Gender differences in cortical representation of rectal distension in healthy humans.

Cerebral cortical processing of information relayed via visceral afferents is poorly understood. We determined and compared cortical activity caused by various levels of rectal distension in healthy male and female subjects. Twenty-eight healthy, young (20-44 yr) volunteer subjects (13 male, 15 female) were studied with a paradigm-driven functional magnetic resonance imaging (fMRI) technique during barostat-controlled rectal distension at perception threshold and 10 mmHg below and above perception threshold. Male subjects showed localized clusters of fMRI activity primarily in the sensory and parietooccipital regions, whereas female subjects also showed activity in the anterior cingulate and insular regions. A progressive increase in maximum percent fMRI signal change and total volume of cortical activity was associated with the intensity of rectal distension pressure in both genders. Regions of cortical activity for below-threshold stimuli showed less substantial signal intensity and volume than responses for threshold and above-threshold stimuli. Volume of cortical activity during rectal distension in women was significantly higher than that for men for all distensions. We conclude that 1) there are substantial differences in female cortical activation topography during rectal distension compared with males; 2) intensity and volume of registered cortical activity due to rectal stimulation are directly related to stimulus strength; and 3) rectal stimulation below perception level is registered in the cerebral cortex.

High-resolution fMRI using multislice partial k-space GR-EPI with cubic voxels.

The premises of this work are: 1) the limit of spatial resolution in fMRI is determined by anatomy of the microcirculation; 2) because of cortical gray matter tortuosity, fMRI experiments should (in principle) be carried out using cubic voxels; and 3) the noise in fMRI experiments is dominated by low-frequency BOLD fluctuations that are a consequence of spontaneous neuronal events and are pixel-wise dependent. A new model is proposed for fMRI contrast which predicts that the contrast-to-noise ratio (CNR) tends to be independent of voxel dimensions (in the absence of partial voluming of activated tissue), TE, and scanner bandwidth. These predictions have been tested at 3 T, and results support the model. Scatter plots of fMRI signal intensities and low-frequency fluctuations for activated pixels in a finger-tapping paradigm demonstrated a linear relationship between signal and noise that was independent of TE. The R(2) value was about 0.9 across eight subjects studied. The CNR tended to be constant across pixels within a subject but varied across subjects: CNR = 3.2 +/- 1.0. fMRI statistics at 20- and 40-ms TE values were indistinguishable, and TE values as short as 10 ms were used successfully. Robust fMRI data were obtained across all subjects using 1 x 1 x 1 mm(3) cubic voxels with 10 contiguous slices, although 1.5 x 1.5 x 1.5 mm(3) was found to be optimum. Magn Reson Med 46:114-125, 2001.

Swallow-related cerebral cortical activity maps are not specific to deglutition.

Cortical representation of swallow-related motor tasks has not been systematically investigated. In this study, we elucidated and compared these cortical representations to those of volitional swallow using block-trial and single-trial methods. Fourteen volunteers were studied by functional magnetic resonance imaging. Cortical activation during both swallowing and swallow-related motor tasks that can be performed independent of swallowing, such as jaw clenching, lip pursing, and tongue rolling, was found in four general areas: the anterior cingulate, motor/premotor cortex, insula, and occipital/parietal region corresponding to Brodmann's areas 7, 19, and 31. Regions of activity, volume of activated voxels, and increases in signal intensity were found to be similar between volitional swallow and swallow-related motor tasks. These findings, using both block-trial and single-trial techniques, suggest that cerebral cortical regions activated during swallowing may not be specific to deglutitive function.

B(0)-fluctuation-induced temporal variation in EPI image series due to the disturbance of steady-state free precession.

Steady-state free precession (SSFP) can develop under a train of RF pulses, given the condition TR < T(2). SSFP in multi-shot imaging sequences has been well studied. It is shown here that serial single-shot echo-planar imaging (EPI) acquisition can also develop SSFP, and the SSFP can be disturbed by B(0) fluctuation, causing voxel-wise temporal variation. This SSFP disturbance is predominantly present in cerebrospinal fluid (CSF) regions due to the long T(2) value. By applying a sufficiently strong crusher gradient in the EPI pulse sequence, the temporal variation induced by SSFP disturbance can be suppressed due to diffusion. Evidence is provided to indicate that physiological motions such as cardiac pulsation and respiration could affect the voxel-wise time courses through the mechanism of SSFP disturbance. It is advised that if the disturbance is observed in serial EPI images, the crusher should be made stronger to eliminate the unwanted temporal variation.

Real-time 3D image registration for functional MRI.

Subject head movements are one of the main practical difficulties with brain functional MRI. A fast, accurate method for rotating and shifting a three-dimensional (3D) image using a shear factorization of the rotation matrix is described. Combined with gradient descent (repeated linearization) on a least squares objective function, 3D image realignment for small movements can be computed as rapidly as whole brain images can be acquired on current scanners. Magn Reson Med 42:1014-1018, 1999.

Current-induced magnetic resonance phase imaging.

Electric current-induced phase alternations have been imaged by fast magnetic resonance image (MRI) technology. We measured the magnetic resonance phase images induced by pulsed current stimulation from a phantom and detected its sensitivity. The pulsed current-induced phase image demonstrated the feasibility to detect phase changes of the proton magnetic resonance signal that could mimic neuronal firing. At the present experimental setting, a magnetic field strength change of 1.7 +/- 0.3 nT can be detected. We also calculated the averaged value of the magnetic flux density BT parallel to B0 produced by electric current I inside the voxel as a function of the wire position. The results of the calculation were consistent with our observation that for the same experimental setting the current-induced phase change could vary with location of the wire inside the voxel. We discuss our findings in terms of possible direct MRI detection of neuronal activity.

Identification and characterization of cerebral cortical response to esophageal mucosal acid exposure and distention.

BACKGROUND & AIMS: Esophageal acid exposure is a common occurrence in healthy individuals and patients with esophagitis. Clinically, perception of this exposure ranges from no perception to severe heartburn and chest pain. Cerebral cortical response to esophageal mucosal contact to acid has not been systematically studied. The aim of this study was to elucidate cerebral cortical response to esophageal acid exposure in normal individuals by functional magnetic resonance imaging (FMRI). METHODS: We studied 10 normal healthy volunteers. Cortical FMRI response to 10 minutes of intraesophageal perfusion of 0.1N HCl (1 mL/min) was determined, and the results were compared with those of saline infusion and balloon distention. RESULTS: Acid perfusion did not induce heartburn or chest pain but increased FMRI signal intensity by 6.7% +/- 2.0% over the preperfusion values. No increase was detected for saline infusion. FMRI signal intensity to balloon distention was similar to that of acid perfusion. Activation latency, activation to peak, and the deactivation periods for response to acid perfusion were significantly longer than those of balloon distention (P < 0.05). CONCLUSIONS: Contact of esophageal mucosa with acid, before inducing heartburn, evokes a cerebral cortical response detectable by FMRI. Temporal characteristics of this response are significantly different from those induced by esophageal balloon distention.

Single-shot half k-space high-resolution gradient-recalled EPI for fMRI at 3 Tesla.

Half k-space gradient-recalled echo-planar imaging (GR-EPI) is discussed in detail. T2* decay during full k-space GR-EPI gives rise to unequal weighting of the lines of k-space, loss of signal intensity at the center of k-space, and a point-spread function that limits resolution. In addition, the long readout time for high-resolution full k-space acquisition gives rise to severe susceptibility effects. These problems are substantially reduced by acquiring only half of k-space and filling the empty side by Hermitian conjugate formation. Details of the pulse sequence and image reconstruction are presented. The point-spread function is 3(1/2) times narrower for half than full k-space acquisition. Experiments as well as theoretical considerations were carried out in a context of fMRI using a whole-brain local gradient and an RF coil at 3 Tesla. Using a bandwidth of +/-83 kHz, well-resolved single-shot images of the human brain, as well as good quality fMRI data sets were obtained with a matrix of 192 x 192 over 16 x 16 cm2 FOV using half k-space techniques. The combination of high spatial resolution using the methods presented in this article and the high temporal resolution of EPI opens opportunities for research into fMRI contrast mechanisms. Increase of percent signal change as the resolution increases is attributed to reduction of partial volume effects of activated voxels. Histograms of fMRI pixel responses are progressively weighted to higher percent signal changes as the resolution increases. The conclusion has been reached that half k-space GR-EPI is generally superior to full k-space GR-EPI and should be used even for low-resolution (64 x 64) EPI.

Magnetic field changes in the human brain due to swallowing or speaking.

Variations in the magnetic field in the human brain caused by the processes of swallowing or speaking are measured. In both processes, motion of the pharyngeal muscles, especially the tongue and jaw, alter the susceptibility-induced magnetic field distribution at the brain slice being imaged. This leads to image warping, compromising the analysis of a time series of images, such as in functional magnetic resonance imaging (fMRI). These dynamic changes are assessed by acquiring a time series of images using a gradient-echo asymmetric-spin-echo sequence (GREASE), a technique in which two images are acquired for each excitation--one during the gradient echo, and one during the latter part of the spin echo. The NMR phase difference between the two images is a measure of the magnetic field distribution. A series of brain images, acquired with this sequence while the subject either swallows or speaks, indicated negative magnetic field changes of up to 0.087 ppm in the inferior region of the brain for both speaking and swallowing, and in some speech, additional positive changes of up to 0.056 ppm in the frontal region of the brain were indicated.

Functional MRI of brain activation induced by scanner acoustic noise.

A method is introduced by which brain activation caused by the acoustic noise associated with echo planar imaging (EPI) is mapped. Two types of time series were compared. The first time series, considered the "task," involved applying only EPI gradients for 20 s without the application of RF pulses, then, without pause, starting image collection. The second, considered the "control," involved typical sequential image acquisition without the prior gradient pulses. Subtraction of the first 5 s of the two time series revealed signal enhancement mainly in the primary auditory cortex. The technique was validated using a motor cortex task that mimicked the hypothesized scanner noise induced activation.

Artifacts in functional magnetic resonance imaging from gaseous oxygen.

Unexpectedly large fluctuations in signal intensity were identified in the functional MRI (FMRI) of normal subjects breathing pure oxygen intermittently. To test the hypothesis that the signal changes were due to fluctuating concentrations of gaseous (paramagnetic) oxygen in the magnetic field, echo planar gradient echo images were acquired of a phantom contiguous to an oxygen mask through which pure oxygen was administered intermittently via plastic tubing. As a control, room air was administered intermittently or oxygen continuously in the same experimental protocol. Signal intensity changes of up to 60% temporally correlated with the administration of oxygen were produced in the phantom. In functional images prepared from the echo planar images, the signal intensity changes resulted in artifacts especially at interfaces in the phantom. The intermittent administration of pure oxygen during acquisition of data for FMRI may produce signal intensity changes that stimulate or obscure function.

Lateralized human brain language systems demonstrated by task subtraction functional magnetic resonance imaging.

OBJECTIVE: To develop a procedure for noninvasive measurement of language lateralization with functional magnetic resonance imaging (MRI). DESIGN: Functional neuroimaging using time-series echo-planar MRI. SETTING: University medical center research facility. SUBJECTS: Five healthy, right-handed, young adults. MAIN OUTCOME MEASURES: Number of MRI voxels in left and right hemispheres showing task-related signal increases during two contrasting auditory processing tasks. The nonlinguistic task involved processing of pure tones, while the linguistic task involved processing of single words based on semantic content. RESULTS: The pure-tone processing task activated temporal lobe auditory areas and dorsolateral frontal regions bilaterally. Using this task as a control condition, the semantic processing task resulted in lateralized activity in distributed regions of the left hemisphere. A significant effect of task on intrahemispheric activity pattern was demonstrated in every subject. Results were reproduced in preliminary studies of test-retest reliability. CONCLUSIONS: The results demonstrate the lateralized anatomy of semantic linguistic systems in contrast to non-linguistic auditory sensory processors and introduce a task subtraction technique adapted for functional MRI as a noninvasive measure of language lateralization.

Real-time functional magnetic resonance imaging.

A recursive algorithm suitable for functional magnetic resonance imaging (FMRI) calculations is presented. The correlation coefficient of a time course of images with a reference time series, with the mean and any linear trend projected out, may be computed with 22 operations per voxel, per image; the storage overhead is four numbers per voxel. A statistical model for the FMRI signal is presented, and thresholds for the correlation coefficient are derived from it. Selected images from the first real-time functional neuroimaging experiment (at 3 Tesla) are presented. Using a 50-MHz workstation equipped with a 14-bit analog-to-digital converter, each echo planar image was acquired, reconstructed, correlated, thresholded, and displayed in pseudocolor (highlighting active regions in the brain) within 500 ms of the RF pulse.

Functional magnetic resonance imaging in partial epilepsy.

Functional magnetic resonance imaging (FMRI) detects signal changes in brain that accompany regional changes in neuronal activity. In normal human brain, FMRI shows changes in signal in the postcentral gyrus or superior temporal gyrus that correlate with voluntary motor activity or language processing, respectively. The model used to explain the changes in signal linked temporally with cerebral activity is a reduction in cerebral capillary deoxyhemoglobin concentration due to the increased blood flow that accompanies neuronal activity in the cerebrum. FMRI has been used in normal subjects but not extensively in patients. To determine the feasibility of using FMRI to map cerebral functions in patients with partial epilepsy syndromes, we performed a pilot study, using FMRI to identify signal changes in motor and language areas in response to tasks that activate those areas. Signal changes in epilepsy patients approximated those observed in volunteers. We conclude that FMRI can be developed as a method for functional cerebral mapping in partial epilepsies.

Physiology of the lower eyelid retractors: tight linkage of the anterior capsulopalpebral fascia demonstrated using dynamic ultrafine surface coil MRI.

Although the histologic anatomy of the lower eyelid retractors is well defined, the physiology of the lower retractors has been determined only by interference based on anatomic and clinical findings. In this study, in five normal subjects we investigated the physiology of the lower eyelid retractors utilizing dynamic high resolution magnetic resonance imaging (MRI) with a custom designed surface coil. Measurements of the excursion of the cornea, lower eyelid margin, and anterior edge of the inferior oblique muscle were made from scans taken in upgaze and downgaze. We found that the corneal movement was substantially greater than the movement of the eyelid margin, a finding that can readily be made clinically. A more important result was that the movement of the eyelid margin and the movement of the inferior oblique margin were similar in all cases. Thus, the length of the anterior capsulopalpebral fascia between the tarsus and inferior oblique muscle remains constant in downgaze, and the source of the stretch in the lower eyelid retractors lies in the posterior capsulopalpebral system, at the capsulopalpebral head. High resolution eyelid MR has great potential to allow investigation of essential aspects of normal and pathologic eyelid anatomy and physiology.

Functional magnetic resonance imaging of human auditory cortex.

Magnetic resonance imaging methods recently demonstrated regional cerebral signal changes in response to limb movement and visual stimulation, attributed to blood flow enhancement. We studied 5 normal subjects scanned while listening to auditory stimuli including nonspeech noise, meaningless speech sounds, single words, and narrative text. Imaged regions included the lateral aspects of both hemispheres. Signal changes in the superior temporal gyrus and superior temporal sulcus were observed bilaterally in all subjects. Speech stimuli were associated with significantly more widespread signal changes than was the noise stimulus, while no consistent differences were observed between responses to different speech stimuli. Considerable intersubject variability in the topography of signal changes was observed. These observations confirm the utility of magnetic resonance imaging in the study of human brain structure-function relationships and emphasize the role of the superior temporal gyrus in perception of acoustic-phonetic features of speech, rather than processing of semantic features.

Spin-echo and gradient-echo EPI of human brain activation using BOLD contrast: a comparative study at 1.5 T.

In this study, Blood Oxygenation Level Dependent (BOLD) contrast in the detection of human brain activation was compared between spin-echo and gradient-echo echo-planar sequences at 1.5 T. Time course series of spin-echo and gradient-echo images containing the primary motor cortex were collected during rest (no finger movement) and activation (finger movement). Each time course series was collected using a different TE. Resting and active state signal intensities at each TE were measured in identical regions in the motor cortex. From these data, resting and active state R2 (1/T2) and R2* (1/T2*) values were obtained. Across four subjects, brain activation produced an average R2 change of -0.16 +/- 0.02/s (+/- SE), and an average R2* change of -0.55 +/- 0.08/s. The average delta R2*/delta R2 ratio was 3.52 +/- 0.56. The average gradient-echo/spin-echo ratio of activation-induced signal changes at the TE for maximal BOLD contrast for each sequence (TE approximately T2* and T2) was calculated to be 1.87 +/- 0.40.

Contrast enhancement of normal intervertebral disks: time and dose dependence.

PURPOSE: To determine the dose of contrast medium and the imaging strategy sufficient to detect diffusion of low-molecular-weight gadolinium-containing contrast media into normal intervertebral disks. METHODS: In 11 rabbits, sequential MR images were obtained of the spine for 120 minutes after intravenous injection of gadopentetate dimeglumine in doses of 0.1 to 2.8 mmol/kg. Images were inspected for evidence of contrast enhancement. Signal intensity was measured and plotted as a function of time and dose. RESULTS: Contrast enhancement was detected by inspection of images and by measurement in animals receiving doses of 0.3 mmol/kg and larger. CONCLUSIONS: Diffusion of gadolinium-containing chelates into the intervertebral disk can be detected with clinically used doses of commercially available contrast medium. Therefore, with MR and a gadolinium-containing contrast medium, diffusion into intervertebral disks can be studied.

Functional magnetic resonance imaging of complex human movements.

Functional magnetic resonance imaging (FMRI) is a new, noninvasive imaging tool thought to measure changes related to regional cerebral blood flow (rCBF). Previous FMRI studies have demonstrated functional changes within the primary cerebral cortex in response to simple activation tasks, but it is unknown whether FMRI can also detect changes within the nonprimary cortex in response to complex mental activities. We therefore scanned six right-handed healthy subjects while they performed self-paced simple and complex finger movements with the right and left hands. Some subjects also performed the tasks at a fixed rate (2 Hz) or imagined performing the complex task. Functional changes occurred (1) in the contralateral primary motor cortex during simple, self-paced movements; (2) in the contralateral (and occasionally ipsilateral) primary motor cortex, the supplementary motor area (SMA), the premotor cortex of both hemispheres, and the contralateral somatosensory cortex during complex, self-paced movements; (3) with less intensity during paced movements, presumably due to the slower movement rates associated with the paced (relative to self-paced) condition; and (4) in the SMA and, to a lesser degree, the premotor cortex during imagined complex movements. These preliminary results are consistent with hierarchical models of voluntary motor control.

Processing strategies for time-course data sets in functional MRI of the human brain.

Image processing strategies for functional magnetic resonance imaging (FMRI) data sets acquired using a gradient-recalled echo-planar imaging sequence are considered. The analysis is carried out using the mathematics of vector spaces. Data sets consisting of N sequential images of the same slice of brain tissue are analyzed in the time-domain and also, after Fourier transformation, in the frequency domain. A technique for thresholding is introduced that uses the shape of the response in a pixel compared with the shape of a reference waveform as the decision criterion. A method is presented to eliminate drifts in data that arise from subject movement. The methods are applied to experimental FMRI data from the motor-cortex and compared with more conventional image-subtraction methods. Several finger motion paradigms are considered in the context of the various image processing strategies. The most effective method for image processing involves thresholding by shape as characterized by the correlation coefficient of the data with respect to a reference waveform followed by formation of a cross-correlation image. Emphasis is placed not only on image formation, but also on the use of signal processing techniques to characterize the temporal response of the brain to the paradigm.