Whole‐brain perfusion imaging with balanced steady‐state free precession arterial spin labeling

Recently, balanced steady‐state free precession (bSSFP) readout has been proposed for arterial spin labeling (ASL) perfusion imaging to reduce susceptibility artifacts at a relatively high spatial resolution and signal‐to‐noise ratio (SNR). However, the main limitation of bSSFP‐ASL is the low spatial coverage. In this work, methods to increase the spatial coverage of bSSFP‐ASL are proposed for distortion‐free, high‐resolution, whole‐brain perfusion imaging. Three strategies of (i) segmentation, (ii) compressed sensing (CS) and (iii) a hybrid approach combining the two methods were tested to increase the spatial coverage of pseudo‐continuous ASL (pCASL) with three‐dimensional bSSFP readout. The spatial coverage was increased by factors of two, four and six using each of the three approaches, whilst maintaining the same total scan time (5.3 min). The number of segments and/or CS acceleration rate (R) correspondingly increased to maintain the same bSSFP readout time (1.2 s). The segmentation approach allowed whole‐brain perfusion imaging for pCASL‐bSSFP with no penalty in SNR and/or total scan time. The CS approach increased the spatial coverage of pCASL‐bSSFP whilst maintaining the temporal resolution, with minimal impact on the image quality. The hybrid approach provided compromised effects between the two methods. Balanced SSFP‐based ASL allows the acquisition of perfusion images with wide spatial coverage, high spatial resolution and SNR, and reduced susceptibility artifacts, and thus may become a good choice for clinical and neurological studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of diffusion in inhomogeneous magnetic fields on balanced steady-state free precession

In this work, the effects of susceptibility variation from dilute, micron-sized spherical field perturbers and diffusion on balanced steady-state free precession (bSSFP) are analyzed. Predictions from Monte Carlo simulations are in good agreement with experiments and reveal that, for diffusing protons, susceptibility variation becomes apparent as a reduction in the overall bSSFP signal intensity. This reduction depends on microsphere parameters (radius and susceptibility difference), as well as on sequence-related parameters (repetition time and flip angle) and on relaxation times. Specific Monte Carlo results from one set of parameter values can be extrapolated to another set of values by means of a scaling law and a substitution model. The scaling law, derived from the Bloch-Torrey equation, captures the dependencies of bSSFP signal reduction on susceptibility and diffusion-related changes and on repetition time, whereas the substitution model describes those on flip angle and relaxation times.     

Phase imaging with multiple phase‐cycled balanced steady‐state free precession at 9.4 T

While phase imaging with a gradient echo (GRE) sequence is popular, phase imaging with balanced steady‐state free precession (bSSFP) has been underexplored. The purpose of this study was to investigate anatomical and functional phase imaging with multiple phase‐cycled bSSFP, in expectation of increasing spatial coverage of steep phase‐change regions of bSSFP. Eight different dynamic 2D pass‐band bSSFP studies at four phase‐cycling (PC) angles and two T_E/T_R values were performed on rat brains at 9.4 T with electrical forepaw stimulation, in comparison with dynamic 2D GRE. Anatomical and functional phase images were obtained by averaging the dynamic phase images and mapping correlation between the dynamic images and the stimulation paradigm, and were compared with their corresponding magnitude images. Phase imaging with 3D pass‐band and 3D transition‐band bSSFP was also performed for comparison with 3D GRE phase imaging. Two strategies of combining the multiple phase‐cycled bSSFP phase images were also proposed. Contrast between white matter and gray matter in bSSFP phase images significantly varied with PC angle and became twice as high as that of GRE phase images at a specific PC angle. With the same total scan time, the combined bSSFP phase images provided stronger phase contrast and visualized neuronal fiber‐like structures more clearly than the GRE phase images. The combined phase images of both 3D pass‐band and 3D transition‐band bSSFP showed phase contrasts stronger than those of the GRE phase images in overall brain regions, even at a longer T_E of 20 ms. In contrast, phase functional MRI (fMRI) signals were weak overall and mostly located in draining veins for both bSSFP and GRE. Multiple phase‐cycled bSSFP phase imaging is a promising anatomical imaging technique, while its usage as fMRI does not seem desirable with the current approach.     

Optimization of 4D vessel‐selective arterial spin labeling angiography using balanced steady‐state free precession and vessel‐encoding

Vessel‐selective dynamic angiograms provide a wealth of useful information about the anatomical and functional status of arteries, including information about collateral flow and blood supply to lesions. Conventional x‐ray techniques are invasive and carry some risks to the patient, so non‐invasive alternatives are desirable. Previously, non‐contrast dynamic MRI angiograms based on arterial spin labeling (ASL) have been demonstrated using both spoiled gradient echo (SPGR) and balanced steady‐state free precession (bSSFP) readout modules, but no direct comparison has been made, and bSSFP optimization over a long readout period has not been fully explored. In this study bSSFP and SPGR are theoretically and experimentally compared for dynamic ASL angiography. Unlike SPGR, bSSFP was found to have a very low ASL signal attenuation rate, even when a relatively large flip angle and short repetition time were used, leading to a threefold improvement in the measured signal‐to‐noise ratio (SNR) efficiency compared with SPGR. For vessel‐selective applications, SNR efficiency can be further improved over single‐artery labeling methods by using a vessel‐encoded pseudo‐continuous ASL (VEPCASL) approach. The combination of a VEPCASL preparation with a time‐resolved bSSFP readout allowed the generation of four‐dimensional (4D; time‐resolved three‐dimensional, 3D) vessel‐selective cerebral angiograms in healthy volunteers with 59 ms temporal resolution. Good quality 4D angiograms were obtained in all subjects, providing comparable structural information to 3D time‐of‐flight images, as well as dynamic information and vessel selectivity, which was shown to be high. A rapid 1.5 min dynamic two‐dimensional version of the sequence yielded similar image features and would be suitable for a busy clinical protocol. Preliminary experiments with bSSFP that included the extracranial vessels showed signal loss in regions of poor magnetic field homogeneity. However, for intracranial vessel‐selective angiography, the proposed bSSFP VEPCASL sequence is highly SNR efficient and could provide useful information in a range of cerebrovascular diseases. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

High-resolution neural network-driven mapping of multiple diffusion metrics leveraging asymmetries in the balanced steady-state free precession frequency profile

We propose to utilize the rich information content about microstructural tissue properties entangled in asymmetric balanced steady-state free precession (bSSFP) profiles to estimate multiple diffusion metrics simultaneously by neural network (NN) parameter quantification. A 12-point bSSFP phase-cycling scheme with high-resolution whole-brain coverage is employed at 3 and 9.4 T for NN input. Low-resolution target diffusion data are derived based on diffusion-weighted spin-echo echo-planar-imaging (SE-EPI) scans, that is, mean, axial, and radial diffusivity (MD, AD, and RD), fractional anisotropy (FA), as well as the spherical coordinates (azimuth Φ and inclination ϴ) of the principal diffusion eigenvector. A feedforward NN is trained with incorporated probabilistic uncertainty estimation. The NN predictions yielded highly reliable results in white matter (WM) and gray matter structures for MD. The quantification of FA, AD, and RD was overall in good agreement with the reference but the dependence of these parameters on WM anisotropy was somewhat biased (e.g. in corpus callosum). The inclination ϴ was well predicted for anisotropic WM structures, while the azimuth Φ was overall poorly predicted. The findings were highly consistent across both field strengths. Application of the optimized NN to high-resolution input data provided whole-brain maps with rich structural details. In conclusion, the proposed NN-driven approach showed potential to provide distortion-free high-resolution whole-brain maps of multiple diffusion metrics at high to ultrahigh field strengths in clinically relevant scan times.     

Improved cine displacement-encoded MRI using balanced steady-state free precession and time-adaptive sensitivity encoding parallel imaging at 3 T

Cine displacement-encoded MRI is a promising modality for quantifying regional myocardial function. However, it has two major limitations: low signal-to-noise ratio (SNR) and data acquisition efficiency. The purpose of this study was to incrementally improve the SNR and the data acquisition efficiency of cine displacement-encoded MRI through the combined use of balanced steady-state free precession (b-SSFP) imaging, 3T imaging, echo-combination image reconstruction, and time-adaptive sensitivity encoding (TSENSE) parallel imaging. Phantom experiments were performed to empirically determine the optimal excitation angle (alpha) and to estimate the measurement errors in the presence of 130 Hz peak-to-peak static magnetic field (B0) variation. The optimal alpha was determined to be 20 degrees . The intrinsic phase correction in the echo-combination effectively reduced the phase error, which produced small displacement errors (0.11 versus 0.11 mm) and negligible strain errors (-0.001 versus -0.002). Six healthy volunteers were imaged in three short-axis levels of the heart to evaluate the SNR and the relative accuracy of strain calculations. Compared with the 24-heartbeat cine echo-planar imaging acquisition, the 24-heartbeat non-accelerated b-SSFP acquisition yielded approximately 65% higher SNR, and the 12-heartbeat twofold accelerated b-SSFP acquisition yielded approximately 28% higher SNR. The 12-heartbeat twofold accelerated b-SSFP acquisition yielded functional maps with spatial resolution of 3.6 x 3.6 mm, temporal resolution of 35 ms, and relatively high SNR (31.2 +/- 5.4 at end diastole; 19.9 +/- 3.6 at end systole; 10.3 +/- 1.1 at late diastole; mean +/- SD). The left ventricular strain values between the non-accelerated and twofold accelerated b-SSFP acquisitions correlated strongly (slope = 0.99; bias = 0.00; R2 = 0.91) and were in excellent agreement. The combined implementation of b-SSFP imaging, 3T imaging, echo-combination image reconstruction, and TSENSE parallel imaging can be used to incrementally improve the cine displacement-encoded MRI pulse sequence.     

Comparison of spoiled gradient echo and steady‐state free‐precession imaging for native myocardial T1 mapping using the slice‐interleaved T1 mapping (STONE) sequence

Cardiac T₁ mapping allows non‐invasive imaging of interstitial diffuse fibrosis. Myocardial T₁ is commonly calculated by voxel‐wise fitting of the images acquired using balanced steady‐state free precession (SSFP) after an inversion pulse. However, SSFP imaging is sensitive to B₁ and B₀ imperfection, which may result in additional artifacts. A gradient echo (GRE) imaging sequence has been used for myocardial T₁ mapping; however, its use has been limited to higher magnetic field to compensate for the lower signal‐to‐noise ratio (SNR) of GRE versus SSFP imaging. A slice‐interleaved T₁ mapping (STONE) sequence with SSFP readout (STONE–SSFP) has been recently proposed for native myocardial T₁ mapping, which allows longer recovery of magnetization (>8 R–R) after each inversion pulse. In this study, we hypothesize that a longer recovery allows higher SNR and enables native myocardial T₁ mapping using STONE with GRE imaging readout (STONE–GRE) at 1.5T. Numerical simulations and phantom and in vivo imaging were performed to compare the performance of STONE–GRE and STONE–SSFP for native myocardial T₁ mapping at 1.5T. In numerical simulations, STONE–SSFP shows sensitivity to both T₂ and off resonance. Despite the insensitivity of GRE imaging to T₂, STONE–GRE remains sensitive to T₂ due to the dependence of the inversion pulse performance on T₂. In the phantom study, STONE–GRE had inferior accuracy and precision and similar repeatability as compared with STONE–SSFP. In in vivo studies, STONE–GRE and STONE–SSFP had similar myocardial native T₁ times, precisions, repeatabilities and subjective T₁ map qualities. Despite the lower SNR of the GRE imaging readout compared with SSFP, STONE–GRE provides similar native myocardial T₁ measurements, precision, repeatability, and subjective image quality when compared with STONE–SSFP at 1.5T.     

Targeted vessel reconstruction in non‐contrast‐enhanced steady‐state free precession angiography

Image quality in non‐contrast‐enhanced (NCE) angiograms is often limited by scan time constraints. An effective solution is to undersample angiographic acquisitions and to recover vessel images with penalized reconstructions. However, conventional methods leverage penalty terms with uniform spatial weighting, which typically yield insufficient suppression of aliasing interference and suboptimal blood/background contrast. Here we propose a two‐stage strategy where a tractographic segmentation is employed to auto‐extract vasculature maps from undersampled data. These maps are then used to incur spatially adaptive sparsity penalties on vascular and background regions. In vivo steady‐state free precession angiograms were acquired in the hand, lower leg and foot. Compared with regular non‐adaptive compressed sensing (CS) reconstructions (CS_low), the proposed strategy improves blood/background contrast by 71.3 ± 28.9% in the hand (mean ± s.d. across acceleration factors 1–8), 30.6 ± 11.3% in the lower leg and 28.1 ± 7.0% in the foot (signed‐rank test, P < 0.05 at each acceleration). The proposed targeted reconstruction can relax trade‐offs between image contrast, resolution and scan efficiency without compromising vessel depiction. Copyright © 2016 John Wiley & Sons, Ltd.     

Arterial spin labeling‐fast imaging with steady‐state free precession (ASL‐FISP): a rapid and quantitative perfusion technique for high‐field MRI

Arterial spin labeling (ASL) is a valuable non‐contrast perfusion MRI technique with numerous clinical applications. Many previous ASL MRI studies have utilized either echo‐planar imaging (EPI) or true fast imaging with steady‐state free precession (true FISP) readouts, which are prone to off‐resonance artifacts on high‐field MRI scanners. We have developed a rapid ASL‐FISP MRI acquisition for high‐field preclinical MRI scanners providing perfusion‐weighted images with little or no artifacts in less than 2 s. In this initial implementation, a flow‐sensitive alternating inversion recovery (FAIR) ASL preparation was combined with a rapid, centrically encoded FISP readout. Validation studies on healthy C57/BL6 mice provided consistent estimation of in vivo mouse brain perfusion at 7 and 9.4 T (249 ± 38 and 241 ± 17 mL/min/100 g, respectively). The utility of this method was further demonstrated in the detection of significant perfusion deficits in a C57/BL6 mouse model of ischemic stroke. Reasonable kidney perfusion estimates were also obtained for a healthy C57/BL6 mouse exhibiting differential perfusion in the renal cortex and medulla. Overall, the ASL‐FISP technique provides a rapid and quantitative in vivo assessment of tissue perfusion for high‐field MRI scanners with minimal image artifacts. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis of errors in diffusion kurtosis imaging caused by slice crosstalk in simultaneous multi‐slice imaging

Simultaneous multi‐slice (SMS) imaging techniques accelerate diffusion MRI data acquisition. However, slice separation is imperfect and results in residual signal leakage between the simultaneously excited slices. The resulting consistent bias may adversely affect diffusion model parameter estimation. Although this bias is usually small and might not affect the simplified diffusion tensor model significantly, higher order diffusion models such as kurtosis are likely to be more susceptible to such effects. In this work, two SMS reconstruction techniques and an alternative acquisition approach were tested to quantify the effects of slice crosstalk on diffusion kurtosis parameters. In reconstruction, two popular slice separation algorithms, slice GRAPPA and split‐slice GRAPPA, are evaluated to determine the effect of slice leakage on diffusion kurtosis metrics. For the alternative acquisition, the slice pairings were varied across diffusion weighted images such that the signal leakage does not come from the same overlapped slice for all diffusion encodings. Simulation results demonstrated the potential benefits of randomizing the slice pairings. However, various experimental factors confounded the advantages of slice pair randomization. In volunteer experiments, region‐of‐interest analyses found high metric errors with each of the SMS acquisitions and reconstructions in the brain white matter.     

Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart

Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field (B₁⁺) inhomogeneities. Parallel transmission MRI offers the ability to correct for both issues at the level of individual radiofrequency (RF) pulses, but must operate within strict hardware and safety constraints. The constraints are themselves affected by sequence parameters, such as the RF pulse duration and TR, meaning that an overall optimal operating point exists for a given sequence. This work seeks to obtain optimal performance by performing a ‘sequence‐level’ optimization in which pulse sequence parameters are included as part of an RF shimming calculation. The method is applied to balanced steady‐state free precession cardiac MRI with the objective of minimizing TR, hence reducing the imaging duration. Results are demonstrated using an eight‐channel parallel transmit system operating at 3 T, with an in vivo study carried out on seven male subjects of varying body mass index (BMI). Compared with single‐channel operation, a mean‐squared‐error shimming approach leads to reduced imaging durations of 32 ± 3% with simultaneous improvement in flip angle homogeneity of 32 ± 8% within the myocardium.     

Dynamic 2D and 3D mapping of hyperpolarized pyruvate to lactate conversion in vivo with efficient multi‐echo balanced steady‐state free precession at 3 T

The aim of this study was to acquire the transient MRI signal of hyperpolarized tracers and their metabolites efficiently, for which specialized imaging sequences are required. In this work, a multi‐echo balanced steady‐state free precession (me‐bSSFP) sequence with Iterative Decomposition with Echo Asymmetry and Least squares estimation (IDEAL) reconstruction was implemented on a clinical 3 T positron‐emission tomography/MRI system for fast 2D and 3D metabolic imaging. Simulations were conducted to obtain signal‐efficient sequence protocols for the metabolic imaging of hyperpolarized biomolecules. The sequence was applied in vitro and in vivo for probing the enzymatic exchange of hyperpolarized [1–¹³C]pyruvate and [1–¹³C]lactate. Chemical shift resolution was achieved using a least‐square, iterative chemical species separation algorithm in the reconstruction. In vitro, metabolic conversion rate measurements from me‐bSSFP were compared with NMR spectroscopy and free induction decay‐chemical shift imaging (FID‐CSI). In vivo, a rat MAT‐B‐III tumor model was imaged with me‐bSSFP and FID‐CSI. 2D metabolite maps of [1–¹³C]pyruvate and [1–¹³C]lactate acquired with me‐bSSFP showed the same spatial distributions as FID‐CSI. The pyruvate‐lactate conversion kinetics measured with me‐bSSFP and NMR corresponded well. Dynamic 2D metabolite mapping with me‐bSSFP enabled the acquisition of up to 420 time frames (scan time: 180‐350 ms/frame) before the hyperpolarized [1–¹³C]pyruvate was relaxed below noise level. 3D metabolite mapping with a large field of view (180 × 180 × 48 mm³) and high spatial resolution (5.6 × 5.6 × 2 mm³) was conducted with me‐bSSFP in a scan time of 8.2 seconds. It was concluded that Me‐bSSFP improves the spatial and temporal resolution for metabolic imaging of hyperpolarized [1–¹³C]pyruvate and [1–¹³C]lactate compared with either of the FID‐CSI or EPSI methods reported at 3 T, providing new possibilities for clinical and preclinical applications.     

Prostate MR elastography with transperineal electromagnetic actuation and a fast fractionally encoded steady‐state gradient echo sequence

Our aim is to develop a clinically viable, fast‐acquisition, prostate MR elastography (MRE) system with transperineal excitation. We developed a new actively shielded electromagnetic transducer, designed to enable quick deployment and positioning within the scanner. The shielding of the transducer was optimized using simulations. We also employed a new rapid pulse sequence that encodes the three‐dimensional displacement field in the prostate gland using a fractionally encoded steady‐state gradient echo sequence, thereby shortening the acquisition time to a clinically acceptable 8–10 min. The methods were tested in two phantoms and seven human subjects (six volunteers and one patient with prostate cancer). The MRE acquisition time for 24 slices, with an isotropic resolution of 2 mm and eight phase offsets, was 8 min, and the total scan, including positioning and set‐up, was performed in 15–20 min. The phantom study demonstrated that the transducer does not interfere with the acquisition process and that it generates displacement amplitudes that exceed 100 µm even at frequencies as high as 300 Hz. In the in vivo human study, average wave amplitudes of 30 µm (46 µm at the apex) were routinely achieved within the prostate gland at 70 Hz. No pain or discomfort was reported. Results in a single patient suggest that MRE can identify cancer tumors, although this result is preliminary. The proposed methods allow the integration of prostate MRE with other multiparametric MRI methods. The results of this study clearly motivate the clinical evaluation of transperineal MRE in patients. Copyright © 2014 John Wiley & Sons, Ltd.     

Characterizing intra‐axonal water diffusion with direction‐averaged triple diffusion encoding MRI

For large diffusion weightings, the direction‐averaged diffusion MRI (dMRI) signal from white matter is typically dominated by the contribution of water confined to axons. This fact can be exploited to characterize intra‐axonal diffusion properties, which may be valuable for interpreting the biophysical meaning of diffusion changes associated with pathology. However, using just the classic Stejskal‐Tanner pulse sequence, it has proven challenging to obtain reliable estimates for both the intrinsic intra‐axonal diffusivity and the intra‐axonal water fraction. Here we propose to apply a modification of the Stejskal‐Tanner sequence designed for achieving such estimates. The key feature of the sequence is the addition of a set of extra diffusion encoding gradients that are orthogonal to the direction of the primary gradients, which corresponds to a specific type of triple diffusion encoding (TDE) MRI sequence. Given direction‐averaged dMRI data for this TDE sequence, it is shown how the intra‐axonal diffusivity and the intra‐axonal water fraction can be determined by applying simple, analytic formulae. The method is illustrated with numerical simulations, which suggest that it should be accurate for b‐values of about 4000 s/mm² or higher.     

Precision and accuracy of diffusion kurtosis estimation and the influence of b‐value selection

Diffusion kurtosis imaging (DKI) is an extension of diffusion tensor imaging that accounts for leading non‐Gaussian diffusion effects. In DKI studies, a wide range of different gradient strengths (b‐values) is used, which is known to affect the estimated diffusivity and kurtosis parameters. Hence there is a need to assess the accuracy and precision of the estimated parameters as a function of b‐value. This work examines the error in the estimation of mean of the kurtosis tensor (MKT) with respect to the ground truth, using simulations based on a biophysical model for both gray (GM) and white (WM) matter. Model parameters are derived from densely sampled experimental data acquired in ex vivo rat brain and in vivo human brain. Additionally, the variability of MKT is studied using the experimental data. Prevalent fitting protocols are implemented and investigated. The results show strong dependence on the maximum b‐value of both net relative error and standard deviation of error for all of the employed fitting protocols. The choice of b‐values with minimum MKT estimation error and standard deviation of error was found to depend on the protocol type and the tissue. Protocols that utilize two terms of the cumulant expansion (DKI) were found to achieve minimum error in GM at b‐values less than 1 ms/μm², whereas maximal b‐values of about 2.5 ms/μm² were found to be optimal in WM. Protocols including additional higher order terms of the cumulant expansion were found to provide higher accuracy for the more commonly used b‐value regime in GM, but were associated with higher error in WM. Averaged over multiple voxels, a net average error of around 15% for both WM and GM was observed for the optimal b‐value choice. These results suggest caution when using DKI generated metrics for microstructural modeling and when comparing results obtained using different fitting techniques and b‐values.     

MRI DWI序列显示健康志愿者骶髂关节成像的最佳b值

背景：MRI DWI序列是一种简单易行、特异性较强、敏感度较高且能早期诊断活动性骶髂关节炎、监测其活动性的检查方法之一,但DWI序列对骶髂关节显示的最佳b值尚无统一标准。 目的：通过比较10-20岁健康志愿者骶髂关节MRI不同b值时DWI及表观弥散系数差异,寻找MRI DWI序列显示骶髂关节的最佳b值。  方法：随机选择21名10-20岁无骶髂关节疾病的健康志愿者作为研究对象,行骶髂关节轴位T1WI、STIR及DWI扫描(b值为0,300,600,900 s/mm²),观察不同b值对骶髂关节的显示情况,对图像进行评价;同时,分别测量双侧骶髂关节的骶侧、髂侧关节旁骨髓的表观弥散系数,对不同b值双侧骶髂关节旁骨髓表观弥散系数进行统计学分析。   结果与结论：21名健康志愿者42个骶髂关节在b值为300,600 s/mm²时,图像显示清晰,对比度好,能清晰显示骶髂关节关节。b值为900 s/mm²时,图像伪影较大,图像质量模糊,对比度差,无法完成表观弥散系数值的测量。b值为600 s/mm²时,双侧骶髂关节旁骨髓所测得的表观弥散系数值变化范围小,而b值为300 s/mm²时,表观弥散系数值差异性较大。结果显示600 s/mm²是MRI DWI序列显示骶髂关节的最适宜b值,不仅能清晰显示双侧骶髂关节,而且所测得关节旁骨髓的表观弥散系数值精确度较高。     

Multiband diffusion‐weighted MRI of the eye and orbit free of geometric distortions using a RARE‐EPI hybrid

Diffusion‐weighted imaging (DWI) provides information on tissue microstructure. Single‐shot echo planar imaging (EPI) is the most common technique for DWI applications in the brain, but is prone to geometric distortions and signal voids. Rapid acquisition with relaxation enhancement [RARE, also known as fast spin echo (FSE)] imaging presents a valuable alternative to DWI with high anatomical accuracy. This work proposes a multi‐shot diffusion‐weighted RARE‐EPI hybrid pulse sequence, combining the anatomical integrity of RARE with the imaging speed and radiofrequency (RF) power deposition advantage of EPI. The anatomical integrity of RARE‐EPI was demonstrated and quantified by center of gravity analysis for both morphological images and diffusion‐weighted acquisitions in phantom and in vivo experiments at 3.0 T and 7.0 T. The results indicate that half of the RARE echoes in the echo train can be replaced by EPI echoes whilst maintaining anatomical accuracy. The reduced RF power deposition of RARE‐EPI enabled multiband RF pulses facilitating simultaneous multi‐slice imaging. This study shows that diffusion‐weighted RARE‐EPI has the capability to acquire high fidelity, distortion‐free images of the eye and the orbit. It is shown that RARE‐EPI maintains the immunity to B₀ inhomogeneities reported for RARE imaging. This benefit can be exploited for the assessment of ocular masses and pathological changes of the eye and the orbit.     

Investigation of field and diffusion time dependence of the diffusion‐weighted signal at ultrahigh magnetic fields

Over the last decade, there has been a significant increase in the number of high‐magnetic‐field MRI magnets. However, the exact effect of a high magnetic field strength (B₀) on diffusion‐weighted MR signals is not yet fully understood. The goal of this study was to investigate the influence of different high magnetic field strengths (9.4 T and 14.1 T) and diffusion times (9, 11, 13, 15, 17 and 24 ms) on the diffusion‐weighted signal in rat brain white matter. At a short diffusion time (9 ms), fractional anisotropy values were found to be lower at 14.1 T than at 9.4 T, but this difference disappeared at longer diffusion times. A simple two‐pool model was used to explain these findings. The model describes the white matter as a first hindered compartment (often associated with the extra‐axonal space), characterized by a faster orthogonal diffusion and a lower fractional anisotropy, and a second restricted compartment (often associated with the intra‐axonal space), characterized by a slower orthogonal diffusion (i.e. orthogonal to the axon direction) and a higher fractional anisotropy. Apparent T₂ relaxation time measurements of the hindered and restricted pools were performed. The shortening of the pseudo‐T₂ value from the restricted compartment with B₀ is likely to be more pronounced than the apparent T₂ changes in the hindered compartment. This study suggests that the observed differences in diffusion tensor imaging parameters between the two magnetic field strengths at short diffusion time may be related to differences in the apparent T₂ values between the pools. Copyright © 2013 John Wiley & Sons, Ltd.     

Regulation of the tolerogenic function of steady‐state DCs

Dendritic cells (DCs) are master regulators of T‐cell responses. After sensing pathogen‐derived molecular patterns (PAMPs), or signals of inflammation and cellular stress, DCs differentiate into potent activators of naïve CD4⁺ and CD8⁺ T cells through a process that is termed DC maturation. By contrast, DCs induce and maintain peripheral T‐cell tolerance in the steady state, that is in the absence of overt infection or inflammation. However, the immunological steady state is not devoid of DC‐activating stimuli, such as commensal microorganisms, subclinical infections, or basal levels of proinflammatory mediators. In the presence of these activating stimuli, DC maturation must be calibrated to ensure self‐tolerance yet allow for adequate T‐cell responses to infections. Here, we review the factors that are known to control DC maturation in the steady state and discuss their effect on the tolerogenic function of steady‐state DCs.