1D spectroscopic imaging with rf echo planar (SIRFEN) methods

Fast spectroscopic imaging strategies for potential applications in fMRI

Technical aspects of two general fast spectroscopic imaging (SI) strategies, one based on gradient echo trains and the other on spin echo trains, are reviewed within the context of potential applications in the field of functional magnetic resonance imaging (fMRI). Fast spectroscopic imaging of water may prove useful for identifying mechanisms underlying the blood oxygenation level dependence (BOLD) of the water signal during brain activation studies. Reasonably rapid mapping of changes in proton signals from brain metabolites, like lactate, creatine or even neurotransmitter associated metabolites like GABA, is substantially more challenging but technically feasible particularly as higher field strengths become available. Fast spectroscopic methods directed towards the 31P signals from phosphocreatine (PCr) and adenosine tri-phosphates (ATP) are also technically feasible and may prove useful for studying cerebral energetics within fMRI contexts.

Fast imaging of phosphocreatine in the normal human myocardium using a three-dimensional RARE pulse sequence at 4 Tesla

PURPOSE

To investigate the use of a three-dimensional rapid acquisition with relaxation enhancement (RARE) pulse sequence for direct acquisition of phosphocreatine (PCr) images of the human myocardium.

MATERIALS AND METHODS

A short elliptical birdcage radiofrequency (RF) body coil was constructed to produce a uniform flip angle throughout the chest cavity. In vivo images using a spectrally-selective RARE sequence with a spatial resolution of 1.2 cm x 1.2 cm x 2.5 cm (4 cm(3)) were acquired in nine minutes and 40 seconds.

RESULTS

Scans of phantoms demonstrated excellent spectral selectivity. The signal-to-noise ratio in the myocardium ranged from 12.6 in the anterior wall to 5.3 in the mid septum.

CONCLUSION

This study demonstrates that PCr data can be acquired using a three-dimensional RARE sequence with greater spatial and temporal resolution than spectroscopic techniques.

Multiple-echo proton spectroscopic imaging using time domain parametric spectral analysis

A multiple-echo MR spectroscopic imaging (MRSI) method is presented that enables improved metabolite imaging in the presence of local field inhomogeneities and measurement of transverse relaxation parameters. Short echo spacing is used to maximize signal energy from inhomogeneously line-broadened resonances, and time domain parametric spectral analysis of the entire echo train is used to obtain sufficient spectral resolution from the shortened sampling periods. Optimal sequence parameters for 1H MRSI are determined by computer simulation, and performance is compared with conventional single-echo acquisition using phantom studies at a field strength of 4.7 T. A preliminary example for use at 1.5 T is also presented using phantom and human brain MRSI studies. This technique is shown to offer improved performance relative to single-echo MRSI for imaging of metabolites with shortened T2* values due to the presence of local field inhomogeneities. Additional advantages are the intrinsic measurement of metabolite T2 values and determination of metabolite integrals without T2 weighting, thereby facilitating quantitative metabolite imaging.

Temperature monitoring of ultrasonically heated muscle with RARE chemical shift imaging

The ability to monitor tissue temperature in ultrasonically heated rabbit muscle is demonstrated using a chemical shift imaging approach based on the rapid acquisition with relaxation enhancement (RARE) fast imaging method [Hennig et al., Magn. Reson. Med. 3, 823-833 (1986)] applied in a line scan format. A three echo sequence with a 16 Hz spectral resolution with 64 ms echo readouts and 78 ms echo spacings is shown capable of measuring relevantly small water frequency shifts in phantoms. Applied to the in vivo model of ultrasonically heated rabbit muscle, water resonance frequencies at the ultrasonic focal point were found to be linearly related to temperature with a slope of -0.007 +/- 0.001 ppm/degree C (N = 6 studies). Measurements of the frequency shift in unheated tissue located away from the ultrasonically heated tissue varied by approximately 0.011 ppm over the course of the experiments, leading to an estimated temperature accuracy of +/- 1.6 degrees C in vivo.

In vivo bone marrow lipid characterization with line scan Carr-Purcell-Meiboom-Gill proton spectroscopic imaging

Line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging sequences have been used to extract lipid chemical composition indices in healthy adult bone marrow in the knee at 1.5 T. Since several spectroscopic echo readouts follow each excitation, the information acquired reflects a balance between spectral T2 decay processes and spectral resolution. To examine this balance in detail, data sets with two different echo spacings and spectral resolutions have been acquired to compare the information available from each in studies of bone marrow. Oils for which high field (7 T) proton spectra were recorded were used to evaluate the accuracy of lipid chemical composition indices extracted from the line scan Carr-Purcell-Meiboom-Gill spectroscopic imaging methods at 1.5 T. The extension of the method to fast spectroscopic imaging of bone marrow with multiple echoes is demonstrated.

Multiecho approaches to spectroscopic imaging of the brain

Spectroscopic imaging (SI) with nuclear magnetic resonance (NMR) is one of the most powerful tools available for studying brain chemistry in vivo. Both proton (1H) and phosphorus (31P) NMR offer valuable biochemical information that can in principle be mapped throughout the entire brain, thereby enhancing our understanding of brain function. With the exception of protons from tissue water and the triglycerides of adipose tissue, however, nuclei contributing to the NMR signals of living tissue are in relatively small (millimolar) concentrations. The low concentration of metabolite nuclei reduces the overall sensitivity of conventional SI techniques, making high-quality metabolite mapping a lengthy procedure. This problem has led to the development and testing of nonconventional methods for reducing SI scan times, including techniques based on the collection of multiple spin-echoes. The extent to which multiecho methods can be used to decrease SI scan times and maintain high-quality metabolite mapping depends on several factors. These include the spectral transverse relaxation times, the spectral resolution required, and J-coupling interactions. We have discussed these various technical aspects of multiecho SI methods as applied to 1H and 31P spectroscopic imaging of the living brain.

Multi-echo 31P spectroscopic imaging of ATP: a scan time reduction strategy

Spectroscopic imaging of 31P metabolites and adenosine triphosphate (ATP) in particular with multiple spin echoes may prove useful for reducing data acquisition times. The usual T2 decay processes that degrade multi-echo spectroscopic imaging methods, however, are further compounded by J-coupling modulations in the case of ATP. We determine how these modulations affect multi-echo spectroscopic imaging k-space data and produce systematic spatial misregistrations of the ATP resonances. The specific J-coupling modulations of ATP are determined to identify echo-spacing effects in multi-echo spectroscopic imaging of ATP and to determine appropriate post-processing correction schemes to address the spatial misregistration problem. An in vivo demonstration of the technique that offers a threefold reduction in scan time compared to conventional SI methods is provided and compared with the conventional SI approach.

Line scan imaging of brain metabolites with CPMG sequences at 1.5 tesla

A line scan Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic imaging sequence has been implemented on a standard 1.5 T clinical scanner to map metabolite signals at multiple echo times from voxels along selected tissue columns through the brain. The CPMG multiecho spectroscopic image data sets are used to estimate brain metabolite T2 decay parameters in a group of healthy volunteers and in one tumor patient. Inherent trade-offs between T2 decay, spectral resolution, and echo spacing prove to be important limiting factors. In particular, separate quantitation of choline and creatine resonances at 1.5 T was not achieved in the present implementation. However, the ability to collect data sets suitable for T2 decay analyses of combined choline and creatine resonances and N-acetyl aspartate resonances in under 10 minutes may prove of clinical utility in the study of brain pathology.

Temporomandibular joint disc replacement made by tissue-engineered growth of cartilage

OBJECTIVE

To test the effectiveness of the new technique of tissue-engineered growth of cartilage, temporomandibular joint (TMJ) disc replacements were created by seeding dissociated chondrocytes on synthetic, three-dimensional, bioresorbable polymer constructs of a predetermined anatomic shape, incubating the cell-polymer constructs in vitro, and transplanting them into test animals.

MATERIALS AND METHODS

Twelve highly porous and bioresorbable cell-transplantation devices in the shape of TMJ discs were created using biodegradable polylactid and polyglycolic acid fibers. Bovine articular cartilage was dissociated into chondrocytes and the cells were allowed to attach to the three-dimensional polymer scaffolds and multiply in vitro. After 1 week, the cell-polymer constructs were implanted subcutaneously into nude mice. The neocartilage was assessed by magnetic resonance imaging (MRI) techniques, gross inspection, histology, and biomechanical and biochemical analysis after 12 weeks.

RESULTS

All implants seeded with chondrocytes showed gross evidence of histologically organized hyaline cartilage. The scaffolds maintained their specific shape. They not only showed appropriate intrinsic stability during neomorphogenesis of cartilage in vitro and in vivo, but also seemed to guide the growth of cartilage. The presence of sulfated glycosaminoglycans was shown by aldehyde fuchsin alcian blue staining of the specimens. Type II collagen, considered to be indicative of cartilage formation, was found in the specimens tested. MRI showed signal characteristics similar to those of hyaline cartilage. Analysis of neocartilage force/displacement curves and aqueous phase compliance using a closed compression chamber suggested that the ability of the constructs to resist deformation was similar to that of native donor cartilage.

CONCLUSION

The technology of tissue-engineered growth of cartilage on individually designed scaffolds may have many applications not only in reconstructive surgery of the TMJ, but also in craniomaxillofacial, plastic, and orthopedic surgery.

Band-selective spin echoes for in vivo localized 1H NMR spectroscopy

This study describes a new single spin-echo spatial localization sequence, BASSALE or BAnd-Selective Spin echo Acquisition for Localized Editing, that overcomes a number of the limitations of the STEAM and PRESS volume selection pulse sequences. It achieves conformal volume localization in a single shot by spatially tailored suppression of all magnetization outside a 2D region of interest followed by selection of a single orthogonal slice. This separation of spatial localization from the echo formation process has permitted use of a spectrally selective cosine-modulated sinc refocusing pulse to acquire localized 1H spectra with the water suppression efficiency of STEAM and the sensitivity of PRESS. Echoes formed by such spectrally selective pulses have been termed band-selective spin echoes. The BASSALE sequence attains shorter echo times than PRESS, inhibits scalar spin-spin interactions to permit localized editing and T2 relaxometry of metabolites with J-coupled spins (e.g., lactate), is insensitive to homonuclear multiple-quantum and polarization transfer effects, and can be made sensitive or insensitive to spin displacement effects. Applications are shown both with phantoms and in situ in the rat brain.

Bone marrow characterization in the lumbar spine with inner volume spectroscopic CPMG imaging studies

The authors describe new magnetic resonance (MR) spectroscopic and postprocessing methods for characterizing major proton peaks and their spectral T2 values in many small voxels throughout extensive regions of bone marrow within the adult lumbar spine. The techniques are based on spectroscopic interrogation of 128 voxels along columns oriented through the spine at eight TE values. Mean fat content measurements, based on quantification of the proton peaks of water and saturated fat (-CH2-)n, corrected for T2 decay, ranged from 40% to 60%. The mean T2 value of the lipid peak, 113 msec +/- 21, was significantly longer (P < .001) than that of water (71 msec +/- 14). The techniques combine features of MR spectroscopy and imaging most suited for spatially efficient coverage of bone marrow at spectral resolutions sufficient for intra-voxel fat or water content measurements. The methods introduced provide a practical, quantitative means for characterizing vertebral marrow in diseases affecting marrow cellularity.

Rapid fat/water assessment in knee bone marrow with inner-volume RARE spectroscopic imaging

An inner-volume modification of a spectroscopic imaging technique based on RARE (rapid acquisition with relaxation enhancement) sequences is shown to provide, in less than 1 minute, spectra suitable for fat/water peak-area integration from selected imaging columns containing more than 1,000 5 x 5 x 1-mm voxels. A limitation is the unavoidable T2 weighting of the spectral peaks, which influences estimations of true fat/water composition from the spectra. Studies obtained in the knees of healthy volunteers demonstrate the ability of the technique to enable characterization of the two major proton peaks in bone marrow. Data collected at two or more effective TEs are suitable for extrapolation of fat/water estimates to zero TE.