Planar Ventilation-Perfusion Imaging for Pulmonary Embolism: The Case for “Outcomes” Medicine

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Single-photon emission computed tomography (SPECT) has been a significant advancement in scintigraphy, impacting many areas of diagnosis. It has begun to find use in ventilation-perfusion (V/Q) scintigraphy. However, its utility has been limited in the United States because of a lack of an optimal and Food and Drug Administration-approved SPECT ventilatory agent. Although SPECT V/Q can show more and smaller mismatches than planar studies, there is persistent debate regarding the clinical significance of these smaller pulmonary emboli (PE); they may be neither clinically significant nor require treatment. Available data suggest that planar V/Q, SPECT V/Q, and computed tomographic pulmonary angiography (CTPA) have similar false-negative rates and thus have a similar impact on outcomes. In most cases, emergency department physicians are the first to encounter patients who may have PE, and they frequently use an imaging study as part of the evaluation. We discuss the rational for triaging patients to different imaging modalities with the use of chest radiography and the strengths and weaknesses of each modality. Detailed anatomy is an advantage of CTPA, breast radiation dose is reduced with scintigraphy, and imaging is quicker and more detailed with SPECT. We also review planar and SPECT V/Q and CTPA from the differing vantage points of diagnostic accuracy vs patient outcomes. Whatever modality their patients require, physicians can be confident that they are all similarly efficacious at diagnosing clinically relevant emboli.

Section snippets

Overuse of Both V/Q and Computed Tomographic Pulmonary Angiography (CTPA)

Emergency department (ED) physicians generally are the first clinicians to examine patients with signs or symptoms suggestive of PE. They are under great pressure to triage rapidly and decide whether to discharge or hospitalize patients. In cases of suspected PE, use of either V/Q (or increasingly so) CTPA represents an attractive triage method. Most patients presenting to the ED have minimal risk factors. However, even with a low subjective or objective clinical probability, ED physicians are

Evidence of Overdiagnosis of PE

The 2006 Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II report evaluated the role of CTPA as a diagnostic study for PE16 with the use of a composite reference standard, including the Wells score (Table 1)17 and additional imaging. CTPA did not perform well when its results were discordant with the clinical probability. The negative predictive value (NPV) of CTPA was 60% when the clinical probability of PE was high and the positive predictive value (PPV) of CTPA was 58%

Why V/Q Instead of CTPA?

There has been attention in both the medical literature and the lay press about the increase in population radiation exposure from medical imaging, with a large proportion related to CT.24, 25, 26, 27, 28, 29 Breast radiation in young women (who represent a very significant proportion of patients studied for suspected PE) is a concern with regard to chest CTPA, and the lung remains susceptible to carcinogenic effects of radiation into old age. In fact, when dose-reduction strategies are not

Improvement in Interpreting V/Q Studies

Proponents of SPECT certainly will point to cross-sectional imaging's greater capability of detecting smaller lesions. We agree with this argument but, once again, it deals with accuracy rather than outcome. With the aforementioned similar NPV for planar V/Q and CTPA, the case for the adequacy of planar imaging remains convincing. Although CTPA and SPECT V/Q derive from differing sources, both are tomographic imaging modalities, and it is logical to extrapolate the points about CTPA regarding

The Problems with the PIOPED Study

One important problem with the initial 1990 PIOPED study was that 68% of the subjects studied were inpatients. The patients had a much greater incidence of underlying cardiopulmonary disease (with associated chest radiographic abnormalities) than a similar-sized outpatient population. Chest radiograph abnormalities make interpretation of V/Q scans more difficult. In retrospect, the PIOPED population was a suboptimal group to be imaged with V/Q scanning. This flaw in the study design resulted in

The Good Thing About PIOPED

The computer database generated by the PIOPED study has been used to greatly enhance our understanding and interpretation of the V/Q study.35

A Safe, Simple, and Accurate Algorithm to Study Patients With Suspected PE

In late 2006, a joint decision was made between 3 hospital departments (Emergency Medicine, Nuclear Medicine, and Radiology) at Montefiore Medical Center to reduce the number of CTPAs being performed for suspected PE. The Nuclear Medicine service took the lead in holding educational sessions with the support of our chest radiologists and emergency department physicians. After drawing upon lessons learned from the 1990 PIOPED report, it was decided that chest radiography could serve as an

The Language of Lung Scan Interpretation

The language of lung scan interpretation has long been a source of confusion both among requesting clinicians and the interpreters. This confusion was nicely documented by Scotland's Dr Harry Gray almost 2 decades ago.50, 51 He conducted separate surveys for both physician groups. Biello's original formulation36 associated low-probability interpretations with a <10% likelihood of PE. PIOPED, unfortunately, expanded this to a <20% likelihood of PE. Gray's survey of clinicians showed an

Conclusion: Planar Imaging Is “Good Enough”

Current practical barriers to implementation of SPECT V/Q include the current lack of an optimal ventilatory agent in the United States. Secondarily to this, most U.S. nuclear medicine physicians and radiologists are unfamiliar with SPECT V/Q interpretation on a practical basis and U.S. technologists generally lack sufficient experience to perform such studies. This is obviously not the case with CTPA and is often much less so with traditional V/Q. If Technegas becomes available in the United

Acknowledgments

The authors would like to thank Louisa Olmo for her invaluable assistance in the preparation of this manuscript.

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