The following is an interview I conducted with Dr. Lee Mitsumori, Assistant Professor of Radiology at the University of Washington.
Dr. Shuman: Please give us a brief overview of your work involving weight-based selection of CT parameters.
Dr. Mitsumori: We have been investigating the use of body-size-dependent CT scan parameters to optimize the radiation dose of each scan. Several imaging parameters can be modified at the time of the CT that can alter the amount of radiation of the scan (tube current, tube potential, scan range, scan phases, image reconstruction techniques). The challenge is to adjust each so that the final set of patient images are of diagnostic image quality and were obtained with the lowest possible radiation dose. The risk is that incorrect settings or too aggressive dose reduction can create a scan that is of poor image quality that does not allow a correct interpretation. Current work at University of Washington Medical Center involves studies comparing different CT image reconstruction techniques (adaptive iterative reconstruction, model based iterative reconstruction) that can significantly reduce CT image noise and how these can be best implemented in patient CT exams.
Dr. Shuman: Why is weight an important concern regarding CT? How does weight affect CT?
Dr. Mitsumori: A patient’s body size, as reflected by weight, affects two important feature of a CT scan. The first is the amount of photons needed to generate an interpretable CT image. A CT is a projection technique, where an x-ray source emits photons that then pass through the subject before being recorded by a detector on the other side of the patient. Larger patients need more photons with higher energy than smaller patients to create images that can be correctly interpreted. If the CT parameters are not correctly set and only a small number of photons are recorded by the detector, the images created are noisy. If an image is too noisy, the interpreting physician may not be able to see if a problem or disease process is present. Unfortunately, the more photons used the higher the radiation exposure of the CT exam. While large patients are at risk for having noisy images, small patients can have scans done with parameters selected for an average sized patient, in which case too many photons are used than what would be needed to create diagnostic images. Thus, the importance of matching the radiation dose (number and energy of photons emitted) with the patient’s body size to prevent non-interpretable studies in larger patients, and avoid excessive radiation dose in the smaller patient.
The second feature of a CT scan where patient size is important is in the amount of iodinated contrast needed for the scan. Iodinated CT contrast is injected intravenously during the scan and improves the depiction of the different organs and vascular structures in the body. With CT imaging, the use of contrast greatly improves the radiologist’s ability to differentiate diseased (tumors, infection, inflammation) from normal tissues. Similar to radiation dose, the amount of contrast needed to enhance the organs is dependent upon patient body size. Small patients will have smaller organs, less blood volume, and be shorter than larger patients. Thus the amount of CT contrast needed may not be the same for every patient.
Dr. Shuman: What are the risks involved when a hospital or imaging center does NOT take a patient’s weight into account?
Dr. Mitsumori: The risks for scanning with fixed CT parameters are that when compared to an average sized patient, the large patient may have scans with noisy CT images that could lead to interpretation errors, or a non-diagnostic scan in which the scan may need to be repeated or an alternative imaging test performed. For the smaller patient, the risk would be that more radiation is used than what is needed to generate adequate, diagnostic images.
Similar risks occur with CT contrast administration. If a fixed amount of contrast is used for every scan that is based on an “average” patient size, then large patients may not receive enough contrast to adequately enhance the internal organs rendering the scan potentially non-diagnostic, and the small patients are given more than the amount of contrast needed to obtain an adequate scan.
Dr. Shuman: When discussing CT exams with their doctors, are there questions a patient should ask regarding their weight? In other words, what would make an “informed patient” as it relates to weight and CT?
Dr. Mitsumori: An informed patient would want to ask if their CT scans will be performed with “patient tailored” or “patient specific” protocols that use the amount of radiation and iodinated contrast that best match the patient’s body size.