![]() This is made possible because the BOLD contrast is sensitive to changes in the tissue concentration and the amount of paramagnetic deoxyhemoglobin which, in turn, reflects perturbations in CBF, CBV and/or CMRO 2. We did not observe any temporal differences in the highly diffusion weighted data compared to BOLD, however, in any case, although the changes may likely be vascular in nature, they are highly localized to the tissue areas.įunctional MRI BOLD (Blood oxygenation level dependent) signal monitors changes in neural activity in the brain via alterations in cerebral blood flow (CBF), blood volume (CBV) and oxygen consumption (CMRO 2) 1 – 5. No significant changes in ADC were observed in surface areas containing mostly large vessels for any diffusion weighting. When using low b-values, delayed increases in the tissue ADC during activation were observed. In this work, we were able to measure a small but significant ADC decrease in tissue areas in conjunction with brain activation in the cat visual cortex at 9.4 T when using highly diffusion weighted images (b > 1200 s/mm 2) where the intravascular effects are minimal. At high fields, SNR increases, tissue signals are enhanced, and signal changes inside the blood are significantly reduced compared to lower fields. At high magnetic fields, and in animal model studies, these limitations are alleviated. Human studies also have the potential confound of motion. The low SNR can lead to significant partial volume effects because of the lower spatial resolutions required to attain sufficient SNR in diffusion weighted images. All of these studies have been conducted in humans and at lower magnetic fields, which can be limited by signal to noise ratios (SNR). Most recently, the ADC decreases have been suggested as originating from transient cell swelling caused by either shrinkage of the extracellular space or by some intracellular neuronal process which precedes the hemodynamic response. The changes, in either case, are hypothesized to provide the ability to improve the spatial specificity of fMRI over conventional BOLD methods. Recent studies in the human visual cortex using diffusion weighted fMRI have suggested that the apparent water diffusion coefficient (ADC) decreases in contrast to earlier studies which consistently reported ADC increases during neuronal activation. ![]()
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