The Silver Bullet: Cardiac Imaging with 2nd Generation Dual-Source CT (J. Schoepf)

Just when the noise over radiation exposure at cardiac CT was about to become unbearable, patients were getting scared to the point of refusing life saving imaging tests for fear of radiation, and the benefit of non-invasive assessment of the heart was about to be droned out by public discussions on radiation risks, a silver lining appeared on the horizon: 2nd generation dual-source CT promising complete coronary evaluation with sub-milliSievert radiation dose.

No hollow promises

Marketing claims accompanying the launch of a new high-end imaging system are always to be taken with a grain of salt. In the case of coronary CT angiography with 2nd generation dual-source CT, however, the emerging literature and our own experiences using this system largely support the pre-launch projections. These include technical performance, radiation exposure, as well as novel applications.

The established advantages of 1st generation dual-source CT have been consolidated, expanded, and further enhanced with the latest edition of dual-source CT technology. 2nd generation dual-source CT remains a full-body imaging system, albeit with very special capabilities for the assessment of the heart and vascular system.

Further decreased radiation exposure

In our experience, radiation exposure at cardiac CT and image acquisition times are drastically decreased with 2nd generation dual-source CT. This has been made possible by the doubling of detector elements which enables much higher table speeds. With 1st generation dual-source CT, retrospective ECG-gating and prospective ECG-triggering were the traditional cardiac CT image acquisition methods. The latest iteration of this technology now offers a unique scan mode that allows acquiring the entire cardiac anatomy within a quarter of a second, i.e. a single diastolic phase. Combined with 100kV tube potential, the radiation exposure of such a prospectively ECG-triggered, high-pitch spiral acquisition reliably remains under 1 mSv.

Subject to regular heart rates

Not all patients are eligible for this acquisition mode. Successful application of this method requires slow (i.e. < 65 bpm) and regular heart rates to ensure a sufficiently long duration and predictability of the diastolic phase, during which the scan is performed. This prompted us to re-institute the use of beta-blockade in patients with faster heart rates, a practice which we had abandoned with 1st generation dual-source CT. However, in our opinion the ability of reducing radiation to a bare minimum greatly outweighs the very minor inconvenience associated with pharmaceutical rate control.

Also, the method finds its limitation in severely obese individuals, requiring us to fall back onto the traditional methods of prospective or retrospective ECG synchronization in some patients Regardless of the acquisition protocol, radiation doses are substantially lower across the board, compared with other scanner platforms.

Accordingly, 2nd generation dual-source CT and the availability of other low dose image acquisition methods should effectively eliminate prevailing concerns over radiation exposure from this non-invasive coronary CTA.

Integrative imaging of coronary heart disease – Convincing advantages

In addition, the introduction of 2nd generation dual-source CT has brought about several improvements for integrative imaging of coronary heart disease: i.e. the ability to assess coronary artery anatomy, function, perfusion, and viability with a single, stand-alone modality. Dual-energy CT, based on dual-source CT has shown great promise for this purpose, as it enables analyzing the iodine (and thus blood) distribution within healthy and diseased myocardium.

While first generation dual-source CT was limited to a temporal resolution of 165 msec in dual-energy mode, the full temporal resolution of 75 msec for motion-free imaging of the coronary arteries can now also be realized for dual-energy cardiac examinations. Advanced selective energy filtering techniques have improved the discriminatory power of dual-energy CT image data. This allows better characterization of tissues and of iodine distribution within the myocardium.

Lastly, while dual-energy CT statically images the myocardial blood supply during first pass arterial perfusion, technical advancements now also enable the rapid, time-resolved, quantitative assessment of myocardial perfusion during rest and stress.

Outlook

The efforts to refine CT into an instrument for integrative imaging of coronary heart disease are likely to continue and intensify. Whether these efforts will eventually challenge the role of traditional physiological testing remains to be seen. However, the signs bode well for CT: Patient evaluation with a single, non-invasive modality is likely safer, cheaper, and has less radiation than the routine combination of nuclear myocardial perfusion imaging and invasive coronary catheterization, which is ordinarily needed to obtain comparable complementary information.

References

Achenbach S, Marwan M, Schepis T, Pflederer T, Bruder H, Allmendinger T, Petersilka M, Anders K, Lell M, Kuettner A, Ropers D, Daniel WG, Flohr T. High-pitch spiral acquisition: a new scan mode for coronary CT angiography. J Cardiovasc Comput Tomogr. 2009;3:117-21.

Leschka S, Stolzmann P, Desbiolles L, Baumueller S, Goetti R, Schertler T, Scheffel H, Plass A, Falk V, Feuchtner G, Marincek B, Alkadhi H. Diagnostic accuracy of high-pitch dual-source CT for the assessment of coronary stenoses: first experience. Eur Radiol. 2009. [Epub ahead of print].

Achenbach S, Marwan M, Ropers D, Schepis T, Pflederer T, Anders K, Kuettner A, Daniel WG, Uder M, Lell MM. Coronary computed tomography angiography with a consistent dose below 1 mSv using prospectively electrocardiogram-triggered high-pitch spiral acquisition. Eur Heart J. 2009 Nov 5. [Epub ahead of print]

Lell M, Marwan M, Schepis T, Pflederer T, Anders K, Flohr T, Allmendinger T, Kalender W, Ertel D, Thierfelder C, Kuettner A, Ropers D, Daniel WG, Achenbach S. Prospectively ECG-triggered high-pitch spiral acquisition for coronary CT angiography using dual source CT: technique and initial experience. Eur Radiol. 2009;19:2576-83.

Bastarrika G, Lee YS, Huda W, Ruzsics B, Costello P, Schoepf UJ. CT of coronary artery disease. Radiology. 2009;253:317-38.