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.     

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.     

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.     

A novel method for simultaneous 3D B(1) and T(1) mapping: the method of slopes (MoS)

A novel three-dimensional simultaneous B(1) and T(1) mapping method is introduced: the method of slopes (MoS). The linearity of the spoiled gradient recalled echo (SPGR) signal vs flip angle relation is exploited: B(1) mapping is achieved by a two-point extrapolation to signal null with a correction scheme while T(1) mapping uses the slopes of the SPGR signal vs flip angle curves near the origin and near the signal null. This new method improves upon the existing variable flip angle (VFA) T(1)-mapping method in that (i) consistency between B(1) and T(1) maps is ensured (ii) the sampling scheme is T(1)-independent (iii) the noise bias and singularity, associated with using a linear form for the SPGR signal equation, is eliminated by using the full equation. The method is shown to yield accurate and robust results via simulations. Initial estimates of B(1) and T(1) values are obtained from three data points via simple computations and straight line approximations. Initial estimates of B(1) values, for a range of values, are shown to be accurate due to the proposed B(1) correction scheme. The accuracy and robustness of T(1) values is achieved via a non-linear fitting algorithm which includes a fourth data point sampled at high SNR. The MoS was validated by comparing resulting B(1) and T(1) maps with those obtained using other standard methods. Finally, the ability to obtain brain B(1) and T(1) maps using the MoS was demonstrated by in vivo experiments. The MoS is expected to perform well on other motion-free anatomical regions as well.     

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.     

Triple‐echo steady‐state T2 relaxometry of the human brain at high to ultra‐high fields

Quantitative MRI techniques, such as T₂ relaxometry, have demonstrated the potential to detect changes in the tissue microstructure of the human brain with higher specificity to the underlying pathology than in conventional morphological imaging. At high to ultra‐high field strengths, quantitative MR‐based tissue characterization benefits from the higher signal‐to‐noise ratio traded for either improved resolution or reduced scan time, but is impaired by severe static (B₀) and transmit (B₁) field heterogeneities. The objective of this study was to derive a robust relaxometry technique for fast T₂ mapping of the human brain at high to ultra‐high fields, which is highly insensitive to B₀ and B₁ field variations. The proposed method relies on a recently presented three‐dimensional (3D) triple‐echo steady‐state (TESS) imaging approach that has proven to be suitable for fast intrinsically B₁‐insensitive T₂ relaxometry of rigid targets. In this work, 3D TESS imaging is adapted for rapid high‐ to ultra‐high‐field two‐dimensional (2D) acquisitions. The achieved short scan times of 2D TESS measurements reduce motion sensitivity and make TESS‐based T₂ quantification feasible in the brain. After validation in vitro and in vivo at 3 T, T₂ maps of the human brain were obtained at 7 and 9.4 T. Excellent agreement between TESS‐based T₂ measurements and reference single‐echo spin‐echo data was found in vitro and in vivo at 3 T, and T₂ relaxometry based on TESS imaging was proven to be feasible and reliable in the human brain at 7 and 9.4 T. Although prominent B₀ and B₁ field variations occur at ultra‐high fields, the T₂ maps obtained show no B₀‐ or B₁‐related degradations. In conclusion, as a result of the observed robustness, TESS T₂ may emerge as a valuable measure for the early diagnosis and progression monitoring of brain diseases in high‐resolution 2D acquisitions at high to ultra‐high fields. Copyright © 2014 John Wiley & Sons, Ltd.     

Rapid simultaneous acquisition of T1 and T2 mapping images using multishot double spin‐echo EPI and automated variations of TR and TE (ms‐DSEPI‐T12)

A rapid method of simultaneous T₁ and T₂ measurement is presented which uses a segmented echo‐planar readout with varying repetition times (TR) and echo times (TE). This method is useful in T₁ mapping for analysis of dynamic contrast enhanced MRI (DCE‐MRI), where T₁ can be used to estimate contrast agent concentration. In the application of this method to dynamic imaging, the equilibrium magnetization is measured on pre‐contrast images and incorporated into post‐contrast T₁ calculations for improved accuracy. Simultaneous T₂ measurement allows correction of T₂ effects in the T₁ map which may occur at high contrast agent concentrations, and is performed without significant imaging time penalty. Phantom and in vivo results show the usefulness of this technique for analysis of contrast enhancement kinetics. Accurate rapid contrast agent concentration measurement may be useful for analyzing the distribution and kinetics of contrast agents or labeled pharmaceuticals. Copyright © 2009 John Wiley & Sons, Ltd.     

The quest for high spatial resolution diffusion‐weighted imaging of the human brain in vivo

Diffusion‐weighted imaging, a contrast unique to MRI, is used for assessment of tissue microstructure in vivo. However, this exquisite sensitivity to finer scales far above imaging resolution comes at the cost of vulnerability to errors caused by sources of motion other than diffusion motion. Addressing the issue of motion has traditionally limited diffusion‐weighted imaging to a few acquisition techniques and, as a consequence, to poorer spatial resolution than other MRI applications. Advances in MRI imaging methodology have allowed diffusion‐weighted MRI to push to ever higher spatial resolution. In this review we focus on the pulse sequences and associated techniques under development that have pushed the limits of image quality and spatial resolution in diffusion‐weighted MRI.     

Two‐dimensional spectroscopic imaging with combined free induction decay and long‐TE acquisition (FID echo spectroscopic imaging,FIDESI) for the detection of intramyocellular lipids in calf muscle at 7 T

The aim of this study was to introduce a two‐dimensional chemical shift imaging (2D CSI) sequence, with simultaneous acquisition of free induction decay (FID) and long TEs, for the detection and quantification of intramyocellular lipids (IMCLs) in the calf at 7 T. The feasibility of the new 2D CSI sequence, which acquires FID (acquisition delay, 1.3 ms) and an echo (long TE) in one measurement, was evaluated in phantoms and volunteers (n = 5): TR/TE*/TE = 800/1.3/156 ms; 48 × 48 matrix; field of view, 200 × 200 × 20 mm³; Hamming filter; no water suppression; measurement time, 22 min 2 s. The IMCL concentration and subcutaneous lipid contamination were assessed. Spectra in the tibialis anterior (TA), gastrocnemius (GM) and soleus (SOL) muscles were analyzed. The water signal from the FID acquisition was used as an internal concentration reference. In the spectra from subcutaneous adipose tissue (SUB) and bone marrow (BM), an unsaturation index (UI) of the vinyl‐H (5.3 ppm) to methyl‐CH₃ ratio, and a polyunsaturation index (pUI) of the diallylic‐H (2.77 ppm) to ‐CH₃ ratio, were calculated. Long‐TE spectra from muscles showed a simplified spectral pattern with well‐separated IMCL for several muscle groups in the same scan. The IMCL to water ratio was largest in SOL (0.66% ± 0.23%), and lower in GM (0.37% ± 0.14%) and TA (0.36% ± 0.12%). UI and pUI for SUB were 0.65 ± 0.06 and 0.18 ± 0.04, respectively, and for BM were 0.60 ± 0.16 and 0.18 ± 0.08, respectively. The new sequence, with the proposed name ‘free induction decay echo spectroscopic imaging’ (FIDESI), provides information on both specific lipid resonances and water signal from different tissues in the calf, with high spectral and spatial resolution, as well as minimal voxel bleeding and subcutaneous lipid contamination, in clinically acceptable measurement times. Copyright © 2014 John Wiley & Sons, Ltd.