Volume 33, Issue 6, June 2006
Index of content:
- Imaging Scientific Session: Room 330D
- Imaging Dosimery
TU‐E‐330D‐01: TLD‐100 Measurement and Assessment of Internal Mouse Dosimetry During Micro‐CT Analysis33(2006); http://dx.doi.org/10.1118/1.2241612View Description Hide Description
Purpose: Because Micro‐CT utilizes ionizing radiation for image formation,radiation exposure during imaging is a concern. The objective of this study is to quantify the radiation exposure delivered during a Micro‐CT scan and to assess potential therapeutic effects associated with this radiationdose in a murine cancer model. Materials and Methods:Radiation exposure was measured using calibrated thermoluminescent dosimeters (TLD‐100) irradiated during a typical Micro‐CT scan protocol. TLD calibration curves were generated with a Cs‐137 irradiator. TLD's were implanted into a euthanized mouse and was imaged with Micro‐CT. TLD's were removed post‐scan and analyzed. Internal exposures were converted to dose in water. A C57BL/6 mouse lung tumor model derived by IV injection of 400,000 B16F10 murine melanoma cells was assessed for survivability and potential therapeutic effects due to absorbed radiationdoses during Micro‐CT imaging.Results: A single Micro‐CT scan dose of 7.8±0.5 cGy was achieved when using a lucite anesthesia support module and a dose of 9.2±0.6 cGy with out the use of the anesthesia module. TLD data was validated using an ion chamber, providing measured radiation exposures of 8.1±0.4 cGy and 9.7±0.5 cGy with and with out the anesthesia module, respectively. Internal TLDanalysis demonstrated an average mouse organ absorbed dose of 7.3±0.6 cGy. Conclusions: Survival analysis demonstrated a mean survival of nontreated control animals of 29±2 days, with animals receiving up to five sequential Micro‐CT studies surviving a mean of 30.5±1.5 days (total estimated dose of 39±2.5 cGy). The calculated cell survival fraction for a 9.2 cGy Micro‐CT scan was 99.25%. Therefore, negligible therapeutic effect from the radiation exposure delivered during Micro‐CT analysis was observed in the animal model investigated.
TU‐E‐330D‐02: Monte Carlo Simulation to Assess Fetal Dose From MDCT Imaging Using Patient Based Voxelized Models33(2006); http://dx.doi.org/10.1118/1.2241613View Description Hide Description
Purpose: To use detailed Monte Carlo simulations to investigate fetal dose from a multidetector CT (MDCT) using voxelized models created from actual patient images including early and late term pregnancies. Method and Materials: Detailed voxelized models of anatomy were created based on image data from a cohort of pregnant patients who had previously undergone abdomen/pelvis CT scans. The gestational ages ranged from less than 5 weeks to 33.7 weeks. Three regions, corresponding to fetus, gestational sac, and uterus, were contoured on each image series by a radiologist.
A MDCT model was created using details about the source spectra, filtration, collimation, and geometry. To simulate an actual scan, a helical source path was defined and particles were transported through the anatomy of the voxelized patient models; radiation dose was tallied in voxels belonging to the three regions of interest. The simulated abdomen/pelvis scan used a helical scan of 120kVp, pitch 1, and 4 × 5 mm total nominal beam collimation.
Dose on a per mAs basis was separately calculated for the fetus when the fetus was distinguishable from the gestational sac and uterus in the original image. These doses were then compared to two generally accepted fetus dose estimations: the Felmlee et al method and the ImPact estimation of dose to a uterus (for fetuses < 8 weeks). Results: The radiation dose to the fetus in the models with gestational ages of <5 weeks, 6.6 weeks, 7.1 weeks, and 28.3 weeks, were 8.31 mGy/100mAs, 9.67 mGy/100mAs, 14.22 mGy/100mAs, and 11.70 mGy/100mAs, respectively. The fetus dose estimate using the Felmlee technique was 11.30 mGy/100mAs. The ImPact dose estimate to a fetus was 13.0mGy/100mAs. Conclusion: Radiation dose to the fetus was successfully estimated at different gestational ages using detailed models of actual patient and fetus morphology, scanner geometry, and acquisition protocols.
33(2006); http://dx.doi.org/10.1118/1.2241614View Description Hide Description
Purpose: Recent development of wide‐beam multidetector CT and conebeam CT demands alternative methodology to CTDI. In this study, the Optically Stimulated Luminescence(OSL) technique was evaluated for CTquality assurance and dose optimization.Method and Materials:CT scans were performed on 38‐mm2 thin Luxel™ dosimeters oriented in axial plane of a GE LightSpeed Ultra and at the center of a CT body dosimetry phantom. A 5‐mm beam collimation was selected to determine the energy response of the OSLdetector at 80, 120, and 140 kVp stations. mA response of the OSLdetector was evaluated from 60 to 350 mA. Helical scans of varying length coverage were also performed with the detector placed at the isocenter. The exposed OSL discs were measured using a Riso TL/OSL‐DA‐15 reader. Each OSL measurement was followed by a standard beta source irradiation and subsequent OSL measurement to normalize for the differing mass of the disks. The normalized OSL signal reading was used in the data analysis. Results: Good mA linearity was observed at all 3 kVp stations. There are discontinuities seen at ∼ 250 mA and it is known to be caused by tube focal spot change. The 120 and 140 kVp data show good correlation with ionization chamber reading. For 80 kVp, OSL signals to ion chamber readings are all higher, particularly at low mA range, indicating higher sensitivity of the OSLdetector over the ion chamber. The helical scans show an increased OSL signal with scan length and a leveling of the signal at large scan coverage. Conclusion: The OSLdetectors are of small size and responded well to CT exposure. It provides a practical technique for quantifying the dose at any location of a phantom for quality assurance and hence exhibits a potential for estimating patient organ dose.
TU‐E‐330D‐04: Estimation of Cardiologists Radiation Doses Received During Interventional Examinations33(2006); http://dx.doi.org/10.1118/1.2241615View Description Hide Description
Purpose: The aim of this work is to suggest a simple method for the estimation of cardiologist extremity doses. Method and Materials: The extremity and effective doses of nine cardiologists working at five different angiographic units were measured for 157 interventional examinations. Simultaneous measurement of patient doses were also carried out using a DAP meter separately for each projection. Fluoroscopy time (Tfl), number of radiographic frames (Nrf) were recorded on‐line during these measurements.. A Rando phantom was exposed at similar projections with patient studies and one minute of fluoroscopic exposure (DFLx,n) and one frame of radiographic exposure (DRNx,n) were determined for each projection. Scatter radiations from these exposures were also measured at 50, 100 and 150 cm above the floor level at the cardiologist positions for the estimation of legs, wrists and thyroid (or eye) doses. Weighting of projections were determined for the patient group of each cardiologist using the recorded values of Tfl and Nrf Extremity doses, Dx were calculated from the following formula: n gives the projection number and x is the distance from the floor level. Results: Measured and calculated extremity doses for each cardiologist were in good agreement (R= 0.75 for thyroid). The calculated doses for 50cm and 100cm were found within the measured values of left and right legs and wrists. The use of dominant projection data alone still provided comparable results. Conclusion: If there is a lock of personal dosimetry for cardiologists, it could be possible to make an estimation of extremity doses from the of total fluoroscopy time and frame numbers used in the examinations together with the knowledge of scatter radiations at cardiologist positions.
33(2006); http://dx.doi.org/10.1118/1.2241616View Description Hide Description
Purpose: An International Atomic Energy Coordinated Research Program investigated the feasibility of providing guidance levels for fluoroscopically guided invasive cardiology procedures. Method and Materials: A sample of 6,000 cases of coronary angiography (CA), coronary angioplasty (PTCA) or combined CA and PTCA from ten laboratories in four different countries were used for the analysis. Dosimetric and patient demographic data were collected. Intercalibration of kerma area product (KAP) meters and quality control tests were periodically performed. Image quality and skin dose distributions were evaluated in a small sample of procedures. Procedure complexity was evaluated in a subset of 1,000 PTCA cases. Results: Classification into three clean categories proved difficult due to the rapid evolution of interventional cardiology and considerable variability in routine clinical practice among the centers. It may be better to categorize cases as either coronary angiography only (DX) or interventional, with a variable CA component (RX).
Median values of KAP (for similar procedures) varied by a factor of three between centers. Complexity has a factor of two influence on KAP. The data suggest that a laboratory can normalize its data by complexity scaling.
Dose inefficient laboratories were identified by noting that the median value of procedures exceeded the third‐quartile value for the entire pool. One laboratory exceeded the guidance level because of the use of 25 fps imaging in place of the more common 12.5 fps. Another laboratory, not under routine QA testing, had high fluoroscopic and cinefluorographic dose rates. Conclusion: Guidance levels for invasive cardiology appear to be feasible. Suggested KAP guidance levels for a facility performing procedures of moderate complexity are 50 Gycm2 for DX and 120 Gycm2 for RX. These values should identify dose inefficient laboratories. Guidance for fluoro time and cine frame count is being developed. Further research is needed as clinical procedures and technologies evolve.
TU‐E‐330D‐06: Assessing Patient Radiation Exposure From Fluoro‐Guided Procedures Based On Direct Dose and Dose‐Area Products33(2006); http://dx.doi.org/10.1118/1.2241617View Description Hide Description
Purpose: To assess patient radiation exposure from fluoro‐guided procedures based on direct dose and dose‐area product. Method and Materials: Various procedures performed by members of Cardiology and Radiology Departments at SUNY Stony Brook were monitored for radiation exposure to patients' skin.Dose,dose‐area product, fluoro‐time, and beam parameters were recorded for each fluoro‐cine run. Along with clinical data, dosimetry print‐out was reviewed after each procedure to identify possible high doses to any given site of the exposed area and to have appropriate medical care provided timely. Accrued data were analyzed and evaluated in detail. Results: Data from a total of 3040 consecutive cases, 1883 diagnostic and 1157 interventional, performed by 16 and 10 physicians respectively were analyzed. Based on dosimetry print‐out, the total skindose averaged 87 rads for diagnostic procedures, with maximum skindose below 100 rads to any given site found in over 90% of cases, that for interventional procedures respectively were 223 rads, with maximum skindose over 300 rads to any given site found in over 15% of cases. Conclusions: Our experience demonstrated the value of careful monitoring and thorough assessment of radiationdose and dose distribution in keeping medical staff informed timely of their patients' exposures.
33(2006); http://dx.doi.org/10.1118/1.2241618View Description Hide Description
Purpose: To quantify the dependence of effective dose on subcutaneous fat thickness in x‐ray radiography and develop a set of relative radiation risk factors for comparing overweight to “lean” patients. Materials and Methods: Using the MCNP Monte Carlo code and geometric phantoms patterned after Christy et al, effective dose calculations were performed for abdominal and chest radiographs of a “normal weight” adult male phantom. The effective dose, Eo, was normalized to the number of x‐rays exiting the patient to model a constant exposure to the image receptor. Varying thicknesses of adipose tissue were then added to the anterior, lateral, and /or posterior regions of the torso of the phantom and the normalized effective dose, E, calculated at the same kVp and source to image receptor distance. The ratio E/Eo provides an index of the increased stochastic risk. Anterior:lateral:posterior fat ratios used ranged from 6:5:1 to 1:3:6 with total anterior plus posterior additional fat thicknesses extending from 1 to 30 cm. Results: For AP and PA projections, both the lateral fat and the fat layer proximal to the x‐ray beam had negligible effect on E/Eo. E/Eo was shown to depend only on the distal fat layer thickness with an exponential dependence of the form exp(kx), where k is an exam/kVp specific constant, and x the distal fat layer thickness. For the AP abdominal projection, k values were 0.127, 0.119, 0.106, and 0.094 cm−1 at 80, 100, 120, and 140 kVp, respectively. R2 for all fits were better than 0.96. E/Eo ranged as high as 12.4 for an extremely obese patient with 20 cm of posterior fat at 80 kVp. Conclusions: Overweight and obese patients incur significantly elevated stochastic risks from radiographic procedures as compared to their lean counterparts, depending upon the kVp and fat layer thickness closest to the image receptor.