Purpose
Clinical use of dedicated breast computed tomography (bCT) requires relatively short scan times necessitating systems with high frame rates. This in turn impacts the x‐ray tube operating ...range. We characterize the effects of tube voltage, beam filtration, dose, and object size on contrast and noise properties related to soft tissue and iodine contrast agents as a way to optimize imaging protocols for soft tissue and iodine contrast at high frame rates.
Methods
This study design uses the signal‐difference‐to‐noise ratio (SDNR), noise‐equivalent quanta (NEQ), and detectability (d´) as measures of imaging performance for a prototype breast CT scanner that utilizes a pulsed x‐ray tube (with a 4 ms pulse width) at 43.5 fps acquisition rate. We assess a range of kV, filtration, breast phantom size, and mean glandular dose (MGD). Performance measures are estimated from images of adipose‐equivalent breast phantoms machined to have a representative size and shape of small, medium, and large breasts. Water (glandular tissue equivalent) and iodine contrast (5 mg/ml) were used to fill two cylindrical wells in the phantoms.
Results
Air kerma levels required for obtaining an MGD of 6 mGy ranged from 7.1 to 9.1 mGy and are reported across all kV, filtration, and breast phantom sizes. However, at 50 kV, the thick filters (0.3 mm of Cu or Gd) exceeded the maximum available mA of the x‐ray generator, and hence, these conditions were excluded from subsequent analysis. There was a strong positive association between measurements of SDNR and d’ (R2 > 0.97) within the range of parameters investigated in this work. A significant decrease in soft tissue SDNR was observed for increasing phantom size and increasing kV with a maximum SDNR at 50 kV with 0.2 mm Cu or 0.2 mm Gd filtration. For iodine contrast SDNR, a significant decrease was observed with increasing phantom size, but a decrease in SDNR for increasing kV was only observed for 70 kV (50 and 60 kV were not significantly different). Thicker Gd filtration (0.3 mm Gd) resulted in a significant increase in iodine SDNR and decrease in soft tissue SDNR but requires significantly more tube current to deliver the same MGD.
Conclusions
The choice of 60 kV with 0.2 mm Gd filtration provides a good trade‐off for maximizing both soft tissue and iodine contrast. This scanning technique takes advantage of the ~50 keV Gd k‐edge to produce contrast and can be achieved within operating range of the x‐ray generator used in this work. Imaging at 60 kV allows for a greater range in dose delivered to the large breast sizes when uniform image quality is desired across all breast sizes. While imaging performance metrics (i.e., detectability index and SDNR) were shown to be strongly correlated, the methodologies presented in this work for the estimation of NEQ (and subsequently d') provides a meaningful description of the spatial resolution and noise characteristics of this prototype bCT system across a range of beam quality, dose, and object sizes.
Purpose
The 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.
Methods
A 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.
Results
The 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.
Conclusion
The 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.
The inadequate visibility of microcalcifications-small calcium deposits that cue radiologists to early stages of cancer-is a major limitation in current designs of dedicated breast computed ...tomography (bCT). This limitation has previously been attributed to the constituent components, spatial resolution, and utilized dose. Scattered radiation has been considered an occurrence with low-frequency impacts that can be compensated for in post-processing. We hypothesized, however, that the acquisition of scattered radiation has a far more detrimental impact on clinically relevant features than has previously been understood. Critically, acquisition of scatter leads to the reduced visibility of microcalcifications. This hypothesis was investigated and supported via mathematical derivations and simulation studies. We conducted a series of comparative studies in which four bCT systems were simulated under iso-dose and iso-resolution conditions, characterizing the dependencies of microcalcification contrast on accumulated scatter. Included among the simulated systems is a novel bCT design-narrow beam bCT (NB-bCT)-that captures nearly zero scatter. We find that current bCT systems suffer from significant levels of scatter. As validated in theory, depending on the system and size of microcalcifications, between 25% and over 70% of contrast resolution is lost due to scatter. The results in NB-bCT, however, provide evidence that by removing scatter build-up in projections, the contrast of microcalcifications in a bCT image is preserved, regardless of their size or location in the breast.
Purpose
To introduce an auxiliary apparatus of fluence modulation and scatter shielding for dedicated breast computed tomography (bCT) and the corresponding patient‐specific method of image ...acquisition.
Methods
The apparatus is composed of two assemblies, referred to herein as the “Dynamic Fluence Gate” (FG) and “Scatter Shield” (SS), that work in synchrony to form a narrow beam sweeping the entire fan angle coverage of the imaging system during a projection. The apparatus, as a whole, is referred to as FG‐SS. FG and SS are pre‐patient and post‐patient units, respectively. Each is composed of a rotating drum, on top of which are installed two sheets of high x‐ray attenuating material, a sensory system, and the constituent robotics. The sheets of each unit are positioned such that an opening — a window Fluence Modulation and Scatter Shielding is formed between them. The rotations of the drums and positioning of the sheets are synchronized and adjusted such that a line of sight is created between the source, FG window, the breast, and the SS window. With line of sight achieved, the narrow beam transitions from the source to the detector. The fluence of the narrow beam during a projection depends on the size, shape, and positioning of the breast. The FG‐SS method of imaging is discussed mathematically and demonstrated using computer simulations. A series of Monte Carlo simulations are conducted to evaluate the performance of the system as relates to its impact on the imager’s dynamic range, dose distribution to the breast, noise inhomogeneity in reconstructed images, and scatter buildup in projections within small, medium, and large breasts composed of homogeneous medium breast tissue.
Results
Implementation of FG‐SS results in near scatter‐free projections, reduction in both dose and the required dynamic range of the imager, and equalization of the quantum noise distribution in the reconstructed image. Using the disclosed design, the dynamic range was reduced by factors ranging from 1.6 to 5.5 across the range of breast sizes studied. A reduction in the acquisition of the scattered rays, varying between the factors of 6.1 (in the small breast) and 9.8 (in that large breast) was achieved and consequently, shading artifacts were suppressed. Noise in the CT image was equalized by reducing the overall spatial variation from 29% to <5% in small breast and from 45% to 14% in the large breast. An overall reduction in deposited dose to the breast was achieved — between 26% and 39% depending on the breast size.
Conclusions
Utilization of the FG‐SS apparatus and technique was demonstrated via simulations to result in: significant reductions in dose to the breast, reductions in scatter uptake in projections, reduced required dynamic range of the imager, and homogenizing of quantum noise throughout the reconstructed image.
Purpose
In cone‐beam breast CT, scattered photons form a large portion of the acquired signal, adversely impacting image quality throughout the frequency response of the imaging system. Prior ...simulation studies provided proof of concept for utilization of a hardware solution to prevent scatter acquisition. Here, we report the design, implementation, and characterization of an auxiliary apparatus of fluence modulation and scatter shielding that does indeed lead to projections with a reduced level of scatter.
Methods
An apparatus was designed for permanent installation within an existing cone‐beam CT system. The apparatus is composed of two primary assemblies: a “Fluence Modulator” (FM) and a “Scatter Shield” (SS). The design of the assemblies enables them to operate in synchrony during image acquisition, converting the sourced x‐rays into a moving narrow beam. During a projection, this narrow beam sweeps the entire fan angle coverage of the imaging system. As the two assemblies are contingent on one another, their joint implementation is described in the singular as apparatus FM–SS. The FM and the SS assemblies are each comprised a metal housing, a sensory system, and a robotic system. A controller unit handles their relative movements. A series of comparative studies were conducted to evaluate the performance of a cone‐beam CT system in two “modes” of operation: with and without FM–SS installed, and to compare the results of physical implementation with those previously simulated. The dynamic range requirements of the utilized detector in the cone‐beam CT imaging system were first characterized, independent of the mode of operation. We then characterized and compared the spatial resolution of the imaging system with, and without, FM–SS. A physical breast phantom, representative of an average size breast, was developed and imaged. Actual differences in signal level obtained with, versus without, FM–SS were then compared to the expected level gains based on previously reported simulations. Following these initial assessments, the scatter acquisition in each projection in both modes of operation was investigated. Finally, as an initial study of the impact of FM–SS on radiation dose in an average size breast, a series of Monte Carlo simulations were coupled with physical measurements of air kerma, with and without FM–SS.
Results
With implementation of FM‐SS, the detector's required dynamic range was reduced by a factor of 5.5. Substantial reduction in the acquisition of the scattered rays, by a factor of 5.1 was achieved. With the implementation of FM–SS, deposited dose was reduced by 27% in the studied breast.
Conclusions
The disclosed implementation of FM–SS, within a cone‐beam breast CT system, results in reduction of scatter‐components in acquired projections, reduction of dose deposit to the breast, and relaxation of requirements for the detector's dynamic range. Controlling or correcting for patient motion occurring during image acquisition remains an open problem to be solved prior to practical clinical usage of FM–SS cone‐beam breast CT.
Purpose
To introduce a novel methodology for developing anthropomorphic breast phantoms for use in X‐ray‐based imaging modalities.
Methods
“Hyperization” is a quasi‐stippling mapping operation in ...which regions of varying grayscale values in a 2D image are transformed into regions of varying holes on a surface. The holes can be cut or engraved on the sheet of paper using a high‐resolution laser cutter/engraver. In hyperization, the main parameters are the size and the distance between the holes. Here, we introduce the concept and chronicle the development and characterization of a proof‐of‐concept prototype. In this study, we hypothesized that a resulting “Hyperia” phantom would be a realistic representative of a patient's breast tissue: it would exhibit similar X‐ray properties and show textural complexities. We used breast computed tomography (bCT) images of real patients as the input models. Using a previously developed segmentation method, the input CT images were segmented into different tissue classes (skin, adipose, and fibroglandular). The segmented images were then “Hyperized”. A series of Monte Carlo simulations were conducted to find the optimal hyperization parameters. Different laser cutter/engraver systems and substrate materials were explored to find a viable option for developing an entire Hyperia breast phantom. The resulting phantom was imaged on a prototype breast CT system, and the resulting images were evaluated based on physical properties and similarity to the original patient data.
Results
The simulation results indicate close similarities – both in the distribution of different tissue types and the resulting CT numbers – between the patient bCT image and the bCT of the Hyperia phantom, regardless of the breast size and density: the Pearson correlation coefficient (ρ) ranged from 0.88 in a BIRADS A breast to 0.94 in BIRADS C and D breasts (ρ of 1.00 suggests perfect structural similarity), and the volumetric mean squared error ranged from 0.0033 (in BIRADS D breast) to 0.0059 (in BIRADS A), suggesting good agreement between the resulting CT numbers. For fabricating the slices, the office paper was found to be an optimal substrate material, with the Hyperization parameters of (α, β) = (0.200 mm, 0.400 mm).
Conclusion
A novel phantom can be used for X‐ray‐based breast cancer imaging systems. The main advantage is that only one material is used for creating a contrast between different tissue types in an image.
Narrow beam breast CT: proof‐of‐concept Ghazi, Peymon; Fu, Guanhao; Ghazi, Tara ...
Medical physics (Lancaster),
June 2023, 2023-Jun, 2023-06-00, 20230601, Letnik:
50, Številka:
6
Journal Article
Recenzirano
Background
In breast CT, scattered photons form a large portion of the acquired signal, adversely impacting image quality throughout the frequency response of the imaging system. Prior studies ...provided evidence for a new image acquisition design, dubbed Narrow Beam Breast CT (NB‐bCT), in preventing scatter acquisition.
Purpose
Here, we report the design, implementation, and initial characterization of the first NB‐bCT prototype.
Methods
The imaging system's apparatus is composed of two primary assemblies: a dynamic Fluence Modulator (collimator) and a photon‐counting line detector. The design of the assemblies enables them to operate in lockstep during image acquisition, converting sourced x‐rays into a moving narrow beam. During a projection, this narrow beam sweeps the entire fan angle coverage of the imaging system. The assemblies are each comprised of a metal housing, a sensory system, and a robotic system. A controller unit handles their relative movements. To study the impact of fluence modulation on the signal received in the detector, three physical breast phantoms, representative of small, average, and large size breasts, were developed and imaged, and acquired projections analyzed. The scatter acquisition in each projection as a function of breast phantom size was investigated. The imaging system's spatial resolution at the center and periphery of the field of view was measured.
Results
Minimal acquisition of scattered rays occurs during image acquisition with NB‐bCT; results in minimal scatter to primary ratios in small, average, and large breast phantoms imaged were 0.05, 0.07, and 0.9, respectively. System spatial resolution of 5.2 lp/mm at 10% max MTF and 2.9 lp/mm at 50% max MTF at the center of the field of view was achieved, with minimal loss with the shift toward the corner (5.0 lp/mm at 10% max MTF and 2.5 lp/mm at 50% max MTF).
Conclusion
The disclosed development, implementation, and characterization of a physical NB‐bCT prototype system demonstrates a new method of CT‐based image acquisition that yields high spatial resolution while minimizing scatter‐components in acquired projections. This methodology holds promise for high‐resolution CT‐imaging applications in which reduction of scatter contamination is desirable.
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