Image quality / radiation doses for CT examinations

An important area for a medical physicist is the optimization of the radiation dose / image quality. The challenge is to establish sufficient image quality for a specific diagnostic task with the lowest effective dose to the patient. Within radiology, the use of computed tomography (CT) has greatly increased in recent years. CT is associated with higher radiation doses than other radiation-based medical examinations such as plain radiography and most nuclear medicine examinations. Radiation dose from a general CT examination of the thorax (chest) is approximately five times the natural background radiation a person is exposed to in a year. Consequently it is of great importance to minimize radiation doses.

We have performed a number of studies for various CT examinations where we have varied the dose level, by lowering the tube current, or simulated dose levels by manipulating the raw data or using commercial algorithms for manipulating DICOM data after reconstruction (so called post-processing). Radiologists have reviewed the images by searching for pathological structures or assessed specific image quality criteria. The results of these studies show that it is potential for dose reduction.

In order to improve the diagnostic information, contrast medium commonly based on iodine, is used. Such contrast medium can negatively affect the kidney function. The lifetime risk for cancer is greater the younger the patient is at the time of exposure, however, the risk of contrast-induced impairment of kidney function increases with age. By adjusting the amount of contrast to the radiation dose, age-specific risks are minimized. We have performed studies in which the relationship between radiation dose / contrast medium dose and image quality was studied. By balancing radiation and contrast media doses the radiation dose for younger patients was reduced by more than 50% while the contrast-to-noise ratio was maintained. In elderly patients the amount of contrast decreased by about 20% and the radiation dose increased by about 50% without affecting image quality.