A method to acquire CT organ dose map using OSL dosimeters and ATOM anthropomorphic phantoms

Abstract

Purpose: To present the design and procedure of an experimental method for acquiring densely sampled organ dose map for CT applications, based on optically stimulated luminescence (OSL) dosimeters "nanoDots" and standard ATOM anthropomorphic phantoms; and to provide the results of applying the method-a dose data set with good statistics for the comparison with Monte Carlo simulation result in the future. Methods: A standard ATOM phantom has densely located holes (in 3 × 3 cm or 1.5 × 1.5 cm grids), which are too small (5 mm in diameter) to host many types of dosimeters, including the nanoDots. The authors modified the conventional way in which nanoDots are used, by removing the OSL disks from the holders before inserting them inside a standard ATOM phantom for dose measurements. The authors solved three technical difficulties introduced by this modification: (1) energy dependent dose calibration for raw OSL readings; (2) influence of the brief background exposure of OSL disks to dimmed room light; (3) correct pairing between the dose readings and measurement locations. The authors acquired 100 dose measurements at various positions in the phantom, which was scanned using a clinical chest protocol with both angular and z-axis tube current modulations. Results: Dose calibration was performed according to the beam qualities inside the phantom as determined from an established Monte Carlo model of the scanner. The influence of the brief exposure to dimmed room light was evaluated and deemed negligible. Pairing between the OSL readings and measurement locations was ensured by the experimental design. The organ doses measured for a routine adult chest scan protocol ranged from 9.4 to 18.8 mGy, depending on the composition, location, and surrounding anatomy of the organs. The dose distribution across different slices of the phantom strongly depended on the z-axis mA modulation. In the same slice, doses to the soft tissues other than the spinal cord demonstrated relatively small variations, with the maximum COV around 11.4%. This might be attributed to the angular mA modulation, the placement of the dosimeters, the chest cavity of the scanned region, and the size of the phantom. Doses to the spinal cord were consistently lower than those to other soft tissues. Conclusions: The method is suited for acquiring densely sampled organ dose maps, and can be used for studying dose distributions relevant to subject size, organ location, and clinical CT protocols.

Radiation dose calculations for CT scans with tube current modulation using the approach to equilibrium function

“…The OSL dosimeter calibration and readout were detailed elsewhere. 8 Figure 3 illustrates that the agreement between the OSL measurements and the calculations was much better with the TCM tube current distribution than with the constant tube current. The latter resulted in symmetric dose distributions, largely different from the measured dose profiles.…”

Location of radiosensitive organs inside pediatric anthropomorphic phantoms: Data required for dosimetry

“…Pediatric anthropomorphic phantoms have been widely used in medical dosimetry [7e30]. Experimental direct dose measurements on anthropomorphic phantoms may be considered more accurate than other methods that use simple phantoms such as a cylinders or sets of slabs [31].…”

In vitro dose measurements in a human cadaver with abdomen/pelvis CT scans

“…5 The calibration procedure of OSLDs was detailed in a previous study which reported a method for obtaining densely sampled dose maps inside an anthropomorphic phantom with OSLDs. 40 Figure 3 showed the cross-system calibration factors of all the six Thimble chamber systems, with dosimeter system #3 being the reference system (the calibration factors for this system are one for all the kVp settings). The response varied from system to system, with the highest deviation below 0.05, and most of the deviations within 0.02.…”

“…These OSLDs were read twice to reduce the read-out fluctuation, and the background readings were subtracted from the average of the two readings. 40 For both nipples which were close to the boundary of the scanned region, the beam quality was considered the same as the primary beam because of overscan. 41 For the other three locations of the OSLDs, we adopted the conservative estimation that the scattered rays had the same beam qualities as the primary beams.…”