Interest in cardiac imaging with multislice CT is growing, as evidenced by the large number of studies that have been published on this topic. Advances in cardiac MSCT have also been aided by the introduction of extremely fast, user-friendly scanners.

The radiation dose associated with ECG-gated cardiac MSCT, however, has generated considerable debate.¹ The International Commission on Radiological Protection recommends that "all exposures [are kept] as low as reasonably achievable."² This goal is known as the ALARA principle, and many strategies have been introduced to achieve it. These have been based on x-ray emissions and/or scanning parameters such as mAs, kV, pitch, and collimation and on the individual patient's characteristics (i.e., automatic exposure control systems and ECG-pulsing techniques for ECG-gated acquisitions).

The adjustment of scanning parameters can lead to an increase in image noise, which may, in turn, affect the diagnostic acceptability of images. This has been a major concern. Scan parameters should be tailored to provide diagnostic images, but without exposing patients to unnecessary radiation.

FOCUS ON DOSE

The fundamental parameter for measures of radiation dose is the CT dose index. The CTDI, the radiation dose absorbed by a dose phantom, is measured in either grays (Gy) or rads (1 Gy = 100 rad). It represents the integrated dose along the z-axis from a single axial CT scan (one rotation of the x-ray tube).

The weighted CTDI (CTDIw) accounts for differences in absorbed dose within the scanned region. Absorption is approximately twice as high at the surface as at the center of the field-of-view. When considering volumetric scan protocols, it is also essential to consider any gaps or overlaps between radiation dose profiles from consecutive rotations of the x-ray tube. The volume CTDI (CTDIvol) can be used to express the average dose delivered to the scan volume for a specific examination. This value can be read directly from the scanner, allowing a direct and rapid estimate of the amount of radiation delivered to patients.

The absorbed dose does not account for differences in the sensitivity of target organs to radiation damage.³ This assessment is achieved with a second parameter: the equivalent dose, which is calculated by multiplying the absorbed dose to a specific tissue with a radiation weighting factor. The weighting factor for x-rays is approximately unity, so the equivalent dose has the same numerical value as absorbed dose. Equivalent dose is measured in sieverts (Sv) or rems (1 rem = 10 mSv).

Another important parameter is the effective dose, which is useful when assessing the potential biological risk from specific x-ray examinations. It is calculated by summing the absorbed doses in individual organs, weighted by their radiation sensitivity. This provides an estimate of the whole-body dose required to produce the same risk as a partial-body dose delivered by a localized radiological procedure. The effective dose allows comparisons among different x-ray examinations. Doses associated with medical imaging examinations can also be compared with those received from other sources; for example, natural background radiation.