Hyung-Joong Kim

Detection of cerebral metabolites in a canine model of ischemic stroke using 1H magnetic resonance spectroscopy.

Proton magnetic resonance spectroscopy ((1)H MRS) provides in vivo biochemical information on tissue metabolites. The purpose of this study was to investigate the serial metabolic changes of (1)H MRS in the cerebrum of ischemic dogs. An ischemic stroke was induced in five health laboratory beagle dogs by permanent middle cerebral artery occlusion using a silicone plug. (1)H MRS was serially performed three times with a 1.5-T MR system: before, three days after and 10days after the stroke. Immunohistochemical staining was performed to determine the expression of neuronal nuclei (NeuN) and glial fibrillary acidic protein (GFAP) at both the ipsilateral and contralateral cerebral cortex. Reduced levels of N-acetyl-asparate (p<0.05), choline (Cho), creatine (Cr) and myo-inositol (mI), and a marked increase in the lactate (Lac) level (p<0.01) were found at three days after the stroke. At 10days after the stroke, the levels of Lac significantly increased (p<0.01); however, the other metabolites were partially elevated. The changes of Cr, Cho and mI were not statistically significant (p>0.05) when the before and after stroke values were compared. There was a significant loss of NeuN and GFAP immunoreactivity at the ischemic core. (1)H MRS may be to a useful diagnostic tool for the evaluation of ischemic stroke in dogs.

MREIT conductivity imaging of canine head using multi-echo pulse sequence

In magnetic resonance electrical impedance tomography (MREIT), we measure induced magnetic flux densities subject to multiple injection currents to reconstruct cross-sectional conductivity images. Spin echo pulse sequence has been widely used in MREIT and produce postmortem and in vivo conductivity images of animal and human subjects. The image quality depends on the SNR of the measured magnetic flux density image. In order to reduce the scan time and current amplitude while keeping the image quality, we have developed a multi-echo pulse sequence for MREIT. In this study, we show results of canine head MREIT imaging experiments using the multi-echo pulse sequence. Compared to the injection current nonlinear encoding (ICNE) pulse sequence, it provides a higher SNR of MR magnitude images by combining multiple echo signals. Noise in measured magnetic flux density data is significantly reduced due to an increased current injection time over multiple echo signals. These allow us to significantly decrease the total scan time. Reconstructed conductivity images of a canine head show enhanced conductivity contrast between gray and white matter using the multi-echo pulse sequence. In our future work, we will focus on in vivo human and disease model animal experiments using the new MREIT pulse sequence.

In vivo conductivity imaging of canine male pelvis using a 3T MREIT system

The prostate is an imaging area of growing concern related with aging. Prostate cancer and benign prostatic hyperplasia are the most common diseases and significant cause of death for elderly men. Hence, the conductivity imaging of the male pelvis is a challenging task with a clinical significance. In this study, we performed in vivo MREIT imaging experiments of the canine male pelvis using a 3T MRI scanner. Adopting carbon-hydrogel electrodes and a multi-echo pulse sequence, we could inject as much as 10 mA current in a form of 51 ms pulse into the pelvis. Collecting magnetic flux density data inside the pelvis subject to multiple injection currents, we reconstructed cross-sectional conductivity images using a MREIT software package CoReHA. Scaled conductivity images of the prostate show a clear contrast between the central and peripheral zones which are related with prostate diseases including cancer and benign prostatic hyperplasia. In our future work, we will focus on prostate cancer model animal experiments.

In vivo conductivity imaging of human knee using 3 mA injection current in MREIT

Recent in vivo human leg MREIT experiments showed successful conductivity image reconstructions using carbon-hydrogel electrodes and optimized RF coils. However, it is still difficult to perform in vivo human and disease model animal experiments primarily due to a long scan time and high injection current of about 9 mA. Compared to previous MREIT pulse sequences, a newly developed multi-echo pulse sequence provides a higher SNR of MR magnitude image and better quality of magnetic flux density data. Unlike the human calf, the knee has sensitive nerve bundles and mainly consists of the bone. In this study, we tried to obtain high-resolution conductivity images of in vivo human knees using the multi-echo pulse sequence. We injected as much as 3 mA current in the form of an 81 ms pulse into the knee without producing a painful sensation and motion artifacts. Reconstructed conductivity images well distinguish different parts of the subcutaneous adipose tissue, muscle, synovial capsule, cartilage and bone inside the knee. Considering clinical applications, future work should be focused on in vivo human and disease model animal experiments.

Electric Field Mapping in ex vivo Anisotropic Muscle Tissue Using DT-MREIT

Accurate coverage of tissue with a sufficiently large electric field is one of the key conditions for successful electroporation. Magnetic resonance electrical impedance tomography (MREIT) provides a means to map the electric filed distribution during electroporation. To estimate the electric field strength, the magnetic flux density data induced by the electroporation pulses are measured from MREIT scans during electroporation. Since biological tissues such as skeletal muscle are anisotropic, we propose a novel MREIT technique to map the electric field in anisotropic as well as isotropic regions. We utilize the anisotropic conductivity estimation method based on the lately developed DT-MREIT technique where diffusion tensor imaging is combined with MREIT. To estimate the current density in an optimal way, we adopted the projected current density estimation algorithm. From ex vivo experiments using bovine muscle tissues, we found that the new method produces electric field maps with a wider coverage of electroporation than the previous method. The results suggest that it is important to properly handle the effects of the tissue anisotropy for more accurate mapping of electric field during electroporation.

Chemical shift artifact correction in MREIT using iterative least square estimation method

Human and animal imaging in magnetic resonance electrical impedance tomography (MREIT) demands high signal-to-noise ratio (SNR) data. We therefore perform MREIT experiments with a higher bandwidth per pixel. This leads to bigger chemical shift artifacts in MR images from fat regions. We may correct such artifacts in MREIT using a recently proposed method based on the three-point Dixon technique. This method is however not suitable for fast imaging pulse sequences. It has a poor SNR and also sometimes leads to swapping of water and fat signals in certain pixels when the field inhomogeneity phase unwrapping algorithm fails. This work demonstrates a new chemical shift artifact correction method in MREIT using a least square estimation method. Iterative separations of water and fat complex images obviate the phase unwrapping step. We present the separated water and fat images using the conventional and also the least square method. These two algorithms are compared in terms of the SNR and their water-fat separation capability. We propose the new method for future studies of fast MREIT imaging experiments.