Contrast behavior and relaxation effects of conventional and hyperecho-turbo spin echo sequences at 1.5 and 3 T.

To overcome specific absorption rate (SAR) limitations of spin-echo-based MR imaging techniques, especially at (ultra) high fields, rapid acquisition relaxation enhancement/TSE (turbo spin echo)/fast spin echo sequences in combination with constant or variable low flip angles such as hyperechoes and TRAPS (hyperTSE) have been introduced. Due to the multiple spin echo and stimulated echo pathways involved in the signal formation, the contrast behavior of such sequences depends on both T2 and T1 relaxation times. In this work, constant and various variable flip angle sequences were analyzed in a volunteer study. It is demonstrated that a single effective echo time parameter TE(eff) can be calculated that accurately describes the overall T2 weighted image contrast. TE(eff) can be determined by means of the extended phase graph concept and is practically independent of field strength. Using the described formalism, the contrast of any TSE sequence can be predicted. HyperTSE sequences are demonstrated to show a robust and well-defined T2 contrast allowing clinical routine MRI to be performed with SAR reductions of typically at least 70%.     

Time‐efficient fast spin echo imaging at 4.7 T with low refocusing angles

An implementation of fast spin echo at 4.7 T designed for versatile and time‐efficient T₂‐weighted imaging of the human brain is presented. Reduced refocusing angles (α < 180°) were employed to overcome specific absorption rate (SAR) constraints and their effects on image quality assessed. Image intensity and tissue contrast variations from heterogeneous RF transmit fields and incidental magnetization transfer effects were investigated at reduced refocusing angles. We found that intraslice signal variations are minimized with refocusing angles near 180°, but apparent gray/white matter contrast is independent of refocusing angle. Incidental magnetization transfer effects from multislice acquisitions were shown to attenuate white matter intensity by 25% and gray matter intensity by 15% at 180°; less than 5% attenuation was seen in all tissues at flip angles below 60°. We present multislice images acquired without excess delay time for SAR mitigation using a variety of protocols. Subsecond half Fourier acquisition single‐shot turbo spin echo (HASTE) images were obtained with a novel variable refocusing angle echo train (20° < α < 58°) and high‐resolution scans with a voxel volume of 0.18 mm³ were acquired in 6.5 min with refocusing angles of 100°. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS).

A variation of the rapid acquisition with relaxation enhancement (RARE) sequence (also called turbo spin-echo (TSE) or fast spin-echo (FSE)) is presented. This technique uses variable flip angles along the echo train such that magnetization is initially prepared into the static pseudo steady state (PSS) for a low refocusing flip angle (alpha < 180 degrees ). It is shown that after such a preparation, magnetization will always stay very close to the static PSS even after significant variation of the subsequent refocusing flip angles. This allows the design of TSE sequences in which high refocusing flip angles yielding 100% of the attainable signal are applied only for the important echoes encoding for the center of k-space. It is demonstrated that a reduction of the RF power (RFP) by a factor of 2.5-6 can be achieved without any loss in signal intensity. The contribution of stimulated-echo pathways leads to a reduction of the effective TE by a factor f(t), which for typical implementations is on the order of 0.5-0.8. This allows the use of longer echo readout times, and thus longer echo trains, for acquiring images with a given T(2) contrast.     

Inversion recovery prepared turbo spin echo sequences with reduced SAR using smooth transitions between pseudo steady states.

This study investigates the contrast behavior of 2D inversion recovery (IR) prepared turbo spin echo (TSE) sequences that use RF pulse schemes with variable low flip angles (hyperTSE) to reduce RF power deposition. A framework of equations and calculations for adapting the sequence parameters is presented by which equivalent image contrast is achieved compared to conventional IR-TSE imaging. Although the inversion time (TI) and repetition time (TR) do not need to be changed, the echo time (TE) has to be prolonged such that the effective TE (TE(eff)) is preserved. Measurements in healthy volunteers confirmed this finding for IR-TSE sequences using different TIs: fluid attenuated inversion recovery (FLAIR), gray matter (GM)-white matter (WM)-IR, and short tau IR (STIR). The results demonstrate that hyperTSE sequences enable high-quality IR-prepared imaging with a considerably reduced specific absorption rate (SAR).     

Modeling the influence of TR and excitation flip angle on the magnetization transfer ratio (MTR) in human brain obtained from 3D spoiled gradient echo MRI

Attempts to optimize the magnetization transfer ratio (MTR) obtained from spoiled gradient echo MRI have focused on the properties of the magnetization transfer pulse. In particular, continuous‐wave models do not explicitly account for the effects of excitation and relaxation on the MTR. In this work, these were modeled by an approximation of free relaxation between the radiofrequency pulses and of an instantaneous saturation event describing the magnetization transfer pulse. An algebraic approximation of the signal equation can be obtained for short pulse repetition time and small flip angles. This greatly facilitated the mathematical treatment and understanding of the MTR. The influence of inhomogeneous radiofrequency fields could be readily incorporated. The model was verified on the human brain in vivo at 3 T by variation of flip angle and pulse repetition time. The corresponding range in MTR was similar to that observed by a 4‐fold increase of magnetization transfer pulse power. Choice of short pulse repetition time and larger flip angles improved the MTR contrast and reduced the influence of radiofrequency inhomogeneity. Optimal contrast is obtained around an MTR of 50%, and noise progression is reduced when a high reference signal is obtained. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Reduced RF power without blurring: correcting for modulation of refocusing flip angle in FSE sequences.

In order to reduce the RF power deposition of fast spin echo sequences operated at high field strength, the flip angles of the refocusing pulse train are varied from pulse to pulse using a modulated angle refocusing train method. The technique employs high flip angle pulses prior to sampling the center of k-space in order to preserve T(2) contrast, low flip angles after sampling the center of k-space to reduce power and prolong relaxation, and a smooth transition between the high and low flip angle regimes in order to maintain the pseudosteady-state, maximizing signal and avoiding artifact-inducing oscillations. An analytical expression is used to predict and correct for the flip angle dependence of the signal, thus eliminating any deleterious effects of flip angle modulation on the point spread function. Analysis of resolution and SNR were performed in simulation and phantom studies. In human imaging studies, it is shown that RF energy deposition per slice in a single-shot fast spin echo application can be reduced by up to 75%, making the sequence as practical at 3 T as it is has been at 1.5 T.     

3D fast spin echo with out‐of‐slab cancellation: A technique for high‐resolution structural imaging of trabecular bone at 7 tesla

Spin‐echo‐based pulse sequences are desirable for the application of high‐resolution imaging of trabecular bone but tend to involve high‐power deposition. Increased availability of ultrahigh field scanners has opened new possibilities for imaging with increased signal‐to‐noise ratio (SNR) efficiency, but many pulse sequences that are standard at 1.5 and 3 T exceed specific absorption rate limits at 7 T. A modified, reduced specific absorption rate, three‐dimensional, fast spin‐echo pulse sequence optimized specifically for in vivo trabecular bone imaging at 7 T is introduced. The sequence involves a slab‐selective excitation pulse, low‐power nonselective refocusing pulses, and phase cycling to cancel undesired out‐of‐slab signal. In vivo images of the distal tibia were acquired using the technique at 1.5, 3, and 7 T field strengths, and SNR was found to increase at least linearly using receive coils of identical geometry. Signal dependence on the choice of refocusing flip angles in the echo train was analyzed experimentally and theoretically by combining the signal from hundreds of coherence pathways, and it is shown that a significant specific absorption rate reduction can be achieved with negligible SNR loss. Magn Reson Med 63:719–727, 2010. © 2010 Wiley‐Liss, Inc.     

Diffusion sensitivity of turbo spin echo sequences

This article introduces an effective b-factor b(TSE) for turbo spin echo (TSE) sequences to quantify their inherent diffusion sensitivity. b(TSE) is investigated for a broad variety of two-dimensional- and three-dimensional-TSE sequences using constant and varying flip angles (transitions between pseudo steady states, SPACE, VISTA, Cube, etc.). The inherent TSE diffusion sensitivity becomes important for high-resolution protocols, which can lead to subtle contrast modifications or even fluid suppressions in a clinical setting or animal imaging regime. The b(TSE) values obtained considerably depend on the relaxation times and diffusion coefficient and, thus, on the tissue under observation. The fractional b(TSE) contributions per TSE imaging encoding axis are highly anisotropic. Further noteworthy effects such as decreasing b-factors along a TSE train are pointed out and explained. The results are also discussed in combination with recent findings regarding contrast properties and possible diffusion sensitivity of TSE sequences. Identical but well more pronounced b(TSE) effects are observed in the animal imaging regime due to smaller field of view and higher resolutions.     

Comparison of two-dimensional gradient echo, turbo spin echo and two-dimensional turbo gradient spin echo sequences in MRI of the cervical spinal cord anatomy

The aim of this study was to assess the detectability and distinguishability of the cervical spinal cord, the anterior and posterior spinal roots and of the internal anatomy of the cord (distinction of grey and white matter). For this purpose 20 healthy volunteers were examined using a 1.5 T MR unit with 20 mT/m gradient strength and a dedicated circular polarized neck array coil. Three T2* weighted (w). 2D gradient echo sequences, two T2 w. 2D turbo spin echo (TSE) sequences and one T2 w. 2D turbo gradient spin echo (TGSE) sequence were compared. The multiecho 2D fast low angle shot (FLASH) sequence with magnetization transfer saturation pulse (me FLASH+MTS) yielded the best results for liquor/compact bone, liquor/spinal cord and grey/white matter contrast, as found with regions of interest (ROI) analysis. The single echo 2D FLASH sequence was significantly poorer than the two me FLASH+/-MTS sequences. Two-dimensional TGSE as well as 2D TSE with a 256 matrix and with a 512 matrix yielded the poorest results. In the visual analysis the contrast between liquor and compact bone, liquor and cord as well as liquor and roots was best with me FLASH+MTS, whereas grey/white matter distinction was best using me FLASH-MTS. In conclusion, we would therefore recommend the inclusion of an axial T2* w. multiecho 2D spoiled gradient echo sequence with magnetization transfer saturation pulse and gradient motion rephasing in a MR imaging protocol of the cervical spine.     

Spin‐locked balanced steady‐state free‐precession (slSSFP)

A spin‐locked balanced steady‐state free‐precession (slSSFP) pulse sequence is described that combines a balanced gradient‐echo acquisition with an off‐resonance spin‐lock pulse for fast MRI. The transient and steady‐state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady‐state free‐precession (bSSFP) for a range of T₂/T₁ and flip angles, although the slSSFP steady‐state could be maintained with considerably lower radio frequency (RF) power. In both simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal‐to‐noise ratio (SNR) efficiency to bSSFP, but with significantly lower power. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Design and use of variable flip angle schedules in transient balanced SSFP subtractive imaging

In subtractive imaging modalities, the differential longitudinal magnetization decays with time, necessitating signal‐efficient scanning methods. Balanced steady‐state free precession pulse sequences offer greater signal strength than conventional spoiled gradient echo sequences, even during the transient approach to steady state. Although traditional balanced steady‐state free precession requires that each excitation pulse use the same flip angle, operating in the transient regimen permits the application of variable flip angle schedules that can be tailored to optimize certain signal characteristics. A computationally efficient technique is presented to generate variable flip angle schedules efficiently for any optimization metric. The validity of the technique is shown using two phantoms, and its potential is demonstrated in vivo with a variable angle schedule to increase the signal‐to‐noise ratio (SNR) in myocardial tissue. Using variable flip angles, the mean SNR improvement in subtractive imaging of myocardial tissue was 18.2% compared to conventional, constant flip angle, balanced steady‐state free precession (P = 0.0078). Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Incidental magnetization transfer contrast by fat saturation preparation pulses in multislice look‐locker echo planar imaging

In this study, it is demonstrated that fat saturation (FS) preparation (prep) pulses generate incidental magnetization transfer contrast (MTC) in multislice Look‐Locker (LL) imaging. It is shown that frequency‐selective FS prep pulses can invoke MTC through the exchange between free and motion‐restricted protons. Simulation reveals that the fractional signal loss by these MTC effects is more severe for smaller flip angles (FAs), shorter repetition times (TRs), and greater number of slices (SN). These incidental MTC effects result in a signal attenuation at a steady state (up to 30%) and a T₁ measurement bias (up to 20%) when using inversion recovery (IR) LL echo‐planar imaging (EPI) sequences. Furthermore, it is shown that water‐selective MRI using binomial pulses has the potential to minimize the signal attenuation and provide unbiased T₁ measurement without fat artifacts in MR images. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Imaging with positive T(1) -contrast using superstimulated echoes

This article presents the basic principles of the superstimulated echo mechanism and shows preliminary results of its application to T₁‐weighted imaging with positive T₁‐contrast. A superstimulated echo scheme uses a preparation of square‐wave modulated, periodically inverted z‐magnetization, which after signal evolution during the mixing time TM is fully converted into transverse magnetization. This avoids the 50% signal loss of a conventional stimulated echo. Furthermore, its implementation as a preparation module for standard turbo spin echo (TSE) imaging allows producing images with positive T₁‐contrast. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Whole-brain black-blood imaging with magnetization-transfer prepared spin echo-like contrast: a novel sequence for contrast-enhanced brain metastasis screening at 3T

In contrast-enhanced (CE) brain metastasis screening, coexistence of enhanced blood vessel suppression and higher tumor-to-parenchyma contrast may improve radiologists’ performances in detecting brain metastases compared with conventional sequences. In this study, we propose a new scheme, allowing both suppression of blood signals and improvement of tumor-to-parenchyma contrast, using motion-sensitized driven equilibrium prepared 3D low-refocusing flip-angle turbo spin echo (TSE) (“magnetization transfer prepared spin echo”-like contrast volume examination: MATLVE) for brain metastasis screening at 3.0 T, and we compare MATLVE to conventional three-dimensional (3D)-gradient recalled echo (GRE) and 3D-TSE sequences. With the use of MATLVE, the signal intensity of CE blood decreased substantially. Furthermore, the contrast ratio of tumor-to-white matter was significantly higher than in either conventional 3D-GRE or 3D-TSE. MATLVE can be used for 3D volumetric post-CE black-blood imaging, and it may be effective in detecting small brain metastases by selectively enhancing tumor signals while suppressing blood signals.     

Rapid estimation of cartilage T2 based on double echo at steady state (DESS) with 3 Tesla

The double‐echo‐steady‐state (DESS) sequence generates two signal echoes that are characterized by a different contrast behavior. Based on these two contrasts, the underlying T2 can be calculated. For a flip‐angle of 90°, the calculated T2 becomes independent of T1, but with very low signal‐to‐noise ratio. In the present study, the estimation of cartilage T2, based on DESS with a reduced flip‐angle, was investigated, with the goal of optimizing SNR, and simultaneously minimizing the error in T2. This approach was validated in phantoms and on volunteers. T2 estimations based on DESS at different flip‐angles were compared with standard multiecho, spin‐echo T2. Furthermore, DESS‐T2 estimations were used in a volunteer and in an initial study on patients after cartilage repair of the knee. A flip‐angle of 33° was the best compromise for the combination of DESS‐T2 mapping and morphological imaging. For this flip angle, the Pearson correlation was 0.993 in the phantom study (～20% relative difference between SE‐T2 and DESS‐T2); and varied between 0.429 and 0.514 in the volunteer study. Measurements in patients showed comparable results for both techniques with regard to zonal assessment. This DESS‐T2 approach represents an opportunity to combine morphological and quantitative cartilage MRI in a rapid one‐step examination. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Hyperecho-turbo spin-echo sequences at 3T: clinical application in neuroradiology

BACKGROUND AND PURPOSE: Hyperecho-turbo spin-echo (hyperTSE) sequences were developed to reduce the specific absorption rate (SAR), especially at high fields such as 3T and above. The purpose of this study was to quantitatively and qualitatively assess the detection of neuroradiologic pathologies by hyperTSE in comparison with standard turbo spin-echo (TSE180°) sequences.MATERIALS AND METHODS: TSE180° and hyperTSE images with parameters adapted for equal T2 contrast were acquired on a 3T whole-body system in 51 patients with 54 cerebral pathologies. Region-of-interest analysis was performed of signal intensities of pathologies, normal white and gray matter, CSF, and the SD of noise. Signal intensity-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) for healthy tissues and pathologies were determined. A qualitative rating concerning artifacts, lesion conspicuity, and image quality was performed by 2 experienced neuroradiologists.RESULTS: HyperTSE sequences were equivalent to standard TSE180° sequences for the CNR of pathologies and of the contrast between gray and white matter. The SNR of gray and white matter and CSF were also the same. The CNRs of the pathologies in hyperTSE and TSE180° images were strongly correlated with each other (r = 0.93, P = .001). The visual rating of images revealed no significant differences between hyperTSE and TSE180°.CONCLUSION: HyperTSE sequences proved to be qualitatively and quantitatively equivalent to TSE180° sequences in the detection of high- and low-signal-intensity lesions. They provide equal CNR of pathologies and of gray minus white matter and reduce the imaging restrictions of conventional TSE180° imposed by SAR limitations at 3T.

The use of standard turbo spin-echo (TSE180°) sequences¹ is well established in clinical routine at lower magnetic fields. At higher fields, the inherent robustness of spin-echo sequences against field inhomogeneities and susceptibility artifacts is even more desirable. Tolerable specific absorption rate (SAR) levels, however, are very soon exceeded by using multiple refocusing pulses with a 180° flip angle. Common approaches in clinical routine MR imaging to overcome SAR limitations have several disadvantages: Reducing the number of sections keeps the acquisition time constant but compromises volume coverage. Prolonging TR keeps volume coverage unchanged but increases the acquisition time. An overall reduction of the (constant) refocusing flip angle reduces SAR but also markedly reduces signal intensity to noise. Moreover, refocusing flip angles that deviate from 180° cause T1-weighted stimulated echo contributions to participate in echo formation and, thus, lead to a reduced T2 contrast in the image.^2-4The generic hyperecho scheme is a possible solution to this problem because it allows a considerable reduction of SAR while preserving full signal intensity-to-noise ratios (SNRs).^3-6 Variable refocusing flip angles are used so that high flip angles produce a high signal for the encoding of the central k-space, and lower flip angles are applied for the acquisition of outer parts of the k-space. Asymmetric hyperechoes (smooth transitions between pseudo-steady states) are the most flexible hyperecho approach, which allows the flip angles to be varied freely for an optimized signal-intensity behavior (hyperecho-turbo spin-echo sequences [hyperTSE]) (Fig 1).^3,6 Due to their low flip angles, hyperTSE sequences also display, however, a reduced T2 contrast compared with standard TSE180° sequences with constant 180° flip angles at a given TE.^3,4Open in a separate windowFig 1.The basic principle of TSE180° and hyperTSE. A sketch of the radio-frequency (RF) of the refocusing pulses applying flip angles α_ref is shown with time in TSE180° (upper graph) and hyperTSE (lower graph). In TSE180°, a constant α_ref = 180° is used, whereas in hyperTSE, α_ref is varied along the echo train. The box indicates the echo written in the center of the k-space, which corresponds to the respective TE. Note that the hyperTSE uses a substantial later echo than the TSE180°. α_exec indicates excitation pulse.Recently, it has been shown by calculations, simulations, and a thorough volunteer study that a defined prolongation of TE can compensate for this reduced T2 contrast in hyperTSE.^3,4 Thus, hyperTSE sequences are highly suitable for low SAR imaging of volunteers with full SNR and equal T2-contrast as TSE180°.The purpose of this study was to assess quantitatively the application of such T2 contrast–adapted hyperTSE sequences in comparison with the corresponding T2-weighted standard TSE180° sequences, regarding contrast-to-noise ratios (CNRs) for the pathologies and for gray-white matter contrast, in patients with neurologic or neurosurgical diseases. Furthermore, a qualitative rating of the images presented in random order to 2 experienced neuroradiologists blinded to the methods was intended to provide a subjective visual assessment.     

Optimization of on‐resonant magnetization transfer contrast in coronary vein MRI

Magnetization transfer contrast has been used commonly for endogenous tissue contrast improvements in angiography, brain, body, and cardiac imaging. Both off‐resonant and on‐resonant RF pulses can be used to generate magnetization transfer based contrast. In this study, on‐resonant magnetization transfer preparation using binomial pulses were optimized and compared with off‐resonant magnetization transfer for imaging of coronary veins. Three parameters were studied with simulations and in vivo measurements: flip angle, pulse repetitions, and binomial pulse order. Subsequently, first or second order binomial on‐resonant magnetization transfer pulses with eight repetitions of 720° and 240° flip angle were used for coronary vein MRI. Flip angles of 720° yielded contrast enhancement of 115% (P < 0.0006) for first order on‐resonant and 95% (P < 0.0006) for off‐resonant magnetization transfer. There was no statistically significance difference between off‐resonant and on‐resonant first order binomial Magnetization transfer at 720°. However, for off‐resonance pulses, much more preparation time is needed when compared with the binomials but with considerably reduced specific absorption rate. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Contrast‐enhanced,three‐dimensional,whole‐brain,black‐blood imaging: Application to small brain metastases

Contrast‐enhanced three‐dimensional T₁‐weighted imaging based on magnetization‐prepared rapid‐gradient recalled echo is widely used for detecting small brain metastases. However, since contrast materials remain in both blood and the tumor parenchyma and thus increase the signal intensity of both regions, it is often challenging to distinguish brain tumors from blood. In this work, we develop a T₁‐weighted, black‐blood version of single‐slab three‐dimensional turbo/fast spin echo whole‐brain imaging, in which the signal intensity of the brain tumor is selectively enhanced while that of blood is suppressed. For blood suppression, variable refocusing flip angles with flow‐sensitizing gradients are employed. To avoid a signal loss resulting from the flow‐sensitizing scheme, the first refocusing flip angle is forced to 180°. Composite restore pulses at the end of refocusing pulse train are applied to achieve partial inversion recovery for the T₁‐weighted contrast. Simulations and in vivo volunteer and patient experiments are performed, demonstrating that this approach is highly efficient in detecting small brain metastases. Magn Reson Med 63:553–561, 2010. © 2010 Wiley‐Liss, Inc.     

Magnetization transfer contrast MRI of musculoskeletal neoplasms

Magnetic resonance imaging (MRI) examinations were performed in 15 patients with musculoskeletal neoplasms to assess the value of magnetization transfer contrast in tumor characterization. Multiplanar gradient-recalled echo sequences (TR 500-600/TE 15-20/flip angle 20–30°) were performed first without and then with magnetization transfer contrast generated by a zero degree binomial pulse (MPGR and MTMPGR). Standard T1-weighted spin echo images (SE; TR 300-400/TE 12-20) and either T2-weighted SE (TR 2000-2900/TE 70-80) or T2-weighted fast spin echo (FSE; TR 4000-5000/TE 100-119 effective) images were also obtained. Signal intensities on MTMPGR scans were compared to those on MPGR scans for both tumors and normal tissues. Signal intensity ratios (SIR) and contrast-to-noise ratios (CNR) were also compared for all sequences. MTMPGR images provided better contrast between pathologic tissues and muscle than did standard MPGR images, increasing both conspicuity of lesions and definition of tumor/muscle interfaces. Benign and malignant tumors, with the exception of lipoma, underwent similar degrees of magnetization transfer and could not be distinguished by this technique.     

Fast CPMG‐based Bloch‐Siegert B1+ mapping

A novel method for B mapping based on the Bloch‐Siegert (BS) shift was recently presented. This method applies off‐resonant pulses before signal acquisition to encode B₁ information into the signal phase. BS‐based methods possess significant advantages in measurement time and accuracy compared to magnitude‐based B methods. This study extends the idea of BS B mapping to Carr, Purcell, Meiboom, Gill (CPMG)‐based multi‐spin‐echo (BS‐CPMG‐MSE) and turbo‐spin‐echo (BS‐CPMG‐TSE) imaging. Compared to BS‐based spin echo imaging (BS‐SE), faster acquisition of the B information was possible using the BS‐CPMG‐TSE sequence. Furthermore, signal loss by T₂* effects could be minimized using these spin echo‐based techniques. These effects are critical for gradient echo‐based BS methods at high field strengths. However, multi‐spin‐echo‐based BS B₁ methods inherently possess high specific absorption rates. Thus, the relative specific absorption rate of BS‐CPMG‐TSE sequences was estimated and compared with the specific absorption rate produced by BS‐SE sequences. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.