A digitally controlled analog phase shifter is described for the application in magnetic resonance (MR) imaging and spectroscopy. The design is based on a single side band modulator scheme. A phase resolution of 1.4°and a switching time of 400 ns are achieved. The device controls the phase of the intermediate frequency in the MR instrument, and hence can be used over the entire observe frequency range of interest.
With recent advances in the field, magnetic resonance imaging (MRI) has become a powerful quantitative imaging modality for the study of neurological disorders. The quantitative power of MRI is significantly enhanced with multi-contrast and high-resolution techniques. However, those techniques generate large volumes of data which, combined with the sophisticated state-of-the-art image analysis methods, result in a very high computational load. In order to keep the scanner workflow uninterrupted, processing has to be performed off-line leading to delayed access to the quantitative results. This time delay also precludes the evaluation of data quality, and prevents the care giver from using the results of quantitative analysis to guide subsequent studies. We developed a scanner-integrated system for fast online processing of dual-echo fast spin-echo and fluid-attenuated inversion recovery images to quickly classify different brain tissues and generate white matter lesion maps in patients with multiple sclerosis (MS). The segmented tissues were imported back into the patient database on the scanner for clinical interpretation by the radiologist. The analysis pipeline included rigid-body registration, skull stripping, nonuniformity correction, and tissue segmentation. In six MS patients, the average time taken by the processing pipeline to the final segmentation of the brain into white matter, grey matter, cerebrospinal fluid, and white matter lesions was ∼2 min, making it feasible to generate lesion maps immediately after the scan.
Background: Chronic cerebrospinal venous insufficiency (CCSVI) was implicated in the pathophysiology of multiple sclerosis (MS). Objective: We evaluated neurosonography (NS), magnetic resonance venography (MRV), and transluminal venography (TLV) in subsets of MS patients drawn from a single-center, prospective, case-control study of 206 MS and 70 non-MS volunteers. Methods: As previously reported, findings on high-resolution B-mode NS imaging with color and spectral Doppler of the extracranial and intracranial venous drainage consistent with CCSVI were similar among MS and non-MS volunteers (3.88% vs 7.14%; p = 0.266). Ninety-nine MS participants consented to intravascular contrast-enhanced 3D MRV to assess their major systemic and intracranial venous circulation, and 40 advanced to TLV that included pressure measurements of the superior vena cava, internal jugular, brachiocephalic, and azygous veins. Results: NS findings and MRV patterns were discrepant for 26/98 evaluable subjects, including four with abnormal findings on NS that had normal venous anatomy by MRV. In no instance were TLV pressure gradients indicative of clinically significant functional stenosis encountered. The three imaging approaches provided generally consistent data with discrepancies referable to inherent technique properties. Conclusions: Our findings lend no support for altered venous outflow dynamics as common among MS patients, nor do they likely contribute to the disease process.
Abstract In this report, we applied diffusion tensor imaging (DTI) methods in 36 patients with uncomplicated mild traumatic brain injury (mTBI) and a comparison group of 37 participants with orthopedic injury. Our aim was to characterize regional and global macro- and microstructural attributes of white matter (WM), gray matter (GM), in addition to volume and diffusivity of cerebrospinal fluid (CSF) to identify and differentiate patterns of acute and short-term recovery. Given that previous DTI reports on mTBI in adults using a region-of-interest approach implicated the corona radiata (CR), corpus callosum, and hippocampus, we analyzed and quantified DTI metrics of these regions using atlas-based methods. The normalized volume percentages of global CSF, GM, and WM were not different between the mTBI and orthopedic comparison (OC) groups at either the baseline or follow-up time points or between the baseline and follow-up time points within the OC group (p>0.17; uncorrected for multiple comparisons). The DTI metrics did not differ between groups at either occasion. However, an increase was noted on follow-up in the OC group in the global mean diffusivity of GM (uncorrected p=0.003) and WM (uncorrected p=0.02), indicating a decrease in diffusivity at the 3-month postinjury, as compared to the baseline scan. An analysis of the DTI data collected longitudinally in the CR show insignificant changes in the OC group (p>0.08; N=37). CR radial diffusivity was found to be elevated in the between-group comparison at baseline (mTBI1 vs. OC1), but did not differ in the within-group comparison (mTBI1 vs. mTBI2; N=19), suggesting the possible resolution of edema. Our analysis of the cross-sectional and follow-up data, which is uncorrected for multiple comparisons, demonstrates dissociation between volumetric (macrostructural) and tissue integrity (microstructural) attributes and shows the potential utility of DTI to capture transient edema in the CR.
Abstract Studies have been performed to evaluate the reproducibility of longitudinal acquisitions with short TE spectroscopic magnetic resonance (MR) imaging of human brain. In healthy volunteers, the ratios of N ‐acetyl‐aspartate (NAA) and choline (Cho) with respect to total creatine (Cr) (creatine and phosphocreatine) have been examined in terms of voxel‐to‐voxel, acquisition‐to‐acquisition, and subject‐to‐subject variation. Overall coefficients of variation for NAA/Cr ratios and Cho/Cr ratios were 18% and 16%, respectively. The interacquisition variation was not statistically significant ( P = .05) for either ratio. In terms of F ratios, the dominant source of variation in the NAA/Cr ratio was intersubject variability, whereas for the Cho/Cr ratio the dominant source of variation was intervoxel variability. Consequences of spectroscopic processing with spatial apodization and postacquisition water suppression and resolution enhancement were also examined. Such reproducibility studies are crucial if longitudinal spectroscopic evaluations are to be performed in a clinical setting to follow the natural course of disease states or the effects of therapy.
We have measured the ultrasonic attenuation parameters alpha o and n of several biological fluids in vitro in the frequency range 2.8-6.8 MHz, using frequency shift and/or a sinc (.) sidelobe ratio technique. The parameters alpha 0 and n describe the frequency-dependent attenuation via a power-law model, i.e., alpha(f) = alpha 0 fn, where f is frequency. The samples investigated were blood, pus, cyst fluid, bile, and infected hematoma. It was found that the values of alpha o span approximately the range between 0.03-0.3 dB cm-1 MHz-n, and values of n range from about 1.1-1.3.
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