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Title: MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT

Abstract

Purpose: Advanced dosimetry in CT (e.g. the Monte Carlo method) requires an accurate characterization of the shaped filter and radiation quality used during a scan. The purpose of this work was to develop a method where half value layer (HVL) profiles along shaped filters could be made. From the HVL profiles the beam shaping properties and effective photon spectrum for a particular scan can be inferred. Methods: A measurement rig was developed to allow determinations of the HVL under a scatter-free narrow-beam geometry and constant focal spot to ionization chamber distance for different fan angles. For each fan angle the HVL is obtained by fitting the transmission of radiation through different thicknesses of an Al absorber (type 1100) using an appropriate model. The effective Al thickness of shaped filters and effective photon spectra are estimated using a model of photon emission from a Tungsten anode. This method is used to obtain the effective photon spectra and effective Al thickness of shaped filters for a CT scanner recently introduced to the market. Results: This study resulted in a set of effective photon spectra (central ray) for each kVp along with effective Al thicknesses of the different shaped filters. The effective photonmore » spectra and effective Al thicknesses of shaped filters were used to obtain numerically approximated HVL profiles and compared to measured HVL profiles (mean absolute percentage error = 0.02). The central axis HVL found in the vendor’s technical documentation were compared to approximated HVL values (mean absolute percentage error = 0.03). Conclusion: This work has resulted in a unique method of measuring HVL profiles along shaped filters in CT. Further the effective photon spectra and the effective Al thicknesses of shaped filters that were obtained can be incorporated into Monte Carlo simulations.« less

Authors:
 [1];  [2];  [3]; ;  [1];  [4]
  1. Karolinska University Hospital, Stockholm (Sweden)
  2. (Sweden)
  3. Sunderby Hospital, Lulea (Sweden)
  4. Umea University, Umea (Sweden)
Publication Date:
OSTI Identifier:
22653897
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; BEAM SHAPING; COMPUTERIZED SIMULATION; IONIZATION CHAMBERS; MONTE CARLO METHOD; PHOTON EMISSION; RADIATION QUALITY; SPECTRA; THICKNESS

Citation Formats

Bujila, R, Royal Institute of Technology, Stockholm, Kull, L, Nowik, P, Poludniowski, G, and Andersson, J. MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT. United States: N. p., 2016. Web. doi:10.1118/1.4957349.
Bujila, R, Royal Institute of Technology, Stockholm, Kull, L, Nowik, P, Poludniowski, G, & Andersson, J. MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT. United States. doi:10.1118/1.4957349.
Bujila, R, Royal Institute of Technology, Stockholm, Kull, L, Nowik, P, Poludniowski, G, and Andersson, J. 2016. "MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT". United States. doi:10.1118/1.4957349.
@article{osti_22653897,
title = {MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT},
author = {Bujila, R and Royal Institute of Technology, Stockholm and Kull, L and Nowik, P and Poludniowski, G and Andersson, J},
abstractNote = {Purpose: Advanced dosimetry in CT (e.g. the Monte Carlo method) requires an accurate characterization of the shaped filter and radiation quality used during a scan. The purpose of this work was to develop a method where half value layer (HVL) profiles along shaped filters could be made. From the HVL profiles the beam shaping properties and effective photon spectrum for a particular scan can be inferred. Methods: A measurement rig was developed to allow determinations of the HVL under a scatter-free narrow-beam geometry and constant focal spot to ionization chamber distance for different fan angles. For each fan angle the HVL is obtained by fitting the transmission of radiation through different thicknesses of an Al absorber (type 1100) using an appropriate model. The effective Al thickness of shaped filters and effective photon spectra are estimated using a model of photon emission from a Tungsten anode. This method is used to obtain the effective photon spectra and effective Al thickness of shaped filters for a CT scanner recently introduced to the market. Results: This study resulted in a set of effective photon spectra (central ray) for each kVp along with effective Al thicknesses of the different shaped filters. The effective photon spectra and effective Al thicknesses of shaped filters were used to obtain numerically approximated HVL profiles and compared to measured HVL profiles (mean absolute percentage error = 0.02). The central axis HVL found in the vendor’s technical documentation were compared to approximated HVL values (mean absolute percentage error = 0.03). Conclusion: This work has resulted in a unique method of measuring HVL profiles along shaped filters in CT. Further the effective photon spectra and the effective Al thicknesses of shaped filters that were obtained can be incorporated into Monte Carlo simulations.},
doi = {10.1118/1.4957349},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: To validate Computed Tomography Fractional Flow Reserve (CT-FFR) measurements with accurate 3D printed coronary phantoms. Methods: DICOM data from four phases in two patients imaged with a standard 320 × 0.5mm coronary CT acquisition (70–80% cardiac cycle) underwent semi-automated segmentation using a research workstation. Both patients had a >50% stenosis from the clinical image interpretation. Each volume was saved as a Stereo Lithographic (STL) file with 250 micron resolution. The 3D geometries were qualitatively assessed; the best of the four phases was 3D printed using a Stratasys Eden260V printer in Tango+, a rubber-like material that roughly emulates mechanical propertiesmore » of human vasculature. We connected the model to a programmable pump and measured the pressure drop using pressure sensors embedded proximal and distal to the arterial stenosis. Next, the STL files used for the 3D printed models were uploaded in the ANSYS meshing tool (ICEM CFD 16.1). A standard meshing process was applied and the meshed geometry was directly imported in the ANSYS Fluent for Computational Flow Dynamics simulations. The CFD simulations were used to calculate the CT-FFR and compared to the bench top FFR measured in the 3D printed phantoms. Results: FFR-CT measurements and phantoms were completed in within an hour after the segmentation. Patient 1 had a 60% stenosis that resulted in a CT-FFR of 0.68. The second case had a 50% stenosis and a CT-FFR of 0.75. The average bench top FFR measurements were 0.72 and 0.80, respectively. Conclusion: This pilot investigation demonstrated the use of a bench-top coronary model for CT-FFR validation. The measurements and the CFD simulations agreed within 6%. Project supported by Support: Toshiba America Medical Systems Corp.and NIH grant R01-EB002873. Project supported by Toshiba America Medical Systems Corp.and partial support from NIH grant R01-EB002873.« less
  • Purpose: To evaluate the accuracy of size-specific dose estimates (SSDE) in CT in the presence of simulated metal prostheses. Methods: Radiation dose in tissue (f-factor = 0.94) was measured at various chamber positions in a conventional nested CTDI phantom with nominal 0.5 inch metal rods inserted to simulate the presence of prosthetic implant(s). An average weighted tissue dose (AWTD) was calculated in a manner similar to CTDIw. Subsequent scans were performed with varying phantom diameter, number of metal rods and type of metal. The scan acquisition parameters were fixed for all such measurements (i.e. CTDIvol was constant). Axial CT imagesmore » reconstructed both with and without a metal artifact reduction algorithm (SEMAR) were used to calculate the water-equivalent diameter (Dw) per AAPM TG Report 220. The Dw values were subsequently used to determine the SSDE from the known CTDIvol. In addition SSDE was also calculated from the effective diameter per AAPM TG Report 204. Accuracy of the calculated SSDE values were assessed by comparing to the AWTD measurements. Results: In the 32-cm diameter phantom the SSDE calculations from Dw (TG-220) were within ±1% of the AWTD measurements regardless of type of metal and number of metal rods while SSDE calculated from effective diameter (TG-204) overestimated the AWTD by 7–10%. In the 16-cm diameter phantom the SSDE calculations from Dw (TG-220) were within ±4% of the AWTD measurement. The Dw calculations used to determine SSDE varied by less than 0.2% between the images reconstructed with and without the metal artifact reduction algorithm. Conclusion: The TG-220 SSDE method can provide an accurate estimation of tissue dose in the presence of metal while TG-204 SSDE method overestimates the dose. The determination of Dw is independent of reconstruction algorithm.« less
  • Purpose: To discover if a previously published methodology for estimating patient-specific organ dose in a pediatric population (5–55kg) is translatable to the adult sized patient population (> 55 kg). Methods: An adult male anthropomorphic phantom was scanned with metal oxide semiconductor field effect transistor (MOSFET) dosimeters placed at 23 organ locations in the chest and abdominopelvic regions to determine absolute organ dose. Organ-dose-to-SSDE correlation factors were developed by dividing individual phantom organ doses by SSDE of the phantom; where SSDE was calculated at the center of the scan volume of the chest and abdomen/pelvis separately. Organ dose correlation factors developedmore » in phantom were multiplied by 28 chest and 22 abdominopelvic patient SSDE values to estimate organ dose. The median patient weight from the CT examinations was 68.9 kg (range 57–87 kg) and median age was 17 years (range 13–28 years). Calculated organ dose estimates were compared to published Monte Carlo simulated patient and phantom results. Results: Organ-dose-to-SSDE correlation was determined for a total of 23 organs in the chest and abdominopelvic regions. For organs fully covered by the scan volume, correlation in the chest (median 1.3; range 1.1–1.5) and abdominopelvic (median 0.9; range 0.7–1.0) was 1.0 ± 10%. For organs that extended beyond the scan volume (i.e. skin bone marrow and bone surface) correlation was determined to be a median of 0.3 (range 0.1–0.4). Calculated patient organ dose using patient SSDE agreed to better than 6% (chest) and 15% (abdominopelvic) to published values. Conclusion: This study demonstrated that our previous published methodology for calculating organ dose using patient-specific SSDE for the chest and abdominopelvic regions is translatable to adult sized patients for organs fully covered by the scan volume.« less
  • Purpose: To combine total variation (TV) and Hessian penalty in a structure adaptive way for cone-beam CT (CBCT) reconstruction. Methods: TV is a widely used first order penalty with good ability in suppressing noise and preserving edges but leads to the staircase effect in regions with smooth intensity transition. The second order Hessian penalty can effectively suppress the staircase effect with extra cost of blurring object edges. To take the best of both penalties we proposed a novel method to combine both for CBCT reconstruction in a structure adaptive way. The proposed method adaptively determined the weight of each penaltymore » according to the geometry of local regions. An specially-designed exponent term with image gradient involved was used to characterize the local geometry such that the weights for Hessian and TV were 1 and 0 respectively at uniform local regions and 0 and 1 at edge regions. For other local regions the weights varied from 0 to 1. The objective functional was minimized using the majorzationminimization approach. We evaluated the proposed method on a modified 3D shepp-logan and a CatPhan 600 phantom. The full-width-at-halfmaximum (FWHM) and contrast-to-noise (CNR) were calculated. Results: For 3D shepp-logan the reconstructed images using TV had an obvious staircase effect while those using the proposed method and Hessian preserved the smooth transition regions well. FWHMs of the proposed method TV and Hessian penalty were 1.75 1.61 and 3.16 respectively, indicating that both TV and the proposed method is able to preserve edges. For CatPhan 600 CNR values of the proposed method were similar to those of TV and Hessian. Conclusion: The proposed method retains favorable properties of TV like preserving edges and also has the ability in better preserving gradual transition structure as Hessian does. All methods performs similarly in suppressing noise. This work was supported in part by National Natural Science Foundation of China (NNSFC) under Grant Nos.60971112 and 61375018 grants from the Cancer Prevention and Research Institute of Texas (RP130109 and RP110562-P2) National Institute of Biomedical Imaging and Bioengineering (R01 EB020366) and a grant from the American Cancer Society (RSG-13-326-01-CCE).« less
  • Purpose: To demonstrate the possibility and quantify the impact of operating a clinical CT scanner at exceptionally high x-ray tube voltage for better penetration through metal objects and facilitating metal artifact reduction. Methods: We categorize metal objects according to the data corruption severeness (level of distortion and complete photon starvation fraction). To demonstrate feasibility and investigate the impact of high voltage scanning we modified a commercial GE LightSpeed VCT scanner (generator and software) to enable CT scans with x-ray tube voltages as high as 175 kVp. A 20 cm diameter water phantom with two metal rods (10 mm stainless andmore » 25 mm titanium) and a water phantom with realistic metal object (spine cage) were used to evaluate the data corruption and image artifacts in the absence of any algorithm correction. We also performed simulations to confirm our understanding of the transmitted photon levels through metal objects with different size and composition. Results: The reconstructed images at 175 kVp still have significant dark shading artifacts, as expected since no special scatter correction or beam hardening was performed but show substantially lower noise and photon starvation than at lower kVp due to better beam penetration. Analysis of the raw data shows that the photon starved data is reduced from over 4% at 140 kVp to below 0.2% at 175 kVp. The simulations indicate that for clinically relevant titanium and stainless objects a 175 kVp tube voltage effectively avoids photon starvation. Conclusion: The use of exceptionally high tube voltage on a clinical CT system is a practical and effective solution to avoid photon starvation caused by certain metal implants. Sparse and hybrid high-voltage protocols are being considered to maintain low patient dose. This opens the door to algorithmic physics-based corrections rather than treating the data as missing and relying on missing data algorithms. Some of the authors are employees of General Electric.« less