Cardiac: Bypass Grafts
Dual Source coronary CT angiography (cCTA) is not only faster than previous scanner generations and less invasive than cardiac catheterization, it also may in some cases more realiably localize proximal bypass anastomoses and provide more anatomic information crucial to the success of potential redo coronary artery bypass graft (CABG) surgery.
Dual Source CT allows evaluation prior to intervention
Examples of useful pre-surgery information are severe calcification of the coronary arteries, pleural or pericardial fluid, or sternal infection. Occluded grafts usually present as tubular hypoattenuating structures that are denser than epicardial fat but hypoattenuating relative to myocardium. The proximal anastomoses may be found with the help of “bypass markers” (metallic rings) or subtle outpouchings of the aortic wall.
Essential considerations when imaging CABG patients
In patients where the internal mammary arteries (IMA) have been used as arterial grafts or are planned to be used during redo surgery, it is important extend the scan range and evaluate for ostial IMA stenosis at their origin from the subclavian arteries.
It is also useful to know whether the patient is already on beta-blockers or nitrates, as well as about his blood pressure and LV ejection fraction. Due to the high temporal resolution of DSCT, however, it is possible to generate excellent quality diagnostic images even in patients with heart rates > 100 beats per minute. Thus, in our practice, we have abandoned the use of rate-controlling agents entirely with DSCT. Administration of nitrates may still improve visualization.
Routine ECG pulsing lowers radiation for all patients
While use of ECG pulsing with 64-slice CT to reduce radiation had been reserved for patients with slow and steady heart rates (HR), DSCT now enables routine application of this tool to all patients. In patients with HR > 65, a wider pulsing window is used (35 – 75% of the R-R interval) while for lower heart rates a narrower window may be used (55 – 75%). Some experts recommend scanning at 70 – 70% only, which results in full current at ±5% of the cardiac cycle. This yields diagnostic results in most patients with slow and steady heart rates.
Include myocardial viability check with DSCT
One may consider acquiring low-kV delayed images 5 – 10 minutes following contrast media injection and review cCTA images for potential hypoattenuation in the myocardium, while the patient is still on the scanner table. Automated segmentation of the heart should be disabled, as segmentation algorithms might not recognize grafts and sculpt them away from the 3D renderings, which is very useful to understand the anatomy of more complex grafts.
Authors: Pal Suranyi, Christian Thilo, Heon Lee, U. Joseph Schoepf
See corresponding cases: Thrombosed venous bypass graft – pseudoaneurysm following coil embolization, Quadruple, patent coronary artery bypass grafts
See corresponding protocol: Cardiac: Bypass Grafts
