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R M Henkelman

Publications and source records attributed to R M Henkelman.

At least 37 records · Page 2Linked to original sources
PubMed1999

Pulsed magnetization transfer imaging: evaluation of technique.

With use of an established physical model, numeric simulations were performed to evaluate current imaging protocols for the two primary applications of magnetization transfer: cerebral magnetic resonance angiography and neuroimaging of white matter disease. The authors found that the current technique is appropriate in the former but suboptimal in the latter. Further clinical investigations could potentially improve magnetization protocols for neuroimaging.

Animals↗
PubMed1998

Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle.

Detailed measurements of the T1-weighted, T2-weighted, and MT-weighted signal were performed for ex vivo muscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase in T2 relaxation time and an increase in MT-weighted signal for temperatures above 60 degrees C. The relationship for T1 relaxation was more complex, showing a decrease in T1 relaxation from 40 to 60 degrees C and an increase above 60 degrees C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in target tissues.

Animals↗
PubMed1998

Water dynamics in human blood via combined measurements of T2 relaxation and diffusion in the presence of gadolinium.

An analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr-Purcell Meiboom-Gill and pulsed-field-gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also discussed.

Blood↗
PubMed1998

Integrated analysis of diffusion and relaxation of water in blood.

Diffusion and T2 relaxation of water both inside and outside red blood cells (RBCs) in human blood were investigated using a hybrid NMR pulse sequence to obtain a more quantitative understanding of the diffusion and relaxation behavior of water in paramagnetic-doped blood samples. The data were analyzed by both examining the relaxation properties of the system after each diffusion weighting and looking at the diffusion properties at each echo time. The results illustrate how diffusion-sensitizing gradients affect the T2 spectra of blood and how relaxation weighting changes the curvature of the diffusion curves, thereby demonstrating the close coupling between diffusion and T2 relaxation. A three-pool model, consisting of RBCs, plasma, and macromolecular protons, was used to model the data from the diffusion-relaxation hybrid experiments. The model was found to describe all the characteristic features of the experimental data well and was used to evaluate the approximations involved in the conventional analysis methods and elucidate the nature of the relaxing and diffusing components. Compared with the separate diffusion and relaxation experiments, the diffusion-relaxation hybrid experiments are less time-consuming, result in better parameter determinations, and may be useful in analyzing diffusion-T2 coupling in tissues with more complicated multiexponential T2 behavior.

Biological Transport↗
PubMed1998

Diffusional anisotropy of T2 components in bovine optic nerve.

A diffusion-CPMG hybrid experiment was used to analyze the diffusion characteristics of different T2 relaxation components in bovine optic nerve. Data were collected using a pulsed field gradient (PFG) multi spin echo (MSE) CPMG sequence for parallel and perpendicular axon orientation and four diffusion times. The apparent diffusion coefficient (ADC) was evaluated for two observed T2 components as a function of axonal orientation and diffusion time delta. The short T2 component exhibited minor diffusional anisotropy and larger ADC, whereas the long T2 component showed significant anisotropy effects. This is consistent with the hypothesis that the short T2 component is associated with water within the myelin sheath, which is less restricted than axonal water that is limited by cell membrane permeability.

Animals↗
PubMed1998

Ex vivo tissue-type independence in proton-resonance frequency shift MR thermometry.

The temperature sensitivity of the proton-resonance frequency (PRF) has proven valuable for the monitoring of MR image-guided thermal coagulation therapy. However, there is significant inconsistency in reported values of the PRF-thermal coefficient, as measured from experiments encompassing a range of in vivo and ex vivo tissue types and experimental conditions. A method of calibrating the temperature dependence of the PRF is described and results are presented that indicate a tissue-type independence. To this end, other possible mechanisms for variations in the PRF-thermal coefficient are suggested, including physiological perturbations and volume magnetic susceptibility effects from geometry and orientation.

Animals↗
PubMed1997

An analytical model of restricted diffusion in bovine optic nerve.

An analytical model of restricted diffusion in bovine optic nerve is presented. The nerve tissue model is composed of two different objects: prolate ellipsoids (axons) and spheres (glial cells) surrounded by partially permeable membranes. The free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to diffusion in the pulsed gradient spin echo (PGSE) experiment for this tissue model are derived. The model is fitted to experimental data for bovine optic nerve. The obtained model parameters are shown to be reasonable. The model describes all of the characteristics of the PGSE data: anisotropy, upward curvature of decay curves, and diffusion time dependence. The validity and sensitivity of the model are also discussed.

Animals↗
PubMed1997

A flexible magnetization transfer line shape derived from tissue experimental data.

To summarize and compare magnetization transfer data from biological tissues, a method was developed to extract the average absorption lineshape of the semi-solid pool directly from magnetization transfer experimental data along with the four other parameters that characterize the two-pool model of exchange. Magnetization transfer data for several biological tissues were analyzed using this method and the resulting "flexible" lineshapes were compared with super-Lorentzian and "Kubo-Tomita" lineshapes. The use of flexible lineshapes noticeably improves the fit of the two-pool model to the data. The derived flexible lineshapes of all the tissues analyzed are physically realistic and show remarkably consistent behavior.

Animals↗
PubMed1997

Magnetic resonance imaging of temperature changes during interstitial microwave heating: a phantom study.

Changes in magnetic resonance (MR) signals during interstitial microwave heating are reported, and correlated with simultaneously acquired temperature readings from three fiber-optic probes implanted in a polyacrylamide gel phantom. The heating by a MR-compatible microwave antenna did not interfere with simultaneous MR image data acquisition. MR phase-difference images were obtained using a fast two-dimensional-gradient echo sequence. From these images the temperature-sensitive resonant frequency of the 1H nuclei was found to decrease approximately by 0.008 ppm/ degree C. The method and results presented here demonstrate that noninvasive MR-temperature imaging can be performed simultaneously with interstitial microwave thermal treatment.

Biophysical Phenomena↗
PubMed1997

Image resolution and signal-to-noise ratio requirements for MR imaging of degenerative cartilage.

OBJECTIVE: The purpose of this study was to determine the MR image resolution and signal-to-noise ratio (SNR) required to reveal morphologic abnormalities in degenerative cartilage. MATERIALS AND METHODS: In vitro MR microscopy of healthy bovine and degenerative human patellar cartilage was performed and image resolution degraded to simulate resolution achievable in routine and optimized clinical MR images. Noise was then added to images as in-plane resolution was increased to simulate the MR imaging appearance of cartilage with improved resolution, which is possible using a standard magnetic field strength. MR images of patellar cartilage from a healthy volunteer were also obtained to determine optimal SNR and image resolution achievable at 1.5 T; these images were compared with in vitro images to determine those features of abnormal cartilage that can be identified using available clinical MR imaging techniques. RESULTS: In-plane resolution of 39 microns exquisitely defines degenerative changes in articular cartilage. As in-plane resolution decreases to 600 microns, only gross cartilage thinning and signal intensity alterations are seen. MR images simulating progressively improved resolution using higher field strength gradients with conventional field strengths and RF coils provided suboptimal information because of low SNR. CONCLUSION: Routine clinical MR images do not accurately reveal early degenerative changes in articular cartilage; only large defects and distorted changes in the signal intensity of cartilage are seen.

Adult↗
PubMed1996

Velocity encoding using ghost artifacts.

Motion artifacts represent a significant limitation of MRI, and an ideal solution to that problem has proved elusive. However, in this paper, motion artifacts are not considered as the usual enemy and are not suppressed; on the contrary, they are deliberately created to encode flow information. In MRI, velocity is encoded readily into the phase of a pixel. However, if the pixel contains overlapping signals, the phase of one of these signals now has consequences on both the magnitude and phase of the resulting pixel. It is shown here that an overlap of information may be used to encode velocity both in the phase and in the magnitude of an image, making the velocity-encoding process faster. The overlap of information is obtained by superposing ghosting artifacts of different orders and information is retrieved about complex intensity and velocity in two dimensions using the equivalent of two images instead of the usual three images. The price to pay to do so is some loss of simplicity in the equations involved, an increase in reconstruction computing time requirements, and a factor of 4 decrease in signal-to-noise ratio in the velocity measurements.

Artifacts↗
PubMed1996

A new way of averaging with applications to MRI.

Averaging is often used to increase the quality of an image degraded by noise or artifacts. A method is developed in which several degrees of freedom are introduced in the averaging process, this freedom making possible the choice of different weighting factors for different portions of the Fourier space. If a weighting factor is associated with each line of a magnetic resonance acquisition, we show that we obtain some freedom to eliminate motion artifacts. The process minimizes a quantity called the gradient energy over a region of interest in the image plane. A processed image is obtained from a mosaic of such regions of interest scanned over the whole image plane. The method is shown to yield greater motion artifact suppression in magnetic resonance images than that achieved with regular averaging. The main strength of the method is probably its ability to diminish the intensity of unstructured artifacts which are usually poorly managed by other postprocessing methods of artifacts suppression.

Biophysical Phenomena↗
PubMed1996

Effects of compression and recovery on bovine articular cartilage: appearance on MR images.

PURPOSE: To determine the influence of compression and decompression on bovine articular cartilage as it appears on magnetic resonance (MR) images. MATERIALS AND METHODS: Consecutive spin-echo MR images (repetition time msec/echo time msec = 800/33) of normal bovine cartilage were obtained with increasing increments of pressure, to a maximum of 4.14 MPa (600 psi); this was followed by consecutive MR imaging after release of the pressure. RESULTS: Before compression, cartilage showed a laminated appearance. After incremental pressure, cartilage thickness progressively decreased; a short T2, low-signal-intensity lamina became thicker and more distinct at the articular surface; signal intensity of the deep cartilage zone initially was high and then gradually decreased; and at maximum pressure, the cartilage showed uniform low signal intensity. After release of pressure, signal intensity changes in the cartilage were sequentially reversed from those observed with compression. Image resolution and echo time influenced the appearance of these findings, particularly the low-signal-intensity line at the cartilage interface. CONCLUSION: The varying appearance and signal intensity characteristics of cartilage under pressure are hypothesized to result from a combination of net water loss and alteration in collagen orientational structure.

Animals↗
PubMed1995

Rotating frame RF current density imaging.

RF current density imaging (RF-CDI) is a new MRI technique for imaging the Larmor frequency current density parallel to B0 in electrolytic media. To extend the use of RF-CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a large B1 field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three-compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time and B1, whereas the avoidance of phase artifacts imposes a lower bound on B1.

Algorithms↗
PubMed1995

A model for magnetization transfer in tissues.

Magnetization transfer in several tissues is measured and successfully modeled using a two-pool model of exchange. The line shape for the semi-solid pool is characterized by a superLorentzian and the liquid pool by a Lorentzian. The tissues investigated were white and gray matter, optic nerve, muscle, and liver. All tissues the authors studied are characterized by the same model but differ in the parameter values of the model. Blood and cerebral spinal fluid (CSF) were also investigated. The two-pool model with a Lorentzian line shape for both the semi-solid and liquid pools modeled the magnetization transfer in blood. In CSF, as expected, there is no measurable exchange of magnetization. The T2B associated with the semi-solid pool was short (approximately 10 microseconds) for all tissues indicating a fairly rigid semi-solid pool. In addition characterization of the line shape as superLorentzian indicates molecules such as integral membrane proteins or lipids in membranes are likely molecules participating in the exchange. Conversely, in blood large globular proteins are indicated due to the Lorentzian nature of the semi-solid pool and a T2B approximately 300 microseconds.

Animals↗
PubMed1995

Magnetization transfer and T2 relaxation components in tissue.

T2 relaxation makes an important contribution to tissue contrast in magnetic resonance (MR) imaging. Many tissues are known to exhibit multicomponent T2 relaxation that suggests some compartmental segregation of mobile protons on a T2 timescale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water-macromolecular interactions. To investigate the relationship between multicomponent T2 relaxation and the MT process, multiecho T2 measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter. For muscle, short T2 components show greater MT than long T2 components, consistent with the belief that they represent distinct water environments. For white matter, quantitative MT measurements were identical for the two major T2 components, apparently because of exchange between the T2 compartments on a time-scale characteristic of the MT experiment. Implications for accurate modeling of MT in tissue and the use of MT for MR image contrast are discussed.

Animals↗
PubMed1995

Phase constrained encoding (PACE): a technique for MRI in large static field inhomogeneities.

In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echo train and alternating readout gradients. Small artifactual side-bands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.

Artifacts↗