MRI of spontaneous fluctuations after acute cerebral ischemia in nonhuman primates

PURPOSE: To study the spontaneous low-frequency blood oxygenation level-dependent (BOLD) functional MRI (fMRI) signal fluctuations during hyperacute focal cerebral ischemia. MATERIALS AND METHODS: A stroke model in nonhuman primates (macaques) was used in this study. Spontaneous fluctuations were recorded using a series of gradient-recalled echo (GRE) echo-planar imaging (EPI) images. Fast Fourier transformation (FFT) was performed on the serial EPI data to calculate the frequency and magnitude of the spontaneous fluctuations. Diffusion tensor imaging (DTI) and perfusion-weighted imaging (PWI) were preformed to detect the ischemic lesion. RESULTS: The frequency of these fluctuations decreased in the periinfarct tissue in the ipsilateral hemisphere, while their magnitude increased. This area of abnormal signal fluctuations often extended beyond the hyperacute diffusion/perfusion abnormality. CONCLUSION: This study suggests that measurement of the spontaneous fMRI signal fluctuations provides different information than is available from diffusion/perfusion or T2-weighted MRI.     

复发性与首发性抑郁症的静息态脑功能MRI低频振幅初步研究

目的 基于低频振幅（ALFF）方法研究复发性抑郁症（RDE）和首发性抑郁症（FDE）脑功能活动改变特征。 方法 前瞻性纳入2家医院确诊的20例RDE病人、18例FDE病人,同期纳入性别、年龄相匹配的20例健康志愿者作为对照组（HC组）。3组受试者均行静息态功能MRI（rs-fMRI）检查,并完成汉密顿抑郁量表（HAMD-17）、汉密尔顿焦虑量表（HAMA-14）、焦虑自评量表（SAS）、抑郁自评量表（SDS）、中文版冗思量表（RRS）评定。采用Matlab平台上的DPARSF 5.0工具包进行rs-fMRI数据预处理,采用单因素方差分析比较3组间ALFF差异,并对结果进行高斯随机场（GRF）校正。提取3组间ALFF有差异脑区的时间序列均值（ALFF值）,对ALFF值的事后两两比较采用LSD-t检验,所得结果进行Bonferroni校正（P<0.016）。最后对3组间有统计学差异脑区的ALFF值与临床量表评分进行Pearson相关分析。 结果 RDE组与FDE组的HAMD-17、HAMA-14、SDS、SAS、RRS评分均高于HC组（均P<0.05）,RDE组的SDS、SAS评分高于FDE组（均P<0.05）。与FDE组相比,RDE组左侧杏仁核、右侧海马的ALFF值增高,而右侧额中回、左侧三角部额下回的ALFF值减低（均P<0.016）。与HC组相比,RDE组左侧楔前叶、右侧舌回的ALFF值减低（均P<0.016）,FDE组右侧额中回、左侧三角部额下回的ALFF值增高,而FDE组在左侧楔前叶、左侧杏仁核、右侧海马、右侧舌回的ALFF值减低（均P<0.016）。相关性分析显示,RDE组右侧海马ALFF值与HAMA-14量表评分呈负相关（r=-0.53,P=0.014）。 结论 RDE与FDE病人的ALFF值差异脑区与前额叶-边缘神经环路密切相关,尤其在右侧额中回、左侧三角部额下回、左侧杏仁核、右侧海马表现明显,这有助于区别RDE组和FDE组的神经病理机制差异。     

Task-modulation of functional synchrony between spontaneous low-frequency oscillations in the human brain detected by fMRI.

Recent developments in functional MRI (fMRI) technology with high spatial and temporal resolution have made it possible to noninvasively detect spontaneous low-frequency oscillations (SLOs) and quantify their functional synchrony in the human brain. In the present fMRI study the dynamic characteristics of the functional synchrony between SLOs were quantitatively determined by the phase shift index (PSI). With the use of an fMRI-guided voxel-selection method, the SLOs and their functional synchrony were found to be modulated by different memory tasks. The results demonstrate that SLOs in episodic memory-related circuitry have significantly higher synchrony during the performance of declarative memory-encoding activities compared to nondeclarative memory-encoding activities. It is suggested that the dynamic property of SLOs and the quantitative assessment of their functional synchrony could be utilized as a biomarker to noninvasively characterize localized pathophysiological functions in the human brain.     

Spatiotemporal dynamics of low frequency fluctuations in BOLD fMRI of the rat

Purpose

To examine spatiotemporal dynamics of low frequency fluctuations in rat cortex.

Materials and Methods

Gradient‐echo echo‐planar imaging images were acquired from anesthetized rats (repetition time = 100 ms). Power spectral analysis was performed to detect different frequency peaks. Functional connectivity maps were obtained for the frequency peaks of interest. The images in the filtered time‐series were displayed as a movie to study spatiotemporal patterns in the data for frequency bands of interest.

Results

High temporal and spectral resolution allowed separation of primary components of physiological noise and visualization of spectral details. Two low frequency peaks with distinct characteristics were observed. Selective visualization of the second low frequency peak revealed waves of activity that typically began in the secondary somatosensory cortex and propagated to the primary motor cortex.

Conclusion

To date, analysis of these fluctuations has focused on the detection of functional networks assuming steady state conditions. These results suggest that detailed examination of the spatiotemporal dynamics of the low frequency fluctuations may provide more insight into brain function, and add a new perspective to the analysis of resting state fMRI data. J. Magn. Reson. Imaging 2009;30:384–393. © 2009 Wiley‐Liss, Inc.     

Investigating the effects of low dose alcohol on neural timing using functional MRI

Purpose:

To examine human brain function related to the perception of short time intervals before and after ingesting a low dosage of alcohol (0.25 g/kg).

Materials and Methods:

The experiment used a novel paradigm that required participants to view a virtual traffic‐light (TL) stimulus while estimating the length of a short time interval (Timing task) and counting several flashes (Counting task). The influence of alcohol was evaluated by measuring behavioral performance in terms of accuracy and reaction time as well as simultaneously measuring changes in the blood oxygenation level‐dependent (BOLD) signal using functional MRI.

Results:

Our results indicated that, for both the Pre‐Drink and Post‐Drink conditions, the left cerebellum, right inferior parietal lobe, right insula, and medial frontal gyrus, revealed greater BOLD signal increases for Timing than for Counting. In the Pre‐Drink state, the Timing task demonstrated increased BOLD signal changes relative to the Counting task in the bilateral prefrontal cortex, left insula, right SMA, and in the left ventrolateral thalamus. Most notably, the right superior parietal lobe (BA 5/7) showed a BOLD signal increase in the Post‐Drink state for both the Counting and Timing tasks, thus possibly suggesting the recruitment of additional resources to sustain accurate neural timing.

Conclusion:

Understanding the harmful impact of short‐term alcohol administration on tasks that heavily rely on accurate temporal processing will hopefully contribute to the long‐term prevention of its unfortunate and deleterious consequences. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Perfusion imaging using spin-labeling methods: contrast-to-noise comparison in functional MRI applications.

In this study the performance of FLASH imaging with selective inversion preparation for functional perfusion studies was investigated. In addition to the absolute quantification of perfusion by measurement of the longitudinal relaxation times with global (T(1glob)) and selective (T(1sel)) inversion, the measurement of absolute (BASE) and relative (FAIR) perfusion increases by subtraction of appropriately weighted images was also considered. The subject averages of absolute perfusion obtained by the quantitative method were 70.7 +/- 4.0 ml/100g/min in gray matter, 10.2 +/- 3.4 ml/100g/min in white matter, and 89.0 +/- 3.1 ml/100g/min in visual cortex. These values, as well as the average increase of perfusion due to visual stimulation (44.4 +/- 3.7 ml/100g/min), agree well with respective data reported by PET and other MRI studies. However, for individual subjects the standard deviations over single ROIs inside the visual cortex lay around 100% which prevented the detection of significant activation. BASE and FAIR, on the other hand, were able to detect significant activation in single subjects. The measured average perfusion increases were 51.7 +/- 6.6 ml/100g/min and 56.5 +/- 13.8%, respectively. Magn Reson Med 46:172-182, 2001.     

New strategy for reconstructing partial-Fourier imaging data in functional MRI.

Most partial Fourier (PF) approaches use a low-resolution phase estimate in the reconstruction to account for non-zero phases of the image. These methods may fail when there are large phase errors, a situation commonly encountered in T(*)(2)-weighted functional MRI (fMRI). To mitigate this problem, a method was developed based on the inversion of a matrix formulated according to a phase map derived from iterative reconstruction. To make this method computationally practical for fMRI, a strategy was introduced such that the matrix inversion is performed only once for each slice in the time series, assuming that the phase map remains constant in the time series. To ensure the temporal phase invariance, physiological noise correction and global phase correction were applied to the data before the reconstruction. This method was demonstrated to be robust and efficient for fMRI. Magn Reson Med 46:1045-1048, 2001.     

The correlation between blood oxygenation level-dependent signal strength and latency

PURPOSE: To investigate the relationship between signal strength and latency of the blood oxygenation level-dependent (BOLD) signal. MATERIALS AND METHODS: Several correlation analyses were performed on data obtained in a functional magnetic resonance imaging (fMRI) experiment, where subjects were presented with a simple visual stimulus. The BOLD signal strength was correlated with both the phase shift of the spectral density matrix and time-to-peak calculated from trial-averaged time courses. Correlation coefficients were calculated for visual stimuli of 2, 6, and 15 seconds in duration. RESULTS: Analyzing all functional runs for the same subject separately, i.e., including for each run all significantly activated voxels, we observed that correlations between phase shift and signal strength, as well as between time-to-peak and signal strength, decreased with increasing stimulus length. However, when analyses were restricted to voxels found activated in all functional runs, we observed similar correlations between BOLD signal strength and latency in all runs, independent of the length of stimulation. This result was again obtained for both latency measures: the spectral density phase shift and time-to-peak. CONCLUSION: For both latency measures, phase shift and time-to-peak, a high correlation between BOLD signal strength and latency was observed. We have shown that this correlation is independent of the length of visual stimulation. Thus, the correlation between BOLD signal strength and latency seems to be an inherent property of the BOLD response that is independent of the length of stimulation and can be observed using different methods for determining signal latency.     

Limitations of temporal resolution in functional MRI

In fMRI, images can be collected in a very short time; therefore, high temporal resolution is possible in principle. However, the temporal resolution is limited by a blurred intrinsic hemodynamic response and a finite signal-to-noise ratio. To determine the upper limit of temporal resolution in a single area during repeated tasks, motor cortex activity was investigated during visually instructed finger movements. Without averaging, a sequence of four single-finger movements with an execution time of approximately 2 s can be resolved when the delay time between consecutive sequences is at least 3 s. The hemodynamic response time is constant for each subject, but not among different subjects. The temporal resolution can be better when the signal from spatially distinct regions is examined. For a series of experiments involving a visually instructed delayed cued finger movement task with a well-defined, independently determined, variable delay time, time courses in the motor area are distinct from each other in two experiments if the difference in delay time is as little as 2 s. The activation in the visual area due to the presentation of the task serves here as an internal time reference. By comparing a set of fMRI time courses in multiple distinct areas, serial neural processing may be investigated.