Purpose: Advances in computed tomography (CT) acquisition techniques, primarily multi‐detector CT (MDCT) and cone beam CT (CBCT), make it highly desirable to develop measurement techniques that provide a more physically meaningful measurement of patient dose than the traditional CT dose index (CTDI). The goal of this work was to accurately quantify the organ doses delivered to adult patients during CT exams as a fundamental tool for developing patient dose histories. Method and Materials: A tissue‐equivalent anthropomorphic adult phantom based on a tomographic data set was developed and fabricated at the University of Florida. The adult phantom's physical and radiological parameters are designed to mimic ICRP reference man, and include tissue‐equivalent materials for soft tissue, bone, and lung. Fiber optic‐coupled (FOC) dosimeters, which have been calibrated and characterized across the CT energy range, provide point dose measurements for multiple organ locations. The combination of phantom and FOC dosimeters establishes a dosimetry system to characterize both internal organ doses and effective dose. Results: The calibrated point detector system provides remote, real‐time dose measurements and in conjunction with the adult physical phantom allows for the quantification of absorbed organ dose measurements for a wide range of MDCT acquisition protocols. Average organ doses were quantified for the prostate, kidneys, liver, and lungs. The point dosimeter measurement illustrates the significant dose contributions that originate from scattering outside of tissue regions represented by the 15 cm length of CTDI phantoms. FOC dosimeters permit the dose to be displayed as a function of longitudinal position for a variety of scan parameters, aiding in the interpretation of dose comparisons. Conclusion: FOC dosimeters demonstrate high sensitivity, reproducibility, excellent dose linearity, and combined with their small physical size permit accurate point‐dose measurements. The organ doses vary significantly from interpretations of the CTDI, depending strongly on position and scan volume.
Purpose: This study's aim was to develop an easily reproducible clinical protocol to predict the average glandular dose (AGD) delivered to patients during routine mammography screening. It incorporates an evaluation of patient specific features, including glandularity, to predict the clinically delivered dose for both the cranio‐caudal (CC) and the medio‐lateral oblique (MLO) views. Method and Materials: The development of a modified homogenous dosimetric breast tissue equivalent phantom series (BRTES‐MOD) based on anthropomorphic measurements of the screening mammography population is central in evaluating the patient's fibroglandular content. It has been constructed with reference to the breast tissue elemental composition tabulated in the International Commission on Radiation Units and Measurements ‐ Report 44, and simulates the compression and variable content of patient's tissue characteristics. This study calculates the average glandular dose using entrance skin exposure and dose conversion factors based on fibroglandular content, compressed breast thickness, volumetric and anatomical factors, mammographic unit parameters and modifiable parameters of the BRTES‐MOD phantom. Results: Dose conversion factors were successfully calculated from the patient's fibroglandular content, compressed thickness, unit parameters, and spectral half value layer. An anthropometric population study facilitated the derivation of clinically usable equations to determine patient whole breast area, estimate patient skin layer thickness, and assess optimal placement for the automatic exposure control ionization chamber location. Dose distributions for the study population are presented for both CC and MLO views and compare well with those derived from previous population studies. Conclusion: The designed protocol can be performed within the time of a typical mammography screening appointment, and allows the determination of patient‐specific average glandular dose. The BRTES‐MOD method also provides a quantitative measure of patient specific AGD for the multiple projections comprising screening mammography examinations.
Purpose: Advances in CT acquisition techniques, primarily multidetector CT (MDCT) and cone beam CT (CBCT), make it highly desirable to develop measurement techniques that provide a more physically meaningful measurement of dose than the traditional CT dose index (CTDI). This study presents data based on a point dosimetry system utilizing fiber‐optic‐coupled (FOC) radioluminescent dosimeters to measure fundamental parameters associated with CT dosimetry. This point detector approach provides remote, real‐time dose measurements and allows direct recording of single‐scan dose profiles that contain the essential information required to determine dosimetric quantities for MDCT. Method and Materials: FOC dosimeters based on sensitive elements of either a copper‐doped quartz or coupled scintillation phosphor are characterized for their performance across the CT energy range based on energy dependence, dose linearity, and angular response. A custom Labview program provides a user‐friendly interface to control the system. Measurements were made using traditional CTDI as well as FOC dosimeter measurements for a variety of MDCT acquisition protocols. Results: Measurements along the central axis of a CTDI phantom provide a direct evaluation of the single scan dose profile. FOC peripheral point measurements detect intensity variation with tube rotation, a dependence on scanner pitch, and permit the correlation of scan parameters and dose profiles. While CTDI remains an accurate prediction of MSAD for axial CT, it is empirically demonstrated to fail for multiple scan dose profiles when pitch is not equal to one. The development of a small dosimeter that can directly measure the helical dose profile provides a useful characterization of MDCT scanning performance and an accurate prediction of MSAD. Conclusion: FOC dosimeters demonstrate high sensitivity, reproducibility, excellent dose linearity, and combined with their small physical size permit accurate point‐dose measurements. These properties provide a useful tool for the characterization of the dosimetry quantities fundamental to MDCT.
Purpose: The purpose of this work was to compare clinically relevant point dose measurements in both homogenous geometric and anthropomorphic phantoms in order to compare the effects of MDCT tube‐current modulation on the dose distributions in each. These dose measurements were compared to corresponding CTDIvol measurements to determine this metric's effectiveness in describing clinical scan doses in MDCT. Method and Materials: A fiber optic coupled dosimeter system was utilized with both a homogenous elliptical phantom and an anthropomorphic phantom incorporating accurate anatomical features scaled to represent ICRP reference man. Point dose measurements were taken at various points in both phantoms using routine abdominal scan techniques. Both modulated and fixed tube‐current scans were performed, and the resulting data was used to create an isodose map for each phantom. CTDI measurements were taken using the same scan techniques, and the resulting CTDIvol was compared to the doses measured using the elliptical and anthropomorphic phantoms. Results: Tube‐current modulated scans produced an average point dose reduction of 7.6% in a homogeneous elliptical phantom. This reduction was accompanied by a more uniform dose distribution across the transverse slice, with the most reduction apparent across the minor elliptical axis. The average point dose reduction was slightly less in the anthropomorphic phantom, where tube modulation resulted in an average dose reduction of 4.5% and a similar improvement in dose uniformity. CTDIvol was found to underestimate the average point dose measured from the anthropomorphic phantom, with measured doses being 90% higher than the predicted CTDIvol. Conclusion: Tube‐current modulation was found to reduce both the average dose and the dose variation for an individual slice in both a homogeneous geometric phantom and a heterogeneous anthropomorphic phantom. CTDIvol was found to be a poor predictor of organ doses in MDCT.
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