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R V Mulkern

Publications and source records attributed to R V Mulkern.

At least 19 recordsLinked to original sources
PubMed1992

Magnetization transfer effects in multislice RARE sequences.

Magnetization transfer effects are demonstrated to be significant in determining the signal intensity from brain tissues on images acquired with multislice rapid acquisition relaxation enhanced (RARE) sequences. We report studies designed to determine how the signal intensities vary with slice number or, equivalently, off-resonance power deposition. The results obtained in fat, gray matter, and white matter are similar in form to those reported in kidney tissues during classic magnetization transfer experiments (J. Eng, T. L. Ceckler, and R. S. Balaban, Magn. Reson. Med. 17, 304 (1991)). Of clinical significance to RARE practitioners is the increase of contrast-to-noise ratios between gray and white matter on proton density-weighted images with increasing slice number.

Artifacts↗
PubMed1992

Partial RF echo planar imaging with the FAISE method. I. Experimental and theoretical assessment of artifact.

The fast acquisition interleaved spin-echo (FAISE) method is a partial RF echo-planar technique which utilizes a specific phase-encode reordering algorithm to manipulate image contrast (Melki et al., J. Magn. Reson. Imaging 1:319, 1991). The technique can generate "spin-echo" like images up to 16 times faster than conventional spin-echo methods. However, the presence of T2 decay throughout the variable k-space trajectories used to manipulate T2 contrast ensures the presence of image artifacts, especially along the phase-encode direction. In this work, we experimentally and theoretically examine the type and extent of artifacts associated with the FAISE technique. We demonstrate the existence of well-defined minima of phase-encode ghost noise for selected k-space trajectories, examine the extent of blurring and edge enhancement artifacts, demonstrate the influence of matrix size and number of echoes per train on phase-encode artifact, and show how proper choice of FAISE sequence parameters can lead to proton density brain images which are practically indistinguishable from conventional spin-echo proton density images. A comparison of contrast between FAISE and standard spin-echo methods is presented in a companion article referred to as II.

Algorithms↗
PubMed1992

Partial RF echo-planar imaging with the FAISE method. II. Contrast equivalence with spin-echo sequences.

The fast acquisition interleaved spin-echo (FAISE) sequence and its dual-echo version (DEFAISE) are partial RF echo-planar methods which utilize a specific phase-encode reordering algorithm to manipulate T2 contrast via an operator-controlled pseudo-echo time, pTE. The repetition time, TR, between successive applications of the Carr-Purcell-Meiboom-Gill (CPMG) echo trains used in FAISE may be reduced to introduce T1 weighting. To quantitatively determine the extent to which FAISE T1 and T2 contrast characteristics agree with spin-echo methods, signal intensities from FAISE acquisitions were compared with signal intensities from equivalent CPMG acquisitions. In phantoms and in human heads, the contrast characteristics of FAISE are found to be highly correlated with that obtained with equivalent CPMG sequences. However, conventional SE sequences generally utilize longer echo spacings than employed with FAISE/CPMG. Thus, echo spacing-dependent mechanisms such as spin-spin coupling and magnetic susceptibility lead to some differences in contrast between conventional SE and FAISE. Finally, FAISE appears to be more sensitive to magnetization transfer effects than conventional SE sequences since more off-resonance irradiation is applied to individual slices during multislice acquisitions.

Adipose Tissue↗
PubMed1992

A novel method for fat suppression in RARE sequences.

Rapid acquisition relaxation-enhanced (RARE) sequences (Hennig et al., Magn. Reson. Med. 3, 823 (1986)) utilize one or several Carr-Purcell-Meiboom-Gill (CPMG) echo trains to sample a number of k-space lines each repetition time TR. The technique can rapidly generate multislice T2-weighted images which, as a rule, are strikingly similar in contrast to conventional T2-weighted spin-echo (SE) images. An exception to this rule is the appearance of very bright signal from fat in T2-weighted RARE images as compared to conventional T2-weighted SE images. To reduce this fat signal, we introduce a time delay, tau c, between the 90 degrees x and first 180 degrees y pulse of each echo train such that a phase angle of pi/2 develops between fat and the reference (water) line at echo maxima. The technique leads to single-acquisition fat suppression without the use of frequency-selective saturation pulses and concomitant loss of slices per TR. A Bloch equation analysis is used to identify two major mechanisms contributing to suppression of off-resonance spins such that w tau c = pi/2. Namely, the CPMG sequence becomes a CP sequence with no self-correction properties for imperfect 180 degrees pulses leading to enhanced signal decay, and the raw k-space data matrix become segmented into blocks alternately multiplied by +/- i, leading to signal dispersion following Fourier transformation.

Abdomen↗
PubMed1992

Characterization of cerebral infarction by multicomponent analysis of transverse magnetization decay curves.

RATIONALE AND OBJECTIVES: Individual components of the transverse magnetization decay curve (TDC) were assessed for their ability to characterize ischemia in photochemically induced cerebral infarcts. METHODS: Fifty rats were randomly divided into equal-sized experimental and control groups, which were subdivided into groups studied at five different time points, ranging from 6 hours to 22 days. All the rats received transcalvarial irradiation with 560-nm light. Five rats in each time group also received a sensitizing dye before irradiation. In these latter animals, lesions of uniform size and location developed. Lesions were compared with tissue of similar volume and location from the contralateral cortex of the experimental animals and with tissue from both hemispheres of the control animals. TDCs of all the samples were measured and fit with mono- and bi-exponential functions. RESULTS: Unlike the control tissue, infarcted tissue displayed definitive two-component TDC behavior. The time course of the bi-exponential parameters yielded information unavailable from mono-exponential analyses. CONCLUSIONS: Bi-exponential analysis of TDCs may have diagnostic use as a more sensitive indicator of cerebral infarction than mono-exponential analyses.

Animals↗
PubMed1992

Brain hemorrhage: evaluation with fast spin-echo and conventional dual spin-echo images.

Signal intensity of blood products on proton-density- and T2-weighted images obtained with spin-echo (SE) and fast SE (FSE) sequences was evaluated in 15 patients with central nervous system hemorrhage to determine the extent of differences between the two techniques when signal loss from magnetic susceptibility effects in hemorrhagic lesions is considered. Within operator-defined regions of interest, signal intensity of hemorrhage, iron-containing nuclei, white matter, scalp fat, and noise was measured along the phase-encoding direction. Hemosiderin, deoxyhemoglobin, and iron-containing nuclei had slightly higher signal intensity on FSE images than on SE images, but the differences were not statistically significant. Signal intensity of methemoglobin was similar with both sequences, whereas that of scalp fat was higher on FSE images. Signal intensity measurements for most tissues studied were comparable, but the signal-to-noise ratios with FSE imaging were less than those with SE imaging. Although paramagnetic blood products may show slightly higher signal intensity with FSE imaging, contrast with the two sequences was comparable and lesion conspicuity was nearly identical.

Adult↗
PubMed1991

Regional 1H transverse magnetization studies in perfused rabbit kidney.

A Carr-Purcell-Meiboom-Gill imaging sequence consisting of 128 echoes is used to extract transverse magnetization decay curves (TDCs) at 1.9 T from 1.7 x 1.7 x 5-mm3 voxels within the cortex, outer medulla, and inner medulla of perfused rabbit kidneys. The spatially localized TDCs within each tissue type are found to be better approximated by biexponential, as opposed to monoexponential, functions. The biexponential parameters characterizing the TDCs demonstrate an improved degree of tissue specificity over that available from monoexponential analyses. The fraction of the quickly relaxing TDC component and the relaxation rate of this component are observed to decrease from cortex to inner medulla. A two-site exchange analysis is used to convert biexponential TDC parameters into water volume fractions and exchange rates. The exchange rates between the fast and slowly relaxing pools increased from cortex to inner medulla. All exchange rates were less than 1.5 Hz, indicating a relatively slow water exchange process. The imaging methods and subsequent analyses offer the potential to generate unconventional MR images with tissue contrast dependent upon water compartmentation and exchange.

Animals↗
PubMed1991

1D spectroscopic imaging with rf echo planar (SIRFEN) methods.

A recently developed rf echo planar imaging method has been modified to rapidly generate spectroscopic information along one in-plane axis and spatial information along the other. The method allows the production of one-dimensional chemical shift images (1D CSIs) in acquisition times of 18 sec or less. A specific phase-encode-reordering algorithm provides convenient manipulation of T2 weighting, yielding partial suppression of short T2 species like muscle water. The method is demonstrated in phantoms and in vivo with 1D CSIs of human brain and limbs. Abnormal fat distribution is demonstrated in the calf of a patient with aggressive fibromatosis. The advantages of short acquisition times obtainable with SIRFEN are offset by limited spectral resolution, suggesting that primary applications will be confined to rapid spatial mapping of major spectral components.

Algorithms↗
PubMed1990

Compatibility of the two-site exchange model and 1H NMR relaxation rates.

We demonstrate a formalism for determining whether 1H longitudinal and transverse magnetic relaxation decay curves are compatible with a model of proton exchange between two sites with inherently different relaxation rates. We apply the method to published data in which both decay curves have been fit to normalized biexponential functions, a prediction of the simplest two-site exchange model. The analysis is applied to relaxation data of adsorbed water vapor on silica gel (J.R. Zimmerman and J.A. Lasater, J. Phys. Chem. 62, 1157 (1958] and to relaxation data reported in mycelia of Botrytis cinerea Persoon and pupae of the tobacco cutworm (M. Yoshida and K. Nose, Agric. Biol. Chem. 51, 3399 (1987]. Exchange parameters are calculated form these data sets and the compatibility of the two-site exchange model with the data is discussed for each case. The sensitivity of the calculated exchange parameters to errors in the bioexponential fitting parameters is treated in some detail.

Animals↗
PubMed1990

CPMG imaging sequences for high field in vivo transverse relaxation studies.

Two-dimensional Fourier transform Carr-Purcell-Meiboom-Gill (CPMG) imaging sequences were implemented on 1.9-T and 1.5-T imaging systems in order to test their ability to characterize in vivo transverse decay curves (TDCs). Both hard- and soft-pulse CPMG imaging sequences, consisting of up to 128 echoes with echo spacings of approximately 10 ms were developed, implemented, and tested. These sequences provide one of the most detailed samplings of TDCs from image data sets reported to date. Good agreement between image-extracted T2 values and spectroscopically obtained T2 values of NiCl2-doped saline solutions was found with both hard- and soft-pulse sequences. In vivo TDCs were extracted from rabbit and human image data sets. For several tissues, biexponential TDC fits provided considerable improvement over monoexponential fits and the sensitivity of the fitting parameters to positive baseline offsets was examined. With the time coverage of the relaxation decay curves available from these sequences, the TDCs from white matter in humans appear largely monoexponential while those from cortical grey matter demonstrate biexponential behavior.

Adipose Tissue↗
PubMed1990

MR studies of brain oedema in the developing animal.

Assessment of perinatal brain oedema is complicated by normal changes in brain water that accompany the marked physiological, biochemical and morphological alterations occurring during this phase of development. Multiexponential analysis of transverse decay curves (TDCs), derived from 128 echo CPMG images, of white matter (WM) made oedematous by either exposure of animals to triethyltin (TET) or cryogenic cortical lesions revealed a second, slower decay component not apparent in controls. More significantly, an obvious difference was noted between the TET and cryogenic lesion fast decay components which might serve as a basis to differentiate non-invasively cytotoxic and vasogenic oedemas.

Aging↗
PubMed1990

Contrast manipulation and artifact assessment of 2D and 3D RARE sequences.

The extent of contrast manipulation and the assessment of characteristic artifacts in imaging studies of brain and knee as performed with novel variants of the Rapid Acquisition Relaxation Enhanced (RARE) sequence are reported. Methods of ordering the phase encoding within one or two echo trains are proposed for manipulating T2 contrast. Options for minimizing artifacts associated with the various schemes are discussed. The extent of T1 contrast manipulation in RARE sequences is explored by varying repetition rates in a signal averaging scheme and by applying inversion pulses prior to data acquisition. The results demonstrate that RARE sequences can be utilized for obtaining good quality images with a range of tissue contrast options similar to those associated with slower spin-echo methods. They also suggest that RARE applications need not be confined to highlighting long T2 fluid spaces, an application already well documented.

Adult↗
PubMed1989

The effect of gadolinium DTPA on tissue water compartments in slow- and fast-twitch rabbit muscles.

Proton T2 relaxation and its biexponential components have been determined in rabbit skeletal muscle in the presence and absence of GdDTPA. The effect of GdDTPA, which distributes only in the extracellular space, was greatest in the longer-relaxing T2 component (T22). A 27% reduction in T22 was measured for slow-twitch (red) muscle and 17% for fast-twitch (white) muscle, consistent with the larger extracellular space of the former. Magnetic resonance images demonstrated apparent contrast between red and white rabbit muscles. This contrast was instantaneously enhanced by administration of GdDTPA and returned to near normal levels after approximately 30 min. These functional changes in tissue contrast are consistent with differences in blood perfusion and biological water compartmentation between fast- and slow-twitch skeletal muscles.

Animals↗
PubMed1989

Two-site exchange revisited: a new method for extracting exchange parameters in biological systems.

A new analysis is presented which links real volume fractions, relaxation rates, and intracompartmental exchange rates directly with apparent volume fractions and relaxation rates obtained from biexponential fits of transverse magnetization decay curves. The analysis differs from previous methods in that measurements from two paramagnetic doping levels are used to close the two-site exchange equations. Both the new method and one previously described by Herbst and Goldstein (HG) have been applied to paramagnetically doped whole-blood data sets. Significant differences in the calculated exchange parameters are found between the two methods. A small dependence of the intracellular relaxation rate on extracellular paramagnetic agent concentration, assumed nonexistent with the HG method, is inferred from the new analysis. The analysis was also applied to published data on perfused rat hearts, and we obtained a limited assessment of two-site exchange in this system.

Body Fluids↗
PubMed1989

Response to and control of destructive energy by magnetic resonance.

Magnetic resonance imaging techniques can be used to control and monitor the deposition of destructive energy. The authors evaluated the feasibility of phosphorus-31 magnetic resonance spectroscopy for the control, monitoring, and prediction of the three-dimensional extent of tissue destruction during interstitial laser surgery. Characteristic metabolic changes were demonstrated within the lesion and in the adjacent normal tissue during the deposition of thermal energy.

Adenosine Triphosphate↗
PubMedDate not supplied

Comparing the FAISE method with conventional dual-echo sequences.

The FAISE (fast-acquisition interleaved spin-echo) technique consists of a hybrid rapid-acquisition relaxation-enhanced (RARE) sequence combined with a specific phase-encode reordering method. Implemented on a 1.5-T unit, this multisection, high-resolution technique permits convenient contrast manipulation similar to that of spin-echo imaging, with selection of a pseudo-echo-time parameter and a TR interval. With a TR of 2 seconds, eight 256 x 256 images are obtained in 34 seconds with either T2 or proton-density weighting. A direct comparison between FAISE and spin echo for obtaining T2-weighted head images in healthy subjects indicates that FAISE and spin-echo images are qualitatively and quantitatively similar. Image artifacts are more pronounced on "proton-density" FAISE images than on the T2-weighted FAISE images. T1 contrast can be obtained with inversion recovery and short TR FAISE images. Preliminary temperature measurements in saline phantoms do not indicate excessive temperature increases with extended FAISE acquisitions. However, extensive studies of radio-frequency power deposition effects should be performed if the FAISE technique is to be fully exploited.

Artifacts↗
PubMedDate not supplied

T2-weighted thin-section imaging with the multislab three-dimensional RARE technique.

A novel three-dimensional (3D) RARE (rapid acquisition with relaxation enhancement) sequence was implemented on a clinical imager. In this technique, multiple slabs are excited in the same way as in the multisection spin-echo sequence, and each slab is further phase encoded into eight sections along the section-slab direction. With a 16-echo RARE sequence, 128 excitations cover the 256 X 256 X 8 3D k space. With a TR of 2,500 msec, 10 slabs can be excited sequentially at each TR, yielding 80 sections in 5 minutes. Slabs were overlapped to give contiguous sections after discarding of the aliased sections at slab edges. This relatively fast sequence makes contiguous thin-section T2-weighted imaging possible, an impractical achievement with the much longer spin-echo method. Compared with 3D Fourier transform gradient-echo imaging, the sensitivity of 3D RARE sequences to magnetic susceptibility is reduced. The clinical potential of T2-weighted 3D imaging is illustrated with high-resolution brain, spine, and temporomandibular joint images.

Brain↗