Understanding the relationship between fMRI signal changes and activated cortex is paramount to successful mapping of neuronal activity. To this end, the relative extravascular and intravascular contribution to fMRI signal change from capillaries (localized), venules (less localized) and macrovessels (remote, draining veins) must be determined.
Brain MR signal has been observed to decrease during cessation of breathing due to the increase of deoxyhemoglobin in the blood. However, for both animal and human studies, we have demonstrated that if the subjects breathed 100% oxygen in advance of apnea for a short time, T2*-weighted MR brain signal increased when breathing was stopped for a period of 30-60 s. This demonstrates the possibility of measuring responses to hemodynamic change throughout the entire brain with a single respiratory perturbation in a rapid, reliable, and robust manner.
MRI was extended to the measurement of changes in oxidative metabolism in the normal human during functionally induced changes in cellular activity. A noninvasive MRI method that is model-independent calibrates the blood oxygen level dependent (BOLD) signal of functional MRI (fMRI) against perfusion-sensitive MRI, using carbon dioxide breathing as a physiological reference standard. This calibration procedure provides a regional measurement of the expected sensitivity of the fMRI BOLD signal to changes in the cellular activity of the brain.
In this report, a review is made of the mechanism behind the functional MRI (fMRI) signals and the advances made in many different areas of fMRI. Such areas include understanding and compensating for artifacts specific to fMRI, postprocessing of fMRI images, the application of fMRI to areas of interest in neuroscience, some interesting new glimpses on the frontiers of studying hemodynamics and metabolic responses, and the effect to combine fMRI results with magnetoencephalography (MEG) and electroencephalography (EEG).
Magnetic resonance imaging (MRI) studies of human brain activity are described. Task-induced changes in brain cognitive state were measured using high-speed MRI techniques sensitive to changes in cerebral blood volume (CBV), blood flow (CBF), and blood oxygenation. These techniques were used to generate the first functional MRI maps of human task activation, by using a visual stimulus paradigm. The methodology of MRI brain mapping and results from the investigation of the functional organization and frequency response of human primary visual cortex (V1) are presented.
Dynamic images are temporal sequences of images, where the intensities of certain regions of interest (ROI's) change with time, whereas anatomical structures remain stationary. Here, new applications of dynamic image analysis, called similarity mapping, are reviewed. Similarity mapping identifies regions in a dynamic image sequence according to their temporal similarity or dissimilarity with respect to a reference ROI.
Past neuroimaging studies of acupuncture have demonstrated variable results for important brainstem nuclei. We have employed cardiac-gated fMRI with T1-variability correction to study the processing of acupuncture by the human brain. Furthermore, our imaging experiments collected simultaneous ECG data in order to correlate heart rate variability (HRV) with fMRI signal intensity.
RATIONALE AND OBJECTIVES: The authors determine the utility of relative blood volume measurements (rBV) using a blood pool marker for magnetic resonance imaging (MRI) in detection of early testicular torsion.
BACKGROUND: Functional MRI (fMRI) is of potential value in determining hemisphere dominance for language in epileptic patients.
OBJECTIVE: To develop and validate an fMRI-based method of determining language dominance for patients with a wide range of potentially operable brain lesions in addition to epilepsy.
Functional magnetic resonance imaging (fMRI) was used to map cortical areas that control eye blinking. T2*-weighted asymmetric spin-echo MRI (or BOLD-blood oxygen level dependent-MRI) was used to detect changes caused by focal variations in blood oxygenation. Six normal volunteers and two cases of dry eye (less than 5-mm Schirmer's test) entered the study. The experimental scheme consisted of three cycles of a two-step sequence: (eyes closed)-(blink or blink inhibition).
