Field camera versus phantom-based measurement of the gradient system transfer function (GSTF) with dwell time compensation.

PURPOSE: The gradient system transfer function (GSTF) can be used to describe the dynamic gradient system and applied for trajectory correction in non-Cartesian MRI. This study compares the field camera and the phantom-based methods to measure the GSTF and implements a compensation for the difference in measurement dwell time. METHODS: The self-term GSTFs of a MR system were determined with two approaches: 1) using a dynamic field camera and 2) using a spherical phantom-based measurement with standard MR hardware. The phantom-based GSTF was convolved with a box function to compensate for the dwell time dependence of the measurement. The field camera and phantom-based GSTFs were used for trajectory prediction during retrospective image reconstruction of 3D wave-CAIPI phantom images. RESULTS: Differences in the GSTF magnitude response were observed between the two measurement methods. For the wave-CAIPI sequence, this led to deviations in the GSTF predicted trajectories of 4% compared to measured trajectories, and residual distortions in the reconstructed phantom images generated with the phantom-based GSTF. Following dwell-time compensation, deviations in the GSTF magnitudes, GSTF-predicted trajectories, and resulting image artifacts were eliminated (< 0.5% deviation in trajectories). CONCLUSION: With dwell time compensation, both the field camera and the phantom-based GSTF self-terms show negligible deviations and lead to strong artifact reduction when they are used for trajectory correction in image reconstruction.

[Modulation of cortical pain processing by cyclooxygenase inhibition: a functional MRI study]. / Modulation der kortikalen Schmerzverarbeitung durch Cyclooxygenase-Hemmung: Eine funktionelle MRT-Studie.

BACKGROUND: Little is known about changes in brain activity with pharmacological modulation of hyperalgesia. Therefore, we sought to investigate the cerebral processing of hyperalgesia and acute pain using functional magnetic resonance imaging (fMRI) and pharmacological modulation with cyclooxygenase (COX) inhibitors. METHODS: As an experimental model, we used UVB-induced mechanical hyperalgesia. In a double-blind, placebo-controlled, randomized study, acetylsalicylic acid (ASA) and parecoxib were administered to 14 healthy volunteers. Corresponding brain activity changes were assessed using fMRI. RESULTS: Psychophysically, parecoxib showed both analgesic and antihyperalgesia effects, whereas ASA had only antihyperalgesic effects in the experimental model. Analgesic effects were found in primary (S1) and secondary (S2) somatosensory cortices and anterior parts of the anterior cingulate cortex (ACC). In contrast, antihyperalgesic effects were mainly detected in S1, parietal association cortices, and the ACC. CONCLUSION: This study provides new evidence for the involvement of COX inhibitors in modulating the cerebral activity associated with acute pain and hyperalgesia. Our results hint at a differential modulation of brain areas under either analgesia or antihyperalgesia.

Cortical representation of experimental tooth pain in humans.

Cortical processing of electrically induced pain from the tooth pulp was studied in healthy volunteers with fMRI. In a first experiment, cortical representation of tooth pain was compared with that of painful mechanical stimulation to the hand. The contralateral S1 cortex was activated during painful mechanical stimulation of the hand, whereas tooth pain lead to bilateral activation of S1. The S2 and insular region were bilaterally activated by both stimuli. In S2, the center of gravity of the activation during painful mechanical stimulation was more medial/posterior compared to tooth pain. In the insular region, tooth pain induced a stronger activation of the anterior and medial parts. The posterior part of the anterior cingulate gyrus was more strongly activated by painful stimulation of the hand. Differential activations were also found in motor and frontal areas including the orbital frontal cortex where tooth pain lead to greater activations. In a second experiment, we compared the effect of weak with strong tooth pain. A significantly greater activation by more painful tooth stimuli was found in most of those areas in which tooth pain had induced more activation than hand pain. In the medial frontal and right superior frontal gyri, we found an inverse relationship between pain intensity and BOLD contrast. We concluded that tooth pain activates a cortical network which is in several respects different from that activated by painful mechanical stimulation of the hand, not only in the somatotopically organized somatosensory areas but also in parts of the 'medial' pain projection system.

BOLD effects in different areas of the cerebral cortex during painful mechanical stimulation.

The contribution of four cortical areas (S1, S2, insular cortex and gyrus cinguli) to pain processing was assessed by functional magnetic resonance imaging (fMRI). Phasic (mechanical impact) and tonic stimuli (squeezing) were applied to the back of a finger, both at two different strengths. Stimuli were adjusted to inflict weak and strong pain sensations. It had been shown before that stronger noxious mechanical stimuli induce a weaker input from myelinated mechanoreceptors, but a more vigorous input from nociceptive primary afferents, and vice versa. Sizes of activation clusters and percent increase of the blood oxygenation level dependent (BOLD) signal during activation were compared in the areas of interest. Phasic stimulus patterns were more closely reflected in the time course of the MR signal in S1, S2 and the cingulate than tonic patterns, since the tonic stimuli tended to induce slow MR signal increase also during the resting periods which is in parallel to the persisting character of the tonic pain sensations. In S1 only the contralateral side was activated in most cases, and the more painful stimuli did not induce greater BOLD responses compared to the less painful stimuli in this area. Paradoxically, more painful stimuli produced smaller activation clusters in S1, both in tonic and phasic stimulus trials. In contralateral S2 more painful phasic stimuli induced significantly stronger BOLD responses than the weaker stimuli. The responses to tonic stimuli did not differentiate painfulness and were significantly smaller than the phasic. Activation clusters in this area were also smaller for tonic stimuli. In the gyrus cinguli more painful phasic stimuli induced stronger BOLD responses, but no difference was seen between tonic stimulation of different strength. Though the insular cortex was often bilaterally activated, no significant differences between stimulus quality or intensity were found. Our results provide evidence for a contribution of the S2 projection area and of the cingulate cortex to the processing of the intensity dimension of phasic mechanical pain. Such evidence was not found for the S1 area, which probably receives dominant input from non-nociceptive mechanoreceptors.

Brain stem afferents of hypoglossal neurons in the rat.

The origin of afferent connections of the hypoglossal nucleus in rats was investigated using horseradish peroxidase (HRP) as a retrograde tracer. Pressure injections (0.15-0.17 mu1) of 15% HRP were introduced into the rostral, middle and caudal portions of the nucleus. Projections to the hypoglossal nucleus originated from 3 regions of the brainstem: the reticular formation, the spinal V complex and the nucleus of the solitary tract. Bilateral projections with ipsilateral predominance came from the lateral reticular formation: the dorsal aspect of the nucleus reticularis parvocellularis and its caudal continuation, the nucleus reticularis dorsalis. Fewer projections emerged from two nuclei of the medial reticular formation. The dorsal part of the nucleus reticularis ventralis at the spinomedullary junction contributed bilateral with mainly contralateral input to hypoglossal neurons. A few labeled neurons were situated bilaterally in the nucleus reticularis gigantocellularis of the rostral medulla. The input from the spinal V complex originated from the dorsal aspect along most of its length but particularly from the pars interpolaris and oralis subdivisions. Labeled neurons were located primarily in the posterior portion of the nucleus of the solitary tract. Projections from the spinal V complex and the solitary nucleus exhibited ipsilateral predominance. These results suggest that somatic and visceral centers of the rat brainstem play an important role in the control of the activity of hypoglossal motoneurons.

Cerebral representation of the anorectum using functional magnetic resonance imaging.

BACKGROUND: Anorectal continence depends not only on the organs of continence but also on cerebral control. There are relatively few data regarding cerebral processing of anorectal continence. METHODS: Thirteen healthy subjects underwent rectal distension to cause urge increasing to discomfort during functional magnetic resonance imaging (fMRI). In addition, a painful heat stimulus was applied to the skin of the anterior abdominal wall in the dermatome corresponding to the rectum. Voluntary contraction of the anal sphincter was also performed. Subjective rating of stimulus intensity was recorded. Evaluation of the data used a general linear model with Brain Voyager(trade mark). RESULTS: Subjective sensation of discomfort increased during repeated rectal distension and caused activation in the anterior cingulate gyrus, insula, thalamus and secondary somatosensory cortex seen on fMRI. Perception of rectal urge and discomfort activated the same cerebral regions with differing intensity. Application of a painful thermal stimulus in the corresponding dermatome showed a modification of the response. Voluntary contraction of the anal sphincter led to activation of the motor cortex and increased activity in the supplementary motor cortex and the insula. CONCLUSION: Cerebral representation of the anorectum as mapped by fMRI is intricate and reflects the complexity of the continence mechanism.

Neonatal circumcision techniques.

Neonatal circumcision is most commonly performed using one of three techniques--the Mogen clamp, the Gomco clamp or the Plastibell device. With all three techniques, careful selection and preparation of patients is essential. Informed consent must be obtained from parents or guardians, based on an objective understanding of the medical and social implications of circumcision, including potential complications from the procedure. Measures for creating an aseptic field, anesthesia and positioning of the infant do not vary with the technique selected. Both the Mogen and Gomco clamps protect the glans while producing crush injury to the prepuce, which is then surgically removed. The Plastibell device induces necrotic tissue, which is sloughed off, along with the plastic shield, within a week or so. Although complications from neonatal circumcision are rare, hemorrhage, local infection, sepsis, meatal ulceration and poor cosmetic results have been reported.