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In an effort to both reduce and improve the monitoring of radiation dose delivered to patients during computed tomography (CT) procedures, the Dosimetry Group has an ongoing project that involves the characterization of clinical dosimeters and reference quality ionization chambers.
The Dosimetry Group of the Radiation Physics Division has an on-going collaboration with the FDA, NIH, and the Walter Reed National Military Medical Center to characterize clinical dosimeters for the purpose of monitoring the radiation dose delivered to patients during computed tomography (CT) exams. To date, this work has focused primarily on head CT exams, but the methods used are equally valid for scans of the chest or abdomen. A proper assessment of patient radiation exposure risk should consider both the deep tissue and surface dose. To this end, measurements were performed by placing dosimeters inside and on the surface of physical phantoms which were then imaged using typical scanning protocols. The physical phantoms included an acrylic cylinder from NIST and an anthropomorphic phantom from the FDA, and the dosimeters included optically stimulated luminescent dosimeters (OSLs) and radiochromic film.
The results of the measurements from the head scans (made with OSLs) provided an estimate of the dose to the primary critical organs - the eye lenses, skin, and brain. The measured doses ranged between 36 mGy and 47 mGy for the set of CT scanner parameters used during these experiments (tube current, voltage, scan type, number of slices, etc…) The scanner parameters used in these experiments corresponded to those used routinely in the clinic for patient procedures. Instead, the measurements made with films explored dose values delivered to the surface of the head phantom with routine and non-routine scanner parameters. Doses measured on the surface of the head phantom correspond to the peak skin dose (PSD). These film measurements showed that the PSD values for the head phantom varied between 27 mGy and 136 mGy. In addition to these measurements, an ionization chamber was placed inside the acrylic cylinder phantom to measure the volume CT dose index (CTDIvol), a machine parameter which serves as a measure of the radiation output of the CT scanner. The measurements were performed using CT machines from 4 different manufacturers and, in each case, the measured CTDIvol value was compared to the nominal planned value displayed by the CT scanner prior to initiating the scan. The CTDIvol for different values of the x-ray tube current setting were correlated to PSD measurements performed simultaneously using radiochromic film. Such a relationship is useful because it allows one to directly relate a machine parameter displayed on the CT console to a more accurate measure of patient dose. The ratio was determined for the measured PSD values to the CTDIvol displayed on the CT for all four machines studied. The values obtained were: 0.84, 0.94, 0.75 and 0.87. These values compared quite well with recent Monte Carlo calculations performed by other groups. Therefore these measurements help provide a validation to those calculations. The results from this ongoing effort have been published recently (Heras et al, Radiation Protection Dosimetry (2013), Vol. 157, No. 4, pp. 536–542; Brunner et al, Medical Physics, Vol. 40, No. 8, August 2013).
End Date:August 2013
Lead Organizational Unit:pml