Merits of V/Q SPECT Scintigraphy Compared with CTPA in Imaging of Pulmonary Embolism

TO THE EDITOR: We refer to the recent Invited Perspective which proposes that there is now a reduced role for V/Q scintigraphy in the detection of pulmonary embolism (PE) given the emergence of CT pulmonary angiography (CTPA) (1). Although the article addressed many of the strengths and limitations of CTPA in the evaluation of suspected PE, we do not agree with the suggestion that there is now a role for ventilation/perfusion (V/Q) scanning only in a very limited number of patient groups. It is surprising that this review article makes no mention of the use of V/Q SPECT, which has been shown to be superior to planar V/Q scintigraphy (2,3) and CTPA (2). As with other areas of nuclear medicine, the transition from planar techniques to SPECT has led to improvements in sensitivity and diagnostic accuracy. The published literature on V/Q SPECT has consistently shown improvements in sensitivity (2,3), specificity (3,4), and intraobserver reproducibility (2–4), as well as reducing the number of intermediate or inconclusive results to less than 5% (3,5). In a direct comparison of CTPA and V/Q SPECT in 83 patients with suspected PE, Reinartz et al. demonstrated a sensitivity of 97% for SPECT, compared with 86% for CTPA (2). The CTPA sensitivity in that paper is consistent with the 83% result quoted in the recent prospective multicenter PIOPED 2 study (6). These publications suggest that, even with current-generation CT technology, CTPA fails to diagnose PE in approximately 1 in every 6 patients. Given this failure rate, we consider that few clinicians would agree with Dr. Strashun in his statement that CTPA is ‘‘very sensitive’’ in the diagnosis of PE (1). As he correctly states, PE is a potentially fatal condition and its accurate diagnosis is essential. The limitations of CTPA in the detection of smaller emboli (particularly at the segmental and subsegmental levels) are well recognized (7); however, larger emboli can also be missed (6). Although it has been suggested that emboli not detected by CTPA are not clinically significant, this may not be the case in patients with cardiorespiratory disease (7), and in these patients in particular, accurate detection is crucial. Furthermore, the PIOPED 2 study demonstrated that the CTPA accuracy deteriorates further if the scan results do not correlate with the clinical likelihood of disease, and in these circumstances, the incidence of false-positive and false-negative results is significant (6). In this paper, 40% of negative CTPA results were false-negative if the clinical suspicion was high, and 42% of positive scan results were false-positive if the clinical suspicion was low. Dr. Strashun states that radiation dose and the risks of contrast media are the only 2 reasons why V/Q scintigraphy should be used in preference to CTPA (1).