Smaller FOV cuts dose but maintains sensitivity for detecting pulmonary emboli

An abbreviated field-of-view can lead to a 48% cut in radiation dose for CT pulmonary embolism exams and 96% dose reduction compared to full-view 64-slice CT triple rule-out without affecting the diagnostic sensitivity of either procedure.

Dr. Hetal Patel, a radiologist with Hartford Hospital-Jefferson Radiology in Hartford, CT, presented results from a retrospective evaluation of 600 CTA exams for pulmonary emboli studies that demonstrated the value of condensing the FOV by eliminating areas above the aortic arch and below the heart.

Fifty (8.3%) of the studies were positive for PE, 534 (89%) were negative, and 16 (2.7%) were deemed suboptimal because of patient motion or poor arterial tree opacification, Patel said. Of those 50 positive CT pulmonary angiography scans (CTPA), no pulmonary embolus was seen only in the lungs above the arch and below the heart, the region excluded from the abbreviated-FOV exams.

The standard CTPA with beam modulation covered 330 mm longitudinally. It exposed patients to 5.4 mSv of radiation, compared to 186 mm of longitudinal coverage that generated 2.8 mSv of exposure, Patel said. A full-chest contrast triple rule-out exam involved 21.9 mSv, compared to the 12.7 mSv for the same procedure in a more limited FOV.

Patel also found that the coronary CTA approach adopted for the triple rule-out scan generated significantly more radiation than CT pulmonary angiography. The abbreviated FOV at least partially mitigates that higher radiation exposure.

"If the sole concern is pulmonary embolus detection, limiting the scan field from the top of the aortic arch to the bottom of the heart would lower the dose without lowering the exam's diagnostic sensitivity," Patel said.