Author + information
- Oliver E. Gosling, BM⁎ ( and )
- Carl A. Roobottom, MB
- ↵⁎C/O Professor Roobottoms' Office, Department of Radiology, Plymouth Hospitals NHS Trust, Derriford Hospital, Derriford Road, Plymouth PL6 8DH, United Kingdom
We read with interest the article by Ho et al. (1) in the recent edition of iJACC. Computed tomography (CT) myocardial perfusion imaging (MPI) offers a new, noninvasive functional assessment of myocardial ischemia. When combined with CT coronary angiography, it may offer the strong negative predictive value of an anatomical test and the specificity of functional testing in a “1-stop shop.”
CT MPI accuracy and radiation dose has been compared with nuclear MPI as a reference.
The effective radiation dose from a medical exposure is measured in mSv. This value takes into account the different radiation sources and the potential biological harm from exposure to a particular organ. Tissues with a high susceptibility to harm from ionizing radiation are allocated a higher weighting in the calculation of effective dose—a higher tissue weighting factor. In 2007, the International Commission on Radiological Protection (ICRP) updated the tissue weighting factors in light of further epidemiological studies; of importance is the increase in the breast-tissue weighting factor from 0.05 to 0.12 (2).
There is now increasing evidence that previously published chest conversion factors (when applied to cardiac CT) significantly underestimate the effective dose to the patient. This is due to 2 factors: 1) the change in the ICRP tissue weighting factors mentioned earlier; and 2) the marked difference in scan volume between cardiac and whole-chest CT scans. Cardiac CT scans only irradiate the lower chest and upper abdomen, a scan field that involves irradiating the breast tissue for the majority of the scan volume, rather than including the relatively radio-insensitive tissues of the upper chest.
Work in our institution (3) using computer-based anthropomorphic phantoms has demonstrated that the conversion factor for cardiac CT is at least double that previously reported; this has been confirmed by other groups (4–6).
We suggest a conversion factor of 0.028 (3) for prospectively gated cardiac CT—which would result in a doubling of the reported dose to 36.5 mSv for the stress and rest examination in the paper by Ho et al. (1). With increasing evidence of the risk of ionizing radiation from medical exposure (7), further dose reduction strategies will be needed before CT MPI becomes the primary choice for functional imaging over established techniques such as stress echocardiography and cardiac magnetic resonance.
- American College of Cardiology Foundation
- Ho K.T.,
- Chua K.C.,
- Klotz E.,
- Panknin C.
- ↵(2007) The 2007 Recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann ICRP 37:1–332.
- Gosling O.,
- Loader R.,
- Venables P.,
- Rowles N.,
- Morgan-Hughes G.,
- Roobottom C.
- Christner J.A.,
- Kofler J.M.,
- McCollough C.H.