TLD assessment of mouse dosimetry during microCT imaging

Abstract

Advances in laboratory animal imaging have provided new resources for noninvasive biomedical research. Among these technologies is microcomputed tomography (microCT) which is widely used to obtain high resolution anatomic images of small animals. Because microCT utilizes ionizing radiation for image formation, radiation exposure during imaging is a concern. The objective of this study was to quantify the radiation dose delivered during a standard microCT scan. Radiation dose was measured using thermoluminescent dosimeters (TLDs), which were irradiated employing an 80 kVp x-ray source, with 0.5 mm A1 filtration and a total of 54 mA s for a full 360 deg rotation of the unit. The TLD data were validated using a 3.2 cm3 CT ion chamber probe. TLD results showed a single microCT scan air kerma of 78.0 +/- 5.0 mGy when using a poly(methylmethacrylate) (PMMA) anesthesia support module and an air kerma of 92.0 +/- 6.0 mGy without the use of the anesthesia module. The validation CT ion chamber study provided a measured radiation air kerma of 81.0 +/- 4.0 mGy and 97.0 +/- 5.0 mGy with and without the PMMA anesthesia module, respectively. Internal TLD analysis demonstrated an average mouse organ radiation absorbed dose of 76.0 +/- 5.0 mGy. The author's results have defined x-ray exposure for a routine microCT study which must be taken into consideration when performing serial molecular imaging studies involving the microCT imaging modality.

Figure 1

MicroCT PMMA anesthesia support module. Radiation exposures were defined for microCT scans when conducted with and without the use of the microCT PMMA anesthesia support module. The anesthesia module provides gas anesthesia under a HEPA filtered pathogen-free environment.

Figure 2

Calibration curve to convert TLD output into radiation dose. We used this calibration plot to determine the exposure response of the TLD chips which was 0.00542 C∕kg∕μC. This calculation allowed us to convert TLD output into radiation dose for the remainder of the experiments.

Figure 3

C∕kg delivered per radiograph. This graph shows the charge imparted to the field of view isocenter as a function of radiograph acquisition angle. The radiation dose is relatively constant from 0 to ∼200 deg angle of acquisition, but then decreases from 200 to ∼350 deg. This reduction is due to the plastic animal bed within the anesthesia module which absorbs some of the x-ray dose and therefore reduces the radiation exposure measured by the CT probe ion chamber.

Figure 4

C∕kg delivered as a function of kVp. This graph shows the linear relationship between dose and kVp setting of the x-ray source. As the kVp increases, the dose increases linearly. For all three conditions, the slopes of the lines are very similar confirming that TLD radiation measurements are comparable to ion chamber measurements.

Figure 5

MicroCT images of implanted TLDs. MicroCT reconstructed images highlighting the precise locations of implanted TLDs. Figure shows (A) a sagittal microCT slice, (B) a coronal microCT slice, and (C) the axial microCT slice corresponding to the intersected lines in (A) and (B).