CT Angiography in the Assessment of Saphenous Vein Graft Disease

There is interest in the non-invasive assessment of SVG patency using CT imaging. In a recent study that used SVG patency as the primary endpoint, 224/249  (90.3%) of patients returned for follow-up CT at 3 months. There were 704 grafts in these 224 patients. Only 11 segments were non-diagnostic (1.6%). In this relatively modest sized study, there was a significant improvement in SVG patency to 91.6% (219/239) among patients treated  with both aspirin and clopidogrel versus 85.7% (198/231) in those  patients treated with aspirin alone (relative risk: 1.707; 95%  confidence interval: 1.010 to 2.886; p = 0.043).

Early Studies Using 16 Slice CT
Early studies demonstrated a sensitivity of 92% to 100% and a specificity of  89% to 100% in the evaluation of SVG patency with invasive angiography  as the gold standard. In the same studies, in the detection of IMA patency, the sensitivity has ranged from 80% to 100% and the specificity  has ranged from 82% to 100%.

Methodologic Issues
There are methodologic issues that should be highlighted when evaluating  comparisons of CT to angiography as the gold standard. In the above studies, between 4% and 29% of the SVGs were unevaluable. In the study by Anand, only those patients with disease on CT underwent angiography. More than half of the patients in the study did not undergo invasive angiography, so statements regarding sensitivity and specificity cannot  be made. 6 out of 102 SVGs could not be evaluated. Among those selected patients who underwent angiography, there were two false positive  diagnoses of a stenosis, and there was "100% predictice accuracy in  detecting graft occlusion, and 85% predictive accuracy in detecting  graft stenosis". Again, this is an overstatement given that not all patients underwent protocol mandated angiography.

Contemporary Studies Using 64 Slice CT
The resolution of CT angiography improved with the introduction of 64 slice  CT. The assessment of the presence of either a stenosis or occlusion of  SVG has been associated with high rates of sensitivity of (97%),  specificity (97%), and positive and negative predictive values of 93%  and of 99%, respectively. Arrhythmias and heart rates > 65 beats/minute during scanning were associated with errors in assessment. It should be noted that in modern studies such as the one above, 12 segments were excluded from analysis  because of the presence of stents which are associated with blooming  artifact. IT should also be noted that 9 out of 406 grafts demonstrated insufficient image quality for the assessment of the SVGs due either to  motion artifacts (8 SVGs) or numerous metallic clips adjacent to the  bypass graft (1 SVG). When the data is analyzed on a per patient basis rather than a per SVG basis, among evaluable patients the following were  observed :


 * Sensitivity: 100% (95% CI 94% to 100%)
 * Specificity: 92% (95% CI 82% to 97%)
 * Positive predictive value (PPV): 93% (95% CI 85% to 97%)
 * Negative predictive value (NPV): 100% (95% CI 93% to 100%)

When all patients were included in the analysis (including those in whom the CT was unevaluable) the rates were as follows :


 * Sensitivity: 100% (95% CI 94% to 100%)
 * Specificity: 87% (95% CI 76% to 93%)
 * Positive predictive value (PPV): 89% (95% CI 79% to 94%)
 * Negative predictive value (NPV): 100% (95% CI 93% to 100%)

It should also be noted that these patients were symptomatic and the  pre-test probability was high. These rates may be poorer in an asymptomatic population with a low pre-test probability.

Radiation Dose with CT Scans
It should be noted that the radiation dose is more than twice as high for  CT scans as it was for invasive angiography (17.8 +/- 5.4 mSv and 8.8  +/- 4.5 mSv, respectively, p < 0.05).

Simultaneous Assessment of Native and SVG Disease
The assessment of SVGs is simplified by the fact that the SVGs are  stationary while there is motion artifact in the assessment of native  coronary arteries. The assessment of native coronary artery disease in  addition to SVG conduits was investigated by Ropers et al. Fifty patients with a total of 138 arterial and venous conduits were  assessed a mean of 106 months after CABG. All the arterial and the venous conduits were both evaluable and  were correctly classified as  either being occluded (n=38) or patent (n=100) when compared with  angiography as the "gold standard". With respect to SVG stenosis severity, Sensitivity for stenosis detection in patent grafts was 100%  (16/16)  with a specificity of 94% (79/84). For the per-segment evaluation of  native coronary arteries and distal runoff vessels,  sensitivity in  evaluable segments (91%) was 86% (87/101) with a  specificity of 76%  (354/465). If evaluation was restricted to nongrafted arteries and  distal runoff vessels, sensitivity was 86%  (38/44) with a specificity of  90% (302/334). On a per-patient basis, classifying patients with at  least 1 detected stenosis in a CABG, a  distal runoff vessel, or a  nongrafted artery or with at least 1  unevaluable segment as "positive,"  MDCT yielded a sensitivity of 97%  (35/36) and specificity of 86%  (12/14).

Incidental Findings on CT
One study indicated that about 20% of patients undergoing cardiac CT have  an incidental finding at the time of the imaging. 9.3% of patients had a cardiac finding such as a pseudoaneurysm or intracardiac thrombus, and  13.1% of patients had a noncardiac finding including pulmonary embolism,  lung cancer, or pneumonia.