DOI:10.2214/AJR.06.5026.1
AJR 2006; 186:1200-1201
© American Roentgen Ray Society
Reply
Smita Patel and
Ella A. Kazerooni
Department of Radiology University of Michigan Health System Ann
Arbor, MI 48109-0326
We thank Drs. Kashif Ashraf and Omer Ashraf for their letter regarding our
review of CT pulmonary angiography and venography for the evaluation of venous
thromboembolic disease [1], in
which they raise concern for the use of CT as a primary screening tool for the
diagnosis of acute pulmonary embolism. Specifically, they refer to one report
of greater sensitivity for ventilation–perfusion (V/Q) SPECT
scintigraphy (97%) imaging with ultrafine aerosol, compared to CT pulmonary
angiography (86%) [2]. In the
referenced study of 83 patients, 37 patients or 44.6% had pulmonary embolism,
which is much higher than seen in clinical practice, suggesting that the
sample population was biased, which could artificially alter the accuracy data
for any or all of the reported tests. Furthermore, there was no significant
difference in overall diagnostic accuracy between the two techniques (94% for
V/Q SPECT vs 93% for CT) and the specificity for the diagnosis of pulmonary
embolism was higher for CT (98% vs 91%).
With respect to the greater sensitivity of SPECT for PE to which the
authors refer, this was compared with an older generation of MDCT scanners
(4-MDCT). The authors quote increased detection of subsegmental embolism as
one of the strong points of V/Q scintigraphy; however, in that study
[2] an intermediate probability
V/Q result was seen in 26.5% with SPECT when compared to 28.9% with planar
imaging, requiring an additional test to rule PE either in or out. While there
are well-developed schemes to interpret V/Q scans that are associated with
probabilities of PE, such as the Biello criteria
[3,
4], there are no data to
indicate what the probabilities are for V/Q SPECT findings. All of the
existing probability schemes were developed from planar V/Q scans. It would be
incorrect to infer the same probabilities seen with small-, medium-, and
large-sized defects of various numbers from V/Q planar scans to V/Q SPECT
scans.
Most of the published V/Q data to date have used conventional planar
imaging rather than SPECT for the diagnosis of PE. We acknowledge that
state-of-the-art SPECT imaging may be superior to planar V/Q imaging; however,
this requires validation in larger samples of patients to draw conclusions.
Currently, the use of V/Q SPECT is not universal; with most V/Q scans
performed using the planar technique. Bajc et al.
[5] reported a higher
sensitivity for V/Q SPECT over V/Q planar imaging using pulmonary angiography
as the reference standard in an experimental porcine model; however, the
pulmonary arterial branching pattern in that model is substantially different
from humans, and this type of evaluation requires validation in humans. When
such data are available, the role of SPECT imaging in the diagnostic algorithm
of PE diagnosis will be established.
As with newer V/Q technology, with newer generation MDCT scanners there may
be increased sensitivity at both the segmental and subsegmental levels using
16-MDCT (Patel S, et al.,16-slice MDCT optimization of small pulmonary artery
visualization for pulmonary embolism detection vs 4-slice MDCT, presented at
SCBT/MR Annual Meeting, March 2003). The ability to perform CT venography at
the same sitting increases the overall diagnostic yield for combined CT
pulmonary angiography/venography (CTPA/CTV) over CTPA alone for venous
thromboembolic disease, something not possible with V/Q scintigraphy alone. In
our thoracic CT practice, CTPA/CTV is performed exclusively on 16-MDCT and
greater scanners.
We welcome the correspondence from the authors and hope that this will lead
to further research in which state-of-the-art V/Q SPECT and the latest
generation of MDCT scanners are compared, to further improve our understanding
of these techniques in the diagnosis of venous thromboembolic disease.
References
- Patel S, Kazerooni EA. Helical CT for the evaluation of acute
pulmonary embolism. AJR 2005;185
: 135-149[Abstract/Free Full Text]
- Reinartz P, Wildberger JE, Schaefer W, Nowak B, Mahnken AH, Buell
U. Tomographic imaging in the diagnosis of pulmonary embolism: a comparison
between V/Q lung scintigraphy in SPECT technique and multislice spiral CT.
J Nucl Med 2004;45
: 1501-1508[Abstract/Free Full Text]
- Biello DR, Mattar AG, Osei-Wusu A, Alderson PO, McNeil BJ, Siegel
BA. Interpretation of indeterminate lung scintigrams.
Radiology 1979;133
: 189-194[Abstract]
- Biello DR, Mattar AG, McKnight RC, Siegel BA. Ventilation-perfusion
studies in suspected pulmonary embolism. AJR1979; 133:1033
-1037[Abstract]
- Bajc M, Bitzen U, Olsson B, Perez de Sa V, Palmer J, Jonson B. Lung
ventilation/perfusion SPECT in the artificially embolized pig. J
Nucl Med 2002; 43:640
-647[Abstract/Free Full Text]
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