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.

     

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.     

Comparison of 1H blood oxygen level–dependent (BOLD) and 19F MRI to investigate tumor oxygenation

Fluorine‐19 [¹⁹F] MRI oximetry and ¹H blood oxygen level–dependent (BOLD) MRI were used to investigate tumor oxygenation in rat breast 13762NF carcinomas, and correlations between the techniques were examined. A range of tissue oxygen partial pressure (pO₂) values was found in the nine tumors while the anesthetized rats breathed air, with individual tumor pO₂ ranging from a mean of 1 to 36 torr and hypoxic fraction (HF10) (<10 torr) ranging from 0% to 75%, indicating a large intra‐ and intertumor heterogeneity. Breathing oxygen produced significant increase in tumor pO₂ (mean ΔpO₂ = 50 torr) and decrease in HF₁₀ (P < 0.01). ¹H BOLD MRI observed using a spin echo‐planar imaging (EPI) sequence revealed a heterogeneous response and significant increase in mean tumor signal intensity (SI) (ΔSI = 7%, P < 0.01). R measured by multigradient‐echo (MGRE) MRI decreased significantly in response to oxygen (mean ΔR = ?4 s^?1; P < 0.05). A significant correlation was found between changes in mean tumor pO₂ and mean EPI BOLD ΔSI accompanying oxygen breathing (r² > 0.7, P < 0.001). Our results suggest that BOLD MRI provides information about tumor oxygenation and may be useful to predict pO₂ changes accompanying interventions. Significantly, the magnitude of the BOLD response appears to be predictive for residual tumor HFs. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Functional magnetic resonance imaging of the human brain based on signal enhancement by extravascular protons (SEEP fMRI).

Functional magnetic resonance imaging (fMRI) studies of the human brain were carried out at 3 Tesla to investigate an fMRI contrast mechanism that does not arise from the blood oxygen-level dependent (BOLD) effect. This contrast mechanism, signal enhancement by extravascular protons (SEEP), involves only proton-density changes and was recently demonstrated to contribute to fMRI signal changes in the spinal cord. In the present study it is hypothesized that SEEP fMRI can be used to identify areas of neuronal activity in the brain with as much sensitivity and precision as can be achieved with BOLD fMRI. A detailed analysis of the areas of activity, signal intensity time courses, and the contrast-to-noise ratio (CNR), is also presented and compared with the BOLD fMRI results. Experiments were carried out with subjects performing a simple finger-touching task, or observing an alternating checkerboard pattern. Data were acquired using a conventional BOLD fMRI method (gradient-echo (GE) EPI, TE = 30 ms), a conventional method with reduced BOLD sensitivity (GE-EPI, TE = 12 ms), and SEEP fMRI (spin-echo (SE) EPI, TE = 22 ms). The results of this study demonstrate that SEEP fMRI may provide better spatial localization of areas of neuronal activity, and a higher CNR than conventional BOLD fMRI, and has the added benefit of lower sensitivity to field inhomogeneities.     

kT‐points: Short three‐dimensional tailored RF pulses for flip‐angle homogenization over an extended volume

With Transmit SENSE, we demonstrate the feasibility of uniformly exciting a volume such as the human brain at 7T through the use of an original minimalist transmit k‐space coverage, referred to as “k_T‐points.” Radio‐frequency energy is deposited only at a limited number of k‐space locations in the vicinity of the center to counteract transmit sensitivity inhomogeneities. The resulting nonselective pulses are short and need little energy compared to adiabatic or other B‐robust pulses available in the literature, making them good candidates for short‐repetition time 3D sequences at high field. Experimental verification was performed on three human volunteers at 7T by means of an 8‐channel transmit array system. On average, whereas the standard circularly polarized excitation resulted in a 33%‐flip angle spread (standard deviation over mean) throughout the brain, and a static radio‐frequency shim showed flip angle variations of 17% and up, application of k_T‐point‐based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub‐millisecond durations. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Optimization and validation of methods for mapping of the radiofrequency transmit field at 3T

MRI techniques such as quantitative imaging and parallel transmit require precise knowledge of the radio‐frequency transmit field (B). Three published methods were optimized for robust B mapping at 3T in the human brain: three‐dimensional (3D) actual flip angle imaging (AFI), 3D echo‐planar imaging (EPI), and two‐dimensional (2D) stimulated echo acquisition mode (STEAM). We performed a comprehensive comparison of the methods, focusing on artifacts, reproducibility, and accuracy compared to a reference 2D double angle method. For the 3D AFI method, the addition of flow‐compensated gradients for diffusion damping reduced the level of physiological artifacts and improved spoiling of transverse coherences. Correction of susceptibility‐induced artifacts alleviated image distortions and improved the accuracy of the 3D EPI imaging method. For the 2D STEAM method, averaging over multiple acquisitions reduced the impact of physiological noise and a new calibration method enhanced the accuracy of the B maps. After optimization, all methods yielded low noise B maps (below 2 percentage units), of the nominal flip angle value (p.u.) with a systematic bias less than 5 p.u. units. Full brain coverage was obtained in less than 5 min. The 3D AFI method required minimal postprocessing and showed little sensitivity to off‐resonance and physiological effects. The 3D EPI method showed the highest level of reproducibility. The 2D STEAM method was the most time‐efficient technique. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast radiofrequency flip angle calibration by Bloch–Siegert shift

In a recent work, we presented a novel method for B field mapping based on the Bloch–Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off‐resonance radiofrequency pulses were added to a slice‐selective spin echo sequence. The off‐resonance pulses induce a Bloch–Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B₁². The signal is further spatially localized by a readout gradient, and the signal‐weighted average B₁ field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch–Siegert‐based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

RF pulse optimization for Bloch‐Siegert B mapping

The Bloch–Siegert (B–S) method of B mapping has been shown to be fast and accurate, yet has high SAR and moderately long TE. These limitations can lengthen scan times and incur signal loss due to B₀ inhomogeneity, particularly at high field. The B–S method relies on applying a band‐limited off‐resonant B–S radiofrequency pulse to induce a B‐dependent frequency‐shift for resonant spins. A method for optimizing the B–S radiofrequency pulse is presented here, which maximizes B–S B measurement sensitivity for a given SAR and T₂. A 4‐ms optimized pulse is shown to have 35% less SAR compared with the conventional 6‐ms Fermi pulse while still improving B map angle‐to‐noise ratio by 22%. The optimized pulse performance is validated both in phantom and in vivo brain imaging at 7 T. Magn Reson Med, 2012. © 2011 Wiley Periodicals, 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.     

Ultrashort T relaxometry for quantitation of highly concentrated superparamagnetic iron oxide (SPIO) nanoparticle labeled cells

A new method was developed to measure ultrashort T relaxation in tissues containing a focal area of superparamagnetic iron oxide (SPIO) nanoparticle‐labeled cells in which the T decay is too short to be accurately measured using regular gradient echo T mapping. The proposed method utilizes the relatively long T₂ relaxation of SPIO‐labeled cells and acquires a series of spin echo images with the readout echo shifted to sample the T decay curve. MRI experiments in phantoms and rats with SPIO‐labeled tumors demonstrated that it can detect ultrashort T down to 1 ms or less. The measured T values were about 10% higher than those from the ultrashort TE (UTE) technique. The shorter the TE, the less the measurements deviated from the UTE T mapping. Combined with the regular T mapping, this technique is expected to provide quantitation of highly concentrated iron‐labeled cells from direct cell transplantation. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

B1 mapping by Bloch‐Siegert shift

A novel method for amplitude of radiofrequency field (B) mapping based on the Bloch‐Siegert shift is presented. Unlike conventionally applied double‐angle or other signal magnitude–based methods, it encodes the B₁ information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch‐Siegert frequency shift is caused by irradiating with an off‐resonance radiofrequency pulse following conventional spin excitation. When applying the off‐resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of B. Adding gradient image encoding following the off‐resonance pulse allows one to acquire spatially resolved B₁ maps. The frequency shift from the Bloch‐Siegert effect gives a phase shift in the image that is proportional to B. The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off‐resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch‐Siegert B₁ mapping with 25 sec/slice is demonstrated to be quantitatively comparable to a 21‐min double‐angle map. As such, this method enables robust, high‐resolution B mapping in a clinically acceptable time frame. Magn Reson Med 63:1315–1322, 2010. © 2010 Wiley‐Liss, Inc.     

Sodium long‐component T mapping in human brain at 7 Tesla

Sodium (²³Na) MRI may provide unique information about the cellular and metabolic integrity of the brain. The quantification of tissue sodium concentration from ²³Na images with nonzero echo time (TE) requires knowledge of tissue‐specific parameters that influence the single‐quantum sodium signal such as transverse (T₂) relaxation times. We report the sodium (²³Na) long component of the effective transverse relaxation time T values obtained at 7 T in several brain regions from six healthy volunteers. A two‐point protocol based on a gradient‐echo sequence optimized for the least error per given imaging time was used (TE₁ = 12 ms; TE₂ = 37 ms; averaged N₁ = 5; N₂ = 15 times; pulse repetition time = 130 ms). The results reveal that long T component of tissue sodium (mean ± standard deviation) varied between cerebrospinal fluid (54 ± 4 ms) and gray (28 ± 2 ms) and white (29 ± 2 ms) matter structures. The results also show that the long T component increases as a function of the main static field B₀, indicating that correlation time of sodium ion motion is smaller than the time‐scale defined by the Larmor frequency. These results are a prerequisite for the quantification of tissue sodium concentration from ²³Na MRI scans with nonzero echo time, will contribute to the design of future measurements (such as triple‐quantum imaging), and themselves may be of clinical utility. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Spectral‐spatial pulse design for through‐plane phase precompensatory slice selection in T‐weighted functional MRI

T‐weighted functional MR images suffer from signal loss artifacts caused by the magnetic susceptibility differences between air cavities and brain tissues. We propose a novel spectral‐spatial pulse design that is slice‐selective and capable of mitigating the signal loss. The two‐dimensional spectral–spatial pulses create precompensatory phase variations that counteract through‐plane dephasing, relying on the assumption that resonance frequency offset and through‐plane field gradient are spatially correlated. The pulses can be precomputed before functional MRI experiments and used repeatedly for different slices in different subjects. Experiments with human subjects showed that the pulses were effective in slice selection and loss mitigation at different brain regions. Magn Reson Med 61:1137–1147, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

Fuzzy clustering of gradient-echo functional MRI in the human visual cortex. Part II: Quantification

Fuzzy cluster analysis (FCA) is a new exploratory method for analyzing fMRI data. Using simulated functional MRI (fMRI) data, the performance of FCA, as implemented in the software package Evident, was tested and a quantitative comparison with correlation analysis is presented. Furthermore, the fMRI model fit allows separation and quantification of flow and blood oxygen level dependent (BOLD) contributions in the human visual cortex. In gradient-recalled echo fMRI at 1.5 T (TR = 60 ms, TE = 42 ms, radiofrequency excitation flip angle [?] = 10^°–60^°) total signal enhancement in the human visual cortex, ie, flow-enhanced BOLD plus inflow contributions, on average varies from 5% to 10% in or close to the visual cortex (average cerebral blood volume [CBV] = 4%) and from 10% to 20% in areas containing medium-sized vessels (ie, average CBV = 12% per voxel), respectively. Inflow enhancement, however, is restricted to intravascular space (= CBV) and increases with increasing radiofrequency (RF) flip angle, whereas BOLD contributions may be obtained from a region up to three times larger and, applying an unspoiled gradient-echo (GRE) sequence, also show a flip angle dependency with a minimum at approximately 30^°. This result suggests that a localized hemodynamic response from the microvasculature at 1.5 T maybe extracted via fuzzy clustering. In summary, fuzzy clustering of fMRI data, as realized in the Evident software, is a robust and efficient method to (a) separate functional brain activation from noise or other sources resulting in time-dependent signal changes as proven by simulated fMRI data analysis and in vivo data from the visual cortex, and (b) allows separation of different levels of activation even if the temporal pattern is indistinguishable. Combining fuzzy cluster separation of brain activation with appropriate model calculations allows quantification of flow and (flow-enhanced) BOLD contributions in areas with different vascularization.     

Feasibility of T mapping for the evaluation of hip joint cartilage at 1.5T using a three‐dimensional (3D), gradient‐echo (GRE) sequence: A prospective study

This study defines the feasibility of utilizing three‐dimensional (3D) gradient‐echo (GRE) MRI at 1.5T for T mapping to assess hip joint cartilage degenerative changes using standard morphological MR grading while comparing it to delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC). MRI was obtained from 10 asymptomatic young adult volunteers and 33 patients with symptomatic femoroacetabular impingement (FAI). The protocol included T mapping without gadolinium‐enhancement utilizing a 3D‐GRE sequence with six echoes, and after gadolinium injection, routine hip sequences, and a dual‐flip‐angle 3D‐GRE sequence for dGEMRIC T₁ mapping. Cartilage was classified as normal, with mild changes, or with severe degenerative changes based on morphological MRI. T₁ and T findings were subsequently correlated. There were significant differences between volunteers and patients in normally‐rated cartilage only for T₁ values. Both T₁ and T values decreased significantly with the various grades of cartilage damage. There was a statistically significant correlation between standard MRI and T (T₁) (P < 0.05). High intraclass correlation was noted for both T₁ and T. Correlation factor was 0.860 to 0.954 (T‐T₁ intraobserver) and 0.826 to 0.867 (T‐T₁ interobserver). It is feasible to gather further information about cartilage status within the hip joint using GRE T mapping at 1.5T. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Systematic investigation of balanced steady-state free precession for functional MRI in the human visual cortex at 3 Tesla.

Previous studies have applied balanced steady-state free precession (bSSFP) to functional brain imaging. Methods that exploit the strong frequency dependence of the MR signal in the bSSFP transition band are strongly affected by field inhomogeneity and frequency drifts. Recent bSSFP studies using "on-resonance" (in the bSSFP passband) acquisition claimed that higher sensitivity was achieved compared to traditional fMRI methods. However, the contrast mechanism that generates activation-related signal changes in bSSFP imaging is not yet fully understood. We performed a systematic study of on-resonance bSSFP signal behavior using a multiecho balanced SSFP sequence with different TRs at 3 Tesla. We conclude that intravoxel dephasing, or the off-resonance averaged steady state, dominates the bSSFP signal decay and determines the bSSFP fMRI contrast. Experimental findings were confirmed by simulations based on existing theories for signal formation around blood vessels in inhomogeneous tissues. The activation-induced signal change in on-resonance bSSFP increases with TE, and the TE dependence of the contrast-to-noise ratio (CNR) in bSSFP is similar to that in gradient echo-planar imaging (GE-EPI). However, GE-EPI has a significantly higher CNR efficiency.     

B interferometry for the calibration of RF transmitter arrays

Multiple‐channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B, of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B fields of individual transmitter elements are measured indirectly by observing their interference with a SAR‐efficient baseline RF field. In this fashion even small RF fields can be observed in the B ‐sensitive large‐flip‐angle regime. Based on a set of such experiments B maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off‐resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8‐channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T₁ or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B maps accurately reflect the linearity of RF superposition. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

On the measurement of absolute cerebral blood volume (CBV) using vascular‐space‐occupancy (VASO) MRI

Recently, a vascular‐space‐occupancy (VASO) MRI technique was developed for quantitative assessment of cerebral blood volume (CBV). This method uses the T₁‐shortening effect of gadolinium diethylenetriamine pentaacetic acid (Gd‐DTPA) with imaging parameters chosen that null the precontrast blood magnetization but allow the postcontrast blood magnetization to recover to equilibrium. A key advantage of VASO CBV estimation is that it provides a straightforward procedure for converting MR signals to absolute physiologic values. However, as with other T₁‐based steady‐state approaches, several important factors need to be considered that influence the accuracy of CBV values obtained with VASO MRI. Here, the transverse relaxation (T₂/T) effect in VASO MRI was investigated using multiecho spin‐echo and gradient‐echo experiments, resulting in underestimation of CBV by 14.9% ± 1.1% and 16.0% ± 2.5% for spin echo (TE = 10 ms) and gradient echo (TE = 6 ms), respectively. In addition, the influence of contrast agent clearance was studied by acquiring multiple postcontrast VASO images at 2.2‐min intervals, which showed that the concentration of Gd‐DTPA in the first 14 min (single dose) was sufficient for the blood magnetization to fully recover to equilibrium. Finally, the effect of vascular Gd‐DTPA leakage was assessed for scalp tissue, and signal extrapolation as a function of postinjection time was demonstrated to be useful in minimizing the associated errors. Specific recommendations for VASO MRI acquisition and processing strategies are provided. Magn Reson Med, 2009. © 2008 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.