Search PubMed⌕ Search

Biomedical subjects

Z H Cho

Publications and source records attributed to Z H Cho.

At least 55 records · Page 3Linked to original sources
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↗
PubMed1988

Fast gradient-echo chemical-shift imaging.

A fast chemical-shift imaging using the gradient-echo technique is proposed for the separation of the water and fat in human in vivo imaging. This technique also incorporates magnetic field inhomogeneity correction in the fast chemical-shift imaging. Experimental results of the proposed imaging technique demonstrate that the method would be useful for quick in vivo water-fat separation imaging with a substantially shorter time than the conventional spin-echo imaging sequence.

Adipose Tissue↗
PubMed1988

Fast SSFP gradient echo sequence for simultaneous acquisitions of FID and echo signals.

Recently, the gradient echo imaging sequence in conjunction with small flip angle excitations and short repetition times has been widely used for fast NMR imaging. As the repetition time decreases to an extent comparable to the spin-spin relaxation time T2, the residual phase coherency of the transverse spin magnetization tends to form another nuclear signal which is heavily weighted by T2 and similar to a long TR/long TE spin-echo signal. This effect, although expected, has not been utilized in the conventional fast gradient echo imaging. When this residual phase coherency is utilized in conjunction with the fast SSFP (steady-state free precession) technique, both the FID and the echo signals can be obtained. In this paper, we have proposed a new technique by which simultaneous acquisitions of both the FID and the echo signals are possible. Experiments on this fast SSFP mode imaging have shown that the FID signal is T1-weighted while the echo signal is strongly T2-weighted. The flip angle optimal for maximizing signal and related contrast are also studied in conjunction with the proposed sequence and the experimental results are presented.

Brain↗
PubMed1988

Measurement of the magnetic susceptibility effect in high-field NMR imaging.

The magnetic susceptibility effect often obtained in clinical NMR imaging is analysed and a measurement method of the magnetic susceptibility effect is proposed. The method of extraction of the susceptibility effect alone in high-field NMR imaging where usually the chemical shift and main magnetic field inhomogeneity effects are intermingled with the susceptibility effect is discussed. The proposed susceptibility measurement uses the susceptibility weighted echo-time encoding technique (SWEET) to modulate the NMR signal as a function of echo time with which the image intensity can be weighted. Both phantom-oriented physical experiments and human volunteer experiments were performed to demonstrate the usefulness of the technique for human imaging.

Humans↗
PubMed1987

Chemical-shift imaging with large magnetic field inhomogeneity.

A new echo-time-encoded chemical-shift imaging technique applicable to dual-peak spectroscopic imaging with large magnetic field inhomogeneity is proposed and studied. The basic concept and its applications to modest field homogeneity (approximately 3.0 ppm) as well as to relatively large field inhomogeneity (approximately 10.5 ppm) are discussed. Actual pulse sequences are given and some experimental results on human volunteers obtained with a 2.0-T KAIS NMR system are also presented.

Adipose Tissue↗
PubMed1987

Gradient reversal technique and its applications to chemical-shift-related NMR imaging.

The chemical-shift effect often degrades slice selectivity thereby degrading the contrast and spatial resolutions in a high-field NMR imaging. We propose a new pulse sequence which can compensate for this chemical-shift effect during slice selection. This technique provides correct slice definition as well as clear separation between water and lipid protons in high-field proton NMR imaging. Particular applications of this technique are 2D imaging with a thin-slice selection and chunk 3D imaging, where correction of the chemical-shift effect in slice selection is important. Another interesting application of the technique is chemical-shift-selective (CHESS) imaging. This technique does not require spectral-selective rf pulses and it takes advantage of the chemical-shift effect during slice selection. The latter is found to be useful for human in vivo chemical-shift imaging in high-field NMR. In this paper, the theoretical derivation of the proposed gradient reversal technique, its applications, and related experimental results are presented.

Humans↗
PubMed1987

Whole body slotted tube resonator (STR) for proton NMR imaging at 2.0 Tesla.

Design principles of a whole body Slotted Tube Resonator (STR) RF coil are given and fabrication details of the RF coil for a 2.0 Tesla NMR imaging system are presented. Experimental study proved efficient operation of the coil at 2.0 Tesla field and indicated potential for the extension of the technique to the even higher fields.

Equipment Design↗
PubMed1987

Methods and algorithms for Fourier-transform nuclear magnetic resonance tomography.

In nuclear magnetic resonance (NMR) imaging there are two main image formation approaches, the direct Fourier imaging method proposed by Kumar et al. [J. Mag. Res. 88, 69 (1975)] and the projection reconstruction technique commonly used in conventional computed tomography. In this paper these two approaches are reviewed with particular reference to Fourier-transform NMR imaging or tomography. Although Fourier imaging is the most conventional imaging method, useful applications for projection reconstruction techniques have often been found in NMR imaging. Therefore, in this paper, the emphasis is placed more on the projection reconstruction techniques that may not be readily available in NMR imaging.

Algorithms↗
PubMed1986

Real-value representation in inversion-recovery NMR imaging by use of a phase-correction method.

A new technique of real-value representation in inversion-recovery (IR) imaging by use of a phase-correction method is proposed. In this scheme, negative magnetizations at the beginning of the inversion point (T1 approximately equal to 0) are correctly represented as negative values rather than positive. By use of this new scheme, a consistent IR image set as a function of several inversion times (T1) can be obtained. The latter, i.e., consistent image set which represents the real value of T1 weighted images at several inversion times is important in the search for tumors and abnormalities since the inversion time (T1) in the pulse sequence strongly affects T1 contrast.

Brain↗
PubMed1986

Quadrature-multiplexed rf excitation for information doubling in NMR imaging.

A quadrature multiplexed pulse sequence to double the data information in a given acquisition time is proposed for NMR tomography. This technique takes advantage of the Hermitian property of the Fourier transform operation in NMR imaging. The proposed technique uses two rf pulses with 90 degrees phase difference simultaneously applied for the excitation of the spins in two different slices. The method results in truly simultaneous data acquisition coming from two different anatomical slices which can be separated into the real and imaginary parts of the image domain. This technique, therefore, also allows us either to reduce the experiment time by half or double the number of slices in the same examination time, depending on the applied imaging scheme. As a preliminary study, simultaneous dual-slice imaging of a human head using a conventional single-slice imaging scheme was performed.

Fourier Analysis↗
PubMed1986

A method for in vivo MR imaging of the short T2 component of sodium-23.

A magnetic resonance imaging pulse sequence featuring a short echo time of 3.6 ms is described and used clinically for in vivo imaging of sodium-23 in the human head. In living tissues, sodium exhibits at least two transverse relaxation constants, namely, (a) a short component T2s = 0.7-3.0 ms and (b) a long component T2e = 16-30 ms. Since our first in vivo imaging of sodium, an echo time of 10-15 ms has been used by us and by other investigators. This echo time is adequate for the observation of the long T2 component of sodium. However, a substantial fraction of the sodium, namely, the short T2 component, representing about 40%, has remained undetected by the imaging method with this echo time and is now observable by the new pulse sequence. The proposed method is a hybrid technique combining both projection reconstruction and Fourier encoding schemes. The projection reconstruction is used for the xy plane imaging while the Fourier encoding is used for slice separation. Sodium MR images of the human head produced with a short and a long TE are presented and analyzed. There is an increase in the MR signal and improved visibility of intraparenchymal sodium with the new scheme of short TE.

Brain↗
PubMed1986

Some results of high-flow-velocity NMR imaging using selection gradient.

A simple and new flow velocity measurement technique using conventional spin-echo sequence is proposed and its applications to a preclinical result are presented. This technique utilizes the phase velocity encoding effect due to 180 degrees rf and its corresponding selection gradient. This phase encoding and its phase velocity relations have been obtained by numerical solution of the Bloch equation. A flow velocity measurement obtained with a volunteer using this proposed technique indicates close agreement with other previously measured values.

Aorta↗
PubMed1986

A multinuclear magnetic resonance imaging technique--simultaneous proton and sodium imaging.

Simultaneous imaging of proton and sodium is achieved using a new two coil system and a time-multiplexing technique with a 1.5 T NMR imaging system. Distinctly different NMR parameters of protons and sodium, such as resonant frequencies and T1 relaxation rates, are incorporated in the imaging with a new dual coil arrangement. Since the T1 relaxation time of sodium is substantially shorter than that of protons, a set of fast repeating sodium pulse sequences is inserted between the proton pulse repetition intervals. The different resonant frequencies of the nuclei provide good isolation between the two RF coils. In this paper, the system configuration and the pulse sequence employed for the proposed simultaneous multinuclear imaging of protons and sodium are presented along with some preliminary results.

Humans↗
PubMed1985

Chemical-shift artifact correction scheme using echo-time encoding technique.

An investigation has been undertaken to study the effects of chemical shift on NMR image obtained by the projection reconstruction technique. Mathematical simulation studies indicate that chemical shift causes blurring in NMR images regardless of the image reconstruction technique. A systematic study of such artifacts is presented and possible correction schemes are proposed. Exemplary results with and without correction of the chemical-shift artifact and pulse schemes employed for projection reconstruction are presented. The present chemical-shift correction scheme is basically the same as a spectroscopic imaging technique such as the existing 4-D NMR imaging method except that the encoding steps required can be reduced substantially in the case of modest spectroscopic resolution.

Magnetic Resonance Spectroscopy↗
PubMed1985

Time-multiplexed multislice inversion-recovery techniques for NMR imaging.

Multislice imaging techniques effectively applicable to inversion-recovery (IR) imaging are developed and applied to human imaging. These new multislice IR imaging sequences employ the time-multiplexing (TM) technique conventionally used in the slice-by-slice saturation-recovery (SR) imaging. Two new time-multiplexed multislice (TMM) IR imaging sequences are proposed and some of the experimental results obtained with the methods are presented.

Humans↗
PubMed1984

Limits to signal-to-noise improvement by FID averaging in NMR imaging.

We studied the improvement of signal-to-noise ratio (SNR) in NMR images by free induction decay (FID) averaging. We found that as the number of FIDS averaged increases, a region is reached where the image SNR levels off asymptotically. A theoretical explanation for this behaviour is offered. Agreement between experimental results and theoretical prediction is excellent. It is also pointed out that the exact location of the point at which further averaging does not pay-off is dependent on the individual NMR imaging system used.

Information Theory↗