Biomedical subjects

S A Bobman

Publications and source records attributed to S A Bobman.

9 recordsLinked to original sources

PubMed1992

Thin-section, three-dimensional Fourier transform, steady-state free precession MR imaging of the brain.

The authors evaluated a three-dimensional Fourier transform implementation of a very short repetition time (TR) (24 msec), steady-state free precession (SSFP) pulse sequence for clinical imaging of the brain and compared it with a conventional two-dimensional Fourier transform long TR/echo time (TE) spin-echo sequence. First, the optimal flip angle of 10 degrees for generating images with contrast similar to that of long TR/TE spin-echo images was determined. Then, 29 patients with suspected brain lesions were studied with both techniques. Although the SSFP images did not exhibit the magnetic susceptibility artifacts that plague other rapid-imaging techniques, the conspicuity of most parenchymal lesions was often less than that on the spin-echo images. Also, the visibility of paramagnetic effects, such as the low signal intensity of brain iron, was less obvious at SSFP imaging. These substantial limitations may relegate the SSFP sequence to an adjunctive role, perhaps mainly demonstration of the cystic nature of mass lesions, because of its extreme sensitivity to slow flow.

Adolescent↗

PubMed1991

Postoperative lumbar spine: contrast-enhanced chemical shift MR imaging.

A modified fat-suppression pulse sequence (consisting of combined frequency-selective fat presaturation followed by a spin-echo acquisition when fat and water magnetization vectors have opposite phase) was used to optimize the conspicuity of intravenous enhancement by gadopentetate dimeglumine on magnetic resonance images in 10 patients previously operated on for lumbar discogenic disease as well as in two patients with herniated disks who had not previously undergone surgery. This technique produced the greatest degree of fat suppression in the phantom study. In six of the patients who had previously undergone surgery, epidural enhancement was more obvious on the fat-suppressed images than on conventional spin-echo images, while in four patients, enhancement was equivalent. The herniated disks in two patients not previously operated on were not enhanced with either technique. Contrast enhancement was universally distinguishable from fat signal and from nonenhancing water-containing tissue on the fat-suppressed images obtained after contrast material administration. This technique may reduce the need for precontrast imaging. Furthermore, postoperative enhancement of nerve roots was more obvious on fat-suppressed images in seven of eight patients. This finding might represent previously undiagnosed degrees of arachnoidal inflammation, which may be a factor in the failed back syndrome.

Adipose Tissue↗

PubMed1986

Instrumentation for rapid MR image synthesis.

MR image synthesis has previously been developed as a means of retrospectively optimizing contrast of arbitrary materials in MR images. The first step of this process is to form computed N(H), T1, and T2 images from source images acquired at a variety of echo delay and repetition times. The second step is to take these computed images, along with operator-selected timing parameters, and mathematically generate a synthesized image. Computation is carried out pixel by pixel according to the equation describing the chosen pulse sequence. This paper presents a study of design considerations for a digital image processor capable of rapidly performing the second step, the actual synthesis. In this work the computations inherent to image synthesis are identified, and the feasibility of performing them in high-speed hardware examined. An analysis of the imprecision due to bit-limited calculations shows that an error bound of 0.4% is possible with a 16-bit processor design. A method is described which uses a commercially available image processor by which images can be synthesized according to any of the standard pulse sequences in less than 600 ms.

Computers↗

PubMed1986

Pulse sequence extrapolation with MR image synthesis.

Previous reports have presented validation studies of magnetic resonance (MR) image synthesis in which multiple spin-echo (MSE) source data were used to generate spin-echo images for various echo times and repetition times (TRs). A new method-"pulse sequence extrapolation" -synthesizes images for pulse sequences different from that of the acquisition. MSE data acquired in a time equivalent to a TR of 2,000 msec can be used to generate inversion-recovery (IR) images for arbitrarily chosen TI inversion times. Other combinations of pulse sequences were also studied, and synthetic images were compared visually and quantitatively to directly acquired images with corresponding parameters. Synthetic IR signals of the brain parenchyma consistently matched directly acquired signals to within 6%, with respect to the full magnetization signal. The noise level of synthetic signals was generally no more than twice that of direct acquisition signals, as predicted. This method can achieve selective fat suppression and enhancement in IR imaging.

Biophysical Phenomena↗

PubMed1985

Synthesized MR images: comparison with acquired images.

Synthesized and directly acquired spin-echo images were compared in order to assess the validity of magnetic resonance (MR) image synthesis as a method enabling retrospective formation of images by interactive manipulation of scan parameters. Synthetic images subjectively compared favorably in both accuracy and precision with acquired images when formed for the same values of echo (TE) and repetition times (TR) and for interpolated and extrapolated values of both TE and TR. Plots of synthetic and acquired signals within the same pixel sectors quantitatively showed comparable values for several regions of interest in the brain. Percent error and noise-normalized differences between acquired and synthetic images were tested as a quantitative measure of accuracy. Percent error was consistently less than 5% for brain parenchyma, and synthetic signals were accurate to within four times the noise level at acquisition. The apparent signal-to-noise ratio of synthetic images was comparable, superior, or inferior to similar acquired images, depending on the values of TE and TR. Total acquisition time required for synthetic formation of images for arbitrary values of TE and TR was equivalent to that of a single direct acquisition with a TR of 2,500 msec.

Brain↗

PubMed1984

Automated MR image synthesis: feasibility studies.

The authors describe an automated technique of magnetic resonance (MR) image synthesis. Given a specific pulse sequence, MR signals are acquired for several pulse delay and/or repetition times and used to compute images of intrinsic parameters T1, T2, and N(H). Both the computed images and operator-specified pulse delay and repetition times are then used to "synthesize" a new image based on equations descriptive of MR signal behavior and comparable to that acquired by using the operator-specified parameters in an actual MR study. Instrumentation enabling rapid operator-interactive generation of synthesized images is described and initial results presented, allowing for dependence of the signal on T2 in spin echo images. Extension to full T1, T2, and N(H) dependence for arbitrary pulse sequences is described. Major advantages of this technique include retrospective optimization of contrast between arbitrary materials, rapid and systematic image analysis, and reduced scanning time; potential limitations include accuracy, noise, motion artifacts, and multicomponent behavior.

Computers↗

PubMedDate not supplied

Improved precision in calculated T1 MR images using multiple spin-echo acquisition.

Calculated T1 images require that magnetic resonance signals be detected at several inversion or repetition times (TR). Multiple spin-echo (SE) acquisitions provide several measurements of the magnetization at each TR, the signal size diminishing according to T2 decay. In this work we review one method (Case 1) for estimating T1 from single echoes and present four new methods (Cases 2-5) in which multiple acquired echoes are used. For Case 2 a fit is performed using the first echo at each TR, repeated using second echoes, etc., and the final T1 estimate is the simple average of the individual fits at each echo time (TE). For Case 3 the optimum weighted average is performed. For Cases 4 and 5 synthetic SE images are generated at each TR prior to the T1 fit, Case 4 using a synthetic TE of zero, and Case 5 using a TE providing maximum signal-to-noise ratio in the synthetic image. The relative precision in T1 provided by each method is calculated rigorously. It is proven that Cases 3 and 5 are optimum and equivalent and can theoretically reduce the noise in T1 images by as much as 40% over Case 1 with no increase in scanning time. Approximations are proposed that enable the optimum methods to be implemented in a practical fashion. Experimental images are presented that verify the relative predicted behavior.

Brain↗

PubMedDate not supplied

The precision of TR extrapolation in magnetic resonance image synthesis.

We present a model of noise propagation from acquired magnetic resonance (MR) images to TR-extrapolated synthetic images. This model assumes that images acquired at two repetition times TR1 and TR2 are used to generate synthetic images at arbitrary repetition times TR. The predictions of the model are compared with experimentally acquired phantom data, and show excellent agreement. The model is utilized in an analysis of two applications of MR image synthesis: scan time reduction and multiple-image synthesis. Scan time is reduced by acquiring data at two short repetition times, and synthesizing at a longer repetition time, with TR1 + TR2 less than TR. For T1 = 800 ms, a reduction of 20% in scan time results in a 45% reduction in signal-to-noise ratio SNR, when compared to direct acquisition. Reducing scan time by much more than 20% produces large noise levels in the synthetic image, and is unlikely to be useful. In multiple-image synthesis, images are synthesized at any repetition time in the range 0 to TR1 + TR2, for contrast optimization. If T1 = 800 ms, and TR1 + TR2 = 2000 ms, the optimum combination of TR1, TR2 results in synthetic images whose SNR is at worst 22% less than the SNR of directly acquired images. For many values of TR, the synthetic images have SNR superior to that obtainable by direct acquisition.

Brain↗

PubMedDate not supplied

Cerebral magnetic resonance image synthesis.

The authors previously described magnetic resonance (MR) image synthesis, a process that enables the investigator to manipulate imaging parameters retrospectively and generate or "synthesize" the image that corresponds to various arbitrary scanning factors. They demonstrate the validity and utility of synthetic spin-echo images in cerebral imaging. As a test of their method, spin-echo images are synthesized for echo times identical to those of the original acquired images as well as for alternate values. Subjectively, the quality of synthetic and acquired images is comparable. It is shown quantitatively for several tissue types that the reconstructed synthetic signal matches the acquired signal within the uncertainty of the acquired images. Observed and measured noise levels in the acquired and synthetic images are comparable. Because of a signal-averaging effect, the synthetic images can have a higher signal-to-noise ratio than the source images, thereby providing improved boundary definition. Applications of MR image synthesis are discussed with respect to potential reduction in scanning time. The advantages of image synthesis versus analysis of computed images are discussed.

Adult↗