Purpose: We develop an Active Shape Model (ASM) framework for automated bone segmentation and anatomical landmark localization in weight-bearing Cone-Beam CT (CBCT). To achieve a robust shape model fit in narrow joint spaces of the foot (0.5 – 1 mm), a new approach for incorporating proximity constraints in ASM (coupled ASM, cASM) is proposed. Methods: In cASM, shape models of multiple adjacent foot bones are jointly fit to the CBCT volume. This coupling enables checking for proximity between the evolving shapes to avoid situations where a conventional single-bone ASM might erroneously fit to articular surfaces of neighbouring bones. We used 21 extremity CBCT scans of the weight-bearing foot to compare segmentation and landmark localization accuracy of ASM and cASM in leave-one-out validation. Each scan was used as a test image once; shape models of calcaneus, talus, navicular, and cuboid were built from manual surface segmentations of the remaining 20 scans. The models were augmented with seven anatomical landmarks used for common measurements of foot alignment. The landmarks were identified in the original CBCT volumes and mapped onto mean bone shape surfaces. ASM and cASM were run for 100 iterations, and the number of principal shape components was increased every 10 iterations. Automated landmark localization was achieved by applying known point correspondences between landmark vertices on the mean shape and vertices of the final active shape segmentation of the test image. Results: Root Mean Squared (RMS) error of bone surface segmentation improved from 3.6 mm with conventional ASM to 2.7 mm with cASM. Furthermore, cASM achieved convergence (no change in RMS error with iteration) after ~40 iterations of shape fitting, compared to ~60 iterations for ASM. Distance error in landmark localization was 25% to 55% lower (depending on the landmark) with cASM than with ASM. The importance of using a coupled model is underscored by the finding that cASM detected and corrected collisions between evolving shapes in 50% to 80% (depending on the bone) of shape model fits. Conclusion: The proposed cASM framework improves accuracy of shape model fits, especially in complexes of tightly interlocking, articulated joints. The approach enables automated anatomical analysis in volumetric imaging of the foot and ankle, where narrow joint spaces challenge conventional shape models.

To assess visualization tasks using cone-beam CT (CBCT) compared to multi-detector CT (MDCT) for musculoskeletal extremity imaging.

Purpose: Clinical performance studies of an extremity cone-beam CT (CBCT) system indicate excellent bone visualization, but point to the need for improvement of soft-tissue image quality. To this end, a rapid Monte Carlo (MC) scatter correction is proposed, and Penalized Likelihood (PL) reconstruction is evaluated for noise management.Methods: The accelerated MC scatter correction involved fast MC simulation with low number of photons implemented on a GPU (10(7) photons/sec), followed by Gaussian kernel smoothing in the detector plane and across projection angles. PL reconstructions were investigated for reduction of imaging dose for projections acquired at similar to 2 mGy.Results: The rapid scatter estimation yielded root-mean-squared-errors of scatter projections of similar to 15% of peak scatter intensity for 5.10(6) photons/projection (runtime similar to 0.5 sec/projection) and 25% improvement in fat-muscle contrast in reconstructions of a cadaveric knee. PL reconstruction largely restored soft-tissue visualization at 2 mGy dose to that of 10 mGy FBP image.Conclusion: The combination of rapid (5-10 minutes/scan) MC-based, patient-specific scatter correction and PL reconstruction offers an important means to overcome the current limitations of extremity CBCT in soft-tissue imaging.

In Cone Beam CT Imaging, metallic and other dense objects, such as implantable orthopedic appliances, surgical clips and staples, and dental fillings, are often acquired as part of the image dataset. These high-density, high atomic mass objects attenuate X-rays in the diagnostic energy range much more strongly than soft tissue or bony structures, resulting in photon starvation at the detector. In addition, signal behind the metal objects suffer from increased quantum noise, scattered radiation, and beam hardening. All of these effects combine to create nonlinearities which are further amplified by the reconstruction algorithm, such as conventional filtered back-projection (FBP), producing strong artifacts in the form of streaking. They reduce image quality by masking soft tissue structures, not only in the immediate vicinity of the dense object, but also throughout the entire image volume. A novel, physical-model-based, metal-artifact reduction scheme (MAR) is proposed to mitigate the metal-induced artifacts. The metal objects are segmented in the projection domain, and a physical model based method is adopted to fill in the segmented area. The FDK1 reconstruction algorithm is then used for the final reconstruction.

Purpose: We describe the initial assessment of the peripheral quantitative CT (pQCT) imaging capabilities of a conebeam CT (CBCT) scanner dedicated to musculoskeletal extremity imaging. The aim is to accurately measure and quantify bone and joint morphology using information automatically acquired with each CBCT scan, thereby reducing the need for a separate pQCT exam. Methods: A prototype CBCT scanner providing isotropic, sub-millimeter spatial resolution and soft-tissue contrast resolution comparable or superior to standard multi-detector CT (MDCT) has been developed for extremity imaging, including the capability for weight-bearing exams and multi-mode (radiography, fluoroscopy, and volumetric) imaging. Assessment of pQCT performance included measurement of bone mineral density (BMD), morphometric parameters of subchondral bone architecture, and joint space analysis. Measurements employed phantoms, cadavers, and patients from an ongoing pilot study imaged with the CBCT prototype (at various acquisition, calibration, and reconstruction techniques) in comparison to MDCT (using pQCT protocols for analysis of BMD) and micro-CT (for analysis of subchondral morphometry). Results: The CBCT extremity scanner yielded BMD measurement within ±2-3% error in both phantom studies and cadaver extremity specimens. Subchondral bone architecture (bone volume fraction, trabecular thickness, degree of anisotropy, and structure model index) exhibited good correlation with gold standard micro-CT (error ~5%), surpassing the conventional limitations of spatial resolution in clinical MDCT scanners. Joint space analysis demonstrated the potential for sensitive 3D joint space mapping beyond that of qualitative radiographic scores in application to non-weight-bearing versus weight-bearing lower extremities and assessment of phalangeal joint space integrity in the upper extremities. Conclusion: The CBCT extremity scanner demonstrated promising initial results in accurate pQCT analysis from images acquired with each CBCT scan. Future studies will include improved x-ray scatter correction and image reconstruction techniques to further improve accuracy and to correlate pQCT metrics with known pathology.

A flat-panel, detector-based cone beam CT system can provide advantages over a fan beam CT system in terms of 3D isotropic spatial resolution. However, as a result of increased X-ray coverage along the rotation axis, there is also an increase in scatter. This can lead to a decrease in low-contrast resolution as well as the appearance of non-uniform artifacts across the reconstructed image. These effects can be minimized with the use of an anti-scatter grid; however, further software corrections are often desirable. Software scatter correction is generally achieved through the subtraction of an estimate of the scatter distribution from the corresponding original projection data in the linear space. While the non-uniform artifacts effect is generally improved, a side effect of this subtractive process can be an undesirable amplification of the apparent noise, which makes the image quality, in terms of contrast-to-noise ratio (CNR), much worse than the images produced by fan beam CT systems. In this work, a novel modified imaging chain has been proposed to apply separate, non-linear noise-reduction algorithms on bone and soft tissues to improve the CNR for soft tissue as well as to maintain a high spatial resolution for the display of boney structures.

PURPOSE To assess the diagnostic performance of a prototype cone-beam CT (CBCT) scanner developed for musculoskeletal extremity imaging. Studies involved controlled observer studies conducted subsequent to rigorous technical assessment as well as patient images from the first clinical trial in imaging the hand and knee. METHODS Performance assessment included: 1.) rigorous technical assessment; 2.) controlled observer studies using CBCT images of cadaveric specimens; and 3.) first clinical images. Technical assessment included measurement of spatial resolution (MTF), constrast, and noise (SDNR) versus kVp and dose using standard CT phantoms. Diagnostic performance in comparison to multi- detector CT (MDCT) was assessed in controlled observer studies involving 12 cadaveric hands and knees scanned with and without abnormality (fracture). Observer studies involved five radiologists rating pertinent diagnostics tasks in 9-point preference and 10-point diagnostic satisfaction scales. Finally, the first clinical images from an ongoing pilot study were assessed in terms of diagnostic utility in disease assessment and overall workflow in patient setup. RESULTS Quantitative assessment demonstrated sub-mm spatial resolution (MTF exceeding 10% out to 15-20 cm-1) and SDNR sufficient for relevant soft-tissue visualization tasks at dose <10 mGy. Observer studies confirmed optimal acquisition techniques and demonstrated superior utility of combined soft-tissue visualization and isotropic spatial resolution in diagnostic tasks. Images from the patient trial demonstrate exquisite contrast and detail and the ability to detect tissue impingement in weight-bearing exams. CONCLUSIONS The prototype CBCT scanner provides isotropic spatial resolution superior to standard-protocol MDCT with soft-tissue visibility sufficient for a broad range of diagnostic tasks in musculoskeletal radiology. Dosimetry and workflow were advantageous in comparison to whole-body MDCT. Multi-mode and weight-bearing capabilities add valuable functionality. An ongoing clinical study further assesses diagnostic utility and defines the role of such technology in the diagnostic arsenal. - Research Grant, Carestream Health - Research Grant, National Institutes of Health 2R01-CA-112163.

A novel cone-beam CT (CBCT) system has been developed with promising capabilities for musculoskeletal imaging (e.g., weight-bearing extremities and combined radiographic / volumetric imaging). The prototype system demonstrates diagnostic-quality imaging performance, while the compact geometry and short scan orbit raise new considerations for scatter management and dose characterization that challenge conventional methods. The compact geometry leads to elevated, heterogeneous x-ray scatter distributions - even for small anatomical sites (e.g., knee or wrist), and the short scan orbit results in a non-uniform dose distribution. These complex dose and scatter distributions were investigated via experimental measurements and GPU-accelerated Monte Carlo (MC) simulation. The combination provided a powerful basis for characterizing dose distributions in patient-specific anatomy, investigating the benefits of an antiscatter grid, and examining distinct contributions of coherent and incoherent scatter in artifact correction. Measurements with a 16 cm CTDI phantom show that the dose from the short-scan orbit (0.09 mGy/mAs at isocenter) varies from 0.16 to 0.05 mGy/mAs at various locations on the periphery (all obtained at 80 kVp). MC estimation agreed with dose measurements within 10-15%. Dose distribution in patient-specific anatomy was computed with MC, confirming such heterogeneity and highlighting the elevated energy deposition in bone (factor of ~5-10) compared to soft-tissue. Scatter-to-primary ratio (SPR) up to ~1.5-2 was evident in some regions of the knee. A 10:1 antiscatter grid was found earlier to result in significant improvement in soft-tissue imaging performance without increase in dose. The results of MC simulations elucidated the mechanism behind scatter reduction in the presence of a grid. A ~3-fold reduction in average SPR was found in the MC simulations; however, a linear grid was found to impart additional heterogeneity in the scatter distribution, mainly due to the increase in the contribution of coherent scatter with increased spatial variation. Scatter correction using MC-generated scatter distributions demonstrated significant improvement in cupping and streaks. Physical experimentation combined with GPU-accelerated MC simulation provided a sophisticated, yet practical approach in identifying low-dose acquisition techniques, optimizing scatter correction methods, and evaluating patientspecific dose.

Purpose: A dedicated cone‐beam CT(CBCT)system for musculoskeletal extremities imaging has recently been developed to complement existing technologies and provide new capabilities in weight‐bearing imaging, combined planar and volumetric imaging, and improved workflow. This paper presents the first physical performance characterization of the scanner prototype in terms of imaging dose, contrast resolution, spatial resolution and image quality. Methods: The system employs a Varian 3030+ flat‐panel detector (0.194 mm pixel pitch) and a 0.875 kW fixed anode x‐ray source. The scanning orbit is 220o, and the magnification factor is 1.3. Performance characterization involved measurements of HVL and imaging dose in a 16 cm CTDI phantom as a function of tube voltage (60–120 kVp) and position with respect to the scanning orbit. Contrast, noise, and line‐pair resolution were investigated using a Catphan phantom. Anthropomorphic knee and hand phantoms with simulated soft tissues provided initial qualitative assessment of spatial and contrast resolution. Results: Effective dose at the isocenter was 0.14 mSv for a nominal knee scan (90 kvp, 93.1 mAs/scan) and 0.07 mSv for a nominal hand scan (90 kVp, 49 mAs/scan), compared to 0.03–0.16 mSv in conventional CT of the extremities. The entrance surface dose for the knee varied 0.08–0.22 mSv, depending on location. Spatial resolution exceeded 15 lp/cm (0.33 mm−1) for full‐resolution readout. Soft‐ tissue contrasts inserts in the range ∼+−20 HU were readily discernable. Exquisite visualization of bone architecture and soft‐tissue visibility approaching that of conventional CT was observed in anthropomorphic phantoms. Conclusion: The prototype scanner provides high spatial resolution, volumetric images with soft‐tissue resolution at doses less than or comparable to conventional CT. The capability for weight‐bearing and multi‐mode imaging could benefit a wide variety of musculoskeletal applications. The prototype system is being deployed in a clinical feasibility study This research was supported in collaboration with Carestream Health, Inc., Rochester, NY

PURPOSE:This paper reports on the design and initial imaging performance of a dedicated cone-beam CT (CBCT) system for musculoskeletal (MSK) extremities. The system complements conventional CT and MR and offers a variety of potential clinical and logistical advantages that are likely to be of benefit to diagnosis, treatment planning, and assessment of therapy response in MSK radiology, orthopaedic surgery, and rheumatology. METHODS:The scanner design incorporated a host of clinical requirements (e.g., ability to scan the weight-bearing knee in a natural stance) and was guided by theoretical and experimental analysis of image quality and dose. Such criteria identified the following basic scanner components and system configuration: a flat-panel detector (FPD, Varian 3030+, 0.194 mm pixels); and a low-power, fixed anode x-ray source with 0.5 mm focal spot (SourceRay XRS-125-7K-P, 0.875 kW) mounted on a retractable C-arm allowing for two scanning orientations with the capability for side entry, viz. a standing configuration for imaging of weight-bearing lower extremities and a sitting configuration for imaging of tensioned upper extremity and unloaded lower extremity. Theoretical modeling employed cascaded systems analysis of modulation transfer function (MTF) and detective quantum efficiency (DQE) computed as a function of system geometry, kVp and filtration, dose, source power, etc. Physical experimentation utilized an imaging bench simulating the scanner geometry for verification of theoretical results and investigation of other factors, such as antiscatter grid selection and 3D image quality in phantom and cadaver, including qualitative comparison to conventional CT. RESULTS:Theoretical modeling and benchtop experimentation confirmed the basic suitability of the FPD and x-ray source mentioned above. Clinical requirements combined with analysis of MTF and DQE yielded the following system geometry: a -55 cm source-to-detector distance; 1.3 magnification; a 20 cm diameter bore (20 x 20 x 20 cm3 field of view); total acquisition arc of -240 degrees. The system MTF declines to 50% at -1.3 mm(-1) and to 10% at -2.7 mm(-1), consistent with sub-millimeter spatial resolution. Analysis of DQE suggested a nominal technique of 90 kVp (+0.3 mm Cu added filtration) to provide high imaging performance from -500 projections at less than -0.5 kW power, implying -6.4 mGy (0.064 mSv) for low-dose protocols and -15 mGy (0.15 mSv) for high-quality protocols. The experimental studies show improved image uniformity and contrast-to-noise ratio (without increase in dose) through incorporation of a custom 10:1 GR antiscatter grid. Cadaver images demonstrate exquisite bone detail, visualization of articular morphology, and soft-tissue visibility comparable to diagnostic CT (10-20 HU contrast resolution). CONCLUSIONS:The results indicate that the proposed system will deliver volumetric images of the extremities with soft-tissue contrast resolution comparable to diagnostic CT and improved spatial resolution at potentially reduced dose. Cascaded systems analysis provided a useful basis for system design and optimization without costly repeated experimentation. A combined process of design specification, image quality analysis, clinical feedback, and revision yielded a prototype that is now awaiting clinical pilot studies. Potential advantages of the proposed system include reduced space and cost, imaging of load-bearing extremities, and combined volumetric imaging with real-time fluoroscopy and digital radiography.

Purpose: High-quality volumetric imaging with isotropic resolution, soft-tissue visualization, and the ability to image load-bearing extremities would be of major benefit to diagnosis and planning in musculoskeletal radiology, orthopaedic surgery, and rheumatology. This paper reports the design and initial performance of an innovative cone-beam CT system under development to address such clinical needs. Methods: The scanner employs a flat-panel detector (Varian3030+), with source-detector distance of 53 cm, source-isocenter distance of 41 cm, and field of view ∼(20×20×20) cm. Gantry orientations (vertical and horizontal) permit imaging of weight-bearing knee and ankle (patient standing) and imaging of tensioned elbow, wrist, or hand (patient seated). Optimization of parameters such as kVp and beam filtration and characterization of detective quantum efficiency, resolution, and required dose levels were performed using cascaded systems analysis. An experimental CBCT bench simulated scanner operation, guided system design, and provided initial assessment of image quality in cadaveric specimens. Results: Analysis indicates sub-milimeter resolution (∼0.3–0.5 mm) and optimal performance for an x-ray technique of ∼90 kVp + 0.2 mm Cu added filtration, giving <3 mGy patient dose and requiring ∼0.5 kW power. At this dose, signal levels at the detector are ∼ 100 times above the electronic noise floor, giving input-quantum-limited performance and facilitating soft-tissue imaging. Benchtop studies demonstrate exquisite detail in bony trabeculae, and excellent visualization of joint spaces. Soft-tissue visibility approaches that of diagnostic CT (∼ 10–20 HU contrast), with methods for improvement underway in scatter correction and novel reconstruction techniques. Conclusions: Results indicate that the proposed system delivers low dose, high resolution, volumetric images of the extremities with soft-tissue visualization. The unique characteristic of the design in permitting imaging of loaded extremities is of value in a broad spectrum of applications. A prototype scanner for deployment in clinical trials is now under construction. Research sponsored by NIH and Carestream Health.