At present, no established methods exist for dosimetry in micro computed tomography (micro-CT). The purpose of this study was therefore to investigate practical concepts for both dosimetric scanner quality assurance and tissuedose assessment for micro-CT.Methods:
The computed tomography dose index(CTDI) was adapted to micro-CT and measurements of the CTDI both free in air and in the center of cylindrical polymethyl methacrylate (PMMA) phantoms of 20 and 32 mm diameter were performed in a 6 month interval with a 100 mm pencil ionization chamber calibrated for low tube voltages. For tissuedose assessment, z-profile measurements using thermoluminescence dosimeters (TLDs) were performed and both profile and CTDImeasurements were compared to Monte Carlo (MC)dose calculations to validate an existing MC tool for use in micro-CT. The consistency of MC calculations and TLDmeasurements was further investigated in two mice cadavers.Results:
CTDI was found to be a reproducible quantity for constancy tests on the micro-CT system under study, showing a linear dependence on tube voltage and being by definition proportional to mAs setting and z-collimation. The CTDImeasured free in air showed larger systematic deviations after the 6 month interval compared to the CTDImeasured in PMMA phantoms. MC calculations were found to match CTDImeasurements within 3% when using x-ray spectrameasured at our micro-CT installation and better than 10% when using x-ray spectra calculated from semi-empirical models. Visual inspection revealed good agreement for all z-profiles. The consistency of MC calculations and TLDmeasurements in mice was found to be better than 10% with a mean deviation of 4.5%.Conclusions:
Our results show the CTDI implemented for micro-CT to be a promising candidate for dosimetric quality assurance measurements as it linearly reflects changes in tube voltage, mAs setting, and collimation used during the scan, encouraging further studies on a variety of systems. For tissuedose assessment, MC calculations offer an accurate and fast alternative to TLDmeasurements allowing for dose calculations specific to any geometry and scan protocol.
This research was supported by the German Science Foundation “Forschergruppe 661: Multimodal Imaging in preclinical research” (DFG: KA 1254/11-1). The authors thank Indra Yohannes and Wei Chen for their assistance in preparing this paper and Adam Dabkowski for the measurement of the x-ray spectra.
II. MATERIALS AND METHODS
II.A. Micro-CT scanner
II.B. Scan protocols
II.D. Acceptance and constancy testing
II.F. Dose calculations
II.G. Stoichiometric calibration of the MC tool
II.H. Comparison of TLDmeasurements in mice with MC results
III.A. Constancy testing
III.B. Validation of the Monte Carlo tool for small animal imaging
III.C. Dose calculations using Monte Carlo methods
III.C.1. Stoichiometric calibration
III.C.2. Comparison of MC calculations to TLDmeasurements
IV. DISCUSSION AND CONCLUSIONS
- Thermoluminescent dosimeters
- Computed tomography
- Monte Carlo methods
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