Sarah E. McKenney

The characterization of breast anatomical metrics using dedicated breast CT

PURPOSE Accurate anatomical characterization of the breast is useful in breast phantom development and computer modeling of breast imaging technologies. Capitalizing on the three-dimensional capabilities of dedicated breast CT (bCT), a number of parameters which describe breast shape and fibroglandular distribution are defined. METHODS Among 219 bCT data sets, the effective diameter and length of the pendant breast as well as the breast volume were measured and characterized for each bra cup size. The volume glandular fraction (VGF) was determined as a function of patient age, BIRADS density, bra cup size, and breast diameter. The glandular fraction was examined in coronal and sagittal planes of the breast, and the radial distribution of breast glandular fraction within a coronal bCT image was examined for three breast regions. The areal glandular fraction (AGF) was estimated from two-dimensional projections of the breast (simulated by projecting bCT data sets) and was compared to the corresponding VGF. RESULTS The effective breast diameter and length increase with increasing bra cup size. The mean breast diameters (+/- standard error) of bra cup sizes A/AA, B, C, and D/DD were 11.1 +/- 0.5, 11.4 +/- 0.3, 13.0 +/- 0.2, and 13.7 +/- 0.2 cm, respectively. VGF was lower among older women and those with larger breast diameter and larger bra cup size. VGF increased as a function of the reported BIRADS density. AGF increased with VGF. Fibroglandular tissue was distributed primarily in the central portion of the breast. CONCLUSIONS Breast metrics were examined and a number of parameters were defined which may be useful for breast modeling. The reported data may provide researchers with useful information for characterizing the breast for various imaging or dosimetry tasks.

Methods for CT Automatic Exposure Control Protocol Translation between Scanner Platforms

PURPOSE An imaging facility with a diverse fleet of CT scanners faces considerable challenges when propagating CT protocols with consistent image quality and patient dose across scanner makes and models. Although some protocol parameters can comfortably remain constant among scanners (eg, tube voltage, gantry rotation time), the automatic exposure control (AEC) parameter, which selects the overall mA level during tube current modulation, is difficult to match among scanners, especially from different CT manufacturers. METHODS Objective methods for converting tube current modulation protocols among CT scanners were developed. Three CT scanners were investigated, a GE LightSpeed 16 scanner, a GE VCT scanner, and a Siemens Definition AS+ scanner. Translation of the AEC parameters such as noise index and quality reference mAs across CT scanners was specifically investigated. A variable-diameter poly(methyl methacrylate) phantom was imaged on the 3 scanners using a range of AEC parameters for each scanner. The phantom consisted of 5 cylindrical sections with diameters of 13, 16, 20, 25, and 32 cm. The protocol translation scheme was based on matching either the volumetric CT dose index or image noise (in Hounsfield units) between two different CT scanners. A series of analytic fit functions, corresponding to different patient sizes (phantom diameters), were developed from the measured CT data. These functions relate the AEC metric of the reference scanner, the GE LightSpeed 16 in this case, to the AEC metric of a secondary scanner. RESULTS When translating protocols between different models of CT scanners (from the GE LightSpeed 16 reference scanner to the GE VCT system), the translation functions were linear. However, a power-law function was necessary to convert the AEC functions of the GE LightSpeed 16 reference scanner to the Siemens Definition AS+ secondary scanner, because of differences in the AEC functionality designed by these two companies. CONCLUSIONS Protocol translation on the basis of quantitative metrics (volumetric CT dose index or measured image noise) is feasible. Protocol translation has a dependency on patient size, especially between the GE and Siemens systems. Translation schemes that preserve dose levels may not produce identical image quality.

Kilovoltage Rotational External Beam Radiotherapy on a Breast Computed Tomography Platform: A Feasibility Study

PURPOSE To demonstrate the feasibility of a dedicated breast computed tomography (bCT) platform to deliver rotational kilovoltage (kV) external beam radiotherapy (RT) for partial breast irradiation, whole breast irradiation, and dose painting. METHODS AND MATERIALS Rotational kV-external beam RT using the geometry of a prototype bCT platform was evaluated using a Monte Carlo simulator. A point source emitting 178 keV photons (approximating a 320-kVp spectrum with 4-mm copper filtration) was rotated around a 14-cm voxelized polyethylene disk (0.1 cm tall) or cylinder (9 cm tall) to simulate primary and primary plus scattered photon interactions, respectively. Simulations were also performed using voxelized bCT patient images. Beam collimation was varied in the x-y plane (1-14 cm) and in the z-direction (0.1-10 cm). Dose painting for multiple foci, line, and ring distributions was demonstrated using multiple rotations with varying beam collimation. Simulations using the scanners native hardware (120 kVp filtered by 0.2-mm copper) were validated experimentally. RESULTS As the x-y collimator was narrowed, the two-dimensional dose profiles shifted from a cupped profile with a high edge dose to an increasingly peaked central dose distribution with a sharp dose falloff. Using a 1-cm beam, the cylinder edge dose was <7% of the dose deposition at the cylinder center. Simulations using 120-kVp X-rays showed distributions similar to the experimental measurements. A homogeneous dose distribution (<2.5% dose fluctuation) with a 20% decrease in dose deposition at the cylinder edge (i.e., skin sparing) was demonstrated by weighted summation of four dose profiles using different collimation widths. Simulations using patient bCT images demonstrated the potential for treatment planning and image-guided RT. CONCLUSIONS Rotational kV-external beam RT for partial breast irradiation, dose painting, and whole breast irradiation with skin sparing is feasible on a bCT platform with the potential for high-resolution image-guided RT.

SU-F-18C-11: Diameter Dependency of the Radial Dose Distribution in a Long Polyethylene Cylinder

PURPOSE The radial dose distribution in the central plane of a long cylinder following a long CT scan depends upon the diameter and composition of the cylinder. An understanding of this behavior is required for determining the spatial average of the dose in the central plane. Polyethylene, the material for construction of the TG200/ICRU phantom (30 cm in diameter) was used for this study. Size effects are germane to the principles incorporated in size specific dose estimates (SSDE); thus diameter dependency was explored as well. METHOD ssuming a uniform cylinder and cylindrically symmetric conditions of irradiation, the dose distribution can be described using a radial function. This function must be an even function of the radial distance due to the conditions of symmetry. Two effects are accounted for: The direct beam makes its weakest contribution at the center while the contribution due to scatter is strongest at the center and drops off abruptly at the outer radius. An analytic function incorporating these features was fit to Monte Carlo results determined for infinite polyethylene cylinders of various diameters. A further feature of this function is that it is integrable. RESULTS Symmetry and continuity dictate a local extremum at the center which is a minimum for the larger sizes. The competing effects described above can Resultin an absolute maximum occurring between the center and outer edge of the cylinders. For the smallest cylinders, the maximum dose may occur at the center. CONCLUSION An integrable, analytic function can be used to characterize the radial dependency of dose for cylindrical CT phantoms of various sizes. One use for this is to help determine average dose distribution over the central cylinder plane when equilibrium dose has been reached.

Quantitative characterization of liver tumor radiodensity in CT images: a phantom study between two scanners.

Quantitative assessment of tumor radiodensity is important for the clinical evaluation of contrast enhancement and treatment response, as well as for the extraction of texture-related features for image analysis or radiomics. Radiodensity estimation, Hounsfield Units (HU) in CT images, can be affected by patient factors such as tumor size, and by system factors such as acquisition and reconstruction protocols. In this project, we quantified the measurability of liver tumor HU using a 3D-printed phantom, imaged with two CT systems: Siemens Somatom Force and GE Lightspeed VCT. The phantom was printed by dithering two materials to create spherical tumors (10, 14 mm) with uniform densities (90, 95, 100, 105 HU). Image datasets were acquired at 120 kVp including 15 repeats using two matching exposures across the CT systems, and reconstructed using comparable algorithms. The radiodensity of each tumor was measured using an automated matched-filter method. We assessed the performance of each protocol using the area under the ROC curve (AUC) as the metric for distinguishing between tumors with different radiodensities. The AUC ranged from 0.8 to 1.0 and was affected by tumor size, radiodensity, and scanner; the lowest AUC values corresponded to low dose measurements of 10 mm tumors with less than 5 HU difference. The two scanners exhibited similar performance >0.9 AUC for large lesions with contrast above 7 HU, though differences were observed for the smallest and lowest contrast tumors. These results show that HU estimation should be carefully examined, considering that uncertainty in the tumor radiodensity may propagate to quantification of other characteristics, such as size and texture.

Lead Apron Inspection Using Infrared Light: A Model Validation Study

PURPOSE To evaluate defect detection in radiation protective apparel, typically called lead aprons, using infrared (IR) thermal imaging. The use of IR lighting eliminates the need for access to x-ray-emitting equipment and radiation dose to the inspector. MATERIALS AND METHODS The performance of radiation workers was prospectively assessed using both a tactile inspection and the IR inspection with a lead apron phantom over a 2-month period. The phantom was a modified lead apron with a series of nine holes of increasing diameter ranging from 2 to 35 mm in accordance with typical rejection criteria. Using the tactile method, a radiation worker would feel for the defects in the lead apron. For the IR inspection, a 250-W IR light source was used to illuminate the lead apron phantom; an IR camera detected the transmitted radiation. The radiation workers evaluated two stills from the IR camera. RESULTS From the 31 participants inspecting the lead apron phantom with the tactile method, only 2 participants (6%) correctly discovered all 9 holes and 1 participant reported a defect that was not there; 10 of the 20 participants (50%) correctly identified all 9 holes using the IR method. Using a weighted average, 5.4 defects were detected with the tactile method and 7.5 defects were detected with the IR method. CONCLUSION IR light can penetrate an aprons protective outer fabric and illuminate defects below the current standard rejection size criteria. The IR method improves defect detectability as compared with the tactile method.

SU-C-12A-06: A Universal Definition for CT Irradiated Length

PURPOSE The length of scan shown or calculated from the console is the distance between the center of the starting and ending images. The irradiated length for such a set of images can vary substantially from this distance, depending on the acquisition mode. We propose that the rise to equilibrium function h(λ) be used to uniquely define an irradiated length independent of the details of the scanning process. This method also seamlessly accommodates recent developments in active collimation. METHOD h(λ) is determined along the central axis of a long cylindrical phantom, ideally from integration of the dose spread function. For practical reasons h(λ) may be approximated from helical scans with real-time dose rate measurements or from a series of central dose measurements using finite irradiated length exposures. For a particular scan then, the irradiated length λ is calculated from a central dose measurement of h and then applying the inverse function h(λ)-1 . RESULTS h(λ) was determined from measurements made in a prototype ICRU/TG200 phantom, 60 cm long and 30 cm in diameter. The resultant λ generally agreed with indicated scan length plus one beam width for axial and indicated scan length for helical scans if overscanning was accounted for. For most scanners, DLP/CTDIvol is a good measure of irradiated length allowing for another index of comparison. CONCLUSION Central dose measurements with a small chamber along with the rise to equilibrium function h(λ) provide us with a robust and consistent means of determining scan length independent of the means of irradiation over a variety of scanners and scanning conditions. The method is limited to λ significantly less than the equilibrium.

WE‐A‐218‐03: Translating X‐Ray Tube Modulation Parameters between Different CT Scanner Models Using Dose Metrics

Purpose: While auto tube current modulation is an effective dose reduction technique, manufacturer‐specific implementation of the technique leads to inconsistencies between scans obtained on different scanners. This research describes efforts to translate comprehensive scan protocols from a reference CTscanner to progeny scanners by matching x‐ray output using a variable diameter phantom. Methods: The phantom consisted of six cylinders of PMMA with increasingly large diameters, much like a wedding cake. The diameters of the cylinders were 10, 13, 16, 20, 25, and 32 cm. The reference CTscanner was a GE LightSpeed 16; the progeny scanners were a GE LightSpeed VCT and Siemens Definition AS+. The phantom was scanned while varying the tube modulation metric (either noise index or reference mAs). Using custom software, the mAs was determined on a slice‐by‐slice basis and then averaged along the length of each phantom. Two conversion metrics were examined as a way to translate x‐ray output between scanners: (1) CTDIw or (2) air exposure. The modulation metric of the progeny CTscanner that produced the output closest to that of the reference CTscanner was then selected as the optimal metric for each phantom size. The metric was then compared to the ad hoc scan protocols currently used for the progeny scanners.Results: The range of appropriate tube modulation metrics is highly dependent on patient diameter. When applied to patients of varying sizes, tube current modulation may be hitting the maximum or minimum limits. Conclusions: Both CTDIw and air exposure are reasonable conversion metrics to match x‐ray output between CTscanners. While matching x‐ray output is the first step towards standardizing scan protocols across different types of scanners, thorough image quality analysis must be done to ensure that clinical tasks can be performed sufficiently

TU‐E‐BRB‐07: Feasibility of a Dedicated Breast CT Platform for Orthovoltage Rotational External Beam Radiation Therapy: A Simulation Study

Purpose: To evaluate the feasibility of a dedicated breast computed tomography (bCT) platform to delivertherapeuticdose distributions, specifically for partial breast irradiation (PBI), whole breast irradiation (WBI), and dose painting, through orthovoltage rotational external beam radiation therapy (EBRT).Methods: Rotational EBRT using the geometry of a prototype bCT platform was evaluated by means of MCNPXMonte Carlo simulator. A 178 keV monoenergetic photon source was used to approximate 320 kVp photons filtered by 4 mm of copper, as validated by depth‐dose characteristics in polyethylene. The source was rotated around a 14 cm voxelized polyethylene disk (0.1 cm tall) or cylinder (9 cm tall) to simulate primary and primary + scattered photon interactions, respectively. Simulations were also performed using bCT patient images. Beam collimation was varied in the x‐y plane of rotation (1–14 cm) and in the z‐direction (0.1–10 cm).Results: As x‐y collimation narrowed, 2D dose profiles shifted from a cupped profile with high edge dose (14 cm beam) to an increasingly peaked central dose. A 1 cm beam had a center‐to‐cylinder edge dose ratio of 14.8. Similar distributions were observed experimentally. Dose painting for multiple foci, a line distribution, and a ring distribution was demonstrated using multiple rotations with varying collimation. A homogeneous dose distribution (<5% fluctuation) with skin sparing was demonstrated by weighted summation of four dose profiles. Using 2 cm z‐collimation, scatter tails decreased exponentially towards the cylinder top/bottom to 3% of maximum dose. Simulations using patient bCT images demonstrated the potential for treatment planning and image‐guided therapy. Conclusions: A bCT platform can feasibly deliver orthovoltage rotational EBRT for the treatment of breast cancer. A variety of dose distributions can be generated allowing for PBI to a single focus, dose painting, and WBI with skin sparing. Dedicated bCT is a potential platform for image‐guidedradiation therapy. One author is a consultant to Varian Imaging Systems (Palo Alto, Calif) and Artemis (Erlangen, Germany) and receives funding from Varian Imaging Systems, Fuji Medical Systems (Stamford, Conn), and Hologic (Bedford, Mass).

SU‐E‐I‐85: Application of Real‐Time Dosimetry to Characterize the X‐Ray Penetrability of CT Scanners

Purpose: Beam quality measurement on commercial whole body CT scanners generally requires the presence of the service engineer to stop the gantrys rotation using a service mode; consequently, the half‐value layer (HVL) is not regularly assessed. An apparatus was constructed and allows for HVL measurements to be performed using a standard axial scan with a prototype real‐time (RT) isotropic dose probe.Method: The HVL apparatus consisted of eight plates of high purity aluminum (type 1100) equally spaced around the probes detective volume. The thickness of the plates ranged from 0.81 mm to 13.51 mm; the x‐ray beam was attenuated by varying amounts as the gantry rotated. From the acquired signal train, the relative signal of the attenuated beam to the unattenuated beam was used to estimate the HVL. The HVL measurements made with the prototype apparatus were validated against conventional methods on a prototype dedicated breast CT (bCT) scanner over a range of x‐ray tube voltages. The energy dependence of the prototype probes output was compared to that of an ion chamber. The HVL measurements were then demonstrated on a clinical whole body CT scanner at four discrete x‐ray tube potentials. Results: The energy dependence of the RT probe was found to be non‐linear as compared to the conventional ion chamber, and a calibration factor for the RT solid‐state probe was produced. After converting the probes signal into units of air kerma, the HVL measurements using the proposed techniques were found to be equivalent to those found using traditional techniques. Conclusions: A proposed new method and apparatus for measurement of the HVL in whole body CT scanners provides for a fast, noninvasive measurement of x‐ray beam quality within a single scan. Using this approach, it is anticipated that HVL measurements across kV settings can be routine in CT scanner evaluation.