Biomedical subjects

Nan-kuei Chen

Publications and source records attributed to Nan-kuei Chen.

5 recordsLinked to original sources

PubMed2004

Removal of EPI Nyquist ghost artifacts with two-dimensional phase correction.

Odd-even echo inconsistencies result in Nyquist ghost artifacts in the reconstructed EPI images. The ghost artifacts reduce the image signal-to-noise ratio and make it difficult to correctly interpret the EPI data. In this article a new 2D phase mapping protocol and a postprocessing algorithm are presented for an effective Nyquist ghost artifacts removal. After an appropriate k-space data regrouping, a 2D map accurately encoding low- and high-order phase errors is derived from two phase-encoded reference scans, which were originally proposed by Hu and Le (Magn Reson Med 36:166-171;1996) for their 1D nonlinear correction method. The measured phase map can be used in the postprocessing algorithm developed to remove ghost artifacts in subsequent EPI experiments. Experimental results from phantom, animal, and human studies suggest that the new technique is more effective than previously reported methods and has a better tolerance to signal intensity differences between reference and actual EPI scans. The proposed method may potentially be applied to repeated EPI measurements without subject movements, such as functional MRI and diffusion coefficient mapping.

Animals↗

PubMed2004

Spatially selective T2 and T2 * measurement with line-scan echo-planar spectroscopic imaging.

Line-scan echo planar spectroscopic imaging (LSEPSI) is applied to quickly measure the T2 and T2* relaxation time constants in pre-selected 2D or 3D regions. Results from brain imaging studies at 3T suggest that the proposed method may prove valuable for both basic research (e.g., quantifying the changes of T2/T2* values in functional MRI with blood oxygenation level-dependent contrast) and clinical studies (e.g., measuring the T2' shortening due to iron deposition). The proposed spatially selective T2 and T2* mapping technique is especially well suited for studies, where T2/T2* quantification needs to be performed dynamically in a pre-selected 2D or 3D region.

Brain↗

PubMed2004

Fast spectroscopic imaging strategies for potential applications in fMRI.

Technical aspects of two general fast spectroscopic imaging (SI) strategies, one based on gradient echo trains and the other on spin echo trains, are reviewed within the context of potential applications in the field of functional magnetic resonance imaging (fMRI). Fast spectroscopic imaging of water may prove useful for identifying mechanisms underlying the blood oxygenation level dependence (BOLD) of the water signal during brain activation studies. Reasonably rapid mapping of changes in proton signals from brain metabolites, like lactate, creatine or even neurotransmitter associated metabolites like GABA, is substantially more challenging but technically feasible particularly as higher field strengths become available. Fast spectroscopic methods directed towards the 31P signals from phosphocreatine (PCr) and adenosine tri-phosphates (ATP) are also technically feasible and may prove useful for studying cerebral energetics within fMRI contexts.

Adult↗

PubMed2003

fMRI of the conscious rabbit during unilateral classical eyeblink conditioning reveals bilateral cerebellar activation.

The relative contributions of the ipsilateral and contralateral cerebellar cortex and deep nuclei to delay eyeblink conditioning have been debated and are difficult to survey entirely using typical electrophysiological and lesion techniques. To address these issues, we used single-event functional magnetic resonance imaging (fMRI) in the conscious rabbit to visualize the entire cerebellum simultaneously during eyeblink conditioning sessions. Examination of the blood oxygenation level-dependent (BOLD) response to a visual conditioning stimulus early in training revealed significant bilateral learning-related increases in the BOLD response in the anterior interpositus nucleus (IPA) and significant bilateral deactivation in hemispheric lobule VI (HVI) of the cerebellar cortex. Later in training, the BOLD response remained bilateral in the cortex and predominantly ipsilateral in the IPA. Conditioning stimulus-alone trials after conditioning revealed that both sides of HVI were affected similarly but that only the ipsilateral interpositus nucleus was activated. These results suggest that both sides of HVI normally influence the side of the IPA being conditioned and illustrate how fMRI can be used to examine multiple brain regions simultaneously in an awake, behaving animal to discover more rapidly the neural substrates of learning and memory.

Animals↗

PubMed2003

Functional MRI with variable echo time acquisition.

A new functional MRI protocol that integrates variable echo time (TE) acquisition and a block-design paradigm is proposed and evaluated with finger-tapping motor task. Simulations and experimental data show that the blood oxygenation level-dependent (BOLD) sensitivity achieved with this approach is comparable to that achieved using a conventional constant-TE protocol. The proposed variable-TE fMRI protocol provides valuable information that cannot be obtained with the constant-TE protocol. First, a field inhomogeneity map can be derived from the multi-TE data and used to correct EPI geometric distortions. Second, changes of T2* values due to the BOLD effect can be quantified. Third, for brain regions with pronounced susceptibility field gradients, the reduced BOLD sensitivity may be compensated for when the acquired multi-TE data are processed appropriately (e.g., with weighted summation). Fourth, large venules and veins may possibly be identified (depending on the vessel orientation and volume fraction) by evaluating the phase values of the multi-TE data. Finally, magnetic field drift over time can be measured from dynamic field maps available with this protocol.

Algorithms↗