Review Article
Established and emerging dose reduction methods in cardiac computed tomography

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Abstract

Cardiac computed tomography (CT) is a non-invasive modality that is commonly used as an alternative to invasive coronary angiography for the investigation of coronary artery disease. The enthusiasm for this technology has been tempered by a growing appreciation of the potential risks of malignancy associated with the use of ionising radiation. In the spirit of minimizing patient risk, the medical profession and industry have worked hard to developed methods and protocols to reduce patient radiation exposure while maintaining excellent diagnostic accuracy. A complete understanding of radiation reduction techniques will allow clinicians to reduce patient risk while providing an important diagnostic service. This review will consider the established and emerging techniques that may be adopted to reduce patient absorbed doses from x-ray CT. By modifying (1) x-ray tube output, (2) imaging time (scan duration), (3) imaging distance (scan length) and (4) the appropriate use of shielding, clinicians will be able to adhere to the ‘as low as reasonably achievable (ALARA)’ principle.

Introduction

Cardiac computed tomography is an accurate non-invasive method for detecting coronary artery disease (CAD)1, 2, 3 and is gaining favour as evidenced by the increasing numbers of coronary CT angiograms (CCTA) performed.4 Though invasive coronary angiography (ICA) remains the cornerstone for evaluating anatomical CAD, ICA is an invasive procedure and has inherent complications associated with arterial cannulation.5 Although CCTA does not have the same immediate complications as ICA, concerns have been raised regarding the potential long-term complications of radiation exposure.6, 7, 8

The lifetime attributable risk of developing cancer in younger populations following a CCTA has been estimated to be as high as 1/143 (Table 1) and is of similar incidence as acute complications from ICA.5,6 Therefore, CCTA may not be as benign a procedure as once thought. Still controversial is the absorbed dose calculations for CCTA (Table 2) and the estimated risks of malignancy attributable to CCTA and other procedures using ionizing radiation.9 Whether the projected estimates of future malignancies truly transpire requires further monitoring.

Nevertheless, there is uniform agreement that the medical community needs to adhere to the ALARA principle.8,10 There have been considerable efforts by vendors to develop software, hardware and novel imaging protocols that reduce CCTA radiation dose while maintaining diagnostic image quality.11

We review radiation reduction techniques available for clinical use and those that are being developed. Dose reduction techniques can be broadly categorized into methods that: (1) reduce tube output (tube current and tube voltage), (2) minimize duration (time) of radiation exposure, (3) limit the amount of tissue irradiated (scan length) and (4) improve x-ray beam focus and the application of shielding.10

Section snippets

Ionising Radiation

X-ray radiation consists of high-energy photons that have been released by electrons through the Bremsstrahlung effect (Figure 1). The characteristics of the x-ray beam can be modified by changes in tube current and tube voltage. The ‘number’ or ‘intensity of photons can be altered with the modification tube current (mA). Increases in tube current (higher mA) increase the number of photons emitted from the x-ray tube thereby increasing radiation exposure. Similarly, modifications in tube

Quantification of Radiation Dose

CT scanners estimate the absorbed radiation dose delivered to each patient using the volume CT dose index (CTDIVOL) and dose length product (DLP). CTDIVOL refers to the average radiation dose to a standard phantom over the volume scanned and is measured in milliGrays (mGy). DLP (mGy · cm) is calculated as the product of CTDIVOL and scan length mGy · cm.14,16 DLP can then be multiplied by a coefficient which estimates the effective dose (mSv).

Attempts have been made to standardize the reporting

Reduce Tube Output: Automated Exposure Control

Automatic tube current modulation adapts the tube current (mA) according to patient body habitus.19 Tube current can be modulated in the x-y plane (angular modulation) and in the z-axis (axial modulation). Angular modulation alters the tube current according to attenuation in the x-y plane. It reduces tube current in projections with less attenuation (e.g., when imaging antero-posteriorly) and increases tube current when in planes (lateral) with highest attenuation, while maintaining a constant

Emerging Techniques: Iterative Reconstruction

Traditionally, CT 3D reconstructions have used filtered back projection (FBP) methods which rely upon the overlying projections to create an image. Conversely, iterative reconstruction uses mathematical modelling to predict and modify reconstructed images. The magnitude of computing power required to construct 512 × 512 matrices using iterative reconstruction has limited its applicability. Vendors have developed less intensive algorithms that reduce computing needs. Adaptive statistical

Dual Source and Beyond 64 Slice MDCT

Dual source CT scanning systems offer improvements in temporal resolution which can also be advantageous for radiation dose reduction. The improved temporal resolution of these systems enables prospective studies to be performed in patients with high hearts rates.11

Dual source systems permit image acquisition with high pitch with significant reductions in radiation exposure. Using 128 dual source MDCT doses of radiation as low as 1 mSV are possible with ECG gated high pitch protocols.42 A

Projections and Conclusions

Coronary CT angiography is an emerging diagnostic tool that is being adopted widely in the clinical arena. Initial evaluations of the radiation exposure have revealed major variations in dose across different centres and different vendors.43 This has brought to light the significant variations in practice and the potential resultant adverse effects of the different radiation doses. Studies suggest that it is now possible to reduce CT x-ray doses below those of diagnostic coronary angiography.

Acknowledgment

The authors have indicated that they have no financial conflicts of interest.

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    Gary Small is supported by the University of Ottawa Cardiology Research Endowment Foundation. Benjamin Chow is supported by CIHR New Investigator Award #MSH-83718.

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