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R V Mulkern

Publications and source records attributed to R V Mulkern.

At least 19 recordsLinked to original sources
PubMed1999

Multi-component apparent diffusion coefficients in human brain.

The signal decay with increasing b-factor at fixed echo time from brain tissue in vivo has been measured using a line scan Stejskal-Tanner spin echo diffusion approach in eight healthy adult volunteers. The use of a 175 ms echo time and maximum gradient strengths of 10 mT/m allowed 64 b-factors to be sampled, ranging from 5 to 6000 s/ mm2, a maximum some three times larger than that typically used for diffusion imaging. The signal decay with b-factor over this extended range showed a decidedly non-exponential behavior well-suited to biexponential modeling. Statistical analyses of the fitted biexponential parameters from over 125 brain voxels (15 x 15 x 1 mm3 volume) per volunteer yielded a mean volume fraction of 0.74 which decayed with a typical apparent diffusion coefficient around 1.4 microm2/ms. The remaining fraction had an apparent diffusion coefficient of approximately 0.25 microm2/ms. Simple models which might explain the non-exponential behavior, such as intra- and extracellular water compartmentation with slow exchange, appear inadequate for a complete description. For typical diffusion imaging with b-factors below 2000 s/mm2, the standard model of monoexponential signal decay with b-factor, apparent diffusion coefficient values around 0.7 microm2/ms, and a sensitivity to diffusion gradient direction may appear appropriate. Over a more extended but readily accessible b-factor range, however, the complexity of brain signal decay with b-factor increases, offering a greater parametrization of the water diffusion process for tissue characterization.

Adult↗
PubMed1999

Partial wavelet encoding: a new approach for accelerating temporal resolution in contrast-enhanced MR imaging.

We propose a new approach using wavelet encoding to improve temporal resolution in contrast-enhanced magnetic resonance (MR) imaging. Exploiting the unique property of wavelets localized in space and frequency, we construct an efficient encoding scheme to capture signal changes due to contrast agent uptake, which in general is spatially localized with low- and mid-range frequency components. On the basis of space-frequency analysis, we describe mathematical formulations of our method and discuss its theoretical advantages over Fourier-based phase-encoding methods (the keyhole and reduced-encoding imaging by generalized-series reconstruction [RIGR] techniques). The results obtained in computer simulations and a phantom study demonstrate the feasibility and practical advantages of our approach.

Computer Simulation↗
PubMed1999

Legg-Calvé-Perthes disease: MR imaging evaluation during manual positioning of the hip--comparison with conventional arthrography.

PURPOSE: To evaluate the use of magnetic resonance (MR) imaging during manual positioning of the hip, or multipositional MR imaging, in an open-magnet configuration to study femoral head containment, articular congruency, and femoral head deformity in Legg-Calvé-Perthes disease. MATERIALS AND METHODS: In 12 children with advanced Legg-Calvé-Perthes disease, multipositional MR imaging and conventional arthrography were compared in the assessment of containment, femoroacetabular congruency, and femoral head deformity. Images of the hips in several positions were compared subjectively and objectively. RESULTS: MR imging correlated well with arthrography for overall subjective assessment of severity of disease (r = 0.71, P = .01), with good interobserver agreement (kappa = 0.65, P < .001). MR images demonstrated all cases of hinge abduction shown arthrographically. However, MR imaging failed to depict one case of femoral head flattening. MR imaging correlated well with arthrography in the objective evaluation of joint fluid and lateral subluxation (r = 0.80, P < .01). MR imaging correlated poorly with arthrography in the measurement of sphericity of the femoral head. CONCLUSION: Multipositional MR imaging with an open-magnet configuration was comparable to arthrography for demonstration of femoral head containment and congruency of the articular surfaces of the hip. In the evaluation of deformity, it performed less well.

Arthrography↗
PubMed1999

MR line scan diffusion imaging of the brain in children.

BACKGROUND AND PURPOSE: MR imaging of the self-diffusion of water has become increasingly popular for the early detection of cerebral infarction in adults. The purpose of this study was to evaluate MR line scan diffusion imaging (LSDI) of the brain in children. METHODS: LSDI was performed in four volunteers and 12 patients by using an effective TR/TE of 2736/89.4 and a maximum b value of 450 to 600 s/mm2 applied in the x, y, and z directions. In the volunteers, single-shot echo planar imaging of diffusion (EPID) was also performed. The patients (10 boys and two girls) ranged in age from 2 days to 16 years (average age, 6.6 years). Diagnoses included acute cerebral infarction, seizure disorder, posttraumatic confusion syndrome, complicated migraine, residual astrocytoma, encephalitis, hypoxia without cerebral infarction, cerebral contusion, and conversion disorder. In all patients, routine spin-echo images were also acquired. Trace images and apparent diffusion coefficient maps were produced for each location scanned with LSDI. RESULTS: In the volunteers, LSDI showed less chemical-shift and magnetic-susceptibility artifact and less geometric distortion than did EPID. LSDI was of diagnostic quality in all studies. Diffusion abnormalities were present in five patients. Restricted diffusion was present in the lesions of the three patients with acute cerebral infarction. Mildly increased diffusion was present in the lesions of encephalitis and residual cerebellar astrocytoma. No diffusion abnormalities were seen in the remaining seven children. CONCLUSION: LSDI is feasible in children, provides high-quality diffusion images with less chemical-shift and magnetic-susceptibility artifact and less geometric distortion than does EPID, and complements the routine MR examination.

Acute Disease↗
PubMed1998

Optimization of chemical shift selective suppression of fat.

Strategies to optimize flip angles for chemical shift selective fat suppression are discussed. Mathematical models for fat suppression in spoiled gradient recalled acquisition, spin echo, and RARE, which incorporate steady state conditions and multiple spectral components of fat, are developed. The optimal suppression flip angle is found to be larger than that determined with a single fat component model by more than 10 degrees due to contributions from unflipped components such as olefinic and glycerol protons that lie outside the suppression band.

Adipose Tissue↗
PubMed1998

Developing epiphysis: MR imaging characteristics and histologic correlation in the newborn lamb.

PURPOSE: To correlate magnetic resonance (MR) signal characteristics of developing regions at the ends of bones with the histologic findings. MATERIALS AND METHODS: In 36 newborn lamb epiphyses (including cartilage of the epiphysis and physis, and bone of the secondary ossification center and juxtaphyseal metaphysis), signal intensity and thickness of cartilaginous regions seen on MR images were correlated with architecture and thickness of zones shown in corresponding histologic sections. Possible effects of T2, magnetization transfer, fat or water content, chemical shift, and anisotropy on differences in regional signal intensity were evaluated. RESULTS: MR imaging depicted five regions between the secondary ossification center and the metaphysis corresponding histologically to (a) zone of provisional calcification of the secondary ossification center, (b) physis of the secondary ossification center, (c) epiphyseal cartilage, (d) physis, and (e) zone of provisional calcification. The thickness of regions shown on T1- and T2-weighted images correlated with that of histologic zones (r = .9). T2 signal intensity and relaxation time were greater in physeal than in epiphyseal cartilage (P < .01). No regional differences due to fat or water content, magnetization transfer, chemical shift, or anisotropy were found. CONCLUSION: MR imaging findings differentiate epiphyseal and physeal regions and correlate with histologic findings. T2 is slower in physeal than in epiphyseal cartilage, probably reflecting differences in water binding.

Animals↗
PubMed1998

MR imaging of pelvic floor continence mechanisms in the supine and sitting positions.

OBJECTIVE: Our goal was to determine the anatomic differences in pelvic floor anatomy for continent women and women with stress incontinence using MR imaging and to assess whether these anatomic differences depend on the position of the subject during imaging. SUBJECTS AND METHODS: Eight continent volunteers and eight women with stress incontinence underwent MR imaging in the supine and sitting positions. For imaging, we used an open-configuration 0.5-T magnet. T2-weighted images were obtained in the midline sagittal plane with subjects at rest and at maximal pelvic floor strain in 5 sec using a modified fast spin-echo sequence. In the axial plane, thin-section T2-weighted images were obtained with subjects at rest using a standard fast spin-echo technique. Images were evaluated for mobility of the urethra and bladder neck and for integrity of the vagina, levator ani, and supporting fascia. RESULTS: Pelvic floor laxity and abnormalities of the supporting fascia were more common in incontinent women than in continent women. Both descent of the bladder neck when subjects strained and the posterior urethrovesical angle were not significantly greater when subjects were measured in the sitting position (p < .1). CONCLUSION: Changes of pelvic floor laxity related to incontinence can be seen with subjects in both the supine and the sitting positions but are increased in the sitting position.

Adult↗
PubMed1998

Diffusion-weighted MR imaging in hypertensive encephalopathy: clues to pathogenesis.

PURPOSE: Hypertensive encephalopathy, a complex of cerebral disorders, including headache, seizures, visual disturbances, and other neurologic manifestations, is associated with a variety of conditions in which blood pressure rises acutely. It has been ascribed to either exuberant vasospasm with ischemia/infarction or breakthrough of autoregulation with interstitial edema. Diffusion-weighted MR imaging may be used to determine whether the edema in hypertensive encephalopathy is cytotoxic or vasogenic in origin. METHODS: Diffusion-weighted imaging was performed using the double line scan diffusion imaging technique on a 1.5-T MR system. Seven patients with hypertensive encephalopathy were imaged within 1 day of the onset of their symptoms. Apparent diffusion coefficient maps as well as low and high b-factor images were acquired. The two-tailed paired Student's t-test was used to compare the apparent diffusion coefficients in edematous brain regions with those of normal white matter. RESULTS: In all cases the apparent diffusion coefficient maps of the patients with hypertensive encephalopathy showed increased signal in regions corresponding to increased T2 signal on standard T2-weighted (low b-factor) images. Quantitative apparent diffusion coefficients in regions of abnormal T2 signal were 1.36 +/- 0.14 microm2/ms, compared with 0.80 +/- 0.05 microm2/ms in normal white matter. Diffusion-weighted (high b-factor) T2-weighted images did not show abnormal signal. CONCLUSION: Diffusion-weighted MR imaging shows that the edema in hypertensive encephalopathy is of vasogenic origin and does not represent ischemia or infarction. This finding may have therapeutic implications.

Adult↗
PubMed1997

Multiecho approaches to spectroscopic imaging of the brain.

Spectroscopic imaging (SI) with nuclear magnetic resonance (NMR) is one of the most powerful tools available for studying brain chemistry in vivo. Both proton (1H) and phosphorus (31P) NMR offer valuable biochemical information that can in principle be mapped throughout the entire brain, thereby enhancing our understanding of brain function. With the exception of protons from tissue water and the triglycerides of adipose tissue, however, nuclei contributing to the NMR signals of living tissue are in relatively small (millimolar) concentrations. The low concentration of metabolite nuclei reduces the overall sensitivity of conventional SI techniques, making high-quality metabolite mapping a lengthy procedure. This problem has led to the development and testing of nonconventional methods for reducing SI scan times, including techniques based on the collection of multiple spin-echoes. The extent to which multiecho methods can be used to decrease SI scan times and maintain high-quality metabolite mapping depends on several factors. These include the spectral transverse relaxation times, the spectral resolution required, and J-coupling interactions. We have discussed these various technical aspects of multiecho SI methods as applied to 1H and 31P spectroscopic imaging of the living brain.

Animals↗
PubMed1997

Non-Fourier encoding with multiple spin echoes.

The advantages and limitations of multiple spin-echo sequences for non-Fourier encoding are investigated. Complications caused by improper encoding of alternate magnetization pathways due to imperfect refocusing pulses are analyzed. It is shown that mirror image ghosts result if the encoding RF pulse matrix is real-valued. These ghosts can be avoided as long as the rows of the RF pulse matrix are conjugate symmetric, which implies that spatial profiles are real valued. Non-Fourier encoding using bases derived from wavelet, Hadamard, and other real-valued orthogonal functions does not result in a mirror ghost artifact. A RARE sequence for non-Fourier encoding has been implemented on a clinical imaging system and successfully applied for brain imaging.

Artifacts↗
PubMed1997

In vivo bone marrow lipid characterization with line scan Carr-Purcell-Meiboom-Gill proton spectroscopic imaging.

Line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging sequences have been used to extract lipid chemical composition indices in healthy adult bone marrow in the knee at 1.5 T. Since several spectroscopic echo readouts follow each excitation, the information acquired reflects a balance between spectral T2 decay processes and spectral resolution. To examine this balance in detail, data sets with two different echo spacings and spectral resolutions have been acquired to compare the information available from each in studies of bone marrow. Oils for which high field (7 T) proton spectra were recorded were used to evaluate the accuracy of lipid chemical composition indices extracted from the line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging methods at 1.5 T. The extension of the method to fast spectroscopic imaging of bone marrow with multiple echoes is demonstrated.

Adult↗
PubMed1997

Temperature monitoring of ultrasonically heated muscle with RARE chemical shift imaging.

The ability to monitor tissue temperature in ultrasonically heated rabbit muscle is demonstrated using a chemical shift imaging approach based on the rapid acquisition with relaxation enhancement (RARE) fast imaging method [Hennig et al., Magn. Reson. Med. 3, 823-833 (1986)] applied in a line scan format. A three echo sequence with a 16 Hz spectral resolution with 64 ms echo readouts and 78 ms echo spacings is shown capable of measuring relevantly small water frequency shifts in phantoms. Applied to the in vivo model of ultrasonically heated rabbit muscle, water resonance frequencies at the ultrasonic focal point were found to be linearly related to temperature with a slope of -0.007 +/- 0.001 ppm/degree C (N = 6 studies). Measurements of the frequency shift in unheated tissue located away from the ultrasonically heated tissue varied by approximately 0.011 ppm over the course of the experiments, leading to an estimated temperature accuracy of +/- 1.6 degrees C in vivo.

Animals↗
PubMed1996

A multiparameter analysis of sickle erythrocytes in patients undergoing hydroxyurea therapy.

During 24 weeks of hydroxyurea treatment, we monitored red blood cell (RBC) parameters in three patients with sickle cell disease, including F-cell and F-reticulocyte profiles, distributions of delay times for intracellular polymerization, sickle erythrocyte adherence to human umbilical vein endothelial cells in a laminar flow chamber, RBC phthalate density profiles, mean corpuscular hemoglobin concentration and cation content, reticulocyte mean corpuscular hemoglobin concentration, 1H-nuclear magnetic resonance transverse relaxation rates of packed RBCs, and plasma membrane lateral and rotational mobilities of band 3 and glycophorins. Hydroxyurea increases the fraction of cells with sufficiently long delay times to escape the microcirculation before polymerization begins. Furthermore, high pretreatment adherence to human umbilical vein endothelial cells of sickle RBCs decreased to normal after only 2 weeks of hydroxyurea treatment, preceding the increase in fetal hemoglobin levels. The lower adhesion of sickle RBCs to endothelium would facilitate escape from the microcirculation before polymerization begins. Hydroxyurea shifted several biochemical and biophysical parameters of sickle erythrocytes toward values observed with hemoglobin SC disease, suggesting that hydroxyurea moderates sickle cell disease toward the milder, but still clinically significant, hemoglobin SC disease. The 50% reduction in sickle crises documented in the Multicenter Study of Hydroxyurea in Sickle Cell Disease is consistent with this degree of erythrocyte improvement.

Adult↗
PubMed1996

Relative phosphocreatine and nucleoside triphosphate concentrations in cerebral gray and white matter measured in vivo by 31P nuclear magnetic resonance.

Rates of ATP metabolism generally are higher in cerebral gray matter compared to white matter. In order to study the physiology of this regional difference in vivo, the 1-dimensional chemical shift imaging technique (1D-CSI) was used to acquire 31P nuclear magnetic resonance spectra from 2.5 mm slices of 4-week old piglet brains. Spectra from predominantly gray matter slices (estimated 76% gray matter, 7 mm below the scalp) were compared to predominantly white matter slices (56% estimated white matter, 13 mm below the scalp) as assessed by magnetic resonance images. The 1D-CSI technique introduced no systematic changes in the ratio of signals from a single chamber phantom containing a phosphocreatine (PCr) and ATP solution. Gray matter slices showed a PCr/NTP ratio of 0.93 +/- 0.11 (mean +/- S.D.) using a 2 s interpulse interval, a value very close to the ratio in surface coil localized spectra. The predominantly white matter slices showed a PCr/NTP ratio of 1.32 +/- 0.18 (P < 0.02 for gray versus white matter). Using the estimated percentages of gray and white matter in the two slices and calculated concentrations from fully relaxed spectra, the gray matter PCr/NTP ratio is approximately 0.77, while the ratio in white matter is approximately 2.18. The difference in PCr/NTP measured in vivo suggests that either the total NTP concentration is higher or the steady state PCr concentration is lower in gray matter than in white matter in the piglet brain.

Adenosine Triphosphate↗
PubMed1996

Line scan diffusion imaging.

A novel line scan diffusion imaging sequence (LSDI) is introduced. LSDI is inherently insensitive to motion artifacts and high quality diffusion maps of the brain can be obtained rapidly without the use of head restraints or cardiac gating. Results from a stroke study and abdominal diffusion images are presented. The results indicate that it is feasible to use the LSDI technique for clinical evaluation of acute ischemic stroke. In contrast to echo-planar diffusion imaging, LSDI does not require modified gradient hardware and can be implemented on conventional scanners. Thus, LSDI should dramatically increase the general availability of robust clinical diffusion imaging.

Abdomen↗
PubMed1996

Citrate signal enhancement with a homonuclear J-refocusing modification to double-echo PRESS sequences.

The citrate signal at field strengths of whole body imagers arises from two sets of two strongly coupled methylene protons. This causes citrate spectra acquired with standard in vivo localization schemes like the double-echo point resolved echo spectroscopy (PRESS) sequence to have complicated dependencies on timing parameters. A homonuclear J-refocused version of the double-echo PRESS sequence that has previously been shown to completely remove J-modulations from weakly coupled AX systems is considered for its potential in acquiring signal from the strongly coupled AB system of citrate. An analytic solution to the problem is derived with the density matrix formalism and verified both numerically and experimentally for 7 T conditions. The general expression for the AB signal is applied to study the 1.5 T citrate signal where a substantial signal enhancement over conventional double-echo PRESS sequences is predicted and verified for echo times in the 150 to 300 ms range.

Citrates↗
PubMed1996

Density-matrix calculations of the 1.5 T citrate signal acquired with volume-localized STEAM sequences.

Citrate detection and quantitation with proton spectroscopic methods are of current interest as potential tools in the diagnosis and staging of prostate cancer. The stimulated echo acquisition mode (STEAM) sequence is a commonly used volume-localization method for detecting citrate signal. Since the 1H citrate resonance at clinically available field strengths arises from a strongly coupled two-spin system, the 90 degrees RF pulses and localizing gradients used in STEAM sequences result in a complicated dependence of signal intensity on timing intervals and gradient amplitudes. The density-matrix formalism has been applied to arrive at a general solution to this problem. Citrate-signal properties at 1.5 T for different gradient localization schemes are examined with the solution. Optimal interpulse delays, deleterious gradient balances, zero-quantum oscillations with mixing time, and a low-frequency, large-amplitude oscillation with echo time are identified for signals acquired with the standard disposition of gradients in STEAM. The generality of the solution also allows for an examination of non-standard gradient disposition schemes for enhancing citrate signal and for quantifying the sensitivity of such approaches to both field inhomogeneities and off-resonance effects.

Citric Acid↗