Purpose: To demonstrate the utility of high-resolution micro-computed tomography (μCT) for determining ground-truth size and shape properties of calcium grains for evaluation of detection performance in breast CT (bCT).
Approach: Calcium carbonate grains (∼200 μm) were suspended in 1% agar solution to emulate microcalcifications (μCalcs) within a fibroglandular tissue background. Ground-truth imaging was performed on a commercial μCT scanner and was used for assessing calcium-grain size and shape, and for generating μCalc signal profiles. Calcium grains were placed within a realistic breast-shaped phantom and imaged on a prototype bCT system at 3- and 6-mGy mean glandular dose (MGD) levels, and the non-prewhitening detectability was assessed. Additionally, the μCT-derived signal profiles were used in conjunction with the bCT system characterization (MTF and NPS) to obtain predictions of bCT detectability.
Results: Estimated detectability of the calcium grains on the bCT system ranged from 2.5 to 10.6 for 3 mGy and from 3.8 to 15.3 for 6 mGy with large fractions of the grains meeting the Rose criterion for visibility. Segmentation of μCT images based on morphological operations produced accurate results in terms of segmentation boundaries and segmented region size. A regression model linking bCT detectability to μCalc parameters indicated significant effects of μCalc size and vertical position within the breast phantom. Detectability using μCT-derived detection templates and bCT statistical properties (MTF and NPS) were in good correspondence with those measured directly from bCT (R2 > 0.88).
Conclusions: Parameters derived from μCT ground-truth data were shown to produce useful characterizations of detectability when compared to estimates derived directly from bCT. Signal profiles derived from μCT imaging can be used in conjunction with measured or hypothesized statistical properties to evaluate the performance of a system, or system component, that may not currently be available.
This study introduces a methodology for generating high resolution signal profiles of microcalcification (MC) grains for validating breast CT (bCT) systems. A physical MC phantom was constructed by suspending calcium carbonate grains in an agar solution emulating MCs in a fibroglandular tissue background. Additionally, small Teflon spheres (2.4 mm diameter) were embedded in the agar solution for the purpose of fiducial marking and assessment of segmentation accuracy. The MC phantom was imaged on a high resolution (34 μm) commercial small-bore μCT scanner at high dose, and the images were used as the gold-standard for assessing MC size and for generating high resolution signal profiles of each MC. High-dose bCT scans of the MC phantom suspended in-air were acquired using 1×1 binning mode (75 μm dexel pitch) by averaging three repeat scans to produce a single low-noise reconstruction of the MC phantom. The high resolution μCT volume data set was then registered with the corresponding bCT data set after correcting for the bCT system spatial resolution. Microcalcification signal profiles constructed using low-noise bCT images were found to be in good agreement with those generated using the μCT scanner with all differences <10% within the VOI surrounding each MC. The MC signal profiles were used as detection templates for a non-prewhitening-matched-filter model observer for scans acquired in a realistic breast phantom at 3, 6, and 9 mGy mean glandular dose. MC detectability using signal templates derived from bCT were shown to be in good agreement with those generated using μCT.
Field-emission x-ray source arrays have been studied for both tomosynthesis and CT applications, however these arrays tend to have limited output. We propose the use of multi-source x-ray arrays using thermionic cathodes, contained within a single vacuum housing. A prototype 3-source x-ray array has been fabricated and tested, and the utility of multi-x-ray-source arrays has been demonstrated using physical simulations in both tomosynthesis and in cone beam CT. The prototype x-ray tube made use of a cylindrical molybdenum anode, machined to have 3 specific focal tracks. Grid-controlled cathode assemblies were fabricated and aligned to each focal tract, and the individual x-ray focal spots were evaluated with a star pattern at 35 kV and 40 mA. The 3-source assembly was used to physically simulate tomosynthesis imaging geometry, and tomosynthesis images of a lemon were obtained. Physical simulations using a cone beam breast CT scanner were also performed, by vertically moving the single x-ray source into 5 different locations – simulating 5 different source positions. A new geometry for cone beam CT imaging is proposed, where each source of a multi-x-ray source array is individually collimated to eliminate rays involving large cone angles. This geometry also allows three sources to be simultaneously pulsed onto a single flat panel detector, achieving better duty cycle and view sampling in cone beam CT. A reconstruction algorithm was written to accommodate the different source positions, and phantoms designed to demonstrate cone beam artifacts were imaged. The tomosynthesis images illustrate appropriate depth resolution in the test object. Analysis of the CT data demonstrate marked improvement compared to one source. We conclude that multi-source x-ray arrays using thermionic cathodes will have important applications in medical imaging, especially breast tomosynthesis and cone beam computed tomography.
This study examines the potential of a multisource x-ray system to reduce cone beam artifacts in a dedicated breast CT acquisition geometry. A breast CT scanner (Doheny), built at our institution, was used to demonstrate the potential of multiple x-ray sources in a single x-ray tube housing. Both 3 focal spot and 5 focal spot thermionic systems were physically simulated in this study. The x-ray tube is mounted on a vertical actuator on the breast CT system gantry, allowing the single x-ray source to be positioned at different vertical locations in the field of view. Five acquisition geometries were used to acquire raw cone beam CT data with the x-ray source locations placed at 2 cm intervals. Data was collected using a 15-cm tall Defrise phantom. The individual acquisitions of raw CT data were reconstructed using filtered back projection, aligned and summed. The reconstructed CT volume data set using three sources and five sources were compared to that produced from a single source. Both multi-source datasets demonstrated less visible cone beam artifact, and the contrast clearly improved. The resolvable field of view in the vertical direction was extended by 50% when comparing the one source to the three source geometry and extended by 120% when comparing the one source to the five source geometry. This physical simulation of a multisource x-ray CT system successfully demonstrated that a reduction in cone beam CT artifacts could be achieved using a multi-source x-ray tube on a breast CT scanner.
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