Inhomogeneity correction for in vivo spectroscopy by high-resolution water referencing

Sequence building block for magnetic resonance spectroscopy on Siemens VE-series scanners

We present a sequence building block (SBB) that embeds magnetic resonance spectroscopy (MRS) into another sequence on the Siemens VE platform without any custom hardware. This enables dynamic studies such as functional MRS (fMRS), dynamic shimming and frequency correction, and acquisition of navigator images for motion correction. The SBB supports nonlocalised spectroscopy (free induction decay), STimulated Echo Acquisition Mode single voxel spectroscopy, and 1D, 2D and 3D phase-encoded chemical shift imaging. It can embed 1H or X-nuclear MRS into a 1H sequence; and 1H-MRS into an X-nuclear sequence. We demonstrate integration into the vendor's gradient-recalled echo sequence. We acquire test data in phantoms with three coils (31P/1H, 13C/1H and 2H/1H) and in two volunteers on a 7-T Terra MRI scanner. Fifteen lines of code are required to insert the SBB into a sequence. Spectra and images are acquired successfully in all cases in phantoms, and in human abdomen and calf muscle. Phantom comparison of signal-to-noise ratio and linewidth showed that the SBB has negligible effects on image and spectral quality, except that it sometimes produces a nuclear Overhauser effect (NOE) signal enhancement for multinuclear applications in line with conventional 1H NOE pulses. Our new SBB embeds MRS into a host imaging or spectroscopy sequence in 15 lines of code. It allows homonuclear and heteronuclear interleaving. The package is available through the standard C2P procedure. We hope this will lower the barrier for entry to studies applying dynamic fMRS and for online motion correction and B0-shim updating.

A method for measuring B0 field inhomogeneity using quantitative double-echo in steady-state

PURPOSE

To develop and validate a method forB 0 $$ {B}_0 $$ mapping for knee imaging using the quantitative Double-Echo in Steady-State (qDESS) exploiting the phase difference (Δ θ $$ \Delta \theta $$ ) between the two echoes acquired. Contrary to a two-gradient-echo (2-GRE) method,Δ θ $$ \Delta \theta $$ depends only on the first echo time.

METHODS

Bloch simulations were applied to investigate robustness to noise of the proposed methodology and all imaging studies were validated with phantoms and in vivo simultaneous bilateral knee acquisitions. Two phantoms and five healthy subjects were scanned using qDESS, water saturation shift referencing (WASSR), and multi-GRE sequences.Δ B 0 $$ \Delta {B}_0 $$ maps were calculated with the qDESS and the 2-GRE methods and compared against those obtained with WASSR. The comparison was quantitatively assessed exploiting pixel-wise difference maps, Bland-Altman (BA) analysis, and Lin's concordance coefficient (ρ c $$ {\rho}_c $$ ). For in vivo subjects, the comparison was assessed in cartilage using average values in six subregions.

RESULTS

The proposed method for measuringΔ B 0 $$ \Delta {B}_0 $$ inhomogeneities from a qDESS acquisition providedΔ B 0 $$ \Delta {B}_0 $$ maps that were in good agreement with those obtained using WASSR.Δ B 0 $$ \Delta {B}_0 $$ ρ c $$ {\rho}_c $$ values were≥ $$ \ge $$ 0.98 and 0.90 in phantoms and in vivo, respectively. The agreement between qDESS and WASSR was comparable to that of a 2-GRE method.

CONCLUSION

The proposed method may allow B0 correction for qDESST 2 $$ {T}_2 $$ mapping using an inherently co-registeredΔ B 0 $$ \Delta {B}_0 $$ map without requiring an additional B0 measurement sequence. More generally, the method may help shorten knee imaging protocols that require an auxiliaryΔ B 0 $$ \Delta {B}_0 $$ map by exploiting a qDESS acquisition that also providesT 2 $$ {T}_2 $$ measurements and high-quality morphological imaging.

Improvement of water saturation shift referencing by sequence and analysis optimization to enhance chemical exchange saturation transfer imaging

PURPOSE

To optimize B0-field inhomogeneity correction for chemical exchange saturation transfer (CEST) imaging by investigating different water saturation shift referencing (WASSR) Z-spectrum shapes and different frequency correction techniques.

METHODS

WASSR Z-spectra were simulated for different B1-fields and pulse durations (PD). Two parameter settings were used for further simulations and experiments (WASSR1: B1=0.1 μT, PD=50ms; WASSR2: B1=0.3 μT, PD=40ms). Four frequency correction techniques were investigated: 1) MinW: Minimum of the spline-interpolated WASSR-spectrum; 2) MSCF: maximum symmetry center frequency algorithm; 3) PMSCF: further development of MSCF algorithm; 4) BFit: fit with Bloch equations. Performance of frequency correction was assessed with Monte-Carlo simulations and in-vivo MR examinations in the brain and intervertebral disks.

RESULTS

Different shapes of WASSR-Z-spectra were obtained by changing B1 and PD including spectra with one (1-Peak) or two (2-Peak) minima. WASSR1 resulted in 1-Peak WASSR-spectrum, whereas WASSR2 resulted in 2-Peak WASSR-spectrum. Both Monte-Carlo simulations and in-vivo MR examinations revealed highest accuracy of field-inhomogeneity correction with WASSR1 combined with PMSCF or BFit.

CONCLUSION

Using a WASSR sequence, which results in a Z-spectrum with a single absorption peak, in combination with advanced postprocessing algorithms enables improved B0-field inhomogeneity correction for CEST imaging.

Improving the spectral resolution and spectral fitting of (1) H MRSI data from human calf muscle by the SPREAD technique

Proton magnetic resonance spectroscopic imaging ((1) H MRSI) has been used for the in vivo measurement of intramyocellular lipids (IMCLs) in human calf muscle for almost two decades, but the low spectral resolution between extramyocellular lipids (EMCLs) and IMCLs, partially caused by the magnetic field inhomogeneity, has hindered the accuracy of spectral fitting. The purpose of this paper was to enhance the spectral resolution of (1) H MRSI data from human calf muscle using the SPREAD (spectral resolution amelioration by deconvolution) technique and to assess the influence of improved spectral resolution on the accuracy of spectral fitting and on in vivo measurement of IMCLs. We acquired MRI and (1) H MRSI data from calf muscles of three healthy volunteers. We reconstructed spectral lineshapes of the (1) H MRSI data based on field maps and used the lineshapes to deconvolve the measured MRS spectra, thereby eliminating the line broadening caused by field inhomogeneities and improving the spectral resolution of the (1) H MRSI data. We employed Monte Carlo (MC) simulations with 200 noise realizations to measure the variations of spectral fitting parameters and used an F-test to evaluate the significance of the differences of the variations between the spectra before SPREAD and after SPREAD. We also used Cramer-Rao lower bounds (CRLBs) to assess the improvements of spectral fitting after SPREAD. The use of SPREAD enhanced the separation between EMCL and IMCL peaks in (1) H MRSI spectra from human calf muscle. MC simulations and F-tests showed that the use of SPREAD significantly reduced the standard deviations of the estimated IMCL peak areas (p < 10(-8) ), and the CRLBs were strongly reduced (by ~37%).

Quantitative comparison of post-processing methods for reduction of frequency modulation sidebands in non-water suppression 1H MRS

Non-water suppression MRS (NWS MRS) has several advantages. First, the unsuppressed water signal can be used as internal calibration for metabolite quantification and as a reliable frequency/phase reference for retrospective motion correction. Second, it avoids the potential artifacts caused by incomplete water suppression (WS) and extra radiofrequency deposition from WS pulses. However, the frequency modulation (FM) sidebands originating from a large water signal will distort the spectrum. Among the methods proposed to solve the problems caused by FM sidebands, post-acquisition processing methods are superior in flexibility for general use compared with experimental methods. In this study, we propose two algorithms based on advanced matrix decomposition to remove the FM sidebands. These methods, the simultaneous diagonalization (QZ) algorithm and its subsequent variant, the simultaneously generalized Schur decomposition (SGSD) algorithm, were numerically evaluated using computer simulations. In addition, we quantitatively compared the performance of these methods and the modulus method in an in vitro experiment and in vivo NWS MRS against conventional WS data. Our results show that the proposed SGSD algorithm can reduce the FM sidebands to achieve superior estimation of concentration on three major metabolites. This method can be applied directly to spectra pre-acquired under various experimental conditions without modifying the acquisition sequences.

Role of very high order and degree B0 shimming for spectroscopic imaging of the human brain at 7 tesla

With the advent of ultrahigh field systems (7 T), significant improvements in spectroscopic imaging (SI) studies of the human brain have been anticipated. These gains are dependent upon the achievable B0 homogeneity, both globally (σB0Global, over the entire regions of interest or slice) and locally (σB0Global, influencing the linewidth of individual SI voxels within the regions of interest). Typically the B0 homogeneity is adjusted using shim coils with spatial distributions modeled on spherical harmonics which can be characterized by a degree (radial dependence) and order (azimuthal symmetry). However, the role of very high order and degree shimming (e.g., 3rd and 4th degree) in MRSI studies has been controversial. Measurements of σB0Global and σB0Local were determined from B0 field maps of 64×64 resolution. In a 10 mm thick slice taken through the region of the subcortical nuclei, we find that in comparison to 1st-2nd degree shims, use of 1st-3rd and 1st-4th degree shims reduces σB0Global by 29% and 55%, respectively. Using a SI voxel size of ∼1cc with an estimate of σB0Local from 3×3×3 B0 map pixels in this subcortical region, the number of pixels with σB0Local of less than 5 Hz increased from 24 to 59% with 1st-3rd and 1st-4th over 1st-2nd degree shims, respectively.

Chemical exchange saturation transfer magnetic resonance imaging of human knee cartilage at 3 T and 7 T

The sensitivity of chemical exchange saturation transfer (CEST) on glycosaminoglycans (GAGs) in human knee cartilage (gagCEST) in vivo was evaluated at 3 and 7 T field strengths. Calculated gagCEST values without accounting for B(0) inhomogeneity (~0.6 ppm) were >20%. After B(0) inhomogeneity correction, calculated gagCEST values were negligible at 3 T and ~6% at 7 T. These results suggest that accurate B(0) correction is a prerequisite for observing reliable gagCEST. Results obtained with varying saturation pulse durations and amplitudes as well as the consistency between numerical simulations and our experimental results indicate that the negligible gagCEST observed at 3 T is due to direct saturation effects and fast exchange rate. As GAG loss from cartilage is expected to result in a further reduction in gagCEST, gagCEST method is not expected to be clinically useful at 3 T. At high fields such as 7 T, this method holds promise as a viable clinical technique.

Real-time motion and B0 corrected single voxel spectroscopy using volumetric navigators

In population groups where head pose cannot be assumed to be constant during a magnetic resonance spectroscopy examination or in difficult-to-shim regions of the brain, real-time volume of interest, frequency, and shim optimization may be necessary. We investigate the effect of pose change on the B0 homogeneity of a (2 cm)3 volume and observe typical first-order shim changes of 1 μT/m per 1° rotation (chin down to up) in four different volumes of interest in a single volunteer. An echo planar imaging volume navigator was constructed to measure and apply in real-time within each pulse repetition time: volume of interest positioning, frequency adjustment, and first-order shim adjustment. This volume navigator is demonstrated in six healthy volunteers and achieved a mean linewidth of 4.4 Hz, similar to that obtained by manual shim adjustment of 4.9 Hz. Furthermore, this linewidth is maintained by the volume navigator at 4.9 Hz in the presence of pose change. By comparison, a mean linewidth of 7.5 Hz was observed, when no correction was applied.

High-throughput screening of chemical exchange saturation transfer MR contrast agents

A new high-throughput MRI method for screening chemical exchange saturation transfer (CEST) agents is demonstrated, allowing simultaneous testing of multiple samples with minimal attention to sample configuration and shimming of the main magnetic field (B(0)). This approach, which is applicable to diamagnetic, paramagnetic and liposome CEST agents, employs a set of inexpensive glass or plastic capillary tubes containing CEST agents put together in a cheap plastic tube holder, without the need for liquid between the tubes to reduce magnetic susceptibility effects. In this setup, a reference image of direct water saturation spectra is acquired in order to map the absolute water frequency for each volume element (voxel) in the sample image, followed by an image of saturation transfer spectra to determine the CEST properties. Even though the field over the total sample is very inhomogeneous due to air-tube interfaces, the shape of the direct saturation spectra is not affected, allowing removal of susceptibility shift effects from the CEST data by using the absolute water frequencies from the reference map. As a result, quantitative information such as the mean CEST intensity for each sample can be extracted for multiple CEST agents at once. As an initial application, we demonstrate rapid screening of a library of 16 polypeptides for their CEST properties, but in principle the number of tubes is limited only by the available signal-noise-ratio, field of view and gradient strength for imaging.

Water saturation shift referencing (WASSR) for chemical exchange saturation transfer (CEST) experiments

Chemical exchange saturation transfer (CEST) is a contrast mechanism that exploits exchange-based magnetization transfer (MT) between solute and water protons. CEST effects compete with direct water saturation and conventional MT processes, and generally can only be quantified through an asymmetry analysis of the water saturation spectrum (Z-spectrum) with respect to the water frequency, a process that is exquisitely sensitive to magnetic field inhomogeneities. Here it is shown that direct water saturation imaging allows measurement of the absolute water frequency in each voxel, allowing proper centering of Z-spectra on a voxel-by-voxel basis independently of spatial B(0) field variations. Optimal acquisition parameters for this "water saturation shift referencing" (WASSR) approach were estimated using Monte Carlo simulations and later confirmed experimentally. The optimal ratio of the WASSR sweep width to the linewidth of the direct saturation curve was found to be 3.3-4.0, requiring a sampling of 16-32 points. The frequency error was smaller than 1 Hz at signal-to-noise ratios of 40 or higher. The WASSR method was applied to study glycogen, where the chemical shift difference between the hydroxyl (OH) protons and bulk water protons at 3T is so small (0.75-1.25 ppm) that the CEST spectrum is inconclusive without proper referencing.

Spectral resolution amelioration by deconvolution (SPREAD) in MR spectroscopic imaging

PURPOSE

To develop, implement, and evaluate a novel postprocessing method for enhancing the spectral resolution of in vivo MR spectroscopic imaging (MRSI) data.

MATERIALS AND METHODS

Magnetic field inhomogeneity across the imaging volume was determined by acquiring MRI datasets with two differing echo times. The lineshapes of the MRSI spectra were derived from these field maps by simulating an MRSI scan of a virtual sample whose resonance frequencies varied according to the observed variations in the magnetic field. By deconvolving the lineshapes from the measured MRSI spectra, the linebroadening effects of the field inhomogeneities were reduced significantly.

RESULTS

Both phantom and in vivo proton MRSI spectra exhibited significantly enhanced spectral resolutions and improved spectral lineshapes following application of our method. Quantitative studies on a phantom show that, on average, the full width at half maximum of water peaks was reduced 42%, the full width at tenth maximum was reduced 38%, and the asymmetries of the peaks were reduced 86%.

CONCLUSION

Our method reduces the linebroadening and lineshape distortions caused by magnetic field inhomogeneities. It substantially improves the spectral resolution and lineshape of MRSI data.

Improved calibration technique for in vivo proton MRS thermometry for brain temperature measurement

The most common MR-based approach to noninvasively measure brain temperature relies on the linear relationship between the (1)H MR resonance frequency of tissue water and the tissue's temperature. Herein we provide the most accurate in vivo assessment existing thus far of such a relationship. It was derived by acquiring in vivo MR spectra from a rat brain using a high field (11.74 Tesla [T]) MRI scanner and a single-voxel MR spectroscopy technique based on a LASER pulse sequence. Data were analyzed using three different methods to estimate the (1)H resonance frequencies of water and the metabolites NAA, Cho, and Cr, which are used as temperature-independent internal (frequency) references. Standard modeling of frequency-domain data as composed of resonances characterized by Lorentzian line shapes gave the tightest resonance-frequency versus temperature correlation. An analysis of the uncertainty in temperature estimation has shown that the major limiting factor is an error in estimating the metabolite frequency. For example, for a metabolite resonance linewidth of 8 Hz, signal sampling rate of 2 Hz and SNR of 5, an accuracy of approximately 0.5 degrees C can be achieved at a magnetic field of 3T. For comparison, in the current study conducted at 11.74T, the temperature estimation error was approximately 0.1 degrees C.

Optimization of fast spiral chemical shift imaging using least squares reconstruction: application for hyperpolarized (13)C metabolic imaging

A least-squares-based optimization and reconstruction algorithm has been developed for rapid metabolic imaging in the context of hyperpolarized (13)C. The algorithm uses a priori knowledge of resonance frequencies, J-coupling constants, and T(2)* values to enable acquisition of high-quality metabolic images with imaging times of approximately 100 ms for an 8-cm field of view (FOV) and 0.5 cm isotropic resolution. A root-mean-square error (rMSE) analysis is introduced to optimize metabolic image quality by appropriate choice of pulse sequence parameters, echo times, and signal model. By performing the reconstruction in k-space, the algorithm also allows the inclusion of the effect of chemical shift evolution during the readout period. Single-interleaf multiecho spiral chemical shift imaging (spCSI) is analyzed in detail as an illustrative example for the use of the new reconstruction and optimization algorithm. Simulation of the in vivo spectrum following the bolus injection of hyperpolarized (13)C(1) pyruvate shows that single-interleaf spiral spectroscopic imaging can achieve image quality in 100 ms, comparable to the performance of a 13-s phase-encoded chemical shift imaging (FIDCSI) experiment. Single-interleaf spCSI was also tested at a 3-T MR scanner using a phantom containing approximately 0.5-M solutions of alanine, lactate, and a pyruvate-pyruvate hydrate C(1)-C(2) ester at thermal equilibrium polarization, all enriched to 99% (13)C in the C(1) carbonyl positions. Upon reconstruction using the k-space-based least-squares technique, metabolite ratios obtained using the spCSI method were comparable to those obtained using a reference FIDCSI acquisition.

Natural linewidth chemical shift imaging (NL-CSI)

The discrete Fourier transform (FT) is a conventional method for spatial reconstruction of chemical shifting imaging (CSI) data. Due to point spread function (PSF) effects, FT reconstruction leads to intervoxel signal leakage (Gibbs ringing). Spectral localization by imaging (SLIM) reconstruction was previously proposed to overcome this intervoxel signal contamination. However, the existence of magnetic field inhomogeneities creates an additional source of intervoxel signal leakage. It is demonstrated herein that even small field inhomogeneities substantially amplify intervoxel signal leakage in both FT and SLIM reconstruction approaches. A new CSI data acquisition strategy and reconstruction algorithm (natural linewidth (NL) CSI) is presented that eliminates effects of magnetic field inhomogeneity-induced intervoxel signal leakage and intravoxel phase dispersion on acquired data. The approach is based on acquired CSI data, high-resolution images, and magnetic field maps. The data are reconstructed based on the imaged object structure (as in the SLIM approach) and a reconstruction matrix that takes into account the inhomogeneous field distribution inside anatomically homogeneous compartments. Phantom and in vivo results show that the new method allows field inhomogeneity effects from the acquired MR signal to be removed so that the signal decay is determined only by the "natural" R2 relaxation rate constant (hence the term "natural linewidth" CSI).

1H MR spectroscopy of the brain: absolute quantification of metabolites

Hydrogen 1 (1H) magnetic resonance (MR) spectroscopy enables noninvasive in vivo quantification of metabolite concentrations in the brain. Currently, metabolite concentrations are most often presented as ratios (eg, relative to creatine) rather than as absolute concentrations. Despite the success of this approach, it has recently been suggested that relative quantification may introduce substantial errors and can lead to misinterpretation of spectral data and to erroneous metabolite values. The present review discusses relevant methods to obtain absolute metabolite concentrations with a clinical MR system by using single-voxel spectroscopy or chemical shift imaging. Important methodological aspects in an absolute quantification strategy are addressed, including radiofrequency coil properties, calibration procedures, spectral fitting methods, cerebrospinal fluid content correction, macromolecule suppression, and spectral editing. Techniques to obtain absolute concentrations are now available and can be successfully applied in clinical practice. Although the present review is focused on 1H MR spectroscopy of the brain, a large part of the methodology described can be applied to other tissues as well.

Evaluation of variable line-shape models and prior information in automated 1H spectroscopic imaging analysis

Analysis of in vivo short TE 1H spectra is complicated by broad baseline signal contributions and resonance line-shape distortions. Although the assumptions of ideal metabolite resonance line-shapes and slowly varying baseline signals can be used to separate these signals, the presence of broad or asymmetric line-shapes can invalidate this model. More complex line-shape models are computationally expensive or difficult to constrain, particularly for the low signal-to-noise commonly found for in vivo MR spectroscopic imaging applications. In this study, two time-domain models for fitting variable spectral line-shapes are examined, one using B-splines and another using summed sinusoids. The methods were verified using both phantom and human data, and Monte Carlo simulations were used to evaluate variations in calculated metabolite amplitudes due to interactions between the baseline and line-shape estimations. Additional studies investigated the use of prior line-shape information, obtained from either a water MRSI measurement or calculations from B(0) maps, to determine parameter starting values or optimization constraints. Both line-shape models showed the ability to fit the variety of line-shapes present in both the phantom and human MRSI data, with similar or improved accuracy over a Gaussian line-shape model; however, this improvement resulted in only minor improvement for the high-SNR phantom data and moderate improvements in regions with asymmetry for the fitted in vivo metabolite images. The use of prior line-shape information was of most benefit when applied toward setting optimization constraints but was of limited benefit when used to define initial starting values.

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.

Common processing of in vivo MR spectra

This introductory article addresses approaches currently in use to process in vivo spectra. First, a brief overview is given of the information content represented by the parameters of MR signals. Subsequently, common steps in the processing of MR spectra such as pre-processing, normalisation and quantification and the use of prior knowledge are described. Finally, some prospects for more advanced processing are given.

MR spectroscopy quantitation: a review of time-domain methods

In this article an overview of time-domain quantitation methods is given. Advantages of processing the data in the measurement domain are discussed. The basic underlying principles of the methods are outlined and from them the situations under which these algorithms perform well are derived. Also an overview of methods to preprocess the data is given. In that respect, signal-to-noise and/or resolution enhancement, the removal of unwanted components and corrections for model imperfections are discussed.

A fast variant of (1)H spectroscopic U-FLARE imaging using adjusted chemical shift phase encoding

So far, fast spectroscopic imaging (SI) using the U-FLARE sequence has provided metabolic maps indirectly via Fourier transformation (FT) along the chemical shift (CS) dimension and subsequent peak integration. However, a large number of CS encoding steps N(omega) is needed to cover the spectral bandwidth and to achieve sufficient spectral resolution for peak integration even if the number of resonance lines is small compared to N(omega) and even if only metabolic images are of interest and not the spectra in each voxel. Other reconstruction algorithms require extensive prior knowledge, starting values, and/or model functions. An adjusted CS phase encoding scheme (APE) can be used to overcome these drawbacks. It incorporates prior knowledge only about the resonance frequencies present in the sample. Thus, N(omega) can be reduced by a factor of 4 for many (1)H in vivo studies while no spectra have to be reconstructed, and no additional user interaction, prior knowledge, starting values, or model function are required. Phantom measurements and in vivo experiments on rat brain have been performed at 4.7 T to test the feasibility of the method for proton SI.

Feasibility study of lactate imaging of head and neck tumors

A proton spectroscopic imaging sequence was used to investigate the feasibility of lactate imaging in head and neck tumors. The sequence employs a two-shot lactate editing method with inversion recovery for additional lipid suppression, and a restricted field of view to suppress motion artifacts. Variations in acquisition parameters and two different receive coils were investigated on twelve patients. Elevated lactate was detected in three patients, no lactate was observed in seven patients, and two studies were inconclusive because of severe motion or inhomogeneity artifacts. Best results were obtained with an anterior/posterior neck coil at a 288 ms echo time (TE).

Versatile frequency domain fitting using time domain models and prior knowledge

An iterative nonlinear least-squares fitting algorithm in the frequency domain using time domain models for quantification of complex frequency domain MR spectra is presented. The algorithm allows incorporation of prior knowledge and has both the advantage of time-domain fitting with respect to handling the problem of missing data points and truncated data sets and of frequency-domain fitting with respect to multiple frequency-selective fitting. The described algorithm can handle, in addition to Lorentzian and Gaussian lineshapes, Voigt and nonanalytic lineshapes. The program allows the user the design of his own fitting strategy to optimize the probability of reaching the global least-squares minimum. The application of the fitting program is illustrated with examples from in vivo 1H-, 31P-, and 13C-MR spectroscopy.

1H-spectroscopic imaging with read gradient during acquisition in inhomogeneous fields: analysis, measurement strategy, and data processing

The proton magnetic resonance spectroscopic imaging techniques that use read gradient during acquisition produce proton spectra with high spatial and moderately high spectroscopic resolution in a reasonable time for in vivo applications. These techniques suffer mainly from the spatial and spectral distortions caused by the convolution of spectral/spatial information (chemical-shift artifacts) and from the spectral shifts caused by static magnetic field inhomogeneities. The investigators analyze the chemical-shift artifacts in the presence of nonnegligible static magnetic field inhomogeneities and propose a postdetection processing scheme to correct for such effects. Spectral artifacts caused by chemical shifts, spectral line overlapping, streak broadening, and magnetic field inhomogeneities are discussed. The postdetection data processing scheme is demonstrated on measurements of a phantom as well as a human leg.

Modeling NMR lineshapes using logspline density functions

Distortions in the FID and spin echo due to magnetic field inhomogeneity are proved to have a representation as the characteristic function of some probability distribution. In the special case that the distribution is Cauchy, the model reduces to the conventional Lorentzian model. A more general and flexible representation is presented using the Fourier transform of a logspline density. An algorithm for fitting the model is described, the performance of the model and algorithm is investigated in applications to real and simulated data sets, and the logspline approach is compared to a previous Hermitian spline approach and to the Lorentzian model. The logspline model is more parsimonious than the Hermitian spline model, provides a better fit to real data, and is much less biased than the Lorentzian model.

Time course of the apparent diffusion coefficient (ADC) abnormality in human stroke

Diffusion-weighted MRI can rapidly detect acute cerebral ischemic injury as hyperintense signal changes, reflecting a decline in the apparent diffusion coefficient (ADC) of water through brain parenchyma, whereas ADC is elevated in the chronic stage because of increased extracellular water content. To determine the time course of these ADC changes, we analyzed 157 diffusion-weighted MRI studies performed at varying time points from the initial ischemic event from 101 patients. Data were expressed as the relative ADC (rADC), the ratio of lesion to control regions of interest. We observed two phases in the time course of rADC changes in acute human stroke: a significant (p < 0.005) reduction in rADC lasting for at least 96 hours from stroke onset (mean, 58.3% of control; SEM, 1.47) and an increasing trend from reduction to pseudonormalization to elevation of rADC values at later subacute to chronic time points (> or = 7 days). We suggest that the persistent reduction of rADC within the first four days may reflect ongoing or progressive cytotoxic edema to a greater degree than extracellular edema and cell lysis.

Reproducibility of high spatial resolution proton magnetic resonance spectroscopic imaging in the human brain

The application of proton (1H) magnetic resonance spectroscopic imaging (MRSI) allows for noninvasive, localized analyses of brain biochemistry; however, minimal work has been devoted to the evaluation of 1H MRSI reproducibility. This study examined the reproducibility of 1H MRSI from five normal subjects on two occasions, separated by 10 days. Reproducibility of the MR signal was evaluated in the context of automated shimming, automated processing, and accurate subject repositioning. Reliability measures for physicochemical indices (choline moieties, creatine, N-acetylaspartate, and myo-inositol) were moderately concordant across repeat studies. Gain variation and repositioning results were excellent. It has been concluded that 1H MRSI reproducibility is adequate for serial studies of brain metabolism.

A computer simulation of the static magnetic field distribution in the human head

Distortion of the static magnetic field inside the human head is dependent on regional tissue susceptibility variations and geometrical shape. These effects result in resonance line broadening and frequency shifts and consequently, intensity and spatial errors in both magnetic resonance imaging (MRI) and magnetic resonance (MR) spectroscopy. To calculate the field distortion due to the susceptibility's geometry, two dimensional (2D) finite element analysis was applied to simulate the field distribution in a 2D model of the human head, placed in a uniform magnetic field. The model contains air-filled cavities and sinuses, and the remainder is treated as water. The magnetic field deviation was evaluated using gray scale plots and histograms of the magnetic field. The shifts in parts/million and broadening of the histograms correspond to the NMR of the sampled region. The field distribution of the human head was also experimentally mapped using the DANTE tagging sequence. The calculated and experimental field maps are in good agreement. Thus, geometric considerations with uniform susceptibilities are sufficient to explain most of the static magnetic field distribution in the human head.

Magnetic resonance spectroscopic imaging of ethanol in the human brain: a feasibility study

The in vivo distribution of ethanol in normal human brain following the consumption of a moderate amount of alcohol was measured using magnetic resonance spectroscopic imaging. Three subjects were studied, and the spatial distribution of brain ethanol, 60-min postingestion and measured at a spatial resolution of 1.5 cm, was found to be highly nonuniform with the relative ethanol signal in cerebral spinal fluid, gray matter, and white matter determined to be 1.00, 0.72, and 0.37, respectively. These spectroscopic imaging results indicate that whereas in vivo magnetic resonance studies of ethanol are feasible, quantitative studies of alcohol need to account carefully for the various tissue types within the observed volume.

Correction of phase effects produced by eddy currents in solvent suppressed 1H-CSI

Accurate phasing of MRS spectra is often difficult unless time varying phase effects produced by gradient-induced eddy currents that persist during data acquisition are eliminated. This effect is particularly problematic in 1H-CSI spectra where frequency shifts produced by static field inhomogeneity and phase shifts produced by eddy currents combine. In this paper we present a method that corrects both shifts and eliminates manual phasing of individual CSI spectra typically required to recover a pure absorption line shape. The method uses a time domain phase correction derived from the ambient water signal acquired under identical conditions (i.e., acquisition parameters, gradient sequence) as the solvent-suppressed CSI data. Results from CSI experiments on phantoms and in vivo solvent suppressed 1H-CSI spectra from normal human brain are presented demonstrating the capabilities of the technique.

Lipid-suppressed single- and multisection proton spectroscopic imaging of the human brain

Spectroscopic images of the brain have great potential in disease diagnosis and treatment monitoring. Unfortunately, interfering lipid signals from subcutaneous fat and poor water suppression due to magnetic field inhomogeneities can make such images difficult to obtain. A pulse sequence that uses inversion recovery for lipid suppression and a spectral-spatial refocusing pulse for water suppression is introduced. In contrast to methods that eliminate fat signal by restricting the excited volume to lie completely within the brain, inversion-recovery techniques allow imaging of an entire section without such restrictions. In addition, the spectral-spatial pulse was designed to provide water suppression insensitive to a reasonable range of B0 and B1 inhomogeneities. Several data processing algorithms have also been developed and used in conjunction with the new pulse sequence to produce metabolite maps covering large volumes of the human brain. Images from single- and multisection studies demonstrate the performance of these techniques.