Detection of Pulmonary Embolism: A Comparison Study of 3 Modalities

Hossein Jadvar, MD, PhD, MPH, MBA


July 22, 2010

Detection of Pulmonary Embolism with Combined Ventilation-Perfusion SPECT and Low-Dose CT: Head-to-Head Comparison with Multidetector CT Angiography

Gutte H, Mortensen J, Jensen CV, et al
J Nucl Med. 2009;50:1987-1992

Study Summary

The aim of this Danish prospective study was to compare the diagnostic abilities of ventilation/perfusion (V/Q) single-photon emission computed tomography (SPECT), V/Q SPECT combined with low-dose CT, and pulmonary multidetector computed tomography (MDCT) angiography in 196 patients suspected of having pulmonary embolism (PE). All tests were obtained during a single imaging session using a combined SPECT/MDCT camera system, and patients were followed for 6 months.

PE was clinically suspected on the basis of positive D-dimer results (> 0.5 mmol/mL) or clinical assessment resulting in a Wells score greater than 2.[1] A total of 81 simultaneous studies were actually analyzed, 38% of which were from patients with documented PE.

The sensitivities and specificities of the 3 imaging modalities are shown in the Table.

Table. Diagnostic Performance of Pulmonary Embolism
Imaging Studies

Imaging Modality Sensitivity Specificity
V/Q SPECT 97% 88%
V/Q SPECT with low-dose CT 97% 100%
MDCT pulmonary angiography 68% 100%

The authors concluded that V/Q SPECT combined with low-dose CT (without intravenous contrast) was most accurate in the imaging assessment of patients with suspected PE and should probably be considered as the first-line imaging modality in this clinical setting.


Acute PE, if untreated, has a high mortality rate. Accurate diagnosis of PE is challenging. The typical assessments include clinical evaluation, D-dimer blood sampling, and imaging. Currently, MDCT pulmonary angiography appears to be the first-line modality in the imaging evaluation of PE because this test is fast and available, may offer alternative diagnoses, and has largely replaced planar V/Q scanning. With the recent development of three-dimensional SPECT-CT technology, interest in V/Q scanning as a one-stop shop imaging modality is renewed, not only for detection of emboli, but also for assessment of their perfusion and functional consequences.

This investigation is important in that it compared the new three-dimensional SPECT/low-dose CT with pulmonary CT angiography, using the same camera system in the same imaging session and in a prospective manner. Of the 196 consecutive patients that were recruited from 2 hospitals in Copenhagen, Denmark, 100 were eligible for the study. Six patients were excluded because of poor pulmonary arterial enhancements, 8 were excluded because of poor technical quality, and another 5 patients were excluded for lack of a final diagnosis, leaving 81 cases for the final analysis.

The investigators concluded that in this clinical setting, the most accurate imaging strategy with the lowest associated radiation dose may be V/Q SPECT with low-dose, unenhanced CT. This conclusion, however, is tempered by a number of potential limitations that underlie this investigation. The researchers excluded 46 patients (24%) from the study because of renal dysfunction. Although this situation favors the scintigraphic approach, as the investigators noted, the overall results may not hold for all patients with suspected PE. In addition, all mismatched defects on SPECT were considered to be pulmonary emboli. Although the follow-up and the CT portion of the study probably reduced the false-positive rate with respect to other causes of VQ mismatch (eg, vasculitis, extrinsic vascular compression, vessel stenosis), the details were not systematically reported. Moreover, all women younger than age 40 (14 patients) were excluded, presumably to minimize radiation exposure, but this group may, in fact be an important risk group due to their potential use of birth control pills. The exact reasons for the exclusion of these patients were not well described. Additional studies will be needed to confirm these enthusiastic findings for VQ SPECT combined with low-dose CT.