Biomedical subjects

T J Lawry

Publications and source records attributed to T J Lawry.

6 recordsLinked to original sources

PubMed1990

Computer modeling of surface coil sensitivity.

A simple model is presented for the calculation of relative signal-to-noise (S/N) ratios of coils of different sizes and configurations when applied to in vivo MRS. Axial symmetry is assumed, which enables rather simple expressions to be used for the calculation of coil loading by the tissue. The model is calibrated to experiments through measurement of the loaded and unloaded coil Q's. Applications of the model demonstrate that for small, superficial regions of interest (ROI), small surface coils can provide a S/N much improved over that of a larger coil. However, for very deep ROIs, larger coils or coils producing uniform B1 provide improved S/N.

Computer Simulation↗

PubMed1989

Computer simulation of MRS localization techniques: an analysis of ISIS.

Computer simulations were used to evaluate the ISIS localization technique as implemented with both head and surface coils. The effects of chemical shift, B1 inhomogeneity, repetition time, T2 relaxation, a postacquisition saturation pulse, and a B1 insensitive observation pulse were examined. Integrals of ISIS signals over the sample volume showed that significant signal loss from the volume of interest (VOI) and contamination from outside the VOI can occur for both head and surface coil ISIS experiments. The results showed that the saturation pulse, order of the various ISIS acquisitions, and repetition time affect contamination but not signal loss. In addition, short T2 and high RF power can combine synergistically to degrade the selective inversion pulses, causing further contamination and signal loss.

Computer Simulation↗

PubMed1989

Improvement of the rotating frame experiment by detection of residual Z magnetization: a 31P MRS study of metabolite levels in a Meth-A sarcoma.

The radio frequency field (B1) gradient of a surface coil can be used to obtain spectra from a series of sample regions which experience different B1 field strengths. We previously reported that the sensitivity of this method, known as the surface coil rotating frame experiment (SCRFE), can be enhanced by applying a composite pulse immediately after signal acquisition to sample residual Z magnetization which is normally undetected. Initially this modified SCRFE was used to obtain spatially resolved spectra across a B1 gradient of a factor of 2.5. Here we demonstrate the extension of this method to map phosphorylated metabolites across a B1 gradient of a factor of close to 5. Computer simulations were used to evaluate the performance of the composite pulse, and to assist in analyzing the data. The method was used to obtain 31P spectra in vivo from various tissue layers within and beneath a murine Meth-A tumor. The spectra differentiated between metabolite levels in tumor tissue and underlying skeletal muscle. Metabolic heterogeneity within the tumor itself was also evident.

Animals↗

PubMed1988

Detection of motion using B1 gradients.

An NMR method which makes use of a radiofrequency (RF) field gradient to detect diffusion, perfusion, or flow is demonstrated. The technique is analogous to the detection of motion using pulsed B0 gradients. The simplest form of the experiment is as follows: A spatially imhomogeneous RF field generated by a surface coil produces dispersal of magnetization in the Y Z plane. After a delay, the RF field is applied within reversed phase to restore polarization along +Z, which is then sampled using an observe pulse or pulse sequence. If molecular motion occurs during the delay period the amplitude of Z magnetization will be reduced. The lengths of the RF pulses and the delay period are varied so that the effects of relaxation, flow, and diffusion or perfusion can be distinguished. The present study demonstrates the use of this method to detect slow fluid flow. Advantages of this method include the availability of large RF gradients, and the avoidance of eddy currents. It is suggested that the method may have application to the study of perfusion and flow in vivo.

Diffusion↗

PubMed1988

Application of image-guided surface coil P-31 MR spectroscopy to human liver, heart, and kidney.

Localized phosphorus-31 magnetic resonance (MR) spectroscopy in humans has previously been accomplished with surface coils by means of depth-resolved surface coil spectroscopy or rotating frame experiments, in which the extent of tissue sampled critically depends on surface coil placement. The authors' goal was to modify the surface coil image-selected in vivo spectroscopy (ISIS) experiment to accomplish three-dimensional volume selection through application of selective pulses in the presence of B0 gradients. Advantages of ISIS include the ability to use proton images to define the volume of interest (VOI) and reduced dependence on exact positioning of the surface coil. However, rapid replication of the surface coil ISIS experiment can cause spectral contamination from signals originating outside the VOI. A modified version of the ISIS experiment was developed to alleviate contamination under conditions of rapid replication. Applications of localized P-31 MR spectroscopy for observation of high-energy phosphorus metabolites are presented in human liver, heart, and transplanted and normal kidney.

Computer Simulation↗

PubMedDate not supplied

Use of computer simulations for quantitation of 31P ISIS MRS results.

The difficulties in quantitation of in vivo 31P spectra are exacerbated by the fact that, in general, coils with inhomogeneous B1 fields are used with in vivo samples. A general method for quantitation of in vivo 31P MRS results obtained with the ISIS localization method was developed using computer simulations. The simulation calculates the preparation of the sample magnetization throughout the sample by the ISIS pulse sequence, as well as the sensitivity of signal reception. The calculation accounts for both the B1 field and the B0 gradients applied to the sample. The sensitivity of the experiment is expressed by integration of the simulated signal over the sample, assuming a homogeneous sample. The primary advantage of this approach is that a separate localization experiment on a phantom of known concentration is not required each time parameters of the localization experiment, such as dimensions or location of the localized volume, are altered. In addition, the simulations indicate the degree of contamination (signal from outside of the localized volume) that occurs, and provide a means of comparing different executions of the ISIS experiment. Experiments were performed on phantoms to verify the simulations, and experimental results on human brain and liver are reproduced to show that this approach provides reasonable estimates of metabolite levels in terms of molar concentrations.

Computer Simulation↗