Chrit T. W. Moonen

Gradient-enhanced heteronuclear correlation spectroscopy : theory and experimental aspects

Abstract A detailed theoretical and experimental treatment is given for gradient-enhanced heteronuclear correlation spectroscopy. Both multiple-quantum and single-quantum sequences are described. In addition to a comparison with conventional experiments using phase cycling, the effects of different gradient combinations are examined with respect to artifacts occurring in the heteronuclear dimension. The influence of gradient performance and diffusion on sensitivity is discussed. Approaches to attain phase-sensitive spectra are also analyzed.

Rapid recording of solvent-suppressed 2D COSY spectra with inherent quadrature detection using pulsed field gradients

Multidimensional NMR experiments have drawn much attention, particularly due to their ability to elucidate the structure of complex mo lecules. One of the ma jor drawbacks of many mu ltidimensional experiments is the long time required to record a dataset. In many cases, the m inimal experimental time is not lim ited by sensitivity, but rather by the number of repetitions needed to complete a phase-cycl ing protocol. Phase cycling may be necessary to select the desired coherence pathway ( 2 ) , to obtain absorptive l ineshapes (2-4, and to eliminate undesired signals such as axial peaks or quadrature artifacts (5). In order to reduce experimental time, efforts have been undertaken to design reduced phase-cycl ing schemes (6, 7). Spectral artifacts may also appear if the spin system is not in a constant state at the beginning of each pulse sequence, for example, in the case of a fast repetition rate. This problem may be solved by the use of a saturation-recovery sequence (8). Coherence pathways can also be selected in a single scan with the application of field gradients (9-11). Sotak et al. ( 12) have applied this principle to obtain in vivo 2D double-quantum spectra, and Hurd recently reported gradient-enhanced doublequantum-fi ltered COSY and gradient-enhanced TOCSY ( 13). In this contribution, we discuss how standard COSY spectra can be recorded without phase cycling if pulsed field gradients are available. Furthermore, it will be shown that quadrature detection in thef; doma in is attained inherently. F inally, solvent suppression is accomplished by using an alternative pulse sequence, and an improved deconvolut ion method for postprocessing. A similar approach, applied to in vivo 2D spectroscopy, has independently been reported by Z iegler et al. ( 14). F igure 1 a shows the two-pulse sequence for the basic COSY experiment (IS). Using the product-operator formalism to describe the evolution of a weakly coupled twospin system, the observable terms of the density operator after the second 90” pulse at the beginning of the detection period t2 are (16)

Brain regional distribution pattern of metabolite signal intensities in young adults by proton magnetic resonance spectroscopic imaging

Proton magnetic resonance spectroscopy (1H-MRS) is evolving from single-volume localized acquisitions to multiple-volume acquisitions using magnetic resonance spectroscopic imaging (lH-MRSI). The normal regional patterns of 1H-MRSI-detectable metabolite signal intensities have yet to be established. We studied 13 healthy young adults with a multiple-section lH-MRSI technique. The metabolite signals measured were N-acetylaspartate (NA), cho-line-containing compounds (CHO), creatine-phosphocreatine (CRE), and lactate. Ten neuroanatomic regions (nine bilateral) were identified in gray matter, white matter, and basal nuclei. Analysis of the data led to the following conclusions: (1) NA and CHO signals from centrum semiovale (CSO) can be used as a normalizing factor to reduce intersub-ject variability due to external causes; (2) in normal human brain, there is no left versus right asymmetry in the regions studied; (3) statistically significant patterns of signal distribution of NA, CHO, and CRE can be identified in normal human brain; and (4) CSO-normalized metabolite signal intensities and metabolite ratios complement each other for the detection of significant regional differences.

Measuring random microscopic motion of water in tissues with MR imaging: a cat brain study.

Cat brain images sensitized to incoherent motion by additional gradient pulses were obtained on a 4.7 T magnetic resonance unit equipped with shielded gradient coils. The apparent diffusion coefficient of water in gray and white matter was accurately determined and imaged from the signal attenuation curve obtained as a function of gradient strength. Contrast in calculated diffusion images differed from typical T2-weighted contrast. Furthermore, in gray matter and in areas containing flowing CSF the attenuation curve was found to be biexponential. These results are interpreted in terms of a simple voxel model with microcirculation and diffusion contributions.

Proton spectroscopic imaging of human brain

Abstract Signals from water and fat can cause artifacts in proton spectroscopic imaging in the human brain. The major problem is variation of the B0 field over a range of several ppm within the sensitive volume of the standard whole-head coil. Here, the coherence-pathway formalism is used to describe and evaluate the origin of artifacts in a double spin-echo (PRESS) sequence. The attenuation of unwanted coherences using pulsed field gradients is described for homogeneous and inhomogeneous B0 fields. The effect of the following parameters on the quality of the spectroscopic images is analyzed: (a) directional order of plane selection, (b) positioning of phase-encode gradients in the sequence, (c) postprocessing spatial windowing, and (d) motion. It is shown that, for a typical echo time of 272 ms, it is not necessary to first select a region of interest within the brain borders when sufficient phase-encode steps are used. Examples of 2D proton spectroscopic images with a nominal voxel volume of 0.85 ml are given for a healthy volunteer and a patient with a low-grade glioma.

Complete water suppression for solutions of large molecules based on diffusional differences between solute and solvent (DRYCLEAN)

Abstract A method is outlined for suppressing water in proton spectra using differences in its diffusion properties with the solutes. Solute protons at the water resonance frequency can be studied, the spectrum is not phase-distorted, and success of the technique does not depend on B1 homogeneity or water linewidth. No phase cycling is necessary and suppression is easier for larger solutes.

Homonuclear J refocusing in echo spectroscopy

Abstract A simple pulse scheme is given for obtaining echoes with both chemical shift and homonuclear scalar coupling refocused. The procedure is illustrated for some biologically important nuclei, namely 31 P in ATP and 1 H in lactate. The refocusing of the scalar coupling J is perfect for AX systems, but incomplete for multiply coupled spins. However, for the latter, a gain in in-phase intensity is still obtained at relatively long echo times ( J ). Theory, in terms of the operator formalism, and experiment are presented for AX n systems and are in good agreement. Strategies to remove spectral artifacts resulting from pulse angle deviations are given.

Functional brain MR imaging based on bolus tracking with a fast T2∗-sensitized gradient-echo method

Dynamic physiological scanning, based on temporary changes in local field homogeneity during the passage of a contrast agent bolus, has been performed hitherto with echo-planar imaging (EPI) or conventional gradient-recalled techniques (FLASH). Here, it is shown that the T2* sensitivity of conventional FLASH techniques can be improved drastically on a conventional whole body instrument by delaying the gradient-echo until the subsequent TR-period without increasing total imaging time. Examples are given for a full k-space matrix (128 x 256) obtained within 2 s with a TE of 25 ms, resulting in images free of artifacts. The method is applied to bolus tracking through the brain of healthy volunteers during visual stimulation and in the dark. An average increase of regional cerebral blood volume (rCBV) in the visual cortex of 10.9% (n = 9, p = .001) was found.

Diffusion Spectroscopy in Living Systems

NMR spectroscopy offers the opportunity for molecular diffusion to be noninvasively studied in situ in living animals. Diffusion imaging studies of water have provided insight in the nature of membrane permeability and of changes in water properties on induction of insults, e.g., ischemia. Diffusion spectroscopy offers the possibility of, in addition to water, studying mobility of metabolites. In contrast to water, many metabolites are restricted to the intracellular space and metabolite diffusion spectroscopy can therefore provide unique information about cell size and intracellular transport and exchange processes. Early work in the field has demonstrated these restriction effects in cells and in excised tissue, but it was not until recently that metabolite diffusion studies were reported in animals and humans. In this limited chapter we review the in vivo data that we believe are the most accurate available at present and compare them to in vitro and ex vivo studies. A critical discussion of experimental problems and of the possibility of separating different compartments in situ is also provided. Although the initial in vivo results are promising the data show that, at present, comparison between the results of different research groups is difficult due to differences in experimental parameters such as gradient direction, animal orientation, selected tissue regions, and diffusion time. Possible strategies necessary to increase the accuracy and compatibility of future studies are given.

3d bolus tracking with frequency-shifted burst mri

Objective Our goal was to develop and test a 3D bolus-tracking MR technique for perfusion imaging of normal and pathological (infarcted) human brain. Materials and Methods All experiments were performed on standard 1.5 T GE/Signa clinical scanners. Five normal volunteers and one patient with a subacute brain infarct were studied. Modified [frequency-shifted (FS)] BURST MRI was performed during injection of a bolus of Gd-DTPA (0.13 mmol/kg) in the antecubital vein. The 3D datasets were acquired with a time resolution of 2.2 s and an effective spatial resolution of 4.3 ± 4.3 ± 6.4 mm. Three-dimensional maps of blood volume and bolus arrival time were determined by fitting a synthetic curve to the intensity time course on a voxel-by-voxel basis. Results Both relative cerebral blood volume and arrival time maps demonstrated sensitivity to regional differences in blood supply in both normal brain and in the subacute brain infarction. The transit time maps showed arrival time delays of 5–7 s within and around the infarct and confirmed the diagnosis of left middle cerebral artery occlusion. Conclusion The results of the measurements on both normal and diseased human brain demonstrated the ability to acquire valuable 3D information about brain perfusion using FS BURST MRI.