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Biomedical subjects

L E Crooks

Publications and source records attributed to L E Crooks.

At least 19 recordsLinked to original sources
PubMed1992

In vivo relaxation times and hydrogen density at 0.063-4.85 T in rats with implanted mammary adenocarcinomas.

Magnetic resonance (MR) imaging at 0.063 T, 0.17 T, 0.35 T, 2.94 T, and 4.85 T was performed in rats with implanted mammary adenocarcinomas (and in control rats) to study hydrogen density and the effects of magnetic field strength on T1 and T2 relaxation times in vivo. T1 increased by an order of magnitude and T2 decreased by a factor of two between the lowest and highest field strengths. Only about half of the increased water content of tumor, compared with that of muscle, was reflected in increased hydrogen density. The sensitivity of relaxation time change was increased at the lower field strengths. These results are of significance in understanding the effects of field strength on lesion detectability, contrast, and signal-to-noise ratio.

Adenocarcinoma↗
PubMed1989

Measuring signal-to-noise ratios in MR imaging.

The signal-to-noise ratio (S/N) in magnetic resonance imagining is one of the variables that must be measured when comparing the relative performance of different techniques. Although various investigators and official groups have proposed different methods for measuring S/N, these are generally not practical for use by a physician working in a clinical situation. The authors present a simple method that should serve for estimating S/N in most cases.

Magnetic Resonance Imaging↗
PubMed1989

Physics of thin-section MR imaging at low field strength.

Thin-section magnetic resonance imaging at low field strengths requires analysis of the relative merits of data collection techniques for direct three-dimensional versus multisection two-dimensional imaging. This analysis was done with specific emphasis on shortened relaxation times and the use of reduced magnetic field gradients. Three-dimensional Fourier transform techniques can provide thin-section images of good diagnostic quality when combined with partial flip-angle gradient-reversal techniques.

Humans↗
PubMed1989

Directions in magnetic resonance imaging technology.

Over the past few years there have been substantial improvements in the performance of magnetic resonance (MR) imagers. As image quality improved it became possible to perform studies in less time, increasing the throughput and the availability of the technique. A crucial contributor has been improvements in signal/noise. Techniques can now give signal/noise levels that a few years ago would have required much longer imaging times. With partial flip angle imaging techniques, it is possible to maintain image contrast and signal/noise while using reduced values of TR which decrease imaging time. It is also possible to decrease the number of acquired data lines and replace these lines mathematically at reconstruction time. Signal/noise is sacrificed, but the benefit is almost a factor of two in acquisition time. Echo planar techniques provide even higher speed imaging. In addition to the trade-off of signal/noise versus acquisition time, signal/noise can be traded for reduction in magnetic field strength. This results in reduced cost, improved patient access and also offers reduction in motion artifacts.

Humans↗
PubMed1988

Echo-planar pediatric imager.

Practical constraints make it difficult to build large-aperture echo-planar magnetic resonance (MR) imagers. The implementation of a pediatric imager and its performance are described. Spatial resolution and signal-to-noise levels comparable to those of 1982 state-of-the-art MR imagers have been achieved in imaging times of 0.05-0.15 seconds. T1 and T2 information are obtainable in the echo-planar mode. A major issue is that of chemical-shift displacements.

Child↗
PubMed1987

The value of relaxation times and density measurements in clinical MRI.

The hope that MRI relaxation time signatures would identify tissues, specifically, malignancies, has not been realized. This is due much less to measurement inaccuracies than to a large intrinsic variability and overlaps between malignancies and many benign pathologies. Neither has there been success in predicting relaxation times from basic tissue compositions. Nevertheless, MRI provides a qualitative measure of tissue hydration, and of flow, on the basis of relaxation times. Furthermore, pixel-by-pixel maps of relaxation times have proven useful in understanding the MRI process, in predicting the efficacy of untried techniques, and replace, in many circumstances, the need for acquisition of images with diverse sequencing parameters.

Body Water↗
PubMed1987

Albumin labeled with Gd-DTPA as an intravascular, blood pool-enhancing agent for MR imaging: biodistribution and imaging studies.

Albumin is a macromolecule that remains largely confined to the vascular space after intravenous administration. Human serum albumin was paramagnetically labeled by covalently binding from nine to 18 gadolinium-DTPA (diethylenetriaminepentaacetic acid) chelates per protein molecule. This conjugate was tested in varying doses for in vivo biodistribution and effectiveness in tissue relaxation. After intravenous injection of the agent in rats, T1 relaxation times were significantly reduced in samples of the blood and in lung, heart, spleen, kidney, and brain tissue. These effects persisted at a relatively constant level for the next 30 minutes. In vivo magnetic resonance imaging of the heart and lungs of rats and rabbits confirmed the prolonged contrast-enhancing effect of the labeled albumin. These preliminary studies indicate that paramagnetically labeled macromolecules that distribute in the intravascular space may be effective for MR imaging evaluation of tissue blood volume.

Animals↗
PubMed1987

Partial flip angle MR imaging.

Theoretical analysis predicts that performing magnetic resonance (MR) imaging with partial (less than 90 degrees) flip angles can reduce imaging times two- to fourfold when lesions with elevated T1 values are being examined. This time savings occurs because repetition time (TR) is reduced when imaging is performed with partial flips. Partial flip MR imaging can also improve signal-to-noise ratio (S/N) in fast body imaging. For this study, analytical tools were used to predict image contrast and S/N for short TR, partial flip sequences. Experimental implementation of the short TR, partial flip sequences that analytical work had predicted would be optimal supported the analytical predictions and demonstrated their validity. Partial flip MR imaging is applicable to reducing imaging time only when the ratio of signal differences to noise exceeds threshold values in conventional MR images. Partial flip sequences can be used to advantage in MR imaging of both the head and the body, and the observed effects are predictable through theoretical analysis.

Brain↗
PubMed1987

Complex and simple renal cysts: comparative evaluation with MR imaging.

Magnetic resonance (MR) imaging was performed in two groups of patients to determine its usefulness in evaluating fluid-containing renal masses deemed complex with computed tomography (CT). Twenty-two patients in group 1 had indeterminate renal masses by CT, five of which were also indeterminate by ultrasound (US). The results in this group were compared with histologic findings. Group 2 consisted of 20 patients with simple renal cysts diagnosed according to rigid CT criteria. On MR imaging, 11 of the 23 masses in group 1 and 19 of the 20 in group 2 were diagnosed as benign cysts. Fluid within the cyst had long T1 and T2, resulting in a low signal intensity on T1-weighted images. In the 12 remaining lesions in group 1 and in one lesion in group 2, the fluid content was indeterminate and MR did not permit differentiation of cystic renal carcinoma from old hemorrhage or adenoma. When fluid within the cystic mass did not have the MR characteristics of simple fluid, MR was not helpful in characterizing the mass, but when the fluid intensity was similar to normal urine, the cyst was benign.

Adenoma↗
PubMed1986

Contiguous thin multisection MR imaging by two-dimensional Fourier transform techniques.

Section thickness in two-dimensional Fourier transform (FT) imaging is dependent on gradient strength and the shape of the radio-frequency pulses used to excite the nuclei. By manipulation of these parameters, it is possible to obtain 2.5-mm-thick sections in contiguous, multisection imaging. Because this method is efficient in imaging with long repetition times (TR), it effectively complements three-dimensional FT thin-section imaging techniques, which require imaging with short TRs. Fifteen double-echo, contiguous images of 0.9 X 0.9 X 2.5-mm resolution were obtained in 17.1 minutes for a TR of 2 seconds.

Fourier Analysis↗
PubMed1986

MRI of blood flow: correlation of image appearance with spin-echo phase shift and signal intensity.

Phase-sensitive imaging was used to correlate signal distribution with phase shift and velocity distribution in spin-echo magnetic resonance imaging (MRI). Flow-dependent, changing intensity patterns that were seen in a constant-flow phantom study were explained by the simultaneous effects of inflow signal enhancement, first-echo dephasing, and outflow signal loss occurring during laminar flow. In clinical studies, first-echo dephasing was shown during laminar flow in the inferior vena cava. Turbulent flow was demonstrated in the descending thoracic aorta during late systolic flow, and turbulent dephasing-rephasing was shown in the abdominal aorta.

Aorta, Thoracic↗
PubMed1985

Magnetic resonance imaging the velocity vector components of fluid flow.

Encoding the precession phase angle of proton nuclei for Fourier analysis has produced accurate measurement of fluid velocity vector components by MRI. A pair of identical gradient pulses separated in time by exactly 1/2 TE, are used to linearly encode the phase of flow velocity vector components without changing the phase of stationary nuclei. Two-dimensional Fourier transformation of signals gave velocity density images of laminar flow in angled tubes which were in agreement with the laws of vector addition. These velocity profile images provide a quantitative method for the investigation of fluid dynamics and hemodynamics.

Fourier Analysis↗