Quantitative assessment of improved homogeneity using higher-order shims for spectroscopic imaging of the brain

Mitochondrial toxicity before and after combination antiretroviral therapy, a Magnetic Resonance Spectroscopy study

The aim of this study was to quantify, via Magnetic Resonance Spectroscopy (MRS), the effect of combination antiretroviral therapy (cART) on brain metabolites and characterize any possible associations between changes in metabolites, age, blood biomarkers of neuronal damage, functional connectivity and cognitive performance. As cART has dramatically increased the life expectancy of HIV-infected (HIV + ) individuals and unmasked an increase in HIV-associated neurocognitive disorders, it is still not clear whether cART neurotoxicity contributes to these disorders. We hypothesized a bimodal effect, with early cART treatment of HIV infection decreasing inflammation as measured by MRS metabolites and improving cognitive performance, and chronic exposure to cART contributing to persistence of cognitive impairment via its effect on mitochondrial function. Basal ganglia metabolites, functional connectivity, cognitive scores, as well as plasma levels of neurofilament light chain (NfL) and tau protein were measured before and after 12 weeks, 1 year and 2 years of cART in a cohort of 50 cART-naïve HIV + subjects and 72 age matched HIV- healthy controls. Glutamate (Glu) levels were lower in the cART naïve patients than in healthy controls and were inversely correlated with plasma levels of NfL. There were no other significant metabolite differences between HIV + and uninfected individuals. Treatment improved Glu levels in HIV+, however, no associations were found between Glu, functional connectivity and cognitive performance. Stable brain metabolites and plasma levels of NfL and Tau over two-years of follow-ups suggest there are no signs of cART neurotoxicity in this relatively young cohort of HIV + individuals.

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

Proton MRS (1 H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good-quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi-adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0 ) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.

Gradient and shim technologies for ultra high field MRI

Ultra High Field (UHF) MRI requires improved gradient and shim performance to fully realize the promised gains (SNR as well as spatial, spectral, diffusion resolution) that higher main magnetic fields offer. Both the more challenging UHF environment by itself, as well as the higher currents used in high performance coils, require a deeper understanding combined with sophisticated engineering modeling and construction, to optimize gradient and shim hardware for safe operation and for highest image quality. This review summarizes the basics of gradient and shim technologies, and outlines a number of UHF-related challenges and solutions. In particular, Lorentz forces, vibroacoustics, eddy currents, and peripheral nerve stimulation are discussed. Several promising UHF-relevant gradient concepts are described, including insertable gradient coils aimed at higher performance neuroimaging.

In vivo B0 field shimming methods for MRI at 7T

Functional MRI (fMRI) at 7T and above provides improved Signal-to-Noise Ratio and Contrast-to-Noise Ratio compared to 3T acquisitions. In addition to the beneficial effects on spin polarization and magnetization of deoxyhemoglobin, the increased applied field also further magnetizes air and tissue. While the magnets themselves typically provide a static B0 field with sufficient spatial homogeneity, the diamagnetism of tissue and the paramagnetism of air causes local field deviations inside the human head. These spatially-varying field offsets (ΔB0) cause image artifacts, especially in single shot EPI, including geometric distortion, signal dropout, and blurring. These effects are particularly strong near air-tissue interfaces such as the frontal sinus, and ear canals. Furthermore, if the field offsets are dynamically modulated through physiological processes such as respiration or motion, then the effect on the image time-series can be even more problematic. While post-processing methods have been developed to mitigate these effects, the ideal solution would be to reduce the ΔB0 variations at their source. Typically 7T scanners contain 2nd and some 3rd order spherical harmonic shim coil terms to cancel static ΔB0 variations of low spatial order. In this article, we will motivate the need for improved, higher-order compensation for B0 inhomogeneity and potentially add dynamic control of these fields. We discuss and compare several promising hardware approaches for static and dynamic B0 shimming using either higher-order spherical harmonic shim coils or multi-coil shim arrays as well as passive shimming approaches, and active variants such and adaptive current networks.

Comparision of new element designs for combined RF-Shim arrays at 7 T

PURPOSE

To identify novel concepts for RF-shim loop architectures suitable for 7T made of two concentric conducting loops fulfilling RF and DC functions, respectively, and to determine their relative SNR performance. The goal is to minimize interference between the two systems while making efficient use of the space closest to the body.

THEORY

We show by means of theoretical derivation of the frequency spectrum that the proposed two-loop structure exhibits an anti-resonant null and a resonant peak in the frequency domain.

METHODS

The proposed structure is comprised of two concentric wire loops either arranged as nested loops or in the form of a coaxial cable, in which the two conductors carry the RF and shim signals, respectively. We use theory, simulation, and phantom measurements to obtain frequency spectra and SNR maps for the proposed structures.

RESULTS

Retained SNR is found to be 75% in the coaxial loop and ranges from 57% to 67% in three different coaxial configurations. We have found both implementations to be a viable concept for the use in RF-shim devices if remaining SNR limitations can be overcome.

CONCLUSIONS

We have investigated two new design modalities in 7T RF-shim coil design that separate the RF and shim conductors such that the previously proposed toroidal chokes are eliminated - thereby removing undesired additional loss, bulk, and design complexity.

Fast magnetic resonance spectroscopic imaging techniques in human brain- applications in multiple sclerosis

Multi voxel magnetic resonance spectroscopic imaging (MRSI) is an important imaging tool that combines imaging and spectroscopic techniques. MRSI of the human brain has been beneficially applied to different clinical applications in neurology, particularly in neurooncology but also in multiple sclerosis, stroke and epilepsy. However, a major challenge in conventional MRSI is the longer acquisition time required for adequate signal to be collected. Fast MRSI of the brain in vivo is an alternative approach to reduce scanning time and make MRSI more clinically suitable.Fast MRSI can be categorised into spiral, echo-planar, parallel and turbo imaging techniques, each with its own strengths. After a brief introduction on the basics of non-invasive examination (¹H-MRS) and localization techniques principles, different fast MRSI techniques will be discussed from their initial development to the recent innovations with particular emphasis on their capacity to record neurochemical changes in the brain in a variety of pathologies.The clinical applications of whole brain fast spectroscopic techniques, can assist in the assessment of neurochemical changes in the human brain and help in understanding the roles they play in disease. To give a good example of the utilities of these techniques in clinical context, MRSI application in multiple sclerosis was chosen. The available up to date and relevant literature is discussed and an outline of future research is presented.

Multivendor implementation and comparison of volumetric whole-brain echo-planar MR spectroscopic imaging

PURPOSE

To assess volumetric proton MR spectroscopic imaging (MRSI) of the human brain on multivendor MRI instruments.

METHODS

Echo-planar spectroscopic imaging was developed on instruments from three manufacturers, with matched specifications and acquisition protocols that accounted for differences in sampling performance, radiofrequency (RF) power, and data formats. Intersite reproducibility was evaluated for signal-normalized maps of N-acetylaspartate (NAA), creatine (Cre), and choline using phantom and human subject measurements. Comparative analyses included metrics for spectral quality, spatial coverage, and mean values in atlas-registered brain regions.

RESULTS

Intersite differences for phantom measurements were less than 1.7% for individual metabolites and less than 0.2% for ratio measurements. Spatial uniformity ranged from 79% to 91%. The human studies found differences of mean values in the temporal lobe, but good agreement in other white matter regions, with maximum differences relative to their mean of under 3.2%. For NAA/Cre, the maximum difference was 1.8%. In gray matter, a significant difference was observed for frontal lobe NAA. Primary causes of intersite differences were attributed to shim quality, B0 drift, and accuracy of RF excitation. Correlation coefficients for measurements at each site were over 0.60, indicating good reliability.

CONCLUSION

A volumetric intensity-normalized MRSI acquisition can be implemented in a comparable manner across multivendor MR instruments.

Non-contrast-enhanced renal and abdominal MR angiography using velocity-selective inversion preparation

Non-contrast-enhanced MR angiography is a promising alternative to the established contrast-enhanced approach as it reduces patient discomfort and examination costs and avoids the risk of nephrogenic systemic fibrosis. Inflow-sensitive slab-selective inversion recovery imaging has been used with great promise, particularly for abdominal applications, but has limited craniocaudal coverage due to inflow time constraints. In this work, a new non-contrast-enhanced MR angiography method using velocity-selective inversion preparation is developed and applied to renal and abdominal angiography. Based on the excitation k-space formalism and Shinnar-Le-Roux transform, a velocity-selective excitation pulse is designed that inverts stationary tissues and venous blood while preserving inferiorly flowing arterial blood. As the magnetization of the arterial blood in the abdominal aorta and iliac arteries is well preserved during the magnetization preparation, artery visualization over a large abdominal field of view is achievable with an inversion delay time that is chosen for optimal background suppression. Healthy volunteer tests demonstrate that the proposed method significantly increases the extent of visible arteries compared with the slab-selective approach, covering renal arteries through iliac arteries over a craniocaudal field of view of 340 mm.

Altered brain function underlying verbal memory encoding and retrieval in psychotic major depression

Psychotic major depression (PMD) is associated with deficits in verbal memory as well as other cognitive impairments. This study investigated brain function in individuals with PMD during a verbal declarative memory task. Participants included 16 subjects with PMD, 15 subjects with non-psychotic major depression (NPMD) and 16 healthy controls (HC). Functional magnetic resonance imaging (fMRI) data were acquired while subjects performed verbal memory encoding and retrieval tasks. During the explicit encoding task, subjects semantically categorized words as either "man-made" or "not man-made." For the retrieval task, subjects identified whether words had been presented during the encoding task. Functional MRI data were processed using SPM5 and a group by condition ANOVA. Clusters of activation showing either a significant main effect of group or an interaction of group by condition were further examined using t-tests to identify group differences. During the encoding task, the PMD group showed lower hippocampus, insula, and prefrontal activation compared to HC. During the retrieval task, the PMD group showed lower recognition accuracy and higher prefrontal and parietal cortex activation compared to both HC and NPMD groups. Verbal retrieval deficits in PMD may be associated with deficient hippocampus function during encoding. Increased brain activation during retrieval may reflect an attempt to compensate for encoding deficits.

Dynamic Shimming of the Human Brain at 7 Tesla

Dynamic shim updating (DSU) of the zero- to second-order spherical harmonic field terms has previously been shown to improve the magnetic field homogeneity in the human brain at 4 Tesla. The increased magnetic field inhomogeneity at 7 Tesla can benefit from inclusion of third-order shims during DSU. However, pulsed higher-order shims can generate a multitude of temporally varying magnetic fields arising from eddy-currents that can strongly degrade the magnetic field homogeneity.The first realization of zero- to third-order DSU with full preemphasis and B(0) compensation enabled improved shimming of the human brain at 7 Tesla not only in comparison with global (i.e. static) shimming, but also when compared to state-of-the-art zero- to second-order DSU. Temporal shim-to-shim interactions were measured for each of the 16 zero- to third-order shim coils along 1D column projections on a spherical phantom. The decomposition into up to 3 exponentials allowed full preemphasis and B(0) compensation of all 16 shims covering 67 potential shim-to-shim interactions. Despite the significant improvements achievable with DSU, the magnetic field homogeneity is still not perfect even when updating all zero- through third-order shims. This is because DSU is still inherently limited by the shallowness of the low order spherical harmonic fields and their inability to compensate the higher-order inhomogeneities encountered in vivo. However, DSU maximizes the usefulness of conventional shim coil systems and provides magnetic field homogeneity that is adequate for a wide range of applications.

Regional myo-inositol concentration in mild cognitive impairment Using 1H magnetic resonance spectroscopic imaging

The goal was to assess regional patterns of metabolite abnormalities in mild cognitive impairment (MCI) and Alzheimer disease (AD) patients using proton magnetic resonance spectroscopy imaging at 1.5 Tesla. Fourteen MCI, 17 AD, and 16 healthy control (HC) subjects were studied. MCI was associated with higher myo-inositol (mIn) concentration in right parietal white matter compared with HC and lower mIn levels in frontal white matter compared with AD. AD was associated with higher mIn concentration in frontal and parietal white matter compared with HC. N-acetylaspartate (NAA) concentration of white matter was similar in all groups, whereas NAA concentration of gray matter showed a trend toward lower values in the right parietal lobe in AD compared with MCI and HC. A mIn increase in white matter in absence of significant NAA reduction suggests that mIn is a more robust and sensitive marker of white matter pathology in AD and MCI than NAA. Furthermore, the dissociation between mIn and NAA alterations in white matter could provide important information regarding the role of glial and neuronal damage in MCI and AD.

Proton MRS of the breast in the clinical setting

Information for determining whether a primary breast lesion is invasive and its receptor status and grade can be obtained before surgery by performing proton MRS on a fine-needle aspiration biopsy (FNAB) specimen and analyzing the MRS information by a pattern recognition method. Two-dimensional MRS, on either specimens or cells, allows the unambiguous assignment of most resonances. When correlated with the spectral regions selected by the pattern recognition method, there are strong indications for the biochemical markers responsible for prognostic information of invasive capacity and metastatic spread. Spectral assignments and biological correlations can be made using cell models. In vivo MRS can distinguish invasive from benign lesions. This pathological distinction can be made from the presence of resonances at discrete frequencies. To achieve this level of spectral resolution and signal-to-noise ratio, there are stringent requirements when acquiring and processing the data. The challenge now is to implement two-dimensional MRS in vivo. Until this is realized, the combination of in vivo MR, for diagnosis and spatial location, and MRS, for image-guided biopsy to provide information on tumor spread, promises to provide a higher level of preoperative diagnosis than previously achieved.

DWI of the spinal cord with reduced FOV single-shot EPI

Single-shot echo-planar imaging (ss-EPI) has not been used widely for diffusion-weighted imaging (DWI) of the spinal cord, because of the magnetic field inhomogeneities around the spine, the small cross-sectional size of the spinal cord, and the increased motion in that area due to breathing, swallowing, and cerebrospinal fluid (CSF) pulsation. These result in artifacts with the usually long readout duration of the ss-EPI method. Reduced field-of-view (FOV) methods decrease the required readout duration for ss-EPI, thereby enabling its practical application to imaging of the spine. In this work, a reduced FOV single-shot diffusion-weighted echo-planar imaging (ss-DWEPI) method is proposed, in which a 2D spatially selective echo-planar RF excitation pulse and a 180 degrees refocusing pulse reduce the FOV in the phase-encode (PE) direction, while suppressing the signal from fat simultaneously. With this method, multi slice images with higher in-plane resolutions (0.94 x 0.94 mm(2) for sagittal and 0.62 x 0.62 mm(2) for axial images) are achieved at 1.5 T, without the need for a longer readout.

Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

The use of high magnetic fields increases the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS) of brain metabolites. Practical limitations arise, however, from susceptibility-induced field distortions, which are increased at higher magnetic field strengths. Solutions to this problem include optimized shimming, provided that active, i.e., electronic, shimming can operate over a sufficient range. To meet our shim requirements, which were an order of magnitude greater than the active shim capacity of our 7T MR system, we developed a combined passive and active shim approach. Simple geometries of ferromagnetic shim elements were derived and numerically optimized to generate a complete set of second-order spherical harmonic shim functions in a modular manner. The major goals of the shim design were maximization of shim field accuracy and ease of practical implementation. The theoretically optimized ferro-shim geometries were mounted on a cylindrical surface and placed inside the magnet bore, surrounding the subject's head and the RF coil. Passive shimming generated very strong shim fields and eliminated the worst of the field distortions, after which the field was further optimized by flexible and highly accurate active shimming. Here, the passive-shimming procedure was first evaluated theoretically, then applied in phantom studies and subsequently validated for in vivo 1H MRS in the macaque visual cortex. No artifacts due to the passive shim setup were observed; adjustments were reproducible between sessions. The modularity and the reduction to two pieces per shim term in this study is an important simplification that makes the method applicable also for passive shimming within single sessions. The feasibility of very strong, flexible and high-quality shimming via a combined approach of passive and active shimming is of great practical relevance for MR imaging and spectroscopy at high field strengths where shim power is limited or where shimming of specific anatomical regions inherently requires strong shim fields.

Routine testing of magnetic field homogeneity on clinical MRI systems

Poor main magnetic field (B0) homogeneity (H(B)) leads to artifacts and signal losses in magnetic resonance imaging (MRI). The American College of Radiology's MRI quality control manual mandates annual checks of H(B), suggesting tests using spectral linewidth and phase-difference (delta phi) maps. A new method, the bandwidth-difference (deltaBW) method, which compares the distortion for small and large BW acquisitions to determine the HB, is proposed. The deltaBW method has the advantage that it can be used to measure multiple diameters of spherical volumes (DSV) in a single phantom. A phantom has been developed to exploit this method and results obtained with it are compared to those using three standard methods. Small receiver BW in the presence of poor H(B) leads to geometric distortions because gradients are reduced to the level of the B0 inhomogeneities. Data were acquired using seven MRI systems from different manufacturers, ranging in field strength from 0.2 to 3.0 T. Fast gradient echo pulse sequences were scanned twice using small and large BWs. H(B) was measured from the shift of landmarks between the two BW acquisitions. Results were compared with data from the full width at half maximum (FWHM) method, the delta phi method and one manufacturer's resonant frequency mapping data. The FWHM method was available on two systems and the detla phi method was available on one. The deltaBW method could be performed in all scanners investigated. The H(B) measured ranged 0.11-0.32 ppm to 6.7-12.9 ppm for DSV of 13-22.6 cm. Direct comparisons of the data obtained using the deltaBW method showed good agreement with data obtained using the FWHM method. Data obtained using the deltaBW method compared favorably with the manufacturer's resonant frequency map. The deltaBW method produces measurements of H(B) at various DSV values that can be obtained from a single set of phantom images. The accuracy of deltaBW B0 homogeneity measurements are comparable to the other methods tested.

The role of the human ventral premotor cortex in counting successive stimuli

Adult humans have the ability to count large numbers of successive stimuli exactly. What brain areas underlie this uniquely human process? To identify the candidate brain areas, we first used functional magnetic resonance imaging, and found that the upper part of the left ventral premotor cortex was preferentially activated during counting of successive sensory stimuli presented 10-22 times, while the area was not activated during small number counting up to 4. We then used transcranial magnetic stimulation to assess the necessity of this area, and found that stimulation of this area preferentially disrupted subjects' exact large number enumeration. Stimulation to the area affected neither subjects' number word perception nor their ability to perform a non-numerical sequential letter task. While further investigation is necessary to determine the precise role of the left ventral premotor cortex, the results suggest that the area is indispensably involved for large number counting of successive stimuli, at least for the types of tasks in this study.

Neural correlates of counting of sequential sensory and motor events in the human brain

Little is known about the ability to enumerate small numbers of successive stimuli and movements. It is possible that there exist neural substrates that are consistently recruited both to count sensory stimuli from different modalities and for counting movements executed by different effectors. Here, we identify a network of areas that was involved in enumerating small numbers of auditory, visual, and somatosensory stimuli, and in enumerating sequential movements of hands and feet, in the bilateral premotor cortex, presupplementary motor area, posterior temporal cortex, and thalamus. The most significant consistent activation across sensory and motor counting conditions was found in the lateral premotor cortex. Lateral premotor activation was not dependent on movement preparation, stimulus presentation timing, or number word verbalization. Movement counting, but not sensory counting, activated the anterior parietal cortex. This anterior parietal area may correspond to an area recruited for movement counting identified by recent single-neuron studies in monkeys. These results suggest that overlapping but not identical networks of areas are involved in counting sequences of sensory stimuli and sequences of movements in the human brain.

Articular Cartilage of the Knee: Evaluation with Fluctuating Equilibrium MR Imaging—Initial Experience in Healthy Volunteers

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study, whose purpose was to prospectively compare three magnetic resonance (MR) imaging techniques-fluctuating equilibrium, three-dimensional (3D) spoiled gradient-recalled acquisition in the steady state (SPGR), and two-dimensional (2D) fast spin echo (SE)-for evaluating articular cartilage in the knee. The study cohort consisted of 10 healthy volunteers (four men, six women; age range, 26-42 years). Cartilage signal-to-noise ratio (SNR), SNR efficiency, cartilage-fluid contrast-to-noise ratio (CNR), CNR efficiency, image quality, cartilage visibility, and fat suppression were compared. Cartilage volume was compared for the fluctuating equilibrium and 3D SPGR techniques. Compared with 3D SPGR and 2D fast SE, fluctuating equilibrium yielded the highest cartilage SNR efficiency and cartilage-fluid CNR efficiency (P < .01 for both). Image quality was similar with all sequences. Fluctuating equilibrium imaging yielded higher cartilage visibility than did 2D fast SE imaging (P <. 01) but worse fat suppression than did 3D SPGR and 2D fast SE imaging (P < .04). Cartilage volume measurements with fluctuating equilibrium and 3D SPGR were similar. Fluctuating equilibrium MR imaging is a promising method for evaluating articular cartilage in the knee.

Cortical activity in multiple motor areas during sequential finger movements: An application of independent component analysis

Multiple cortical regions such as the supplementary motor area (SMA), premotor cortex (PM), and primary motor cortex (M1) are involved in the sequential execution of hand movements, but it is unclear how these areas collaborate in the preparation and execution of ipsilateral and contralateral hand movements. In this study, we used right-handed subjects to examine the spatial distribution and temporal profiles of motor-related activity during visually cued sequential finger movements by applying independent component analysis (ICA) to event-related functional magnetic resonance imaging (fMRI) signals. The particular merit of the ICA method is that it allows brain activity in individual subjects to be elucidated without making a priori assumptions about the anatomical areas that are activated or the temporal profile of activity. By applying ICA, we found that (1) the SMA contributed to both the preparation and execution of movements of the right and left hand; (2) the left M1 and dorsal premotor cortex (PMd) contributed to both the preparation and execution of movements of the right and left hand, whereas the right M1 and PMd contributed mainly to the execution of movements of the left hand; (3) pre-SMA areas were activated in some subjects in concert with the posterior parietal and prefrontal cortex; and (4) fMRI signals over superficial cortical draining veins could be distinguished from cortical activation. We suggest that ICA is useful for categorizing distributed task-related activities in individual subjects into several spatially independent activities that represent functional units in motor control.

Implementation issues of multivoxel STEAM-localized1H spectroscopy

Single-voxel STEAM-localized spectroscopy studies of neuropsychiatric patients yield high-quality data at short echo times, but are often limited to only a few regions of interest due to the linear increase of acquisition time with the number of regions examined. A multivoxel STEAM approach increases the number of regions of interest examined with a less than linear increase in acquisition time. Several implementation issues were considered, especially the signal contribution of outer voxel stimulated echoes (OVSE), which can lead to systematic errors in the quantification of relative metabolite concentrations. The relative signal contribution of OVSEs was found to be as great as 30% in phantoms. Gradient polarity switching completely canceled the contribution of OVSEs. A two-voxel STEAM approach produces phantom and in vivo data quality comparable to single-voxel STEAM in practically half the time. Quantification precision and accuracy are preserved in phantoms and in vivo.

Shim design using a linear programming algorithm

The advent of open magnetic resonance imaging (MRI) scanners and dedicated MRI scanners tailored to specific body parts has led to an increasing number of noncylindrical MRI scanner geometries, for which noncylindrical gradients and shims are needed. These new scanner geometries are driving the need for fast, flexible shim design methods that can design shim coils for any geometry. A linear programming (LP) algorithm was developed to design minimum-power resistive shim coils on an arbitrary surface. These coils can be designed to produce any order shim field over an arbitrarily shaped target region, which can be placed anywhere within the coil. The resulting designs are relatively sparse and can be readily constructed. This algorithm was used to design and construct a seven-coil cylindrical shim set for a knee imaging magnet with a cylindrical homogeneous region. The algorithm was then used to design shim coils for a biradial head imager with an asymmetrically located spherical target region for brain imaging.

Region and volume dependencies in spectral line width assessed by 1H 2D MR chemical shift imaging in the monkey brain at 7 T

High magnetic fields increase the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS). In contrast, spectral peaks are broadened in vivo at higher field strength due to stronger susceptibility-induced effects. Strategies to minimize the spectral line width are therefore of critical importance. In the present study, 1H 2D chemical shift imaging at short echo times was performed in the macaque monkey brain at 7 T. Large brain coverage was obtained at high spatial resolution with voxel sizes down to 50 microl being able to quantify up to nine metabolites in vivo with good reliability. Measured line widths of metabolites decreased from 14.2 to 7.6 Hz with voxel volumes of 3.14 ml to 50 microl (at increased spatial resolution). The line width distribution of the metabolites (7.6+/-1.6 Hz, ranging from 5.5 to 10 Hz) was considerably smaller compared to that of water (10.6+/-2.4 Hz) and was also smaller than reported in 1H MRS at 7 T in the human brain. Our study showed that even in well-shimmed areas assumed to have minimal macroscopic susceptibility variations, spectral line widths are tissue-specific exhibiting considerable regional variation. Therefore, an overall improvement of a gross spectral line width--directly correlated with improved spectral quality--can only be achieved when voxel volumes are significantly reduced. Our line width optimization was sufficient to permit clear glutamate (Glu)-glutamine separation, yielding distinct Glu maps for brain areas including regions of greatly different Glu concentration (e.g., ventricles vs. surrounding tissue).

A shared neural network for simple reaction time

Simple reaction time, a simple model of sensory-to-motor behavior, has been extensively investigated and its role in inferring elementary mental organization has been postulated. However, little is known about the neuronal mechanisms underlying it. To elucidate the neuronal substrates, functional magnetic resonance imaging (fMRI) signals were collected during a simple reaction task paradigm using simple cues consisting of different modalities and simple triggered movements executed by different effectors. We hypothesized that a specific neural network that characterizes simple reaction time would be activated irrespective of the input modalities and output effectors. Such a neural network was found in the right posterior superior temporal cortex, right premotor cortex, left ventral premotor cortex, cerebellar vermis, and medial frontal gyrus. The right posterior superior temporal cortex and right premotor cortex were also activated by different modality sensory cues in the absence of movements. The shared neural network may play a role in sensory triggered movements.

Quality assurance of magnetic resonance spectroscopic imaging–derived metabolic data

PURPOSE

Spatially resolved metabolite maps, as measured by magnetic resonance spectroscopic imaging (MRSI) methods, are being increasingly used to acquire metabolic information to guide therapy, with metabolite ratio maps perhaps providing the most diagnostic information. We present a quality assurance procedure for MRSI-derived metabolic data acquired ultimately for guiding conformal radiotherapy.

METHODS AND MATERIALS

An MRSI phantom filled with brain-mimicking solutions was custom-built with an insert holding eight vials containing calibration solutions of precisely varying metabolite concentrations that emulated increasing grade/density of brain tumor. Phantom metabolite ratios calculated from fully relaxed 1D, 2D, and 3D MRS data for each vial were compared with calibrated metabolite ratios acquired at 9.4 T. Additionally, 3D ratio maps were "discretized" to eight pseudoabnormality levels on a slice-by-slice basis and the accuracy of this procedure was verified.

RESULTS

Regression analysis revealed expected linear relationships between experimental and calibration metabolite ratios with intercepts close to zero for the three acquisition modes. 1D MRS data agreed most with theoretical considerations (regression coefficient, b = 0.969; intercept 0.008). The 2D (b = 1.049; intercept -0.199) and 3D (correlation coefficient r(2) = 0.9978-0.7336 for five slices) MRSI indicated reduced MRS data quality in regions of degraded B(0) and B(1) homogeneity. Pseudoabnormality levels were found to be consistent with expectations within regions of adequate B(0) homogeneity.

CONCLUSIONS

This simple phantom-based approach to generate baseline calibration curves for all MRS acquisition modes may be useful to identify temporal deviations from acceptable data quality in a routine clinical environment or for testing new MRS and MRSI acquisition software.

Regional analysis of hippocampal activation during memory encoding and retrieval: fMRI study

Investigators have recently begun to examine the differential role of subregions of the hippocampus in episodic memory. Two distinct models have gained prominence in the field. One model, outlined by Moser and Moser (Hippocampus 1998;8:608-619), based mainly on animal studies, has proposed that episodic memory is subserved by the posterior two-thirds of the hippocampus alone. A second model, derived by Lepage et al. (Hippocampus 1998;8:313-322) from their review of 52 PET studies, has suggested that the anterior hippocampus is activated by memory encoding while the posterior hippocampus is activated by memory retrieval. Functional magnetic resonance imaging (fMRI) studies have tended to show limited activation in the anteriormost regions of the hippocampus, providing support for the Moser and Moser model. A potential confounding factor in these fMRI studies, however, is that susceptibility artifact may differentially reduce signal in the anterior versus the posterior hippocampus. In the present study, we examined activation differences between hippocampal subregions during encoding and retrieval of words and interpreted our findings within the context of these two models. We also examined the extent to which susceptibility artifact affects the analysis and interpretation of hippocampal activation by demonstrating its differential effect on the anterior versus the posterior hippocampus. Both voxel-by-voxel and region-of-interest analyses were conducted, allowing us to quantify differences between the anterior and posterior aspects of the hippocampus. We detected significant hippocampal activation in both the encoding and retrieval conditions. Our data do not provide evidence for regional anatomic differences in activation between encoding and retrieval. The data do suggest that, even after accounting for susceptibility artifact, both encoding and retrieval of verbal stimuli activate the middle and posterior hippocampus more strongly than the anterior hippocampus. Finally, this study is the first to quantify the effects of susceptibility-induced signal loss on hippocampal activation and suggests that this artifact has significantly biased the interpretation of earlier fMRI studies.

Improved spectral quality for 3D MR spectroscopic imaging using a high spatial resolution acquisition strategy

Spectral quality in (1)H MR spectroscopic imaging (MRSI) of the brain is often significantly degraded in regions subject to local magnetic susceptibility variations, which results in broadened and distorted spectral lineshapes. In this report, a modified acquisition strategy for volumetric echo-planar spectroscopic imaging (3D EPSI) is presented that extends the region of the brain that can be observed. The data are sampled at higher spatial resolution, then corrected for local B(0) shifts and reconstructed such that the final spatial resolution matches that of 3D EPSI data acquired with the conventional lower spatial resolution. Comparison of in vivo data obtained at 1.5 T with these two acquisition schemes shows that the high spatial resolution acquisition provides considerable reduction of spectral linewidths in many problematic brain regions, though with a reduction in signal-to-noise ratio by a factor of approximately 1.4 to 1.6 for the matrix sizes used in this study. However, the effect of the increased noise was largely offset by the improved spectral quality, leading to an overall improvement of the metabolite image quality obtained using automated spectral analysis.

Quantification of B0 homogeneity variation with head pitch by registered three-dimensional field mapping

In this study, we quantify the extent to which B(0) homogeneity in adult humans is dependent on head pitch relative to the B(0) vector. Three-dimensional, whole-brain B(0) field maps were acquired in five normal subjects for three generalized head pitch angles. Optimal first- and second-order shimming of the experimental B(0) maps were simulated numerically. The spatial B(0) distribution within the brain was analyzed following automated volumetric co-registration of all data. Increasing head pitch improves both the resonance offset and local homogeneity in the inferior frontal lobes, but introduces inhomogeneities in other regions of the brain which cannot be compensated by first-order shimming but are further improved by second-order shimming.

Optimization of static field homogeneity in human brain using diamagnetic passive shims

These preliminary studies demonstrate that static field inhomogeneity in the human inferior frontal cortex (IFC) is significantly diminished through placement of a small amount of strongly diamagnetic material in the roof of the mouth. As a result, susceptibility-related image artifacts in this region, as observed in blood oxygen level dependent (BOLD) functional MRI (fMRI), are considerably decreased without compromising the spatial or temporal resolution of the study. Simulations of the static field utilizing perturbation theory are shown, which support the experimental results. The limitations and possible future developments of the technique are described. The application of diamagnetic passive shimming on other regions of the brain is also discussed. Routine use of the proposed method within fMRI studies is practicable through subject-specific optimization of the technique utilizing the simulation algorithm.

Inverse planning for functional image-guided intensity-modulated radiation therapy

Radiation therapy is an image-guided process whose success critically depends on the imaging modality used for treatment planning and the level of integration of the available imaging information. In this work, we establish a dose optimization framework for incorporating metabolic information from functional imaging modalities into the intensity-modulated radiation therapy (IMRT) inverse planning process and to demonstrate the technical feasibility of planning deliberately non-uniform dose distributions in accordance with functional imaging data. For this purpose, a metabolic map from functional images is discretized into a number of abnormality levels (ALs) and then fused with CT images. To escalate dose to the metabolically abnormal regions, we assume, for a given spatial point, a linear relation between the AL and the prescribed dose. But the formalism developed here is independent of the assumption and any other relation between AL and prescription is applicable. For a given AL and prescription relation, it is only necessary to prescribe the dose to the lowest AL in the target and the desired doses to other regions with higher AL values are scaled accordingly. To accomplish differential sparing of a sensitive structure when its functional importance (FI) distribution is known, we individualize the tolerance doses of the voxels within the structure according to their Fl levels. An iterative inverse planning algorithm in voxel domain is used to optimize the system with in homogeneous dose prescription. To model intra-structural trade-off, a mechanism is introduced through the use of voxel-dependent weighting factors, in addition to the conventional structure specific weighting factors which model the inter-structural trade-off. The system is used to plan a phantom case with a few hypothetical functional distributions and a brain tumour treatment with incorporation of magnetic resonance spectroscopic imaging data. The results indicated that it is technically feasible to produce deliberately non-uniform dose distributions according to the functional imaging requirements. Integration of functional imaging information into radiation therapy dose optimization allows for consideration of patient-specific biologic information and provides a significant opportunity to truly individualize radiation treatment. This should enhance our capability to safely and intelligently escalate dose and lays the technical foundation for future clinical studies of the efficacy of functional imaging-guided IMRT.

Regularized higher-order in vivo shimming

A regularized algorithm is presented for localized in vivo shimming. The technique uses first- (X,Y,Z), second- (Z(2), ZX, ZY, X(2)-Y(2), XY), and third-order (Z(3)) shim coils, and is robust when applied to arbitrarily-shaped, as well as off-center, regions of interest (ROIs). A single-shot spiral pulse sequence is used for rapid field map acquisition, and a least-squares calculation of the shim currents is performed to minimize the root-mean-square (RMS) value of the B(0) inhomogeneity over a user-selected ROI. The use of a singular value decomposition (SVD) in combination with a regularization algorithm significantly improves the numerical stability of the least-squares fitting procedure. The fully automated shimming package is implemented on a 3 T GE Signa system and its robust performance is demonstrated in phantom and in vivo studies.

Retinotopic hemodynamic activation of the human V5/MT area during optokinetic responses

To detect retinotopic activation in the human V5/MT, we obtained fMRI signals during optokinetic responses (OKR). We used two types of patterns, consisting of random dots plotted in either the central or peripheral regions, to stimulate the central and peripheral visual fields, respectively. These patterns moved at a constant speed of 20 degrees/s rightward and leftward alternately. Subjects were required to track the patterns with their eyes. The two types of visual stimuli elicited different patterns of brain activation; the area with the most significant response to central visual field stimuli was located posteriorly to that responding to peripheral visual field stimuli.

Assessment of 3D proton MR echo-planar spectroscopic imaging using automated spectral analysis

For many clinical applications of proton MR spectroscopic imaging (MRSI) of the brain, diagnostic assessment is limited by insufficient coverage provided by single- or multislice acquisition methods as well as by the use of volume preselection methods. Additionally, traditional spectral analysis methods may limit the operator to detailed analysis of only a few selected brain regions. It is therefore highly desirable to use a fully 3D approach, combined with spectral analysis procedures that enable automated assessment of 3D metabolite distributions over the whole brain. In this study, a 3D echo-planar MRSI technique has been implemented without volume preselection to provide sufficient spatial resolution with maximum coverage of the brain. Using MRSI acquisitions in normal subjects at 1.5T and a fully automated spectral analysis procedure, an assessment of the resultant spectral quality and the extent of viable data in human brain was carried out. The analysis found that 69% of brain voxels were obtained with acceptable spectral quality at TE = 135 ms, and 52% at TE = 25 ms. Most of the rejected voxels were located near the sinuses or temporal bones and demonstrated poor B0 homogeneity and additional regions were affected by stronger lipid contamination at TE = 25 ms.

Volumetric multishot echo-planar spectroscopic imaging

Rapid volumetric magnetic resonance spectroscopic imaging (MRSI) is potentially of great relevance to the diagnosis and treatment of focal cerebral diseases such as cancer and epilepsy. A strategy for volumetric multishot echo-planar spectroscopic imaging (MEPSI) is described which allows whole-brain metabolite mapping in approximately 20 min. A multishot trajectory is used in both the spatial and temporal domains which reduces the accumulated phase during each echo train and tolerates conventional Fourier reconstruction without regridding. Also described is a generalized correction for phase discontinuities arising from the multishot acquisition of the time domain, which is independent of the spatial k-space trajectory and is therefore also applicable to multishot spiral MRSI. Whole-brain, lipid-suppressed MEPSI data were acquired from five normal subjects. The mean signal-to-noise ratios (SNRs) (+/-SE) for the n-acetylaspartate (NAA), choline (Cho), and creatine (Cr) maps across all subjects were 21.3 +/- 1.8, 11.7 +/- 0.6, and 9.2 +/- 0.6, respectively, with a computed voxel size of 2.33 ml.

The accuracy of whole brain N-acetylaspartate quantification

A non-localizing pulse sequence to quantify the total amount of N-acetylaspartate (NAA) in the whole brain (WBNAA) was introduced recently [Magn. Reson. Med. 40, 684-689 (1998)]. However, it is known that regional magnetic field inhomogeneities, deltaB0s, arising from susceptibility differences at tissue interfaces, shift and broaden local resonances to outside the integration window, leading to an underestimation of the true amount of NAA in the entire brain. To quantify the upper limit of this loss, the whole-head proton MR spectrum (1H-MRS) of the water was integrated over the same frequency width as the NAA. The ratio of this area/total-water-line was 75 +/- 5% in 5 volunteers. The procedure was repeated with the brain-only water peak, obtained by summing signals only from voxels within that organ from a three-dimensional chemical-shift-imaging (3D CSI) set. It indicated that <10% of the water signal loss occurred in the brain. Therefore, by analogy, WBNAA accounts for >90% of that metabolite.

Optimal voxel size for measuring global gray and white matter proton metabolite concentrations using chemical shift imaging

Quantification of gray and white matter levels of spectroscopically visible metabolites can provide important insights into brain development and pathological conditions. Chemical shift imaging offers a gain in efficiency for estimation of global gray and white matter metabolite concentrations compared to single voxel methods. In the present study, the optimal voxel size is calculated from segmented human brain data and accompanying field maps. The optimal voxel size is found to be approximately 8 cc, but a wide range of values, 4-64 cc, can be chosen with little increase in estimated concentration error (<15%). Magn Reson Med 44:10-18, 2000.

Field mapping without reference scan using asymmetric echo-planar techniques

Improvements in Bo mapping and shimming were achieved by measuring the static field information in multiple subsequent echoes generated by an asymmetric echo-planar readout gradient train. With careful compensation, eddy current effects were shown to affect the adjustment of the shim coils minimally. In addition to reducing the time required for field mapping by two-fold, the sensitivity was simultaneously optimized irrespective of the prevalent T2* present, thereby minimizing the error of the static field measurement to below 0.1 Hz. With adiabatic low flip-angle excitation, the time required for field mapping was below 1 second.