Radiation Minimisation and Outcome Maximisation in Cardiac Computed Tomography

maximum of 1.5 to ensure gap-less coverage in the z-axis. A DSCT has two X-ray tubes and two detectors arranged at a 90º angle, allowing a high temporal resolution and high-pitch data acquisition. In the high- pitch scan mode, the second tube detector of a DSCT system is used to essentially fill the data gaps, enabling the helical pitch factor of 3 to be used. Due to the short sub-second radiation exposure and little overlap between spiral acquisitions, considerable reduction in radiation dose has been found.

This technique is not without limitations. Similar to prospective triggering, no functional data are available, and the technique is susceptible to artifacts due to ectopy. Use of the high-pitch spiral technique has been limited to heart rates <60bpm, and it has been primarily used with non-obese (<100kg) patients. Proof-of-concept trials were initially performed on the 64-row DSCT system; however, optimally the latest 128-detector row DSCT platform should be used. Nevertheless, early clinical results show good potential and the technique represents another method for performing low-dose CCTA.

Iterative Reconstruction

Image reconstruction for CT has traditionally been performed using filtered back projection (FBP). FBP is fast and mathematically simple,

requiring limited computational power, but has limitations that negatively affect image quality. Adaptive statistical iterative reconstruction (ASIR) is a new technique that may be used to lower the radiation exposure of all CT studies, including cardiac.23–25,54 ASIR uses iterative comparisons of each acquired image to a synthesised projection incorporating modelling of both system optics and system statistics. Compared with FBP, images reconstructed with ASIR have lower image noise. By using ASIR to reduce image noise, one can reduce tube current and maintain relative image noise, achieving lower radiation doses.

Hara et al. reported that use of ASIR was associated with 32–65% effective dose reductions compared with FBP without degradation of image quality for abdominal imaging.23

In CCTA, Leipsic et al.

evaluated 62 patients using an ASIR-capable 64-detector scanner and a low-dose CCTA technique.55

In comparison with FBP (0% ASIR),

increasing ASIR reduced image noise by up to 43%, but there was no effect on signal (see Figure 4). Reconstruction using 40 or 60% ASIR significantly improved image quality and the proportion of interpretable segments when using a low mA technique.

1. Brenner DJ, Hall EJ, Computed tomography-an increasing source of radiation exposure, N Engl J Med, 2007;357(22): 2277–84.

2. Einstein AJ, Henzlova MJ, Rajagopalan S, Estimating risk of cancer associated with radiation exposure from 64- slice computed tomography coronary angiography, JAMA, 2007;298(3):317–23.

3. Smith-Bindman R, Lipson J, Marcus R, et al., Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer, Arch Intern Med, 2009;169(22):2078–86.

4. Mettler FA, Thomadsen BR, Bhargavan M, et al., Medical radiation exposure in the U.S. in 2006: preliminary

results, Health Phys, 2008;95(5):502–7.

5. Budoff MJ, Achenbach S, Blumenthal RS, et al., Assessment of coronary artery disease by cardiac computed tomography: a scientific statement from the American Heart Association Committee on Cardiovascular Imaging and Intervention, Council on Cardiovascular Radiology and Intervention, and Committee on Cardiac Imaging, Council on Clinical

Use of ASIR was recently reported in a multicentre analysis in 1,202 consecutive patients in three arms: routine 64-detector CCTA with FBP (n=753), 64-detector CCTA with ASIR (n=247) and the latter following initiation of a protocol using 100kV (in patients with a BMI <30) and a BMI-based mA protocol (n=202).37

The mean effective

radiation of the three groups was 3.8, 2.6 and 1.3mSv, respectively. Despite the overall radiation dose reduction, image interpretability and signal-to-noise ratio of post-protocol ASIR and FBP were similar.

Conclusion

There has been heightened concern of a risk from increasing radiation exposure from cardiac and general diagnostic CT imaging. This has led to a great deal of research on new CT techniques that are capable of imaging the heart at lower radiation doses. In clinical practice, optimised selection of user-defined parameters such as tube current and voltage, as well as use of new technologies such as prospective triggering, can lead to a substantial reduction in radiation dose without loss of diagnostic accuracy. n

James P Earls is Director of Cardiovascular Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) at Fairfax Radiological Consultants and Co-Director of the Cardiac CT Lab at the Inova Heart and Vascular Institute in Falls Church, VA. His research interests include new technologies and methods for minimising the associated radiation dose and improving the accuracy of cardiac CT. Dr Earls is a member of the Board of Directors of both the Society of Cardiovascular

Computed Tomography (SCCT) and the North American Society of Cardiovascular Imaging (NASCI). He is the author of over 100 articles, book chapters, abstracts and other publications on advanced imaging technologies. Dr Earls lectures extensively on cardiac CT and has been course director for both national and international cardiovascular imaging meetings.

Jonathon A Leipsic is Chairman of the Department of Radiology for Providence Health Care and an Assistant Professor of Radiology and Cardiology at the University of British Columbia. He is also Co-Director of Advanced Cardiac Imaging for the Providence Health Care Heart Center at St Paul’s Hospital. Dr Leipsic is actively involved in cardiac computed tomography (CT) research, with previous involvement in a multicentre trial evaluating coronary CT angiography versus

quantitative coronary angiography (QCA). He has published numerous peer-reviewed manuscripts and has a number currently in press focusing on a variety of aspects of coronary CT angiography. These include, but are not limited to, diagnostic performance and radiation reduction.

Cardiology, Circulation, 2006;114(16):1761–91.

6. Leber AW, Knez A, von Ziegler F, et al., Quantification of obstructive and nonobstructive coronary lesions by 64- slice computed tomography: a comparative study with quantitative coronary angiography and intravascular

ultrasound, J Am Coll Cardiol, 2005;46(1):147–54.

7. Motoyama S, Kondo T, Anno H, et al., Atherosclerotic plaque characterization by 0.5-mm-slice multislice computed tomographic imaging, Circ J, 2007;71(3):363–6.

8. Cheng V, Gutstein A, Wolak A, et al., Moving beyond binary grading of coronary arterial stenoses on coronary computed tomographic angiography: insights for the imager and referring clinician, JACC Cardiovasc Imaging, 2008;1(4):460–71.

9. Hausleiter J, Meyer T, Hermann F, et al., Estimated radiation dose associated with cardiac CT angiography,

JAMA, 2009;301(5):500–507.

10. Abada HT, Larchez C, Daoud B, Set al., MDCT of the coronary arteries: feasibility of low-dose CT with ECG- pulsed tube current modulation to reduce radiation

dose, AJR Am J Roentgenol, 2006;186(6 Suppl. 2):S387–90.

11. Leschka S, Stolzmann P, Schmid FT, et al., Low kilovoltage cardiac dual-source CT: attenuation, noise, and radiation

dose, Eur Radiol, 2008;18(9):1809–17.

12. Jakobs TF, Becker CR, Ohnesorge B, et al., Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation, Eur Radiol, 2002;12(5):1081–6.

13. Earls JP, Berman EL, Urban BA, et al., Prospectively gated transverse coronary CT angiography versus retrospectively gated helical technique: improved image quality and reduced radiation dose, Radiology, 2008;246(3):742-53.

14. Shuman WP, Branch KR, May JM, et al., Prospective versus retrospective ECG gating for 64-detector CT of the coronary arteries: comparison of image quality and patient radiation dose, Radiology, 2008;248(2):431–7.

15. Stolzmann P, Leschka S, Scheffel H, et al., Dual-source CT in step-and-shoot mode: noninvasive coronary angiography with low radiation dose, Radiology, 2008;249(1):71–80.

16. Husmann L, Herzog BA, Burkhard N, et al., Low-dose

EUROPEAN CARDIOLOGY

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