Validation of MRI biomarkers of hepatic steatosis in the presence of iron overload in the ob/ob mouse

Purpose:

To validate the utility and performance of a T correction method for hepatic fat quantification in an animal model of both steatosis and iron overload.

Materials and Methods:

Mice with low (n = 6), medium (n = 6), and high (n = 8) levels of steatosis were sedated and imaged using a chemical shift‐based fat‐water separation method to obtain magnetic resonance imaging (MRI) fat‐fraction measurements. Imaging was performed before and after each of two superparamagnetic iron oxide (SPIO) injections to create hepatic iron overload. Fat‐fraction maps were reconstructed with and without T correction. Fat‐fraction with and without T correction and T measurements were compared after each injection. Liver tissue was harvested and imaging results were compared to triglyceride extraction and histology grading.

Results:

Excellent correlation was seen between MRI fat‐fraction and tissue‐based fat quantification. Injections of SPIOs led to increases in R (=1/T). Measured fat‐fraction was unaffected by the presence of iron when T correction was used, whereas measured fat‐fraction dramatically increased without T correction.

Conclusion:

Hepatic fat‐fraction measured using a T‐corrected chemical shift‐based fat‐water separation method was validated in an animal model of steatosis and iron overload. T correction enables robust fat‐fraction estimation in both the presence and absence of iron, and is necessary for accurate hepatic fat quantification. J. Magn. Reson. Imaging 2012;35:844–851. © 2011 Wiley Periodicals, Inc.     

Correcting for the echo‐time effect after measuring the cerebral blood flow by arterial spin labeling

Purpose:

To take into account the echo time (TE) influence on arterial spin labeling (ASL) signal when converting it in regional cerebral blood flow (rCBF). Gray matter ASL signal decrease with increasing TE as a consequence of the difference in the apparent transverse relaxation rates between labeled water in capillaries and nonlabeled water in the tissue (δR). We aimed to measure ASL/rCBF changes in different parts of the brain and correct them.

Materials and Methods:

Fifteen participants underwent ASL measurements at TEs of 9.7–30 ms. Decreases in ASL values were localized by statistical parametric mapping. The corrections assessed were a subject‐per‐subject adjustment, an average δR value adjustment, and a two‐compartment model adjustment.

Results:

rCBF decreases associated with increasing TEs were found for gray matter and were corrected using an average δR value of 20 s^?1. Conversely, for white matter, rCBF values increased with increasing TEs (δR = ?23 s^?1).

Conclusion:

Our correction was as good as using a two‐compartment model. However, it must be done separately for the gray and white matter rCBF values because the capillary R values are, respectively, larger and smaller than those of surrounding tissues. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.

     

Quantification of holmium-166 loaded microspheres: estimating high local concentrations using a conventional multiple gradient echo sequence with S₀-fitting

Purpose:

To provide a best estimate of the R value and hence of the local concentration of highly paramagnetic holmium‐166 loaded microspheres (HoMS) in voxels for which R cannot be characterized by conventional fitting of multigradient echo (MGE) data because of fast signal decay due to high local concentrations.

Materials and Methods:

A postprocessing method, S₀‐fitting, was implemented in a conventional R fitting method that is used for quantification of HoMS. S₀‐fitting incorporates the estimated initial amplitude of the free induction decay (FID) curve, S₀, of neighboring voxels into the fitting procedure for voxels for which the conventional algorithm failed. The method was applied to HoMS in vitro and ex vivo in a rabbit liver. The performance of the S₀‐fitting method was evaluated by comparing results qualitatively and quantitatively with results obtained with quantitative ultrashort TE imaging (qUTE).

Results:

Applying S₀‐fitting provided a best estimate for R up to a value of about 2300 s^?1 compared with a maximum value of about 1000 s^?1 that could be characterized using conventional fitting. A good agreement was observed both qualitatively and quantitatively for in vitro experiments as well as for ex vivo rabbit liver experiments between results obtained with S₀‐fitting and results obtained with qUTE imaging.

Conclusion:

S₀‐fitting is a postprocessing method that can provide a best estimate of high R values that cannot be characterized by conventional relaxometry. The method can be applied to conventional MGE datasets and was shown to be beneficial for quantification of high local concentrations of holmium‐loaded microspheres. J. Magn. Reson. Imaging 2012;35:1453–1461. © 2012 Wiley Periodicals, Inc.

     

Quantitative MRI measurement of lung density must account for the change in T with lung inflation

Purpose

To evaluate lung water density at three different levels of lung inflation in normal lungs using a fast gradient echo sequence developed for rapid imaging.

Materials and Methods

Ten healthy volunteers were imaged with a fast gradient echo sequence that collects 12 images alternating between two closely spaced echoes in a single 9‐s breathhold. Data were fit to a single exponential to determine lung water density and T. Data were evaluated in a single imaging slice at total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). Analysis of variance for repeated measures was used to statistically evaluate changes in T and lung water density across lung volumes, imaging plane, and spatial locations in the lung.

Results

In normal subjects (n = 10), T (and [lung density/water density]) was 1.2 ± 0.1 msec (0.10 ± 0.02), 1.8 ± 0.2 ms (0.25 ± 0.04), and 2.0 ± 0.2 msec (0.27 ± 0.03) at TLC, FRC, and RV, respectively. Results also show that there is a considerable intersubject variability in the values of T.

Conclusion

Data show that T in the lung is very short, and varies considerably with lung volume. Thus, if quantitative assessment of lung density within a breathhold is to be measured accurately, then it is necessary to also determine T. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.     

MRI and histological analysis of beta‐amyloid plaques in both human Alzheimer's disease and APP/PS1 transgenic mice

Purpose

To investigate the relationship between MR image contrast associated with beta‐amyloid (Aβ) plaques and their histology and compare the histopathological basis of image contrast and the relaxation mechanism associated with Aβ plaques in human Alzheimer's disease (AD) and transgenic APP/PS1 mouse tissues.

Materials and Methods

With the aid of the previously developed histological coil, T‐weighted images and R parametric maps were directly compared with histology stains acquired from the same set of Alzheimer's and APP/PS1 tissue slices.

Results

The electron microscopy and histology images revealed significant differences in plaque morphology and associated iron concentration between AD and transgenic APP/PS1 mice tissue samples. For AD tissues, T contrast of Aβ‐plaques was directly associated with the gradation of iron concentration. Plaques with significantly less iron load in the APP/PS1 animal tissues are equally conspicuous as the human plaques in the MR images.

Conclusion

These data suggest a duality in the relaxation mechanism where both high focal iron concentration and highly compact fibrillar beta‐amyloid masses cause rapid proton transverse magnetization decay. For human tissues, the former mechanism is likely the dominant source of R relaxation; for APP/PS1 animals, the latter is likely the major cause of increased transverse proton relaxation rate in Aβ plaques. The data presented are essential for understanding the histopathological underpinning of MRI measurement associated with Aβ plaques in humans and animals. J. Magn. Reson. Imaging 2009;29:997–1007. © 2009 Wiley‐Liss, Inc.     

Susceptibility effects in hyperpolarized 3He lung MRI at 1.5T and 3T

Purpose

To compare susceptibility effects in hyperpolarized ³He lung MRI at the clinically relevant field strengths of 1.5T and 3T.

Materials and Methods

Susceptibility‐related B₀ inhomogeneity was evaluated on a macroscopic scale by B₀ field mapping via phase difference. Subpixel susceptibility effects were quantified by mapping T. Comparison was made between ventilation images obtained from the same volunteers at both field strengths.

Results

The B₀ maps at 3T show enhanced off‐resonance effects close to the diaphragm and the ribs due to susceptibility differences. The average T from a voxel (20 × 4 × 4) mm³ was determined as T = 27.8 msec ± 1.2 msec at 1.5T compared to T = 14.4 msec ± 2.6 msec at 3T. In ventilation images the most prominent effect is increased signal attenuation close to the intrapulmonary blood vessels at higher B₀.

Conclusion

Image homogeneity and T are lower at 3T due to increased B₀ inhomogeneity as a consequence of susceptibility differences. These findings indicate that ³He imaging at 3T has no obvious benefit over imaging at 1.5T, as signal‐to‐noise ratio (SNR) was comparable for both fields in this work. J. Magn. Reson. Imaging 2009;30:418–423. © 2009 Wiley‐Liss, Inc.     

Increased SNR and reduced distortions by averaging multiple gradient echo signals in 3D FLASH imaging of the human brain at 3T

Purpose

To demonstrate how averaging of multiple gradient echoes can improve high‐resolution FLASH (fast low angle shot) magnetic resonance imaging (MRI) of the human brain.

Materials and Methods

3D‐FLASH with multiple bipolar echoes was studied by simulation and in three experiments on human brain at 3T. First, the repetition time (TR) was increased by the square of the flip angle to maintain contrast as derived by theory. Then the number of echoes was increased at constant TR with bandwidths between 110 and 1370 Hz/pixel. Finally, signals of a 12‐echo acquisition train (echo times 4.9–59 msec) were averaged consecutively to study the increase in SNR.

Results

At unchanged contrast, the signal increased proportionally with flip angle and sqrt(TR). Increasing the bandwidth improved delineation of the basal cortex and vessels, while most of the loss in the signal‐to‐noise ratio (SNR) was recovered by averaging. Consecutive averaging increased the SNR to reach maximum efficiency at an echo train length corresponding roughly to T.

Conclusion

SNR is gained efficiently by acquiring additional echoes and increasing TR (and flip angle accordingly to maintain contrast) until the associated T loss in the averaged signal consumes the sqrt(TR) increase in the steady state. A bandwidth of 350 Hz/pixel or higher and echo trains shorter than T are recommended. J. Magn. Reson. Imaging 2009;29:198–204. © 2008 Wiley‐Liss, Inc.     

Myocardial T mapping free of distortion using susceptibility‐weighted fast spin‐echo imaging: A feasibility study at 1.5 T and 3.0 T

This study demonstrates the feasibility of applying free‐breathing, cardiac‐gated, susceptibility‐weighted fast spin‐echo imaging together with black blood preparation and navigator‐gated respiratory motion compensation for anatomically accurate T mapping of the heart. First, T maps are presented for oil phantoms without and with respiratory motion emulation (T = (22.1 ± 1.7) ms at 1.5 T and T = (22.65 ± 0.89) ms at 3.0 T). T relaxometry of a ferrofluid revealed relaxivities of R = (477.9 ± 17) mM^?1s^?1 and R = (449.6 ± 13) mM^?1s^?1 for UFLARE and multiecho gradient‐echo imaging at 1.5 T. For inferoseptal myocardial regions mean T values of 29.9 ± 6.6 ms (1.5 T) and 22.3 ± 4.8 ms (3.0 T) were estimated. For posterior myocardial areas close to the vena cava T‐values of 24.0 ± 6.4 ms (1.5 T) and 15.4 ± 1.8 ms (3.0 T) were observed. The merits and limitations of the proposed approach are discussed and its implications for cardiac and vascular T‐mapping are considered. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Improved in vivo measurement of myocardial transverse relaxation with 3 Tesla magnetic resonance imaging

Purpose

To develop practical methods at 3 Tesla (T) for measuring myocardial transverse relaxation in normal human myocardium.

Materials and Methods

Ten healthy volunteers were investigated with four multi‐echo, turbo spin‐echo (TSE) methods. Each method traded acquired phase encoding lines per image for echo‐image sample points obtained along the T₂ decay curve. Four multi‐echo turbo field‐echo (TFE) methods were also tested. The TFE methods highlighted differences between achievable receiver bandwidth and echo time constraints versus the number of sample points obtained along the T decay curve.

Results

Measured transverse relaxation values were consistent in reported means across all scan methods. T₂ for the ventricular septum was measured as 58.8 ± 7.7 ms (N = 10). T for the ventricular septum was 31.6 ± 5.8 ms (N = 10). The variation of mean T₂ or T within an region of interest improved significantly with increases in acquired echoes. Therefore, four or more echoes may provide for clear distinctions between regions of altered tissue composition within a subject.

Conclusion

These results suggest that the 4‐echo methods are best suited for measuring variations in transverse relaxation values in the mid‐ventricular septum. J. Magn. Reson. Imaging 2009;30:684–689. © 2009 Wiley‐Liss, Inc.     

Field strength dependence of R1 and R relaxivities of human whole blood to prohance,vasovist, and deoxyhemoglobin

This study has measured the longitudinal and transverse (T) relaxivity curves for ProHance (Gadoteridol), Vasovist (Gadofosveset) and deoxyhemoglobin at 1.5, 3.0, and 7.0 Tesla. The plots of R₁ versus both contrast agent and deoxyhemoglobin concentration were linear. The plots of R versus deoxyhemoglobin concentration showed a quadratic dependence. R versus contrast agent concentration showed a parabolic dependence with a minimum occurring at contrast agent concentrations of approximately 1.5 mM, corresponding to an accessible concentration in vivo. Monte Carlo simulations were performed to support the hypothesis that the minimum results from the susceptibility of the red blood cells being matched to the susceptibility of the plasma. Relaxivity values (s^?1mM^?1) for R and R₁ for all agents and all three field strengths are given. Magn Reson Med 60:1313–1320, 2008. © 2008 Wiley‐Liss, Inc.     

Differential effect of isoflurane, medetomidine, and urethane on BOLD responses to acute levo-tetrahydropalmatine in the rat

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B phase and the homogeneous Helmholtz equation. This new MRI technique might open future opportunities for tumor and lesion characterization based on conductivity differences, while it may also find application in radio frequency safety assessment. Prerequisites for conductivity mapping using only the B phase (instead of the complex B field) are addressed. Furthermore it was found that the B phase can be derived directly from the measurable transceive phase arg(B B) in the head. Validation for a human head excited by a 7 T‐birdcage coil using simulations and measurements showed that it is possible to measure in vivo conductivity patterns in the brain using B phase information only. Conductivity contrast between different brain tissues is clearly observed. The measured mean values for white matter, gray matter and cerebrospinal fluid differed 54%, 26%, and ?13% respectively from literature values. The proposed method for B phase measurements is very suited for in vivo applications, as the measurement is short (less than a minute per imaged slice) and exposes the patient to low RF power, contrary to earlier proposed approaches. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Conditions for achieving artifact-free T preparation in MRI

A theoretical analysis of the stimulated gradient-echo method for achieving T-preparation in magnetic resonance imaging is performed. The results show that T-weighting without the “shading effect” due to magnetic field variation can only be achieved if the dephasing requirement of the stimulated echo is satisfied. It is also shown that the effective echo-time for T-weighting depends on the polarities of the dephasing and the rephasing gradient pulses that generate the stimulated echo. The results from the theoretical analysis are experimentally validated.     

Measuring bone mineral density with fat–water MRI: comparison with computed tomography

Purpose:

To develop a method for measuring bone mineral density (BMD) with MRI, and to validate this method against quantitative computed tomography (QCT).

Materials and Methods:

A mathematical relationship between signal intensities from proton‐density‐weighted in‐phase images generated by multi‐fat‐peak T‐IDEAL MRI and BMD was derived using a set of calibration standards constructed from various concentrations of hydroxyapatite in water. Using these standards, the relationship between hydroxyapatite concentration and MRI signal intensity was examined. A T‐IDEAL protocol was performed on the patella of 5 volunteers and the signal model was used to compute BMD of all voxels of the patella. The BMD data were validated by obtaining QCT scans of the same patella, computing QCT BMD of all voxels, and comparing the MRI and QCT BMD data by performing linear regression analysis on a voxel‐by‐voxel basis.

Results:

A strong linear correlation between hydroxyapatite concentration of the calibration standards and MRI signal intensities was observed (r = 0.98; P < 0.01). In the patella, BMD measurements (N = 28796 voxels) from the MRI signal model were significantly correlated with those from QCT (r = 0.82; P < 0.001; slope = 1.02; and intercept = ?0.26).

Conclusion:

A standardized phantom consisting of hydroxyapatite and water can be used to accurately quantify BMD in vivo using MRI. J. Magn. Reson. Imaging 2013;37:237–242. © 2012 Wiley Periodicals, Inc.

     

Rapid B1+ mapping using a preconditioning RF pulse with TurboFLASH readout

In MRI, the transmit radiofrequency field (B) inhomogeneity can lead to signal intensity variations and quantitative measurement errors. By independently mapping the local B variation, the radiofrequency‐related signal variations can be corrected for. In this study, we present a new fast B mapping method using a slice‐selective preconditioning radiofrequency pulse. Immediately after applying a slice‐selective preconditioning pulse, a turbo fast low‐angle‐shot imaging sequence with centric k‐space reordering is performed to capture the residual longitudinal magnetization left behind by the slice‐selective preconditioning pulse due to B variation. Compared to the reference double‐angle method, this method is considerably faster. Specifically, the total scan time for the double‐angle method is equal to the product of 2 (number of images), the number of phase‐encoding lines, and approximately 5T₁, whereas the slice‐selective preconditioning method takes approximately 5T₁. This method was validated in vitro and in vivo with a 3‐T whole‐body MRI system. The combined brain and pelvis B measurements showed excellent agreement and strong correlation with those by the double‐angle method (mean difference = 0.025; upper and lower 95% limits of agreement were ?0.07 and 0.12; R = 0.93; P < 0.001). This fast B mapping method can be used for a variety of applications, including body imaging where fast imaging is desirable. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

T(1) independent, T(2) (*) corrected chemical shift based fat-water separation with multi-peak fat spectral modeling is an accurate and precise measure of hepatic steatosis

Purpose:

To determine the precision and accuracy of hepatic fat‐fraction measured with a chemical shift‐based MRI fat‐water separation method, using single‐voxel MR spectroscopy (MRS) as a reference standard.

Materials and Methods:

In 42 patients, two repeated measurements were made using a T₁‐independent, T‐corrected chemical shift‐based fat‐water separation method with multi‐peak spectral modeling of fat, and T₂‐corrected single voxel MR spectroscopy. Precision was assessed through calculation of Bland‐Altman plots and concordance correlation intervals. Accuracy was assessed through linear regression between MRI and MRS. Sensitivity and specificity of MRI fat‐fractions for diagnosis of steatosis using MRS as a reference standard were also calculated.

Results:

Statistical analysis demonstrated excellent precision of MRI and MRS fat‐fractions, indicated by 95% confidence intervals (units of absolute percent) of [?2.66%,2.64%] for single MRI ROI measurements, [?0.81%,0.80%] for averaged MRI ROI, and [?2.70%,2.87%] for single‐voxel MRS. Linear regression between MRI and MRS indicated that the MRI method is highly accurate. Sensitivity and specificity for detection of steatosis using averaged MRI ROI were 100% and 94%, respectively. The relationship between hepatic fat‐fraction and body mass index was examined.

Conclusion:

Fat‐fraction measured with T₁‐independent T‐corrected MRI and multi‐peak spectral modeling of fat is a highly precise and accurate method of quantifying hepatic steatosis. J. Magn. Reson. Imaging 2011;33:873–881. © 2011 Wiley‐Liss, Inc.

     

Cerebral blood oxygenation in rat brain during hypoxic hypoxia. Quantitative MRI of effective transverse relaxation rates

Oxygenation-sensitive MRI of respiratory challenges in the brain of experimental animals will considerably benefit from a quantitative relationship between cerebral blood oxygenation and MRI parameters. Here, a multi-echo gradient-echo MRI technique was used to determine effective transverse relaxation rates R = 1/T of rat brain in vivo during short periods of hypoxia and interleaved normoxic phases. The differential contribution ΔR observed during hypoxia was found to increase linearly with arterial blood deoxygenation for mild to moderate conditions. Severe deoxygenation resulted in a plateau most likely due to enhanced cerebral blood flow.     

Design and application of a four-channel transmit/receive surface coil for functional cardiac imaging at 7T

Purpose

To design and evaluate a four‐channel cardiac transceiver coil array for functional cardiac imaging at 7T.

Materials and Methods

A four‐element cardiac transceiver surface coil array was developed with two rectangular loops mounted on an anterior former and two rectangular loops on a posterior former. specific absorption rate (SAR) simulations were performed and a B calibration method was applied prior to obtain 2D FLASH CINE (mSENSE, R = 2) images from nine healthy volunteers with a spatial resolution of up to 1 × 1 × 2.5 mm³.

Results

Tuning and matching was found to be better than 10 dB for all subjects. The decoupling (S₂₁) was measured to be >18 dB between neighboring loops, >20 dB for opposite loops, and >30 dB for other loop combinations. SAR values were well within the limits provided by the IEC. Imaging provided clinically acceptable signal homogeneity with an excellent blood‐myocardium contrast applying the B calibration approach.

Conclusion

A four‐channel cardiac transceiver coil array for 7T was built, allowing for cardiac imaging with clinically acceptable signal homogeneity and an excellent blood‐myocardium contrast. Minor anatomic structures, such as pericardium, mitral, and tricuspid valves and their apparatus, as well as trabeculae, were accurately delineated. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Postprocessing correction for distortions in T2* decay caused by quadratic cross‐slice B0 inhomogeneity

Relaxometric measurement of the effective transverse relaxation rate R plays an important role in the quantitative evaluation of brain function, perfusion, and tissue iron content. However, accurate measurement of R is prone to macroscopic background field inhomogeneity. In clinical applications and systems, postprocessing correction techniques are more flexible in implementation than unsupported protocol or hardware modifications. The current postprocessing correction approach assumes the cross‐slice background field inhomogeneity can be approximated by a linear gradient and corrects for a sinc modulation function. The importance of the high‐order terms in background field inhomogeneity has increased with the fast development of high‐ and ultrahigh‐field scanners in recent years. In this study, we derived an analytical expression of the free induction decay signal modulation in the presence of a quadratic cross‐slice background field inhomogeneity. The proposed quadratic correction method was applied to phantom and volunteer studies and demonstrated to be superior to the classic monoexponential model, monoexponential‐plus‐constant model, and the linear sinc correction method in recovering background field inhomogeneity‐induced. R overestimations with visual inspection of R parametric maps and a statistical model selection technique. We also tabulated 7‐T T/R measurements of several human brain structures and MnCl₂ solutions with various concentrations for fellow researchers' reference. Magn Reson Med 63:1258–1268, 2010. © 2010 Wiley‐Liss, Inc.     

A single‐scan T mapping method based on two gradient‐echo images with compensation for macroscopic field inhomogeneity

T‐weighted imaging (TWI) and quantitative T mapping with conventional gradient‐echo acquisition are often hindered by severe signal loss induced by macroscopic field inhomogeneity. Various z‐shimming approaches have been developed for TWI/T mapping in which the effects of macroscopic field inhomogeneity are suppressed while the sensitivity of T‐related signal intensity to alterations in the microscopic susceptibility is maintained. However, this is often done at the cost of significantly increased imaging time. In this work, a fast T mapping method with compensation for macroscopic field inhomogeneity was developed. A proton density‐weighted image and a composite T‐weighted image, both of which were essentially free from macroscopic field inhomogeneity‐induced signal loss, were used for the T calculation. The composite T‐weighted image was reconstructed from a number of gradient‐echo images acquired with successively incremented z‐shimming compensation. Because acquisition of the two images and z‐shimming compensation were realized in a single scan, the total acquisition time for obtaining a T map with the proposed method is the same as the time taken for a conventional multiecho gradient‐echo imaging sequence without compensation. The performance and efficiency of the proposed method were demonstrated and evaluated at 4.7 T. Magn Reson Med 60:1388–1395, 2008. © 2008 Wiley‐Liss, Inc.     

B(1) mapping with selective pulses

Knowledge of B distribution is crucial for many applications, such as quantitative MRI. A novel method has been developed to improve the accuracy of the conventionally applied double‐angle method for B mapping. It solves the remaining issues raised by the use of selective pulses for slice selection to accelerate the acquisition process. A general approach for reconstructing B maps is presented first. It takes B‐induced slice profile distortions over off‐resonance frequencies into account. It is then shown how the ratio between the prescribed flip angles can be adjusted to reach a compromise between the level of noise propagated onto B maps and the width of the range in which the field can be mapped. Lastly, several solutions are proposed for reducing the B‐dependent pollution of regions distal to the image slice which participates significantly in the inaccuracy of B mapping. These methods were experimentally tested by comparison with gold standard B maps obtained on a phantom using a non‐selective and thus much slower technique. As they are independent and lead to significant improvements, these solutions can be combined to achieve high precision and fast B mapping using spin‐echo DAM. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.