STEAM Sequence Research Articles

Quality assessment in in vivo NMR spectroscopy: IV. A multicentre trial of test objects and protocols for performance assessment in clinical NMR spectroscopy

A multicentre trial of test objects and protocols for performance assessment in single volume and slice selective magnetic resonance spectroscopy (MRS) was conducted by the European Community Concerted Action on MRI and MRS. The trial assessed phosphorus and proton localisation techniques implemented on commercially available MR systems at ten sites in Europe. At each site, a number of parameters devised by the Concerted Action were measured using prototype test objects. Some of these parameters related to the quality of localisation and others to the overall performance of the spectrometer. Results were obtained for the ISIS, DRESS, STEAM, and PRESS sequences with a range of acquisition parameters, allowing evaluation of the assessment methodology and comparison of the efficacy of various implementations of these localisation techniques. The results of this trial have been important in the development of the Concerted Action's final recommendations for MRS performance assessment, and demonstrate that such assessment provides valuable information in the comparison of spectroscopy data from different sites and in the development of new localisation sequences, and provides a means of quality assurance in MRS.

A gradient scheme suitable for localized shimming and in vivo 1H/31P STEAM and ISIS NMR spectroscopy.

A gradient scheme is presented which may be used for STEAM or ISIS localization. One application of the scheme is to perform single-shot STEAM shimming prior to data acquisition with STEAM and ISIS, using identical gradient amplitudes and durations. Using conventional STEAM to shim for ISIS can produce line-shape distortions induced by different eddy currents in the two sequences; with this gradient scheme the problem is minimized. Line-shape improvements of STEAM and ISIS localized data obtained after volume localized shimming with the proposed STEAM sequence are demonstrated. The localization performance of the STEAM and ISIS sequences are demonstrated on phantoms and in vivo for 1H and 31P metabolites.

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.

Localized proton MR spectroscopy of the human kidney in vivo by means of short echo time STEAM sequences

In order to obtain proton magnetic resonance spectra from the normal human kidney in vivo, we employed a STEAM sequence with delay times TE = 10 ms and TM = 30 ms. Signals are attenuated during STEAM sequences by J-coupling effects and by macroscopic movement of the sample. The combination of short echo times and respiratory triggering ensured that the kidney was stationary during the pulse sequence, and allowed us to detect strongly coupled resonances between 3 and 4.2 ppm. Analysis of spectra of extracts of bovine kidneys suggested that the renal MR-visible metabolites could include the osmolytes betaine, myo-inositol, and glycerophosphocholine. Four volunteers were subjected to overnight dehydration followed by rehydration, and we found that these signals increased significantly after dehydration, and decreased significantly 4 h after rehydration, thus supporting the assignment of the resonances as osmotically active metabolites.

Phosphorous and Proton Spectroscopy in Relation to near Incarceration and Incarceration of the Human Brain

We report 3 cases of 31P and 1H MR spectroscopy (MRS) performed at different stages on patients with clinical signs of near or fulminant incarceration of the brain. The measurements were made on a whole body, 1.5 T scanner. 1H-MRS was obtained with the STEAM sequence and 31P-spectra were obtained using the chemical shift imaging technique. Medical treatment including controlled ventilation and sedation of the patients was carried out during the examination. The first patient was evaluated on days 6 and 10 after evacuation of an acute subdural haematoma. An intracranial pressure of 35 mm Hg was registered during the first examination. The 2nd patient had suffered a spontaneous intracerebral haemorrhage and showed clinical signs of imminent incarceration during the examination. The 3rd patient showed clinical signs of incarceration just prior to the examination. In the 1st patient 1H-MRS showed a 3-fold increase in the concentration of choline-containing compounds and a small decrease in N-acetyl aspartate from the 1st to the 2nd examination, which we interpret as a loss of neurones. In case 2 only small changes in metabolism could be detected, indicating that, despite signs of imminent clinical incarceration, the energy supply to the brain was substantial. 1H-MRS of the 3rd patient showed massive lactate concentration, and 31P-MRS revealed the total absence of high-energy phosphorous compounds leaving only one single peak of inorganic phosphate, indicating irreversible brain death.

Scalar coupling and zero-quantum coherence relaxation in STEAM: implications for spectral editing of lactate.

Accurate values were obtained for the lactate zero-quantum coherence frequency, omega ZQ identical to omega 1-omega s = CH3 - CH chemical shift difference, and scalar coupling constant, J, by using the methyl signal's amplitude modulation during the TM period of a STEAM sequence, 90 degrees - TE/2 - 90 degrees - TM - 90 degrees - TE/2 - Acquire. Although most previous work has used J = 7.35 Hz, or 1/J = 136 ms, the actual value is J = 6.93 +/- 0.05 Hz or 1/J = 144.3 +/- 1 ms. In addition, the CH3 - CH chemical shift difference = 2.7956 +/- 0.0005 ppm, and the relaxation time for zero-quantum coherence, TZQ, was much shorter than either T2 or T1 for the methyl resonance. A small component of the signal with TE = 144 ms, which was not modulated at the zero-quantum coherence frequency or by scalar coupling, was assigned to longitudinal two-spin order magnetization (IzSz) created by imperfect radio frequency pulse profiles. This information will allow improved editing of the lactate signal and more accurate quantitation of lactate concentrations.

Proton spectroscopy of human stroke: Assessment of transverse relaxation times and partial volume effects in single volume STEAM MRS

Proton T 2 relaxation times were measured in 13 stroke patients and 13 aged-matched normal subjects at 2.1 T. Spectra were acquired from an 8-cc volume using the STEAM sequence with echo times (TE) of 30.4 ms and 270.0 ms and repetition time of 2.8 s. Transverse relaxation times were estimated using two-point calculations. Percentage volume of infarct in the STEAM voxel was measured on spin-echo MRI encompassing the infarct and correlated with the peak amplitude of N-acetylated compounds (NA). T 2 values of NA, creatine, and choline resonances showed no significant difference between patients and controls. T 2 for lactate in patients was 780 ± 257 ms, respectively (mean ± SE, n = 7). In stroke patients, high inverse correlation was found between the absolute NA signal and partial volume of normal brain contributing to each spectrum ( p < .001, r = 0.97). Together with unchanged T 2, this suggests that NAA largely disappears from infarcted tissue within 24 hr postinfarct.

Phosphorous and proton spectroscopy in relation to near incarceration and incarceration of the human brain

We report 3 cases of 31P and 1H MR spectroscopy (MRS) performed at different stages on patients with clinical signs of near or fulminant incarceration of the brain. The measurements were made on a whole body, 1.5 T scanner. 1H-MRS was obtained with the STEAM sequence and 31P-spectra were obtained using the chemical shift imaging technique. Medical treatment including controlled ventilation and sedation of the patients was carried out during the examination. The first patient was evaluated on days 6 and 10 after evacuation of an acute subdural haematoma. An intracranial pressure of 35 mm Hg was registered during the first examination. The 2nd patient had suffered a spontaneous intracerebral haemorrhage and showed clinical signs of imminent incarceration during the examination. The 3rd patient showed clinical signs of incarceration just prior to the examination. In the 1st patient 1H-MRS showed a 3-fold increase in the concentration of choline-containing compounds and a small decrease in N-acetyl aspartate from the 1st to the 2nd examination, which we interpret as a loss of neurones. In case 2 only small changes in metabolism could be detected, indicating that, despite signs of imminent clinical incarceration, the energy supply to the brain was substantial. 1H-MRS of the 3rd patient showed massive lactate concentration, and 31P-MRS revealed the total absence of high-energy phosphorous compounds leaving only one single peak of inorganic phosphate, indicating irreversible brain death.

Absolute Quantitation of Water and Metabolites in the Human Brain. I. Compartments and Water

A method is presented to determine the compartmentation of a localized region in the human brain in terms of CSF, tissue water, and an NMR-invisible rest, using a PRESS or STEAM sequence. Discrimination between CSF and tissue water is based on differences in their T 2 relaxation times. The NMR-invisible compartment is assessed using an external standard. The composition of three regions in the human brain is determined. The CSF content of specific regions can be used to quantify cortical atrophy. The method provides a means for measuring the water content of brain tissue in vivo with a precision of 1.5%. After appropriate corrections, the results are in close agreement with biochemical values. The method has major applications in localized quantitative spectroscopy. The compartmentation model can be used to correct for the CSF content of the selected volume and to properly define and interconvert all major concentration units.

Dissecting and implementing steam spectroscopy

A rapid, simple method for constructing a site-specific STEAM sequence from a basic Hahn three-pulse sequence is presented. The method assures maximum signal available from the hardware.

In vivo quantification of brain metabolites by 1H-MRS using water as an internal standard

The reliability of absolute quantification of average metabolite concentrations in the human brain in vivo by 1H-MRS using the fully relaxed water signal as an internal standard was tested in a number of in vitro as well as in vivo measurements. The experiments were carried out on a SIEMENS HELICON SP 63 84 wholebody MR-scanner operating at 1.5 T using a STEAM sequence. In vitro studies indicate a very high correlation between metabolite signals (area under peaks) and concentration, R = 0.99 as well as between metabolite signals and the volume of the selected voxel, R = 1.00. The error in quantification of N-acetyl aspartate (NAA) concentration was about 1–2 mM (6–12%). Also in vivo a good linearity between water signal and selected voxel size was seen. The same was true for the studied metabolites, N-acetyl aspartate (NAA), creatine/phosphocreatine ( Cr PCr ), and choline (Cho). Calculated average concentrations of NAA, Cr PCr , and Cho in the occipital lobe of the brain in five healthy volunteers were (mean ± 1 SD) 11.6 ± 1.3 mM, 7.6 ± 1.4 mM, and 1.7 ± 0.5 mM. The results indicate that the method presented offers reasonable estimation of metabolite concentrations in the brain in vivo and therefore is useful in clinical research.

In vivo relaxation of N-acetyl-aspartate, creatine plus phosphocreatine, and choline containing compounds during the course of brain infarction: A proton MRS study

Localized water suppressed proton spectroscopy has opened up a new field of pathophysiological studies of severe brain ischemia. The signals obtained with the pulse sequences used so far are both T 1 and T 2 weighted. In order to evaluate the extent to which changes in metabolite signals during the course of infarction can be explained by changes in T 1 and T 2 relaxation times, eight patients with acute stroke were studied. STEAM sequences with varying echo delay times and repetition times were used to measure T 1 and T 2 of N-acetyl-aspartate (NAA), creatine plus phosphocreatine (Cr+PCr) and choline containing compounds (CHO) in a 27-ml voxel located in the affected area of the brain. Ten healthy volunteers served as controls. We found no difference in T 1 or T 2 of the metabolites between the patients and the normal controls. The T 2 of CHO was longer than that of NAA and Cr+PCr. Our results indicate that spectra obtained in brain infarcts and normal tissue with the same acquisition parameters are directly comparable with respect to relative signal intensities as well as signals scaled with internal and external standards.

High‐speed STEAM MRI of the human heart

High-speed STEAM MR images of the normal human heart were obtained from single cardiac cycles using a 2.0-T whole-body system equipped with conventional 10 mT m-1 gradients. The single-shot 90 degrees-TE/2-90 degrees-TM-(alpha-TE/2-Acq)n pulse sequence acquires n differently phase-encoded stimulated echoes. Measuring times of 127-254 ms were achieved using a "repetition time" of 3.96 ms in conjunction with data matrices of 32-64 x 128 pixels covering a field-of-view of 250-350 mm. The sequence provides easy access to anatomical short-axis and long-axis views of the heart by single and double oblique rotation of the image orientation. STEAM images resemble the features of spin-echo images with respect to chemical shifts, susceptibilities, and flow. Thus, no additional techniques are required for the suppression of blood signals. EKG-triggered acquisitions demonstrate that slice-selective STEAM sequences using short TM intervals allow an unambiguous delineation of those parts of the myocardium that remain stationary within the selected plane throughout the entire imaging process. Neither spins leaving nor entering the slice defined by the initial 90 degrees RF pulses give rise to a stimulated echo and therefore do not contribute to the resulting image.

Coupling effects in volume selective 1H spectroscopy of major brain metabolites

The effect of PRESS and STEAM sequences on the spectra of coupled substances are discussed using the examples of weakly coupled AX and A2X systems. Maximum differences compared to uncoupled spins occur if the RF pulses are applied to antiphase magnetization. In this case, the spin echo of the PRESS experiment shows a modified dependence on the refocusing flip angle, which may lead to an attenuation of the acquired signal. In STEAM spectroscopy the evolution within the middle interval tm is dominated by zero quantum coherences and longitudinal polarization, whose maximum efficiencies are 25 and 12.5%, respectively. Zero quantum coherences may lead to strong modulations when the tm value is varied. The effects on the spectra of important coupled metabolites of the human brain such as glutamate, GABA, inositol, and particularly lactate, are demonstrated. The observed modulations seem to make the quantification of the spectra rather difficult at echo times above 50 ms.

Localized in vivo proton spectroscopy of the human kidney

In vivo 1H spectroscopy using the STEAM sequence for localization has been applied to the human kidney in normal volunteers and subjects with successful renal transplants. We show that, within the resolution of our measurements, trimethylamines are present in the spectra from some of the subjects and absent from others. The prominent peak seen at 5.8 ppm in the spectrum is identified as that from urea and not lipid, as previously suggested.

A spin-product-operator analysis of the dephasing requirement in STEAM-localized NMR spectroscopy

The STEAM sequence (I) is frequently used in volume-localized NMR spectroscopy. Its application requires that the spin coherence created by the first 90” pulse be completely dephased before the application of the second pulse (2, 3). Although the fulfillment of this dephasing requirement is essential for the correct interpretation of the STEAM-localized spectra, its importance has not always been fully recognized in the analysis of the sequence. A recent calculation (4)) which ignored the dephasing requirement, yielded undesired modulations of the stimulated-echo (STE) signal by the chemical shift. The purpose of this paper is to analyze the time evolution of a spin system subjected to the STEAM sequence and to demonstrate the importance of the dephasing requirement when using the STEAM sequence for volume-localized NMR spectroscopy. The time evolution of a single spin (I = 4) system is calculated using the spin product operator formalism (5), with the effects of spin relaxation and diffusion ignored. The STEAM sequence used for the analysis is depicted in Fig. 1 with indication of the points on the time axis where the spin density operator is calculated. For simplicity, the gradients in the three time intervals are the averaged gradients. The calculation is performed under the following conditions: (a) the TE gradients are balanced, i.e., GTr( 1) = GTE( 2); (b) the TM gradient is applied to destroy the transverse magnetization after the second RF pulse. Calculation of the density operator at the successive time points yields

Localized proton NMR spectroscopy using stimulated echoes: applications to human skeletal muscle in vivo.

Localized proton NMR spectroscopy using stimulated echoes (STEAM) has been used to study metabolites in different proximal skeletal muscles of normal volunteers at rest. Single scan water-suppressed proton NMR spectra obtained at 1.5 and 2.0 T (Siemens Magnetom) from a 64-ml volume-of-interest (VOI) yield resonances due to triglycerides, phosphocreatine plus a minor contribution from creatine, and betaines comprising carnitine and choline-containing compounds. The observation of the pH-dependent resonances of carnosine required multiple acquisitions and echo times as short as 20 ms. T1 and T2 relaxation times of muscle metabolites were obtained by varying the repetition time and echo time of the STEAM sequence, respectively. Although rather long T2 values such as 180 ms for (phospho-) creatine correspond to natural resonance linewidths of only 2 Hz, the observed linewidths of typically 10-12 Hz are entirely determined by the short T2 relaxation times (25-30 ms) of the water protons used for shimming. The spectroscopic results from 24 muscle studies on 17 young male volunteers show remarkable intra- and interindividual differences in the absolute signal intensities of mobile lipids. Further metabolic variations were observed for the relative concentrations of betaines (by a factor of 2) and carnosine (by a factor of 3) when total creatine is assumed to be constant.

Design and application of prefocused pulses by simulated annealing

A method is proposed for producing inherently refocused selective excitation radio frequency pulses using the numerical technique of simulated annealing (Z-3). The control parameters of the method are discussed and preliminary experimental results are shown, including application to a suitably modified volume-selective STEAM sequence (4). A prefocused pulse is one which provides uniform excitation of nuclear magnetic resonance responses across a selected region of the frequency domain, with minimal phase dispersion. Consequently, in an NMR imaging experiment which uses prefocused pulses to select spatial slices there is no requirement for additional refocusing lobes to the gradient pulses. The advantages of this are twofold. Primarily, the switching demands imposed on the gradient amplifier are reduced, but second, the overall sequence length may be considerably shortened. Figure la shows a conventional sliceselecting pulse and gradient combination and is compared in Fig. 1 b to a method based on prefocused pulses. In this work, existing numerical methods (5, 6) aimed at generating such pulses have been repeated. However, we have incorporated the method of simulated annealing since that algorithm can “escape” local minima in the convergence pathway and thereby arrive at the global minimum. Furthermore, simulated annealing also permits the inclusion into the optimization procedure of constraints based on hardware considerations. Conceptually, the optimization method of simulated annealing is analogous to the thermodynamic process of a molten crystal cooling into a stable, low-energy configuration. The essence of the method is that during the cooling process the system is constantly changing its structural configuration and that, at a given thermal energy, there is a finite chance that a less stable configuration will be adopted (based on Boltzmann statistics). This might be a vital intermediate state allowing a route to a more desirable lower-energy configuration. As the temperature is lowered, the probability of making such upward changes decreases and eventually a stable arrangement is achieved. If the cooling has been sufficiently slow, this final configuration will have the lowest possible associated structural energy. By analogy, these considerations can be applied to the optimization of the pulse shape. It is important to consider the implementation of the optimized pulse within the context of an experimental system. Hence, constraints representing hardware limitations may need to be incorporated.

Region-of-interest selection by outer-volume saturation

As the use of volume-selective nuclear magnetic resonance has become more widespread, both for high-resolution spectroscopy and for “zoom-imaging,” so techniques for extracting signal from a reduced volume of spins within an extended sample have proliferated. The most versatile and generally applicable are those involving magnetic field B,-, gradients in conjunction with selective RF pulses and they fall broadly into two groups: techniques which achieve localization by selectively exciting spins inside the volume of interest and those which saturate all the magnetization except that within the volume of interest. From the former group the STEAM (I) sequence, involving stimulated echoes, is probably the most easily implemented. As with all echo techniques, the results are T2 weighted, which rules out its application to short T2 species such as 31P metabolites, but proves favorable for water and lipid suppression in ‘H metabolite detection. DRESS (2) achieves good sensitivity with surface coils but also suffers from T2 limitations, while the OSIRIS (3) technique is useful for observing fast relaxing magnetization although it suffers from pulse bandwidth limitations and unlike STEAM, it is not feasible to shim on the selected volume. New techniques which achieve 2D selective excitation in a single radiofrequency/gradient pulse and are much less limited by T2 relaxation have recently been developed (4, 5). However, they also suffer from problems such as poor outer volume suppression, difficulty in moving the selected volume under computer control, and off-resonance effects. Localization methods which work by removing signal contributions from outside the volume of interest through saturation are particularly attractive because of their T2 independence, an important factor for 31P in vivo work. Such methods include VSE (6)) SPARS ( 7)) SPACE (8), and DIGGER ( 9). All of these selectively create transverse magnetization outside the volume of interest at some point in the pulse sequence which is then dephased by the applied field gradients, while magnetization inside the volume of interest is stored along the 2 axis. A problem common to all these existing techniques is the high RF power level required to saturate all the spins in the outer volume. Other problems associated with pulse bandwidth and pulse imperfections mean that spins near the edge of the sample are not fully saturated while some magnetization is excited inside the volume of interest. Also, since these tech-

Regional in vivo proton magnetic resonance spectroscopy of brain

In vivo water-suppressed proton magnetic resonance spectra from brain gray and white matter were obtained at 1.5 T in six normal volunteers using an inversion STEAM sequence. From the proton spectral measurements, the concentration of N-acetyl aspartate (NAA) was estimated to be 10.5 ± 2.1 μ M/g in gray matter and 5.9 ± 1.71 μ M/g in white matter. The total concentration of creatine was estimated to be 9.2 ± 1.8 μ M/g in gray matter and 8.9 ± 1.9 μ M/g in white matter. The average ratio [NAA(gray)]/[NAA(white)] was determined to be 1.8 ± 0.2. The possible sources of errors which affect these estimations have been discussed.