Don't Bury the V/Q Scan: It's as Good as Multidetector CT Angiograms with a Lot Less Radiation Exposure

Within the past decade, advances in CT technology have allowed superb angiographic studies for pulmonary embolism (PE) diagnosis. This has been embraced by clinicians and most diagnostic radiologists who are more comfortable with an anatomic demonstration of whether a clot is present or not as compared with looking at ventilation–perfusion (V/Q) mismatches on a lung scan (1). As a result, the number of multidetector CT angiograms (MDCTA or CTA) performed in the past few years has greatly increased with little regard for radiation dose, particularly to the breasts of young women. In addition, sufficient evidence has now been accumulated to confirm that V/Q scintigraphy is as accurate as CTA, making it a viable alternative when studying patients for PE. In a recent Invited Perspective in The Journal of Nuclear Medicine, Dr. Arnold Strashun comments on the MDCTA versus V/Q issue and concludes that ‘‘were it not for definite allergic and nephrotoxic risks of contrast media and the added radiation burden of MDCTA, the ventilation/perfusion scan would virtually disappear from the diagnostic algorithm for pulmonary embolism’’ (2). The CTA problems mentioned by Strashun are innate to the modality and cannot be mitigated. Therefore, it appears quite clear that V/Q imaging will continue to be a very important part of the PE diagnostic algorithm for the foreseeable future. In addition to substantial data confirming that V/Q sensitivity is comparable to that of MDCTA, the radiation risk aspect (particularly to the female breast) requires much stronger emphasis. Young women represent a very significant segment of the population being studied. As pointed out in the recent American College of Radiology (ACR) white paper (3), dose calculation is very complex because absorption in each organ is very variable from patient to patient. Breast radiation estimates made using 4-slice CT vary from 20 to 60 mSv (4–6), whereas that from V/Q is approximately 0.28–0.9 mSv (7). A recent report by Einstein et al. (8) estimated that 64-slice chest CTA delivers a dose of 50–80 mSv to the breast. These reports indicate an enormous 65to 250-fold difference between the 2 procedures. Average estimates generally quoted are a 70to 100-fold difference. Comparatively, a 2-view mammogram is associated with 3 mSv (4), which makes the CTA radiation dose approximately 10 to 20 times greater. In addition, the estimated radiation exposure from CTA suggests that a nonnegligible increase in lifetime attributable risk of cancer exists, particularly to the breasts of young women (1 in 143 for a 20-y-old woman and 1 in 284 in a 40y-old woman) (8). The International Commission on Radiation Protection (ICRP) has reported that CT doses can exceed limits shown to result in an increase in cancer risk (9). These are facts about which we all must be very concerned and be professionally obligated to deal with them. To address this great concern, the ACR white paper (3) strongly emphasizes that it is the responsibility of the imaging physician to be fully educated concerning the radiation risks associated with each procedure and, in turn, educate the clinician requesting the procedure. Presenting them with diagnostically equivalent options also is part of this educational process. This is certainly of paramount importance and pertinence in the MDCTAvs. V/Q issue. One advantage of CTA over V/Q mentioned by Strashun is that a significant number of patients have alternate anatomic diagnoses made that may be the cause of the patient’s symptomatology (2). Most of these serendipitous findings, such as small lung nodules, generally are not related to the patient’s acute problem. However, occasional significant findings, such as dissecting aneurysms or pneumothorax, may be detected (10). In addition to the most important radiation issue, there are several factors that should be considered when deciding between a CTA and V/Q study. These are (a) Is one of the studies clearly superior to the other in the overall context of diagnosing PE as well as in combination with objective clinical assessment (pretest probability)? (b) Are both studies equally available on a 24-h/7-d schedule? (c) Do the physicians interpreting the study possess the appropriate expertise?