Radiation Dose Profiles Research Articles

An investigation into factors affecting the precision of CT radiation dose profile width measurements using radiochromic films.

To investigate the impact of x-ray beam energy, exposure intensity, and flat-bed scanner uniformity and spatial resolution on the precision of computed tomography (CT) beam width measurements using Gafchromic XR-QA2 film and an off-the-shelf document scanner. Small strips of Gafchromic film were placed at isocenter in a CT scanner and exposed at various x-ray beam energies (80-140 kVp), exposure levels (50-400 mA s), and nominal beam widths (1.25, 5, and 10 mm). The films were scanned in reflection mode on a Ricoh MP3501 flat-bed document scanner using several spatial resolution settings (100 to 400 dpi) and at different locations on the scanner bed. Reflection measurements were captured in digital image files and radiation dose profiles generated by converting the image pixel values to air kerma through film calibration. Beam widths were characterized by full width at half maximum (FWHM) and full width at tenth maximum (FWTM) of dose profiles. Dependences of these parameters on the above factors were quantified in percentage change from the baselines. The uncertainties in both FWHM and FWTM caused by varying beam energy, exposure level, and scanner uniformity were all within 4.5% and 7.6%, respectively. Increasing scanner spatial resolution significantly increased the uncertainty in both FWHM and FWTM, with FWTM affected by almost 8 times more than FWHM (48.7% vs 6.5%). When uncalibrated dose profiles were used, FWHM and FWTM were over-estimated by 11.6% and 7.6%, respectively. Narrower beam width appeared more sensitive to the film calibration than the wider ones (R(2) = 0.68 and 0.85 for FWHM and FWTM, respectively). The global and maximum local background variations of the document scanner were 1.2%. The intrinsic film nonuniformity for an unexposed film was 0.3%. Measurement of CT beam widths using Gafchromic XR-QA2 films is robust against x-ray energy, exposure level, and scanner uniformity. With proper film calibration and scanner resolution setting, it can provide adequate precision for meeting ACR and manufacturer's tolerances for the measurement of CT dose profiles.

Source term derivation and radiological safety analysis for the TRICO II research reactor in Kinshasa

The source term from the 1MW TRIGA Mark II research reactor core of the Democratic Republic of the Congo was derived in this study. An atmospheric dispersion modeling followed by radiation dose calculation were performed based on two possible postulated accident scenarios. This derivation was made from an inventory of peak radioisotope activities released in the core by using the Karlsruhe version of isotope generation code KORIGEN. The atmospheric dispersion modeling was performed with HotSpot code, and its application yielded to radiation dose profile around the site using meteorological parameters specific to the area under study. The two accident scenarios were picked from possible accident analyses for TRIGA and TRIGA-fueled reactors, involving the case of destruction of the fuel element with highest activity release and a plane crash on the reactor building as the worst case scenario. Deterministic effects of these scenarios are used to update the Safety Analysis Report (SAR) of the reactor, and for its current version, these scenarios are not yet incorporated. Site-specific meteorological conditions were collected from two meteorological stations: one installed within the Atomic Energy Commission and another at the National Meteorological Agency (METTELSAT), which is not far from the site. Results show that in both accident scenarios, radiation doses remain within the limits, far below the recommended maximum effective (whole body) dose of 20mSv/year for workers and 1mSv/year for the general public in the IAEA Basic Safety Standards 115 and demonstrate the radiation safety of this reactor. This guarantees the safety of workers and the population around the plant site.

TH-C-18A-05: Characteristics and Accuracy of CT Radiation Profile Width Measurements Using Radiochromic Films

Purpose: To investigate the impact of several acquisition and image analysis factors on the accuracy of CT beam width measurement using Gafchromic XR-QA2 film and an off the shelf document scanner. Methods: The film was carefully placed at the iso-center and exposed at various x-ray beam energies (80–140 kVp), exposure levels (50–400 mAs), and nominal beam widths (1.25, 5, and 10 mm). Films were scanned on a RICOH MP3501 flat-bed document scanner using reflection mode and various spatial resolutions (100 to 600 dpi) and locations within the scanner bed. Radiation dose profiles were derived by converting the net reflection measurements to dose values (in mGy) through film calibration (which is also described here). The beam widths were characterized by the FWHM and FWTM of the dose profiles and the deviations from the hybrid reference standards were quantified. The impact of x-ray beam energy, exposure intensity, film calibration, and the uniformity and spatial resolution of the document scanner were investigated. Results: The uncertainties in both FWHM and FWTM caused by varying beam energy, exposure level, and scanner uniformity were all within approximately 5%. Increasing scanner spatial resolution increased the uncertainty in both FWHM and FWTM (R2 = 0.35–0.71), with the FWTM affected by more than five times than the FWHM (28% vs. 4.9%). When uncalibrated dose profiles were used, the FWHM and FWTM were over-estimated by as much as 11.6% and 7.6%, respectively. Narrower beam width was more sensitive to the film calibration than the wider one (R2 = 0.68 and 0.85 for the FWHM and FWTM, respectively). Conclusion: Measurement of CT beam width using Gafchromic XR-QA2 films is robust against x-ray energy, exposure level, and scanner uniformity. It can provide adequate accuracy for all practical purposes with appropriate film calibration and scanner spatial resolution (100 dpi or less).

SU‐F‐18C‐09: Assessment of OSL Dosimeter Technology in the Validation of a Monte Carlo Radiation Transport Code for CT Dosimetry

Purpose:To assess the utility of optically stimulated luminescent (OSL) dosimeter technology in calibrating and validating a Monte Carlo radiation transport code for computed tomography (CT).Methods:Exposure data were taken using both a standard CT 100‐mm pencil ionization chamber and a series of 150‐mm OSL CT dosimeters. Measurements were made at system isocenter in air as well as in standard 16‐cm (head) and 32‐cm (body) CTDI phantoms at isocenter and at the 12 o' clock positions. Scans were performed on a Philips Brilliance 64 CT scanner for 100 and 120 kVp at 300 mAs with a nominal beam width of 40 mm. A radiation transport code to simulate the CT scanner conditions was developed using the GEANT4 physics toolkit. The imaging geometry and associated parameters were simulated for each ionization chamber and phantom combination. Simulated absorbed doses were compared to both CTDI100 values determined from the ion chamber and to CTDI100 values reported from the OSLs. The dose profiles from each simulation were also compared to the physical OSL dose profiles.Results:CTDI100 values reported by the ion chamber and OSLs are generally in good agreement (average percent difference of 9%), and provide a suitable way to calibrate doses obtained from simulation to real absorbed doses. Simulated and real CTDI100 values agree to within 10% or less, and the simulated dose profiles also predict the physical profiles reported by the OSLs.Conclusion:Ionization chambers are generally considered the standard for absolute dose measurements. However, OSL dosimeters may also serve as a useful tool with the significant benefit of also assessing the radiation dose profile. This may offer an advantage to those developing simulations for assessing radiation dosimetry such as verification of spatial dose distribution and beam width.

Effect of iterative reconstructions in low dose computed tomography

Objectives: To assess the patient radiation dose profiles in routine body image acquisitions when using iterative reconstruction (IR) methods and compare them with filtered back projection technique. Material and methods: Iterative dose profiles of 986 patients – Thorax (317), Abdomen/Pelvis (540), Thorax/Abdomen/Pelvis (129) in routine clinical practice have been evaluated and effective patient dose estimated from dose length product (DLP) Values. Within the evaluated samples, distribution of CT Dose Index (CTDI) and DLP values were calculated. Results: In Thorax, average DLP delivered was 286.30 mGy.cm which corresponded to an effective patient dose of 4.008 mSv. In Abdomen/Pelvis group, average value of DLP was 495.5 mGy, estimated dose was 7.43 mSv. In trunk study group average DLP was 570.21 mGy which corresponded to an effective dose of 8.55 mSv. Conclusion: Substantial dose reduction is achieved with iterative techniques. In 986 patients scans evaluated, average dose reduction of 33.2% in Thorax, 32.455% in Abdomen/Pelvis and 49.706% in trunk examinations were estimated.

Overbeaming and overlapping of volume-scan CT with tube current modulation in a 320-detector row CT scanner

The purpose of this study was to evaluate the performance of volume scan tube current modulation (VS-ATCM) with adaptive iterative dose reduction 3D (AIDR3D) technique in abdomen CT examinations. We scanned an elliptical cone-shaped phantom utilizing AIDR3D technique combined with VS-ATCM mode in a 320-detector row CT scanner. The image noise distributions with conventional filtered back-projction (FBP) technique and those with AIDR3D technique were compared. The radiation dose profile and tube current time product (mAs) in three noise levels of VS-ATCM modes were compared. The radiation beam profiles of five preset scan lengths were measured using Gafchromic film strips to assess the effects of overbeaming and everlapping. The results indicated that the image noises with AIDR3D technique was 13–74% lower than those in FBP technique. The mAs distributions can be a prediction for various abdominal sizes when undergoing a VS-ATCM mode scan. Patients can receive the radiation dose of overbeaming and overlapping during the VS-ATCM mode scans.

Human Radiation Dose Estimation of 11C-CFT Using Whole-Body PET

C-Labeled 2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane (C-CFT) is a commonly used positron emission tomography (PET) tracer for dopamine transporters imaging. The present study estimated human radiation absorbed doses of C-CFT based on whole-body PET imaging in healthy subjects. Whole-body PET was performed on 6 subjects after injection of 472.06 ± 116.47 MBq of C-CFT. 7 Frames were acquired for about 70 min in 7 segments of the body. Regions of interest were drawn on PET images of source organs. Residence time was calculated as the area under the time-activity curve. Radiation dosimetry was calculated from organ residence time using the medical internal radiation dosimetry (MIDR) method. The organs with the highest radiation-absorbed doses were the urinary bladder, followed the spleen, pancreas, kidneys, and stomach. The dose-limiting critical organ was the urinary bladder. The effective dose was 8.89E-03 mSv/MBq (22.9 mrem/mCi). Biexponential fitting of mean bladder activity demonstrated that 18% of activity was excreted via the urine. The potential radiation risks of C-CFT associated with in this study are well within accepted limits. C-CFT demonstrates a favorable radiation dose profile in humans and allows multiple PET examinations on the same subject per year.

Technical Note: Precision and accuracy of a commercially available CT optically stimulated luminescent dosimetry system for the measurement of CT dose index

To determine the precision and accuracy of CTDI(100) measurements made using commercially available optically stimulated luminescent (OSL) dosimeters (Landaur, Inc.) as beam width, tube potential, and attenuating material were varied. One hundred forty OSL dosimeters were individually exposed to a single axial CT scan, either in air, a 16-cm (head), or 32-cm (body) CTDI phantom at both center and peripheral positions. Scans were performed using nominal total beam widths of 3.6, 6, 19.2, and 28.8 mm at 120 kV and 28.8 mm at 80 kV. Five measurements were made for each of 28 parameter combinations. Measurements were made under the same conditions using a 100-mm long CTDI ion chamber. Exposed OSL dosimeters were returned to the manufacturer, who reported dose to air (in mGy) as a function of distance along the probe, integrated dose, and CTDI(100). The mean precision averaged over 28 datasets containing five measurements each was 1.4% ± 0.6%, range = 0.6%-2.7% for OSL and 0.08% ± 0.06%, range = 0.02%-0.3% for ion chamber. The root mean square (RMS) percent differences between OSL and ion chamber CTDI(100) values were 13.8%, 6.4%, and 8.7% for in-air, head, and body measurements, respectively, with an overall RMS percent difference of 10.1%. OSL underestimated CTDI(100) relative to the ion chamber 21∕28 times (75%). After manual correction of the 80 kV measurements, the RMS percent differences between OSL and ion chamber measurements were 9.9% and 10.0% for 80 and 120 kV, respectively. Measurements of CTDI(100) with commercially available CT OSL dosimeters had a percent standard deviation of 1.4%. After energy-dependent correction factors were applied, the RMS percent difference in the measured CTDI(100) values was about 10%, with a tendency of OSL to underestimate CTDI relative to the ion chamber. Unlike ion chamber methods, however, OSL dosimeters allow measurement of the radiation dose profile.

Dedicated Cone-beam Breast Computed Tomography and Diagnostic Mammography: Comparison of Radiation Dose, Patient Comfort, And Qualitative Review of Imaging Findings in BI-RADS 4 and 5 Lesions.

Objective:This pilot study was undertaken to compare radiation dose, relative visibility/conspicuity of biopsy-proven lesions, and relative patient comfort in diagnostic mammography and dedicated cone-beam breast computed tomography (CBBCT) in Breast Imaging-Reporting and Data System (BI-RADS)® 4 or 5 lesions.Materials and Methods:Thirty-six consecutive patients (37 breasts) with abnormal mammographic and/or ultrasound categorized as BI-RADS® 4 or 5 lesions were evaluated with CBBCT prior to biopsy. Administered radiation dose was calculated for each modality. Mammograms and CBBCT images were compared side-by-side and lesion visibility/conspicuity was qualitatively scored. Histopathology of lesions was reviewed. Patients were administered a survey for qualitative evaluation of comfort between the two modalities.Results:CBBCT dose was similar to or less than diagnostic mammography, with a mean dose of 9.4 mGy (±3.1 SD) for CBBCT vs. 16.9 mGy (±6.9 SD) for diagnostic mammography in a total of 37 imaged breasts (P<0.001). Thirty-three of 34 mammographic lesions were scored as equally or better visualized in CBBCT relative to diagnostic mammography. Characterization of high-risk lesions was excellent. Patients reported greater comfort in CBBCT imaging relative to mammography.Conclusion:Our experience of side-by-side comparison of CBBCT and diagnostic mammography in BI-RADS® 4 and 5 breast lesions demonstrated a high degree of correlation between the two modalities across a variety of lesion types. Owing to favorable radiation dose profile, excellent visualization of lesions, and qualitative benefits including improved patient comfort, excellent field-of-view, and more anatomical evaluation of lesion margins, CBBCT offers a promising modality for diagnostic evaluation of breast lesions.

SU‐E‐I‐81: Radiation Dose for a 320 Slice Cone Beam CT: Methods and Analysis

Purpose: The new 320 slice CT scanners can irradiate x‐ray beam widths from 2.0 mm to 160 mm length in one rotation, and they present challenges for the measurement and analysis of radiation doses. Various methods to perform and analyze the radiation dose data were examined. Methods: These CT scanners have various modes of operation such as: 320 detector axial volume mode, 64 detector axial and helical scan, and scan mode with 4 narrow axial slices ranging from 4*0.5 mm to 4*8 mm. There are many selectable scan parameters such as: four kVp settings (80, 100, 120 and 135), three x‐ray beam filters, 50 to 550 mA, several scan time settings of 0.35 to 3.0 seconds, various pitch values and automatic mA modulation. These options complicate the radiation dose measurement and analysis procedures. Longer CT dose phantoms were evaluated. Several ionization chambers (100 mm pencil chamber and 0.6 cc thimble) and OSL dosimeters were utilized to perform the measurements and evaluated. Results: The radiation dose profiles were measured, and the narrowest beam width resulted in the largest radiation doses. In the 320 slice volume mode, the utilization of 3 radiation dose phantoms stacked together only changes the measured radiation dose by less than 10%. The large amount of data was reduced to a few simple equations and graphs that could be used to estimate the CT radiation dose for any of the clinical procedures. Both thimble ionization and OSL radiation detectors provide useful contributions in CT dosimetry. Conclusions: Limitations and approaches to CT radiation dosimetry for large cone beam CT scanners are reviewed and guidance is provided. Current dosimetry methods can be employed without significant errors.

SU‐E‐I‐52: Use of Radiochromic Film for the Estimation of CTDI100 and In‐ Phantom Dose Profiling

Purpose: To establish a methodology for the use of radiochromic film in the estimation of CTDI100 and provide an in‐phantom spatial description of the radiation dose profile. Methods: Standard CTDI 32cm body and 16cm head phantoms were used with a modified Acrylic pin to allow for the insertion of radiochromic. Two CT scanners were evaluated at various beam collimations in both center and 12 oˈclock pin locations. For comparison purpose, CTDI 100 measurements were obtained using a 100mm pencil ionization chamber. A 48‐bit optical scanner was used to scan the radiochromic film. Software code was written to analyze the film images. A set of calibration films was acquired on a fluoroscope with similar beam quality (kVp and HVL) as the CT scanner. Results: For reproducibility three films from each of 21 different collimation and pin location combinations were analyzed. Three films were excluded because of prominent moiré patterns observed likely caused by moisture on the film or scanner platform. Three images that were included slightly exceeded the calibration range of the film. The percent differences between the CTDI 100 values obtained with the 100mm pencil chamber and the film ranged from 0– 17.2% with an average difference of 8.6%. The relative standard deviation of the CTDI100 film values ranged between 0.3–3.6% with an average of 1.9%. Though not novel, visual indication of the z axis in phantom beam profile provided very clear evidence that at large collimations radiation dose extends well beyond a 100mm region. Conclusions: Radiochromic film has the potential to estimate CTDI 100 with reasonable accuracy; future calibration methods may bring better accuracy. This method provides additional value by yielding a visual indication of the CT dose profile and potential for easily incorporating dose beyond 100mm.

CT Dose Index and Patient Dose: They AreNotthe Same Thing

Estimates of individual patient risk, and epidemiologic studies assessing potential late effects, must use patient size–specific dose estimates—they cannot use only scanner output (volume CT dose index or dose-length product).

SU-GG-I-63: Radiation Dose of Patient Receiving CTA and CTP Studies of Brain

Purpose: As radiation injury to patients undergoing CT perfusion study of brain was reported in the media, the radiation dose delivered to our own patients receiving the same procedures was being questioned by attending radiologists, physicians and surgeons. The purpose of this paper is to investigate the radiation dose delivered to the stroke patients receiving CTangiography and CT perfusion study of brain at our institution. Materials and Methods: The Landauer CTdosimeter utilizing aluminum oxide, optically stimulated luminescence(OSL) technology, was employed as the radiation detector to measure the radiation dose profile in the standard CTDI head phantom simulating the patient. We performed a CT perfusion acquisition using our clinical protocol (fixed tube current mode, no mA‐modulation). Radiation dose was measured at the 12 O'clock and in the central position to simulate the retina or brain. Measurements were performed for those three component scans in the stroke study; (a) a non‐contrast scan of the brain, (b) a perfusion scan with contrast (the shuttle mode), and (c) a CTangiography (CTA) scan. Results: The total dose for the skin exposure (at 12 O'clock), and the retina/brain (in the center) were 85 mGy and 70 mGy for scan series (a), and 300 mGy and 230 mGy for scan series (b), and 60 mGy and 60 mGy for scan series (c), respectively. A total skindose of 445 mGy and braindose of 360 mGy have been obtained.Conclusion: The results show it is quite possible to reach the erythema dose of 2000 mGy if the same patient had gone through more than 4 times the same procedure. While we were assured that the CTA/CTP procedure at this institution is less than 500 mGy/procedure, it is also prudent to exercise precautionary measures to minimize the radiation as a ongoing practice.

Pseudohelical Scan for the Dose Profile Measurements of 160-mm-Wide Cone-Beam MDCT

Our objective was to determine what the reasonable total phantom length should be for the measurements and determination of length equilibrium, specifically for the Aquilion ONE cone-beam MDCT system. Radiation dose measurements of a 160-mm-wide cone-beam-MDCT scanner and its radiation dose profile require a different approach than the traditional or conventional method using thermoluminescent dosimeters or small ionization chambers, which have been suggested by some investigators. In order to obtain the radiation dose profile of a cone-beam MDCT, two key elements must be addressed: proper instrumentation for the detection of radiation beam and inclusion of the tails of the radiation dose profile. In this study, a small (2 x 2 x 0.3 mm) solid-state detector was used to measure the dose profile, which required the introduction of a stepping motor to pull the detector through the phantom. Inclusion of the tails of the radiation dose profile meant more than one standard CT dose index phantom would be required to encompass the dose profile tails as much as practically possible. In fact, at minimum, a total of five standard CT dose index (CTDI) phantoms would be required to ensure the entire dose profile is included and detected. In the case of Toshiba Aquilion ONE MDCT with the maximum beam width of 160 mm, the phantom length that is required for the radiation dose profile measurement should be at least 750 mm, or 5 standard CTDI body phantoms. Current CTDI measurements utilizing 150 mm or 350 mm phantom lengths significantly underestimate the total dose of wide cone-beam MDCT. The measurement method outlined in this study amounts to an introduction of a new CT dose profile measurement using a pseudohelical scan.

PET-Based Biodistribution and Radiation Dosimetry of Epidermal Growth Factor Receptor–Selective Tracer 11C-PD153035 in Humans

The present study estimated the biodistribution and radiation-absorbed dose of epidermal growth factor receptor (EGFR) radioligand 11C-PD153035 in whole-body PET examinations of healthy volunteers. Two-dimensional whole-body PET was performed on 9 subjects after injection of 11C-PD153035 at 329.3+/-77.8 MBq (mean+/-SD). A total of 12 frames were acquired for approximately 90 min in 7 segments of the body. Regions of interest were drawn on PET images of source organs. Residence time was calculated as the area under the time-activity curve. Radiation dosimetry was calculated from organ residence time by use of MIRDOSE3 software. The renal and hepatobiliary systems played important roles in 11C-PD153035 excretion from the body, accounting for the excretion of approximately 23% and 19% of the injected radioactivity, respectively. Blood-pool activity was only moderate and declined over time. Tracer accumulation in the lungs, bone marrow, and muscles was slight, resulting in low background activity in the chest. The organs with the highest radiation-absorbed doses were the urinary bladder and the gallbladder; the effective doses were 6.08E-02+/-1.85E-02 and 2.40E-02+/-8.01E-03 mGy/MBq, respectively. The effective dose equivalent was 7.43E-03+/-1.10E-03 mSv/MBq, and the dose-limiting organ was the urinary bladder. On the basis of the estimated absorbed dose, 11C-PD153035 displayed a favorable radiation dose profile in humans and therefore could be used in multiple PET examinations of the same subject per year. 11C-PD153035 is a promising ligand for the investigation of EGFR in humans, especially in chest tumors such as non-small cell lung cancer.

CT DOSE: HOW TO MEASURE, HOW TO REDUCE

The fundamental radiation dose parameter in computed tomography (CT) is the CT dose index (CTDI), which is an integral under the radiation dose profile of a single axial scan normalized to the nominal x-ray beam width. It estimates the average dose from a multiple-scan examination and is a directly measurable and standardized quantity. From this information, the dose length product (DLP) is calculated, which estimates the total dose delivered over a specific scan length. Finally, effective dose can be estimated and used to reflect the risk of a non-uniform exposure in terms of a whole-body exposure. To manage dose from CT while maintaining diagnostic image quality, scanner manufacturers have implemented tube current modulation, which may occur angularly around the patient, along the long axis of the patient, or both. Dose reductions of 20 to 50% have been reported using tube current modulation schemes. In the past two decades, the capabilities of CT imaging have expanded tremendously, including narrower image widths, improved temporal and spatial resolution, shorter scan times, and cardiac imaging techniques. Yet, the dose per typical exam (e.g., routine abdominal CT) has decreased by a factor of two or more over the same time period. Therefore, patients should be reassured that the benefits of medically-justified and appropriately-performed CT examinations are associated with radiation doses that continue to decrease as technology continues to evolve.

The trouble with

The computed tomography dose index is typically measured using a long pencil ion chamber with cylindrical polymethyl methacrylate (PMMA) dosimetry phantoms. While this metric was useful in the era of single slice CT scanners with collimated slice thicknesses of or less, the efficiency of this metric in multi‐slice CT scanners with wide collimated x‐ray beams is unknown. Monte Carlo simulations were used to assess the efficiency of the parameter for wider beam collimations. The simulations utilized the geometry of a commercially available CT scanner, with modeled polyenergetic x‐ray spectra. Dose spread functions (DSFs) were computed along the length of diam rods placed at several radii in infinitely long diam (head) and diam (body) PMMA phantoms. The DSFs were used to compute radiation dose profiles for slice thicknesses from . efficiency was defined as the fraction of the dose along a PMMA rod collected in a length centered on the CT slice position, divided by the total dose deposited along an infinitely long PMMA rod. For a slice thickness, a x‐ray spectrum, and the PMMA head phantom, the efficiency of the was 82% and 90% for the center and peripheral holes, respectively. The corresponding efficiency values for the body phantom were 63% and 88%. These values are reduced by only 1% when a slice thickness was studied, so the use of for wide x‐ray beams is no less valid than its use for beam widths. However, these data illustrate that the efficiency of the measurement even with beam widths is low and, consequently, dose computations which are derived from this metric may not be as accurate as desirable.

A geomagnetic cutoff rigidity interpolation tool: Accuracy verification and application to space weather

We have developed a fast and accurate geomagnetic vertical cutoff rigidity interpolation tool that has general space weather applications. The interpolation tool utilizes vertical cutoff rigidity values determined from 5° by 5° world grids of geomagnetic cutoff rigidities obtained by tracing cosmic ray trajectories through the Tsyganenko magnetospheric field model combined with the International Geomagnetic Reference Field. The world grids for each UT hour were calculated for a 450-km satellite altitude for all magnetic conditions parameterized by the integer magnetic Kp index ranging from very quiet to extremely disturbed. The interpolation tool extrapolates from the grid positions at 450 km to any other position or altitude. We demonstrate the accuracy of vertical geomagnetic cutoff values obtained from this interpolation tool by reproducing the orbital radiation dose profile from solar protons recorded during the STS-28 space shuttle mission. We also predict the cutoff latitudes of the 8–15 MeV per nucleon α particle flux and compare these values with those determined by the SAMPEX (Solar Anomalous and Magnetospheric Particle Explorer) spacecraft observations for six independent solar particle events under a variety of geomagnetic conditions. In general our predicted cutoff latitudes are within about one degree of the observed cutoff latitudes for low levels of geomagnetic activity.

Measurement of CT scanner dose profiles in a filmless department

The measurement of the FWHM of the slice thickness radiation dose profile of a CT scanner using a prototype low sensitivity CR imaging plate has been investigated, as an alternative to the traditional method using envelope-packed industrial film. Using a standard Agfa clinical CR system to acquire the image, the FWHM of the dose profile can be accurately measured using readily available Public Domain software. An Agfa 18 x 24 cm CR cassette gives a pixel pitch of 113.5 microm, but with interpolation, the measurement accuracy can be less than 1 pixel. For a nominal 10 mm collimation, 15 successive measurements of the FWHM using CR gave an average width of 10.00 mm with a standard deviation of 0.02 mm. This may be compared with 4 successive measurements using film and a dual exposure technique to define the optical density at half peak height, yielding an average width of 9.98 mm with a SD of 0.03 mm. This prototype NDT plate is not a commercial product, but a radiotherapy plate with a similar sensitivity is available commercially and should give similar results.

The calibration of experimental self‐developing Gafchromic® HXR film for the measurement of radiation dose in computed tomography

A prototype, self-developing Gafchromic HXR film has sensitivity an order of magnitude larger than that of the commercially available Gafchromic XR film used in interventional radiological applications. The higher sensitivity of the HXR film allows the possibility of acquisition of high-resolution calibrated dose profiles within the diagnostic range of exposure levels, below 10 R (87.7 mGy). We employed a commercially available, optical flatbed scanner for digitization of the film and image analysis software to determine the response of the HXR films to ionizing radiation. Spatial uniformity and temporal repeatability of the flatbed scanner were determined and used in optimization of the digitization protocol. The HXR film postexposure density growth and sensitivity to ambient light were determined using multiple scans of two simultaneously exposed sheets, one stored in light-tight conditions and the other continuously illuminated with white light. A calibrated step wedge of the HXR film was obtained by simultaneous irradiation of a portion of a film strip and a calibrated ionization chamber using a radiographic x-ray tube with beam characteristics matched to a typical CT scanner (8 mm Al HVL, 120 kVp). Repeated digitization of the calibration film was used to determine the precision of the film response measurements. The precision, as measured by the standard deviation of multiple measurements, was better than 1% over the full dynamic range of film response. This precision was measured using exposures ranging from 0.5 to 12 R (4.4 to 105.3 mGy). This exposure range is highly relevant to x-ray computed tomography. Preliminary radiation dose profiles demonstrate the utility of this technique.