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PurposeThe purpose of this work was twofold: (a) To provide a robust and accurate method for image segmentation of dedicated breast CT (bCT) volume data sets, and (b) to improve Hounsfield unit (HU) accuracy in bCT by means of a postprocessing method that uses the segmented images to correct for the low-frequency shading artifacts in reconstructed images.MethodsA sequential and iterative application of image segmentation and low-order polynomial fitting to bCT volume data sets was used in the interleaved correction (IC) method. Image segmentation was performed through a deep convolutional neural network (CNN) with a modified U-Net architecture. A total of 45 621 coronal bCT images from 111 patient volume data sets were segmented (using a previously published segmentation algorithm) and used for neural network training, validation, and testing. All patient data sets were selected from scans performed on four different prototype breast CT systems. The adipose voxels for each patient volume data set, segmented using the proposed CNN, were then fit to a three-dimensional low-order polynomial. The polynomial fit was subsequently used to correct for the shading artifacts introduced by scatter and beam hardening in a method termed "flat fielding." An interleaved utilization of image segmentation and flat fielding was repeated until a convergence criterion was satisfied. Mathematical and physical phantom studies were conducted to evaluate the dependence of the proposed algorithm on breast size and the distribution of fibroglandular tissue. In addition, a subset of patient scans (not used in the CNN training, testing or validation) were used to investigate the accuracy of the IC method across different scanner designs and beam qualities.ResultsThe IC method resulted in an accurate classification of different tissue types with an average Dice similarity coefficient > 95%, precision > 97%, recall > 95%, and F1-score > 96% across all tissue types. The flat fielding correction of bCT images resulted in a significant reduction in either cupping or capping artifacts in both mathematical and physical phantom studies as measured by the integral nonuniformity metric with an average reduction of 71% for cupping and 30% for capping across different phantom sizes, and the Uniformity Index with an average reduction of 53% for cupping and 34% for capping.ConclusionThe validation studies demonstrated that the IC method improves Hounsfield Units (HU) accuracy and effectively corrects for shading artifacts caused by scatter contamination and beam hardening. The postprocessing approach described herein is relevant to the broad scope of bCT devices and does not require any modification in hardware or existing scan protocols. The trained CNN parameters and network architecture are available for interested users.

PurposeDedicated breast computed tomography (bCT) technology for the purpose of breast cancer screening has been a focus of research at UC Davis since the late 1990s. Previous studies have shown that improvement in spatial resolution characteristics of this modality correlates with greater microcalcification detection, a factor considered a potential limitation of bCT. The aim of this study is to improve spatial resolution as characterized by the modulation transfer function (MTF) via changes in the scanner hardware components and operational schema.MethodsFour prototypes of pendant-geometry, cone-beam breast CT scanners were designed and developed spanning three generations of design evolution. To improve the system MTF in each bCT generation, modifications were made to the imaging components (x-ray tube and flat-panel detector), system geometry (source-to-isocenter and detector distance), and image acquisition parameters (technique factors, number of projections, system synchronization scheme, and gantry rotational speed).ResultsCharacterization of different generations of bCT systems shows these modifications resulted in a 188% improvement of the limiting MTF properties from the first to second generation and an additional 110% from the second to third. The intrinsic resolution degradation in the azimuthal direction observed in the first generation was corrected by changing the acquisition from continuous to pulsed x-ray acquisition. Utilizing a high resolution detector in the third generation, along with modifications made in system geometry and scan protocol, resulted in a 125% improvement in limiting resolution. An additional 39% improvement was obtained by changing the detector binning mode from 2 × 2 to 1 × 1.ConclusionsThese results underscore the advancement in spatial resolution characteristics of breast CT technology. The combined use of a pulsed x-ray system, higher resolution flat-panel detector and changing the scanner geometry and image acquisition logic resulted in a significant fourfold improvement in MTF.

Background: Mobile C‐arms may be used in fixed locations, and it is recommended that qualified experts evaluate structural shielding. Purpose: To assess clinical workload distributions for mobile C‐arms used in gastrointestinal endoscopy and determine the Archer equation parameters for the C‐arm beam spectra. Methods: Consecutive (30 months) gastrointestinal endoscopic procedures on two Cios Alpha systems (Siemens) were retrospectively analyzed. X‐ray tube voltage, tube current‐time product, reference point air kerma (Ka,r), air kerma‐area product (PKA), and fluoroscopic time were examined. The primary beam half‐value layer (HVL) was measured with an ionization chamber and aluminum 1100 plates. Stray radiation fraction at 1 m from a scattering source (ACR R/F phantom) was directly measured. Monte Carlo (Geant4) simulation was performed to calculate the transmission of broad X‐ray beams through lead, concrete, gypsum, and steel, with X‐ray HVLs matching those of the C‐arm X‐ray beam. The transmission data were fitted to the Archer equation. Results: The number of procedures (3509) was equivalent to 13.48 procedures per room per week. Dose quantities were 54.8 mGy (Ka,r), 18.3 Gy∙cm2 (PKA), and 7.8 min (fluoroscopic time) per procedure. X‐ray beam irradiation events were recorded for 2906 (82.8%) procedures with 160,009 events, whose mA‐minute weighted tube voltage was 91.0 kV and the workload was 0.68 mA‐minute per procedure. The two rooms had a significant difference in the number of procedures per week, 17.3 (29) [mean (maximum)] and 9.6 (16), respectively. The stray radiation fraction was 9.7×10−4 (80 kV) and 1.25×10−3 (120 kV). Transmission fitting parameters were provided for the tube voltage (on average, 90 kV; high end, 120 kV) of the C‐arm. Conclusions: This work provides workload and transmission data for mobile C‐arm fluoroscopy in gastrointestinal endoscopy, which indicates a need for structural shielding evaluation of the procedure rooms. [ABSTRACT FROM AUTHOR]

Background: It is challenging to assess the accuracy of volume CT Dose Index (CTDIvol) when the axial scan modes corresponding to a helical scan protocol are not available. An alternative approach was proposed to directly measure CTDIvolH$CTDI_{vol}^H$ using helical acquisitions and relatively small differences (< 20%) from CTDIvol were observed. Purpose: To visually demonstrate the 3D dose distribution for both axial and helical CT acquisitions and quantitively compare CTDIvolH$CTDI_{vol}^H$ and CTDIvol. Methods: 3D dose distribution within the standard CTDI phantoms (16 and 32 cm diameter) from a single CT projection, Dp(x,y,z) was first generated using Monte Carlo simulation (GEANT4) with 9×108 photons per combination of tube voltage (80–140 kV), collimation width (1–8 cm), and z‐axis location of the central ray of the x‐ray beam, with a spatial resolution of 1 mm3. These dose distributions from one single projection were analytically ensembled to simulate 3D dose volumes DA(x,y,z) and DH(x,y,z) for axial and helical scans, respectively, with different helical pitches (0.3–2) and scan lengths (100–150 mm). 2D planar dose distributions were obtained by integrating the inside 100 mm of the dose volumes. CTDIvol and CTDIvolH$CTDI_{vol}^H\;$were calculated using the planar dose data at corresponding pencil chamber locations and the percentage differences (PD) were reported. Results: High spatial resolution 3D CT dose volumes were generated and visualized. PDs between CTDIvolH$CTDI_{vol}^H$ and CTDIvol had strong dependency on scan length and peripheral chamber locations, with subtle dependency on collimation width and pitch. PDs were mostly within the range of ± 3% for a scan length of 150 mm with four peripheral chamber locations. Conclusions: With a scan length covering the entire phantom length, CTDIvolH$CTDI_{vol}^H$ directly measured from helical scans can serve as an alternative to CTDIvol only if all four peripheral locations were measured. [ABSTRACT FROM AUTHOR]

Purpose: American Association of Physicists in Medicine (AAPM) (Report 204) introduced the size‐specific dose estimate (SSDE) for the average dose to the center of a fixed‐mA scan. International standards establish a method that CT manufacturers and radiation dose index monitoring software may use to calculate SSDE(z) at longitudinal positions of scans with fixed mA or tube current modulation, and its scan range average SSDE(z)¯$\overline {{\rm{SSDE}}(z)} $. We sought to test how accurate SSDE(z) is in tracking the average dose to the transverse slab of an axial image slice (Dslice), evaluated with Monte Carlo calculation, in the chest and abdominopelvic examinations. Methods: We retrospectively identified 65 consecutive adult patients undergoing whole‐body CT for transcatheter aortic valve implantation planning. Examination parameters (kV, mA, CTDIvol) were extracted from the DICOM headers. Patient water equivalent diameter DW(z) was calculated at each image slice, excluding the patient table. A previously validated Monte Carlo simulation (Geant4) program was used to evaluate Dslice from the chest and abdominopelvic examinations. Alternatively, SSDE(z) was calculated at each slice. The results of the two methods were compared with descriptive statistical outcomes (R, version 4.0.2). Results: In chest and abdominopelvic CT examinations, Dslice largely changed with anatomic location and uniformly fell off toward scan range edges. Scan range averages SSDE(z)¯$\overline {{\rm{SSDE}}(z)} $ and Dslice¯$\overline {{D_{{\rm{slice}}}}} \;$were consistent within 2.5%–3.1% (median) and 6.3%–10.4% (maximum) in two examinations. On individual image slices, SSDE(z) could be lower or higher than Dslice, with deviation ranging from −18.3% to 85% in two edges (2 × 5 cm) of scan range and from −35.2% to 18.7% in the remaining central region of the scan. Conclusion: This study provides critical inputs for quality assurance programs. SSDE(z)¯$\overline {{\rm{SSDE}}(z)} $ is useful to track the average dose of all image slices, but further development may be useful for tracking patient dose at the level of individual image slices, especially near a scan range edge. [ABSTRACT FROM AUTHOR]

Background Purpose Methods Results Conclusion Radiography is one of the most widely used x‐ray imaging modalities. In National Council on Radiation Protection and Measurements (NCRP) Report No. 147, transmission data for radiographic systems were evaluated on those installed before 2000. For x‐ray systems (except intraoral dental) manufactured on or after June 10, 2006, the U.S. required minimum half‐value layer (HVL) was increased; for example, 2.9 (2.3) mm Al at 80 kV, where the value in parenthesis denotes the earlier requirement before the above date.To calculate the transmission of the broad x‐ray beam of modern digital radiography (DR) through shielding materials.X‐ray beam HVLs on two DR systems (Agfa DR 600, GE Revolution XR/d) were measured in 10 kV increments between 60 and 120 kV, with a calibrated ionization chamber (Radcal model 10 × 5‐60) and varying thickness of aluminum 1100 plates. Monte Carlo (Geant4) simulation was performed to calculate the transmission of broad x‐ray beams through lead, concrete, gypsum, and steel, with x‐ray HVLs matching those of the DR 600 at two beam filtrations (default, 1 mm Al plus 0.1 mm Cu added filtration). The transmission data were fitted to the Archer equation.HVLs on two DR systems with default beam filtration were consistent (median difference, 2.1%; maximum difference, 5.5%). An additional beam filtration option (1 mm Al plus 0.1 mm Cu) on the DR 600 substantially increased HVLs by 45.2%–61.2%. Transmission fitting parameters were provided for seven tube voltages (60–120 kV) at two beam filtrations.This work presents transmission data for modern DR systems, indicating increased x‐ray beam filtration compared to the primary x‐ray beam in NCRP Report No. 147. The updated transmission data can enhance structural shielding evaluations at individual tube voltages or with a workload distribution. [ABSTRACT FROM AUTHOR]

Purpose: In multidetector CT, the volume CT dose index (CTDIvol) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scannerindependent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length. Methods: A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDIL(water)/CTDIw (PMMA), where L was dose integration length. The ratio was equal to the midpoint dose DL(0) (water) to CTDIvol ratio in a CT scan series with a scan length equal to L. Results: For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of DL(0)/CTDIvol from that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDIvol to SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, DL(0)/CTDIvol decreased considerably as L decreased. DL(0)/CTDIvol was shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, DL(0)/CTDIvol decreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm. Conclusions: The midpoint dose to CTDIvol ratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the feasibility of planning and delivering pulsed low dose radiotherapy (PLRT) using volumetric modulated arc therapy (VMAT) on Elektalinacs. Methods: Ten patients previously treated for glioblastomamultiforme (GBM) were replanned using PLRT VMAT to deliver ten 0.2 Gy pulses separated by 3 min intervals with an effective dose rate of 0.067 Gy/min. VMAT parameters such as leaf speed and arc length were investigated to deliver 2 Gy/fraction to a total of 60 Gy to the target volume in ten subfractions or pulses. Plan quality was assessed using conformity and homogeneity indices. Absolute dose distribution for individual pulses was measured using ArcCHECK diode array. Individual pulses were analyzed for reproducibility and stability using machine log files. Machine characteristics at low monitor units and low dose rate were also investigated. Results: An optimal arc length of 140°-160° and a leaf speed of 0.18-0.25 cm/° were sufficient to provide equivalent plan coverage and stable delivery. The average time and dose rate required to deliver a single 0.2 Gy pulse was 39.5 ± 2.3 s and 49 ± 32.3 cGy/min. Average reductions in MUs for the VMAT PLRT plan compared to IMRT for PTV was 16% (Range: -5.5%-36.1%) and 10.9% (Range: -18.4%-32.3%) for the initial and boost plan. A significant improvement was seen in maximum doses to all sensitive structures when planned with VMAT PLRT. The average absolute dose gamma passing rate for the 10 pulses combined and 2 Gy plan were 91.6 ± 2.5% and 97.3 ± 1.2%, respectively. Cumulative monitor units, dose rate, gantry angles, and leaf positions evaluated using machine log files were within 2% for all pulses. Conclusions: Elekta linacs are capable of delivering reproducible and stable PLRT plans. A prospective clinical study employing PLRT is currently in development. [ABSTRACT FROM AUTHOR]

Background: Dose evaluation at longitudinal positions of body computed tomography (CT) scans is useful for CT quality assurance programs and patient organ dose evaluation. Accurate estimates depend on both patient size and scan length., Purpose: To propose practical evaluation of the average dose to the transverse slab of an axial image slice for adult body CT examinations, considering not only patient size but also scan length, and to compare the results with those of Monte Carlo (Geant4) simulation [D sim (z)] and size-specific dose estimates at longitudinal positions of scans [SSDE(z)] from international standards (IEC publication no. 62985)., Methods: In a scan series, the total dose at each z-axis location was calculated using the input information identical to the SSDE(z) evaluation. Each axial image slice (slice thickness, 2.5 or 5 mm) was first considered independently. Its z-axis coverage and CTDI vol (from the DICOM headers) were used to directly calculate a z-axis dose profile for the average dose over the cross-section of a water phantom, using the approach to equilibrium function. The phantom diameter was taken to be equal to the patient water equivalent diameter at that slice. The above was repeated at all slices and the dose at each z-axis location was accumulated from all profiles, referred to as D calc (z). For validation, we considered a cohort of 65 patients, who underwent chest and abdominopelvic examinations. The resultant D calc (z) was compared with D sim (z) and SSDE(z), both available in a previous paper., Results: D calc (z) evaluation could be used to accurately assess the scan range average dose, with an accuracy of 7.1%-8.7% for 65 patients in two examinations. On individual image slices, the maximum difference in magnitude between D calc (z) and D sim (z) [and between SSDE(z) and D sim (z) in parentheses] was 37.5% (85%) [two edges (2 × 5 cm) of chest scan range], 17.8% (35.2%) (the remaining central region of chest scan), 26.8% (74.1%) [two edges (2 × 5 cm) of abdominopelvic scan range], and 14.2% (22.5%) (the remaining central region of abdominopelvic scan)., Conclusions: Identical input data are used for D calc (z) and SSDE(z) evaluations. The latter is limited to the z-axis locations within scan range. At each image slice, SSDE(z) is equivalent to the midpoint dose of a fixed-mA scan of 15-30 cm (scan length). In contrast, D calc (z) considers dose accumulation from varying scan length (from sub-centimeter to about 1 m) and tube current, and dose profile is also computed outside scan range. Besides greatly improving dose evaluation for individual image slices, D calc (z) allows for evaluating dose accumulation from multiple series, which typically span different scan ranges. Our proposal may assist CT manufacturers and dose index monitoring software in assessing dose at longitudinal positions of body CT scans., (© 2022 American Association of Physicists in Medicine.)

Purpose: In multidetector CT, the volume CT dose index (CTDI vol ) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scanner-independent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length., Methods: A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDI L (water)/CTDI w (PMMA), where L was dose integration length. The ratio was equal to the midpoint dose D L (0) (water) to CTDI vol ratio in a CT scan series with a scan length equal to L., Results: For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of D L (0)/CTDI vol from that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDI vol to SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, D L (0)/CTDI vol decreased considerably as L decreased. D L (0)/CTDI vol was shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, D L (0)/CTDI vol decreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm., Conclusions: The midpoint dose to CTDI vol ratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT.

The purpose of this study was to examine the spatial resolution of a prototype pendant-geometry cone-beam breast computed tomography (CT) system. Modulation transfer functions (MTFs) of the reconstructed image in the coronal (x and y) plane were computed as a function of the cone angle, the radial distance from the axis of rotation, the size of the reconstruction matrix, the back-projection filter used, and the number of projections acquired for the reconstruction. The results show that the cone angle and size of the reconstruction matrix have minimal impact on the MTF, while the MTF degraded radially from the axis of rotation (from 0.76 at 2.6 mm from axis of rotation down to 0.37 at 76.9 mm from axis of rotation at f=0.5 mm(-1)). The Ramp reconstruction filter increases the MTF near the axis of rotation relative to the Shepp-Logan filter, while an increase in the number of projections from 500 to 1000 increased the MTF near the periphery of the reconstructed image. The MTF in the z direction (anterior-posterior direction) was also evaluated. The z-direction MTF values tend to be higher when compared to the coronal MTF (0.85 at f =0.5 mm(-1)), and tend to be very constant throughout the coronal plane direction. The results suggest that an increase in the MTF for the prototype breast CT system is possible by optimizing various scanning and reconstruction parameters.