Echo-Planar Diffusion Spectroscopic Imaging

High-speed diffusion spectroscopic imaging based on an echo-planar technique is presented. A pair of diffusion gradients is applied prior to a rapidly oscillating magnetic field gradient which encodes both chemical shift and spatial information. By applying this technique to a phantom consisting of acetone and water, a diffusion spectroscopic image is obtained in about 15 min, about 64 times faster than the time required in the conventional method. The measured diffusion coefficients show good agreement with previously reported values. This kind of diffusion spectroscopic imaging is expected to provide a way to observe more specific metabolism.     

Three-Dimensional Spectroscopic Imaging with Time-Varying Gradients

A spectroscopic imaging sequence with a time-varying readout gradient in the slice selection direction is used to image multiple contiguous slices. For a given voxel size, the imaging time and signal-to-noise ratio of the three-dimensional spectroscopic sequence are the same as for a single slice acquisition without the oscillating readout gradient. The data reconstruction employs a gridding algorithm in two dimensions to interpolate the nonuniformly sampled data onto a Cartesian grid, and a fast Fourier transform in four dimensions: three spatial dimensions and the spectral dimension. The method is demonstrated by in vivo imaging of NAA in human brain at 1.5 T with 10 slices of 16 x 16 pixels spectroscopic images acquired in a total scan time of 17 min.     

Comparison of k-space sampling schemes for multidimensional MR spectroscopic imaging

For clinical ³¹P MR spectroscopic imaging (MRSI) studies, where signal averaging is necessary, some improvement of sensitivity and spatial response function may be achieved by acquiring data over a spherical k-space volume and varying the number of averages acquired in proportion to the desired spatial filter. Eight different k-space sampling schemes are compared through simulations that provide graphs of the spatial response functions (SRF), and tabulations of voxel volumes, relative signal-to-noise ratios (SNR), and relative data collection efficiencies (SNR per unit volume over the same time). All schemes were based on practical experiments, each of which could be implemented in the same length of time. The results show that in comparison with cubic k-space sampling with the same number of signal averages at each point, spherical and acquisition-weighted k-space sampling can be used to achieve reduced Gibbs ringing along the principal axes directions, and thus reduced contamination from adjacent tissue in these directions, without degradation of voxel volume or SNR.     

Block Regional Interpolation Scheme for k-Space (BRISK): A Rapid Cardiac Imaging Technique

We introduce an acquisition method, “block regional interpolation scheme for k-space” (BRISK), to reduce the acquisition time for cardiac imaging. The method exploits the high degree of correlation that exists between time-resolved cardiac images. For representative k-space data sets, Fourier analysis was applied along the cardiac phase dimension to reveal that different regions of k-space can be effectively sampled at different rates. A reduced sampling strategy was implemented, and unsampled points were generated by Fourier interpolation. Time savings of up to 75% are quite feasible and 25% BRISK scans compare well with 100% scans. Simulations and acquisitions using a normal volunteer and patients are presented.     

Short echo time 1H-MRSI of the human brain at 3T with minimal chemical shift displacement errors using adiabatic refocusing pulses.

The chemical shift displacement error (CSDE) is an often-underestimated problem in slice selection for localized proton spectroscopy at higher fields. With the proposed semi-localized by adiabatic selective refocusing (LASER) pulse sequence, this problem is dealt with by using RF pulses with bandwidths in the order of 5 kHz. A combination of conventional nonadiabatic slice-selective excitation of proton spins, together with double slice-selective refocusing of the spins by two pairs of adiabatic full-passage (APF) pulses, produces a spin echo in a volume of interest (VOI) at an echo time down to 30 ms. An illustration of the CSDE of conventional point-resolved spectroscopy (PRESS) and the semi-LASER sequence is shown with a measurement of the brain of a volunteer at 3T. With one application of the technique to a patient with a glioblastoma multiforme (GBM), its clinical functionality is demonstrated. With sharp selection profiles and a small CSDE, voxels close to the edge of the VOI can also be used for evaluation. With the additional advantage of being relatively insensitive for B(1) inhomogeneities, the semi-LASER technique can be viewed as a superior substitute for conventional PRESS MR spectroscopic imaging (MRSI) at 3T and beyond.     

Phase Constrained Encoding (PACE): A Technique for MRI in Large Static Field Inhomogeneities

In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echotrain and alternating readout gradients. Small artifactual sidebands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.     

3D 31P Spectroscopic Imaging of the Human Heart at 4.1 T

High field (4 Tesla) spectroscopic imaging offers the advantages of increased signal-to-noise ratio and the possibility of acquiring high resolution metabolite images. We have applied a three dimensional spectroscopic imaging sequence using a sparse Gaussian sampling method to acquire phosphocreatine (PCr) images of the human heart with 8-cc voxels. PCr images enabled observation of the septum, left ventricular free wall, apex, and skeletal muscle. Quantitative evaluation of the 50 myocardial voxels acquired from 10 studies of healthy adults revealed a PCr/adenosine triphosphate (ATP) ratio of 1.80 ± 0.32 after correction for saturation effects. Due to the small size of the voxels and the ability to choose the location of the volumes to minimize inclusion of blood, no correction for blood pool ATP was required. The calculated PCr/ATP ratio is in agreement with other studies at 1.5 and 4.0 T.     

Implementation of three‐dimensional wavelet encoding spectroscopic imaging: In vivo application and method comparison

We have recently proposed a two‐dimensional Wavelet Encoding‐Spectroscopic Imaging (WE‐SI) technique as an alternative to Chemical Shift Imaging (CSI), to reduce acquisition time and crossvoxel contamination in magnetic resonance spectroscopic imaging (MRSI). In this article we describe the extension of the WE‐SI technique to three dimensions and its implementation on a clinical 1.5 T General Electric (GE) scanner. Phantom and in vivo studies are carried out to demonstrate the usefulness of this technique for further acquisition time reduction with low voxel contamination. In wavelet encoding, a set of dilated and translated prototype functions called wavelets are used to span a localized space by dividing it into a set of subspaces with predetermined sizes and locations. In spectroscopic imaging, this process is achieved using radiofrequency (RF) pulses with profiles resembling the wavelet shapes. Slice selective excitation and refocusing RF pulses, with single‐band and dual‐band profiles similar to Haar wavelets, are used in a modified PRESS sequence to acquire 3D WE‐SI data. Wavelet dilation and translation are achieved by changing the strength of the localization gradients and frequency shift of the RF pulses, respectively. The desired spatial resolution in each direction sets the corresponding number of dilations (increases in the localization gradients), and consequently, the number of translations (frequency shift) of the Haar wavelets (RF pulses), which are used to collect magnetic resonance (MR) signals from the corresponding subspaces. Data acquisition time is reduced by using the minimum recovery time (TR_min), also called effective time, when successive MR signals from adjacent subspaces are collected. Inverse wavelet transform is performed on the acquired data to produce metabolite maps. The proposed WE‐SI method is compared in terms of acquisition time, pixel bleed, and signal‐to‐noise ratio to the CSI technique. The study outcome shows that 3D WE‐SI provides accurate results while reducing both acquisition time and voxel contamination. Magn Reson Med 61:6–15, 2009. © 2008 Wiley‐Liss, Inc.     

Incorporation of Lactate Measurement in Multi-Spin-Echo Proton Spectroscopic Imaging

An improved multi-slice, multi-spin-echo proton spectro-scopic imaging method for human brain is presented. The technique allows the reconstruction of lactate images, along with choline plus creatine, N-acetylaspartate, and lipid images from one single data set processed in three separate ways. The discrimination between resonances of lipid protons and lactate methyl protons is based on homonuclear spin-spin coupling. The reliability of the separation of the lipid and lactate contribution depends on the T2 of the lipid resonances. Measurements were performed on a standard 1.5 Tesla clinical scanner on healthy volunteers and a patient with high grade CNS lymphoma, demonstrating the ability to obtain high quality metabolite maps within 11 min.     

Selective missing pulse steady state free precession (MP-SSFP): inner volume and chemical shift selective imaging in a steady state sequence

PURPOSE: To examine new sequences that restrict acquisition of spins to those excited by both of the RF pulses in missing pulse steady state free precession (MP-SSFP) MRI. MATERIALS AND METHODS: New MP-SSFP sequences were created by replacing one of the slice selective pulses (SSPs) with an orthogonal SSP for inner volume imaging, and with a chemical shift selective (CHESS) pulse for chemical shift imaging. The inner volume sequence was applied to a reduced field of view at the center of a resolution phantom; resulting images were evaluated for differences in the aliased signal. The CHESS sequence was applied to volunteers, as well as to and fat, water, and acetic acid phantoms. Results were evaluated with SNR measurements. RESULTS: The inner volume sequence eliminated the aliased signal, while nonselected fat and water levels were suppressed to that of noise by the CHESS sequence. CONCLUSION: Results suggest a novel steady state technique for rapid inner volume or chemical shift imaging.     

多体素^1H MRS测定正常脑组织不同区域的代谢物分布

目的: 测定正常人脑组织不同部位代谢物浓度、计算其比值.材料和方法: 用多体素磁共振质子波谱PRESS序列测定100例正常脑组织的额叶、颞叶、顶叶、枕叶、基底节区和小脑代谢物浓度,观察NAA、Cho、Cr的波峰特点,计算和分析NAA/Cr、Cho/Cr、NAA/Cho的比值.结果: 正常脑实质的1H波谱Levene方差分析显示额叶、颞叶、顶叶、枕叶、基底节区和小脑的NAA/Cr(P<0.000)和NAA/Cho(P=0.001)有显著差异,Cho/Cr在上述各部位的浓度无显著差异(P=0.068).进一步用Bonferroni方差分析比较各组间显著差异性显示小脑NAA/Cr低于额叶、顶叶、枕叶的NAA/Cr(P值分别<0.05,<0.01,<0.05),小脑NAA/Cho低于顶叶、枕叶的NAA/Cho(P均<0.01),而额叶、颞叶、顶叶、枕叶、基底节区之间NAA/Cr、NAA/Cho无显著差异.结论: 多体素1H MRS可以测定NAA/Cr、Cho/Cr、NAA/Cho浓度比值,不同解剖部位代谢物浓度不尽相同,为颅脑代谢异常提供参考标准.     

High-resolution (1)H chemical shift imaging in the monkey visual cortex.

Functionally distinct anatomic subdivisions of the brain can often be only a few millimeters in one or more dimensions. The study of metabolic differences in such structures by means of localized in vivo MR spectroscopy is therefore challenging, if not impossible. In fact, the spatial resolution of chemical shift imaging (CSI) in humans is typically in the range of centimeters. The aim of the present study was to optimize (1)H CSI in monkeys and demonstrate the feasibility of high spatial resolutions up to 1.4 x 2 x 1.4 mm(3). The obtained spatial resolution permitted the segregation of gray and white matter in the visual cortex based on the concentration of different metabolites and neurotransmitters like N-acetylaspartate, glutamate, and creatine. Concentration ratios of white matter versus gray matter tissue as well as between metabolites matched those reported in the literature from healthy human brain, demonstrating the consistency and reliability of the procedure.     

High Speed 1H Spectroscopic Imaging in Human Brain by Echo Planar Spatial-Spectral Encoding

We introduce a fast and robust spatial-spectral encoding method, which enables acquisition of high resolution short echo time (13 ms) proton spectroscopic images from human brain with acquisition times as short as 64 s when using surface coils. The encoding scheme, which was implemented on a clinical 1.5 Tesla whole body scanner, is a modification of an echo-planar spectroscopic imaging method originally proposed by Mansfield Magn. Reson. Med. 1, 370–386 (1984), and utilizes a series of read-out gradients to simultaneously encode spatial and spectral information. Superficial lipid signals are suppressed by a novel double outer volume suppression along the contours of the brain. The spectral resolution and the signal-to-noise per unit time and unit volume from resonances such as N-acetyl aspartate, choline, creatine, and inositol are comparable with those obtained with conventional methods. The short encoding time of this technique enhances the flexibility of in vivo spectroscopic imaging by reducing motion artifacts and allowing acquisition of multiple data sets with different parameter settings.     

Temperature mapping using water proton chemical shift obtained with 3D-MRSI: Feasibility in vivo

This article discusses the applicability to a living animal of the temperature mapping method using the water proton chemical shift obtained with three-dimensional magnetic resonance spectroscopic imaging (3D-MRSI). There are several sources of error in obtaining the spectra with 3D-MRSI: signal noise, limitation in the frequency resolution due to the finite signal length, intravoxel inhomogeneity in the static magnetic field, and variation in the magnetic field due to the eddy current magnetic field. A spectral estimation method called phase deduction complex Lorentzian fitting (PD-CLF) was proposed. Numerical simulations demonstrated that this method reduces the error in the chemical shift to one third of that obtained with the simple frequency subtraction method that uses zero-padded first Fourier transformation (FFT). The temperature images obtained using 3D-MRSI with PD-CLF clearly visualized the changes and distribution of temperature in an anesthetized rat.     

Least-squares chemical shift separation for (13)C metabolic imaging

PURPOSE: To describe a new least-squares chemical shift (LSCSI) method for separation of chemical species with widely spaced peaks in a sparse spectrum. The ability to account for species with multiple peaks is addressed. MATERIALS AND METHODS: This method is applied to imaging of (13)C-labeled pyruvate and its metabolites alanine, pyruvate, and lactate. The method relies on a priori knowledge of the resonant frequencies of the different chemical species, as well as the relative signal from the two pyruvate peaks, one of which lies near the alanine peak. With this information a least-squares method was utilized for separation of signal from the three metabolites, facilitating tremendous reductions in the amount of data required to decompose the different chemical species. Optimization of echo spacing for maximum noise performance of the signal separation is also described. RESULTS: Imaging an enriched (13)C phantom at 3.0T, the LSCSI method demonstrates excellent metabolite separation, very similar to echo planar spectroscopic imaging (EPSI), while only using 1/16th as much data. CONCLUSION: This approach may be advantageous for in vivo hyperpolarized (13)C metabolic applications for reduced scan time compared with EPSI.     

Novel rapid fat suppression strategy with spectrally selective pulses.

Short repetition time gradient echo sequences are gaining popularity in clinical applications such as dynamic contrast enhancement imaging, cardiac imaging, and MR angiography. Performing fat suppression in these sequences is usually time consuming and often somewhat ineffective, due to the relatively short T(1) and long T(2) of fat. A novel rapid fat suppression strategy using spectrally selective pulses is introduced and compared with clinically popular sequences such as fat presaturated fast field echo (FFE) and turbo field echo (TFE) and binomial water-selective spatial-spectral excitation (SSE, or SPSP excitation) FFE. The new strategy combines fat presaturation with low-order binomial water-selective SSE pulses in a TFE sequence. This enables the use of a long echo train length to decrease exam time, but without creation of excess fat signal contamination of the resultant images. The fat nullification is also more reliable as fat signals in central k-space data are suppressed twice. An implementation of this strategy is compared with traditional methods in both phantom and human studies, confirming that the new technique provides strong fat suppression with few artifacts despite the short scan duration.     

Chemical-shift artifact reduction in Hadamard-encoded MR spectroscopic imaging at high (3T and 7T) magnetic fields.

Proton MR spectroscopic imaging (MRSI) at higher magnetic fields (B(0)) suffers metabolite localization errors from different chemical-shift displacements (CSDs) if spatially-selective excitation is used. This phenomenon is exacerbated by the decreasing radiofrequency (RF) field strength, B(1), at higher B(0)s, precluding its suppression with stronger gradients. To address this, two new methods are proposed: 1) segmenting the volume-of-interest (VOI) into several slabs, allowing proportionally stronger slice-select gradients; and 2) sequentially cascading rather than superposing the components of the Hadamard selective pulses used for reasons of better point-spread function (PSF) to localize the few slices within each slab. This can reduce the peak B(1) to that of a single slice. Combining these approaches permits us to increase the selective gradient four- to eightfold per given B(1), to 12 or 18mT/m for 4- or 2-cm VOIs. This "brute force" approach reduces the CSD to under 0.05 cm/ppm at 7T, or less than half that at 3T.