David Sowards-Emmerd

Iterative reconstruction for circular cone-beam CT with an offset flat-panel detector

Circular cone-beam computed tomography (CBCT) with a tangentially offset flat-panel X-ray detector offers a large CT field-of-view (FoV) with a relatively small detector. It is used in practice, e.g., for target imaging in image-guided radiotherapy or for localization and attenuation correction in SPECT/CT imaging. The X-ray projections, acquired on a circular source trajectory, each cover roughly half the CT FoV; a central overlap region is imaged by all projections. Offset-detector CBCT reconstruction requires special algorithms. For large detector offsets, previously proposed filtered-backprojection methods can lead to shading artifacts, specifically left/right intensity imbalance. Here, we propose using iterative reconstruction for offset-detector CBCT. To handle the special acquisition geometry, known iterative reconstruction algorithms are modified in terms of axial truncation compensation, redundancy weighting, and algorithm initialization. An efficient implementation using a graphics processing unit (GPU) delivers clinically feasible reconstruction times. Results from patient and phantom studies are presented, showing a clear reduction of artifacts and improvement in image quality.

CBCT-subsystem performance of the multi-modality Brightview XCT system (M09-26)

The new Brightview XCT system uses a flat-panel detector to perform CBCT imaging for attenuation correction and localization. Features include a small footprint due to an offset-detector geometry, advanced scatter correction — both software and hardware — isotropic voxels, and GPU-accelerated reconstruction. System performance characteristics of the CBCT system such as spatial resolution, HU linearity, uniformity, noise, low-contrast detectability, and dose measurements are discussed in this paper.

Investigation of surface treatment of interface for depth of interaction positioning of a 2×2 discrete crystal array

We have previously reported on depth of interaction (DOI) positioning performance of 2×2 discrete LYSO crystal arrays coupled to digital silicon photomultiplier arrays (PDPC arrays). By optimizing the design of the interface between each neighboring crystal, DOI can be estimated from the overall light distribution collected by the 2×2 array of sensors. A significant challenge to the design is controlling how light is shared between neighboring crystals. We have previously used crystals with lapped surfaces to improve light sharing. In this work we investigated four different surface treatments/patterns. The surface treatments were fine etch, coarse etch and sanded. The patterns were treatment for the full length of the interface between crystals and treatment only for the section of the crystals where light was being shared. The LYSO crystals were each 4×4×19 mm3. High index of refraction (i.e., 1.704n) melt mount resin and custom shaped mirror film patterns were used to control light sharing between neighboring crystals. Each detector unit was calibrated using an electronically collimated 511 keV photon flux stepped along the long axis of the crystal. DOI event positioning was done using maximum likelihood. The crystal array fabricated from the crystals with the finely etched surfaces had the best DOI decoding performance. Customizing the surface properties to optimize light sharing can improve DOI positioning for detector designs that utilize light sharing to estimate DOI positioning. The advantage of the etching process used in this work is that it is highly reproducible and can allow for custom patterns to be etched onto crystal surfaces.

Lag correction for flat-panel cone-beam CT in SPECT/CT

MR spectroscopy (MRS) is an analytic technique widely used in chemistry for analyzing the structure of compounds and the composition of mixtures of compounds. Compounds are identified by their unique spectra, based on chemical shifts and coupling constants. MRS allows the noninvasive measurement of selected biologic compounds in vivo. Major technical advances have occurred in MRS over the last several decades, including superconducting magnets and Fourier transform for signal processing. Feasibility was first demonstrated in humans in the mid1980s, and much experience with MRS has accumulated in both research and clinical applications. Nearly all MRI scanners have the capability of performing MRS, and MRS techniques continue to improve even after 2 decades of development. Despite this considerable research effort and the unique biochemical information provided, only limited integration of MRS into clinical practice has occurred, for multiple reasons including nonstandardization of acquisition and analysis protocols, limited vendor support, difficult interpretation, limited perceived added value above conventional MRI, and lack of reimbursement. However, in vivo MRS is increasingly being used in clinical practice, particularly for neurologic disorders. Proton spectroscopy of the human brain is most widely used, but other organ systems such as breast and prostate, and other nuclei including 31P and 13C, have been studied. In the brain, compounds of key importance measured by MRS include N-acetyl aspartate (located predominantly in neurons), choline, myoinositol (located primarily in glial cells), creatine, lactate, glutamate, and glutamine. This book was written by leading MRS experts, and it is an invaluable guide for anyone interested in in vivo MRS, including radiologists, nuclear physicians, neurologists, neurosurgeons, oncologists, and medical researchers. It gives the reader a solid basis for understanding both the techniques and the applications of clinical MRS. The book is organized into 14 chapters. Chapter 1 introduces in vivo MRS, and chapter 2 discusses pulse sequences and protocol design. Chapter 3 addresses spectral analysis methods, quantitation, and common artifacts, and chapter 4 handles normal regional variations, particularly brain development and aging. The rest of the chapters discuss MRS findings in brain tumors; in stroke and hypoxic–ischemic encephalopathy; in infectious, inflammatory and demyelinating lesions; in epilepsy; in neurodegenerative diseases; in traumatic brain injury; in cerebral metabolic disorders; in prostate cancer; in breast cancer; and in musculoskeletal diseases. Each chapter begins with key points and ends with recommendations and a conclusion. References are updated and useful. The aim of this book is to serve as a practical reference work that covers all aspects of in vivo human spectroscopy for clinical purposes. The book explains physical principles and provides a comprehensive and perceptive review of clinical applications. Also discussed are the limitations of MRS, such as its low spatial resolution when compared with MRI, common artifacts, and diagnostic pitfalls. More widespread adoption of MRS into the clinic will lead to better diagnoses and improved outcomes for individual patients. There are 140 figures, which are clear and have detailed legends, and 7 tables that are helpful for readers. The index is convenient and useful. I highly recommend this book to trainees and practitioners in medical physics, radiology, nuclear medicine, oncology, neurology, and cardiology.