Another step forward in CT angiography

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  Curved multiplanar reconstructions of the right coronary artery show comparisons of single- and dual-source CT with and without beta blockers. (A) Single-source CT without heart rate control. Heart rate was 78 beats per minutes; biphasic reconstruction was used. The arrow points to a typical misalignment artifact. (B) Single-source CT with heart rate control. Heart rate during CT was 56 beats per minute. (C) Dual-source CT without heart rate control. Heart rate during CT was 82 beats per minute. (D) Dual-source CT with heart rate control. Heart rate was 54 beats per minute. (Source: Stephan Achenbach, MD)
German researchers have for the first time demonstrated in a randomized fashion that dual-source CT delivers a high rate of evaluable coronary artery images, even if no systematic approach to heart rate lowering is used. The finding could be beneficial for patients who cannot receive specific premedication to lower heart rate, in patients in whom such premedication is logistically challenging, or in patients whose heart rate is not lowered sufficiently despite the use of premedication.

Even with the latest generation of 64-slice CT systems, up to 12 percent of coronary artery segments have to be excluded from analyses because they are categorized as not evaluable. Motion artifacts are a frequent reason for impaired evaluability, so that routine lowering of the heart rate is usually recommended. This is often perceived as a major limitation of the technique, according to lead author Stephan Achenbach, MD, a cardiologist at the University of Erlangen, Germany (J Am Coll Cardiol Img 2008;1:177-86).

While small studies have confirmed the superior image quality of dual-source CT (DSCT) compared to single-source 64-slice CT in patients without beta blockade, no studies have looked at the influence of a systematic approach to lowering the heart rate on diagnostic accuracy. Achenbach and colleagues took up that challenge.

The researchers randomized 200 patients with suspected coronary artery disease to either single-source CT (n = 100) (Sensation 64, Siemens Healthcare) or DSCT (n = 100) (Definition, Siemens). In each group, they further randomized patients to receive either systematic heart rate control (oral and intravenous beta-blockade for a target heart rate <60 beats/min) or receive no premedication. Researchers used invasive angiography as a reference standard.

They found that systematic heart rate control significantly improved image quality for single-source CT, whereas image quality and diagnostic accuracy remained unaffected in the DSCT group. The improved temporal resolution of the dual-source scanner obviates the need for heart rate control, Achenbach said.

“The available studies clearly show that diagnostic image quality can be obtained by DSCT, even without the use of beta blockers, preserving a very high diagnostic accuracy even for high heart rates,” Achenbach told Cardiovascular Business News.

The Siemens single-source 64-slice CT scanner has a temporal resolution of 183 ms. The use of DSCT increases temporal resolution to 83 ms. The DSCT scanner combines two arrays consisting of one tube and one detector each, arranged within the same gantry at a 90° offset, so that one-quarter rotation is sufficient to sample x-ray transmission data over 180° of projections. With a gantry rotation time of 330 ms, the system achieves a temporal resolution of 83 ms in the center of rotation.

In the study, systematic pre-treatment lowered heart rate by an average of 10 beats per minute. The heart rate control significantly improved evaluability in single-source CT (93 percent vs. 69 percent on a per-patient basis), whereas it did not affect the outcome in DSCT (96 percent vs. 98 percent).

In evaluable patients, the sensitivity to detect the presence of at least one coronary stenosis by single-source CT with and without heart rate control was 86 percent and 79 percent, respectively. For DSCT, it was 100 percent and 95 percent, respectively. Neither finding was significant. 

The rate of correctly classified patients was significantly improved by heart rate control in single-source CT (78 percent vs. 57 percent), whereas there was no such influence in DSCT (87 percent vs. 93 percent).

In some situations, superb image quality may be necessary, such as to evaluate complex lesions with a lot of calcium and stented segments, Achenbach said.

“In these situations, DSCT offers the potential to scan at low heart rates and have—in my experience—still tremendously improved image quality over the 64-slice scanners with a single source. No one says you cannot use beta blockers with DSCT. The higher temporal resolution, in my experience, makes a difference even for low heart rates,” he said.

For the single-source CT, researchers acquired the volume dataset in deep inspiration with 64 x 0.6 mm collimation, pitch of 0.2 (reduced to 0.18 for heart rates <45 beats/min), tube voltage of 120 kV, and a maximum tube current of 800 mAs. For patients with a heart rate <65 beats/min, ECG pulsing was used to limit the full tube current to a time window of 450-ms duration in diastole. Outside this window, tube current was reduced by 80 percent.

For the DSCT, the volume dataset was acquired in deep inspiration with 2 x 64 x 0.6 mm collimation, pitch between 0.2 and 0.43 depending on heart rate, tube voltage of 120 kV, and a maximum tube current of 400 mAs per tube. ECG pulsing was used to reduce tube current by 80% outside a time window between 30% and 75% of the cardiac cycle.

The most likely mechanism for the better performance of DSCT in patients with high heart rates is its higher temporal resolution, the authors concluded.

“Although the temporal resolution of 16- and 64-slice single-source CT systems can be improved through multisegment reconstruction, which has been found advantageous in some but not all cases, potential downsides of this approach include varying effectiveness with heart rate and the fact that data acquired during consecutive cardiac cycles are averaged to generate each cross-sectional image,” they wrote.

From a cardiac standpoint, greater temporal resolution from multiple sources (DSCT) and complete coverage (320-detector row AquilionOne, Toshiba Medical Systems) are both highly desirable, wrote João A.C. Lima, MD, director of cardiovascular imaging at Johns Hopkins University, in an accompanying editorial.

“Given the recent acceleration in the development of cardiovascular CT, it is difficult to predict what will be the ultimate system to be employed in the assessment of CAD,” Lima said.

Therefore, he said, it’s important to develop algorithms that limit test redundancy and unnecessary therapy, allow ultimate reductions in cost and radiation, and provide a reliable answer to both the physician and the patient.

“In this regard, the study by Achenbach et al represents another important step forward,” Lima concluded.

Achenbach told Cardiovascular Business News that further research will go towards:
  • exploring the utility of DSCT in more difficult settings, like advanced coronary artery disease, and 
  • using the high image quality of DSCT to improve assessment of non-stenotic plaque.