Reproducibility of continuous arterial spin labeling perfusion MRI after 7 weeks

Background Continuous arterial spin labeling (CASL) is a non-invasive technique for the measurement of cerebral blood flow (CBF). The aim of the present study was to examine the reproducibility of CASL measurements and its suitability to consistently detect differences between groups, regions, and resting states. Materials and methods Thirty-eight healthy subjects (19 female) were examined at 1.5 T on two measurement occasions that were seven weeks apart. Resting CBF was measured with eyes open and eyes closed. Results In different regions of interest (ROIs) the repeatability estimates varied between 9 and 19 ml/100 g/min. There were no significant mean differences between occasions in all ROIs (P > 0.05). Greater CBF in the eyes-open than in the eyes-closed state was consistently present in the primary and secondary visual areas. Furthermore, CBF was consistently greater in the right than in the left hemisphere (P < 0.05) and differed between lobes and between arterial territories (P < 0.001). Finally, we consistently observed greater CBF in women than in men (P < 0.001). Conclusion This study demonstrates the suitability of CASL to consistently detect differences between groups, regions, and resting states even after seven weeks. This emphasizes its usefulness for longitudinal designs.     

Optimization of background suppression for arterial spin labeling perfusion imaging

Object  

To present an algorithm for optimization of background suppression pulse timing for arterial spin labeling (ASL) perfusion imaging.     

Temporal course of perfusion in human masseter muscle during isometric contraction assessed by arterial spin labeling at 3T

Object  

Examination of blood perfusion in the masseter muscle in the course of repetitive isometric contraction by arterial spin-labeling (ASL) MR imaging and additional T2 relaxation time measurements during and after masseter muscle activation.     

Impact of the inversion time on regional brain perfusion estimation with clinical arterial spin labeling protocols

Objective

Evaluating the impact of the Inversion Time (TI) on regional perfusion estimation in a pediatric cohort using Arterial Spin Labeling (ASL).

Materials and methods

Pulsed ASL (PASL) was acquired at 3 T both at TI 1500 ms and 2020 ms from twelve MRI-negative patients (age range 9–17 years). A volume of interest (VOIs) and a voxel-wise approach were employed to evaluate subject-specific TI-dependent Cerebral Blood Flow (CBF) differences, and grey matter CBF Z-score differences. A visual evaluation was also performed.

Results

CBF was higher for TI 1500 ms in the proximal territories of the arteries (PTAs) (e.g. insular cortex and basal ganglia — P < 0.01 and P < 0.05 from the VOI analysis, respectively), and for TI 2020 ms in the distal territories of the arteries (DTAs), including the watershed areas (e.g. posterior parietal and occipital cortex — P < 0.001 and P < 0.01 from the VOI analysis, respectively). Similar differences were also evident when analyzing patient-specific CBF Z-scores and at a visual inspection.

Conclusions

TI influences ASL perfusion estimates with a region-dependent effect. The presence of intraluminal arterial signal in PTAs and the longer arterial transit time in the DTAs (including watershed areas) may account for the TI-dependent differences. Watershed areas exhibiting a lower perfusion signal at short TIs (~ 1500 ms) should not be misinterpreted as focal hypoperfused areas.

     

Magnetic resonance spin-labeling perfusion imaging of synovitis in inflammatory arthritis at 3.0 T

Objective  The aim of this study was to test the feasibility of arterial spin labeling (ASL) perfusion imaging of synovitis in inflammatory joint diseases on a clinical 3.0 T whole-body scanner. Materials and methods  Fifteen patients (geometric mean 47 years, range 8–69 years) with different types of inflammatory arthritis of the finger or wrist joints participated in the study. In addition to conventional spin-echo and dynamic contrast-enhanced FLASH3D sequences, a novel spin-labeling technique (FAIR-TrueFISP) for quantitative assessment of tissue perfusion was applied. Perfusion maps were calculated pixel-wise by means of the extended Bloch equations. Results  Perfusion maps showed good image quality with clear visualization of hyperaemia in synovitis. The computed perfusion maps corresponded best to subtraction images of the dynamic series from 30 to 60 s after contrast-medium injection. The quantitative perfusion values of synovitis showed a good correlation with the disease activity. Perfusion values for inflamed synovium in phase of high activity were up to 230 ml/100 g tissue/min. Conclusion  The proposed modality allows for the assessment of disease activity in arthritis without the application of contrast-medium offering a new tool for therapy monitoring. As the technique provides quantitative information on hyperaemia, it potentially offers new insights in the pathophysiology of arthritic diseases.     

Comparison of 2D simultaneous multi-slice and 3D GRASE readout schemes for pseudo-continuous arterial spin labeling of cerebral perfusion at 3 T

Objective

In this perfusion magnetic resonance imaging study, the performances of different pseudo-continuous arterial spin labeling (PCASL) sequences were compared: two-dimensional (2D) single-shot readout with simultaneous multislice (SMS), 2D single-shot echo-planar imaging (EPI) and multishot three-dimensional (3D) gradient and spin echo (GRASE) sequences combined with a background-suppression (BS) module.

Materials and methods

Whole-brain PCASL images were acquired from seven healthy volunteers. The performance of each protocol was evaluated by extracting regional cerebral blood flow (rCBF) measures using an inline morphometric segmentation prototype. Image data postprocessing and subsequent statistical analyses enabled comparisons at the regional and sub-regional levels.

Results

The main findings were as follows: (i) Mean global CBF obtained across methods was were highly correlated, and these correlations were significantly higher among the same readout sequences. (ii) Temporal signal-to-noise ratio and gray-matter-to-white-matter CBF ratio were found to be equivalent for all 2D variants but lower than those of 3D-GRASE.

Discussion

Our study demonstrates that the accelerated SMS readout can provide increased acquisition efficiency and/or a higher temporal resolution than conventional 2D and 3D readout sequences. Among all of the methods, 3D-GRASE showed the lowest variability in CBF measurements and thus highest robustness against noise.

     

Non-invasive pulmonary perfusion assessment in young patients with cystic fibrosis using an arterial spin labeling MR technique at 1.5 T

Object  

To assess lung perfusion in young patients with cystic fibrosis (CF) using an arterial spin labeling (ASL) technique.     

Quantitative T 2 measurement of a single voxel with arbitrary shape using pinwheel excitation and CPMG acquisition

Objective The aim of this study is to present a new approach for making quantitative single-voxel T ₂ measurements from an arbitrarily shaped region of interest (ROI), where the advantage of the signal-to-noise ratio (SNR) per unit time of the single-voxel approach over conventional imaging approach can be achieved. Materials and methods Two-dimensional (2D) spatially selective radiofrequency (RF) pulses are proposed in this work for T ₂ measurements based on using interleaved spiral trajectories in excitation k-space (pinwheel excitation pulses), combined with a summed Carr–Purcell Meiboom–Gill (CPMG) echo acquisition. The technique is described and compared to standard multi-echo imaging methods, on a two-compartment water phantom and an excised brain tissue. Results The studies show good agreement between imaging and our method. The measured improvement factors of SNR per unit time of our single-voxel approach over imaging approach are close to the predicted values. Conclusion Measuring T ₂ relaxation times from a selected ROI of arbitrary shape using a single-voxel rather than an imaging approach can increase the SNR per unit time, which is critical for dynamic T ₂ or multi-component T ₂ measurements.     

Comparison of arterial transit times estimated using arterial spin labeling

Objective  

To compare arterial transit time estimates from two efficient transit time mapping techniques using arterial spin labeling (ASL)—flow encoded arterial spin tagging (FEAST) and Look-Locker ASL (LL-ASL). The effects of bipolar gradients and label location were investigated.     

The advantages of radial trajectories for vessel-selective dynamic angiography with arterial spin labeling

Objectives

To demonstrate the advantages of radial k-space trajectories over conventional Cartesian approaches for accelerating the acquisition of vessel-selective arterial spin labeling (ASL) dynamic angiograms, which are conventionally time consuming to acquire.

Materials and methods

Vessel-encoded pseudocontinuous ASL was combined with time-resolved balanced steady-state free precession (bSSFP) and spoiled gradient echo (SPGR) readouts to obtain dynamic vessel-selective angiograms arising from the four main brain-feeding arteries. Dynamic 2D protocols with acquisition times of one minute or less were achieved through radial undersampling or a Cartesian parallel imaging approach. For whole-brain dynamic 3D imaging, magnetic field inhomogeneity and the high acceleration factors required rule out the use of bSSFP and Cartesian trajectories, so the feasibility of acquiring 3D radial SPGR angiograms was tested.

Results

The improved SNR efficiency of bSSFP over SPGR was confirmed for 2D dynamic imaging. Radial trajectories had considerable advantages over a Cartesian approach, including a factor of two improvements in the measured SNR (p < 0.00001, N = 6), improved distal vessel delineation and the lack of a need for calibration data. The 3D radial approach produced good quality angiograms with negligible artifacts despite the high acceleration factor (R = 13).

Conclusion

Radial trajectories outperform conventional Cartesian techniques for accelerated vessel-selective ASL dynamic angiography.

     

Denoising of arterial spin labeling data: wavelet-domain filtering compared with Gaussian smoothing

Purpose  

To investigate a wavelet-based filtering scheme for denoising of arterial spin labeling (ASL) data, potentially enabling reduction of the required number of averages and the acquisition time.     

Non-parametric deconvolution using Bézier curves for quantification of cerebral perfusion in dynamic susceptibility contrast MRI

Objective

Deconvolution is an ill-posed inverse problem that tends to yield non-physiological residue functions R(t) in dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI). In this study, the use of Bézier curves is proposed for obtaining physiologically reasonable residue functions in perfusion MRI.

Materials and methods

Cubic Bézier curves were employed, ensuring R(0) = 1, bounded-input, bounded-output stability and a non-negative monotonically decreasing solution, resulting in 5 parameters to be optimized. Bézier deconvolution (BzD), implemented in a Bayesian framework, was tested by simulation under realistic conditions, including effects of arterial delay and dispersion. BzD was also applied to DSC-MRI data from a healthy volunteer.

Results

Bézier deconvolution showed robustness to different underlying residue function shapes. Accurate perfusion estimates were observed, except for boxcar residue functions at low signal-to-noise ratio. BzD involving corrections for delay, dispersion, and delay with dispersion generally returned accurate results, except for some degree of cerebral blood flow (CBF) overestimation at low levels of each effect. Maps of mean transit time and delay were markedly different between BzD and block-circulant singular value decomposition (oSVD) deconvolution.

Discussion

A novel DSC-MRI deconvolution method based on Bézier curves was implemented and evaluated. BzD produced physiologically plausible impulse response, without spurious oscillations, with generally less CBF underestimation than oSVD.

     

Use of spin echo T2 BOLD in assessment of cerebral misery perfusion at 1.5 T

Inadequate blood supply relative to metabolic demand, a haemodynamic condition termed as misery perfusion, often occurs in conjunction with acute ischaemic stroke. Misery perfusion results in adaptive changes in cerebral physiology including increased cerebral blood volume (CBV) and oxygen extraction ratio (OER) to secure substrate supply for the brain. It has been suggested that the presence of misery perfusion may be an indication of reversible ischaemia, thus detection of this condition may have clinical impact in acute stroke imaging. The ability of single spin echo T₂ to detect misery perfusion in the rat brain at 1.5 T owing to its sensitivity to blood oxygenation level dependent (BOLD) contrast was studied both theoretically and experimentally. Based on the known physiology of misery perfusion, tissue morphometry and blood relaxation data, T₂ behaviour in misery perfusion was simulated. The interpretation of these computations was experimentally assessed by quantifying T₂ in a rat model for cerebral misery perfusion. CBF was quantified with the H₂ clearance method. A drop of CBF from 58 ± 8 to 17 ± 3 ml/100 g min in the parieto-frontal cortex caused shortening of T₂. from 66.9 ± 0.4 to 64.6 ± 0.5 ms. Under these conditions, no change in diffusion MRI was detected. In contrast, the cortex with CBF of 42 ± 7 ml/100 g min showed no change in T₂. Computer simulations accurately predicted these T₂, responses. The present study shows that the acute drop of CBF by 70% causes a negative BOLD that is readily detectable by T₂ MRI at 1.5 T. Thus BOLD may serve as an index of misery perfusion thus revealing viable tissue with increased OER.     

Effects of systematic partial volume errors on the estimation of gray matter cerebral blood flow with arterial spin labeling MRI

Objective

Partial volume (PV) correction is an important step in arterial spin labeling (ASL) MRI that is used to separate perfusion from structural effects when computing the mean gray matter (GM) perfusion. There are three main methods for performing this correction: (1) GM-threshold, which includes only voxels with GM volume above a preset threshold; (2) GM-weighted, which uses voxel-wise GM contribution combined with thresholding; and (3) PVC, which applies a spatial linear regression algorithm to estimate the flow contribution of each tissue at a given voxel. In all cases, GM volume is obtained using PV maps extracted from the segmentation of the T1-weighted (T1w) image. As such, PV maps contain errors due to the difference in readout type and spatial resolution between ASL and T1w images. Here, we estimated these errors and evaluated their effect on the performance of each PV correction method in computing GM cerebral blood flow (CBF).

Materials and methods

Twenty-two volunteers underwent scanning using 2D echo planar imaging (EPI) and 3D spiral ASL. For each PV correction method, GM CBF was computed using PV maps simulated to contain estimated errors due to spatial resolution mismatch and geometric distortions which are caused by the mismatch in readout between ASL and T1w images. Results were analyzed to assess the effect of each error on the estimation of GM CBF from ASL data.

Results

Geometric distortion had the largest effect on the 2D EPI data, whereas the 3D spiral was most affected by the resolution mismatch. The PVC method outperformed the GM-threshold even in the presence of combined errors from resolution mismatch and geometric distortions. The quantitative advantage of PVC was 16% without and 10% with the combined errors for both 2D and 3D ASL. Consistent with theoretical expectations, for error-free PV maps, the PVC method extracted the true GM CBF. In contrast, GM-weighted overestimated GM CBF by 5%, while GM-threshold underestimated it by 16%. The presence of PV map errors decreased the calculated GM CBF for all methods.

Conclusion

The quality of PV maps presents no argument for the preferential use of the GM-threshold method over PVC in the clinical application of ASL.

     

Accelerated visualization of selected intracranial arteries by cycled super-selective arterial spin labeling

Objective

To accelerate super-selective arterial spin labeling (ASL) angiography by using a single control condition denoted as cycled super-selective arterial spin labeling.

Materials and methods

A single non-selective control image is acquired that is shared by selective label images. Artery-selective imaging is possible by geometrically changing the position of the labeling focus to more than one artery of interest during measurement. The presented approach is compared to conventional super-selective imaging in terms of its labeling efficiency inside and outside the labeling focus using numerical simulations and in vivo measurements. Additionally, the signal-to-noise ratios of the images are compared to non-selective ASL angiography and analyzed using a two-way ANOVA test and calculating the Pearson’s correlation coefficients.

Results

The results indicate that the labeling efficiency is not reduced within the labeled artery, but can increase as a function of distance to the artery of interest when compared to conventional super-selective ASL. In the final images, no statistically significant difference of image quality can be observed while the acquisition duration could be reduced when the major brain feeding arteries are being tagged.

Conclusion

Using super-selective arterial spin labeling, a single non-selective control acquisition suffices for reconstructing selective angiograms of the cerebral vasculature, thereby accelerating image acquisition of the major intracranial arteries without notable loss of information.

     

Influence of selecting EPI readout-encoding bandwidths on arterial spin labeling perfusion MRI

Object  

The objective of this study was to investigate effects of varying readout bandwidths on the arterial spin labeling (ASL)-perfusion MRI measurements at a high magnetic field MRI system.     

Blind detection of vascular sources and territories using random vessel encoded arterial spin labeling

Object  

The goal of this work is to use vessel encoded arterial spin labeling (VEASL) methods to detect feeding arteries without prior knowledge of their positions, and map the vascular territory of each.     

Quality assurance of PASADENA hyperpolarization for 13C biomolecules

Object  Define MR quality assurance procedures for maximal PASADENA hyperpolarization of a biological ¹³C molecular imaging reagent. Materials and methods  An automated PASADENA polarizer and a parahydrogen generator were installed. ¹³C enriched hydroxyethyl acrylate, 1-¹³C, 2,3,3-d₃ (HEA), was converted to hyperpolarized hydroxyethyl propionate, 1-¹³C, 2,3,3-d₃ (HEP) and fumaric acid, 1-¹³C, 2,3-d₂ (FUM) to hyperpolarized succinic acid, 1-¹³C, 2,3-d₂ (SUC), by reaction with parahydrogen and norbornadiene rhodium catalyst. Incremental optimization of successive steps in PASADENA was implemented. MR spectra and in vivo images of hyperpolarized ¹³C imaging agents were acquired at 1.5 and 4.7 T. Results  Application of quality assurance (QA) criteria resulted in incremental optimization of the individual steps in PASADENA implementation. Optimal hyperpolarization of HEP of P = 20% was achieved by calibration of the NMR unit of the polarizer (B ₀ field strength ± 0.002 mT). Mean hyperpolarization of SUC, P = [15.3 ± 1.9]% (N = 16) in D ₂O, and P = [12.8 ± 3.1]% (N = 12) in H ₂O, was achieved every 5–8 min (range 13–20%). An in vivo ¹³C succinate image of a rat was produced. Conclusion  PASADENA spin hyperpolarization of SUC to 15.3% in average was demonstrated (37,400 fold signal enhancement at 4.7 T). The biological fate of ¹³C succinate, a normally occurring cellular intermediate, might be monitored with enhanced sensitivity.     

Computer-algebra calculations and measurements on AB spin systems for double-spin-echo sequences

The time evolution of the density operator of an AB spin system during a double-spin-echo pulse sequence is evaluated analytically by a computer-algebra system. The computer-algebra system allows one to generate the extensive formulas describing the density operator and yields an expression for the integral of the spectral signals. The simulation of spectra for arbitrary sequence timings can be easily performed by this new tool without risking errors that might occur in conventional calculations. The computer-algebra method can be extended straightforward to other pulse angles and types of sequences. The double-spin-echo pulse sequence is used in the point-resolved spectroscopy (PRESS) method which is often applied for volume selective examinationsin vivo. For verification of the results generated by the computer-algebra system,¹H spectra from a half-liter spherical sample with an aqueous solution that was 0.1 M in sodium citrate and 0.1 M in sodium acetate were recorded after 90°-180°-180° double-spin-echo pulse sequences on a 1.5-T whole-body unit. The measured behavior of the citrate AB spin system corresponds very well with the theoretical predictions. Thus, the theory provides the basis for the optimization of sequence timings for double-spin-echo measurements with high signal gain from AB systems as, for example, citrate. In addition, the theoretically predicted signal modulations could be fitted to the experimental data, providing the transverse relaxation time of the AB-coupled protons.     

计及磁饱和的同步发电机内电势幅值测量方法

在直接测量转子位置获取同步发电机内电势相位的基础上,给出了同步发电机内电势幅值的准确直接计算方法,该方法的准确性只依赖于机端电压、机端电流、发电机功角等电气测量结果的准确性,与同步发电机参数无关。同时,基于内电势幅值和相位的准确测量,给出了能够计及磁路饱和影响的同步发电机定子与转子间互感电抗和同步电抗的计算方法,并基于正态分布参数估计理论,给出电抗计算结果的置信区间和点估计值,实际算例表明,所述方法准确可靠。