PubMed Health⌕ Search

Biomedical subjects

R V Mulkern

Publications and source records attributed to R V Mulkern.

At least 19 recordsLinked to original sources

Magnetic resonance and the human brain: anatomy, function and metabolism.

The introduction and development, over the last three decades, of magnetic resonance (MR) imaging and MR spectroscopy technology for in vivo studies of the human brain represents a truly remarkable achievement, with enormous scientific and clinical ramifications. These effectively non-invasive techniques allow for studies of the anatomy, the function and the metabolism of the living human brain. They have allowed for new understandings of how the healthy brain works and have provided insights into the mechanisms underlying multiple disease processes which affect the brain. Different MR techniques have been developed for studying anatomy, function and metabolism. The primary focus of this review is to describe these different methodologies and to briefly review how they are being employed to more fully appreciate the intricacies associated with the organ, which most distinctly differentiates the human species from the other animal forms on earth.

Brain↗

Echo-time independent signal modulations using PRESS sequences: a new approach to spectral editing of strongly coupled AB spin systems.

In clinical MR spectroscopy, double spin-echo point resolved spectroscopy (PRESS) sequences are routinely used for volume selection. For strongly coupled AB spin systems under PRESS excitation, the dependence of the signal on the echo time TE has been thoroughly investigated, whereas less attention has been paid to the signal modulation which occurs at constant TE with varying interpulse delays. A substantial TE-independent J modulation is here predicted from analytical solutions of the Liouville equation and density matrix simulations, and verified with experiments on citrate at 1.5 and 3T. It is also shown that this modulation effect could be exploited for editing of strongly coupled AB resonances or for removal of singlets in spectra-by means of difference spectroscopy-just using a standard PRESS sequence. The applicability in vivo of this new spectral editing approach is also demonstrated, with selective detection of citrate resonances in the human prostate. This novel approach has the advantages of being simple, and directly applicable on standard clinical MR scanners, provided that the exact behavior of the resonance is known.

Citric Acid↗

DTI and MTR abnormalities in schizophrenia: analysis of white matter integrity.

Diffusion tensor imaging (DTI) studies in schizophrenia demonstrate lower anisotropic diffusion within white matter due either to loss of coherence of white matter fiber tracts, to changes in the number and/or density of interconnecting fiber tracts, or to changes in myelination, although methodology as well as localization of such changes differ between studies. The aim of this study is to localize and to specify further DTI abnormalities in schizophrenia by combining DTI with magnetization transfer imaging (MTI), a technique sensitive to myelin and axonal alterations in order to increase specificity of DTI findings. 21 chronic schizophrenics and 26 controls were scanned using Line-Scan-Diffusion-Imaging and T1-weighted techniques with and without a saturation pulse (MT). Diffusion information was used to normalize co-registered maps of fractional anisotropy (FA) and magnetization transfer ratio (MTR) to a study-specific template, using the multi-channel daemon algorithm, designed specifically to deal with multidirectional tensor information. Diffusion anisotropy was decreased in schizophrenia in the following brain regions: the fornix, the corpus callosum, bilaterally in the cingulum bundle, bilaterally in the superior occipito-frontal fasciculus, bilaterally in the internal capsule, in the right inferior occipito-frontal fasciculus and the left arcuate fasciculus. MTR maps demonstrated changes in the corpus callosum, fornix, right internal capsule, and the superior occipito-frontal fasciculus bilaterally; however, no changes were noted in the anterior cingulum bundle, the left internal capsule, the arcuate fasciculus, or inferior occipito-frontal fasciculus. In addition, the right posterior cingulum bundle showed MTR but not FA changes in schizophrenia. These findings suggest that, while some of the diffusion abnormalities in schizophrenia are likely due to abnormal coherence, or organization of the fiber tracts, some of these abnormalities may, in fact, be attributed to or coincide with myelin/axonal disruption.

Adolescent↗

Dairy cream as a phantom material for biexponential diffusion decay.

Commercially available aliquots of dairy cream are shown to have diffusion decay curves characterized by biexponential functions when studied over a wide range of b-factors. The fast and slow diffusion components responsible for the biexponential decay are attributed to water and lipid protons, respectively. The fast diffusion coefficient and relative fast and slow diffusion component fractions obtained from biexponential fits of cream phantoms over a wide range of b-factors up to 3,000 s/mm2 are similar to those obtained previously for brain. The slow diffusion coefficient from lipid protons is smaller than that found in the brain. Overall, however, the results suggest that dairy cream can serve as a widely available phantom material for testing software and hardware components designed to perform quantitative, biexponential diffusion decay studies.

Animals↗

Prolonged T*2 values in newborn versus adult brain: Implications for fMRI studies of newborns.

The neonatal brain possesses higher water content, lower macromolecular concentration, and reduced synaptic density than is found in the brain of a 1-year-old child. Changes in MRI characteristics of brain such as relaxation times accompany rapid changes in brain during early postnatal development. It was hypothesized that T(*)(2) values found in newborns would be significantly higher than those found in 9-month-old children and adults as measured at 1.5 T. Spoiled gradient echo measurements of T(*)(2) within the brains of newborns, 9-month-olds, and adults confirmed this hypothesis. The results have implications with regard to functional MRI studies in newborns since, in general, BOLD signal optimization is achieved when echo times TE are set equal to the T(*)(2) values of the tissue of interest. Since significantly longer T(*)(2) values are found in newborns, it is suggested that the TE values employed for fMRI studies of newborns should be increased to maximize BOLD signal intensity changes and improve the overall reliability of fMRI results in newborns.

Adult↗

Dynamic imaging with multiple resolutions along phase-encode and slice-select dimensions.

An implementation is reported of an imaging method to obtain MUltiple Resolutions along Phase-encode and Slice-select dimensions (MURPS), which enables dynamic imaging of focal changes using a graded, multiresolution approach. MURPS allows one to trade spatial resolution in part of the volume for improved temporal resolution in dynamic imaging applications. A unique method of Hadamard slice encoding is used, enabling the varying of the phase encode and slice resolution while maintaining a constant effective TR throughout the entire 3-D volume. MURPS was implemented using a gradient-recalled echo sequence, and its utility was demonstrated for MR temperature monitoring. In this preliminary work, it has been shown that changes throughout a large volume can be effectively monitored in times that would normally only permit dynamic imaging in one or a very few slices.

Body Temperature↗

Osmotic effects on the T2 relaxation decay of in vivo muscle.

Saline solutions are commonly employed as a vehicle for drugs administered intramuscularly. In this study, in vivo measurements of spin-spin relaxation (T2) processes by magnetic resonance imaging (MRI) were performed to investigate the distribution of water in rat masseter muscle tissue after intramuscular injection of saline solutions of varying tonicity. Prior to saline injection, image-based T2 relaxation decay of muscle was monoexponential. After injection of saline, the T2 relaxation decay became multiexponential. Non-negative least squares (NNLS) analysis of the decay curves revealed two relaxation components: a fast component (T2 = 20-40 ms) and a slow component (T2 = 150-400 ms), which are assigned to intra- and extracellular water protons, respectively. Injection of hypertonic saline solutions significantly increased the extracellular water component in muscle tissue compared to isotonic saline solutions, an effect which lasted for more than 60 min. These findings suggest that MRI techniques may be useful to investigate the effect of hyper- or hypotonic solutions on muscle tissue in vivo.

Animals↗

Biexponential apparent diffusion coefficient parametrization in adult vs newborn brain.

The decay of brain water signal with b-factor in adult and newborn brains has been measured over an extended b-factor range. Measurements of the apparent diffusion coefficient (ADC) decay curves were made at 16 b-factors from 100 to 5000 s/mm(2) along three orthogonal directions using a line scan diffusion imaging (LSDI) sequence to acquire data from 0.09 ml voxels in a mid-brain axial slice. Regions-of-interest (ROIs) in cortical gray (CG) and white matter in the internal capsule (IC) were selected for ADC decay curve analyses using a biexponential fitting model over this extended b-factor range. Measures of the fast and slow ADC component amplitudes and the traces of the fast and slow diffusion coefficients were obtained from CG and IC ROIs in both adults and newborns. The ADC decay curves from the newborn brain regions were found to have a significantly higher fraction of the fast diffusion ADC component than corresponding regions in the adult brain. The results demonstrate that post-natal brain development has a profound affect on the biexponential parameters which characterize the decay of water signal over an extended b-factor range in both gray and white matter.

Adult↗

Temperature monitoring with line scan echo planar spectroscopic imaging.

UNLABELLED: A new magnetic resonance imaging method, line scan echo planar spectroscopic imaging (LSEPSI), is shown capable of providing rapid, internally referenced temperature monitoring from water and fat chemical shifts. METHODS: Orthogonal 90 degrees and 180 degrees slice selective RF pulses inclined by 45 degrees from the image plane solicit a spin echo from a tissue column. The echo is read by asymmetric sampling of 32 gradient echoes spaced 1.4-1.8 ms apart. Sixty-four adjacent columns are sequentially sampled in 4.2-6.4 s with 4,096 voxels sampled with voxel volumes of 0.08-0.13 cm3. Mixed mayonnaise/water phantoms were used to correlate LSEPSI-derived chemical shifts and thermocouple-based temperature measurements from 23 to 60 degrees C with a 1.5 T scanner. Measurement artifacts unrelated to temperature were investigated with the phantom, as was the feasibility of applying the sequence in human breast in vivo. RESULTS: The correlation between LSEPSI and thermocouple-based temperature measurements in the phantom was excellent (r2>0.99). Field drifts affecting the temperature measurements using the water peak alone were corrected by using the water/lipid peak difference. The sequence had an average temperature resolution of 1.4 degrees C in the phantom. The frequency difference measurement reduced the sensitivity to artifacts related to temperature. Both water and lipid peaks were detectable throughout many locations in the breast, suggesting the applicability of LSEPSI in this organ. DISCUSSION: T1-saturation losses occur in conventional and echo-planar based 2D CSI sequences using phase encoding methods with short TR periods. These losses are eliminated when individual columns are sampled in snapshot fashion with LSEPSI since the effective TR becomes the time between scans rather than excitations. T1 saturation can make small spectral peaks difficult to detect at high temperatures and generally lowers the signal-to-noise ratio of the spectra. The rapid acquisition and insensitivity to T1 saturation effects make LSEPSI an attractive technique for monitoring thermal therapies in breast using the internally referenced fat/water frequency separation.

Algorithms↗

Normal brain and brain tumor: multicomponent apparent diffusion coefficient line scan imaging.

Magnetic resonance line scan diffusion imaging of the brain, with diffusion weighting between 5 and 5,000 sec/mm(2), was performed in healthy subjects and patients with a 1.5-T machine. For each voxel, biexponential signal decay fits produced two apparent diffusion constants and respective signal amplitudes. Images based on these parameters show potential for use in the differentiation of gray and white matter, edema, and tumor.

Brain↗

Fast three-point dixon MR imaging using low-resolution images for phase correction: a comparison with chemical shift selective fat suppression for pediatric musculoskeletal imaging.

OBJECTIVE: The purpose of this study is to describe and to implement a new fast three-point Dixon MR imaging sequence with online image reconstruction, and to compare this sequence with conventional chemical shift selective (CHESS) suppression of fat in pediatric musculoskeletal imaging. SUBJECTS AND METHODS: A three-point Dixon technique using a fast spin-echo sequence with a new phase-correction algorithm providing online image reconstruction was implemented on a 1.5-T scanner. Twelve pediatric patients and young adults were imaged with both the new three-point Dixon and conventional CHESS sequences. Three radiologists un-aware of imaging parameters and clinical information independently scored the homogeneity of fat suppression and conspicuity of abnormality using a four-point system. An additional comparison between the two techniques was made using a phantom. RESULTS: The three-point Dixon method showed superior fat suppression and lesion conspicuity (p < 0.001), particularly in the hands and feet, where CHESS is prone to inconsistent fat suppression. The phantom study showed no significant difference in the ratio of suppressed fat signal to background noise and more homogeneous fat suppression using the three-point Dixon method. CONCLUSION: Compared with CHESS, the new fast three-point Dixon sequence with online image reconstruction provides superior fat suppression and lesion conspicuity and can be routinely used in pediatric musculoskeletal imaging.

Adolescent↗

Fast three-point Dixon MR imaging of the retrobulbar space with low-resolution images for phase correction: comparison with fast spin-echo inversion recovery imaging.

A new phase-correction algorithm for three-point Dixon (3PD) MR imaging allows on-line image reconstruction of three images per section: pure water, pure fat, and water plus fat. When combined with fast spin-echo acquisition, the sequence is suitable for routine MR imaging of the retrobulbar space. The 3PD pure water images have double the image signal-to-noise ratio of fast spin-echo inversion recovery images. The dramatic contrast-to-noise ratio of the 3PD pure fat images may offer improved lesion detection.

Adolescent↗

[Fatty liver quantification with line scan echo planar spectroscopic imaging (LSEPSI)].

INTRODUCTION: Although fatty infiltration of liver is a benign process that generally results from chronic alcohol uptake or obesity, such lifestyle factors may lead to chronic disease. Measuring the fat concentration in liver may therefore prove useful in assessing disease status. In this study, we report the usefulness of line scan echo planar spectroscopic imaging (LSEPSI) for this problem. METHODS: Rapid successive column sampling was accomplished using orthogonal slice-selective 90 degrees and 180 degrees pulses and echo planar spectral/spatial encodings. Phantom and clinical studies of 13 patients suspected of having fatty liver were carried out with LSEPSI. Estimated fat fractions obtained with LSEPSI were compared with ultrasound findings. RESULTS: The results showed a good correlation between the actual fat content of phantoms and the estimated fat fraction obtained with LSEPSI (r = 0.95). In the clinical study, the estimated fat fraction tended to rise as the US grade of fatty liver increased. DISCUSSION: LSEPSI is largely free from T1 and T2 relaxation owing to its infinite TR and minimal T2 weighting. Thus, there is no need for relaxation analysis. In addition, the lack of phase encoding reduces motion-related ghosting artifacts. Rapid fat/water spectral quantification of liver with this technique is useful for fatty liver assessment in a clinical setting.

Adult↗

An image processing strategy for the quantification and visualization of exercise-induced muscle MRI signal enhancement.

Exercise increases the skeletal muscle water signal in T2-weighted images. Potential medical applications of MR studies of exercise-induced muscle signal intensity changes are the assessment of myopathies, sport training regimens, and physical therapy approaches following surgeries. We developed an automated image processing technique that provides volumetric analysis and visualization of exercise-related T2-weighted image intensity changes. The image processing was applied to the segmentation and quantification of activated muscle volumes. Qualitative assessment of muscle activation is demonstrated with three-dimensional surface rendering. Quantitative determination of active muscle volume, signal intensity, and change over time is demonstrated. Visualization of the activated muscles allows functional anatomical assessment of exercise, which in turn allows detection of muscle utilization.

Exercise↗

Temperature mapping using the water proton chemical shift: self-referenced method with echo-planar spectroscopic imaging.

An echo-planar spectroscopic imaging method of temperature mapping is proposed. This method is sufficiently faster than the so-called 3D magnetic resonance spectroscopic imaging (3D-MRSI) method and does not require image subtractions, unlike the conventional phase mapping method when an internal reference signal is detectable. The water proton chemical shift measured by using the tissue lipid as an internal reference clearly visualized the temperature change in a porcine liver sample in vitro. It was also demonstrated that the internally referenced echo-planar spectroscopic imaging method could markedly reduce a temperature error caused by a simple, translational motion between scans compared with the phase-mapping method.

Animals↗

MR-based temperature monitoring for hot saline injection therapy.

We applied magnetic resonance (MR) phase mapping methods to monitor the thermal frequency shift of water in order to study temperature changes from percutaneous hot saline injection therapy (PSIT) using in vitro swine livers and in vivo rabbit livers. The thermal coefficients calculated from the shifts of the water frequency with thermocouple based temperature measurements were -0.0085 +/- 0.0019 ppm/ degrees C for the in vitro studies and -0.0089 ppm/ degrees C for the in vivo studies. The error range was estimated to be +/- 3 degrees C and +/- 4.5 degrees C, respectively. Color-coded temperature maps were compared with macroscopic lesion sizes of the specimen. Regions defined using a 20 degrees C elevation in the initial images following hot saline injection (around 55 degrees C in absolute temperature) closely correlated with visible coagulation in size. We conclude that MR temperature monitoring of PSIT is quite feasible and may be helpful in expanding the clinical use of this thermal therapeutic tool for liver tumors.

Animals↗

Temperature monitoring in fat with MRI.

The aim of the study was to test the hypothesis that fast spin echo T(1)-weighted images can be used to quantify the temperature in fat during thermal therapy in vivo. An MR compatible positioning device was used to manipulate focused ultrasound transducers in an MRI scanner. This system was used to sonicate fat tissue around the kidneys of 12 rabbits at various power levels for 10 to 20 sec. The scan parameters of T(1)-weighted fast spin echo (FSE) sequence were varied to optimize signal intensity characteristics while maintaining short scan times. An invasive optical probe was used to calibrate the temperature related signal intensity changes. For the T(1)-weighted FSE sequence, the signal intensity decreased with the temperature elevation at the rate of 0.97+/-0.02%/ degrees C. The single focused transducer produced a contrast-to-noise ratio more than 10 at power levels below the tissue damage threshold. The signal intensity was linearly dependent on the power, despite the measured temperatures being well above the coagulation threshold. This study demonstrates that T(1)-weighted FSE MRI sequences can be used to quantify the temperature elevation in fat in vivo during short focused ultrasound exposures. This can be very important for breast tumor surgery, fat ablation, and for treating deep seated tumors through superficial fat layers. Magn Reson Med 43:901-904, 2000.

Adipose Tissue↗

Multi-component apparent diffusion coefficients in human brain: relationship to spin-lattice relaxation.

In vivo measurements of the human brain tissue water signal decay with b-factor over an extended b-factor range up to 6,000 s/mm(2) reveal a nonmonoexponential decay behavior for both gray and white matter. Biexponential parametrization of the decay curves from cortical gray (CG) and white matter voxels from the internal capsule (IC) of healthy adult volunteers describes the decay process and serves to differentiate between these two tissues. Inversion recovery experiments performed in conjunction with the extended b-factor signal decay measurements are used to make separate measurements of the spin-lattice relaxation times of the fast and slow apparent diffusion coefficient (ADC) components. Differences between the spin-lattice relaxation times of the fast and slow ADC components were not statistically significant in either the CG or IC voxels. It is possible that the two ADC components observed from the extended b-factor measurements arise from two distinct water compartments with different intrinsic diffusion coefficients. If so, then the relaxation results are consistent with two possibilities. Either the spin-lattice relaxation times within the compartments are similar or the rate of water exchange between compartments is "fast" enough to ensure volume averaged T(1) relaxation yet "slow" enough to allow for the observation of biexponential ADC decay curves over an extended b-factor range. Magn Reson Med 44:292-300, 2000.

Adult↗