSD, PDA,
Transposition of the Great Vessels, Common AV canal
and partial anomalous pulmonary venous drainage.
2. Chronic Hypoxia: Chronic bronchitis, Bronchiolitis
obliterans, advanced pulmonary fibrosis, Sleep apnea,
Cystic fibrosis and chronic high altitude exposure.
3. Vessel obliteration: Emphysema, Primary pulmonary
hypertension, chronic thromboembolic disease,
Vasculitis, sickle cell disease, schistosomiasis and
tumor emboli.
4. Chronic Venous hypertension: Left ventricular or atrial
disease, left sided valvular disease, and pulmonary
venoocclusive disease.
Chest Radiographic Evaluation
Enlargement of the main pulmonary artery is often the first
radiographic manifestation, likely based on the principals from
Laplace’s law. Increased lobar and segmental vessel size, along
with right ventricular chamber and outflow tract enlargement
eventually occur. Rapid tapering (“pruning”) from the central to
peripheral arteries and increased tortuosity of the arterial course
are seen in advanced stages. (2) The increase in the relative
diameter of the arterial and venous ratio is another radiographic
manifestation of PH. (3)
Once the diagnosis of PH is suspected or known, the underlying
etiology may be identified on the radiograph. Increased
pulmonary flow with large arteries and veins along with a
decreased aortic arch size support a left to right shunt (hyperdynamic
flow) as the cause for PH. Hyperinflation, extensive
bronchiectasis, or pulmonary fibrosis support a secondary development
of PH from a primary lung disorder, often secondary to
chronic hypoxia. Patients who are morbidly obese with enlargement
of the pulmonary arteries PH may be secondary to sleep
apnea or potentially weight loss medication. The presence of a
TIPS stent in the right upper quadrant of the abdomen and
increased flow and vessel conspicuity in the lower lobes often
reflects hepatopulmonary syndrome/cirrhotic-associated PH.
Normal lungs with radiographic patterns of PH suggest a primary
vascular cause such as plexogenic arteriopathy, drug
induced or chronic thromboembolic disease (obliteration of ves-
sels). Finally, when radiographic evidence of enlarged upper
lobe vessels and constricted lower vessels are present (“cephalization”),
this supports chronic pulmonary venous hypertension
as the most likely cause. Enlargement of the left atrium and possibly
the left ventricle should be present.
Multidetector CT Evaluation
Multidetector CT is being used increasingly at our institution
for the evaluation of patients with known or suspected PH. It is
useful in documenting the imaging presence or absence of PH,
evaluating for chronic thromboembolic disease and deciding
between a primary lung disease as the main factor versus intrinsic
pulmonary vascular disease. Primary and secondary cardiac
abnormalities are also better seen with the thinner collimation
and increased speed of image acquisition.
The exams are performed as a CT pulmonary angiogram,
beginning from adrenals and scanning cranially through the
apices. Some thin section expiratory images are also performed
to evaluate for air trapping. By beginning the image acquisition
caudally, it is possible to see any initial reflux of hyperdense
contrast into the dilated IVC and hepatic veins. This dynamic
contrast flow is helpful as it often signifies elevated right atrial
and ventricular end diastolic filling pressures. (4) The absence
of this or the presence of unopacified mixing in the right heart
chambers from a decompressed IVC argues against any significantly
elevated right pressures and concurrently, the presence of
PH. Further evaluation with direct right heart catheter pressures
may be required in these patients since it is unlikely they have
significant PH.
Supportive cardiac findings include right atrial and ventricular
chamber and outflow tract hypertrophy. The presence of left
heart chamber dilation and cephalization supports pulmonary
venous hypertension as an important contributor to the PH.
Septal thickening, patchy and/or centrilobular ground glass nodules
are supportive evidence for pulmonary venous abnormalities.
(5) As the collimation of the CT scanners narrows, our ability
to detect septal defects and potentially valve thickening and
calcification should also improve.
The main pulmonary artery size is considered enlarged
when greater than 2.9cm and when lobar arteries are larger than
the accompanying bronchus, the latter being a useful sign for
conventional radiographs too. Chronic pulmonary thromboembolic
disease is well seen with CT, especially in more proximal
vessels. Eccentric mural thickening, asymmetric pulmonary
enlargement, webs, mural calcification and abrupt narrowing of
the lumen at the distal lobar or segmental levels are the most
common imaging findings. (6)
The lungs often demonstrate mosaic lung attenuation, especially
with chronic thromboembolic disease. However, the presence
of severe air trapping on the expiratory images favors an
airways disease such a bronchiolitis obliterans, long-standing
asthma or tracheomalacia as the primary problem with PH a
secondary consequence.
Uncommon Causes
Pulmonary Venoocclusive Disease/Pulmonary Capillary
Hemangiomatosis
Pulmonary venoocclusive disease is an uncommon disorder,
which is caused by the gradual obliteration of pulmonary veins.
Intimal proliferation and fibrosis of the veins are found on
pathology. The chronic elevation of the venous pressures eventually
leads to PH. Multiple etiologies are associated with this
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