Original PaperCardiac CT: are we underestimating the dose? A radiation dose study utilizing the 2007 ICRP tissue weighting factors and a cardiac specific scan volume
Introduction
Coronary artery disease (CAD) is one of the leading causes of mortality and morbidity in the western world.1 Cardiac multidetector computed tomography (MDCT) has an established place in the diagnosis of CAD due to its accuracy and accessibility.2 Despite earlier publications indicating a high radiation dose3, 4 changes in CT algorithms have lead to a significant reduction.5
As the use of cardiac MDCT has increased there have been numerous publications highlighting the radiation dose to patients; these studies use a conversion factor applied to the dose–length product (DLP) to estimate the effective dose of the examination.6, 7, 8 The DLP is a measure of the radiation from a single CT section multiplied by the scan length in the z-axis (head-to-foot plane); it is available from the dose report supplied on the CT console and expressed as milligray centimetres (mGy.cm).
To estimate the radiation risk from a medical examination an effective dose (in millisieverts) is calculated. This figure takes into account the radiation source and the potential biological harm to a particular tissue; different organs are given tissue weighting factors depending on their susceptibility to harm from ionizing radiation. The ICRP (International Commission on Radiological Protection) is an independent non-governmental organization created by the 1928 International Congress of Radiology to advance the science of radiological protection for the benefit of the public. In 2007 the ICRP updated the tissue-weighting factors; based on evidence from cohorts of individuals exposed to ionizing radiation.9 Of note the tissue-weighting factor for breast tissue increased from 0.05 to 0.12. This is significant for cardiac CT as the breasts fall within the primary radiation field and will lead to a net increase in effective dose.
The published chest conversion factors applied to the DLP by other groups have varied between 0.014–0.019 mSv(mGy.cm)−1,6, 7, 8, 10, 11 they are based on total chest CT examinations and use the ICRP 60 (1991) tissue-weighting factors, which were updated 3 years ago (ICRP 103).
Cardiac CT (with the exception of graft cases) does not examine the whole chest. CT examination is of the lower chest and upper abdomen and thus involves irradiating the breast tissue for the majority of the volume. In addition, the way in which cardiac CT is performed has changed. Until recently the majority of cardiac CT was performed using retrospective helical scanning. However, dose-reducing techniques (prospective gating) utilizing step-and-shoot axial scanning12, 13 with a partial gantry rotation and a very short exposure time have become the preferred method. In our own institution prospective gating is used in over 90% of patients. This difference in techniques will change the radiation exposure to the patient and, therefore, a different conversion factor is required.
The radiation dose from CT can be calculated using computer-based anthropomorphic models that simulate the CT environment. Radiation dosimetry measurements within the CT machine can be entered into these programs to calculate the effective dose (mSv) for an individual patient.10, 14, 15 Although risk estimates should not be based on individual effective dose calculations, conclusions and comparisons may be made from the mean effective dose for a patient population16 and the stated examination protocol.
The objective of the study was to use a computer-based simulator to calculate the effective dose from cardiac MDCT and compare this with doses calculated with a chest conversion factor applied to the DLP.
Section snippets
Materials and methods
As part of an ongoing radiation audit within Plymouth Hospitals NHS Trust, the cardiac MDCT examinations of 152 consecutive clinical cases were examined using the ImPACT17 computer-based simulator.
Results
The median effective dose for the ImPACT (ICRP 103) calculator was 4.5 mSv (inter-quartile range 3.5–5.4). The median effective dose for the 0.014 conversion factor was 2.2 mSv (inter-quartile range 1.4–2.7).
There is a significant difference between the radiation dose calculated using the ImPACT calculator (ICRP 103) and the effective dose calculated using the chest CT conversion factors applied to the DLP (p < 0.01; Table 2, Fig. 1).
When the ICRP 60 data are used in the ImPACT calculator the
Discussion
Approximately 5 billion imaging examinations are performed worldwide each year; two-thirds employ ionizing radiation.18 The radiation dose from these investigations is now an important fraction of the dose a population receives per year.
Cardiac MDCT has a proven track record in terms of accuracy and reproducibility;19, 20 however, it has been criticized for the radiation exposure delivered to a relatively well population; the intermediate risk patient.
The results of the present study have
Acknowledgement
O.G. acknowledges the continuing support of the Gawthorn Trust. Dr Morgan-Hughes and Professor Roobottom have received lecture fees from GE Healthcare.
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