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Science to Practice |
1 Department of Radiology, University of California Davis Medical Center, Research Imaging Center, 4701 X St, Sacramento, CA 95817jmboone{at}ucdavis.edu
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For many years, the technique factors for computed tomography (CT) (ie, tube current, peak voltage, pitch, and section thickness) were not adjusted to accommodate for differences in the patients' sizes, but rather were preset values associated with each CT protocol. Although the practice of protocol-driven "push-button" CT was easy, this practice came under scrutiny when it was recognized (1) that children and smaller adults were receiving higher doses than necessary during CT because technique factors were not altered to accommodate their smaller dimensions. Since this problem was identified, a number of authors (2–4), including my colleagues and I (5), have reported methods for determining appropriate CT technique factors depending on the size of the patient to be scanned. In this issue of Radiology, Menke (6) studied a number of body size–related parameters that might be used to predict optimal CT technique settings.
The Science
A generation ago, good-quality radiographic images were acquired with technique factors customized to the patient's measured or estimated dimensions by using so-called technique charts. Automatic exposure control (commonly called "phototiming") gradually replaced the former method of manual control with technique charts, and, currently, almost all radiography (screen-film and digital) makes use of automatic exposure control–determined techniques.
Unlike radiography, the mean gray scale (in Hounsfield units) with CT is not dependent on the CT technique factors because of the water calibration utilized with all CT scanners (7). Consequently, when higher radiation levels are used during CT of small patients, the image quality is not degraded as it is with overexposed, blackened radiographs (indeed, CT images have better noise properties when overexposed).
In a study of parameters that might be used to predict optimal CT technique settings, Menke (6) compared each parameter with the water-equivalent diameter (Dw) of the patient, the Dw representing a reasonable reference standard metric. The Dw can be determined after CT by means of image analysis, and thus it is not available before CT scanning. The dimensions of interest studied by Menke included height, weight, sagittal and coronal body diameters, and other metrics derived from these dimensions, such as body mass index. The study was performed in three areas (thorax, abdomen, and pelvis) that corresponded to the areas of the majority of CT examinations performed on the human torso. Of the parameters studied, body circumference and mean body diameter were found to best predict the most appropriate CT technique factors. For example, Menke found that, for the thorax and pelvis, the mean body diameter is significantly more accurate than is body weight as a predictor of optimal CT factors. While the data for the abdomen did not meet statistical significance, the same parameters can be used for abdominal CT techniques, making the process consistent for all torso CT imaging.
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Clinical use.—All CT manufacturers have developed the equivalent of automatic exposure control for their more recent CT systems. However, for the large installed base of CT scanners without automatic technique modes, CT technologists should use technique charts to adjust technique factors on the basis of the patient's dimensions. Thus, when a patient comes into a facility to undergo CT, what measurable parameter of the patient should be used to adjust the CT settings? While weight is a good parameter and is far better than using patient height or simply using a default CT technique (with no adjustment for body size), the mean body diameter and body circumference are significantly better parameters to use than weight for thoracic and pelvic studies. The topogram-based estimate (Etopo), a parameter introduced in the study by Menke (6), involved region-of-interest analysis of the posteroanterior scout view and delivered the most precise estimate of Dw. These observations suggest that technique charts built on mean body diameter, body circumference, or Etopo will be most accurate for adjusting CT techniques. In practical terms, this means that the CT technologists either should measure the patient's circumference physically with a cloth measuring tape or should use computer software to measure coronal and sagittal diameters on the scout views (5) or to compute Etopo from the posteroanterior scout view and derive Dw from that.
Future opportunities and challenges.—The results discussed in the study by Menke (6) were based on measurements of adult patients aged 18–87 years (50 women and 50 men). Although the results from the experimental approach used by Menke suggest that these results may be applied to pediatric patients, no children were studied by Menke. However, there is some evidence that the same body parameters (body circumference or mean body diameter) would be useful for pediatric patients, as well (5). Additional studies may lend further clarification to those factors best used for the pediatric patients.
Summary
Menke (6) has shown that body circumference and mean body diameter are the best parameters to consider for adjusting CT technique factors. While newer CT scanners can adjust technique factors automatically on the basis of attenuation, technologists using CT scanners without such automatic modes should use patient circumference, mean body diameter, or Etopo (6) with technique charts (5) based on the measured parameter to accommodate the CT technique (principally tube current) to each patient's dimensions.
References
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