Biomedical subjects

I K Mun

Publications and source records attributed to I K Mun.

7 recordsLinked to original sources

PubMed1992

A prospective controlled study of magnetic resonance imaging of the brain in gay men and parenteral drug users with human immunodeficiency virus infection.

To detect the earliest structural changes in the brain in human immunodeficiency virus (HIV) infection, 118 gay men and 115 parenteral drug users enrolled in a study of the natural history of HIV infection underwent magnetic resonance imaging evaluations. Routine T2-weighted and heavily T2-weighted scans for quantification of brain water were obtained, blinded to HIV serostatus. Atrophy and foci of increased signal did not correlate with any medical, immunologic, neurologic, or neuropsychologic parameters in the group as a whole, or in the gay men or parenteral drug user subgroups. Three subjects had progressive multifocal leukoencephalopathy and one had central nervous system lymphoma. In a subgroup in whom intracranial water percent was calculated, correlations were found with CD4 counts and CD4/CD8 ratios. We conclude that standard magnetic resonance imaging of the brain does not differentiate asymptomatic and mildly symptomatic HIV-positive individuals from HIV-negative individuals, regardless of risk group. However, intracranial water percent may distinguish HIV-positive from HIV-negative individuals because it correlates with raw CD4 counts and CD4/CD8 ratios.

Adolescent↗

PubMed1991

New spatial localization method using pulsed high-order field gradients (SHOT: Selection with High-Order gradienT).

A new spatial localization method using an additional set of high-order magnetic field gradients is described. The method uses a nonlinear part of high-order magnetic field gradient patterns which allows us to select a volume in conjunction with the selective radiofrequency (RF) pulse. Unlike the other existing volume selection methods such as ISIS or SPARS, the proposed selection method requires only one RF-gradient pulse pair to select a volume in two directions. The center of the selected volume can be moved to any arbitrary location within the body by the addition of precalculated lower order gradients which are simultaneously pulsed with the high-order gradient. The method also has the potential for localized spectroscopy from the FID signal which can be realized by using oscillating second- and first-order gradients for 3D selection with a single RF pulse. By using the proposed localization method, it is possible to design more flexible pulse sequences, e.g., the shorter echo-time spectroscopic pulse sequence. We have designed and constructed a six-loop r2 (or x2 + y2) gradient coil for initial application. By simultaneously applying this second-order gradient and proper x, y, and/or z gradients, 2D selections were achieved in arbitrarily selected positions in conjunction with a single selective RF pulse. Phantom and animal experiments have been performed and the results appear promising, especially in areas of NMR spectroscopic imaging applications where spatial localization is essential.

Animals↗

PubMed1991

An optimized multislice acquisition sequence for the inversion-recovery MR imaging.

An optimized multislice data acquisition scheme for inversion-recovery MR imaging is proposed and experimental results are presented. In this new scheme, instead of forming a set of multislice inversion-recovery sequences in series for a given phase encoding step, 180 degrees inversion pulses corresponding to different slices are interwoven with the spin echo data acquisition sequence in an optimal way depending on the desired inversion-recovery time. For example, between the 180 degrees inversion RF pulse and the spin-echo imaging sequence, a number of imaging and inversion sequences are inserted with different slice combinations, i.e., long inversion-recovery time is effectively utilized for the other slice pre-inversion and data acquisition. With the optimized sequence, imaging time has been reduced by as much as a factor of four compared with the existing methods.

Brain↗

PubMed1990

Radio frequency field intensity mapping using a composite spin-echo sequence.

A novel radio frequency (RF) field intensity mapping or imaging method using a composite NMR spin-echo sequence is proposed. A composite spin-echo RF pulse with 90 degrees y-180 degrees x-90 degrees y sequence makes phase change in the final image depending on the RF field intensity on the object. The resultant phase change or phase map can be used to obtain the actual RF flip-angle map for a given condition which includes the status of tuning and RF inhomogeneity, etc. Bloch equation has been solved numerically to obtain the effects of the RF field intensity as well as the main magnetic field inhomogeneity and the results are used for the mapping (imaging) of the RF field intensity. Phantom studies have been performed using a 1.5 Tesla whole body MRI system and the results are presented.

Humans↗

PubMed1990

Quadtree-based data compression of abdominal CT images.

An ROC study was performed to evaluate the effect of quadtree-based data compression on the diagnostic yield of CT images. Seventy images were selected from a series of consecutive abdominal/pelvic CT scans. Following the application of quadtree-based compression, all images were reviewed independently by three radiologists. The images were analyzed at six decreasing irreversible compression ratios (30.6:1 to 7.4:1), as well as after reversible compression (2.9:1). ROC curves reveal a gradual decrease in clinical accuracy with increasing compression ratios. At a compression ratio of 7.4:1, sensitivity for all major abnormalities was 99% with a specificity of 93%. As the compression ratio was increased to 30.6:1, sensitivity and specificity dropped to 75% and 83% respectively. Execution times for compression and decompression of the CT images with a PC-AT based digital radiography system varied from 24.7 to 18.5 seconds and from 16.2 to 5.1 seconds respectively, decreasing with higher levels of compression. We conclude that quadtree-based compression of abdominal CT images may have practical applications for a PC based digital radiography system.

Algorithms↗

PubMed1990

Application of region of interest definition to quadtree-based compression of CT images.

A quadtree-based data compression algorithm can provide different levels of compression within and outside of regions of interest (ROIs). The current study shows whether ROI compression can provide greater compression or diagnostic accuracy than uniform quadtree compression. In 75 single CT images from 75 consecutive abdominal examinations, 43 abnormalities were identified and surrounded by ROIs. Three radiologists interpreted the images following (1) 50:1 compression of the entire image; (2) ROI compression at five decreasing compression ratios (with 50:1 compression outside the ROI); and (3) reversible (lossless) compression of the entire image. Reversible compression (compression ratio 3:1) yielded a sensitivity of 96%. ROI compression of 15:1 was achieved with no loss of sensitivity; ROI compression of 28:1 yielded a sensitivity of 91% (not significantly different). At any given compression ratio, diagnostic sensitivity was greater with ROI compression than with uniform quadtree compression. For purposes of image archiving, quadtree-based ROI compression is superior to uniform compression of CT images.

Algorithms↗

PubMed1988

In vivo magnetic resonance imaging of sodium in the human body.

Sodium magnetic resonance imaging of the human body in vivo has been tried and its clinical application is considered. A short T2 imaging algorithm with total volume excitation and a specialized RF coil focused to the region of interest have been adopted to improve the signal-to-noise ratio. Using a 1.5-T human body imaging system, several important organs including heart, liver, gallbladder, kidney, and spine have been examined to demonstrate their sodium concentration in vivo.

Body Composition↗