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Letters to the Editor |
Department of Radiology, Medical University of South Carolina, 169 Ashley Ave, PO Box 250322, Charleston, SC 29425
e-mail: huda{at}musc.edu
In the December 2007 issue of Radiology, Dr Hurwitz and colleagues (1) measured effective doses (EDs) to an adult female phantom and concluded that "use of the DLP [dose-length product] to estimate the ED will underestimate the overall radiation exposure." The normalized ED conversion factor (EDLP) used in their study (ie, 0.017 mSv · mGy–1 · cm–1), however, is inappropriate because it fails to account for the following four important factors:
1. The value of 0.017 mSv · mGy–1 · cm–1 was generated for a 70-kg patient and is not applicable for the 55-kg female anthropomorphic phantom. Doses generally increase with decreasing patient size, and the EDLP for body computed tomographic (CT) scans for 1-year-olds is 100% higher than that for 10-year-olds (2).
2. EDLP varies with scan length. For the VCT scanner used in the study (1), operated at 120 kV, the EDLP is 0.0160 mSv · mGy–1 · cm–1, 0.0193 mSv · mGy–1 · cm–1, and 0.0222 mSv · mGy–1 · cm–1 for scan lengths of 6, 12, and 18 cm, respectively, as measured from the apex of the heart (3,4).
3. EDLP varies with x-ray tube voltage. For a 12-cm–long cardiac scan on a VCT scanner, EDLP increases from 0.0156 mSv · mGy–1 · cm–1 at 80 kV to 0.0200 mSv · mGy–1 · cm–1 at 140 kV (3,4).
4. Tube current modulation in chest CT results in lower EDLP conversion factors because EDs for anteroposterior projections can be twice as high as those for lateral projections at the same incident kerma-area product (5).
DLP values can be converted to patient ED when care is taken in the choice of EDLP (2,6). It is interesting to note that data presented by Dr Hurwitz and colleagues (1) in table 3 may be used to obtain EDLP for adult women for this specific scanner—namely, about 0.025 mSv · mGy–1 · cm–1 for cardiac CT scans and about 0.027 mSv · mGy–1 · cm–1 for pulmonary embolism studies. Use of CT scanner DLP data, together with appropriate EDLP, is a practical method for generating EDs to help operators better understand how much radiation patients receive from CT examinations (7).
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Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710
e-mail: hurwi001{at}mc.duke.edu
We thank Dr Huda for his interest in our work. We concur with Dr Huda that the estimated ED will depend on the phantom size. However, we believe that the main reason for discrepancy between the estimated ED and the ED derived from direct organ measurements comes from inherent shortcomings of the ImPACT tables (1). Groves et al (2) showed that the ED measured with thermoluminescent dosimeters and a Rando phantom was 18% higher than that obtained by using the ImPACT table. They concluded that underestimation by the ImPACT table was due, in part, to the failure to incorporate modern multidetector CT design. Dixon (3) showed that standard CT dose index (CTDI100) measurements underestimated the equilibrium dose by about 20% in a body phantom and by about 10% in a head phantom. Therefore, the weighted CT dose index (CTDI), volume CTDI, and DLP may be underestimated, which results in a lower ED from the DLP calculation. The ImPACT table seems to result in underestimation of the ED.
Dr Huda's items 2 through 4 seem to be a generic description of conversion factor characteristics. For example, in item 3, he points out the effect of tube voltage on the ED; however, standard CTDI already incorporates the tube voltage in the manufacturer's look-up table, thus, weighted CTDI, volume CTDI, and DLP (volume CTDI x scan length) all incorporate the tube voltage variations. In item 4, he points out the effect of tube current modulation on the conversion factor. However, the scanner automatically computes the DLP regardless of the mode. After considering Dr Huda's comments in item 2 and reestimating the ED by using the DLP and the conversion factor, 0.0193 mSv · mGy–1 · cm–1, for our clinical protocols, we have determined the estimated EDs to be 9.8, 13.9, and 24.2 mSV for the pulmonary vein protocol, coronary CT angiography protocol 1, and coronary CT angiography protocol 2, respectively, yielding an underestimation of ED by 21%–24% as compared to the calculated ED from our physical organ measurements. These results confirm our statement that estimation of ED by using the DLP method will underestimate ED.
We believe that metal oxide semiconductor field effect transistor (MOSFET) technology and modern anthropomorphic phantoms make it possible to measure ED directly and more realistically (4).
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This article has been cited by other articles:
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  P. C. Shrimpton, B. F. Wall, T. T. Yoshizumi, L. M. Hurwitz, and P. C. Goodman Effective Dose and Dose-Length Product in CT Radiology, February 1, 2009; 250(2): 604 - 605. [Full Text] [PDF]
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D. Tack and P. A. Gevenois Efforts for Lowering Radiation Dose Delivered with CT: Raising Arms, or Is There More? Radiology, November 1, 2008; 249(2): 413 - 415. [Full Text] [PDF]
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