Quadrature Head Coil Research Articles

Cortical mapping of visceral pain in patients with GI disorders using functional magnetic resonance imaging

OBJECTIVE: We sought to identify central loci that activate in response to visceral stimuli (stool and pain). We had a particular interest in observing the anterior cingulate gyrus and frontal cortex in normals and in patients with intestinal disease, including inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). METHODS: Subjects underwent rectal balloon distention to a sensation of stool and to a sensation of pain while undergoing blood oxygenation level-dependent functional magnetic resonance imaging. Experiments were conducted in a Magnex 3.0-T whole body magnet with a Bruker Biospec console and a quadrature head coil. Four contiguous 5.0-mm oblique axial slices designed to optimize coverage of areas believed to be responsive to noxious stimulation were acquired. Activations were detected by using cross-correlation maps ( p < 0.001) for individual subjects. The experimental groups were compared using both an analysis of variance and profile analysis. RESULTS: A significantly higher percentage of pixels activated in the anterior cingulate gyrus over both pain and stool conditions for the control group than for the IBS group and for the IBS group than for the IBD group ( p < 0.035). Deactivation of left somatosensory cortex was greater for the IBS group than for the IBD group and greater for the IBD group than for the controls ( p < 0.0065) in the boxcar condition. Frontal deactivation in controls compared with disease groups bordered on statistical significance. Profile analysis of the three groups across six regions of interest revealed that the control and IBD groups were distinguished by different profiles of response ( p < 0.005). Nonparametric evaluation of the data suggests that, among the pixels in the anterior cingulate activating to pain, there are two patterns of response to pain—on/off and graded. This was true for both controls and disease groups. CONCLUSIONS: Normal controls and subjects with IBD and IBS share similar loci of activations to visceral sensations of stool and pain. Both activation and deactivation of particular regions of interest differentiate the three groups, as do profiles of patterned response across six of the regions of interest for the control and IBD groups.

Muscle cross-sectional area is associated with specific site of bone in prepubertal girls: a quantitative magnetic resonance imaging study

It is well established that forces applied to bone are the result of muscle contraction. However, data regarding the contribution of muscle cross-sectional area (because muscle area is proportional to muscle strength) to cortical bone area before puberty are controversial. We tested the hypothesis that muscle cross-sectional area is associated with total cortical bone area, and whether there is a region-specific relationship between these parameters in prepubertal and early pubertal girls. Seventeen healthy (9–11 years, Tanner stages I–II) white girls participated in the study. We measured bone loading characteristics (maximal ground reaction forces; GRFs) for a drop jump (50 cm) and side-to-side jump (over a 20-cm-high fence) on a multicomponent force platform. Muscle cross-sectional area and bone cortical area (square centimeters) of the proximal third of the left and right lower leg was measured with a 1.5 T magnetic resonance system using a quadrature head coil. The sequence was T 1 weighted, with spin-echo in transverse (tibial) planes and 3 mm sections with no gap (ten slices). The tibial cross-sectional areas were subdivided into three anatomical sectors (SI–SIII), with the tibial centroid as origin. SI extended from the medial tibial border to the most anterior edge, SII extended from the anterior edge laterally to the interosseous border, and SIII extended posteromedially from the interosseous border to the medial tibial border. The nonparametric bone and muscle volume correlations demonstrated that the total muscle cross-sectional area correlated significantly with the total cortical area in both legs (left leg: r s = 0.59, p = 0.020; right leg: r s = 0.57, p = 0.016). Significant correlations were also found between left and right muscle area and cortical area in SII ( r s = 0.68, p = 0.003, 0.67, and 0.003, respectively). There was no significant association between the muscle area and cortical area in SI or SIII. In addition, there was a significant correlation between GRFs of the side-to-side jump and total cortical area (left leg: r = 0.75, p < 0.01; right leg: r = 0.78, p < 0.01). Thus, we found that muscle area was most highly associated with bone cortical area in SII, the anterolateral sector of the tibia, which emphasizes the specific interplay of muscles and bone in the lower limb. This relationship was present in a regional, site-specific fashion.

FMRI of auditory stimulation with intermolecular double-quantum coherences (iDQCs) at 1.5T

An intermolecular double-quantum coherence (iDQC) imaging technique was used to study auditory activation in the human brain at 1.5T with a dual temporal lobe surface phased array coil and a quadrature head coil. Preliminary results demonstrate that it is feasible to obtain auditory activation maps using iDQC imaging at 1.5T, both in individual subjects using the surface coil array and with multisubject averaging of data using the head coil. The most robust activation map was obtained when a spin-echo (SE) acquisition was combined with an iDQC excitation. Since SE with conventional single quantum coherence (SQC) and similar parameters showed much reduced activation in spite of its higher signal-to-noise ratio (SNR), it was determined that activation resulting from the SE-iDQC acquisition almost entirely originates from iDQCs. In addition, the fact that the robust activation was obtained using signals at an evolution time more sensitive to changes in magnetic susceptibilities also suggests the sensitivity of iDQCs to the BOLD effect upon activation. iDQCs provide a novel MRI method which is potentially more sensitive to the BOLD effect traditionally measured with SQC. Magn Reson Med 45:356-364, 2001.

Computational analysis of the high pass birdcage resonator: finite difference time domain simulations for high-field MRI

In this work, a finite difference time domain (FDTD) algorithm is validated at 1.5 tesla using the standard GE Signa quadrature head coil and a muscle phantom. The electrical characteristics of the birdcage head coil are then calculated for the linear and quadrature cases. Unlike previous computational analysis which assume idealized currents on the end rings and the struts of the resonator, a complete computational analysis is provided. This treatment considers the coupling between the resonator and the sample and includes a real coil excitation, a complete current derivation, and a thorough description of both B 1 fields and RF radiation. With this improvement, electromagnetic phenomena such as radiation, standing wave currents on the wires, and field inhomogeneities due to interactions between the coil and the load inside the coil are observed. At 200 MHz, it is demonstrated that this particular coil does not work well due to radiation and non-uniformities on the struts of the device. Also, at this frequency magnetic field inhomogeneities become large when the coil is loaded with a phantom.

Quality assurance for MRI: practical experience.

The aim of this study is to propose guidelines for quality assurance (QA) in MRI, based on a comprehensive assessment of QA parameters undertaken on a busy clinical MRI scanner over the course of 1 year. QA phantoms supplied by the scanner manufacturer were used together with the Eurospin MRI phantom set. Signal-to-noise ratio (SNR) and image uniformity were measured daily from spin echo images acquired using a quadrature send-receive head coil and from a gradient echo sequence using the Helmholtz body coil. The voltage of the transmit radiofrequency (RF) amplifier was noted. Monthly measurements of slice thickness, geometric distortion, slice position, image resolution and image ghosting were acquired using the head coil. In addition, SNR was measured monthly on a selection of commonly used coils. Apart from some drift of the RF amplifier voltage, all measurements were within acceptable limits and were stable over the course of 1 year. Satisfactory measurements of SNR were possible using the simple phantom supplied with the scanner. The SNR, geometric distortion and RF amplifier voltage are simple to determine and can be measured in less than 15 min by the scanner operator, using the scanner software. Weekly recording of these parameters is recommended for busy clinical MRI scanners, as this should allow deviations from acceptable limits to be identified early. Such in-house checks can usefully be compared with the less frequent estimations performed by the service engineer. Comprehensive QA routines are discussed for systems used for quantitative measurements.

On the reliability of quantitative clinical magnetic resonance spectroscopy of the human brain.

The reliability of a single-voxel, localized proton magnetic resonance spectroscopy protocol suitable for clinical studies was investigated by means of in vitro, single-subject in vivo and multi-subject in vivo examinations of healthy adults aged from 19 to 67 years. The study was performed at 1.5 T using a standard quadrature head coil and a single voxel PRESS sequence (in vitro TR/TE = 1500/30 ms, in vivo TR/TE=2000/35 ms). Eighty-four in vitro and 30 single-subject examinations were statistically evaluated after quantification, including the calculation of the coefficients of variations (CV) for choline (Cho), creatine (Cr), myo-inositol (mI), lactate (Lac), N-acetyl-aspartate (NAA) and unresolved glutamine, glutamate and GABA (Glx). The CVs for absolute concentrations of the main metabolites Cho, Cr and NAA, ranged from 3.3% (3.8) to 4.0% (6.4%) (the in vivo results are given in brackets). Multi-subject CVs of absolute concentrations for Cho, Cr and NAA ranged from 7.6% to 15.0%. CVs of relative in vivo concentrations were found to be higher than CVs of absolute concentrations. Due to the better reproducibility of intra-individual absolute in vivo concentrations, cross-over studies using institutional units are recommended.

A phased array echoplanar imaging system for fMRI.

A fully parallel, simultaneous sampling phased array receiver system for a clinical echoplanar imaging system is described and evaluated for BOLD activation and relative cerebral blood volume (rCBV) experiments. A 4-coil array curved around the occipital lobe improved SNR by factors of 1.5 in the visual cortex and 3.1 in the visual association cortex relative to a 13-cm diameter surface coil, improving the statistical significance and coverage of visual activation maps. A 4-coil bilateral array increased SNR throughout the head relative to a quadrature head coil by up to a factor of 5 in much of the cortex, with proportional improvement in the SNR of rCBV maps.

A novel multi-segment surface coil for neuro-functional magnetic resonance imaging.

The construction of two novel mult-segment (MS) resonators are described. The signal-to-noise ratio (SNR) and B1 homogeneity of the coils are compared with that of a surface coil and a standard quadrature head coil. The images obtained with the MS designs revealed a surface coil-like fall-off in signal with depth. The SNR offered by MS coils was found to be better than the head coil at depths less than approximately 6 cm. Bilateral motor cortex activation on normal subjects performing finger tapping tasks is demonstrated.

Subarachnoid Fluid of the Optic Nerve in Normal Adults

The purpose of the study is to determine the amount of subarachnoid fluid of the optic nerve in normal adults using magnetic resonance (MR) imaging and to investigate whether the subarachnoid fluid is displaced in abduction as assumed by the 30 degree echographic test. The design was a prospective observation study. Twenty-one healthy headache-free adults participated. Ten (48%) were men and 11 (52%) were women with a mean age of 34.3 +/- 7.9 years. Both optic nerves were examined in primary and 45 degrees right and left gazes with T2 fast-spin echo fat-suppressed coronal MR imaging in quadrature head coil. Four MR images, 4 mm apart, starting from 4 mm posterior to the globe were obtained for both nerves simultaneously with an imaging time of 2 minutes and 24 seconds per gaze. Optic nerve and sheath diameters were measured. Mean nerve diameters were 3.2 +/- 0.4 mm anteriorly to 2.6 +/- 0.4 mm posteriorly, and mean sheath diameters were 5.2 +/- 0.9 mm anteriorly to 3.9 +/- 0.4 mm posteriorly. Optic sheath diameters did not change significantly in abduction or adduction. Magnetic resonance imaging can be used effectively to determine the amount of subarachnoid fluid of the optic nerve. In normal adults, the amount of optic nerve subarachnoid fluid is variable and may be substantial. The authors' MR findings show that optic nerve subarachnoid fluid is not displaced significantly with abduction or adduction.

In vivo 19F spin relaxation and localized spectroscopy of fluoxetine in human brain.

Fluorine-19 NMR spectroscopy was used to monitor the anti-depressant drug fluoxetine (and its metabolite norfluoxetine) in vivo in human brain. A quadrature birdcage head coil, developed for operation at 60.1 MHz, yielded a signal from the head 2 to 4 times stronger than for surface coils. It was used to measure the in vivo 19F spin-lattice relaxation time (T1) of fluoxetine for five patients by the inversion-recovery technique. The individual T1s varied from 149 to 386 ms, which was attributed in part to interindividual differences based on the reproducibility of a phantom T1. The individual T1 correlated weakly with approximate brain concentration. A lower limit of 3 to 4 ms was found for the spin-spin relaxation time from line width measurements. Low resolution 4-dimensional spectroscopic imaging confirmed that the single in vivo 19F resonance for fluoxetine arose primarily from brain. The spectrum of a cerebral hemisphere (in formalin) obtained at autopsy from a patient on 40 mg/day of fluoxetine for 19 weeks was comparable with that seen for patients in vivo. The in vivo signal arose about equally from fluoxetine and the active metabolite norfluoxetine, as demonstrated by the in vitro 19F NMR spectrum of the lipophilic extract of a small section of brain. In vitro quantitation of frozen samples from three brain regions yielded combined fluoxetine/norfluoxetine concentrations of 12.3 to 18.6 micrograms/ml, which is higher than typically determined in vivo, and suggests that the fluorinated drugs may not be 100% visible in vivo.

Temporal lobes: surface MR coil phased-array imaging.

The authors describe a four-coil surface phased-array coil for magnetic resonance (MR) imaging of the temporal lobes. On the basis of measurements in a phantom, the signal-to-noise ratio of the array is 1.67 times or more better than that obtained with a standard quadrature head coil in the region of the hippocampi. The higher sensitivity of this phased array permits use of imaging pulse sequences for which the head coil is inadequate.

Lymphatic Imaging in Experimental Filariasis Using Magnetic Resonance

To evaluate acquired lymphatic abnormalities caused by filariasis, the authors examined the peripheral lymphatic system in normal ferrets and those chronically infected with Brugia malayi using magnetic resonance imaging (MRI). The findings were compared with previously obtained lymphangioscintigraphic (LAS) images in ferrets both with and without experimental filariasis. Fifteen ferrets (11 infected with B. malayi and four noninfected controls) underwent whole body coronal MRI using a quadrature transmission-receive head coil at 0.5 Tesla operating at a resonant frequency of 21.5 mHz for protons with a 25-cm field of view. In contrast to normal animals, infected ferrets showed dilated hindlimb dermal lymphatic collaterals, enlarged high-signal intensity groin lymph nodes with punctate low-signal intensity centers and separate low-signal intensity spots with irregular thin channels, suggestive of nests of viable adult nematodes within tortuous lymphatics and nodes. MRI correlated with the LAS findings, and the interpretations were supported by light, scanning electron, and video microscopy. T2-weighted MRI in conjunction with LAS accurately depicts the peripheral lymphatic system in filarial-infected ferrets. These two modalities are useful complementary techniques to examine disorders characterized by lymphatic insufficiency.

A quadrature coil for the adult human head

A quadrature head coil suitable for NMR imaging up to 80 MHz is described. Based on the well known design of Alderman and Grant, the four-quadrant coil has the attributes of distributed tuning capacitance and electrostatic guard rings, while maintaining the √2 improvement in sensitivity and. √2 improvement in effective B 1 inherent in the quadrature design. The appropriate phase shifts for transmission and reception are generated with the aid of a coaxial cable ring hybrid, which is simple in design and affords a considerable degree of isolation between transmitter and receiver.