Robert A. Senn

Dedicated Cone-Beam CT System for Extremity Imaging

PURPOSE To provide initial assessment of image quality and dose for a cone-beam computed tomographic (CT) scanner dedicated to extremity imaging. MATERIALS AND METHODS A prototype cone-beam CT scanner has been developed for imaging the extremities, including the weight-bearing lower extremities. Initial technical assessment included evaluation of radiation dose measured as a function of kilovolt peak and tube output (in milliampere seconds), contrast resolution assessed in terms of the signal difference-to-noise ratio (SDNR), spatial resolution semiquantitatively assessed by using a line-pair module from a phantom, and qualitative evaluation of cadaver images for potential diagnostic value and image artifacts by an expert CT observer (musculoskeletal radiologist). RESULTS The dose for a nominal scan protocol (80 kVp, 108 mAs) was 9 mGy (absolute dose measured at the center of a CT dose index phantom). SDNR was maximized with the 80-kVp scan technique, and contrast resolution was sufficient for visualization of muscle, fat, ligaments and/or tendons, cartilage joint space, and bone. Spatial resolution in the axial plane exceeded 15 line pairs per centimeter. Streaks associated with x-ray scatter (in thicker regions of the patient--eg, the knee), beam hardening (about cortical bone--eg, the femoral shaft), and cone-beam artifacts (at joint space surfaces oriented along the scanning plane--eg, the interphalangeal joints) presented a slight impediment to visualization. Cadaver images (elbow, hand, knee, and foot) demonstrated excellent visibility of bone detail and good soft-tissue visibility suitable to a broad spectrum of musculoskeletal indications. CONCLUSION A dedicated extremity cone-beam CT scanner capable of imaging upper and lower extremities (including weight-bearing examinations) provides sufficient image quality and favorable dose characteristics to warrant further evaluation for clinical use.

Assessment of image quality in soft tissue and bone visualization tasks for a dedicated extremity cone-beam CT system

AbstractObjectiveTo assess visualization tasks using cone-beam CT (CBCT) compared to multi-detector CT (MDCT) for musculoskeletal extremity imaging.MethodsTen cadaveric hands and ten knees were examined using a dedicated CBCT prototype and a clinical multi-detector CT using nominal protocols (80kVp-108mAs for CBCT; 120kVp- 300mAs for MDCT). Soft tissue and bone visualization tasks were assessed by four radiologists using five-point satisfaction (for CBCT and MDCT individually) and five-point preference (side-by-side CBCT versus MDCT image quality comparison) rating tests. Ratings were analyzed using Kruskal–Wallis and Wilcoxon signed-rank tests, and observer agreement was assessed using the Kappa-statistic.ResultsKnee CBCT images were rated “excellent” or “good” (median scores 5 and 4) for “bone” and “soft tissue” visualization tasks. Hand CBCT images were rated “excellent” or “adequate” (median scores 5 and 3) for “bone” and “soft tissue” visualization tasks. Preference tests rated CBCT equivalent or superior to MDCT for bone visualization and favoured the MDCT for soft tissue visualization tasks. Intraobserver agreement for CBCT satisfaction tests was fair to almost perfect (κ ~ 0.26–0.92), and interobserver agreement was fair to moderate (κ ~ 0.27–0.54).ConclusionCBCT provided excellent image quality for bone visualization and adequate image quality for soft tissue visualization tasks.Key Points• CBCT provided adequate image quality for diagnostic tasks in extremity imaging. • CBCT images were “excellent” for “bone” and “good/adequate” for “soft tissue” visualization tasks. • CBCT image quality was equivalent/superior to MDCT for bone visualization tasks.

Model-based super-resolution for MRI

Conventional 1.5T magnetic resonance imaging (MRI) systems suffer from poor out-of-plane resolution (slice dimension), usually with in-plane resolution being several times higher than the former. Post-acquisition, super-resolution (SR) filtering is a viable alternative and a less expensive, off-line image processing approach that is employed to improve tissue resolution and contrast on acquired three-dimensional (3D) MR images. We introduce an SR framework that models a true acquired volume information by taking into account slice thickness and spacing between slices. Previous SR schemes have not considered this type of acquisition information or they have required specialized MR acquisition techniques. Evaluations based on synthetic data and clinical knee MRI data show superior performance of this method over an existing averaging method.

Peripheral Quantitative CT (pQCT) Using a Dedicated Extremity Cone-Beam CT Scanner.

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.

Dose and scatter characteristics of a novel cone beam CT system for musculoskeletal extremities

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.

High-performance soft-tissue imaging in extremity cone-beam CT

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 (107 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 ~2 mGy. Results: The rapid scatter estimation yielded root-mean-squared-errors of scatter projections of ~15% of peak scatter intensity for 5⋅106 photons/projection (runtime ~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.

Physical model-based metal artifact reduction (MAR) scheme for a 3D cone-beam CT extremity imaging system

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.

Image quality improvement through fusion of hybrid bone- and soft-tissue-texture filtering for 3D cone beam CT extremity imaging system

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.

TU‐E‐201B‐03: Design and Initial Performance Characterization of a Dedicated Cone‐Beam CT System for Musculoskeletal Extremities Imaging

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.