Partial Fourier reconstruction for three-dimensional gradient echo functional MRI: comparison of phase correction methods

Partial Fourier (PF) methods take advantage of data symmetry to allow for either faster image acquisition or increased image resolution. Faster acquisition and increased spatial resolution are advantageous for fMRI because of increased temporal resolution and/or reduced partial volume effects, respectively. Standard PF methods, which use a phase reference obtained from a low resolution image, are adequate for the reconstruction of time-stationary images acquired using either spin echoes or short TE gradient echoes. In fMRI, however, multiple images are acquired using long TE gradient echoes, which introduces possible phase drifts in the fMRI data and high spatial frequencies in the phase reference. This work investigates several techniques developed to reconstruct fMRI data obtained with PF acquisitions. PF methods that account for both high-frequency spatial variations and time-dependent drifts in the phase reference are discussed and are quantitatively evaluated using receiver operator characteristic curve analysis.     

Brain functional MRI of the visual cortex with echo planar imaging

Brain functional MR imaging (fMRI) is a non invasive imaging method for detecting neural activity. We performed functional MRI of the visual cortex with gradient-echo echo planar imaging (GE-EPI) and spin-echo EPI (SE-EPI) using 1.5T MRI system. Visual stimuli was performed with a checkerboard patterns. Magnitude and temporal phase of correlation between each pixel's time-course and sine functions at the frequency of the stimulus was calculated. In all subjects, the activation area in visual cortex obtained from SE-EPI was smaller than that from GE-EPI. Temporal phase delay images from both GE-EPI and SE-EPI showed signal spread from the primary visual cortex to peripheral supplementary areas. Temporal phase analysis is important to discriminate the source of the hemodynamic response to neural activation in fMRI.     

Comparison of fat-saturation fast spin echo versus conventional spin-echo MRI in the detection of rotator cuff pathology

The purpose of this study was to compare the diagnostic performance of fat-saturation fast-spin-echo (FSE) T2-weighted (T2W) sequences with conventional spin-echo (CSE) T2W sequences in the detection of rotator cuff pathology using surgery as the reference standard. Oblique coronal dual-echo CSE and FSE T2W images with fat saturation from 50 surgically confirmed MR shoulder examinations were acquired on a 1.5-T MR scanner. Blinded MR readers retrospectively analyzed each imaging sequence separately and ultimately correlated both sequences together with findings at surgery. FSE was 100% sensitive and 94% specific in detection of full-thickness tears (n = 19) and 73% sensitive and 97% specific in the detection of partial-thickness rotator cuff tears (n = 13). There was no statistically significant difference in the performance of FSE with fat saturation compared with CSE. The two discrepancies between imaging sequences related to the extent of partial-thickness tears. Our findings suggest that fat-saturation FSE imaging can effectively replace CSE imaging in the evaluation of rotator cuff pathology.     

Identification of collaterally perfused areas following focal cerebral ischemia in the rat by comparison of gradient echo and diffusion-weighted MRI

Diffusion-weighted (DW) and gradient echo (GE) magnetic resonance images were acquired before and after occlusion of the middle cerebral artery (MCA) in the rat. Upon occlusion, an increase in DW imaging signal intensity was observed in a core area within the MCA territory, most likely reflecting cytotoxic edema. The signal from GE images, which is sensitive to changes in the absolute amount of deoxyhemoglobin, decreased following ischemia within a region that extended beyond the core area observed with DW imaging. This hypointensity is attributed to increases in blood volume and/or oxygen extraction fraction, which result from a decrease in perfusion pressure in the collaterally perfused area. The evolution of the GE imaging signal intensity from different regions was studied for 3.5 h following the occlusion. In the core area, the GE imaging signal returned towards baseline values after approximately 1-2 h, while it remained stable in the surrounding area. This feature may reflect a decrease in hematocrit due to microcirculatory defect and/or a decrease in the oxygen extraction fraction due to ongoing infarction of the tissue and may indicate that tissue recovery is severely compromised. The combined use of DW and GE imaging offers great promise for the noninvasive identification of specific pathological events with high spatial resolution.     

Performance of an elliptical centric view order for signal enhancement and motion artifact suppression in breath-hold three-dimensional gradient echo imaging

An elliptical centric phase-encoding (PE) order is applied to steady-state 3DFT imaging as performed during a single breath-hold or following contrast agent administration. In a set of simulation, phantom, and in vivo experiments, this truly centric PE order is shown to be both more resistant to breathing artifact and more capable of suppressing undesirable venous signals that can arise following peak arterial enhancement than a number of other centric PE techniques that are presently in use. Unlike other PE orders, the elliptical centric ordering changes with the relative dimensions of the two PE fields of view and is optimal based on increasing k-space radius. It thus creates a temporally diminishing importance to the PE order. The specific advantages of such an acquisition are demonstrated.     

On the degree of solute mixing in liver models of drug elimination

One of the fundamental differences between various liver models regards the underlying assumptions on the intrahepatic mixing process. A model-independent method for the evaluation of the departure from the perfectly mixed system is proposed which is based on an application of the relative entropy concept to hepatic transit time distributions of intravascular markers. This approach provides a measure of the distance between two probability distributions. Available data measured in isolated perfused livers indicate that sinusoidal solute mixing is nearly optimal. The suggestion of maximum mixedness in the liver may explain the discrepancy between the apparent validity of the venous equilibrium model and the physiological irrelevance of the underlying well-stirred assumption. In terms of the dispersion model the results are in accordance with the model equation obtained for mixed boundary conditions.     

Human hippocampal activation in the delayed matching-and nonmatching-to-sample memory tasks: An event-related functional MRI approach.

The delayed matching-to-sample (DMS) and delayed nonmatching-to-sample (DNMS) memory tasks are standard tools used to probe visual recognition memory in human and nonhuman primates. Previous research indicates that structures within the medial temporal lobe, including the hippocampus, make up a crucial memory circuit for successful performance on these tasks. In the present investigation, event-related functional magnetic resonance imaging was used to examine activation in the hippocampus proper during these memory tasks relative to a perceptuomotor task involving the same stimuli. The results indicate that both memory tasks elicited greater activation in the right hippocampus during the encoding phase. These findings are consistent with the work from human patients and animal studies, indicating hippocampal involvement in the DMS and DNMS tasks. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Proprioception acts as the main source of input in human S-I activation experiments: a functional MRI study

During tactile exploration cells in human somatosensory cortex S-I receive input from skin receptors and from proprioceptive feedback. To study the extent to which these sources contribute to cell activation we used functional magnetic resonance imaging (fMRI) in order to visualize the spatial extent and amplitude of activation in S-I during active finger movement and passive stimulation of finger tips. In all subjects (n = 6) we measured activation elicited by unilateral single finger tapping (active task) and mechanical stimulation of the palm of the index finger (passive task). In the finger tapping condition all subjects showed a strict contralateral activation of somatosensory cortex S-I and motor cortex M-I. In the passive stimulation experiment we found activation of the contralateral somatosensory cortex S-I only. Although subjects were trained to perform the finger movement with the same frequency and pressure in comparison to the passive stimulation, the activation within S-I induced by finger movements was always significantly larger than that induced by passive stimulation. This result implies that activation of somatosensory cortex originates to a large extent from proprioception while tactile input plays a minor role in S-I excitation.     

Recovery of fMRI activation in motion area MT following storage of the motion aftereffect

We used functional magnetic resonance imaging (fMRI) during storage of the motion aftereffect (MAE) to examine the relationship between motion perception and neural activity in the human cortical motion complex MT+ (including area MT and adjacent motion-selective cortex). MT+ responds not only to physical motion but also to illusory motion, as in the MAE when subjects who have adapted to continuous motion report that a subsequent stationary test stimulus appears to move in the opposite direction. In the phenomenon of storage, the total decay time of the MAE is extended by inserting a dark period between adaptation and test phases. That is, when the static test pattern is presented after a storage period equal in duration to the normal MAE, the illusory motion reappears for almost as long as the original effect despite the delay. We examined fMRI activation in MT+ during and after storage. Seven subjects viewed continuous motion, followed either by an undelayed stationary test (immediate MAE) or by a completely dark storage interval preceding the test (stored MAE). Like the perceptual effect, activity in MT+ dropped during the storage interval then rebounded to reach a level much higher than after the same delay without storage. Although MT+ activity was slightly enhanced during the storage period following adaptation to continuous motion (compared with a control sequence in which the adaptation grating oscillated and no MAE was perceived), this enhancement was much less than that observed during the perceptual phenomenon. These results indicate that following adaptation, activity in MT+ is pronounced only with the presentation of an appropriate visual stimulus, during which the MAE is perceived.     

Differential effects of sympathetic activation on sexual arousal in sexually dysfunctional and functional women.

The effects of sympathetic nervous system (SNS) activation, induced via acute exercise, on sexual arousal in women was studied. In 2 experimental sessions, 36 women viewed a neutral film followed by an erotic film. In 1 session, the women were exposed to 20 min of intense exercise before viewing the films. Twelve women were sexually functional, 12 experienced significant impairments in sexual desire, and 12 experienced primary or secondary anorgasmia. Acute exercise significantly increased vaginal pulse amplitude (VPA) and vaginal blood volume (VBV) responses to an erotic film among sexually functional women and those with low sexual desire. Among anorgasmic women, exercise significantly decreased VPA but had no effect on VBV responses to an erotic film. Acute exercise had no significant effect on the women's perceptions of sexual arousal. Results suggest that increased SNS arousal may affect physiological sexual responding in women. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A recirculatory pharmacokinetic model describing the circulatory mixing, tissue distribution and elimination of antipyrine in dogs

A model of antipyrine disposition from the moment of its injection was developed incorporating the intravascular mixing component as determined by indocyanine green (ICG) kinetics. The simultaneous dispositions of antipyrine and ICG were characterized in five dogs anesthetized with halothane. After injecting antipyrine and ICG into the right atrium, femoral arterial blood samples were collected every 3 sec for the 1st min and less frequently to 20 min for ICG and to 360 min for antipyrine. ICG and antipyrine concentrations were measured by high-performance liquid chromatography and modeled with SAAM 30.1. A fully identifiable recirculatory compartmental model, incorporating the ICG recirculatory model with blood flows and time delays, was used to describe antipyrine disposition. Four distinct antipyrine pharmacokinetic tissue compartments and the distribution clearances assigned to them could be estimated: a pulmonary tissue (0.13 +/- 0.05 I, and 2.51 +/- 0.39 liters/min), a very fast equilibrating tissue (0.12 +/- 0.08 I, and 1.33 +/- 0.22 liters/min), a fast equilibrating tissue (3.21 +/- 0.45 I, and 0.74 +/- 0.09 liters/min) and a slow equilibrating tissue (15.94 +/- 1.8 I, and 0.44 +/- 0.13 liters/min). Although this recirculatory model retains the predominant attributes of traditional pharmacokinetic models, it also can describe completely drug concentrations during the mixing transient when many drugs reach peak effect as well as ascertain the role of cardiac output and its distribution in drug disposition.     

Intrinsic and environmental factors in the development of functional maps in cat visual cortex

In the mammalian visual cortex, key neuronal response properties such as orientation preference and ocular dominance (OD) are mapped in an orderly fashion across the cortical surface. It has been known for some time that manipulating early postnatal visual experience can change the appearance of the OD map. Similar evidence for developmental plasticity of the orientation map has been scarce. We employed optical imaging of intrinsic signals to examine the contribution of intrinsic and environmental factors to the development of cortical maps, using the paradigms of strabismus, reverse occlusion and rearing in a single-orientation environment ('stripe-rearing'). For several weeks after induction of strabismus, the pattern of OD domains remained stable in young kittens. The isotropic magnification of the OD map matched the postnatal growth of the visual cortical surface during the same period. In reverse-occluded and in stripe-reared kittens, orientation preference maps obtained through the left and the right eye were very similar, although the two eyes had never shared any visual experience. We suggest that the geometry of functional maps in the visual cortex is intrinsically determined, while the relative strength of representation of different response properties can be modified through visual experience.     

Registration of neural maps through value-dependent learning: modeling the alignment of auditory and visual maps in the barn owl's optic tectum

In the optic tectum (OT) of the barn owl, visual and auditory maps of space are found in close alignment with each other. Experiments in which such alignment has been disrupted have shown a considerable degree of plasticity in the auditory map. The external nucleus of the inferior colliculus (ICx), an auditory center that projects massively to the tectum, is the main site of plasticity; however, it is unclear by what mechanisms the alignment between the auditory map in the ICx and the visual map in the tectum is established and maintained. In this paper, we propose that such map alignment occurs through a process of value-dependent learning. According to this paradigm, value systems, identifiable with neuromodulatory systems having diffuse projections, respond to innate or acquired salient cues and modulate changes in synaptic efficacy in many brain regions. To test the self-consistency of this proposal, we have developed a computer model of the principal neural structures involved in the process of auditory localization in the barn owl. This is complemented by simulations of aspects of the barn owl phenotype and of the experimental environment. In the model, a value system is activated whenever the owl carries out a foveation toward an auditory stimulus. A term representing the diffuse release of a neuromodulator interacts with local pre- and postsynaptic events to determine synaptic changes in the ICx. Through large-scale simulations, we have replicated a number of experimental observations on the development of spatial alignment between the auditory and visual maps during normal visual experience, after the retinal image is shifted through prismatic goggles, and after the reestablishment of normal visual input. The results suggest that value-dependent learning is sufficient to account for the registration of auditory and visual maps of space in the OT of the barn owl, and they lead to a number of experimental predictions.     

Statistical methods for detecting activated regions in functional MRI of the brain

Two statistical tests for detecting activated pixels in functional MRI (fMRI) data are presented. The first test (t-test) is the optimal solution to the problem of detecting a known activation signal in Gaussian white noise. The results of this test are shown to be equivalent to the cross-correlation method that is widely used for activation detection in fMRI. The second test (F test) is the optimal solution when the measured data are modeled to consist of an unknown activation signal that lies in a known lower dimensional subspace of the measurement space with added Gaussian white noise. A model for the signal subspace based on a truncated trigonometric Fourier series is proposed for periodic activation-baseline imaging paradigms. The advantage of the second method is that it does not assume any information about the shape or delay of the activation signal, except that it is periodic with the same period as the activation-baseline pattern. The two models are applied to experimental echo-planar fMRI data sets and the results are compared.     

Orientation-dependent priming effects in the perception of biological motion.

In a serial 2-choice reaction time (RT) task, Ss discriminated between a biological motion walker and a similar distractor. The point-light walker appeared in 1 of 2 possible in-depth orientations: The figure was walking either to the right or to the left in the sagittal plane. Reliable priming effects were established in consecutive trials but only when priming and primed walkers had the same in-depth orientation. This orientation-dependent priming effect was not tempered when priming and primed figures had different directions of articulatory motion (Exps 1–6), different starting positions in the step cycle (Exp 2), different point-light localizations (Exp 3), or when the figures were translating (Exps 4–6). The data converge with neurophysiological findings that suggest that object recognition is accomplished by accessing high-level, orientation-dependent representations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)