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2009). SPECT identifies the physiologic consequence of the clot rather than the clot itself. Adding low-dose CT without contract agent raises the level of confidence in the reading result. At the same time, it reduces inconclusive studies with SPECT alone from 5% to 0% when combined with low-dose CT. Furthermore, specificity improves with fewer false-positive readings from 18% - 0%. Low-dose findings explaining subtle perfusion defects otherwise perceived as PE on SPECT alone account for the improvement. The study also showed that V/Q SPECT with low-dose CT is useful as a routine method. Immobilization during the SPECT and low-dose CT procedure lasted only 20 minutes. Even critically ill patients can tolerate it. The 11 out of the overall total of 196 volunteer patients were disqualified for participation in the study because of discomfort, shortness of breath or lack of willingness to remain in supine position. Those who qualified were placed in the scanner for approximately 45 minutes. This was longer than if only SPECT was used. Only 2-3% of the volunteers were disqualified because of unwillingness to cooperate with the use of V/Q SPECT, a daily routine schedule (Gutte et al.).
Merits of SPECT
V/Q SPECT has proved superior to planar V/Q scintigraphy and CTPA (Roach 2006). Four members of the Society of Nuclear Medicine made this statement in disagreeing to a recent published invited perspective on the reduced role of V/Q in the detection of PE with the emergence of CTPA. It mentioned many of the strengths and limitations of CTPA in diagnosing and evaluating PE. It also suggested that V/Q scintigraphy now plays only a reduced role in detecting PE in scanning only a very limited number of PE patients. It does not discuss or mention the superior use of V/Q SPECT to planar V/Q scintigraphy and CTPA. Published literature on the transition from planar techniques to SPECT attests to improvements in sensitivity, specificity, intra-observer reproducibility and diagnostic accuracy. It has also reduced the volume of intermediate or inconclusive results to less than 5%. A direct comparison between the capabilities of CTPA and V/Q SPECT using 83 volunteer patients suspected of PE by Reinartz et al. produced a 97% sensitivity for SPECT and only 86% for CTPA. This 86% CTPA sensitivity reflects the result of the recent prospective multi-center PIOPED 2 study. All results suggest that CTPA is unable to diagnose PE in approximately 1 out of every 6 patients despite current CT technology (Roach).
There are at least 100,000 cases of PE every year in the U.S., 30% of which die if untreated. The risk is equal to men and women. CTPA is the most common diagnostic test used with both suspected patients and hospital inpatients. But it also exposes patients to cancers because of its radionuclide feature. Studies show that SPECT has 97% accuracy as compared to CTPA's only 86%. SPECT is the preferred alternative with its minimal radiation risk in reducing PE incidence. It should be made the first-line imaging of PE in most cases.
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