Chemical Shift Selective Research Articles

Solid-State 13C NMR Imaging with Magic-Angle Spinning

By synchronizing a rotating magnetic-field gradient with the rotation of a sample undergoing magic-angle spinning (MAS), solid-state 13C NMR imaging has been carried out. Two-dimensional spatial-spatial 13C NMR images have been demonstrated in natural abundance on organic solids and a rigid polymer with and without chemical-shift selectivity. A 2D spatial-spectral solid-state 13C NMR image is also presented with MAS-enhanced spectral resolution in the chemical-shift dimension. It was found that the dependence of the 13C Signal intensity on 1H frequency offsets in 1H decoupling is one of the major technical problems that must be minimized in order to perform useful solid-state 13C NMR imaging.

Imaging Low-Concentration Metabolites in the Presence of a Large Background Signal

A pulse sequence (CD-CHESS) is described that addresses the challenge of imaging low-concentration metabolites whose chemical shifts are close to that of water. This sequence avoids the problems of the basic chemical- shift- selective (CHESS) imaging sequences, which produce an image contaminated by residual signal from the undesired component. Several signal suppression schemes are combined with the basic 2D CHESS imaging sequences and examined using a specially constructed chemical-shift phantom. The degree of suppression for the new enhanced CHESS sequences is compared with that of the basic CHESS sequences and the SUBMERGE-CHESS sequence. The most effective sequence incorporates cw irradiation and soft-pulse saturation with the basic CHESS sequence. The CDCHESS imaging sequence produces a suppression ratio of better than 1000 to 1 when the desired and undesired peaks are 1 ppm apart. The effectiveness of the CD-CHESS sequence is demonstrated by imaging the sugar protons in a phantom composed of 10% (w/v) sucrose in water. The CD-CHESS sequence is used for the selective imaging of sugars in peas, demonstrating that it can be extended to the broad lines normally encountered in biological specimens.

Silicone-fat differentiation in the breast: exploiting the bright-fat phenomenon in fast spin-echo MR imaging.

Selective suppression of silicone or fat with chemical shift-selective (CHESS) pulses is difficult because of the small chemical shift difference between the primary lipid signal and the primary silicone signal at 1.5 T. Differentiation of these chemically distinct species is, however, an important clinical task in assessing implant rupture and silicone migration in breast tissue. A method uniquely suited for silicone-fat differentiation with fast spin-echo (FSE) sequences is reported. It is based on the dependence of fat signal on echo spacing in FSE imaging and results show that it may provide a clinically robust method for silicone-fat differentiation in magnetic resonance imaging of the breast.

Use of static and dynamic NMR microscopy to investigate the origins of contrast in images of biological tissues

NMR imaging experiments have been carried out on a fruit ( Actinidia deliciosa) and plant stem ( Stachys sylvatica) using a wide range of image contrasts. These included T 1, T 2, T 2*, diffusion, flow and chemical shift selection. In the case of fruit imaging we calculated relaxation time and diffusion maps and established that the imaged parameters varied significantly with fruit ripening. These changes we attribute to changes in water dynamics resulting from elevated sugar concentrations. For the plant stem, water flow has been observed in the xylem vessels with a maximum velocity of 70 μm s −1. The role of image artifacts is considered and, in the case of transverse relaxation, we have demonstrated that it is necessary to use a precursor Carr-Purcell-Meiboom-Gill pulse train if additional diffusive attenuation is to be avoided.

Simultaneous preparation of inversion recovery T1 and chemical shift selective contrast using snapshot-FLASH-MRI

A new technique based on an inversion recovery (IR) snapshot FLASH MRI sequence is presented, which shows the possibility of combining T1 contrast and chemical-shift selectivity on the same images. The introduction of a CHESS preparation allows qualitative visualization of the T1 recovery of separate chemical species that may arise simultaneously within a given tissue. This kind of preparation also leads to a quantitative determination of T1 times of fat and water, even if both components are present in one image voxel. With this method, the inversion recovery behaviour of just one chemical compound can be visualized in a single experiment lasting a few seconds and quantitatively measured in a multishot experiment.

Dynamic nuclear polarization enhanced nuclear magnetic resonance of polymer-blend interfaces

Heterogeneous blends of [3,3′- 13C 2]polycarbonate and [uniform-ring- 12C 6]polystyrene were formed by serial film casting. The polystyrene phase of each blend was homogeneously doped with 2% by weight of a bis (diphenylene) phenylalyl free-radical complex with benzene. Proton polarization enhanced by dynamic nuclear polarization was generated in the polycarbonate phase by dipolar coupling to electrons in the polystyrene phase under 39 GHz microwave irradiation at the difference of the electron and proton Larmor frequencies. Proton magnetization was then transferred to carbons under matched spin-lock conditions for detection with chemical-shift selectivity by magic-angle spinning 13C nuclear magnetic resonance. The 13C signal from polycarbonate arises exclusively from chains which are at the polycarbonate-polystyrene interface. Signals from bulk polycarbonate were suppressed by differencing techniques. The dominant mechanism of polarization transfer from the electrons in the polystyrene phase to the protons in the polycarbonate phase is by direct polarization transfer. The interface signal arises from a 60 Å region which is 2% of the film thickness. As monitored by dynamic nuclear polarization selected dipolar rotational spin-echo 13C nuclear magnetic resonance, polycarbonate chains have less motion at the interface than in the bulk.

Chemical shift imaging in nuclear magnetic resonance: A comparison of methods

AbstractChemical shift imaging techniques are magnetic resonance imaging methods that separately map the spatial distribution of individual peaks or components in a complex NMR spectrum. A range of chemical shift imaging techniques based around the commonly employed spin‐echo Fourier imaging sequences is classified and described. The performance of the methods with regard to field dependence, chemical shift selectivity, signal‐to‐noise ratio, and imaging times is compared as a guide to selecting the most appropriate technique for a particular application.

Selective 19F MR imaging of 5-fluorouracil and α-fluoro-β-alanine

A 19F MR chemical shift imaging (CSI) technique is presented which enables selective imaging of the antineoplastic drug 5-fluorouracil (5-FU) and its major catabolite α-fluoro-β-alanine (FBAL). The CSI sequence employs a chemical shift selective (CHESS) saturation pulse to suppress either the 5-FU or the FBAL resonance before the other component of the two-line 19F MR spectrum is measured. Because the transmitter frequency can always be set to the Larmor frequency of the 19F resonance to be imaged, this approach yields 5-FU and FBAL MR images free of chemical shift artifacts in read-out and slice-selection direction. In phantom experiments, selective 5-FU and FBAL images with a spatial resolution of 15 × 15 × 20 mm 3 (4.5 ml) were obtained in 30 min from a model solution, whose drug and catabolite concentrations were similar to those estimated in the liver of tumor patients undergoing IV chemotherapy with 5-FU. The drug-specific MR imaging technique developed is, therefore, well-suited for the direct and noninvasive monitoring of the up-take and trapping of 5-FU in liver tumors in vivo.

Fat suppression with an improved selective presaturation pulse

Chemical shift selective (CHESS) imaging is an efficient and easily implemented technique for suppression of the fat component in clinical images. A Gaussian pulse is widely used as the selective presaturation pulse in the CHESS technique. However, a Gaussian pulse hardly performs well even if the magnetic field is very carefully shimmed. In clinical applications, the fat and water peaks are always inhomogeneously broadened. We have analyzed the performance of the Gaussian pulse in the presence of magnetic field inhomogeneities and present a more efficient selective presaturation pulse developed by applying the conjugate gradient method. The resulting pulse performs well without any requirement for magnetic field homogeneity greater than needed for routine diagnostic MR imaging.

Combined chemical-shift and phase-selective imaging for fat suppression: theory and initial clinical experience.

Incomplete fat suppression with a chemical-shift-selective (CHESS) or phase-selective (Dixon) technique is partially due to the olefinic fat component, which precesses at the same frequency as water. The authors developed a new method of fat suppression--the opposed-fat saturation (OP-ES) sequence--that combines both techniques to obtain superior fat saturation. Fat suppression was verified in phantom studies, which showed that the CHESS portion can eliminate most of the aliphatic fat signal except for a small residual component because of steady state effects and magnetic field imperfections. This residual component is cancelled by the olefinic fat with the phase-selective opposed portion of the sequence. Furthermore, this sequence was superior to another hybrid technique, chopper Dixon in combination with CHESS. When used in 10 healthy volunteers, the OP-FS sequence showed consistently better suppression of the subcutaneous and retroperitoneal fat compared with CHESS alone. Additional advantages for clinical abdominal imaging include its compatibility with respiratory compensation, use of a single excitation, and ease of implementation with gradient-echo imaging. Preliminary application in 10 patients illustrated other potential advantages, including clarification of fat-containing diseases and increasing conspicuity of some lesions.

Non-invasive histochemistry of plant materials by magnetic resonance microscopy

We have combined nuclear magnetic resonance (NMR) imaging on the microscopic scale with chemical shift selection to demonstrate the application of magnetic resonance imaging (MRI) to plant histochemistry. As an example of the method we have obtained separate images of the distribution of reserve oil and anethole in dried fennel mericarps. The technique can be employed to separately image the distribution of aromatics, carbohydrates, oils, water and possibly fatty acids in suitable plant materials.

Localized high‐resolution proton NMR spectroscopy using stimulated echoes: Initial applications to human brain in vivo

Water-suppressed localized proton NMR spectroscopy using stimulated echoes has been successfully applied to detect metabolites in the human brain in vivo. The STEAM spectroscopy sequence allows single-step localization by exciting three intersecting slices. Water suppression is achieved by preceding chemical-shift-selective (CHESS) rf pulses. High-resolution (0.05 ppm) proton NMR spectra of healthy volunteers have been High-resolution (0.05 ppm) proton NMR spectra of healthy volunteers have been obtained on a conventional 1.5-T whole-body MRI system (Siemens Magnetom). Volumes-of-interest (VOI) of 64 ml (4 x 4 x 4 cm3) were localized in the occipital area of the brain and spectra were recorded within measuring times ranging from 1 s (single scan) to about 10 min. The experimental procedure is described in detail. Resonance assignments include acetate, N-acetyl aspartate, gamma-amino butyrate, glutamine, glutamate, aspartate, creatine and phosphocreatine, choline-containing compounds, taurine, and inositols. Cerebral lactate was found to be at a maximum concentration of 0.5 mM when assuming N-acetyl aspartate in white matter to be 6 mM.

Cartilage disorders: comparison of spin-echo, CHESS, and FLASH sequence MR images.

Magnetic resonance (MR) imaging is known to be a suitable modality for the visualization of the hyaline cartilage and the fibrocartilage joint structures. To compare standard spin-echo (SE) images with water images obtained with the chemical shift selective (CHESS) sequence and with the fast low angle shot (FLASH) sequence, examinations were performed with all three sequences in eight volunteers and 28 patients with inflammatory degenerative and traumatic alterations of the knee, hip, and sacroiliac joints. Arthroscopic and/or surgical correlation were available in 16 patients; bone scanning and computed tomography of the sacroiliac joints were performed in four patients. CHESS-water and FLASH images proved superior to SE images in demonstrating hyaline cartilage disorders. There was no difference between SE, CHESS, and FLASH in the detection of fibrocartilage disorders. Short imaging times and satisfactory depiction of cartilage alterations make FLASH a promising method.

Gradient reversal technique and its applications to chemical-shift-related NMR imaging.

The chemical-shift effect often degrades slice selectivity thereby degrading the contrast and spatial resolutions in a high-field NMR imaging. We propose a new pulse sequence which can compensate for this chemical-shift effect during slice selection. This technique provides correct slice definition as well as clear separation between water and lipid protons in high-field proton NMR imaging. Particular applications of this technique are 2D imaging with a thin-slice selection and chunk 3D imaging, where correction of the chemical-shift effect in slice selection is important. Another interesting application of the technique is chemical-shift-selective (CHESS) imaging. This technique does not require spectral-selective rf pulses and it takes advantage of the chemical-shift effect during slice selection. The latter is found to be useful for human in vivo chemical-shift imaging in high-field NMR. In this paper, the theoretical derivation of the proposed gradient reversal technique, its applications, and related experimental results are presented.

Multiple chemical shift selective (CHESS) MR imaging using stimulated echoes.

Recently, stimulated echo acquisition mode (STEAM) magnetic resonance (MR) imaging has been demonstrated as a new tool for multiparametric MR imaging studies. Applications of the chemical shift selective (CHESS) STEAM technique using a 2.0-T whole-body MR imaging system are reported in which a series of contiguous cross-sectional images of the head and the pelvis were acquired. Because selective excitation of the desired component is employed rather than elimination of the unwanted component, this method yields an improved degree of spectral resolution which is dependent only on the homogeneity of the static magnetic field. For routine medical applications, no sophisticated adjustment of the CHESS pulse is needed, as reported in previous methods.

Multipurpose NMR imaging using stimulated echoes.

STEAM (stimulated-echo acquisition mode) imaging techniques recently introduced by the authors are demonstrated to provide a versatile tool for improving the parametric specificity in NMR imaging. Stimulated echoes can be excited by a sequence of at least three rf pulses with flip angles of 90 degrees or less. The main characteristics of the STEAM method are based on the great functional flexibility of an imaging sequence comprising three rf pulses unequal to 180 degrees and three intervals prior to acquisition of the data. Major advantages are the easy access to contiguous multiplanar images, to CHESS (chemical-shift-selective) images, and to T1 information. Moreover, the rf power deposition is considerably reduced as compared to spin-echo NMR imaging sequences. Here first in vivo results on human extremities are presented including contiguous multislice images, multiple CHESS images, and spin-lattice relaxation time images calculated from a series of simultaneously recorded T1-weighted STEAM images.

NMR imaging of spin-lattice relaxation using stimulated echoes

Abstract A new NMR imaging technique is presented which allows a detailed determination of the spin-lattice relaxation behavior. Both the spatial resolution and the measuring time are the same as for a conventional NMR image. The experiment yields a series of cross-sectional images with intensities declining according to the T1 relaxation curve. The technique is based on the principles of stimulated-echo acquisition-mode (STEAM) imaging which have been described in a preceding paper. For T1 imaging the final 90° rf pulse of the basic three-pulse STEAM imaging sequence is split into a variable number of small-angle rf pulses. Each of these pulses should read out the same amount of the longitudinal magnetization which has been prepared by the two leading 90° pulses. They give rise to a series of stimulated echoes with a T1 weighting which depends on the length of the interval between the second 90° pulse and the readout pulse. STEAM T1, imaging is characterized by an extremely low rf power deposition and, therefore, will be suitable for high-field NMR imaging. The method is easily extended to multislice measurements as well as to chemical-shift-selective (CHESS) T1 imaging. 1H NMR images have been obtained at 100 MHz using a 2.3 T magnet with a bore of 40 cm.

Multiple chemical-shift-selective NMR imaging using stimulated echoes

A new technique for chemical-shift-selective (CHESS) NMR imaging is described which enables cross-sectional imaging of positively selected, i.e., frequency-selective excited, resonance lines. The method relies on NMR sequences comprising three 90° rf pulses and detecting a so-called stimulated echo. Stimulated echo acquisition mode (STEAM) imaging has been described in a preceding paper. Here modified pulse and gradient sequences are reported that allow chemical-shift-selective imaging. Basically, image contributions belonging to a chemical component with a certain resonance frequency within an NMR spectrum can be selected by replacing one of the 90° pulses of a STEAM imaging sequence by a frequency-selective pulse. In particular, a “composite” image and an arbitrary number of CHESS images may be acquired simultaneously from the same slice. The extension of the combined CHESS STEAM experiment to multislice imaging is demonstrated.

Chemical shift selective MR imaging using a whole-body magnet.

We have applied a new method for separating water and fat resonances in proton magnetic resonance (MR) imaging to human studies using a whole-body MR imaging system at 2.0 T. Chemical shift selective (CHESS) MR imaging provides either a water or fat image in a single experimental run within the same time needed for a conventional composite image. Although the technique requires a spectral resolution of about 1 ppm over the entire imaging region, first images of the human head and hip indicated that CHESS MR imaging is extremely promising for use in clinical investigations. Moreover, CHESS MR imaging can be combined arbitrarily with other imaging modalities and is easy to implement in any high-field MR imaging system.

Stimulated echo imaging

A new form of NMR imaging is described using stimulated echoes. The technique, dubbed STEAM ( stimulated echo acquisition mode) imaging, turns out to become a versatile tool for multipurpose NMR imaging. Stimulated echoes can be excited by a sequence of at least three rf pulses, which in the basic experiment have flip angles of 90° or less. Thus no selective or nonselective 180° pulses are needed, which eliminates a variety of problems associated with such pulses in conventional spin-echo NMR imaging. Further advantages of STEAM imaging are concerned with the functional flexibility of an imaging sequence comprising three pulses and three intervals and the possibility of “storing” information prepared during the first interval into the form of longitudinal magnetization during the second interval. In general, the applied rf power is considerably reduced as compared to spin-echo-based imaging sequences. Here the general principles of the technique are outlined and first applications to multislice imaging of directly neighboring slices are demonstrated. Subsequent papers will be concerned with modifications of the basic STEAM sequence which, for example, allow multiple chemical-shift-selective (CHESS) imaging, complete imaging of the spin-lattice relaxation behavior, diffusion imaging, and single-shot real-time imaging.