Table 1.

Dose reduction strategies in coronary CT angiography.

Parameter	Dose Reduction Method
Iterative reconstruction	Improves signal-to-noise and contrast-to-noise ratio making it possible to maintain image quality, even when current is reduced. Starts with an initial estimate of the image, which is improved iteratively by comparing the synthesized image to the one acquired with projection data and improving the previous estimation.
Multi-row detectors Scout view acquisition	z-axis coverage of the scan is linearly proportional to radiation dose. Multi-row detector (or multi-slice) CT uses multiple rows of CT detectors instead of one. Faster scanning times result from an increase in the number of detectors in the z-direction allowing a larger volume of the heart to be covered per gantry rotation. Limits range covered to only part of the thorax required for scan.
High-pitch prospective ECG-triggered helical acquisition (recommended for low and stable heart rates). Increase pitch ≥ 3	Projection data is acquired for only part of the complete gantry rotation (i.e., a partial scan). The minimum projection data required to construct a complete CT image is 180° plus the fan angle of the CT detectors in the axial plane. Full tube current is only applied during a single phase of the cardiac cycle. Radiation dose is inversely proportional to the pitch.
Heart rate reduction with beta-blockers	Minimum cardiac motion is observed during diastolic phase; however, the diastolic phase narrows with increasing heart rate. Desired temporal resolution for motion-free cardiac imaging is 250 ms for heart rates up to 70 beats per minute and up to 150 ms for heart rates greater than 100 beats per minute, at the limit of gantry rotation.
Automated tube current modulation	CT dose decreases linearly with tube current and tube current–time product. Angular-modulation adjusts tube current for each projection angle [antero-posterior vs lateral] according to the size and attenuation characteristics of the human body. z-axis modulation provides noise index to allow users to select x-ray noise level of reconstructed images and attempt to maintain a constant noise level for all images irrespective of patient size and anatomy.
Tube voltage	CT dose is approximately proportional to the square of the tube voltage. Image quality maintained in studies as low as 70 kV in non-obese adult patients (body mass index ≤ 25 kg/m2). Reducing tube voltage increases attenuation of vessel lumen and cardiac chambers with iodinated contrast media resulting in greater image contrast.