Simon Vinitski

Fatty Liver: Chemical Shift Phas and Suppression Magnetic Resonance Imaging Techniques in Animals, Phantoms, and Humans

Mitchell DG, Kim I, Chang TS, et al. Fatty liver: chemical shift phase-difference and suppression magnetic resonance imaging techniques in animals, phantoms, and humans. Invest Radiol 1991;26:1041–1052. In vitro animal and human models were used to evaluate the potential of chemical shift magnetic r

Thickness of patellofemoral articular cartilage as measured on MR imaging: sequence comparison of accuracy, reproducibility, and interobserver variation

ObjectiveSince the thickness of cartilage is an important indicator of the status, progression and response to therapy of articular disorders, assessment of it is desirable. This study was undertaken to assess the accuracy, precision, and reliability of magnetic resonance (MR) measurements of articular cartilage.MethodsFifteen cadaveric patellas were imaged in the axial plane at 1.5 T. Gradient echo and fat-suppressed FSE, T2-weighted, proton density, and T 1-weighted sequences were performed. We measured each 5-mm section separately at three standardized positions, giving a total of 900 measurements. These findings were correlated with independently performed measurements of the corresponding anatomic sections. A hundred random measurements were also evaluated for reproducibility and interobserver variation.ResultsAlthough all sequences were highly accurate (range r=0.78–0.82), the Tl-weighted images were the most accurate, with a mean difference of 0.25 mm and a correlation coefficient of 0.85. All sequences were also highly reproducible (mean difference between -0.09 and 0.05 mm) with little inter-observer variation (mean difference −0.04 and 0.11 mm). In an attempt to improve the accuracy of the MR measurements further, we retrospectively evaluated all measurements with discrepancies greater than 1 mm from the specimen. All these differences were attributable to focal defects causing exaggeration of the thickness on MR imaging.ConclusionMR imaging is accurate, precise, and reliable as a basis for measuring articular cartilage and may potentially be usable to monitor progression of articular disorders. Care must be taken not to overestimate the thickness of areas with surface defects.

Fast tissue segmentation based on a 4D feature map in characterization of intracranial lesions.

The aim of this work was to develop a fast and accurate method for tissue segmentation in magnetic resonance imaging (MRI) based on a four‐dimensional (4D) feature map and compare it with that derived from a 3D feature map. High‐resolution MRI was performed in 5 normal individuals, in 12 patients with brain multiple sclerosis (MS), and 9 patients with malignant brain tumors. Three inputs (proton‐density, T2‐weighted fast spin‐echo, and T1‐weighted spin‐echo MR images) were routinely utilized. As a fourth input, either magnetization transfer MRT was used or T1‐weighted post‐contrast MRI (in patients only). A modified k‐nearest neighbor segmentation algorithm was optimized for maximum computation speed and high‐quality segmentation. In that regard, we a) discarded the redundant seed points; b) discarded the points within 0.5 standard deviation from the cluster center that were non‐overlapping with other tissue; and c) removed outlying seed points outside 5 times the standard deviation from the cluster center of each tissue class. After segmentation, a stack of color‐coded segmented images was created. Our new technique utilizing all four MRI inputs provided better segmentation than that based on three inputs (P < 0.001 for MS and P < 0.001 for tumors). The tissues were smoother due to the reduction of statistical noise, and the delineation of the tissues became sharper. Details that were previously blurred or invisible now became apparent. In normal persons a detailed depiction of deep gray matter nuclei was obtained. In malignant tumors, up to five abnormal tissue types were identified: 1) solid tumor core, 2) cyst, 3) edema in white matter 4) edema in gray matter, and 5) necrosis. Delineation of MS plaque in different stages of demyelination became much sharper. In conclusion, the proposed methodology warrants further development and clinical evaluation. J. Magn. Reson. Imaging 1999;9:768–776.

Bone marrow findings on magnetic resonance images of the knee: Accentuation by fat suppression

Long TR/double spin-echo magnetic resonance images of the knee were obtained with and without the use of fat suppression techniques in seven patients with high signal intramedullary lesions. Comparison between images was performed qualitatively and quantitatively. Contrast-to-noise ratios between focal defects and surrounding fatty marrow were higher with fat suppression in all cases. The mean contrast-to-noise ratio for images obtained with fat suppression was 53.6, while for images obtained without fat suppression the mean contrast-to-noise ratio was 17.3 (p less than 0.01).

Chondromalacia patellae: an in vitro study

Abstract Objective. To develop MR criteria for grades of chondromalacia patellae and to assess the accuracy of these grades. Design. Fat-suppressed T2-weighted double-echo, fat-suppressed T2-weighted fast spin echo, fat-suppressed T1-weighted, and gradient echo sequences were performed at 1.5 T for the evaluation of chondromalacia. A total of 1000 MR, 200 histologic, and 200 surface locations were graded for chondromalacia and statistically compared. Results. Compared with gross inspection as well as with histology the most accurate sequences were fat-suppressed T2-weighted conventional spin echo and fat suppressed T2-weighted fast spin echo, although the T1-weighted and proton density images also correlated well. The most accurate MR criteria applied to the severe grades of chondromalacia, with less accurate results for lesser grades. Conclusions. This study demonstrates that fat-suppressed routine T2-weighted and fast spin echo T2-weighted sequences seem to be more accurate than proton density, T1-weighted, and gradient echo sequences in grading chondromalacia. Good histologic and macroscopic correlation was seen in more severe grades of chondromalacia, but problems remain for the early grades in all sequences studied.

Fat suppression by saturation/opposed-phase hybrid technique: Spin echo versus gradient echo imaging

The objective of the study was to compare hybrid opposed-phase fat suppressed sequences using gradient-echo (GRE) and spin-echo (SE) techniques. Eight normal volunteers had abdominal imaging at 1.5 T using both an opposed-phase fat-suppressed SE sequence (TR/TE 450/14) and an opposed phase GRE sequence (TE = 2.5) with frequency-selective fat suppression. Signal to noise ratios (SNR) and contrast to noise ratios (CNR) relative to fat of pancreas, liver and paraspinal muscle were calculated for each sequence. The GRE sequence with TR of 150 (25 s breath-hold) significantly outperformed the corresponding 3 min SE sequence for SNR and CNR for all tissues (p < .05). Hybrid opposed-phase fat-suppressed GRE sequences can be performed with breath-holding in less overall imaging time than SE sequences, and can achieve superior fat suppression and signal-to-noise if sufficient TR is used.

Assessment of lung water by magnetic resonance in three types of pulmonary edema

SummaryPulmonary edema was produced in nine mongrel dogs by: (a) saline lavage; (b) intravenous injection of oleic acid; and (c) intravenous injection of propranolol followed by ureteral ligation. The resulting effect could be characterized by varying the protein concentration in the pulmonary edema fluid. After induction, all dogs were killed and 20 samples from each passively deflated lung were obtained. Proton T1 and T2 values were measured on a Praxis II NMR spectrometer operated at 10.7 MHz and 37°C. The water content of all samples was determined gravimetrically.Correlation between T1 or T2 measured in vitro and the ratio of wet to dry weight was highly significant (r>0.95,P<0.001) in each pathological state. Regression curves indicate that although all three types of pulmonary edema can be characterized by slightly different slopes, the differences are statistically insignificant. Moreover, the slopes of previous studies, when recast in the same format, are very similar to our findings despite the use of different magnetic field strengths and different animal models. This study indicates that quantitation of pulmonary edema is possible, but in vitro measurements do not give useful information for characterizing the etiology of pulmonary edema.

Tissue Segmentation in MRI as an Informative Indicator of Disease Activity in the Brain

The presented tissue segmentation technique is based on a multispectral analysis approach. The input data were derived from high resolution MR images. Usually, only two inputs, proton density (PD) and T2-weighted images, are utilized to calculate the 2D feature map. In our method, we introduced a third input, T1-weighted MR image, for segmentation based on 3D feature map. k-Nearest Neighborhood segmentation algorithm was utilized. Tissue segmentation was performed in phantoms, normal humans and those with brain tumors and MS. Our technique utilizing all three inputs provided the best segmentation (p<0.001). The inclusion of T1 based images into segmentation produced dramatic improvement in tissue identification. Using our method, we identified the two distinctly different classes of tissue within the same MS plaque. We presume that these tissues represent the different stages involved in the evolution of the MS lesions. Further, our methodology for measuring MS lesion burden was also used to obtain its regional distribution as well as to follow its changes over time. The segmentation results were in full accord with neuropsychological findings.

Tissue segmentation by high resolution MRI: improved accuracy and stability

Tissue segmentation based on 2D and 3D feature maps derived from high resolution MR images was performed in experimental brain edema in cats, normal humans and those with brain tumors and MS. Statistical and anisotropic diffusion filters were applied to the data. The k-nearest neighborhood segmentation algorithm was utilized. Segmentation based on a 3D feature map was found to be much better than that based on a 2D feature map (p<0.01). Inter-observer variability ranged between 6.6% and 9.3% and intra-observer variability ranged between 8.3% and 10.4%. The use of high resolution MRI resulted in detection of new lesions which well correlated with neuropsychological exams. In conclusion, the presented technique is an accurate, stable and promising method of tissue characterization.<<ETX>>

Partial angle inversion recovery (pair) MR imaging: Spin-echo and snapshot implementation

The effects of varying the inversion or excitation RF pulse flip angles on image contrast and imaging time have been investigated in IR imaging theoretically, with phantoms and with normal volunteers. Signal intensity in an IR pulse sequence as a function of excitation, inversion and refocusing pulse flip angles was calculated from the solution to the Bloch equations and was utilized to determine the contrast behavior of a lesion/liver model. Theoretical and experimental results were consistent with each other. With the TI chosen to suppress the fat signal, optimization of the excitation pulse flip angle results in an increase in lesion/liver contrast or allows reduction in imaging time which, in turn, can be traded for an increased number of averages. This, in normal volunteers, improved spleen/liver contrast-to-noise ratio (9.0 vs. 5.7, n = 8, p less than 0.01) and suppressed respiratory ghosts by 33% (p less than 0.01). Reducing or increasing the inversion pulse from 180 degrees results in shorter TI needed to null the signal from the tissue of interest. Although this decreases the contrast-to-noise ratio, it can substantially increase the number of sections which can be imaged per given TR in conventional IR imaging or during breathold in the snapshot IR (turboFLASH) technique. Thus, the optimization of RF pulses is useful in obtaining faster IR images, increasing the contrast and/or increasing the number of imaging planes.