准连续性动脉自旋标记技术在磁共振系统上的序列实现和参数优化

准连续性动脉自旋标记技术(pCASL)是一种新兴的动脉自旋标记脑灌注成像技术(ASL)：一方面,它克服了连续性动脉自旋标记技术(CASL)需要独立发射线圈的硬件限制;另一方面,也避免了脉冲式动脉自旋标记技术(PASL)带来的标记效率低的影响．为了在 1.5 T 磁共振系统上开发一款可稳定应用于临床扫描的 pCASL 序列;并使用该序列准确获得反
应灌注功能的局部脑血流量值(Regional Cerebral Blood Flow, rCBF)．该文利用水模测试pCASL 序列,验证了标记部分的标记性能并通过人体实验,优化了协议中标记位置中心到成像层面中心的距离和标记部分结束点到成像脉冲开始前的等待时间这两项参数．基于优化了参数的 pCASL 协议,扫描 12 组正常志愿者,观测灌注信号分布情况,并对特定灰质区域定量计算,对比不同个体该区域的 rCBF 值．通过人体实验,经验性地确定了延迟时间为 1 200 ms、标记距离为 70 mm 时灌注图像的信噪比达到最优．将两项优化后的参数存入协议中,并使用协议扫描,共获取 12 组结果,其中的 10 组都表明灌注信号稳定均匀,并且灰质区域的 CBF 值同经验结果一致．该工作在1.5 T 的磁共振系统上成功实现了 pCASL序列,经优化参数后的协议扫描,可以获得准确稳定的脑部灌注信号．
     

Test-retest reliability and reproducibility of long-label pseudo-continuous arterial spin labeling

PurposeArterial spin labeling MRI can quantify the cerebral blood flow (CBF) without exogenous tracer. However, the variation of arterial transit time across different brain regions introduces bias for measuring local CBF, especially for those subjects with long arterial transit time (ATT). Long post-labeling delay (PLD) or multi-PLD methods could mitigate the problem of heterogenous ATT at the expense of the signal-to-noise ratio (SNR). Long-label ASL might address the low SNR problem by increasing the amount of labeled arterial blood. Thus, we hypothesized that with the same relatively long PLD, long-label pCASL may be more robust and reproducible than standard-label pCASL in population with potentially prolonged ATT. The purpose of the study was to investigate the reliability and reproducibility of long-label pCASL in the whole brain and vascular regions of interest in an elderly population, compared with standard-label pCASL.MethodTwenty adult volunteers (14 males, 6 females; age, 56.6 ± 17.2 years) were scanned twice on one 3.0 T scanner by standard-label pCASL (label duration (LD) = 1500 ms, PLD = 2000 ms) and long-label pCASL (LD = 3500 ms, PLD = 2000 ms). The intraclass correlation coefficient (ICC), within-subject coefficient of variation (wsCV), random noise and signal coefficient of variation(CoV) were used to assess global and regional reliability and reproducibility. Measurement agreement and difference were compared in different brain regions using correlation coefficient plots and Bland-Altman plots respectively.ResultsCBF value measured by long-label pCASL was overall higher than standard-label pCASL in all ROIs. Long-label pCASL had higher ICC than standard-label pCASL in most ROIs, and lower wsCV, random noise and CoV in all ROIs. Regardless of ASL method used, anterior circulation flow territories (ICC, 0.93–0.97; wsCV, 0.03–0.06) had higher CBF reliability and reproducibility than posterior circulation flow territories (ICC, 0.89; wsCV, 0.06–0.08). In all ROIs, the correlation analysis showed higher test-retest agreement (r_long-label > r_{standard-label}) and the Bland-Altman plots demonstrated lower measurement difference in long-label pCASL.ConclusionThe study demonstrated good reliability and reproducibility of long-label pCASL in anterior brain regions in the elderly population. To further improve CBF quantification in a long-ATT population while proper PLD is already used, increasing the label duration may help.     

Reduced distortion artifact whole brain CBF mapping using blip-reversed non-segmented 3D echo planar imaging with pseudo-continuous arterial spin labeling

PurposeTo implement and evaluate interleaved blip-up, blip-down, non-segmented 3D echo planar imaging (EPI) with pseudo-continuous arterial spin labeling (pCASL) and post-processing for reduced susceptibility artifact cerebral blood flow (CBF) maps.Materials and methods3D EPI non-segmented acquisition with a pCASL labeling sequence was modified to include alternating k-space coverage along phase encoding direction (referred to as “blip-reversed”) for alternating dynamic acquisitions of control and label pairs. Eight volunteers were imaged on a 3T scanner. Images were corrected for distortion using spatial shifting transformation of the underlying field map. CBF maps were calculated and compared with maps obtained without blip reversal using matching gray matter (GM) images from a high resolution 3D scan. Additional benefit of using the correction for alternating blip-up and blip-down acquisitions was assessed by comparing to corrected blip-up only and corrected blip-down only CBF maps. Matched Student t-test of overlapping voxels for the eight volunteers was done to ascertain statistical improvement in distortion.ResultsMean CBF value in GM for the eight volunteers from distortion corrected CBF maps was 50.8 ± 9.9 ml/min/100 gm tissue. Corrected CBF maps had 6.3% and 4.1% more voxels in GM when compared with uncorrected blip up (BU) and blip down (BD) images, respectively. Student t-test showed significant reduction in distortion when compared with blip-up images and blip-down images (p < 0.001). When compared with corrected BU and corrected BD only CBF maps, BU and BD corrected maps had 2.3% and 1% more voxels (p = 0.006 and 0.04, respectively).ConclusionPseudo-continuous arterial spin labeling with non-segmented 3D EPI acquisition using alternating blip-reversed k-space traversal and distortion correction provided significantly better matching GM CBF maps. In addition, employing alternating blip-reversed acquisitions during pCASL acquisition resulted in statistically significant improvement over corrected blip-up and blip-down CBF maps.     

Quantification of cerebral blood flow by bolus tracking and artery spin tagging methods

This study deals with perfusion quantification in healthy volunteers using two types of dynamic magnetic resonance imaging (MRI) methods. Absolute cerebral blood flow (CBF) measurements were performed in 11 subjects by applying both bolus tracking of exogenous contrast agent and non-invasive arterial spin labeling MRI techniques. Both methods produced CBF images with good tissue contrast and CBF values are in good agreement with literature data. The correlation between cerebral blood volume (CBV) and CBF is also discussed.     

Model-free arterial spin labelling for cerebral blood flow quantification: introduction of regional arterial input functions identified by factor analysis

PURPOSE: To identify regional arterial input functions (AIFs) using factor analysis of dynamic studies (FADS) when quantification of perfusion is performed using model-free arterial spin labelling. MATERIAL AND METHODS: Five healthy volunteers and one patient were examined on a 3-T Philips unit using quantitative STAR labelling of arterial regions (QUASAR). Two sets of images were retrieved, one where the arterial signal had been crushed and another where it was retained. FADS was applied to the arterial signal curves to acquire the AIFs. Perfusion maps were obtained using block-circulant SVD deconvolution and regional AIFs obtained by FADS. In the volunteers, the ASL experiment was repeated within 24 h. The patient was also examined using dynamic susceptibility contrast MRI. RESULTS: In the healthy volunteers, CBF was 64+/-10 ml/[min 100 g] (mean+/-S.D.) in GM and 24+/-4 ml/[min 100 g] in WM, while the mean aBV was 0.94% in GM and 0.25% in WM. DISCUSSION: Good CBF image quality and reasonable quantitative CBF values were obtained using the combined QUASAR/FADS technique. We conclude that FADS may be a useful supplement in the evaluation of ASL data using QUASAR.     

Comparison of pulsed arterial spin labeling encoding schemes and absolute perfusion quantification

Arterial spin labeling (ASL) using magnetic resonance imaging (MRI) is a powerful noninvasive technique to investigate the physiological status of brain tissue by measuring cerebral blood flow (CBF). ASL assesses the inflow of magnetically labeled arterial blood into an imaging voxel. In the last 2 decades, various ASL sequences have been proposed which differ in their ease of implementation and their sensitivity to artifacts. In addition, several quantification methods have been developed to determine the absolute value of CBF from ASL magnetization difference images. In this study, we evaluated three pulsed ASL sequences and three absolute quantification schemes. It was found that FAIR-QUIPSSII implementation of ASL yields 10–20% higher signal-to-noise ratio (SNR) and 18% higher CBF as compared with PICORE-Q2TIPS (with FOCI pulses) and PICORE-QUIPSSII (with BASSI pulses). In addition, quantification schemes employed can give rise to up to a 35% difference in CBF values. We conclude that, although all quantitative ASL sequences and CBF calibration methods should in principle result in the similar CBF values and image quality, substantial differences in CBF values and SNR were found. Thus, comparing studies using different ASL sequences and analysis algorithms is likely to result in erroneous intra- and intergroup differences. Therefore, (i) the same quantification schemes should consistently be used, and (ii) quantification using local tissue proton density should yield the most accurate CBF values because, although still requiring definitive demonstration in future studies, the proton density of blood is assumed to be very similar to the value of gray matter.     

Assessment of cerebral blood perfusion reserve with acetazolamide using 3D spiral ASL MRI: Preliminary experience in pediatric patients

PurposeTo demonstrate the clinical feasibility of a new non-Cartesian cylindrically-distributed spiral 3D pseudo-continuous arterial spin labeling (pCASL) magnetic resonance imaging (MRI) pulse sequence in pediatric patients in quantifying cerebral blood flow (CBF) response to an acetazolamide (ACZ) vasodilator challenge.Materials and methodsMRI exams were performed on two 3 Tesla Philips Ingenia systems using 32 channel head coil arrays. After local institutional review board approval, the 3D spiral-based pCASL technique was added to a standard brain MRI exam and evaluated in 13 pediatric patients (average age: 11.7 ± 6.4 years, range: 1.4–22.2 years). All patients were administered ACZ for clinically indicated reasons. Quantitative whole-brain CBF measurements were computed pre- and post-ACZ to assess cerebrovascular reserve.Results3D spiral pCASL data were successfully reconstructed in all 13 cases. In 11 patients, CBF increased 2.8% to 93.2% after administration of ACZ. In the two remaining patients, CBF decreased by 2.4 to 6.0% after ACZ. The group average change in CBF due to ACZ was approximately 25.0% and individual changes were statistically significant (p < 0.01) in all patients using a paired t-test analysis. CBF perfusion data were diagnostically useful in supporting conventional MR angiography and clinical findings.Conclusion3D cylindrically-distributed spiral pCASL MRI provides a robust approach to assess cerebral blood flow and reserve in pediatric patients.     

Validation study of a pulsed arterial spin labeling technique by comparison to perfusion computed tomography

Abnormalities in cerebral blood flow (CBF) are believed to play a significant role in the development of major neonatal neuropathologies. One approach that would appear ideal for measuring CBF in this fragile age group is arterial spin labeling (ASL) since ASL techniques are noninvasive and quantitative. The purpose of this study was to assess the accuracy of a pulsed ASL method implemented on a 3-T scanner dedicated to neonatal imaging. Cerebral blood flow was measured in nine neonatal piglets, the ASL–CBF measurements were acquired at two inversion times (TI) (1200 and 1700 ms), and CBF was measured by perfusion computed tomography (pCT) for validation. Perfusion CT also provided images of cerebral blood volume, which were used to identify large blood vessels, and contrast arrival time, which were used to assess differences in arterial transit times between gray and white matter. Good agreement was found between gray matter CBF values from pCT (76±1 ml/min per 100 g) and ASL at TI=1700 ms (73±1 ml/min per 100 g). At TI=1200 ms, ASL overestimated CBF (91±2 ml/min per 100 g), which was attributed to substantial intravascular signal. No significant differences in white matter CBF from pCT and ASL were observed (average CBF=60±1 ml/min per 100 g), nor was there any difference in contrast arrival times for gray and white matter (0.95±0.04 and 0.99±0.03 s, respectively), which suggests that the arterial transit times for ASL were the same in this animal model. This study verified the accuracy of the implemented ASL technique and showed the value of using pCT to study other factors that can affect ASL–CBF measurements.     

Absolute quantification of cerebral blood flow: correlation between dynamic susceptibility contrast MRI and model-free arterial spin labeling

Purpose

To compare absolute cerebral blood flow (CBF) estimates obtained by model-free arterial spin labeling (ASL) and dynamic susceptibility contrast MRI (DSC-MRI), corrected for partial volume effects (PVEs).

Methods

CBF was measured using DSC-MRI and model-free ASL (quantitative signal targeting with alternating radiofrequency labeling of arterial regions) at 3 T in 15 subjects with brain tumor, and the two modalities were compared with regard to CBF estimates in normal gray matter (GM) and DSC-to-ASL CBF ratios in selected tumor regions. The DSC-MRI CBF maps were calculated using a global arterial input function (AIF) from the sylvian-fissure region, but, in order to minimize PVEs, the AIF time integral was rescaled by a venous output function time integral obtained from the sagittal sinus.

Results

In GM, the average DSC-MRI CBF estimate was 150±45 ml/(min 100 g) (mean±SD) while the corresponding ASL CBF was 44±10 ml/(min 100 g). The linear correlation between GM CBF estimates obtained by DSC-MRI and ASL was r=.89, and observed DSC-to-ASL CBF ratios differed by less than 3% between GM and tumor regions.

Conclusions

A satisfactory positive linear correlation between the CBF estimates obtained by model-free ASL and DSC-MRI was observed, and DSC-to-ASL CBF ratios showed no obvious tissue dependence.     

Intracranial vascular feature changes in time of flight MR angiography in patients undergoing carotid revascularization surgery

PurposeTo characterize the intracranial vascular features extracted from time of flight (TOF) images and their changes from baseline to follow-up in patients undergoing carotid revascularization, using arterial spin labeling (ASL) cerebral blood flow (CBF) measurement as a reference.MethodsIn this retrospective study, brain TOF and ASL images of 99 subjects, acquired before, within 48 h, and/or 6 months after, carotid revascularization surgery were analyzed. TOF images were analyzed using a custom software (iCafe) to quantify intracranial vascular features, including total vessel length, total vessel volume, and number of branches. Mean whole-brain CBF was calculated from ASL images. ASL scans showing low ASL signal in the entire flow territory of an internal carotid artery (ICA), which may be caused by labeling failure, were excluded. Changes and correlations between time points were analyzed separately for TOF intracranial vascular features and ASL CBF.ResultsSimilar to ASL CBF, TOF vascular features (i.e. total vessel length, total vessel volume and number of branches) increased dramatically from baseline to post-surgery, then returned to a level slightly higher than the baseline in long-term follow-up (All P < 0.05). Correlation between time points was observed for all three TOF vascular features but not for ASL CBF.ConclusionIntracranial vascular features, including total vessel length, total vessel volume and number of branches, extracted from TOF images are useful in detecting brain blood flow changes induced by carotid revascularization surgery.     

Pulsed arterial spin labeling perfusion imaging at 3 T: estimating the number of subjects required in common designs of clinical trials

Pulsed arterial spin labeling (PASL) is an increasingly common technique for noninvasively measuring cerebral blood flow (CBF) and has previously been shown to have good repeatability. It is likely to find a place in clinical trials and in particular the investigation of pharmaceutical agents active in the central nervous system. We aimed to estimate the sample sizes necessary to detect regional changes in CBF in common types of clinical trial design including (a) between groups, (b) a two-period crossover and (3) within-session single dosing. Whole brain CBF data were acquired at 3 T in two independent groups of healthy volunteers at rest; one of the groups underwent a repeat scan. Using these data, we were able to estimate between-groups, between-session and within-session variability along with regional mean estimates of CBF. We assessed the number of PASL tag-control image pairs that was needed to provide stable regional estimates of CBF and variability of regional CBF across groups. Forty tag-control image pairs, which take approximately 3 min to acquire using a single inversion label delay time, were adequate for providing stable CBF estimates at the group level. Power calculations based on the variance estimates of regional CBF measurements suggest that comparatively small cohorts are adequate. For example, detecting a 15% change in CBF, depending on the region of interest, requires from 7-15 subjects per group in a crossover design, 6-10 subjects in a within-session design and 20-41 subjects in a between-groups design. Such sample sizes make feasible the use of such CBF measurements in clinical trials of drugs.     

Nonlinear responses of cerebral blood volume, blood flow and blood oxygenation signals during visual stimulation

Hemodynamic-based functional magnetic resonance imaging (fMRI) techniques provide a great utility for noninvasive functional brain mapping. However, because the hemodynamic signals reflect underlying neural activity indirectly, characterization of these signals following brain activation is essential for experimental design and data interpretation. In this report, the linear (or nonlinear) responses to neuronal activation of three hemodynamic parameters based primarily on changes of cerebral blood volume, blood flow and blood oxygenation were investigated by testing these hemodynamic responses' additivity property. Using a recently developed fMRI technique that acquires vascular space occupancy (VASO), arterial spin labeling (ASL) perfusion and blood oxygenation level-dependent (BOLD) signals simultaneously, the additivity property of the three hemodynamic responses in human visual cortex was assessed using various visual stimulus durations. Experiments on healthy volunteers showed that all three hemodynamic-weighted signals responded nonlinearly to stimulus durations less than 4 s, with the degree of nonlinearity becoming more severe as the stimulus duration decreased. Vascular space occupancy and ASL perfusion signals showed similar nonlinearity properties, whereas the BOLD signal was the most nonlinear. These data suggest that caution should be taken in the interpretation of hemodynamic-based signals in fMRI.     

Spin labeling artery method perfusion MRI study of SPG4 and SCA3/MJD

Background

Spinocerebellar ataxia type 3 (SCA3) and Machado–Joseph disease (MJD) are similar diseases that are often referred to jointly as SCA3/MJD. As the most common autosomal-dominantly inherited subtype of hereditary spastic paraplegia (HSP), HSP4 (or SPG4) has overlapping symptoms with SCA3/MJD, which hinders their diagnoses. Arterial spin labeling (ASL) is a noninvasive, contrast-agent free, magnetic resonance perfusion imaging method used to obtain maps of the cerebral blood flow (CBF). Here, we investigated the diagnostic value of ASL in SCA3/MJD and SPG4 patients.

Methods

A total of 13 SPG4 cases, 38 SCA3/MJD cases (22 onset patients and 16 genetic abnormality-only patients), and 27 healthy volunteers were examined by ASL. Data were processed to obtain the regional CBF (rCBF) and comparatively studied.

Results

In the pons, cerebellar dentate nucleus, and cerebellar cortex, rCBF of the onset SCA3/MJD group was significantly lower than that of the normal control group. In the cerebellar dentate nucleus and cerebellar cortex, the rCBF of the non-onset SCA3/MJD group was significantly lower than that of the control group. In the pons and cerebellar cortex, the rCBF of the onset SCA3/MJD group was significantly lower than that of the SPG4 group.

Conclusions

SCA3/MJD lesions are mainly located in the cerebellum and brainstem. Gray matter and white matter were both involved, although the deep cerebellar nuclei may be the earliest involved region. Cerebellar and brainstem lesions of SCA3/MJD were more severe than those of SPG4. ASL can aid the diagnosis of SCA3/MJD and SPG4.     

Denoising arterial spin labeling perfusion MRI with deep machine learning

PurposeArterial spin labeling (ASL) perfusion MRI is a noninvasive technique for measuring cerebral blood flow (CBF) in a quantitative manner. A technical challenge in ASL MRI is data processing because of the inherently low signal-to-noise-ratio (SNR). Deep learning (DL) is an emerging machine learning technique that can learn a nonlinear transform from acquired data without using any explicit hypothesis. Such a high flexibility may be particularly beneficial for ASL denoising. In this paper, we proposed and validated a DL-based ASL MRI denoising algorithm (DL-ASL).MethodsThe DL-ASL network was constructed using convolutional neural networks (CNNs) with dilated convolution and wide activation residual blocks to explicitly take the inter-voxel correlations into account, and preserve spatial resolution of input image during model learning.ResultsDL-ASL substantially improved the quality of ASL CBF in terms of SNR. Based on retrospective analyses, DL-ASL showed a high potential of reducing 75% of the original acquisition time without sacrificing CBF measurement quality.ConclusionDL-ASL achieved improved denoising performance for ASL MRI as compared with current routine methods in terms of higher PSNR, SSIM and Radiologic scores. With the help of DL-ASL, much fewer repetitions may be prescribed in ASL MRI, resulting in a great reduction of the total acquisition time.     

磁振灌流造影:对“流动敏感交互反转恢复”的评论(英文)

磁振造影在过去的数十年內取得了长足的进步,除了可提供生物解剖构造的资讯外,如今更可以进行组织灌流造影.磁振灌流造影主要可分成2种:动态磁感对比(Dynamic susceptibility Contrast)和动脉标记(Arterial Spin Labeling) .相较于动态磁感对比,动脉标记能非侵入性地观测灌流.动脉标记包括了数种技术:如CASL (continu-ous arterial spinlabeling) ,EPISTAR(echo planar imaging and signal targeting with al-ternating radiofrequency) ,PICORE(proxi mal inversion with a control for off-resonance effects)和FAIR(flow-sensitive alternating inversion recovery) .该文主要提供流动磁感交互反转恢复(FAIR)技术的综合介绍,包括其理论基础和实践,特别针对T1法在FAIR定量上的使用.定量上的困难亦将会在文章中被讨论.文章的最后总结FAIR之实际应用情形.     

Atlas-based and DTI-guided quantification of human brain cerebral blood flow: Feasibility,quality assurance,spatial heterogeneity and age effects

Accurate and noninvasive quantification of regional cerebral blood perfusion (CBF) of the human brain tissue would advance the study of the complex interplay between human brain structure and function, in both health and disease. Despite the plethora of works on CBF in gray matter, a detailed quantitative white matter perfusion atlas has not been presented on healthy adults using the International Consortium for Brain Mapping atlases. In this study, we present a host of assurance measures such as temporal stability, spatial heterogeneity and age effects of regional and global CBF in selected deep, cortical gray matter and white matter tracts identified and quantified using diffusion tensor imaging (DTI). We utilized whole brain high-resolution DTI combined with arterial spin labeling to quantify regional CBF on 15 healthy adults aged 23.2–57.1 years. We present total brain and regional CBF, corresponding volume, mean diffusivity and fractional anisotropy spatial heterogeneity, and dependence on age as additional quality assurance measures to compare with published trends using both MRI and nuclear medicine methods. Total CBF showed a steady decrease with age in gray matter (r=?0.58; P= .03), whereas total CBF of white matter did not significantly change with age (r= 0.11; P= .7). This quantitative report offers a preliminary baseline of CBF, volume and DTI measurements for the design of future multicenter and clinical studies utilizing noninvasive perfusion and DT-MRI.     

Evaluation of B0-correction of relative CBF maps using tagging distance dependent Z-spectrum (TADDZ)

Arterial spin labeling (ASL) MRI, based on endogenous contrast from blood water, is used in research and diagnosis of cerebral vascular conditions. However, artifacts due to imperfect imaging conditions such as B₀-inhomogeneity (ΔB₀) could lead to variations in the quantification of relative cerebral blood flow (CBF). In this study, we evaluate a new approach using tagging distance dependent Z-spectrum (TADDZ) data, similar to the ΔB₀ corrections in the chemical exchange saturation transfer (CEST) experiments, to remove the imaging plane B₀ inhomogeneity induced CBF artifacts in ASL MRI. Our results indicate that imaging-plane B₀-inhomogeneity can lead to variations and errors in the relative CBF maps especially under small tagging distances. Along with an acquired B₀ map, TADDZ data helps to eliminate B₀-inhomogeneity induced artifacts in the resulting relative CBF maps. We demonstrated the effective use of TADDZ data to reduce variation while subjected to systematic changes in ΔB₀. In addition, TADDZ corrected ASL MRI, with improved consistency, was shown to outperform conventional ASL MRI by differentiating the subtle CBF difference in Alzheimer's disease (AD) mice brains with different APOE genotypes.     

Balanced steady state free precession for arterial spin labeling MRI: Initial experience for blood flow mapping in human brain,retina, and kidney

We implemented pseudo-continuous ASL (pCASL) with 2D and 3D balanced steady state free precession (bSSFP) readout for mapping blood flow in the human brain, retina, and kidney, free of distortion and signal dropout, which are typically observed in the most commonly used echo-planar imaging acquisition. High resolution functional brain imaging in the human visual cortex was feasible with 3D bSSFP pCASL. Blood flow of the human retina could be imaged with pCASL and bSSFP in conjunction with a phase cycling approach to suppress the banding artifacts associated with bSSFP. Furthermore, bSSFP based pCASL enabled us to map renal blood flow within a single breath hold. Control and test–retest experiments suggested that the measured blood flow values in retina and kidney were reliable. Because there is no specific imaging tool for mapping human retina blood flow and the standard contrast agent technique for mapping renal blood flow can cause problems for patients with kidney dysfunction, bSSFP based pCASL may provide a useful tool for the diagnosis of retinal and renal diseases and can complement existing imaging techniques.     

Improving cerebral blood flow quantification for arterial spin labeled perfusion MRI by removing residual motion artifacts and global signal fluctuations

Denoising is critical to improving the quality and stability of cerebral blood flow (CBF) quantification in arterial spin labeled (ASL) perfusion magnetic resonance imaging (MRI) due to the intrinsic low signal-to-noise-ratio (SNR) of ASL data. Previous studies have been focused on reducing the spatial or temporal noise using standard filtering techniques, and less attention has been paid to two global nuisance effects, the residual motion artifacts and the global signal fluctuations. Since both nuisances affect the whole brain, removing them in advance should enhance the CBF quantification quality for ASL MRI. The purpose of this paper was to assess this potential benefit. Three methods were proposed to suppress each or both of the two global nuisances. Their performances for CBF quantification were validated using ASL data acquired from 13 subjects. Evaluation results showed that covarying out both global nuisances significantly improved temporal SNR and test-retest stability of CBF measurement. Although the concept of removing both nuisances is not technically novel per se, this paper clearly showed the benefits for ASL CBF quantification. Dissemination of the proposed methods in a free ASL data processing toolbox should be of interest to a broad range of ASL users.     

Quantification of cerebral blood flow in nonhuman primates using arterial spin labeling and a two-compartment model

Noninvasive absolute quantification of cerebral blood flow (CBF) with high spatial resolution is still a challenging task. Arterial spin labeling (ASL) is a promising magnetic resonance imaging (MRI) method for accurate perfusion quantification. However, modeling of ASL data is far from being standardized and has not been investigated in great detail. In this study, two-compartment modeling of monkey ASL data in three physiological conditions (baseline, sensory activated and globally elevated CBF) is reported. Absolute perfusion and arterial transit times were derived for gray matter (GM) and white matter (WM) separately. The uncertainties of the model's result were determined by Monte Carlo simulations. The fitted CBF values for GM were 133 ml/min/100 ml at baseline condition, 165 ml/min/100 ml during visual stimulation and 234 ml/min/100 ml for globally elevated CBF after intravenous injection of acetazolamide. The ratio of GM to WM CBF was 2.5 at baseline and was found to decrease to 1.6 after application of acetazolamide. The corresponding arterial transit times decreased from 742 to 607 ms in GM and from 985 to 875 ms in WM. Monte Carlo simulations showed that absolute CBF values can be determined with an error of 11-15%, while the arterial transit time values have a coefficient of variation of 25-31%. With an alternative acquisition scheme, the precision of the arterial transit times can be improved significantly. The CBF values in the occipital lobe of the monkey brain quantified with ASL are higher than previously reported in positron emission tomography studies.