Rationale Integrated PET (Positron Emission Tomography)/MR(magnetic resonance) systems are becoming increasingly popular in clinical and research applications. Quantitative PET reconstruction requires correction for γ photon attenuations using an attenuation coefficient map (μ-map) that is a measure of the electron density. One challenge of PET/MR, in contrast to PET/CT, lies in the accurate computation of μ-maps. Unlike CT, MRI measures physical properties not directly related to electron density. Previous approaches have computed the attenuation coefficients using a segmentation of MR images or using deformable registration of atlas CT images to the space of the subject MRI. Method In this work, we propose a patch-based method to generate whole head μ-maps from Ultra-short Echo Time (UTE) MR imaging sequences. UTE images are preferred to other MR sequences, because of the increased signal from bone). To generate a synthetic CT image, we use patches from a reference dataset, which consists of dual echo UTE images and a co-registered CT from the same subject. By matching patches between the reference and target images, corresponding patches from the reference CT are combined via a Bayesian framework. No registration or segmentation is required. Results For evaluation, UTE, CT and PET data, acquired from 5 patients under an IRB approved protocol, were used. Another patient (with UTE and CT only) was selected to be the reference to generate synthetic CT images for these five patients. PET reconstructions were attenuation corrected using (1) the original CT, (2) our synthetic CT, and Siemens (3) Dixon- and (4) UTE-based μ-maps, and (5) a deformable registration based CT. Our synthetic CT based PET reconstruction shows higher correlation (average ρ = 0.99, R2 = 0.99) to the original CT based PET, as compared to the segmentation and registration based methods. Synthetic CT based reconstruction had minimal bias (regression slope 0.99) as compared to the segmentation based methods (regression slope 0.97). A peak signal-to-noise ratio of 36.0 dB in the reconstructed PET activity is observed, compared with 29.7, 29.3, 27.4 dB for Siemens Dixon, UTE, and registration based μ-maps. Conclusion A patch-matching approach to synthesize CT images from dual echo UTE images leads to significantly more accurate PET reconstruction as compared to actual CT scans. The PET reconstruction is improved over segmentation (Dixon and Siemens UTE) and registration based methods, even in subjects with pathology.

Purpose To evaluate different susceptibility weighted imaging (SWI) phase processing methods and parameter selection, thereby improving understanding of potential artifacts, as well as facilitating choice of methodology in clinical settings. Materials and Methods Two major phase processing methods, Homodyne-filtering and phase unwrapping-high pass (HP) filtering, were investigated with various phase unwrapping approaches, filter sizes, and filter types. Magnitude and phase images were acquired from a healthy subject and brain injury patients on a 3T clinical Siemens MRI system. Results were evaluated based on image contrast to noise ratio and presence of processing artifacts. Results When using a relatively small filter size (32 pixels for the matrix size 512 × 512 pixels), all Homodyne-filtering methods were subject to phase errors leading to 2% to 3% masked brain area in lower and middle axial slices. All phase unwrapping-filtering/smoothing approaches demonstrated fewer phase errors and artifacts compared to the Homodyne-filtering approaches. For performing phase unwrapping, Fourier-based methods, although less accurate, were 2–4 orders of magnitude faster than the PRELUDE, Goldstein and Quality-guide methods. Conclusion Although Homodyne-filtering approaches are faster and more straightforward, phase unwrapping followed by HP filtering approaches perform more accurately in a wider variety of acquisition scenarios.

Susceptibility weighted imaging (SWI) takes advantage of the local variation in susceptibility between different tissues to enable highly detailed visualization of the cerebral venous system and sensitive detection of intracranial hemorrhages. Thus, it has been increasingly used in magnetic resonance imaging studies of traumatic brain injury as well as other intracranial pathologies.In SWI, magnitude information is combined with phase information to enhance the susceptibility induced image contrast. Because of global susceptibility variations across the image, the rate of phase accumulation varies widely across the image resulting in phase wrapping artifacts that interfere with the local assessment of phase variation. Homodyne filtering is a common approach to eliminate this global phase variation. However, filter size requires careful selection in order to preserve image contrast and avoid errors resulting from residual phase wraps. An alternative approach is to apply phase unwrapping prior to high pass filtering. A suitable phase unwrapping algorithm guarantees no residual phase wraps but additional computational steps are required. In this work, we quantitatively evaluate these two phase processing approaches on both simulated and real data using different filters and cutoff frequencies. Our analysis leads to an improved understanding of the relationship between phase wraps, susceptibility effects, and acquisition parameters. Although homodyne filtering approaches are faster and more straightforward, phase unwrapping approaches perform more accurately in a wider variety of acquisition scenarios.

A major image degrading factor in simultaneous Dual Isotope (DI) SPECT or simultaneous Emission-Transmission (ECT-TCT) imaging, is the detection of photons emitted by the higher energy isotope in the energy window used for imaging the lower energy isotope. In Tc-99m/Tl-201 DI-SPECT typically tens of percents of the total detected down-scatter is caused by lead x rays. In Tc-99m/Gd-153 ECT-TCT, a comparable fraction of the down-scatter originates from Tc-99m photons which only partly deposit their energy in the detector crystal (i.e., due to crystal interactions). Efficient simulation methods which model down-scatter can be used to optimize DI-SPECT or ECT-TCT imaging acquisition or reconstruction protocols. In this paper we adapt a previously proposed efficient down-scatter simulation method, to include the interactions of photons with the detector crystal and collimator lead. To this end, point spread function tables including crystal and lead interactions are precalculated. Subsequently, photons are traced through the patient body until their last scatter position, and the precalculated responses are used to project the photons onto the detector plane, while photon attenuation in the patient is taken into account. The approach is evaluated by comparing simulated Tc-99m down-scatter projections with measured projections. Incorporation of photon interaction with crystal and lead leads to significantly improved accuracy of the shape of down-scatter responses, while differences in total counts between simulated and measured projections typically decrease from tens of percents to a couple of percents. Calculating 60 down-scatter projections of an extended distribution on a 64 x 64 x 64 grid takes about three minutes on a PC with two 1.2 GHz processors. We conclude that accurate and efficient simulation of down-scatter is now possible including the major effects of the nonuniform mass density of the patient as well as photon interactions with the crystal and collimator lead.