Improving influx constant and ratio estimation in FDOPA brain PET analysis for Parkinson's disease.

PET studies using l-3,4-dihydroxy-6-(18)F-fluorophenylalanine have been applied to assess the diminished functionality of the striatum in patients with suspected Parkinson's disease. Two techniques for analyzing such studies are ratio methods and graphically computed influx constants. We propose a method for improving the consistency with which scans obtained by either of these techniques are analyzed. The method is based on a fully 3-dimensional analysis of the images. METHODS: Fifty-one dynamically acquired datasets were corrected for motion before analysis. Regions of interest (ROIs) for the analysis were determined by manual affine registration to a standard template, using a separate transformation for each ROI. Indicator values for each ROI were computed by averaging the values of voxels having the highest activity within a specified proportion of the voxels in the ROI, to increase the robustness to perturbations in the ROI position. Sensitivity was analyzed by examining the variation in results obtained when the ROIs were translated by up to 6 mm. RESULTS: We observed significant percentage differences in the computed influx constant before and after motion correction (mean variation +/- SD, -0.75% +/- 9.5% averaged over all regions of all patients). Our method was robust to placement of the cerebellum ROI, whereas a 2- dimensional analysis based on hand-drawn ROIs was associated with a 2- to 3-fold greater percentage variation in uptake for translations of 2 mm or more in ROI position. When we compared the 2 quantification techniques, our influx constants and ratios correlated at r(2) = 0.91, P < 0.0001. CONCLUSION: Motion correction is an important step for computing reliable results in dynamic studies. The robustness of the results can be increased further by using standard normalized volumetric ROIs and by using the average value of a specified proportion of the voxels with the highest activity in the ROI as an indicator for that ROI. Influx constant values computed using our analysis technique closely correlated to values computed with ratio methods using this general approach.     

Complex data analysis in high-resolution SSFP fMRI.

In transition-band steady-state free precession (SSFP) functional MRI (fMRI), functional contrast originates from a bulk frequency shift induced by a deoxygenated hemoglobin concentration change in the activated brain regions. This frequency shift causes a magnitude and/or phase-signal change depending on the off-resonance distribution of a voxel in the balanced-SSFP (bSSFP) profile. However, in early low-resolution studies, only the magnitude signal activations were shown. In this paper the task-correlated phase-signal change is presented in a high-resolution (1 x 1 x 1 mm3) study. To include this phase activation in a functional analysis, a new complex domain data analysis method is proposed. The results show statistically significant phase-signal changes in a large number of voxels comparable to that of the magnitude-activated voxels. The complex-data analysis method successfully includes these phase activations in the activation map and thus provides wider coverage compared to magnitude-data analysis results.     

膝关节类风湿关节炎MRI与X线平片对比

目的　评价MRI与X线平片在膝关节类风湿关节炎中的诊断价值。方法　 34个膝关节行MRI与X线检查 ,13例进行了MRI增强扫描。结果　MRI显示 34个膝均存在骨破坏 ,2 1个膝骨硬化 ,2 2个半月板有信号改变 ,15个膝关节软骨信号改变 ,10个膝胫骨移位 ,8个膝后交叉韧带拉直 ,13例MRI增强扫描显示血管翳附着、滑膜增殖和关节积液。X线显示 1个膝有骨破坏 ,11个膝伴骨硬化 ,16个膝关节间隙狭窄 ,8个膝胫骨移位。结论　MRI诊断膝关节类风湿关节炎优于X线平片     

BOLD Signal in memory paradigms in hippocampal region depends on echo time

Purpose:

To evaluate the hypothesis that the entire hippocampus might be affected by susceptibility artifacts. Previous studies described susceptibility artifacts in the amygdala and the anterior hippocampus.

Materials and Methods:

We investigated 20 subjects with a verbal memory paradigm aiming at testing two different TEs (45 vs. 64 msec) at 1.5 T for hippocampal blood oxygenation level‐dependent (BOLD) activity. T2* maps were calculated from the normalized mean echo‐planar imaging (EPI) of the two echo times (TEs).

Results:

Within the hippocampal region of interest (ROI), the amount of suprathreshold voxels was significantly higher at TE = 64 msec compared to TE = 45 msec. When corrected for multiple comparisons (family‐wise error [FWE] in a small volume of interest, P < 0.05) we no longer found significant activations at TE = 45 msec, while a significant number of voxels remained after the small volume correction (P < 0.05, FWE) within the ROI at TE = 64 msec.

Conclusion:

Although a shorter TE demonstrates advantages, a TE of 45 msec leads to a significant loss of BOLD signal detection in memory functional magnetic resonance imaging (fMRI) studies when compared to 64 msec. We assume that the hippocampal region, even the anterior part, is not strongly affected by susceptibility gradients. J. Magn. Reson. Imaging 2013;37:1064–1071. © 2012 Wiley Periodicals, Inc.     

低场MRI对膝关节痛风的诊断价值

目的:探讨低场MRI对膝关节痛风性关节炎的诊断价值。方法:回顾性分析21例(28膝)经病理或临床随访证实的膝关节痛风的低场MRI资料,并进行总结。结果:21例(28膝)均有不同程度的关节腔积液和滑膜增生。痛风结节累及韧带24膝,累及肌腱24膝;发生骨质侵蚀21膝;单纯骨髓水肿7膝;小囊变影3膝;弧形压迹18膝;发生在髌下脂肪垫内8膝。结论:膝关节痛风性关节炎的低场MRI表现多样。痛风结节形成后,其特征性的低场MRI表现可为该病的及时诊治提供可靠、有价值的影像学依据。     

Assessment of Cortical Visual Impairment in Infants with Periventricular Leukomalacia: a Pilot Event-Related fMRI Study

Objective

We wanted to investigate the usefulness of event-related (ER) functional MRI (fMRI) for the assessment of cortical visual impairment in infants with periventricular leukomalacia (PVL).

Materials and Methods

FMRI data were collected from 24 infants who suffered from PVL and from 12 age-matched normal controls. Slow ER fMRI was performed using a 3.0T MR scanner while visual stimuli were being presented. Data analysis was performed using Statistical Parametric Mapping software (SPM2), the SPM toolbox MarsBar was used to analyze the region of interest data, and the time to peak (TTP) of hemodynamic response functions (HRFs) was estimated for the surviving voxels. The number of activated voxels and the TTP values of HRFs were compared. Pearson correlation analysis was performed to compare visual impairment evaluated by using Teller Acuity Cards (TAC) with the number of activated voxels in the occipital lobes in all patients.

Results

In all 12 control infants, the blood oxygenation level-dependent (BOLD) signal was negative and the maximum response was located in the anterior and superior part of the calcarine fissure, and this might correspond to the anterior region of the primary visual cortex (PVC). In contrast, for the 24 cases of PVL, there were no activated pixels in the PVC in four subjects, small and weak activations in six subjects, deviated activations in seven subjects and both small and deviated activations in three subjects. The number of active voxels in the occipital lobe was significantly correlated with the TAC-evaluated visual impairment (p < 0.001). The mean TTP of the HRFs was significantly delayed in the cases of PVL as compared with that of the normal controls.

Conclusion

Determining the characteristics of both the BOLD response and the ER fMRI activation may play an important role in the cortical visual assessment of infants with PVL.     

High-resolution chemical shift imaging for the assessment of intramuscular lipids.

A new high-resolution MRSI technique was used to measure extracellular lipids (EMCL), intracellular lipids (IMCL), and total muscle lipids (TML). The purpose of this study was to assess the feasibility and reproducibility of this new technique. This study also compared results obtained from small regions of interest (ROIs) vs. a summation of a large ROI of voxels representing the total soleus or anterior tibialis (TA) muscles. Eight volunteers were studied with the use of a conventional single-slice MR spectroscopic imaging (MRSI) sequence run with the following parameters: TR = 145.9 ms, FOV = 16 cm, slice thickness = 1 cm, and 64 x 64 phase encodes. EMCL, IMCL, and TML values from the small ROIs proved to be reproducible (coefficient of variation (CV) = 7.8-13.8% for soleus, and 8.2-18% for TA). EMCL, IMCL, and TML values from the larger soleus ROI proved to be reproducible (CV = 7.3-16.1%), whereas the larger TA ROIs were less reproducible. The small and larger soleus ROIs produced statistically equivalent measures of EMCL and TML per unit area. However, the small soleus and TA ROIs showed a trend toward yielding different IMCL contents as compared to the larger ROIs. This study demonstrates that high-resolution 1H MRSI of the calf muscle is feasible and can reproducibly measure EMCL, IMCL, and TML.     

Fetal sheep brains: findings at functional blood oxygen level-dependent 3-T MR imaging--relationship to maternal oxygen saturation during hypoxia

PURPOSE: To quantify the dependence of the signal intensity (SI) at blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging of fetal sheep brains on maternal oxygen saturation and to investigate the influence of positions of regions of interest (ROIs). MATERIALS AND METHODS: All experimental protocols were reviewed and approved by the local authorities on animal protection. The brains of singleton fetuses of five anesthetized sheep were subjected to rapid sequences (single-shot echo-planar imaging) of BOLD measurements with a 3-T MR imaging unit. Maternal oxygen saturation and heart rate were recorded continuously. After a normoxic phase, hypoxia was induced by reducing the oxygen in a ventilated gas mixture. ROIs were placed in the cerebrum at a reference level and in the cerebellum. Normalized BOLD SI values were calculated from the mean values of steady-state BOLD SIs at the control (SI(c)) and hypoxic (SI(h)) plateaus as follows: normalized BOLD SI = (SI(h)/ SI(c)) x 100. Normalized BOLD SI values were correlated with maternal oxygen saturation, and linear regression (slope) analysis was performed. Additionally, ROIs were varied in section level and position. Differences in normalized BOLD SI values for ROI placements were calculated by using analysis of variance. A t test was performed to evaluate differences. RESULTS: Mean maternal oxygen saturation (as the percentage of oxygen in the blood) was 88% (95% confidence interval [CI]: 80%, 96%) in the control period. During hypoxia, it was reduced to 62% (95% CI: 50%, 75%), while fetal normalized BOLD SI decreased to 64% (95% CI: 44%, 85%) in the cerebrum and 56% (95% CI: 32%, 80%) in the cerebellum. Correlations between normalized BOLD SI values and maternal oxygen saturation were as follows: r2 = 0.84 and slope = 1.27 (95% CI: 1.17, 1.36) in the cerebrum and r2 = 0.83 and slope = 1.54 (95% CI: 1.44, 1.63) in the cerebellum. Normalized BOLD SI was 4% lower in the section above the reference level. Variations in normalized BOLD SI for different ROI positions ranged between 0% and 12%. CONCLUSION: The depletion of oxygen supply is reflected by decreases in fetal brain BOLD SIs that are more distinct in the cerebellum than in the cerebrum. Normalized BOLD SI is influenced only slightly by ROI position.     

Macrophage infiltration into the rat knee detected by MRI in a model of antigen-induced arthritis.

Three-dimensional (3D) MR images were obtained from the knees of rats in a model of antigen-induced arthritis, elicited by the intraarticular administration of methylated bovine serum albumin (mBSA) to previously immunized rats. Superparamagnetic particles of iron oxide (SPIO) were administered i.v. 24 hr before each imaging session. Starting 4 days postantigen injection, images from arthritic knees exhibited distinctive signal attenuation in the synovium. This signal attenuation was significantly smaller in knees from animals treated with dexamethasone, a glucocorticosteroid, and completely absent in contralateral knees that had been challenged with vehicle. A significant negative correlation was found between the MRI signal intensity in the synovium and the histologically determined iron content in macrophages located in the same region. These results suggest the feasibility of detecting macrophage infiltration into the knee synovium in this model of antigen-induced arthritis by labeling the cells with SPIO. This readout could provide an early marker of disease progression, before more aggressive changes like cartilage and bone erosion take place. Monitoring early changes associated with arthritis can have an impact in preclinical studies by shortening the duration of the experimental period and by facilitating the investigation of novel immunomodulatory therapies acting on macrophages. Also, the approach can be potentially adapted to clinical studies.     

The Voxel Sensitivity Function in Fourier Transform Imaging: Applications to Magnetic Resonance Angiography

In this paper the problem of small structure visualization in magnetic resonance imaging (MRI) is considered. The relationship between the structure and the image intensities is defined in terms of the voxel sensitivity function (VSF). Using the VSF, the spatial dependence of the voxel signal for small spheres and cylinders is computed. Although the spatial fluctuation is smaller in the MRI VSF than that which would be obtained from a uniformly sensitive cubical voxel, the deviation still results in significant signal loss near the edges and corners of the voxels. Finally, the VSF formalism is used to demonstrate the improvement in signal uniformity that can be obtained by using zero-filled (band-limited or sinc) interpolation.     

Impact of size of region-of-interest on differentiation of renal cell carcinoma and renal cysts on multi-phase CT: Preliminary findings

Introduction

To assess impact of size of regions-of-interest (ROI) on differentiation of RCC and renal cysts using multi-phase CT, with focus on differentiating papillary RCC (pRCC) and cysts given known hypovascularity of pRCC.

Methods

99 renal lesions (23 pRCC, 47 clear-cell RCC, 7 chromophobe RCC, 22 cysts) underwent multi-phase CT. Subjective presence of visual enhancement was recorded for each lesion. Whole-lesion (WL) ROIs, and small (≤5 mm²), medium (average size of small and large ROIs), and large (half of lesion diameter) peripherally located partial-lesion (PL) ROIs, were placed on non-contrast and nephrographic phases. Impact of ROI size in separating cysts from all RCC and from pRCC based on increased attenuation between phases was assessed using ROC analysis.

Results

Visual enhancement was perceived in 96% of ccRCC, 61% of pRCC, and 9% of cysts. AUCs for separating all RCC and cysts for WL-ROI and small, medium, and large PL-ROIs were 91%, 96%, 91% and 93%, and among lesions without visible enhancement were 60%, 79%, 67% and 67%. AUCs for separating pRCC and cysts for WL-ROI and small, medium, and large PL-ROIs were 78%, 92%, 82% and 84%, and among lesions without visible enhancement were 64%, 88%, 69% and 69%.

Conclusion

Small PL-ROIs had higher accuracy than WL-ROI or other PL-ROIs in separating RCC from cysts, with greater impact in differentiating pRCC from cysts and differentiating lesions without visible enhancement. Thus, when evaluating renal lesions using multi-phase CT, we suggest placing small peripheral ROIs for highest accuracy in distinguishing renal malignancy and benign cysts.     

Physiologic characterization of inflammatory arthritis in a rabbit model with BOLD and DCE MRI at 1.5 Tesla

Objective

Our aim was to test the feasibility of blood oxygen level dependent magnetic resonance imaging (BOLD MRI) and dynamic contrast-enhanced (DCE) MRI to monitor periarticular hypoxic/inflammatory changes over time in a juvenile rabbit model of arthritis.

Methods

We examined arthritic and contralateral nonarthritic knees of 21 juvenile rabbits at baseline and days 1,14, and 28 after induction of arthritis by unilateral intra-articular injection of carrageenin with BOLD and DCE MRI at 1.5 Tesla (T). Nine noninjected rabbits served as controls. Associations between BOLD and DCE-MRI and corresponding intra-articular oxygen pressure (PO₂) and blood flow [blood perfusion units (BPU)] (polarographic probes, reference standards) or clinical–histological data were measured by correlation coefficients.

Results

Percentage BOLD MRI change obtained in contralateral knees correlated moderately with BPU on day 0 (r?=??0.51, p?=?0.02) and excellently on day 28 (r?=??0.84, p?=?0.03). A moderate correlation was observed between peak enhancement DCE MRI (day 1) and BPU measurements in arthritic knees (r?=?0.49, p?=?0.04). In acute arthritis, BOLD and DCE MRI highly correlated (r?=?0.89, p?=?0.04; r?=?1.0, p?Conclusion The proposed techniques are feasible to perform at 1.5 T, and they hold potential as surrogate measures to monitor hypoxic and inflammatory changes over time in arthritis at higher-strength MRI fields.

Key points

? BOLD and DCE MRI detect interval perisynovial changes in a rabbit knee ? BOLD and DCE MRI act as surrogate markers of physiologic changes in arthritis ? BOLD MRI signal represents oxygen extraction compared with intra-articular PO ₂ ? DCE MRI measurements estimate physiologic periarticular vascular properties ? In rabbit knees with acute arthritis, BOLD/DCE MRI highly correlated with histological scores     

fMRI of human olfaction at the individual level: Interindividual variability

Purpose:

To determine whether the range of normal variation of human olfactory functional magnetic resonance imaging (fMRI) activations in healthy single subjects is compatible with the detection of atypical patterns.

Materials and Methods:

In an event‐related olfactory experiment, the variability of fMRI activation in six bilateral olfactory areas known to be affected in neurodegenerative diseases was measured in a region of interest (ROI) analysis in terms of intensity, localization, and overlap on 51 subjects. fMRI measurements were compared against measurements from a visual experiment performed on 25 subjects.

Results:

Olfaction induced activations with low intensity, high variability, and a 4‐fold lower contrast‐to‐noise ratio (CNR) than vision. Even in the best case (piriform cortex), mean pairwise activation overlap was still less than 40%. None of the olfactory ROIs showed significant activation for all subjects at the permissive threshold of P < 0.001. A gender‐dependent significantly stronger activation was found in the bilateral piriform cortex of male subjects.

Conclusion:

Linking t‐statistics and CNR showed that for all olfactory ROIs, CNR is either near or below the estimated threshold of 0.73 found to be necessary to obtain significant activations. In our experimental conditions the low reliability of olfactory activations should prompt major reservations over using fMRI of human olfaction as a diagnostic tool in single subjects. J. Magn. Reson. Imaging 2013;37:92–100. © 2012 Wiley Periodicals, Inc.     

An automated ROI setting method using NEUROSTAT on cerebral blood flow SPECT images

Objective  We have developed a method to automatically set regions of interest (ROI) (automated ROI) on cerebral blood flow single-photon emission computed tomography (SPECT) images with morphological information specific to the subjects. The objective was to set ROIs automatically without losing individual morphological information in the SPECT images and then evaluate its validity and clinical applicability. Methods  We constructed the volume of interest (VOI) template on the standardized brain generated by NEUROSTAT to determine the regions for ROIs to be set. Assuming patients with cerebral vascular disease, the VOI template was constructed so that the ROIs were drawn for the major vascular regions and 17 regions in total within the hemisphere, basal ganglia, thalamus, cerebellar cortex, cerebellar vermis, and pons. By comparing the major vascular occlusion models, the accuracy of region setting by the VOI template was evaluated for validation. Using the anatomical standardization of NEUROSTAT and inverse transformation, the automated ROI transformed the VOI template into the individual brain shape and then the VOI template was extracted from each slice to determine ROIs. An evaluation was made by visually investigating the effect of a different image quality and cerebral blood flow tracers using brain phantom and clinical data. The regional cerebral blood flow (rCBF), determined by the manual setting method of ROI (manual ROI) and automated ROI, was compared. We also compared automated ROI with other morphological images using clinical data. Results  The VOI templates accurately showed the region with the reduced blood flow in the major vascular occlusion model, which validated the proper ROI setting. The brain phantom study demonstrated that ROI settings were least influenced by matrix size, image quality, and image rotation. The observation with the clinical data also indicated that the variation in cerebral blood flow tracers little affected the ROI settings. The comparison with manual ROI revealed a strong correlation between the two ROI settings, and the mean values within both ROIs were similar. The comparative evaluation with morphological images, obtained by magnetic resonance imaging (MRI), verified the accurate setting of ROI. Conclusions  The automated ROI achieved successful automatic ROI settings without distorting individual SPECT images. The automated ROI is not affected by the differences in the image quality or the cerebral blood flow tracers, which suggests versatile applicability. Thus, the use of automated ROI may eliminate the interoperator and interfacility variability in ROI setting and improve objectivity and reproducibility. It also allows comparative evaluation at the same transverse level with images acquired with other modalities such as MRI and is expected to enhance the clinical diagnosis.     

功能MRI在评价对比剂诱导急性肾损伤中的研究进展

对比剂诱导急性肾损伤（CI-AKI）是注射碘对比剂后严重的并发症之一,早期诊断和治疗可以改善或延缓肾损伤。目前多种功能MRI（fMRI）技术可用于肾损伤后肾脏微循环和病理生理学的评估,包括扩散加权成像（DWI）、体素内不相干运动（IVIM）成像、扩散张量成像（DTI）、扩散峰度成像（DKI）、血氧水平依赖（BOLD）成像、动脉自旋标记（ASL）成像等。这些技术不但可以对肾功能损害进行定量分析,还可以在肾损伤的早期诊断和监测方面提供更多信息。就CI-AKI的发病机制及fMRI评价CI-AKI的研究现状做一综述。     

Integrated software for the analysis of brain PET/SPECT studies with partial-volume-effect correction.

We present software for integrated analysis of brain PET studies and coregistered segmented MRI that couples a module for automated placement of regions of interest (ROI) with 4 alternative methods for partial-volume-effect correction (PVEc). The accuracy and precision of these methods have been measured using 4 simulated (18)F-FDG PET studies with increasing degrees of atrophy. METHODS: The software allows the application of a set of labels, defined a priori in the Talairach space, to segmented and coregistered MRI. Resulting ROIs are then transferred onto the PET study, and corresponding values are corrected according to the 4 PVEc techniques under investigation, providing corresponding corrected values. To evaluate the PVEc techniques, the software was applied to 4 simulated (18)F-FDG PET studies, introducing increasingly larger experimental errors, including errors in coregistration (0- to 6-pixel misregistration), segmentation (-13.7% to 14.1% gray matter [GM] volume change) and resolution estimate errors (-16.9% to 26.8% full-width-at-half-maximum mismatch). RESULTS: Even in the absence of segmentation and coregistration errors, uncorrected PET values showed -37.6% GM underestimation and 91.7% WM overestimation. Voxel-based correction only for the loss of GM activity as a result of spill-out onto extraparenchymal tissues left a residual underestimation of GM values (-21.2%). Application of the method that took into account both spill-in and spill-out effects between any possible pair of ROIs (R-PVEc) and of the voxel-based method that corrects also for the WM activity derived from R-PVEC (mMG-PVEc) provided an accuracy above 96%. The coefficient of variation of the GM ROIs, a measure of the imprecision of the GM concentration estimates, was 8.5% for uncorrected PET data and decreased with PVEc, reaching 6.0% for mMG-PVEc. Coregistration errors appeared to be the major determinant of the imprecision. CONCLUSION: Coupling of automated ROI placement and PVEc provides a tool for integrated analysis of brain PET/MRI data, which allows a recovery of true GM ROI values, with a high degree of accuracy when R-PVEc or mMG-PVEc is used. Among the 4 tested PVEc methods, R-PVEc showed the greatest accuracy and is suitable when corrected images are not specifically needed. Otherwise, if corrected images are desired, the mMG-PVEc method appears the most adequate, showing a similar accuracy.     

Effects of the reference tissue setting on the parametric image of 11C-WAY 100635

OBJECTIVES: C-WAY 100635 has been used widely in the PET research field to measure 5-HT1A receptors in the living human brain. Reference tissue model analysis, in which the cerebellum was used as the reference tissue, was employed in clinical studies. However, the reported binding potentials varied greatly among the reports. In this study, regions of interest (ROIs) of the cerebellum for C-WAY 100635 were determined in five different approaches for six healthy male volunteers, and the effects on binding potential parametric images were compared. METHODS: First, ROIs on the cerebellar cortex were decided based on information from magnetic resonance imaging (MRI), and then divided into upper and lower parts. They were then refined according to coregistered positron emission tomography (PET) image information, generating upper-refined and lower-refined parts. In a different approach, circular ROIs were decided upon, based on the PET images, and the areas under the curve (AUCs) of five ROIs were compared. Parametric images of binding potentials were calculated with the five ROIs as the reference, and compared with each other. RESULTS: The AUCs of the lower-refined parts were the lowest among the first four ROIs. The ratio of the AUCs between the upper part and lower-refined part was 1.54+/-0.17, that between the upper-refined part and lower-refined part was 1.27+/-0.21, that between the lower part and lower-refined part was 1.04+/-0.02, and that between the circular ROIs and lower-refined part was 1.01+/-0.04. The differences in AUCs among the five cerebellar parts led to significant differences in the binding potential parametric images. CONCLUSION: The binding potential parametric images of C-WAY100635 could vary significantly according to the different methods of establishing ROIs, using the cerebellum as the reference tissue, because radioactivity in the cerebellum for C-WAY100635 is very low.     

Detecting groups of coherent voxels in functional MRI data using spectral analysis and replicator dynamics

Purpose

To investigate the relationship between functional MRI (fMRI) time series in the human brain, combining fMRI spectral analysis and replicator dynamics.

Materials and Methods

Simulated and real fMRI time courses were investigated using the bivariate spectral coherence. Coherence values were placed in coherence matrices encoding the relationship between the time courses. Groups of maximally coherent voxels were detected using replicator dynamics. Results were compared to a former approach called number of coherent voxels (NCV).

Results

NCV critically depends on a threshold that has to be chosen in advance. The lower this threshold, the larger the detected group. Using higher NCV thresholds in our simulations, the method did not detect all voxels that were constructed to have a high coherence among each other. In contrast, the replicator process found the whole group in all simulations.

Conclusion

The application of replicator dynamics to spectral matrices is a reliable method for detecting groups of maximally coherent voxels. A replicator process is able to determine groups of voxels with the property that each voxel in the group exhibits a high coherence with every other group member. In contrast to the NCV approach, this method is parameter‐free and does not require the a priori selection of a reference voxel. J. Magn. Reson. Imaging 2007. © 2007 Wiley‐Liss, Inc.     

Experimental determination of the BOLD field strength dependence in vessels and tissue

High resolution functional MRI (fMRI) experiments were performed in human visual cortex at 0.5, 1.5, and 4 T to determine the blood oxygenation level dependent (BOLD) field strength response within regions of obvious venous vessels and cortical gray matter (“tissue”). T₂*-weighted FLASH images were collected in single- and multi-echo mode and used to determine the intrinsic BOLD parameters, namely, signal-to-noise ratio (Ψ), the apparent transverse relaxation rate (R₂*) and the change in R₂* (ΔR₂*) between the activated and baseline states. The authors find the average percentage signal change (ΔS/S, measured at TE = T₂*) to be large in vessels (13.3 ± 2.3%, 18.4 ± 4.0%, and 15.1 ± 1.2%) compared with that in tissue (1.4 ± 0.7%, 1.9 ± 0.7%, and 3.3 ± 0.2%) at 0.5, 1.5, and 4 T, respectively. The signal-to-noise ratio in optimized, fully relaxed proton density weighted gradient echo images was found to increase linearly with respect to the static magnetic field strength (B₀). The predicted upper bound on BOLD contrast-to-noise ratio (ΔS/R)_max as a function of field strength was calculated and found to behave less than linearly in voxels containing vessels larger than the voxel itself and greater than linearly in voxels containing a mixture of capillaries and veins/venules with a diameter less than that of the voxel.     

Comparison of BOLD and direct‐MR neuronal detection (DND) in the human visual cortex at 3T

Direct‐MR neuronal detection (DND) of transient magnetic fields has recently been investigated as a novel imaging alternative to the conventional BOLD functional MRI (fMRI) technique. However, there remain controversial issues and debate surrounding this methodology, and this study attempts clarification by comparing BOLD responses in the human visual system with those of DND. BOLD relies on indirectly measuring blood oxygenation and flow changes as a result of neuronal activity, whereas the putative DND method is based on the hypothesis that the components of the in vivo neuronal magnetic fields, which lie parallel to the B₀ field, can potentially modulate the MR signal, thus providing a means of direct detection of nerve impulses. Block paradigms of checkerboard patterns were used for visual stimulation in both DND and BOLD experiments, allowing detection based on different frequency responses. This study shows colocalization of some voxels with slow BOLD responses and putative fast DND responses using General Linear Model (GLM) analysis. Frequency spectra for the activated voxel cluster are also shown for both stimulated and control data. The mean percentage signal change for the DND responses is 0.2%, corresponding to a predicted neuronal field of 0.14 nT. Magn Reson Med 60:1147–1154, 2008. © 2008 Wiley‐Liss, Inc.