Helical CT Pulmonary Angiography

Pulmonary emboli are visualized as low attenuation filling defects within the enhanced pulmonary arterial circulation on HCTPA. Additional findings suggesting acute PE on HCTPA include expanded, unopacified vessels (figure 1b), eccentric filling defects (figure 2), oligemia (figure 3), and peripheral consolidations indicating pulmonary hemorrhage or infarction.

unopacifiedart

Figure 1a. Normal Opacified Vasculature in Acute PE

Image immediately cranial to figure 1b shows well-enhanced vessels.

Figure 1b. Expanded, Unopacified Vasculature in Acute PE

Image immediately caudal to figure 1a shows an enlarged and completely unopacified right lower lobe pulmonary artery, representing an acute PE.

Find the unopacified artery.

thrombusltthrombusrt

Figure 2. Eccentric Thrombus in Acute PE

HCTPA shows embolus adherent to the lateral wall of the left lower lobe pulmonary artery. This pattern is not specific for acute PE because it is commonly encountered with chronic PE also. Acute PE is present within the right lower lobe pulmonary artery.

Find the adherent embolus in the left lower lobe pulmonary artery.

Find the acute PE in the right lower lobe pulmonary artery.

Figure 3. Oligemia with acute PE. HCTPA photographed in lung windows in a patient with a large central PE shows decreased attenuation, representing oligemia, in the peripheral upper lobes. Note how the vessels appear smaller in the affected areas. Find the oligemic areas in each lung.

oligemia1oligemia2

Technical Aspects of HCTPA: Unlike conventional ("stop-and-shoot") CT, with helical (spiral) CT, the x-ray tube rotates 360 degrees around the patient while the patient moves through the gantry. Thus, helical CT allows for the continuous acquisition of scan data, and allows for very rapid scan times. However, with helical CT scanning, only one image is acquired for every rotation of the x-ray tube (which takes about 0.8 seconds). Multislice scanning was introduced in 1998. The first multislice scanners allowed for the simultaneous acquisition of 4 images per x-ray tube rotation; therefore, these scanners were nearly 4 times faster than their single slice helical counterparts. Currently, 16-slice scanners are commercially available, and x-ray tube rotation times are as short as 0.4 seconds. Therefore, the latest generation of multislice scanners are about 32 times as fast as single slice CT scanners. The incredible speed with which multislice scanners acquire data allows patients to be scanned with narrow collimation (slice thickness), thus improving spatial resolution. For example, standard, single slice helical CT scanners performed for suspected pulmonary embolism use 3 mm collimation, which increases to an effective slice thickness of about 4 mm when one accounts for the rapid table transport speed required to cover the necessary imaging volume required for pulmonary embolism studies: about two-thirds of the chest. Such a scan requires about 20 seconds. In contrast, sixteen-channel multislice scanners allow the entire thorax to be scanned using 1 mm slice thickness, with virtually no increase in effective slice thickness, in about 6 seconds.

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Oligemic lung

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