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Original Research |
1 Department of Radiology, Erasmus Medical Center, Dr. Molewaterplein 40,
Rotterdam, The Netherlands.
2 Department of Cardiology, Erasmus Medical Center, Rotterdam, The
Netherlands.
3 Department of Radiology and Cardiology, Azienda Ospedaliero-Universitaria,
Parma, Italy.
4 Department of Radiology, Policlinico "P. Giaccone," Palermo,
Italy.
Received August 12, 2005;
accepted after revision March 30, 2006.
Address correspondence to F. Cademartiri
(filippocademartiri{at}hotmail.com).
Abstract
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MATERIALS AND METHODS. One hundred twenty patients in sinus rhythm
with suspected coronary artery disease who underwent MDCT of the heart and
conventional coronary angiography were retrospectively selected. The
population was divided into two groups depending on median (326 H) coronary
vascular enhancement (i.e., low attenuation and high attenuation). The
diagnostic accuracy of MDCT for the detection of clinically significant
coronary artery lesions (
50% lumen reduction) in both groups was compared
with that of quantitative coronary angiography.
RESULTS. The sensitivity of MDCT was 90% and 93% for the low- and high-attenuation groups, respectively. The specificity was 95% and 97%.
CONCLUSION. Greater intracoronary attenuation leads to higher diagnostic accuracy in the detection of coronary artery stenosis with MDCT.
Keywords: cardiac imaging • cardiovascular disease • contrast media • CT coronary arteriography
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A calcium score scan was obtained with the following parameters: collimation, 16 x 1.5 mm; gantry rotation time, 420 milliseconds; feed/rotation, 6.0 mm; effective slice width, 3 mm; increment, 1.5 mm; effective tube current, 150 mAs at 120 kV. MDCT coronary angiography (Sensation 16, Siemens Medical Solutions) was performed after IV administration of 120 mL of iodinated contrast material (iodixanol, 320 mg I/mL at 4 mL/s, Visipaque, Amersham Health). The parameters were as follows: collimation, 16 x 0.75 mm; gantry rotation time, 420 milliseconds; feed/rotation, 3.0 mm; effective slice width, 1 mm; increment, 0.5 mm; 120 kV; 400-500 mAs. The temporal windows were set at -350 milliseconds, -400 milliseconds, and -450 milliseconds before the next R wave for ECG-gated retrospective reconstruction. The data set with the least residual motion was selected for evaluation.
For evaluation, coronary arteries were divided into segments according to
the American Heart Association classification
[9]. A single observer unaware
of the MDCT results used a quantitative coronary angiography algorithm (CAAS
II, Pie Medical) to classify all coronary segments as < 2 mm or
3 2 mm in diameter. Only segments with a reference diameter 2
mm were considered for comparison with MDCT. The severity of coronary stenosis
was quantified in two orthogonal views. Stenosis was classified as significant
if the mean reduction in lumen diameter was 3 50%.
Coronary calcium score was assessed with a dedicated software application (CaScore, Siemens). The overall Agatston score was recorded for each patient. Coronary vascular enhancement was evaluated as attenuation in Hounsfield units. It was measured in each patient by use of the mean of two regions of interest positioned at the origin of the right and left coronary arteries, respectively. The regions of interest were set as large as possible, but coronary walls, plaques, and calcifications were always avoided.
Two observers blinded to the results of conventional coronary angiography independently evaluated all MDCT coronary angiographic images using different postprocessing techniques (e.g., multiplanar reconstructions and maximum intensity projections). All branches of the coronary tree 3 2 mm in luminal diameter were evaluated for the presence of significant (3 50% diameter reduction) obstructive stenosis. Segments with stents were excluded from analysis. Disagreements were resolved by consensus.
The study cohort was divided into two groups according to median average attenuation value (326 H) of the entire population (60 patients in the low-attenuation group and 60 patients in the high-attenuation group). Interval data were expressed as mean ± SD. Diagnostic accuracy was expressed as percentage and 95% CI. The differences in demographics were tested with an unpaired t test for parametric criteria and with a chi-square test for nonparametric criteria. The differences in diagnostic accuracy were tested with a nonparametric McNemar test. Significance was considered if p was < 0.05.
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Overall, 1,310 coronary segments (mean, 10.9 segments/patient) 2 mm
were available for comparison between MDCT coronary angiography and
conventional coronary angiography (Table
2). Of these, 656 (50%) of the segments were available in the
group with low attenuation and 654 (50%) of the segments in the group with
high attenuation. There were 219 significant coronary lesions available for
analysis. Of these, 108 (49%, 1.8 lesions/patient) were in the group with low
attenuation and 111 (51%, 1.85 lesions/patient) were in the group with high
attenuation. The overall sensitivity, specificity, and positive and negative
predictive values for the detection of significant stenosis were 90%, 93%,
70%, and 98% for the low-attenuation group and 95%, 97%, 87%, and 99% for the
high-attenuation group (p < 0.05).
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The main differences between the two groups were in number of false-positive segments (segments not significantly obstructed overestimated as significant lesions with MDCT coronary angiography). There were 41 false-positive segments (mean degree of stenosis at coronary angiography, 20.9%) in the low-attenuation group and 16 false-positive segments (mean degree of stenosis at coronary angiography, 6.3%) in the high-attenuation group. The number of false-negative segments (segments with significant luminal obstruction missed at MDCT coronary angiography) was 11 (mean degree of stenosis at coronary angiography, 62.3%) for the low-attenuation group and six (mean degree of stenosis at coronary angiography, 62.8%) for the high-attenuation group.
In the group with low vascular attenuation, the false-positive findings were distributed as follows: 23 in proximal segments (segments 1, 2, 5-7, 11) and 18 in distal segments (segments 3, 4, 8-10, 12-15). The false-negative findings were distributed as follows: five in proximal segments and six in distal segments. In the group with high vascular attenuation, the false-positive findings were distributed as follows: nine in proximal segments and six in distal segments. The false-negative findings were distributed as follows: two in proximal segments and four in distal segments.
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The results of our study confirm this observation. When higher
intracoronary attenuation is present, the overall diagnostic accuracy improves
significantly. In the two groups of patients that we retrospectively enrolled,
other parameters that could have affected diagnostic accuracy (e.g., coronary
calcifications and heart rate) were not significantly different. This
difference was found in evaluation of coronary arteries for the presence of
significant coronary lesions ( 50% reduction in luminal diameter) with a
per-segment approach that included all segments with a diameter 2 mm.
More lesions (41 vs 16) were incorrectly diagnosed as significant in the group
with lower intracoronary attenuation.
The increased number of false-positive findings when intracoronary
attenuation is lower can be explained by a reduced contrast-to-noise ratio
that results in reduced sharpness of vessel visualization (Figs.
1A,
1B,
1C,
2A,
2B,
2C, and
2D). In these patients the
observers' scores were more "defensive" because of lack of
confidence. In cases in which lesions were borderline (
50% reduction in
luminal diameter) or when the presence of calcium prominently obscured the
coronary artery lumen, the observers tended to evaluate the segment as a
positive finding. This finding also was confirmed by the mean degree of
stenosis observed at coronary angiography for the false-positive findings in
the low-attenuation group (21%) and in the high-attenuation group (6%).
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The main limitation of the study was that the population was enrolled retrospectively. However, the scan protocol was the same in the study population, and the two groups into which the population was divided were also homogeneous. Another limitation was related to the use of a 16-MDCT scanner. The introduction of 64-MDCT scanners may enhance diagnostic accuracy in the detection of significant coronary artery stenosis, especially in distal segments. It is reasonable to expect that the same concept of improved diagnostic accuracy with higher intracoronary attenuation can be applied to the next generation of MDCT scanners.
In conclusion, higher intracoronary attenuation significantly improves diagnostic accuracy in MDCT coronary angiography. On the basis of our results, we can recommend protocols with a high rate of administration (4-5 mL/s) of IV contrast material, a high iodine concentration (350-400 mg I/mL), or both when MDCT coronary angiography is performed to evaluate coronary artery stenosis.
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