Selective spin inversion in nuclear magnetic resonance and coherent optics through an exact solution of the Bloch-Riccati equation

High-resolution three-dimensional in vivo imaging of atherosclerotic plaque

The internal structure of atherosclerotic-plaque lesions may be a useful predictor of which lesions will rupture and cause sudden events such as heart attack or stroke. With lipid and flow suppression, we obtained high-resolution, three-dimensional (3D) images of atherosclerotic plaque in vivo that show the cap thickness and core size of the lesions. 3D GRASE was used because it provides flexible T(2) contrast and good resistance to off-resonance artifacts. While 2D RARE has similar properties, its resolution in the slice-select direction, which is important because of the irregular geometry of atherosclerotic lesions, is limited by achievable slice-excitation profiles. Also, 2D imaging generally achieves lower SNR than 3D imaging because, for SNR purposes, 3D image data is averaged over all the slices of a corresponding multislice 2D dataset. Although 3D RARE has many of the advantages of 3D GRASE, it requires a longer scan time because it uses more refocusing pulses to acquire the same amount of data. Finally, cardiac gating is an important part of our imaging sequence, but can make the imaging time quite long. To obtain reasonable scan times, a 2D excitation pulse was used to restrict the field of view. Magn Reson Med 42:762-771, 1999.

Hadamard encoding with surface coils for high SNR MR spectroscopy

The advantages of Hadamard over phase encoding in magnetic resonance spectroscopy (MRS) applied with surface coils in the direction perpendicular to the coil are demonstrated experimentally. With the recently introduced, time-shifted adiabatic pulses, the application of Hadamard encoding with surface coils results with almost ideal point spread function for pixels up to a distance of a radius from the coil. Comparison to phase encoding with equal region of interest size shows the significant advantage of Hadamard encoding in slice sharpness, overlapping, and spatial contamination. In addition, since there is no aliasing in Hadamard space, the total experimental time for the same region of interest is much shorter. We conclude that the hybrid of Hadamard encoding in the direction perpendicular to the coil and phase encoding in other directions is the method of choice to obtain reliable high signal to noise ratio MRS in vivo.

QUIPSS II with thin-slice TI1 periodic saturation: a method for improving accuracy of quantitative perfusion imaging using pulsed arterial spin labeling

Quantitative imaging of perfusion using a single subtraction, second version (QUIPSS II) is a pulsed arterial spin labeling (ASL) technique for improving the quantitation of perfusion imaging by minimizing two major systematic errors: the variable transit delay from the distal edge of the tagged region to the imaging slices, and the contamination by intravascular signal from tagged blood that flows through the imaging slices. However, residual errors remain due to incomplete saturation of spins over the slab-shaped tagged region by the QUIPSS II saturation pulse, and spatial mismatch of the distal edge of the saturation and inversion slice profiles. By replacing the original QUIPSS II saturation pulse with a train of thin-slice periodic saturation pulses applied at the distal end of the tagged region, the accuracy of perfusion quantitation is improved. Results of single and multislice studies are reported.

MR perfusion imaging in human brain using the UNFAIR technique. Un-inverted flow-sensitive alternating inversion recovery

Pulsed arterial spin labeling magnetic resonance techniques have been developed recently to estimate cerebral blood flow (CBF). Flow-sensitive alternating inversion recovery (FAIR) is one such technique that has been implemented successfully in humans. Un-inverted FAIR (UNFAIR) is an alternative technique in which the flow-sensitive image is acquired following inversion of all spins outside the slice of interest, and the control image is acquired without any spin labeling. This approach is potentially more efficient than FAIR since the UNFAIR control image is entirely flow independent and need only be acquired once. Here, we describe implementation of the sequence on a clinical 1.5 T magnetic resonance system. Both FAIR and UNFAIR perfusion-weighted images were obtained from six normal volunteers. Wash-in/wash-out curves measured in cortical gray and white matter were practically identical for the two techniques, as predicted by our model.

Localized 1H NMR measurements of 2-pyrrolidinone in human brain in vivo

Localized 1H NMR homonuclear J editing spectroscopy was used to measure the concentration of 2-pyrrolidinone (PRDN) in the human occipital lobe of five normal and six epileptic subjects taking vigabatrin. PRDN is a lactam cyclization product of gamma-aminobutyric acid (GABA). From a localized volume of 13.5 cm3 in the occipital cortex, the concentration of PRDN ranged from 0.2 to 0.3 micromol/g in normal subjects, whereas in epileptic subjects on vigabatrin PRDN was elevated to 0.6 +/- 0.1 micromol/g. The elevated PRDN in patients on vigabatrin was in accord with raised GABA levels compared with normals. 1H NMR measurements of PRDN will be important in assessment of the role of this metabolite for improved seizure control.

Measurement of human myocardial perfusion by double-gated flow alternating inversion recovery EPI

This paper presents a flow-sensitive alternating inversion recovery (FAIR) method for measuring human myocardial perfusion at 1.5 T. Slice-selective/non-selective IR images were collected using a double-gated IR echoplanar imaging sequence. Myocardial perfusion was calculated after T1 fitting and extrapolation of the mean signal difference SI(Sel - SI(NSel). The accuracy of the method was tested in a porcine model using graded intravenous adenosine dose challenge. Comparison with radiolabeled microsphere measurements showed a good correlation (r = 0.84; mean error = 20%, n = 6) over the range of flows tested (0.9-7 ml/g/min). Applied in humans, this method allowed for the measurement of resting myocardial flow (1.04+/-0.37 ml/g/min, n = 11). The noise in our human measurements (SE(flow) = 0.2 ml/g/min) appears to come primarily from residual respiratory motion. Although the current signal-to-noise ratio limits our ability to measure small fluctuations in resting flow accurately, the results indicate that this noninvasive method has great promise for the quantitative assessment of myocardial flow reserve in humans.

Perfusion imaging using FOCI RF pulses

Pulsed arterial spin-tagging techniques for perfusion measurements (e.g., echo planar MR imaging and signal targeting with alternating radiofrequency (EPISTAR), flow-sensitive alternating inversion recovery (FAIR), quantitative imaging of perfusion using a single subtraction (QUIPPS), uninverted FAIR (UNFAIR)) generally use hyperbolic secant (HS) pulses for spin inversion. The performance of these techniques depends on the inversion efficiency, as well as the sharpness of the slice profiles. Frequency offset corrected inversion (FOCI) pulses, a recently proposed HS variant, can provide slice profiles with edges that can be up to 10 times sharper than those obtained with conventional HS pulses. In this communication, the implementation and application of the C-shape FOCI pulse for perfusion imaging in rat brain with the FAIR technique is summarized. Despite providing a more rectangular slice profile than a conventional HS pulse, it is demonstrated both theoretically and experimentally that the FAIR perfusion signal is not increased by using a FOCI tagging pulse. However, the use of a FOCI inversion pulse is shown to significantly minimize static signal subtraction errors that are common with conventional HS pulses. Finally, the suitability of the pulse for perfusion studies is demonstrated, in vivo, on rat brain.

FAIR excluding radiation damping (FAIRER)

It is shown that the flow-sensitive alternating inversion recovery (FAIR) technique is complicated by the effect of radiation damping, leading to problems in calibrating this method on phantoms and to inaccuracies in measured flows. A modified scheme called FAIRER (FAIR excluding radiation damping) is proposed, which suppresses the damping effects by employing very weak magnetic field gradients (0.06 G/cm) during the inversion recovery, spin-echo, and predelay periods. Results on phantoms and in vivo on cat brain are presented that demonstrate that FAIRER effectively solves these problems.

A theoretical and experimental comparison of continuous and pulsed arterial spin labeling techniques for quantitative perfusion imaging

Under ideal conditions, continuous arterial spin labeling (ASL) techniques are higher in SNR than pulsed ASL techniques by a factor of e. Presented here is a direct theoretical and experimental comparison of continuous ASL and pulsed ASL, using versions of both that are amenable to multislice imaging and insensitive to variations in transit times (continuous ASL with a delay before imaging, and QUIPSS II (Quantitative Imaging of Perfusion Using a Single Subtraction-second version)). Perfusion image quality for comparable imaging time was nearly identical for both single-slice and multislice imaging. The measured raw signal was approximately 25% higher with continuous ASL, but the SNR per unit time was identical.

In vivo perfusion measurements in the human placenta using echo planar imaging at 0.5 T

This paper presents the first in vivo measurements of perfusion in the human placenta from 20 weeks gestational age until term, using the non-selective/selective inversion recovery echo-planar imaging sequence, in which data is alternately acquired following a selective and non-selective inversion pulse. Twenty pairs of images were collected, two each at the following inversion times: 20, 310, 610, 910, 1110, 1410, 1910, 2810, 3310, and 4510 ms with the sequence being repeated with a repetition time (TR) of 10 s. The results of these measurements were used to suggest the optimum sequence for future work in terms of the signal to noise ratio in the measured perfusion rate in a given measurement time. The sequence was also analyzed to determine the expected variability in the measurements. In normal pregnancies the average value of perfusion rate was found to be 176 (standard error = +/-24) ml/100 mg/min. (n = 16, standard deviation = 96 ml/100 mg/min). The expected variability in the measured parameters due to signal to noise ratio considerations alone was calculated to be 71%. For a maximum scanning time of 400 s, the optimum sequence for measuring placental perfusion was found to require 8 repetitions at each of 10 inversion times which were geometrically spaced (given by a(o), a(o)r, a(o)r2, a(o)r3, . . .), with a(o) = 850 ms, r = 1.073 and TR = 5 s, giving a pixel variability of 38%. Other timing schemes are recommended for measuring perfusion in other anatomical regions with different values of perfusion rate and longitudinal relaxation time.

Quantitative imaging of perfusion using a single subtraction (QUIPSS and QUIPSS II)

In the pulsed arterial spin labeling (ASL) techniques EPISTAR, PICORE, and FAIR, subtraction of two images in which inflowing blood is first tagged and then not tagged yields a qualitative map of perfusion. An important reason this map is not quantitative is that there is a spatially varying delay in the transit of blood from the tagging region to the imaging slice that cannot be measured from a single subtraction. We introduce here two modifications of pulsed ASL (QUIPSS and QUIPSS II) that avoid this problem by applying additional saturation pulses to control the time duration of the tagged bolus, rendering the technique relatively insensitive to transit delays and improving the quantitation of perfusion.

Simultaneous multislice acquisition with arterial-flow tagging (SMART) using echo planar imaging (EPI)

Arterial spin tagging techniques have been used to image tissue perfusion in MR without contrast injection or ionizing radiation. Currently, spin tagging studies are performed primarily using single-slice imaging sequences, which are time consuming. This note reports a multislice echo-planar arterial spin tagging technique (Simultaneous Multislice Acquisition with aRterial-flow Tagging, or "SMART"). Multiband RF encoding (Hadamard) is used to provide simultaneous multislice acquisition capability for spin tagging techniques (such as echo planar imaging signal targeting with alternating radio frequency and flow-sensitive alternative inversion recovery). The method is illustrated with a two-slice pulse sequence that was implemented using the FAIR technique to generate two perfusion weighted images simultaneously. Compared with single-slice sequences, this two-slice sequence provided similar image quality, signal-to-noise ratio, and twice the spatial coverage compared with the single-slice technique within the same scan time.

Magnetically labeled water perfusion imaging of the uterine arteries and of normal and malignant cervical tissue: initial experiences

PURPOSE

The aim of this pilot study was to evaluate a magnetically labeled water perfusion imaging technique as a non-contrast-enhanced approach to demonstrate the uterine artery, its branches, and to assess the cervical uterine blood flow in healthy volunteers and in patients with advanced uterine cervical carcinoma (FIGO IIB-IVA).

METHODS AND MATERIALS

Seven healthy volunteers (mean age, 29 years) and twenty-two patients (mean age, 52 years) with advanced cancer of the uterine cervix (FIGO IIB-IVA) were prospectively examined by magnetically labeled water perfusion imaging at different inversion delay times (300-900 ms). The magnetic resonance imaging (MRI) findings of all patients were matched to the findings of contrast-enhanced dynamic MRI and multiple biopsies (n = 5) and/or surgical whole mount specimens (n = 17), which were available in all patients.

RESULTS

The uterine artery was well visualized with short inversion delay times of 300-500 ms. It was characterized as single or multiple helical loops before dividing into its intracervical branches. The intracervical branching was observed at inversion delay times of 500-700 ms. With longer inversion delay times, arterial signal enhancement disappeared and cervical tissue enhancement was noted. Enhancement of benign tissue was observed at inversion delay times of 1100-1700 ms and in malignant tissue at shorter inversion delay times of 900-1300 ms. The maximum of this diffuse signal enhancement of benign tissue was seen at inversion delay times of 1500 ms (1100-1700 ms) in malignant tissue at significantly (p < 0.5) shorter inversion delay times of 1100 ms (900-1300 ms).

CONCLUSION

Our preliminary results show that the vascular supply and blood flow of the normal uterine cervix and of advanced cervical cancer can be assessed by magnetically labeled water perfusion imaging and that malignant cervical tissue is earlier and stronger perfused than normal cervical tissue.

In vivo relaxation time measurements in the human placenta using echo planar imaging at 0.5 T

This paper presents the first in vivo measurements of the nuclear magnetic resonance relaxation times T1 and T2 at 0.5 T in the human placenta from 20 weeks gestational age until term, in both normal and compromised pregnancies. T1 measurements were performed by using both an inversion recovery sequence and the Look-Locher echo planar imaging (EPI) sequence on a total of 41 women with normal pregnancies and 11 women with compromised pregnancies. T2 measurements were performed by using a spin-echo EPI sequence on 36 women with normal pregnancies and 14 women with compromised pregnancies. In normal pregnancies, both the T1 values measured with the inversion recovery sequence and the T2 values were found to decrease with gestational age, the linear regression results gave T1 = -9.1t + 1538 r2 = 0.23 p = 0.03. T2 = -4.0t + 338 r2=0.47 p =410(-6) where t is the gestational age in weeks, and T1 and T2 are the relaxation times in milliseconds. T1 values measured very rapidly with the Look-Locher EPI sequence, but, therefore, with a much lower signal-to-noise ratio, showed no significant trends. The T1 values measured in the abnormal group were significantly lower than those measured in the normal group. Four out of eight patients with compromised pregnancies had placental T1 values lying outside the 90% confidence limits for the normal population based about the regression line, significantly more than expected by chance (p = 0.005). Ten out of fourteen of the T2 measurements in the abnormal group were below the regression line established for the normal group, with 4 lying below the 90% confidence interval, although these trends were only just significant (p = 0.06 and p = 0.03).

Analytical description of the effect of adiabatic pulses on IS, I2S, and I3S spin systems

An analytical description of the evolution of magnetization in InS spin systems (1 </= n </= 3) during the course of an adiabatic pulse applied on spin S is provided. Calculations show that multiple-quantum terms are created during the pulse and that the rate at which in-phase and antiphase I-spin magnetization components interchange during spin-echo-based pulse sequences is decreased relative to the case where a hard inversion pulse is substituted for the adiabatic pulse. This has important consequences for purging schemes making use of such frequency-swept pulses. Simulations demonstrate that the evolution of in-phase I magnetization is essentially independent of n.

Decoupling: theory and practice. I. Current methods and recent concepts

Current methods for broadband heteronuclear decoupling are reviewed from a historical perspective. The principal concern is that decoupling should be effective over a wide range of chemical shifts without undue radiofrequency heating of the sample, particularly when human patients are involved. Continuous-wave methods are the least efficient in this respect, followed by noise decoupling. Composite pulse schemes offer a more effective use of radiofrequency power, while adiabatic passage methods are the most efficient of all. Bi-level decoupling employs a low level of radiofrequency irradiation during the relaxation delay to maintain the nuclear Overhauser effect, with a higher level during signal acquisition in order to decouple over a wide frequency band. All decoupling sequences introduce cycling sidebands into the observed spectrum, and schemes are described to minimize the intensity of these artifacts. In part II, practical applications of decoupling methods are examined in the context of in vivo spectroscopy, where the improvements in sensitivity and resolution through broadband decoupling can be critical for solving clinical problems. Attention is focused on the regulatory limits on power deposition in these experiments. A tabulation of the existing work on decoupling in biological tissue is presented, mainly involving 31P and 13C spectroscopy in vivo or in vitro.

A simplified sequence for observing deoxymyoglobin signals in vivo: myoglobin excitation with dynamic unexcitation and saturation of water and fat (MEDUSA)

This paper describes a new, simplified pulse sequence for observing NMR signals from deoxymyoglobin in vivo. Paramagnetically shifted resonances from deoxymyoglobin can be exploited to noninvasively calculate intracellular oxygen tension in striated muscle. However, special sequences are required to observe these weak signals against the larger water and fat signals encountered in vivo. The pulse sequence described here, which is based on inversion recovery sequences, efficiently suppresses both water and fat resonances and can be implemented with short repetition rates. Moreover, it is perfectly suited for studies with surface coils, where RF inhomogeneities render other popular suppression sequences ineffective.

Implementation and evaluation of frequency offset corrected inversion (FOCI) pulses on a clinical MR system

FOCI pulses are a variant of hyperbolic secant pulses in which the RF amplitude, RF frequency, and gradient waveform are all modulated by the same function A(t). This increases the usable gradient amplitude without requiring a corresponding increase in RF amplitude. In this paper the implementation and inversion profiles of FOCI pulses on a clinical MR system are described, showing improved slice definition and chemical-shift offset behavior. Their adiabatic behavior is confirmed, and their use with an ISIS sequence for localized MRS is illustrated. Finally the effects of waveform digitization are considered, and the implications for SAR and dB/dt are discussed.

Slice profile effects in adiabatic inversion: application to multislice perfusion imaging

Imperfections in the slice profile of the adiabatic inversion induced by relaxation effects are shown to cause signal variations in pulsed arterial tagging schemes on the order of magnitude of perfusion changes, and result in gross errors in perfusion quantitation. Significant improvement can be made with minor modifications to the inversion pulse which facilitate the acquisition of quantitative, multislice perfusion images, as demonstrated in both a phantom and a normal human volunteer.

NMR structure of a protein kinase C-gamma phorbol-binding domain and study of protein-lipid micelle interactions

Classical protein kinase C (PKC) family members are activated by the binding of various ligands to one of several cysteine-rich domains of the enzyme. The natural agonist, diacylglycerol (DAG), and the natural product superagonist, phorbol dibutyrate (PDB), activate the enzyme to produce wide-ranging physiological effects. The second cysteine-rich (Cys2) domain of rat brain PKC-gamma was expressed and labeled with 15N and 13C, and the solution structure was determined to high resolution using multidimensional heteronuclear NMR methods. The phorbol binding site was identified by titrating this domain with phorbol-12,13-dibutyrate (PDB) in the presence of organic cosolvents. Titrations of this domain with lipid micelles, in the absence and presence of phorbols, indicate selective broadening of some resonances. The observed behavior indicates conformational exchange between bound and free states upon protein-micelle interaction. The data also suggest that half of the domain, including the phorbol site and one of the zinc sites, is capable of inserting into membranes.

Three-dimensional flow-independent peripheral angiography

A magnetization-prepared sequence, T2-Prep-IR, exploits T1, T2, and chemical shift differences to suppress background tissues relative to arterial blood. The resulting flow-independent angiograms depict vessels with any orientation and flow velocity. No extrinsic contrast agent is required. Muscle is the dominant source of background signal in normal volunteers. However, long-T2 deep venous blood and nonvascular fluids such as edema also contribute background signal in some patients. Three sets of imaging parameters are described to address patient-specific contrast requirements. A rapid, spiral-based, three-dimensional readout is utilized to generate high-resolution angiograms of the lower extremities. Comparisons with x-ray angiography and two-dimensional time-of-flight angiography indicate that this flow-independent technique has unique capabilities to accurately depict stenoses and to visualize slow flow and in-plane vessels.

Improved water and lipid suppression for 3D PRESS CSI using RF band selective inversion with gradient dephasing (BASING)

A T1 insensitive solvent suppression technique-band selective inversion with gradient dephasing (BASING)-was developed to suppress water and lipids for 1H magnetic resonance spectroscopy (MRS). BASING, which consists of a frequency selective RF inversion pulse surrounded by spoiler gradient pulses of opposite signs, was used to dephase stopband resonances and minimally impact passband metabolites. Passband phase linearity was achieved with a dual BASING scheme. Using the Shinnar-Le Roux algorithm, a highpass filter was designed to suppress water and rephase the lactate methyl doublet independently of TE, and water/lipid bandstop filters were designed for the brain and prostate. Phantom and in vivo experimental 3D PRESS CSI data were acquired at 1.5 T to compare BASING with CHESS and STIR suppression. With BASING, the measured suppression factor was over 100 times higher than with CHESS or STIR causing baseline distortions to be removed. It was shown that BASING can be incorporated into a variety of sequences to offer improved suppression in the presence of B1 and T1 inhomogeneites.

Implementation of quantitative perfusion imaging techniques for functional brain mapping using pulsed arterial spin labeling

We describe here experimental considerations in the implementation of quantitative perfusion imaging techniques for functional MRI using pulsed arterial spin labeling. Three tagging techniques: EPISTAR, PICORE, and FAIR are found to give very similar perfusion results despite large differences in static tissue contrast. Two major sources of systematic error in the perfusion measurement are identified: the transit delay from the tagging region to the imaging slice; and the inclusion of intravascular tagged signal. A modified technique called QUIPSS II is described that decreases sensitivity to these effects by explicitly controlling the time width of the tag bolus and imaging after the bolus is entirely deposited into the slice. With appropriate saturation pulses the pulse sequence can be arranged so as to allow for simultaneous collection of perfusion and BOLD data that can be cleanly separated. Such perfusion and BOLD signals reveal differences in spatial location and dynamics that may be useful both for functional brain mapping and for study of the BOLD contrast mechanism. The implementation of multislice perfusion imaging introduces additional complications, primarily in the elimination of signal from static tissue. In pulsed ASL, this appears to be related to the slice profile of the inversion tag pulse in the presence of relaxation, rather than magnetization transfer effects as in continuous arterial spin labeling, and can be alleviated with careful adjustment of inversion pulse parameters.

Design of adiabatic pulses for fat-suppression using analytic solutions of the Bloch equation

Discrimination between signals produced by fat and by water is an important issue in MRI. One efficient approach is to perform fat-suppression by selective inversion. This technique exploits the transition region of a selective RF pulse to invert the longitudinal lipid magnetization while leaving the magnetization of the water protons untouched. The damaging effects of RF field inhomogeneity may be overcome by using pulses based on the adiabatic fast passage principle (AFP). In particular, the well-known sech/tanh adiabatic pulse is a robust and efficient pulse that is obtained as an analytic solution of the Bloch equation. In this paper, a wider class of analytic solutions of the Bloch equation is presented of which the sech/tanh driving function is merely a particular case. The new pulse exhibits an asymmetric distribution of magnetization with one transition sharper than the other. The sharper transition can be used to perform the required selective discrimination between signals. The resulting pulse features excellent adiabatic behavior. Moreover, the transition width of the new pulse can be reduced by a factor of about 2/3 with respect to an equal-duration sech/tanh pulse. The performance of the new pulse is compared with a similar sech/tanh pulse with the aid of a practical design example.

Two methods for peak RF power minimization of multiple inversion-band pulses

Two novel methods to minimize peak RF power for high order longitudinal Hadamard encoding are described and demonstrated experimentally. The first method uses the fact that the choice of a reference phase in an inversion process does not affect the final frequency response. In this method, the different single inversion-band pulses are added together, each with a different reference phase. For a proper phase choice, minimization of the peak RF power is obtained. Scaling laws are defined allowing the use of a given phase-set in multiple cases. In the second method, single inversion-band pulses are added together, each partially shifted in time. This results in a significant reduction in peak power with only a moderate increase in pulse length. Theoretical conditions outlining the optimal addition order are defined. Experimental results verify the theoretical conditions and demonstrate that the frequency response is not affected by the peak power minimization process. With the new low peak RF power, longitudinal Hadamard encoding of 8TH (or 16TH) order can be performed in any clinical setting.

Frequency offset corrected inversion (FOCI) pulses for use in localized spectroscopy

Gradient localized spectroscopy techniques suffer from a well documented spatial localization error caused by the difference in chemical shifts between resonances. This results in the acquisition of spectra from partially overlapping spatial regions of the sample, with each resonance representing a different region. The image-selected in vivo spectroscopy technique uses hyperbolic secant inversion pulses, where the main limitation in reducing this error is in the RF power available for application of the selective RF pulse. This spatial localization error may be dramatically reduced by increasing, and temporally shaping, the gradient pulse during slice-selective spin inversion. The performance of these RF pulses have been experimentally verified.

Calibration of the radio frequency field for magnetic resonance imaging

We have developed and validated the performance of a novel slice selective pulse sequence that allows direct calibration of the RF field using a simple rectangular pulse. The new sequence offers a number of substantial advantages. It operates at steady state and has an accurate calibration response at short repetition times. The slice selection train is insensitive to RF field strength changes caused by patient loading. The issue of patient motion has been addressed in our data collection and analysis routines. The applicability of the method to human scanning has been demonstrated in the automated RF power calibration routine of a commercial imaging system.

Pulses with fixed magnitude and variable phase response profiles

The Shinnar-Le Roux (SLR) method of pulse envelope design provides for the generation of nearly arbitrary magnitude response profiles with such great efficiency that the pulse envelopes may often be calculated at the time of sequence initiation. A significant limitation of the method is that it provides only limited control of the phase of the response profile. In the current manuscript it is demonstrated that the phase of the response profile can be modulated without affecting the magnitude profile by replacement of some of the roots of the SLR polynomials with one over their complex conjugate. This method allows interactive tailoring of the phase profile to the user's needs. Although the method does not allow for the optimization of arbitrary phase profiles, a variety of pulses, which are of general utility, have been generated. Some of these pulses and their response profiles are presented.

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 constant-time (13)C- (1)H HSQC with uniform excitation over the complete (13)C chemical shift range

An improved version of the constant-time HSQC experiment is presented that gives uniform sensitivity over the complete (13)C bandwidth in (13)C-(1)H correlation experiments without creating artifacts in the methyl and aromatic regions of the spectra. The improvement is achieved by replacing the refocussing (13)C 180° pulse in the evolution time by a combination of a full-power (22 kHz) hyperbolic secant 180° pulse that inverts and refocusses the entire (13)C window, immediately followed by a selective 180° pulse on the CO region. Further improvement in signal-to-noise in the aromatic and methyl regions, although less spectacular, is obtained by replacing the other two 180° (13)C pulses in the INEPT parts of the pulse sequence by full-power hyperbolic secant pulses. Results of simulations and experimental data are presented that demonstrate the excellent performance of the hyperbolic secant pulse for broadband inversion and show that refocussing of transverse magnetization occurs over the same bandwidth, albeit with a (13)C signal phase that depends quadratically on offset. A further modification, in which one of the selective pulses on the CO region is omitted, is also presented. Implications for other 2D and 3D experiments performed at high fields, where uniform (13)C inversion and refocussing is desirable, are discussed.

Simultaneous 31P NMR spectroscopy and EMG in exercising and recovering human skeletal muscle: technical aspects

The bioenergetics of human skeletal muscle can be studied by 31P NMR spectroscopy (31P-MRS) and by surface electromyography (SEMG). Simultaneous 31P-MRS and SEMG permit accurate and noninvasive studies of the correlation between metabolic and electrical changes in exercising and recovering human skeletal muscle, a relationship that is still poorly understood. This study describes the optimization of skeletal muscle 31P-MRS in a whole-body magnet, involving surface coil design, utilization of adiabatic radio frequency pulses and advanced time-domain fitting, to the technical design of SEMG. A nonmagnetic ergometer was used for ankle dorsiflexions that activated only the anterior tibial muscle as verified by post exercise imaging. The coil design and the adiabatic sech/tanh pulse improved sensitivity by 45% and 56% respectively, compared with standard techniques. Simultaneous electromyographic recordings did not deteriorate the NMR spectra. The VARPRO time domain fitting routine was very suitable for estimating 31P muscle spectra. With these methods it was possible to accurately estimate parameters describing metabolic and electrical changes during rest, exercise and the entire recovery period with a 20-s time resolution on a standard 1.5 T whole-body NMR scanner.

Determination of saturation factors in 31P NMR spectra of the developing human brain

In order to assess the influence of longitudinal relaxation on previously reported variations in 31P NMR signals during brain development, we used an accelerated two-point technique to determine T1 at 2.35 Tesla in 8 min. Comparison between 10 normal neonates (age range 37-46 weeks postconception) and 10 healthy infants (age range 80-157 weeks postconception) indicated that T1 does not vary substantially during the first year of life, except in the phosphomonoester (PME) region of the spectra. T1 of total PME decreases with age which we could explain by its variable multicomponent nature: The signal from (unresolved) components at the downfield shoulder of PME (attributed mostly to phosphorylethanolamine at 6.72 ppm) with a T1 of at least 6.4 s decreases with age relative to contributions from other phosphomonoester compounds resonating predominantly at the upfield side of the peak (approximately 6.3 ppm), with T1 below 2.9 s. Because the T1 heterogeneity of PME may depend on its relative composition, quantitative 31P NMR spectroscopy may require an assessment of the influence of longitudinal relaxation on the signal amplitudes in each measurement.

Rapid T1 estimation using tagged magnetization-prepared gradient-echo MR imaging

A technique for estimation of the longitudinal relaxation time of a large homogeneous object with an acquisition time of 4 s or less was developed by combining spatially selective rf tagging pulses with a T1-weighted magnetization-prepared gradient-echo sequence. Multiple 5-mm-wide tagged areas are laid orthogonal to the imaging section of interest. The contrast between each tag and the untagged regions differs because each tag is produced at a different time. The T1 value is determined from the nulling time at which tagged and untagged areas have no contrast.

Hadamard spectroscopic imaging technique as applied to study human calf muscles

In vivo results obtained by the B1 insensitive Hadamard spectroscopic imaging multivolume technique used with a surface coil are shown. The functional behavior of different human calf muscles during exercise was determined and the Pi/PCr ratio in each calf muscle, during steady-state conditions, was measured as a function of work level. Different levels of metabolic and physical activity were observed at the three calf muscles.

Two-dimensional selective adiabatic pulses

Using the technique of separable k-space excitation, we have designed a two-dimensional selective adiabatic pulse that inverts magnetization from a square region in the xy plane with insensitivity to RF variations. We also have designed a two-dimensional adiabatic pulse that inverts selectively in frequency and in one spatial dimension. The pulses should be useful for both MR imaging and spectroscopy. We present experimental results to demonstrate that the two-dimensional adiabatic pulses are feasible on commercial MR imaging systems.

Fast angiography using selective inversion recovery

We have developed an enhancement of selective inversion recovery that allows us to obtain high-resolution angiograms in reduced scan time. By applying several read pulses following each tagging inversion pulse, we can obtain several phase encodes in each cardiac cycle, thereby reducing the total scan time required for a complete image. Using this technique, high-resolution angiograms can be obtained in as little as 15 s. Because the phase encodes are collected in short bursts separated by long pauses, care must be taken to maintain uniform signal weighting across phase-encoding views and avoid ghosting. We use an increasing flip-angle sequence to equalize signal level weighting across the readouts. The phase encodes are collected in a special order to minimize ghosting. A postprocessing technique is used to further reduce signal nonuniformity between phase encodes. This fast angiography technique can significantly reduce artifacts due to patient motion during scanning and is especially useful for imaging vasculature in regions of the body where respiratory motion is a problem.

Localized phosphorus NMR spectroscopy: a comparison of the FID, DRESS, CRISIS/CODEX, and STEAM methods in vitro and in vivo using a surface-coil

The FID, DRESS, CRISIS/CODEX, and STEAM techniques for localized 31P NMR spectroscopy were compared using a Siemens Magnetom SP63 1.5 T whole-body imager and a surface-coil, 80 mm in diameter, acting as transmitter and receiver coil. The comparison was performed with phantom experiments and human in vivo investigations on the calf muscle. The phantom experiments which used the same volume size showed a comparable signal-to-noise ratio for FID and DRESS, while the two fully localized techniques showed a reduction in signal-to-noise ratio to 76% for CRISIS/CODEX and 31% for STEAM. The in vivo measurements confirm the phantom results and reveal that CRISIS/CODEX gains a 2.5 fold higher signal-to-noise ratio than STEAM under the same conditions.

Practical implementation and optimization of one-shot T1 imaging

Longitudinal relaxation times (T1) can be measured rapidly in an imaging context using a "one-shot" method based on the pulse sequence originally proposed by D. C. Look and D. R. Locker (Rev. Sci. Instrum. 41, 250 1970). This sequence is significantly faster than either repeated inversion recovery or repeated saturation recovery methods. The method uses a 180 degrees inversion pulse followed by multiple small-angle alpha pulses that sample the longitudinal magnetization during its recovery. Choices of inversion pulse, tip angle, and time intervals are discussed for optimal clinical use. We can produce 29 images sampling the full T1 recovery curve with a 256 x 256 resolution in about 10 min. From this data, T1 images can be calculated with a precision of 10%.