Biomedical subjects

C B Ahn

Publications and source records attributed to C B Ahn.

7 recordsLinked to original sources

PubMed1991

Analysis of eddy currents in nuclear magnetic resonance imaging.

The eddy currents in nuclear magnetic resonance (NMR) imaging are analyzed from the solutions of Maxwell's equations and their effects are examined over various experimental conditions from whole-body diagnostic imaging to recently developed NMR microscopy. The analysis is focused mainly on the frequency characteristics and intensity variations of the eddy-current-induced field which depends on the overall system size, ratio of the gradient coil size to the magnet bore diameter, and the pulse-sequence-dependent parameters such as input current waveform and repetition time. From the analysis, the frequency response of the eddy-current-induced field is that of a high-pass filter whose cutoff frequency is inversely proportional to the square of the overall system size. The intensity ratio of the generated field to the induced field is not affected by the overall system size, but is sensitively related to the ratio of the gradient coil size to the magnet bore diameter.

Equipment Design↗

PubMed1991

Diffusion and perfusion in high resolution NMR imaging and microscopy.

Diffusion and perfusion phenomena under strong gradient fields (approximately 100 G/cm) are examined in high resolution nuclear magnetic resonance (NMR) imaging and microscopy, where diffusion-associated signal attenuation predominates over T1 and T2 relaxation decays. Image contrast based on the diffusion and microcirculation is discussed with experimental results obtained with a 7.0-T microscopy system. Ultimate resolution limit due to diffusion is investigated in high resolution NMR imaging and microscopy.

Diffusion↗

PubMed1989

Nuclear magnetic resonance microscopic ocular imaging for the detection of early-stage cataract.

A nuclear magnetic resonance (NMR) microscopic ocular imaging was performed at 7.0 Tesla to investigate its usefulness in the detection of early-stage cataracts. For this study, galactose cataracts were generated in experimental rabbits through diet (35% galactose), and enucleated eyes were imaged at various times after initiation of the diet. In previous studies using a 0.6 Tesla conventional magnetic resonance imager (MRI), the contrast between normal and cataractous tissues in the lens was not well defined, mainly due to the partial volume effect coming from the limitation of resolution and signal-to-noise ratio (SNR). With resolution of 60 X 60 X 80 microns, early localized precataractous tissue changes were clearly observed after 5 days diet. Precataractous tissue changes were seen histologically but no visible evidence of lens change was detected by the conventional slit lamp biomicroscope at this time. Substantially elongated spin-spin relaxation times (T2) in localized cataractous tissues (72.4 +/- 8.8 msec) were consistently observed compared with those in normal lens region (16.1 +/- 3.2 msec); however, the changes of the spin-lattice relaxation time (T1) were not significant. Some ocular NMR microscopic images with corresponding histological photographs are demonstrated to show the potential of NMR microscopy.

Animals↗

PubMedDate not supplied

An improved nuclear magnetic resonance diffusion coefficient imaging method using an optimized pulse sequence.

Two-dimensional diffusion coefficient maps (images) of a carefully controlled diffusion phantom have been measured by a new diffusion imaging sequence using a 0.6-T whole-body nuclear magnetic resonance (NMR) scanner having a gradient field strength of 2.5 mT/m. The free induction decay (FID) data for the diffusion coefficient images were collected by varying the duration of the readout gradient in the conventional two-dimensional Fourier imaging sequence. The experimental results obtained by the proposed NMR diffusion measurement technique indicate a close agreement with other previous measurements. The selection of optimum spin-echo time for maximum signal-to-noise ratio (SNR) in diffusion imaging is studied and also experimentally confirmed. Finally, a preclinical study with human volunteers has been performed and results are presented.

Humans↗

PubMedDate not supplied

The effects of random directional distributed flow in nuclear magnetic resonance imaging.

Capillary flow or microscopic random directional coherent flow as a model of perfusion is investigated both theoretically and experimentally. In the model, we assumed that molecular motion within a finite resolvable volume element (voxel) is a superposition of flow of randomly oriented small capillaries. In such a case, the observed signal from the capillary flow within a voxel will be attenuated in signal amplitude without any change in phase. Although this attenuation effect is similar to the diffusion phenomenon, it differs basically in the following aspects: since the motion in each capillary segment is coherent, phase cancellation occurs at even echoes due to spin rephasing, while the diffusion phenomenon is a purely random Brownian motion of the thermally agitated molecules, changing both in direction and speed during the measurement period. Because of the random character of diffusion, even-echo rephasing cannot be observed. Thus capillary flow or perfusionlike microscopic flow can be measured based on the above distinct flow characteristics, i.e., signal restoration at even echoes versus signal amplitude attenuation at odd echoes. By applying a suitable mathematical algorithm, information on the capillary flow alone can be extracted from the two separate distinct measurements, i.e., one with a single echo and the other with a double echo. Both a theoretical calculation of the capillary flow, as well as the experimental results with a human volunteer by a 0.6-T nuclear magnetic resonance imager, are presented.

Algorithms↗

PubMedDate not supplied

Nuclear magnetic resonance microscopy with 4-microns resolution: theoretical study and experimental results.

Nuclear magnetic resonance (NMR) microscopy with 4-microns resolution, a step closer to the 1-micron resolution with which in vivo cellular imaging would be possible is described. An analysis of the ultimate resolution and voxel size dependent signal-to-noise ratio (SNR) in NMR microscopy is presented and experimentally verified. For microscopic scale objects (less than 1-mm diameter), the SNR based on the geometrical scale factor(s) is found to be proportional to sn where n less than 2, rather than n = 3 as previously supposed. This comes about because of a drastic reduction in sample noise coupled with a significant sensitivity gain realized in small diameter radiofrequency coils. A new pulse sequence which reduces both diffusion dependent resolution degradation and signal attenuation is presented. The selection of optimal bandwidth and acquisition time for maximal SNR is discussed. Experimental results obtained on both a 2.0-T whole-body system and a 7.0-T small bore system adapted for microscopy indicate the potentials of 4-microns resolution microscopy with the existing magnets.

Animals↗

PubMedDate not supplied

A generalized formulation of diffusion effects in micron resolution nuclear magnetic resonance imaging.

A generalized formulation of the diffusion related nuclear magnetic resonance (NMR) signal is derived from a random walk model. Previous analyses performed in the NMR spectroscopy were the formulations of the diffusion related signal amplitude at a specific time, such as the spin echo formation time. They are, in general, not applicable to continuous time domain analyses. In this paper, we have extended the theory to the two-dimensional imaging case and derived an analytical formula useful for the computation of the diffusion affected signal as a function of continuous time for a time variant gradient. This formulation will be useful in NMR imaging, especially in NMR microscopy where the diffusion associated signal attenuation is serious due to the strong gradient fields (100-1000 G/cm), and at the same time data are acquired continuously for the acquisition period. In addition to the loss of the resolution and signal-to-noise ratio due to the random phase fluctuation by diffusion, the variation of the intensity during the data acquisition period introduces a line broadening whose full width at half-maximum is found to be much larger than the bandwidth-limited resolution or diffusion related intrinsic resolution. This line spreading effect is integrated in a computer simulation and is evaluated as an integral part of the overall diffusion effects in micron resolution NMR imaging or NMR microscopy.

Computer Simulation↗