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Purpose: A scalable multiband and multichannel digital magnetic resonance imaging system has been developed with the goal of reducing the time needed for acquisition of a single volume of gradient-recalled echo-planar images of the brain., Methods: Transmit pulses are created by an offline computer equipped with a Pentek excitation card (PCIe model 78621) that was built around the Texas Instruments D/A converter (DAC5688)., Results: The spectral purity of pulses made in this way surpasses the quality of pulses made by the standard modulators of the scanner, even when using the same pulse-creation algorithm. There is no need to mix reference waveforms with the magnetic resonance imaging signal to obtain inter-k-space coherency for different repetitions. The key was the use of a system clock to create the Larmor frequency used for pulse formation. The 3- and 4-fold slice accelerations were tested using phantoms as well as functional and resting-state magnetic resonance imaging of the human brain., Conclusion: Synthesizers with limited modulation-time steps should be replaced not only because of the improved spectral quality of radiofrequency pulses but also for the exceptional coherence of pulses at different slice-selection frequencies., (Copyright © 2013 Wiley Periodicals, Inc.)

Regional-specific average time courses of spontaneous fluctuations in blood oxygen level dependent (BOLD) MRI contrast at 9.4T in lightly anesthetized resting rat brain are formed, and correlation coefficients between time course pairs are interpreted as measures of connectivity. A hierarchy of regional pairwise correlation coefficients (RPCCs) is observed, with the highest values found in the thalamus and cortex, both intra- and interhemisphere, and lower values between the cortex and thalamus. Independent sensory networks are distinguished by two methods: data driven, where task activation defines regions of interest (ROI), and hypothesis driven, where regions are defined by the rat histological atlas. Success in these studies is attributed in part to the use of medetomidine hydrochloride (Domitor) for anesthesia. Consistent results in two different rat-brain systems, the sensorimotor and visual, strongly support the hypothesis that resting-state BOLD fluctuations are conserved across mammalian species and can be used to map brain systems., ((c) 2008 Wiley-Liss, Inc.)

It is well understood that the different regions of the body have cortical representations in proportion to the degree of innervation. Our current understanding of the rat upper extremity has been enhanced using functional MRI (fMRI), but these studies are often limited to the rat forepaw. The purpose of this study is to describe a new technique that allows us to refine the sensory and motor representations in the cerebral cortex by surgically implanting electrodes on the major nerves of the rat upper extremity and providing direct electrical nerve stimulation while acquiring fMRI images. This technique was used to stimulate the ulnar, median, radial, and musculocutaneous nerves in the rat upper extremity using four different stimulation sequences that varied in frequency (5 Hz vs. 10 Hz) and current (0.5 mA vs. 1.0 mA). A distinct pattern of cortical activation was found for each nerve. The higher stimulation current resulted in a dramatic increase in the level of cortical activation. The higher stimulation frequency resulted in both increases and attenuation of cortical activation in different regions of the brain, depending on which nerve was stimulated., (Copyright 2007 Wiley-Liss, Inc.)

The alpha diffusion-weighted imaging (DWI) method was developed to study heterogeneous water diffusion in the human brain using magnetic resonance imaging (MRI). An advantage of this model is that it does not require an assumption about the shape of the intravoxel distribution of apparent diffusion rates, and it has a calculable relationship to this distribution. The alpha-DWI technique is useful for detecting microstructural tissue changes associated with brain tumor invasion, and may be useful for directing therapy to invading tumor cells. In previous work, alpha-DWI was performed with magnetic field gradients applied along a single direction in order to avoid artificially introducing a source of heterogeneity to the decay. However, it is known that restricted diffusion is anisotropic in the brain, and the alpha-DWI method must take this into account to be complete. In this work the relationship between the applied magnetic field gradients and the fitted stretched-exponential model parameters was studied in the human brain. It was found the distributed diffusion coefficient (DDC) varies with the direction of applied gradients, while the heterogeneity index alpha is relatively direction-insensitive. It is proposed that in clinical use, maps of alpha can be created using diffusion-weighting gradients applied in a single direction that reflect the tissue heterogeneity.

The hypothesis was tested that the intravoxel distribution of water diffusion rates, as measured with a stretched-exponential model of diffusion-weighted imaging (DWI), is a marker of brain tumor invasion. Eight rats underwent intracerebral inoculation of C6 glioma cells. In three rats, cells were labeled with a fluorescent dye for microscopy. One rat was inoculated with a saline solution, and five more rats were imaged without inoculation as controls. Five healthy uninoculated rats were also imaged. DWI was performed 14-15 days after inoculation, with diffusion-weighting factor b = 500 to 6500 sec/mm2, and the resulting signal attenuation was fitted with the stretched-exponential model. The heterogeneity index values were significantly lower (P < 0.05) in the peritumor ROI than in normal gray matter and significantly higher than in normal white matter. The distributed diffusion coefficient values were significantly lower than in normal white matter or normal gray matter. Fluorescence microscopy confirmed the presence of tumors in the peritumor region that could be histologically distinguished from the main tumor mass. There was no change in proton density or T2-weighted images in the peritumor region, making vasogenic edema unlikely as a source of contrast. It is therefore thought that the heterogeneity parameter alpha is a marker of brain tumor invasion., ((c) 2004 Wiley-Liss, Inc.)

The spatial relationship between a measured fMRI signal and its underlying neuronal activity remains unclear. One obstacle is the localization of neuronal activity; another is the spatial resolution of fMRI. In the present study, high-resolution BOLD and CBV fMRI experiments (voxel size: 156 x 156 x 2000 microm3) were conducted in the rat whisker barrel cortex at 3 T; neuronal activity across cortical layers was mapped using the Fos expression technique. Results show that BOLD response is weighted by blood volume and that pixels with high BOLD response can be located at the cortical surface or in deep layers, depending on local vasculature. In contrast to BOLD response, the pixels with high CBV response were consistently clustered in the deep cortical layers. Percentage-CBV change in cortical layers IV-V was 7.3 +/- 1.5%, which was significantly higher than in layers I-III (4.1 +/- 0.9%) and VI (4.3 +/- 0.7%) (mean +/- SEM). The laminar distribution of CBV response correlates well with neuronal activity localized by Fos expression. We conclude that neuronal activity can be inferred from CBV fMRI data with high spatial accuracy. The data indicate that both intracolumn functional connectivity and neurovascular coupling can be studied using CBV fMRI., ((c) 2004 Wiley-Liss, Inc.)

MRI gradient coil design is a type of nonlinear constrained optimization. A practical problem in transverse gradient coil design using the conjugate gradient descent (CGD) method is that wire elements move at different rates along orthogonal directions (r, phi, z), and tend to cross, breaking the constraints. A momentum-weighted conjugate gradient descent (MW-CGD) method is presented to overcome this problem. This method takes advantage of the efficiency of the CGD method combined with momentum weighting, which is also an intrinsic property of the Levenberg-Marquardt algorithm, to adjust step sizes along the three orthogonal directions. A water-cooled, 12.8 cm inner diameter, three axis torque-balanced gradient coil for rat imaging was developed based on this method, with an efficiency of 2.13, 2.08, and 4.12 mT.m(-1).A(-1) along X, Y, and Z, respectively. Experimental data demonstrate that this method can improve efficiency by 40% and field uniformity by 27%. This method has also been applied to the design of a gradient coil for the human brain, employing remote current return paths. The benefits of this design include improved gradient field uniformity and efficiency, with a shorter length than gradient coil designs using coaxial return paths., (Copyright 2003 Wiley-Liss, Inc.)

A multishot partial-k-space EPI technique is presented and validated by fMRI at high spatial resolution. High-resolution phase maps corrected by phase-encoded reference scans have less off-resonance effects. Phantom studies demonstrate that this method can substantially improve partial-k-space EPI image formation. BOLD fMRI at submillimeter spatial resolution (156 x 156 x 2000 microm(3), 0.049 microl) was achieved in a rat whisker barrel stimulation model using this technique. The study included eight rats, five of which were administered an intravascular contrast agent (monocrystalline iron oxide nanocolloid (MION)) after the BOLD experiments. In two rats the highest BOLD responses were in the deep layers (IV-VI), and in six rats the highest responses were on the surface and in the deep cortical layers. Most of the pixels that exhibited high BOLD responses had high blood volume weightings. The benefits of this technique are expected to increase for high-resolution fMRI at higher magnetic fields, where T(2) (*) is shorter., (Copyright 2003 Wiley-Liss, Inc.)

Experience with diffusion-weighted imaging (DWI) shows that signal attenuation is consistent with a multicompartmental theory of water diffusion in the brain. The source of this so-called nonexponential behavior is a topic of debate, because the cerebral cortex contains considerable microscopic heterogeneity and is therefore difficult to model. To account for this heterogeneity and understand its implications for current models of diffusion, a stretched-exponential function was developed to describe diffusion-related signal decay as a continuous distribution of sources decaying at different rates, with no assumptions made about the number of participating sources. DWI experiments were performed using a spin-echo diffusion-weighted pulse sequence with b-values of 500-6500 s/mm(2) in six rats. Signal attenuation curves were fit to a stretched-exponential function, and 20% of the voxels were better fit to the stretched-exponential model than to a biexponential model, even though the latter model had one more adjustable parameter. Based on the calculated intravoxel heterogeneity measure, the cerebral cortex contains considerable heterogeneity in diffusion. The use of a distributed diffusion coefficient (DDC) is suggested to measure mean intravoxel diffusion rates in the presence of such heterogeneity., (Copyright 2003 Wiley-Liss, Inc.)

A technique for acquiring magnetic field maps simultaneously with gradient-recalled echo-planar time-course data is described. This technique uses a trajectory in which the central part of k-space is collected twice. For a 64 x 64 image acquired with a 125-kHz bandwidth, a field map suitable for geometric correction can be collected simultaneously with the echo-planar time-course data in <70 ms. The field maps generated by this technique are registered with the magnitude images because they are calculated using the same data. They do not suffer from errors due to subject motion, or from different geometric distortions that can result from using different pulse sequences. In addition to correcting geometric distortions that resulted from dynamic magnetic field perturbations, this method was used to measure field shifts arising from respiration and jaw motion across five subjects. Values ranged from 0.035 to 0.165 parts per million (ppm)., (Copyright 2003 Wiley-Liss, Inc.)

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., (Copyright 2001 Wiley-Liss, Inc.)

Perfusion and blood oxygenation level-dependent (BOLD) signals were simultaneously measured during a finger-tapping task at 3T using QUIPSS II with thin-slice TI(1) periodic saturation, a modified pulsed arterial spin labeling technique that provides quantitative measurement of perfusion. Perfusion and BOLD signal changes due to motor activation were obtained and correlated with the T(1) values estimated from echo-planar imaging (EPI)-based T(1) maps on a voxel-by-voxel basis. The peak perfusion signal occurs in voxels with a T(1) of brain parenchyma while the peak BOLD signal occurs in voxels with a T(1) characteristic of blood and cerebrospinal fluid. The locations of the peak signals of functional BOLD and perfusion only partially overlap on the order of 40%. Perfusion activation maps will likely represent the sites of neuronal activity better than do BOLD activation maps. Magn Reson Med 44:137-143, 2000., (Copyright 2000 Wiley-Liss, Inc.)

Quantitative imaging of perfusion using a single subtraction, second version (QUIPSS II) is a pulsed arterial spin labeling (ASL) technique for improving the quantitation of perfusion imaging by minimizing two major systematic errors: the variable transit delay from the distal edge of the tagged region to the imaging slices, and the contamination by intravascular signal from tagged blood that flows through the imaging slices. However, residual errors remain due to incomplete saturation of spins over the slab-shaped tagged region by the QUIPSS II saturation pulse, and spatial mismatch of the distal edge of the saturation and inversion slice profiles. By replacing the original QUIPSS II saturation pulse with a train of thin-slice periodic saturation pulses applied at the distal end of the tagged region, the accuracy of perfusion quantitation is improved. Results of single and multislice studies are reported.

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.

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.

Data acquired using functional magnetic resonance imaging are often contaminated by head motion. As a result, optimal information regarding task-induced (or resting-state) signal changes cannot be extracted. Intensity-based registration methods, including intensity correlation or minimum intensity variance techniques, are widely used to register two or more images. It is shown here that intensity-based registration cannot accurately register two or more images in the presence of local intensity changes arising from functional magnetic resonance, fMRI, signals. In this paper, we present a contour-based technique that can be used not only for a more robust registration, but also to help differentiate between task-induced and motion-induced signal changes. Results obtained using both phantom and human brain images demonstrate advantages of this technique compared with a conventional intensity registration technique.

A magnetization transfer (MT)-prepared echo-planar imaging (EPI) pulse sequence was developed to study motor cortex activation, using a finger tapping paradigm. MT weighting resulted in a reduction of both the activated area and, in the majority of activated pixels, the functional MRI signal, regardless of the correlation coefficient threshold used in generating the activation map. The magnetization transfer ratio (MTR) was higher during task activation than during rest. Because the MT effect is strongly tissue-dependent, these results support the hypothesis that incorporation of MT into functional MRI will help to understand the origin of the functional MRI signal.

Functional magnetic resonance imaging (fMRI) is capable of detecting task-induced blood oxygenation changes using susceptibility sensitive pulse sequences such as gradient-recalled echo-planar imaging (EPI). The local signal increases seen in the time course are believed to be due to an increase in oxygen delivery that is incommensurate with oxygen demands. To help isolate the sources of functional signal changes, the authors have incorporated various forms of diffusion weighting into EPI pulse sequences to characterize the apparent mobility of the functionally modulated protons. Results suggest that the majority of the functional signal at 1.5 T arises from protons that have apparent diffusion coefficients that are approximately four or five times higher than that of brain tissue. This implies that significant functional signal sources are either protons within the vascular space or protons from the perivascular space that is occupied by cerebrospinal fluid.

Data obtained from functional magnetic resonance imaging are often limited by a low signal-to-noise ratio. The time-course data obtained from activated regions contain both system noise and physiological noise, primarily linked to the heart and respiratory rates, that are superimposed on task induced signals. Time averaging of a practical number of data sets is not very effective in improving the signal-to-noise ratio because neither system nor physiological noise is truly random. In this paper, a method is presented for filtering unwanted physiological fluctuations, including aliased signals that are formed as a result of long repetition time (TR) values. A pulse oximeter was used to obtain cardiac and respiratory information during the scanning period. Finite impulse response band-reject digital filters were designed to remove the physiological fluctuations. For comparison, cross-correlation analyses were performed at the same level of statistical significance on both filtered and unfiltered data. We demonstrate that this method can improve the detection of weak signals without increasing the probability of false positives.

An MRI time course of 512 echo-planar images (EPI) in resting human brain obtained every 250 ms reveals fluctuations in signal intensity in each pixel that have a physiologic origin. Regions of the sensorimotor cortex that were activated secondary to hand movement were identified using functional MRI methodology (FMRI). Time courses of low frequency (< 0.1 Hz) fluctuations in resting brain were observed to have a high degree of temporal correlation (P < 10(-3)) within these regions and also with time courses in several other regions that can be associated with motor function. It is concluded that correlation of low frequency fluctuations, which may arise from fluctuations in blood oxygenation or flow, is a manifestation of functional connectivity of the brain.

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.

Two single-shot volume imaging techniques are described. The first, single-echo echo-volume imaging, is essentially the echo-volume imaging (EVI) sequence suggested by Mansfield (J. Phys. C. 10, L55 (1977)). The second is a multi-spin-echo approach in which one plane of k-space is collected during each spin echo. In both techniques, phase encoding gradients are applied in the z direction, and three-dimensional k-space is filled by a raster pattern in Cartesian coordinates. Spatial saturation is used to avoid aliasing in the y direction, and a selctive pulse is applied to excite the desired slab of tissue and eliminate aliasing in z. The average echo-times, measured from the center of the 90 degrees pulse to the center of the acquisition k-space (kx = ky = kz = 0), were 45 and 104 ms for single echo and multi-echo methods, respectively. Images of the human brain using both sequences are shown.

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.

Using gradient-echo echo-planar MRI, a local signal increase of 4.3 +/- 0.3% is observed in the human brain during task activation, suggesting a local decrease in blood deoxyhemoglobin concentration and an increase in blood oxygenation. Images highlighting areas of signal enhancement temporally correlated to the task are created.

A flexible iterative algorithm is presented for optimizing gradient and radio frequency coils for MRI. It is based on a model using discrete current elements and direct Biot-Savart calculation of the fields. An error function is defined over a region of interest (ROI) and is minimized by conjugate gradient descent. The choice of error function allows optimization of the field uniformity, the inductance, and the efficiency of the coil in any combination. Neither the coil nor the ROI is restricted to any particular geometry. A 40-turn cylindrical z-gradient coil of radius a and length 4a, designed for ROI of radius 0.7a and length 2a has an average error in the gradient fields generated of 0.85%, an inductance of 0.014a mH/cm, and an efficiency of 6.65a-2 Gcm/A. A 16-turn birdcage-like RF coil of radius 5 cm, designed for a ROI of radius 4 cm has an average error of 0.79%.

Calculations and experiments that provide support for our previously stated theorem are presented: If two coils simultaneously receiving magnetic resonance signals from the same anatomic region exhibit zero mutual inductance, there can be no correlation of the noise. It is shown that correlation does not exist even in the presence of mutual inductance unless the two signal paths have amplifiers prior to signal combination. It is further found that in the presence of mutual inductance with ideal amplifiers (0 dB noise figure) in the two signal paths, there is no correlation of noise. In order to satisfy the condition of zero mutual inductance, it may be necessary to employ a decoupling circuit external to the body. A novel coil assembly, which was used in the experiments, places a single-turn surface coil in the median plane between the two loops of a counter rotating current coil. The signal-to-noise ratio can be improved by combining signals. This is in analogy to quadrature receiving coils, where the mutual inductance is zero because vector reception fields are perpendicular. In the present geometry, vector reception fields are collinear, but are parallel and antiparallel on the two sides of the coil assembly, resulting in zero mutual inductance.

We describe a doubly tuned radiofrequency (RF) local coil probe designed specifically for performing in vivo image-localized spectroscopy. The probe was designed using principles developed in connection with the counter-rotating-current (CRC) and planar-pair loop gap resonators for magnetic resonance imaging (MRI). The probe design satisfies several criteria useful for in vivo 1H/31P experiments at 1.5 T. First, sensitivity on the low-frequency mode is preserved relative to a singly tuned coil. This result was confirmed by bench-test and in vitro MR experimental data. Second, through principles of intrinsic decoupling the probe is isolated from any externally applied uniform excitation field, which is desirable for in vivo 1H imaging and solvent suppression. Third, the regions of sensitivity of the high- and low-frequency modes of the coil are similar, and therefore spectroscopic volumes of interest identified on an image will reflect the same volumes as those selected during spectroscopy. Finally, interface to the MR system is such that the high- or low-frequency circuits may be selected entirely under software control, with no requirement for changing coils or cables or moving the subject.

Small mice (approximately 20 g) were anesthetized and placed in a loop-gap resonator of diameter 25 mm resonating at 1.1 GHz. An oxygen-permeable capsule containing 2.5 X 10(-2) M perdeutero 15N TEMPONE (1-oxyl-2,2,6,6-tetramethyl-4-piperidone) in light paraffin oil was implanted in the peritoneal cavity. Electron spin resonance (ESR) spectra from the capsule are sensitive to the concentration of dissolved oxygen and calibration curves are given. The oxygen concentration was found to rise from a value close to zero when the animal breathes air to a level of 220 microM when the animal breaths pure oxygen. It is speculated that this surprisingly high level is related to the effect of the anesthetic on the cardiovascular system. Encapsulation provides a barrier to spin-label reductants and to paramagnetic metal ions that might confound the spin-label oximetric measurements.

A surface coil assembly consisting of two interleaved coplanar resonators that are intrinsically decoupled from each other is described for imaging at 1.5 T. Vector reception fields on-axis at 3 cm depth are orthogonal and of equal magnitude. Both components of magnetization were received and combined resulting in a 2 1/2 improvement in signal-to-noise for temporomandibular joint images.

High-order radiofrequency transmitter coils are described that permit, in the context of NMR imaging, a knowledgeable trade-off between coil complexity and the degree of inhomogeneity over a specified volume of space. A mathematical formulation of high-order coils is developed for an axially symmetric polarizing dc field and transverse rf field. The field is created by a number of longitudinal current filaments with equal angular separations on the wall of a cylinder. In its most general form, each current can be independently specified. Circularly or linearly polarized fields can be created. Quadrature and saddleshaped coils emerge as special cases of the general formulation. The development has been tested experimentally and good agreement found. High-order coils can also be used as receivers, permitting more equal weighting of the sensitivities of each pixel.

A novel resonator for receiving 31P NMR signals at 1.5 T, which because of its topology has been named a "sectorial resonator," exhibits good radio frequency coupling to the adult human liver. Minimal contamination from nonliver tissue (i.e., muscle) was achieved by the use of depth selective pulses and also because the geometry results in cancellation of signals arising from tissues that are close to the two openings of the structure.

Any number of coils can in principle be used simultaneously and independently in magnetic resonance imaging if the mutual inductances are sufficiently small. Surface coils and head or body coils have equal sensitivity at some crossover depth of the order of 6 to 10 cm. Using a 7.5-cm-diameter surface coil that was intrinsically isolated from the head coil, images were acquired simultaneously from both coils and combined to improve the signal-to-noise ratio at 6 cm depth by 2 1/2. A similar experiment with the body coil showed 2 1/2 improvement at about 8 cm depth.

The design of a piezoelectric-controlled trimmer capacitor and its use as a tuning element of an MRI surface coil at 63.8 MHz are described. It is found that image quality does not depend strongly on match of the coil to the transmission line but that tuning can be a significant factor. The work is intended to lead eventually to an automatic frequency control circuit in which the capacitor serves as the active element in holding the resonant frequency of the nuclei and the resonant frequency of the coil in coincidence.

Proton relaxation enhancement (PRE) values for fatty acid spin labels bound to human serum albumin have been investigated using the inversion-recovery method at 24 MHz. At 0.1 mM protein concentration and a label-to-protein ratio of one-to-one, the PRE value for 12-Doxylsterate-albumin complex is 7.8 +/- 2.3, whereas the PRE values for 5-Doxylstearate and 16-Doxylstearate-albumin complexes are 1.5 +/- 0.6 and 1.7 +/- 0.7, respectively. Addition of 10-fold excess of stearic acid reduced the PRE values nearly to 1, indicating that the strong enhancements arise from direct binding of fatty acid spin labels to human serum albumin. PRE values for all three labels exhibit maxima as a function of the label-to-protein ratio, suggesting multiple binding sites for fatty acid spin labels with labels in the tightest binding sites not resulting in the most effective relaxation. Based on the rates of reduction of ESR signal amplitudes by sodium ascorbate, the difference in PRE values for the three fatty acid spin labels bound to albumin is attributed to the difference in water accessibility of the nitroxide moieties at various positions along the acyl chain, being greater at the C-12 position than at C-5 or C-16 position. The PRE value of 8 for 12-Doxylstearate bound to human serum albumin indicates that this complex may be a suitable paramagnetic contrast agent for in vivo NMR imaging.

A type of local coil (or surface coil) for magnetic resonance imaging and spectroscopy is described in which two circular loops of the same diameter are disposed axially with respect to each other and in which counter rotating currents (CRC) are supported in the two loops. This type of coil has been used for localized reception in a uniform circularly polarized excitation field produced by a whole-body coil of an imager functioning at 1.5 T. The CRC coil is decoupled from the transmitter coil by a combination of intrinsic (or geometrical) isolation (which functions during both excitation and reception) plus passive decoupling (which functions only during excitation). CRC coils of 7.5, 10, 12.5, and 15 cm diameter have been compared on a bench test setup with conventional surface coils of these diameters. Sensitivities are very similar. Q's have been measured as a function of coil diameter and both Q's and frequency shifts have been measured as a function of distance from a saline tank. The CRC coil appears to have advantages with respect to tuning and matching. A high-resolution image of the rotator cuff is shown as an illustration of coil performance.

A strategy for the design of surface coils for the imaging of extended superficial planar anatomic structures including skin and subcutaneous fat is presented. The central concept is to create an array of closely spaced coplanar radiofrequency current filaments parallel to BO, the polarizing field, with opposite current directions in adjacent filaments. A specific coil design with four such filaments for imaging at 1.5 T is presented. It consists of two pairs of counterrotating current (CRC) coils. Phantom images as well as representative images of normal skin are shown.

A new type of loop-gap resonator, which because of its topology is called a "sectorial resonator," is described in considerable analytical detail. It was used for 31P NMR of the livers of two human adult normal volunteers. An effective strategy for suppression of signals from surface tissue was developed. Depth selection was achieved using a separate excitation coil, permitting a reasonable degree of control of the shape of the sensitive region by adjustment of the dimensions of the excitation coil and of the type of pulse delivered. In addition a new principle for cancellation is described: the topology of the sectorial resonator leads to cancellation signals from tissue masses at the two openings of the structure. Using a phase-alternated repeated 90 degrees FID pulse sequence, good spectra were achieved in 2.5 min that are free from contamination by the phosphocreatine peak.

The "prescan" process for setting the gain of an MRI scanner can be confounded by an effect arising from the spatial variation of the angle between the vector reception field of a local or surface coil and the excitation field of a whole-body coil. It is most pronounced when the phase-encoding direction is parallel to the plane of the coil but not along the magnetic field. The largest signal is shown to reside in a data row that is shifted from the one that corresponds to zero phase-encoding gradient. The effect depends on the field-of-view and the dimensions of the receiving coil. Strategies are suggested by which manufacturers of scanners can cope with this intrinsic problem. In the meantime, users of surface coils should be aware of the difficulty.