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St. Christopher's Hospital for Children, Philadelphia, PA 19134
We read with interest the recent articles regarding pediatric CT and radiation dose [1, 2]. Minimizing radiation dose to pediatric patients from CT examinations must indeed have a high priority in every practice where children are examined. Guidelines such as those suggested by Donnelly et al. [2] for selection of tube current (mA) according to patient weight may prove to be very useful. However, we believe the exposure factor of time (in seconds) should be included.
As everyone knows, radiation exposure is the product of tube current (mA) and time (sec). Time of gantry rotation at helical CT is commonly 1 sec, and this factor is often ignored. However, some manufacturers of CT equipment do allow adjustment of the time of gantry rotation. For example, on a Somatom Plus 4 scanner (Siemens, Erlangen, Germany) the time can be set from 0.75 to 1.5 sec. Decrease in gantry rotation time (i.e., faster rotation) reduces patient exposure in direct proportion. For example, a tube rotation time of 0.75 sec would yield a 25% reduction in patient exposure compared with a rotation time of 1 sec at any tube current (mA). Thus, patient exposure (mAs) can be adjusted by altering either tube current or gantry rotation time.
Paterson et al. [1], in evaluating techniques used on CT examinations from multiple referring institutions, looked at tube current (mA) but not time. Their data were not adjusted for gantry rotation cycle speed. To the extent that a 1-sec rotation cycle speed is standard, the data would not be affected. However, it is possible that a substantial number of studies were performed with different rotation cycle speed.
We applaud efforts to increase awareness of the effects and extent of radiation exposure in children, and we welcome creation of guidelines for minimizing radiation dose on CT examination. In addition, we encourage radiologists not to lose track of the time.
References
Duke Children's Hospital and Health Center, Durham, NC
27710
Children's Hospital Medical Center, Cincinnati, OH
45229-3039
We would like to reply to the important comments raised Smergel and Benson. The focus of their letter was gantry rotation time (the amount of time required to complete one revolution measured in seconds) as an additional parameter that should be considered in determining radiation dose in helical CT. As they point out, everybody knows that radiation exposure (mAs) is related to both tube current (mA) and time (sec). Because radiation dose is directly related to gantry rotation speed, it is another important factor to consider when adjusting parameters to minimize radiation dose to children undergoing CT. The authors aptly conclude, "...we encourage radiologists not to lose track of the time." We are all clearly in agreement with the need to attend to all parameters for helical CT to minimize radiation dose. We are also in agreement that the specific parameter of gantry cycle time (time to complete one 360° rotation, in seconds) is not to be overlooked.
The authors state that the data in our articles [1, 2] did not control for gantry cycle time. This is true, for three reasons: First, this information was not available for all CT examinations we reviewed. The data we acquired indicated that approximately 80% of the helical CT examinations were performed on scanners (General Electric Medical Systems, Milwaukee, WI) [2] that had a gantry cycle speed of 0.8-1 sec. At the time the CT examinations were performed, most of these scanners in practice were limited to 1-sec gantry times. Therefore, for most of the CT scans we reviewed, mA equaled mAs. Second, even if all remaining studies had been performed at 0.5-sec rotation speed, the mean mA would have remained greater than 150 for all examinations (at all ages and for both chest and abdomen CT). This value is certainly above recommendations [1, 2]. Finally, the relative lack of adjustments for mA between chest and abdomen CT or within ages would not change on the basis of inclusion of gantry rotation cycle time. The fact remains that few adjustments, or none at all, were made for different ages and between chest and abdomen scanning.
The weight-based table for adjusting tube current [2] was designed for a single-slice helical CT scanner (General Electric Medical Systems) using a gantry rotation of 1 sec. In such a case, the mA and mAs are equal. Rather than dictating exact protocols for all CT scanners, we intended to point out that the tube currents commonly used when performing pediatric CT were grossly higher than necessary [1] and could be radically reduced without compromising diagnostic information [2].
Our own experience continues to be that most outside examinations and pediatric protocols pay relatively less attention to rotation cycle than tube current. Even when a faster cycle is used, the scanner may automatically increase the tube current to adjust the signal-to-noise ratio back to what would have been obtained at a slower cycle time. This is the case on a multidetector scanner (LightSpeed Plus; General Electric Medical Systems). With this scanner, simply reducing the gantry cycle time without resetting the tube current would make no difference in the radiation dose.
We are all in agreement that many factors can control the radiation dose to children, tube current and gantry rotation speed among them. This complexity in using helical CT is increasing: Multidetector technology has literally dozens of options that can be chosen to perform CT examinations of acceptable quality in children. For example, on the General Electric LightSpeed multidetector CT scanner, a study of the abdomen on a 20-kg child could be performed using detector configurations (mm) and table speeds (mm per rotation) of 3.75/11.25, 5/15, 3.75/22.5, or 5/30; kilovoltage settings of 120, 130, or 140 kVp; gantry cycle times of 0.5, 0.6, 0.7, 0.8, or 1 sec; and tube currents of, for example, 80, 90, or 100 mA—for a total of 180 different scanning options. It is impossible to derive a single set of guidelines that suffice for all practices and all clinical questions [2]. This principle applies to single-slice scanning as well, although the options are more limited.
Smergel and Benson are correct, however, in emphasizing that time is an important option in these guidelines, and they are correct in implying that all parameters should be considered. The issues raised in their letter emphasize the heterogeneity that has arisen in CT technology. In addition, the configurations of CT scanners and their associated radiation doses vary from manufacturer to manufacturer. This heterogeneity necessitates making specific adjustments in protocols, depending on which CT scanner is available.
In conclusion, we appreciate the clarification of one of the important factors in determining radiation dose. It is clearly time that we all take the time to understand and adjust all parameters contributing to radiation exposure in CT, in both children and adults.
References
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