Cardiac CT angiography is acknowledged as a useful technology but it does face some limiting factors. Patients with fast heart rates or arrhythmias may be excluded from cardiac CT, as motion artifacts can render the scan unusable. Some patients may not be able to sustain the necessary breath-hold. Finally, slice reconstruction algorithms are advancing, but misregistration and other problems can limit CTA’s effectiveness in imaging perfusion and organ functions.
Cardiac CT angiography is acknowledged as a useful technology but it does face some limiting factors. Patients with fast heart rates or arrhythmias may be excluded from cardiac CT, as motion artifacts can render the scan unusable. Some patients may not be able to sustain the necessary breath-hold. Finally, slice reconstruction algorithms are advancing, but misregistration and other problems can limit CTA's effectiveness in imaging perfusion and organ functions.
New technologies are emerging to address these problems. At the American Heart Association meeting in Chicago in November, researchers presented several papers on the next wave of CTA technology.
Dual-source CT (DSCT) tackles many of the temporal issues limiting cardiac CTA today. The technology, introduced last year, uses two x-ray sources and two detectors simultaneously to scan a patient. Some of the first clinical studies of DSCT's efficacy were presented at the AHA.
DSCT cuts scan times in half, according to Dr. Annick C. Weustink of Erasmus Medical Center in Rotterdam, the Netherlands. DSCT has a temporal resolution of 83 msec, compared to 164 msec for a standard 64-slice scanner.
This increased temporal resolution means that DSCT yields a higher number of evaluable arteries, according to a study presented by Dr. Ulricke Ropers of the University of Erlangen-Nuremberg. Using DSCT, 97% of coronary arteries were evaluable, compared with 91% of coronary arteries using 64-slice CT.
The temporal resolution also makes it possible to scan patients without the use of beta blockers. Both Weustink and Ropers compared the diagnostic sensitivity and specificity of DSCT and 64-slice CT. Patients scanned with 64-slice CT were administered beta blockers while those scanned with DSCT were not. Both studies found the diagnostic accuracy of DSCT without beta blockers to be comparable to 64-slice CT with beta blockers.
In addition, DSCT may be particularly useful for ruling out coronary artery disease in triage situations. A preliminary study of 40 patients presented by Dr. Alexander E. Leber of the University of Munich found a 100% negative predictive value for cardiac CTA with DSCT. The study did find that DSCT significantly overestimates extent of disease in some cases, suggesting that physicians should use caution when scanning patient populations with low to intermediate pretest likelihood of disease.
All presenters at the session acknowledged that more study of DSCT is needed. The technology is currently clinically available, and more data should be available in the coming months.
Another technology profiled at the AHA addresses the temporal issues that affect cardiac CTA and the need for more effective functional imaging of the heart. New 256-slice CT scanners offer the potential for one-beat whole-heart CT imaging, according to a study presented by Dr. Akira Kurata of Ehime University in Japan.
A 64-slice CT can cover 32 mm of body anatomy at once. To capture the entire heart, the scanner must make multiple helical rotations and stitch the resulting images together using reconstruction algorithms. By comparison, a 256-slice scanner can cover 128 mm of body anatomy at one time, allowing physicians to image the entire heart in a single rotation. This allows physicians to acquire images more quickly and can reduce overall radiation dosage, Kurata said.
But 256-slice scanners may prove most useful for providing functional imaging of the heart and other organs. Currently, effective functional imaging using CT is difficult, said Sandra Simon Halliburton, a cardiac imaging physicist at the Cleveland Clinic and session moderator.
"Historically, I think we've yielded to other technologies such as MR that give great functional information without exposing the patient to ionizing radiation," Halliburton said. "But more and more patients show up with hardware - they're living longer, they're receiving implantable devices, so there's a contraindication to use MR. These people may represent the target population for 256."
The 256-slice CT scanner is predicted to be clinically available within two years.