Fast proton spectroscopic imaging with high signal-to-noise ratio: spectroscopic RARE.

A new fast spectroscopic imaging (SI) method is presented which is based on spatial localization by the fast MRI method of rapid acquisition with relaxation enhancement (RARE) and encoding of the chemical shift information by shifting the position of a refocusing 180 pulse in a series of measurements. This method is termed spectroscopic RARE. In contrast to spectroscopic ultrafast low-angle RARE (U-FLARE), the formation of two echo families (odd and even) is suppressed by using a train of 180 RF pulses with an internal four-step phase cycle. By this means a high signal-to-noise ratio (SNR) per unit measurement time is obtained, because the separation of odd and even echoes, as well as dummy echoes to stabilize the echo amplitudes, is not needed anymore. The method is of particular interest for detecting signals of coupled spins, as effective homonuclear decoupling can be achieved by use of constant evolution time chemical shift encoding. The pulse sequence was implemented on a 4.7 T imaging system, tested on phantoms, and applied to the healthy rat brain in vivo. Spectroscopic RARE is particularly useful if T2* double less-than sign T2, which is typically fulfilled for in vivo proton SI measurements at high magnetic field strength.     

A novel method for fat suppression in RARE sequences.

Rapid acquisition relaxation-enhanced (RARE) sequences (Hennig et al., Magn. Reson. Med. 3, 823 (1986)) utilize one or several Carr-Purcell-Meiboom-Gill (CPMG) echo trains to sample a number of k-space lines each repetition time TR. The technique can rapidly generate multislice T2-weighted images which, as a rule, are strikingly similar in contrast to conventional T2-weighted spin-echo (SE) images. An exception to this rule is the appearance of very bright signal from fat in T2-weighted RARE images as compared to conventional T2-weighted SE images. To reduce this fat signal, we introduce a time delay, tau c, between the 90 degrees x and first 180 degrees y pulse of each echo train such that a phase angle of pi/2 develops between fat and the reference (water) line at echo maxima. The technique leads to single-acquisition fat suppression without the use of frequency-selective saturation pulses and concomitant loss of slices per TR. A Bloch equation analysis is used to identify two major mechanisms contributing to suppression of off-resonance spins such that w tau c = pi/2. Namely, the CPMG sequence becomes a CP sequence with no self-correction properties for imperfect 180 degrees pulses leading to enhanced signal decay, and the raw k-space data matrix become segmented into blocks alternately multiplied by +/- i, leading to signal dispersion following Fourier transformation.     

Projection flow imaging by bolus tracking using stimulated echoes

Previous investigators have employed the concept of bolus tracking using either spin echoes or gradient echoes. In this paper we introduce two methods of bolus tracking using planar- and volume-selective stimulated echoes. The planar method employs a selective 90 degrees rf pulse which tags all spins in a particular plane. At a time tau 1 later, a nonselective 90 degrees rf pulse is employed, followed after a time tau 2, by another nonselective rf pulse. Only spins which experience all three rf pulses form a stimulated echo at time tau 1 after the third rf pulse. A balanced pair of flow-compensated dephasing (crusher) gradients further ensures that the stimulated echo is due only to the effect of all three rf pulses while minimizing flow dephasing. The first part of this gradient pair is applied after the initial rf pulse in the first tau 1 period to dephase the tagged spins. The second part of this gradient pair is applied after the third rf pulse to rephase the spins. Since the plane of the excited slice is orthogonal to the readout direction, flowing spins are imaged in an angiographic manner as they move away from the excited slice. A modification to this basic sequence excites only a small volume. In this manner, the suppression of stationary spins is effected by volume-selective excitation. In both the planar- and the volume-selective techniques, the excited spins undergo T1 and T2 relaxation during the tau 1 period but only T1 relaxation in the tau 2 period. In blood, where T1 is much greater than T2, keeping tau 1 as short as possible minimizes signal loss due to T2 dephasing. These methods demonstrate increased sensitivity compared to similar bolus tracking methods using either spin echoes or gradient echoes.     

Fast inversion-recovery MR: the effect of hybrid RARE readout on the null points of fat and cerebrospinal fluid.

PURPOSETo evaluate the effect of the hybrid RARE (rapid acquisition with relaxation enhancement) readout, commonly coupled to inversion-recovery pulse sequences, on the null inversiton time (TI) of fluid and fat using both phantoms and human volunteers.METHODSTwo phantoms, simulating fat (phantom A) and cerebrospinal fluid (phantom B), respectively, were imaged using a fast inversion-recovery sequence that coupled an inversion-recovery preparation pulse to a hybrid RARE readout. At repetition times (TRs) ranging from 700 to 20,000, the TI necessary to null the signal from each phantom (null TI) was determined for an echo train length of 4, 6, 8, 10, 12, 14, 16, 18, and 20, respectively. Plots of null TI versus echo train length at different TRs were generated for both phantoms. Fast inversion-recovery MR imaging of the cervical spine and brain was performed in healthy volunteers. At a fixed TR and TI, the adequacy of signal suppression from bone marrow and cerebrospinal fluid was assessed as a function of echo train length.RESULTSThere was a gradual decrease of null TI for both phantoms with echo train length. This decrease persisted at longer TRs for phantom B (T1 = 3175 +/- 70 milliseconds) than for phantom A (T1 = 218 +/- 5 milliseconds). In the human volunteers, there was a gradual loss of suppression of signal from bone marrow and cerebrospinal fluid, with changes in the hybrid RARE readout.CONCLUSIONTo optimize specific tissue suppression, radiologists implementing fast inversion-recovery MR imaging should be aware of the effects of the hybrid RARE readout on null TI.     

Hybrid RARE: Implementations for abdominal MR imaging

Hybrid RARE (rapid acquisition with relaxation enhancement) is a family of magnetic resonance (MR) imaging techniques whereby a set of images is phase encoded with more than one spin echo per excitation pulse. This increases the efficiency of obtaining T2-weighted images, allowing greater flexibility regarding acquisition time, resolution, signal-to-noise ratio, and tissue contrast. Hybrid RARE techniques involve several important new user-selectable parameters such as effective TE, echo train length, and echo spacing. Choices of other parameters, such as TR, sampling bandwidth, and acquisition matrix, may be different from those of comparable conventional T2-weighted spin-echo images. Different hybrid RARE implementations can be used for abdominal screening, with T2-weighted or T2-weighted and inversion-recovery contrast, or for characterizing liver lesions or imaging the biliary system with an extremely long TE. High-resolution images may be obtained by averaging multiple signals during quiet breathing, or images may be acquired more rapidly during suspended respiration. In this review, the authors discuss the basic principles of hybrid RARE techniques and how various imaging parameters can be manipulated to increase the quality and flexibility of abdominal T2-weighted MR imaging.     

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.     

Thoracic aorta: rapid black-blood MR imaging with half-Fourier rapid acquisition with relaxation enhancement with or without electrocardiographic triggering.

PURPOSE: To evaluate and compare findings for thoracic aortic disease with three black-blood magnetic resonance (MR) pulse sequences: half-Fourier rapid acquisition with relaxation enhancement (RARE), with and without electrocardiographic (ECG) triggering, and ECG-triggered turbo spin echo (SE). MATERIALS AND METHODS: Axial black-blood MR images of the chest acquired at 1.5 T with a phased-array coil were obtained in 38 consecutive patients referred for evaluation of thoracic aortic disease. ECG-triggered and nontriggered half-Fourier RARE images were compared with T1-weighted ECG-triggered turbo SE images. Two readers independently scored images for each of the following parameters: ghosting artifacts; clarity of the mediastinum, cardiac chambers, and aortic wall; conspicuity of abnormality; intraluminal signal void uniformity; and overall image quality. RESULTS: Both half-Fourier RARE sequences outperformed the turbo SE sequence for all measured parameters. Scores for the ECG-triggered half-Fourier RARE sequence were significantly (P < .05) higher than those for the nontriggered version for clarity of the mediastinum and aortic wall, conspicuity of any abnormality other than aortic dissection, and overall image quality. Mean acquisition times for the ECG-triggered (48 seconds) and nontriggered (30 seconds) sequences were significantly shorter than that for the turbo SE sequence (2 minutes 20 seconds). CONCLUSION: Rapid black-blood half-Fourier RARE sequences, with or without ECG triggering, can replace ECG-triggered turbo SE sequences for evaluation of thoracic aortic disease.     

In vivo 1H2O T2+ measurement in the human occipital lobe at 4T and 7T by Carr-Purcell MRI: detection of microscopic susceptibility contrast.

A high-resolution spin-echo imaging method is presented (called CP-LASER) which exploits the spin refocusing capability of an adiabatic Carr-Purcell (CP) pulse sequence to measure apparent 1H2O transverse relaxation (T2+) and generate contrast based on microscopic tissue susceptibility. High-resolution CP-LASER images of the human occipital lobe were acquired at four different echo times from six subjects at 4T and eight subjects at 7T to investigate the effect of magnetic field strength (B(0)) and the CP interpulse time (tau(cp)) on T2+. Susceptibility contrast was identified and T2+ was quantified for long tau(cp) (>10 ms) and short tau(cp) (7 ms at 4T and 6 ms at 7T) in gray matter, white matter, and cerebral spinal fluid. The 1H2O relaxation rate constants (1/T2+) of gray and white matter each increased approximately linearly with field strength and T2+ was inversely related to tau(cp). The average T2+ value of gray matter was 19% and 9% smaller than that of white matter at 4T and 7T, respectively. These results are consistent with higher levels of compartmentalized ferritin and increased blood volume in gray matter compared to white matter in this region of the brain.     

Variable-averaging RARE

The RARE method and its variants have become popular, rapid-imaging alternatives to conventional spin-echo imaging, particularly for long repetition time proton density and T²-weighted imaging. One variant is to generate both early and late echo images using the same pulse sequence, which has the added benefit of reduced edge artifacts and blurring. Described in this paper is variable-averaging RARE (VARARE), a method by which independent amounts of averaging can be set for the early and late echo images generated by a single scanning sequence. Through the use of this method, the signal-to-noise ratio (SNR) of late echo images can be improved without unnecessarily increasing the number of averages for the early echo image, thus saving scanning time. Comparisons to various alternatives are made with respect to scanning time and image quality. Phantom measurements and in vivo images are given to demonstrate the effectiveness of VA-RARE as an efficient method for improving SNR of late echo time images in RARE imaging.     

Perfusion-based functional magnetic resonance imaging with single-shot RARE and GRASE acquisitions.

For perfusion-based functional magnetic resonance imaging, the previously introduced flow-sensitive alternating inversion recovery (FAIR) technique is combined with single-shot RARE (rapid acquisition with relaxation enhancement) and GRASE (gradient and spin echo) imaging sequences. The advantages of these sequences compared to commonly used echo-planar imaging (EPI) are an increased signal-to-noise ratio and the absence of distortions and artifacts due to magnetic field inhomogeneities. RARE- and GRASE-FAIR are applied to functional brain mapping studies in humans during visual stimulation. Results demonstrate that the presented techniques allow for perfusion maps with higher spatial resolution compared to EPI-FAIR. Relative regional cerebral blood flow change in the occipital cortex during visual stimulation was measured to be 41+/-4% (n = 5). The comparison of FAIR data obtained with RARE and GRASE techniques shows that RARE yields images with the higher signal-to-noise ratio. However, the GRASE technique features a shorter acquisition time and less RF power deposition and is thus better suited for multi-slice acquisitions.     

Oscillating steady states.

The signal formation and properties of steady-state free precession (SSFP) in combination with alternating RF pulse phases or alternating spin precession is analyzed. Simulations and experiments demonstrate that the amplitudes of SSFP echo paths are significantly influenced by application of alternating phases either via the exciting RF pulse or via some external mechanism producing alternating spin precession. The influence of alternating phases on echo amplitudes is different for different echo paths. The primary SSFP echo paths F(0) (-) and F(0) (+) exhibit a signal reduction whereas higher-order echoes F(-1) (-) and F(1) (+) show a signal increase upon application of oscillating phases. This behavior can be described using a simple perturbation theory applied to the frequency response profile of balanced SSFP combined with a final signal integration over one balanced SSFP band. The high sensitivity of SSFP echo amplitudes to alternating RF pulse phases or precession is exemplarily used to detect and visualize propagating transverse acoustic shear waves. Detection of flow or alternating currents are further possibilities to apply this unique feature of SSFP.     

TSE-sequences with spin-echo contrast.

The article presents a discussion of the basic signal behavior of contrast-modified RARE(TSE,FSE...)-sequences which have been modified such that the echo train used for image encoding is preceded by a long echo interval in order to introduce the T(2)-contrast of conventional spin-echo sequences while maintaining the high imaging speed of TSE. Sequences aimed at breathhold abdominal imaging as well as for the detection of hemorrhages in the CNS have been implemented and optimized. The significant difference in image contrast at identical echo times compared to unmodified TSE is demonstrated for different tissues.     

Fast MRI by creating multiple spin echoes in a CPMG sequence

A fast multislice imaging technique has been developed. RASTER (Rapid Acquisition with Stimulated Echo Refocusing) is based on RARE (Rapid Acquisition with Relaxation Enhancement), and creates multiple spin echoes/each 180° pulse utilizing stimulated echoes, and phase encode each differently. The sequence can be much faster than RARE while keeping the same spin echo image contrast. The main limitation of the technique is reduced signal-to-noise ratio.     

Multislice T1-weighted hybrid rare in CNS imaging: Assessment of magnetization transfer effects and artifacts

Using a T1-weighted hybrid rapid acquisition with relaxation enhancement (RARE) MR sequence that implements an echo-to-view mapping scheme termed “low-high profile order,” we evaluated signal intensity changes in different brain tissues as a function of number of slices, interslice gap, and echo train length (ETL). We also measured phase-encode and frequency-encode noise as well as blurring artifacts along the phase-encode direction as a function of ETL. Off-resonance magnetization transfer effects were demonstrated to be responsible for signal intensities changes in white matter and gray matter when using multislice techniques. These effects are amplified by increasing the number of slices and ETL. Due to the nature of the implemented echo-to-view mapping scheme, no on-resonance magnetization transfer effects were observed from the intraslice echo train. Selective background (white matter and gray matter) suppression in multislice T1-weighted hybrid RARE, secondary to off-resonance magnetization transfer effects, may provide better contrast resolution of enhancing central nervous system (CNS) lesions at much shorter scan time as compared to conventional spin-echo T1-weighted sequences. This improvement in contrast resolution as a function of ETL may be limited by worsening phase-encode noise and blurring artifacts.     

Comparison of conventional fast spin echo, single-shot two-dimensional and three-dimensional half-fourier RARE for T2-weighted female pelvic imaging

PURPOSE: To evaluate the usefulness of the three-dimensional half-Fourier RARE sequence in comparison with single-shot two-dimensional half-Fourier RARE and conventional fast spin echo (FSE) for female pelvic imaging. MATERIALS AND METHODS: Imaging with all sequences was performed in 146 patients with 166 focal lesions on a 1.5-T system. The images were compared on the basis of quality, lesion conspicuity, and lesion to the uterus contrast-to-noise ratio (CNR). RESULTS: The sharpness of intrapelvic organs on the three-dimensional half-Fourier RARE sequence was better than that on two-dimensional half-Fourier RARE and worse than that on FSE. Motion-related artifacts for three-dimensional half-Fourier RARE were more frequent than those for two-dimensional half-Fourier RARE. There was no statistical difference between the three-dimensional half-Fourier RARE sequence and FSE in regard to lesion conspicuity and overall image quality. The CNR of leiomyoma to myometrium and cervical cancer to cervical stroma was the highest with three-dimensional half-Fourier RARE (P< 0.05). CONCLUSION: The three-dimensional half-Fourier RARE sequence generates images with higher contrast and better image resolution than two-dimensional-RARE. The three-dimensional data set provided images that can be observed in any orientation without acquiring an additional scan by using the multiplanar reconstruction (MPR) method.     

PARACEST MRI with improved temporal resolution

PARAmagnetic Chemical Exchange Saturation Transfer (PARACEST) is a novel contrast mechanism for MRI. A PARACEST MRI methodology with high temporal resolution is highly desired for in vivo MRI applications of molecular imaging. To address this need, a strategy has been developed that includes a long selective saturation period before each repetition of a Rapid Acquisition with Relaxation Enhancement (RARE) pulse sequence. This strategy is suitable for the application of PARACEST contrast agents to environments with long T₁ relaxation times. An alternative strategy uses short selective saturation periods before the acquisition of each k‐space trajectory to maintain steady state conditions, which can be implemented with a Fast Low Angle Shot (FLASH) pulse sequence. These short saturation periods lengthen the total scan time as compared to the first approach but compensate for the loss in PARACEST contrast related to T₁ relaxation. Both approaches have been demonstrated in vitro and in vivo with significantly improved temporal resolutions as compared to a conventional gradient‐echo PARACEST method without sacrificing CNR efficiency. These demonstrations also adopted a strategy for measuring the PARACEST effect that only requires selective saturation at a single MR frequency, which further improves temporal resolution for PARACEST detection. Magn Reson Med 61:399–408, 2009. © 2009 Wiley‐Liss, Inc.     

Evaluation of the RF field uniformity of a double-tuned 31P/1H birdcage RF coil for spin-echo MRI/MRS of the diabetic foot

PURPOSE: To evaluate the B1 field uniformity of a double-tuned birdcage coil designed for (31)P/(1)H MRI/MRS spin-echo (SE) imaging of the metatarsal head region of the foot in neuropathic diabetic patients. MATERIALS AND METHODS: A low-pass double-tuned (31)P/(1)H RF birdcage coil was constructed to fit over the adult forefoot. Flip angle (FA) maps were created from B1 data acquired at the 3T (31)P (four normal subjects) and (1)H (five normal subjects) frequencies. T2-weighted (T2-W) (1)H images, (31)P rapid acquisition with relaxation enhancement (RARE) images, and composite SE pulse CSI data were acquired to demonstrate the uniformity of the resulting images and data. RESULTS: The means and standard deviations (SDs) of the range of FAs across the feet of the volunteer subjects indicated good uniformity (the maximum coefficients of variation (CVs) for all of the (31)P and (1)H FA maps were 7.6% and 7.3%, respectively). The FA values across the metatarsal head region indicated a maximum signal intensity variation of +/-3% in a RARE image acquired using an echo train length of 32. CONCLUSION: A (31)P/(1)H birdcage coil constructed for MRI/MRS studies of the human forefoot provided sufficient signal uniformity of SE data to facilitate accurate (31)P concentration measurements in muscle.     

Spinal dysraphism at MR urography: initial experience

PURPOSE: To prospectively evaluate the role of magnetic resonance (MR) urography in the radiologic assessment of patients with spinal dysraphism. MATERIALS AND METHODS: Fourteen patients with spinal dysraphism were referred for MR urography with half-Fourier rapid acquisition with relaxation enhancement (RARE) (repetition time msec/echo time msec = 11.9/95) and RARE (2,800/1,100) sequences on a 1.5-T MR machine. Six patients did not tolerate MR urography owing to claustrophobia (n = 4) or flexion deformities (n = 2), giving a final success rate of 57% (eight patients). Two patients had a single kidney (one after nephrectomy, one with a crossed-fused ectopic kidney). Images were jointly assessed by two radiologists and compared with excretory urographic studies. The signal intensity ratio and contrast-to-noise ratio were also calculated. RESULTS: Visualization of the kidneys, pelvicaliceal system, and ureters was better with half-Fourier RARE than with RARE imaging, whereas visualization of the bladder was comparable with both sequences. The mean signal intensity ratios for half-Fourier RARE and RARE sequences, respectively, were 16.2 +/- 3.65 and 19.2 +/- 3.65 (P =. 58, factorial analysis of variance model), whereas the mean contrast-to-noise ratios were 55.4 +/- 5.16 and 47.8 +/- 5.16 (P =. 34). Cortical scarring was depicted more clearly at MR urography than at excretory urography, whereas a renal calculus seen at excretory urography was not detected at MR urography. CONCLUSION: MR urography was feasible in 57% of patients with spinal dysraphism and is a safe, accurate method of evaluating the upper urinary tract.     

Fast spin echo and fast gradient echo MRI with low acoustic noise

It has been shown previously that a significant reduction of the acoustic noise in standard, slow magnetic resonance imaging (MRI) sequences is achieved when the gradient pulses have long sinusoidal ramps. An improvement of this method is now presented for fast gradient echo (FLASH) and fast spin echo (RARE) sequences. The new strategy consists of using a sinusoidal readout pulse with no plateau and extending the phase encoding pulses to the entire readout period. The distribution of k-space samples is no longer equidistant, and images have to be reconstructed with gridding. A reduction of the acoustic noise by 20-40 dBA was obtained with respect to standard sequences installed by the scanner manufacturer, which is 10-20 dBA better than that with the previously proposed method of sinusoidal ramps. J. Magn. Reson. Imaging 2001;13:960-966.