Frame-by-frame PRESS 1H-MRS of the brain at 3 T: the effects of physiological motion

Basic Principles and Clinical Applications of Magnetic Resonance Spectroscopy in Neuroradiology

Magnetic resonance spectroscopy is a powerful tool to assist daily clinical diagnostics. This review is intended to give an overview on basic principles of the technology, discuss some of its technical aspects, and present typical applications in daily clinical routine in neuroradiology.

In vivo proton T1 relaxation times of mouse myocardial metabolites at 9.4 T

PURPOSE

Proton magnetic resonance spectroscopy ((1) H-MRS) for quantitative in vivo assessment of mouse myocardial metabolism requires accurate acquisition timing to minimize motion artifacts and corrections for T1 -dependent partial saturation effects. In this study, mouse myocardial water and metabolite T1 relaxation time constants were quantified.

METHODS

Cardiac-triggered and respiratory-gated PRESS-localized (1) H-MRS was employed at 9.4 T to acquire signal from a 4-µL voxel in the septum of healthy mice (n = 10) while maintaining a steady state of magnetization using dummy scans during respiratory gates. Signal stability was assessed via standard deviations (SD) of zero-order phases and amplitudes of water spectra. Saturation-recovery experiments were performed to determine T1 values.

RESULTS

Phase SD did not vary for different repetition times (TR), and was 13.1° ± 4.5°. Maximal amplitude SD was 14.2% ± 5.1% at TR = 500 ms. Myocardial T1 values (mean ± SD) were quantified for water (1.71 ± 0.25 s), taurine (2.18 ± 0.62 s), trimethylamine from choline-containing compounds and carnitine (1.67 ± 0.25 s), creatine-methyl (1.34 ± 0.19 s), triglyceride-methylene (0.60 ± 0.15 s), and triglyceride-methyl (0.90 ± 0.17 s) protons.

CONCLUSION

This work provides in vivo quantifications of proton T1 values for mouse myocardial water and metabolites at 9.4 T.

Magnetic resonance spectroscopy of the breast: current status

In vivo magnetic resonance spectroscopy (MRS) of the breast can be used to measure the level of choline-containing compounds, which is a biomarker of malignancy. In the diagnostic setting, MRS can provide high specificity for distinguishing benign from malignant lesions. MRS also can be used as an early response indicator in patients undergoing neoadjuvant chemotherapy. This article describes the acquisition and analysis methods used for measuring total choline levels in the breast using MRS, reviews the findings from clinical studies of diagnosis and treatment response, and discusses problems, limitations, and future developments for this promising clinical technology.

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.

Metabolite quantification and high-field MRS in breast cancer

In vivo 1H MRS is rapidly developing as a clinical tool for diagnosing and characterizing breast cancers. Many in vivo and in vitro experiments have demonstrated that alterations in concentrations of choline-containing metabolites are associated with malignant transformation. In recent years, considerable efforts have been made to evaluate the role of 1H MRS measurements of total choline-containing compounds in the management of patients with breast cancer. Current technological developments, including the use of high-field MR scanners and quantitative spectroscopic analysis methods, promise to increase the sensitivity and accuracy of breast MRS. This article reviews the literature describing in vivo MRS in breast cancer, with an emphasis on the development of high-field MR scanning and quantitative methods. Potential applications of these technologies for diagnosing suspicious lesions and monitoring response to chemotherapy are discussed.

Spatiotemporal magnetic field monitoring for MR

MR experiments frequently rely on signal encoding by the application of magnetic fields that vary in both space and time. The accurate interpretation of the resulting signals often requires knowledge of the exact spatiotemporal field evolution during the experiment. To better fulfill this need, a new approach is presented that enables measuring the field evolution concurrently with any MR sequence. Miniature NMR probes are used to monitor the MR phase evolution around the object under investigation. Based on these data, a global phase model is calculated that can then be used as a basis for processing the actual image or spectroscopic data. The new method is demonstrated by MRI of a phantom, using spin-warp, spiral, and EPI trajectories. Throughout, the monitoring results enabled highly accurate image reconstruction, even in the presence of massive gradient imperfections.

Proton MR spectroscopic imaging of the medulla and cervical spinal cord

PURPOSE

To demonstrate the feasibility of quantitative, one-dimensional proton MR spectroscopic imaging (1D-MRSI) of the upper cervical spine and medulla at 3.0 Tesla.

MATERIALS AND METHODS

A method was developed for 1D-point-resolved spectroscopy sequence (PRESS)-MRSI, exciting signal in five voxels extending from the pontomedullary junction to the level of the C3 vertebra, and performed in 10 healthy volunteers to generate control data.

RESULTS

High-resolution 1D-MRSI data were obtained from all 10 subjects. Upper cervical spine concentrations of choline, creatine, and N-acetyl aspartate were estimated to be 2.8 +/- 0.5, 8.8 +/- 1.8, and 10.9 +/- 2.7 mM, respectively, while in the medulla they were 2.6 +/- 0.5, 9.1 +/- 1.7, and 10.8 +/- 0.9 mM.

CONCLUSION

Quantitative 1D-MRSI of the upper cervical spine has been shown to be feasible at 3.0 Tesla.

Proton MR spectroscopy of the brain at 3 T: an update

Proton magnetic resonance spectroscopy ((1)H-MRS) provides specific metabolic information not otherwise observable by any other imaging method. (1)H-MRS of the brain at 3 T is a new tool in the modern neuroradiological armamentarium whose main advantages, with respect to the well-established and technologically advanced 1.5-T (1)H-MRS, include a higher signal-to-noise ratio, with a consequent increase in spatial and temporal resolutions, and better spectral resolution. These advantages allow the acquisition of higher quality and more easily quantifiable spectra in smaller voxels and/or in shorter times, and increase the sensitivity in metabolite detection. However, these advantages may be hampered by intrinsic field-dependent technical issues, such as decreased T(2) signal, chemical shift dispersion errors, J-modulation anomalies, increased magnetic susceptibility, eddy current artifacts, challenges in designing and obtaining appropriate radiofrequency coils, magnetic field instability and safety hazards. All these limitations have been tackled by manufacturers and researchers and have received one or more solutions. Furthermore, advanced (1)H-MRS techniques, such as specific spectral editing, fast (1)H-MRS imaging and diffusion tensor (1)H-MRS imaging, have been successfully implemented at 3 T. However, easier and more robust implementations of these techniques are still needed before they can become more widely used and undertake most of the clinical and research (1)H-MRS applications.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Detection and correction of frequency instabilities for volumetric 1H echo-planar spectroscopic imaging

Spectral quality in 1H magnetic resonance spectroscopic imaging (MRSI) critically depends on the stability of the main magnetic field. For echo-planar MRSI implemented at 3 T, temperature variation in the passive steel shims of the magnet system can lead to a significant drift in the resonance frequency. A method is presented that incorporates interleaved measurement of the instantaneous resonance frequency of a reference water signal into a volumetric MRSI sequence and allows correction for the drift during postprocessing. Results from normal human brain at 3 T indicate that the correction largely removes lineshape distortions, recovers metabolite signal loss, and improves spectral quality by reducing the width of spectral lines; however, particularly in inferior regions, other sources of distortion may be present that cause broadening of spectral lines.

Prospective correction of affine motion for arbitrary MR sequences on a clinical scanner

The concept of prospective 3D affine motion correction was generalized, based on the Bloch equations, for signal excitation and sampling using arbitrary MR sequences. The technique was implemented on a clinical MRI scanner for Cartesian, radial, and spiral imaging sequences, as well as for 2D spatially selective RF excitation pulses. A patient-specific motion model steered by real-time navigators was employed to account for the additional degrees of freedom provided by the affine motion model. Different navigator concepts (multiple spatial and temporal navigators, quadratic navigators and other motion sensors) were investigated, with the aim of improving the correlation between navigator information and the motion model. Experiments on moving phantoms are presented to prove the technical feasibility of the approach. In vivo experiments on coronary MRA and renal MRI show the potential of the method for cardiac and abdominal applications hampered by respiratory motion.

Measurement and correction of respiration-inducedB0 variations in breast1H MRS at 4 Tesla

Respiratory motion is well known to cause artifacts in magnetic resonance spectroscopy (MRS). In MRS of the breast, the dominant artifact is not due to motion of the breast itself, but rather it is produced by B0 field distortions associated with respiratory motion of tissues in the chest and abdomen. This susceptibility artifact has been reported to occur in the brain, but it is more apparent in the breast due to the anatomic proximity of the lungs. In the breast, these B0 distortions cause shot-to-shot frequency shifts, which vary an average of 24 Hz during a typical 1H MRS scan at 4 T. This variation can be corrected retrospectively by frequency shifting individual spectra prior to averaging. If not corrected, these shifts reduce spectral resolution and increase peak fitting errors. This work demonstrates the artifact, describes a method for correcting it, and evaluates its impact on quantitative spectroscopy. When the artifact is not corrected, quantification errors increase by an average of 28%, which dramatically impacts the ability to measure metabolite resonances at low signal-to-noise ratios.