Imaging
use of greater amounts of radiation than needed likely will not add significantly to the diagnostic ability of the study.
Techniques used for dose reduction in cardiac CT can be grouped into optimised selection of scanning parameters and use of newer dose- reduction technology. Use of optimal user-selectable options includes methods such as weight- or body mass index (BMI)-dependent tube current, use of lower tube voltage,10,11
minimisation of the scan length
(z-axis) and reduction in the number of phases or series performed. Newer scanning technology includes use of electrocardiogram (ECG)- dependent tube current modulation,12 high-pitched spiral acquisition18–22
algorithms such as iterative reconstruction.23–25
prospective ECG gating,13–17 and use of noise reduction Depending on the CT
scan platform being used and the software version available, one may have many different tools available to lower radiation dose. Many of the techniques described below can and often are used in combination for additional dose reduction.
Measuring Radiation Dose
Radiation dose is proportional to the tube current, the exposure time and the square of tube voltage and is inversely proportional to the pitch for helical acquisition. Estimated radiation doses for CCTA examinations can be expressed in numerous terms. Conventional units of radiation dose (rads and rems) have been replaced with SI units of Grays (Gy) and Sieverts (Sv). Special measures for radiation emitted from CT have been developed: the volume computed
tomography dose index (CTDIvol [in Gy]), dose length product (DLP [in mGy x cm]) and effective dose (E [in mSv]).
The CTDIvol averages radiation dose over x, y and z directions. This is used to express the average dose delivered to the scan volume (3D CT slice) for a specific examination. The DLP is defined as the
CTDIvol times scan length and is an indicator of the integrated radiation dose of an entire CT examination. The CTDIvol and DLP are now reported by most CT systems in current use. E is determined from
dose to individual organs and the associated relative radiation risk assigned to each organ. The estimated effective dose for a patient is obtained by multiplying DLP by a conversion factor, k (in mSv x mGy-1 x cm-1), which varies depending on the body region that is imaged. These normalised effective dose coefficients are determined by the radiation sensitivity of the body region scanned based on exposed organ radiosensitivities. In chest CT, the accepted standard for effective dose quantification is 0.017, which, when multiplied by the DLP in mGy x cm, allows for the calculation of the study effective dose in mSv. It should be noted that while 0.017 is the most commonly used conversion factor for chest imaging, the current recommended conversion coefficient for CCTA is 0.014mSv mGy-1cm-1.26
Techniques for Dose Reduction
Tube Current Optimisation
Current cardiac-capable CT scanners have significantly greater tube power than earlier machines. Cardiac protocols require greater tube current delivered in a significantly shorter period of time than is routinely used for other CT protocols. This power is useful for rapid cardiac scanning of individuals with high BMI. However, if protocols are not individually adjusted, the result may be needlessly high radiation doses.
Anatomy-adapted tube-current modulation, commonly used in non- cardiac applications, is not fully compatible with the ECG-dependent
16
dose-modulation technique currently used in CCTA. The appropriate tube current must be manually selected for each case. The current is tailored according to the patient’s BMI, chest circumference and estimated muscle and breast mass. Reliance on a ‘standard CT protocol’ without altering mA may lead to excessive radiation doses for thin patients and potentially poor image quality for high-BMI patients.
Additionally, a 5cm difference in thoracic diameter
corresponds to a factor of two or more in the dose required to maintain similar image quality.27
Use of weight-adapted mA can
reduce the dose by 18–26%, while constant image noise is achieved and image quality preserved.28
In the PROTOCOL study, LaBounty et al. prospectively evaluated 449 patients undergoing 64-detector CCTA at three centres and compared them pre- (n=247) versus post-initiation (n=202) of a standardised BMI- and heart-rate-based protocol that incorporated multiple dose- reduction strategies, including gating technique, tube voltage, current, padding duration and scan length.29
In multivariate analysis, a 20%
reduction in radiation dose was associated with every 100mA reduction in tube current.
Tube Voltage Optimisation
As the radiation dose varies with the square of the kilovoltage, small reductions in tube voltage enable a marked reduction in effective dose (see Figure 1).10,30–32
Abada et al. reduced the kV in CCTA from the
traditional 120 to 80kV in low-BMI patients undergoing CCTA and found up to an 88% dose reduction.10
I study resulted in a 53% reduction in dose; image quality was maintained.33
Use of 100kV in the PROTECTION Lowering the kV in CCTA exams also increases noise.
The prospective randomised PROTECTION II study randomised 400 patients with a bodyweight <90kg to either standard 120kV or 100kV scan protocol.34
group, with a 31% dose reduction without significant difference in qualitative or quantitative image assessment.
Decreasing voltage to 80 or 100kV has the added benefit of also increasing opacification of vessels due to an increase in the photoelectric effect and a decrease in Compton scattering. Intraluminal CT attenuation (HU) of the coronary arteries is significantly higher at 100kV compared with 120kV.35
Due to this increase in opacification, the iodine dose can be reduced safely.31,36
Scan Coverage Optimisation
Radiation dose is directly proportional to z-axis coverage. Accurate prescription of anatomical coverage is important to minimise dose. If the scan length is too long, unnecessary radiation is delivered to the upper chest and abdominal organs. If the length is too short, part of the coronary tree may be excluded. When minimum z-axis coverage is prescribed, there is a risk of excluding anatomy if the patient does not perform an identical breath-hold to the one used for planning purposes. Teaching the patient to perform a breath-hold in the same manner every time will help avoid truncation of anatomy because of irregular breath-holding.
Minimisation of the z-axis is important. In the PROTOCOL study, multivariate analysis showed a 5% reduction in effective dose for every 1cm of reduced z-axis scan length.29,37
To minimise z-axis
coverage but ensure adequate anatomy is imaged, we prescribe the z-axis from review of the calcium-scoring scan, if performed, or we perform a very-low-dose axial scout. CCTA scanning starts 20mm
EUROPEAN CARDIOLOGY
Radiation dose was significantly lower in the 100kV
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