David H. Foos

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.

Digital Radiography Reject Analysis: Data Collection Methodology, Results, and Recommendations from an In-depth Investigation at Two Hospitals

Reject analysis was performed on 288,000 computed radiography (CR) image records collected from a university hospital (UH) and a large community hospital (CH). Each record contains image information, such as body part and view position, exposure level, technologist identifier, and—if the image was rejected—the reason for rejection. Extensive database filtering was required to ensure the integrity of the reject-rate calculations. The reject rate for CR across all departments and across all exam types was 4.4% at UH and 4.9% at CH. The most frequently occurring exam types with reject rates of 8% or greater were found to be common to both institutions (skull/facial bones, shoulder, hip, spines, in-department chest, pelvis). Positioning errors and anatomy cutoff were the most frequently occurring reasons for rejection, accounting for 45% of rejects at CH and 56% at UH. Improper exposure was the next most frequently occurring reject reason (14% of rejects at CH and 13% at UH), followed by patient motion (11% of rejects at CH and 7% at UH). Chest exams were the most frequently performed exam at both institutions (26% at UH and 45% at CH) with half captured in-department and half captured using portable x-ray equipment. A ninefold greater reject rate was found for in-department (9%) versus portable chest exams (1%). Problems identified with the integrity of the data used for reject analysis can be mitigated in the future by objectifying quality assurance (QA) procedures and by standardizing the nomenclature and definitions for QA deficiencies.

Statistical reconstruction for cone-beam CT with a post-artifact-correction noise model: application to high-quality head imaging

Non-contrast CT reliably detects fresh blood in the brain and is the current front-line imaging modality for intracranial hemorrhage such as that occurring in acute traumatic brain injury (contrast ~40-80 HU, size  >  1 mm). We are developing flat-panel detector (FPD) cone-beam CT (CBCT) to facilitate such diagnosis in a low-cost, mobile platform suitable for point-of-care deployment. Such a system may offer benefits in the ICU, urgent care/concussion clinic, ambulance, and sports and military theatres. However, current FPD-CBCT systems face significant challenges that confound low-contrast, soft-tissue imaging. Artifact correction can overcome major sources of bias in FPD-CBCT but imparts noise amplification in filtered backprojection (FBP). Model-based reconstruction improves soft-tissue image quality compared to FBP by leveraging a high-fidelity forward model and image regularization. In this work, we develop a novel penalized weighted least-squares (PWLS) image reconstruction method with a noise model that includes accurate modeling of the noise characteristics associated with the two dominant artifact corrections (scatter and beam-hardening) in CBCT and utilizes modified weights to compensate for noise amplification imparted by each correction. Experiments included real data acquired on a FPD-CBCT test-bench and an anthropomorphic head phantom emulating intra-parenchymal hemorrhage. The proposed PWLS method demonstrated superior noise-resolution tradeoffs in comparison to FBP and PWLS with conventional weights (viz. at matched 0.50 mm spatial resolution, CNR = 11.9 compared to CNR = 5.6 and CNR = 9.9, respectively) and substantially reduced image noise especially in challenging regions such as skull base. The results support the hypothesis that with high-fidelity artifact correction and statistical reconstruction using an accurate post-artifact-correction noise model, FPD-CBCT can achieve image quality allowing reliable detection of intracranial hemorrhage.

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.

JPEG 2000 compression of medical imagery

A multi-institution effort was conducted to assess the visual quality performance of various JPEG 2000 (Joint Photographic Experts Group) lossy compression options for medical imagery. The purpose of this effort was to provide clinical data to DICOM (Digital Imaging and Communications in Medicine) WG IV to support recommendations to the JPEG 2000 committee regarding the definition of the base standard. A variety of projection radiographic, cross sectional, and visible light images were compressed-reconstructed using various JPEG 2000 options and with the current JPEG standard. The options that were assessed included integer and floating point transforms, scalar and vector quantization, and the use of visual weighting. Experts from various institutions used a sensitive rank order methodology to evaluate the images. The proposed JPEG 2000 scheme appears to offer similar or improved image quality performance relative to the current JPEG standard for compression of medical images, yet has additional features useful for medical applications, indicating that it should be included as an additional standard transfer syntax in DICOM.

New automatic tone-scale method for computed radiography

Computed radiography is used for a wide range of projection radiography examinations. To produce useful diagnostic images it is necessary to apply an appropriate tone scale to the raw CR data. This paper presents a new automated tone scaling method for computed radiography and presents the results of a clinical study of the algorithm encompassing a wide range of clinical examinations.

Statisically lossless image compression for CR and DR

This paper proposes an image compression algorithm that can improve the compression efficiency for digital projection radiographs over current lossless JPEG by utilizing a quantization companding function and a new lossless image compression standard called JPEG-LS. The companding and compression processes can also be augmented by a pre- processing step to first segment the foreground portions of the image and then substitute the foreground pixel values with a uniform code value. The quantization companding function approach is based on a theory that relates the onset of distortion to changes in the second-order statistics in an image. By choosing an appropriate companding function, the properties of the second-order statistics can be retained to within an insignificant error, and the companded image can then be lossless compressed using JPEG-LS; we call the reconstructed image statistically lossless. The approach offers a theoretical basis supporting the integrity of the compressed-reconstructed data relative to the original image, while providing a modest level of compression efficiency. This intermediate level of compression could help to increase the conform level for radiologists that do not currently utilize lossy compression and may also have benefits form a medico-legal perspective.

Enhancement method that provides direct and independent control of fundamental attributes of image quality for radiographic imagery

Image processing is used to transform raw digital radiographic image data, captured using CR (computed radiography) and DR (flat panel direct digital radiography) systems into a display-ready form. Ideally, an image-processing algorithm automatically renders an image for display, based on aims derived from observer performance studies. Establishing the rendering aim for different exam types, however, can be complex because the effects on image appearance introduced by the various steps in the rendering process are interdependent. This paper describes a new rendering algorithm that provides orthogonal control, to the first order, of five fundamental attributes of perceived image quality. These attributes are brightness, latitude, detail contrast, sharpness, and appearance of noise. The detail contrast and sharpness can be controlled in a density-dependent manner. The algorithm uses a multifrequency-band decomposition wherein the bands of the decomposition are manipulated, and the reconstructed image is passed through a tone-scale process that prepares the image for display. The rendering method is implemented in software on a workstation that enables interactive control of these image quality attributes in order to facilitate the determination of rendering aims for different exam types.

Modeling and design of a cone-beam CT head scanner using task-based imaging performance optimization

Detection of acute intracranial hemorrhage (ICH) is important for diagnosis and treatment of traumatic brain injury, stroke, postoperative bleeding, and other head and neck injuries. This paper details the design and development of a cone-beam CT (CBCT) system developed specifically for the detection of low-contrast ICH in a form suitable for application at the point of care. Recognizing such a low-contrast imaging task to be a major challenge in CBCT, the system design began with a rigorous analysis of task-based detectability including critical aspects of system geometry, hardware configuration, and artifact correction. The imaging performance model described the three-dimensional (3D) noise-equivalent quanta using a cascaded systems model that included the effects of scatter, scatter correction, hardware considerations of complementary metal-oxide semiconductor (CMOS) and flat-panel detectors (FPDs), and digitization bit depth. The performance was analyzed with respect to a low-contrast (40-80 HU), medium-frequency task representing acute ICH detection. The task-based detectability index was computed using a non-prewhitening observer model. The optimization was performed with respect to four major design considerations: (1) system geometry (including source-to-detector distance (SDD) and source-to-axis distance (SAD)); (2) factors related to the x-ray source (including focal spot size, kVp, dose, and tube power); (3) scatter correction and selection of an antiscatter grid; and (4) x-ray detector configuration (including pixel size, additive electronics noise, field of view (FOV), and frame rate, including both CMOS and a-Si:H FPDs). Optimal design choices were also considered with respect to practical constraints and available hardware components. The model was verified in comparison to measurements on a CBCT imaging bench as a function of the numerous design parameters mentioned above. An extended geometry (SAD = 750 mm, SDD  = 1100 mm) was found to be advantageous in terms of patient dose (20 mGy) and scatter reduction, while a more isocentric configuration (SAD = 550 mm, SDD  = 1000 mm) was found to give a more compact and mechanically favorable configuration with minor tradeoff in detectability. An x-ray source with a 0.6 mm focal spot size provided the best compromise between spatial resolution requirements and x-ray tube power. Use of a modest anti-scatter grid (8:1 GR) at a 20 mGy dose provided slight improvement (~5-10%) in the detectability index, but the benefit was lost at reduced dose. The potential advantages of CMOS detectors over FPDs were quantified, showing that both detectors provided sufficient spatial resolution for ICH detection, while the former provided a potentially superior low-dose performance, and the latter provided the requisite FOV for volumetric imaging in a centered-detector geometry. Task-based imaging performance modeling provides an important starting point for CBCT system design, especially for the challenging task of ICH detection, which is somewhat beyond the capabilities of existing CBCT platforms. The model identifies important tradeoffs in system geometry and hardware configuration, and it supports the development of a dedicated CBCT system for point-of-care application. A prototype suitable for clinical studies is in development based on this analysis.

Stationary chest tomosynthesis using a CNT x-ray source array

Chest tomosynthesis is an imaging modality that provides 3D sectional information of a patients thoracic cavity using limited angle x-ray projections. Studies show that tomosynthesis can improve the detection of subtle lung nodules comparing to conventional radiography at a lower radiation dose than CT. In the conventional design, the projection images are collected by mechanically moving a single x-ray source to different viewing angles. We investigated the feasibility of stationary chest tomosynthesis using the distributed CNT x-ray source array technology, which can generate a scanning x-ray beam without any mechanical motion. A proof-of-concept system was constructed using a short linear source array and a at panel detector. The performance of the source including the flux was evaluated in the context of chest imaging. The bench-top system was characterized and images of a chest phantom were acquired and reconstructed. The preliminary results demonstrate the feasibility of stationary chest tomosynthesis using the CNT x-ray source array technology.