Browsing by Subject "Scatter"

Coherent scatter imaging techniques have experienced a renaissance in the past two decades from an evolution of detector technology and computational imaging techniques. X-ray diffraction requires a precise knowledge of object location and is time consuming; transforming diffractometry into a practical imaging technique involves spatially resolving the sample in 3-dimensions and speeding up the measurement process. The introduction of a coded aperture in a conventional X-ray diffraction system provides 3D localization of the scatterer as well as drastic reductions in the acquisition time due to the ability to perform multiplexed measurements. This theses document contains two strategies involving coded apertures to address the aforementioned challenges of X-ray coherent scatter measurements.

The first technique places the coded aperture between source and object to structure the incident illumination. A single pixel detector captures temporally modulated coherent scatter data from an object as it travels through the illumination. From these measurements, 2D spatial and 1D spectral information is recovered at each point within a planar slice of an object. Compared to previous techniques, this approach is able to reduce the overall scan time of objects by 1-2 orders of magnitude.

The second measurement technique demonstrates snapshot coherent scatter tomography. A planar slice of an object is illuminated by a fan beam and the scatter data is modulated by a coded aperture between object and detector. The spatially modulated data is captured with a linear array of energy sensitive detectors, and the recovered data shows that the system can image objects that are 13 mm in range and 2 mm in cross range with a fractional momentum transfer resolution of 15\%. The technique also allows a 100x speedup when compared to pencil beam systems using the same components.

Continuing with the theme of snapshot tomography with energy sensitive detectors, I study the impact of detectors properties such as detection area, choice of energies and energy resolution for pencil and fan beam coded aperture coherent scatter systems. I simulate various detector geometries and determine that energy resolution has the largest impact for pencil beam geometries while detector area has the largest impact for fan beam geometries. These results can be used to build detectors which can potentially help implement pencil and/or fan beam coded aperture coherent scatter systems in applications involving medicine and security.

Onboard cone beam computed tomography (CBCT) has become a widespread means of three-dimensional target localization for radiation therapy; however, it is susceptible to metal artifacts and beam-hardening artifacts that can hinder visualization of low contrast anatomy. Dual-CBCT provides easy access to techniques that may reduces such artifacts. Additionally, dual-CBCT can decrease imaging time and provide simultaneous orthogonal projections which may also be useful for fast target localization. However, dual-CBCT will suffer from large increases in scattered radiation due to the addition of the second source.

An experimental bench top dual CBCT system was constructed so that each imaging chain in the dual CBCT system mimics the geometry of gantry-mounted CBCT systems commonly used in the radiation therapy room. The two systems share a common axis of rotation and are mounted orthogonally. Custom control software was developed to ensure reproducible exposure and rotation timings. This software allows the implementation of the acquisition sequences required for the cross scatter avoidance and correction strategies studied.

Utilizing the experimental dual CBCT system cross scatter was characterized from 70-145 kVp in projections and reconstructed images using this system and three cylindrical phantoms (15cm, 20cm, and 30cm) with a common Catphan core. A novel strategy for avoiding cross-scatter in dual-CBCT was developed that utilized interleaved data acquisition on each imaging chain. Contrast and contrast-to-noise-ratio were measured in reconstructions to evaluate the effectiveness of this strategy to avoid the effects of cross scatter.

A novel correction strategy for cross scatter was developed wherein the cross scatter was regularly sampled during the course of data acquisition and these samples were used as the basis for low- and high- frequency corrections for the cross-scatter in projections. The cross scatter sampling interval was determined for an anthropomorphic phantom at three different sites relevant to radiation therapy by estimating the angular Nyquist frequency. The low frequency portion of the cross scatter distribution is interpolated between samples to provide an estimate of the cross scatter distribution at every projection angle and was then subtracted from the projections.

The high-frequency portion of the correction was applied after the low-frequency correction was applied. The novel high-frequency correction utilizes the fact that a direct estimate of the high-frequency components was obtained in the cross scatter samples. The high-frequency components of the measured cross scatter were subtracted from the projections in the Fourier domain, a process referred to as spectral subtraction. Each projection is corrected using the cross scatter sample taken at the closest projection angle. In order to apply this correction in the Fourier domain the high-frequency component of the cross scatter must be approximately stationary. To improve the stationarity of the high-frequency cross scatter component a novel two-dimensional, overlapping window was developed. The spectral subtraction was then applied in each window and the results added to form the final image.

The effectiveness of the correction techniques were evaluated by measuring the contrast and contrast-to-noise-ratio in an image quality phantom. Additionally, the effect of the high-frequency correction on resolution was measured using a line pair phantom.

Cross scatter in dual CBCT was shown for large phantoms to be much higher than forward scatter which has long been known to be one of the largest degrading factors of image quality in CBCT. This results in large losses of contrast and CNR in reconstructed images. The interleaving strategy for avoiding cross scatter during projection acquisition showed similar performance to cross scatter free acquisitions, however, does not acquire projections at the maximum possible rate. For those applications in which maximizing the acquisition rate of projections is important, the low- and high-frequency corrections effectively mitigated the effects of cross scatter in the dual CBCT system.

X-ray mammography has been the gold standard for breast imaging for decades, despite the significant limitations posed by the two dimensional (2D) image acquisitions. Difficulty in diagnosing lesions close to the chest wall and axilla, high amount of structural overlap and patient discomfort due to compression are only some of these limitations. To overcome these drawbacks, three dimensional (3D) breast imaging modalities have been developed including dual modality single photon emission computed tomography (SPECT) and computed tomography (CT) systems. This thesis focuses on the development and integration of the next generation of such a device for dedicated breast imaging. The goals of this dissertation work are to: [1] understand and characterize any effects of fully 3-D trajectories on reconstructed image scatter correction, absorbed dose and Hounsifeld Unit accuracy, and [2] design, develop and implement the fully flexible, third generation hybrid SPECT-CT system capable of traversing complex 3D orbits about a pendant breast volume, without interference from the other. Such a system would overcome artifacts resulting from incompletely sampled divergent cone beam imaging schemes and allow imaging closer to the chest wall, which other systems currently under research and development elsewhere cannot achieve.

The dependence of x-ray scatter radiation on object shape, size, material composition and the CT acquisition trajectory, was investigated with a well-established beam stop array (BSA) scatter correction method. While the 2D scatter to primary ratio (SPR) was the main metric used to characterize total system scatter, a new metric called ‘normalized scatter contribution’ was developed to compare the results of scatter correction on 3D reconstructed volumes. Scatter estimation studies were undertaken with a sinusoidal saddle (±15° polar tilt) orbit and a traditional circular (AZOR) orbit. Clinical studies to acquire data for scatter correction were used to evaluate the 2D SPR on a small set of patients scanned with the AZOR orbit. Clinical SPR results showed clear dependence of scatter on breast composition and glandular tissue distribution, otherwise consistent with the overall phantom-based size and density measurements. Additionally, SPR dependence was also observed on the acquisition trajectory where 2D scatter increased with an increase in the polar tilt angle of the system.

The dose delivered by any imaging system is of primary importance from the patient’s point of view, and therefore trajectory related differences in the dose distribution in a target volume were evaluated. Monte Carlo simulations as well as physical measurements using radiochromic film were undertaken using saddle and AZOR orbits. Results illustrated that both orbits deliver comparable dose to the target volume, and only slightly differ in distribution within the volume. Simulations and measurements showed similar results, and all measured dose values were within the standard screening mammography-specific, 6 mGy dose limit, which is used as a benchmark for dose comparisons.

Hounsfield Units (HU) are used clinically in differentiating tissue types in a reconstructed CT image, and therefore the HU accuracy of a system is very important, especially when using non-traditional trajectories. Uniform phantoms filled with various uniform density fluids were used to investigate differences in HU accuracy between saddle and AZOR orbits. Results illustrate the considerably better performance of the saddle orbit, especially close to the chest and nipple region of what would clinically be a pedant breast volume. The AZOR orbit causes shading artifacts near the nipple, due to insufficient sampling, rendering a major portion of the scanned phantom unusable, whereas the saddle orbit performs exceptionally well and provides a tighter distribution of HU values in reconstructed volumes.

Finally, the third generation, fully-suspended SPECT-CT system was designed in and developed in our lab. A novel mechanical method using a linear motor was developed for tilting the CT system. A new x-ray source and a custom made 40 x 30 cm2 detector were integrated on to this system. The SPECT system was nested, in the center of the gantry, orthogonal to the CT source-detector pair. The SPECT system tilts on a goniometer, and the newly developed CT tilting mechanism allows ±15° maximum polar tilting of the CT system. The entire gantry is mounted on a rotation stage, allowing complex arbitrary trajectories for each system, without interference from the other, while having a common field of view. This hybrid system shows potential to be used clinically as a diagnostic tool for dedicated breast imaging.

Our research group has previously described the development and testing of a coherent-scatter spectral imaging system for identification of cancer using surrogate phantoms, formalin-fixed pathology tissues and, more recently, surgically resected breast tumor. Here we present the implementation of a Monte-Carlo simulation tool for optimization of the imaging system.

MC-GPU, a GPU-enabled Monte Carlo software was modified and used to simulate X-ray diffraction experiments for combinations of X-ray spectra (tungsten and molybdenum anode), kV (15-150), filtration (material and thickness) and phantom geometry and material (normal, adipose, fibroglandular, and cancerous breast tissue). For each combination, a simulated measurement of contrast-to-noise (CNR), signal strength and object detectability were assessed.

Examination of Monte Carlo simulations showed optimal spectrum characterization strategies that exploit spectral and filter characteristics to increase material identification probabilities via momentum transfer measurement. Increased detectability was shown with molybdenum energy spectra, and a higher CNR metric was observed to show better pathological assessments and findings of cancer.

This work demonstrates the utility of Monte Carlo methods and MCGPU in optimizing coherent scatter imaging systems and can be used to provide insightful information regarding the design of coherent scatter imaging systems for material classification breast tissue types.

Scatter in medical imaging is typically cast off as image-related noise that detracts from meaningful diagnosis. It is therefore typically rejected or removed from medical images. However, it has been found that every material, including cancerous tissue, has a unique X-ray coherent scatter signature that can be used to identify the material or tissue. Such scatter-based tissue-identification provides the advantage of locating and identifying particular materials over conventional anatomical imaging through X-ray radiography. A coded aperture X-ray coherent scatter spectral imaging system has been developed in our group to classify different tissue types based on their unique scatter signatures. Previous experiments using our prototype have demonstrated that the depth-resolved coherent scatter spectral imaging system (CACSSI) can discriminate healthy and cancerous tissue present in the path of a non-destructive x-ray beam. A key to the successful optimization of CACSSI as a clinical imaging method is to obtain anatomically accurate phantoms of the human body. This thesis describes the development and fabrication of 3D printed anatomical scatter phantoms of the breast and lung.

The purpose of this work is to accurately model different breast geometries using a tissue equivalent phantom, and to classify these tissues in a coherent x-ray scatter imaging system. Tissue-equivalent anatomical phantoms were designed to assess the capability of the CACSSI system to classify different types of breast tissue (adipose, fibroglandular, malignant). These phantoms were 3D printed based on DICOM data obtained from CT scans of prone breasts. The phantoms were tested through comparison of measured scatter signatures with those of adipose and fibroglandular tissue from literature. Tumors in the phantom were modeled using a variety of biological tissue including actual surgically excised benign and malignant tissue specimens. Lung based phantoms have also been printed for future testing. Our imaging system has been able to define the location and composition of the various materials in the phantom. These phantoms were used to characterize the CACSSI system in terms of beam width and imaging technique. The result of this work showed accurate modeling and characterization of the phantoms through comparison of the tissue-equivalent form factors to those from literature. The physical construction of the phantoms, based on actual patient anatomy, was validated using mammography and computed tomography to visually compare the clinical images to those of actual patient anatomy.