Variation of functional MRI signal in response to frequency of somatosensory stimulation in α-chloralose anesthetized rats

The dependence of functional MRI contrast on the repetition rate (1.5–9 Hz) of a sensory stimulus was investigated with a T₂*-weighted gradient echo method during forepaw stimulation of α-chloralose anesthetized rats (n = 5). An activation area was observed in the left or right somatosensory cortex dependent on the stimulation of the right or left forepaw, respectively. The activation intensity decreased for stimulation frequencies above 3 Hz, and was negligible at 9 Hz. The interpretation is that at low stimulation rates the neurons can respond to each stimulus, but at higher rates there is insufficient recovery time so that the response is progressively occluded.     

右正中神经电刺激早期干预对颅脑损伤昏迷患者的临床疗效观察

目的 观察右正中神经电刺激对颅脑损伤早期昏迷患者的临床疗效. 方法 对2005 - 2011年间颅脑损伤昏迷患者(昏迷时间＞2周)按照随机数字表法分为常规治疗组(对照组)和右正中神经电刺激+常规治疗(治疗组),治疗时间＞2周.观察患者对右正中神经电刺激的治疗依从性以及伤后6个月意识恢复情况. 结果 共纳入465例患者,完成治疗疗程437例,其中治疗组221例,对照组216例,治疗期间无电刺激相关并发症出现.脑血流灌注及脑干诱发电位检测提示治疗组出现明显改善.386例患者获得伤后6个月随访,治疗组患者(204例)意识清醒122例,微意识状态46例,植物状态36例;对照组(182例)意识清醒84例,微意识状态40例,植物状态58例.与对照组比较,治疗组患者意识恢复清醒比例明显高于对照组,植物状态比例明显低于对照组,微意识状态比例与对照组比较差异无统计学意义. 结论 右正中神经电刺激是适宜在颅脑损伤早期昏迷阶段应用的昏迷促醒手段.     

Fast 3D functional magnetic resonance imaging at 1.5 T with spiral acquisition

A new method to perform rapid 3D fMRI in human brain is introduced and evaluated in normal subjects, on a standard clinical scanner at 1.5 Tesla. The method combines a highly stable gradient echo technique with a spiral scan method, to detect brain activation related changes in blood oxygenation with high sensitivity. A motor activation paradigm with a duration of less than 5 min, performed on 10 subjects, consistently showed significant changes in signal intensity in the area of the motor cortex. In all subjects, these changes survived high statistical thresholds.     

BOLD and CBV-weighted functional magnetic resonance imaging of the rat somatosensory system.

A multislice spin echo EPI sequence was used to obtain functional MR images of the entire rat brain with blood oxygenation level dependent (BOLD) and cerebral blood volume (CBV) contrast at 11.7 T. Maps of activation incidence were created by warping each image to the Paxinos rat brain atlas and marking the extent of the activated area. Incidence maps for BOLD and CBV were similar, but activation in draining veins was more prominent in the BOLD images than in the CBV images. Cerebellar activation was observed along the surface in BOLD images, but in deeper regions in the CBV images. Both effects may be explained by increased signal dropout and distortion in the EPI images after administration of the ferumoxtran-10 contrast agent for CBV fMRI. CBV-weighted incidence maps were also created for 10, 20, and 30 mg Fe/kg doses of ferumoxtran-10. The magnitude of the average percentage change during stimulation increased from 4.9% with the 10 mg Fe/kg dose to 8.7% with the 30-mg Fe/kg dose. Incidence of activation followed a similar trend.     

Peripheral nerve stimulation in a whole-body echo-planar imaging system

Echo-planar techniques in MRI use a rapidly oscillating frequency-encoding gradient with the potential to produce peripheral nerve stimulation. To evaluate the incidence, type, and location of stimulation in a commercial whole-body scanner, we studied two groups: (a) 173 consecutive individuals scanned by echo-planar imaging for other purposes and (b) seven subjects who were scanned with an extensive set of 36 echo-planar sequences (with prompting after each scan to report any peripheral nerve stimulation) to test the effects of various parameters. Although only 5% of group A reported symptoms of peripheral nerve stimulation, all in group B experienced some type of stimulation, dependent primarily on direction of the oscillating gradient and location of the body within the gradient coil. Maximum stimulation typically occurred 30 to 40 cm from isocenter in the region of maximum dB/dt. Generally, y gradients produced truncal stimulation, and x gradients produced stimulation in the head. When hands were clasped over the abdomen, a tingling in the hands occasionally was felt. Patients should be instructed to keep their hands apart.     

High-resolution dental magnetic resonance imaging of inferior alveolar nerve responses to the extraction of third molars

The objective of this study was to assess whether signal changes can be detected in the neurovascular bundle of the mandibular canal after the extraction of a third molar. We retrospectively analyzed MRI scans of 30 test subjects with healthy mandibles and 41 patients who had had a wisdom tooth extracted. Signal intensities were measured at particular sites in the neurovascular bundle, which were defined as regions of interest (ROI) in the sagittal T1-weighted images before and after intravenous administration of a paramagnetic contrast agent. On the basis of the signal intensity increases that were measured after contrast agent administration, we compared the signal increases obtained for the patients who had received surgical treatment with the results obtained for the population of test subjects with unremarkable mandibles (t-test, P<0.05). Compared with the healthy test subjects, patients who had received surgical treatment showed significantly higher signal intensity increases at two measurement sites, i.e., the second molar and the second premolar (P<0.05). We found no significant differences when the measurements were performed at the first molar (P=0.06), the third molar (P=0.47) and in the area of the ascending mandibular ramus (P=0.79). Compared with a population of healthy test subjects, patients who had their third molars surgically removed show higher signal intensity increases in the neurovascular bundle after intravenous contrast agent administration. The underlying cause may be the higher blood flow in the arteries and veins and the perineural plexus, which may give evidence of the pathophysiological mechanism of nerve damage in the narrow canal as a result of osteotomy.     

Functional magnetic resonance imaging of the human lumbar spinal cord

PURPOSE: To determine whether consistent regions of activity could be observed in the lumbar spinal cord of single subjects with spin-echo functional MRI (fMRI) if several repeated experiments were performed within a single imaging session. MATERIALS AND METHODS: Repeated fMRI experiments of the human lumbar spinal cord were performed at 1.5 T with a single-shot spin-echo technique (half-Fourier single-shot turbo spin-echo (HASTE)) as used by previous investigators, and a modified method (fluid-attenuated inversion recovery (FLAIR)-HASTE) that nulled the otherwise highly variable signal from the cerebrospinal fluid (CSF). RESULTS: FLAIR-HASTE reduced the variability of the signal in the CSF region to background levels, and presumably reduced associated artifacts in the spinal cord. Consistent areas of activation in the spinal cord in response to a thermal stimulus just below the knee were not observed across the fMRI experiments with either method. CONCLUSION: FLAIR-HASTE was useful for removing artifact in the spinal cord signal induced by variability in the CSF signal. However, with the techniques used in this study, we were not able to confirm the presence of a consistent fMRI response in the lumbar spinal cord because of the signal enhancement by extravascular protons (SEEP) effect during thermal stimulation of the hindlimb.     

Microneurography of human median nerve

PURPOSE: To examine the possibility of performing high-resolution MRI (microneurography) on peripheral nerves. MATERIALS AND METHODS: A specific radio frequency (RF) coil was developed to probe the human median nerve at a magnetic field strength of 9.4 T and tested on three excised samples by acquiring microneurograms. RESULTS: The microneurograms revealed neuronal tissue constituents at subfascicular level. The contrast features on proton-density and T1- and T2-weighted images were described and compared. The microscopic water movement was quantified using diffusion weighting parallel and orthogonal to the neuronal fiber orientation. The characteristics of anisotropic diffusion in the median nerve were comparable to those reported from other biological tissues (white matter and kidney). CONCLUSION: The results overall suggest that microneurography might provide new noninvasive insights into microscopic gross anatomy of the peripheral nerve, injury evaluation, and efficacy of repair, although the feasibility at current clinically relevant field strengths is yet to be determined.     

Functional Mapping of the Human Somatosensory Cortex with Echo-Planar MRI

The somatotopical organization of the human somatosensory cortex was analyzed with echo-planar imaging at 1.5 Tesla, utilizing deoxyhemoglobin as an endogenous contrast medium. Scrubbing stimulation at a frequency of 3 Hz was applied to one of three cutaneous areas: toes, fingertips, and tongue tip. Parasagittal echo-planar slices were obtained every 2 s. We found focal bands of increased signal intensity (4% on average) during the stimulation, with a rise time of 2–6 s. These activated bands were located on the contralateral postcentral gyrus. The cortical responses from the three stimulation sites were anatomically distinct and organized medially-to-laterally in the order of toes, fingertips, and tongue tip.     

Functional diffusion‐weighted magnetic resonance spectroscopy of the human primary visual cortex at 7 T

Purpose:

Microstructural and metabolic changes directly related to neuronal activation have been investigated using functional diffusion‐weighted spectroscopy.

Methods:

The volume of interest was positioned in the primary visual cortex. A time series of alternating diffusion‐ and non‐diffusion‐weighted ¹H spectra was acquired at 7 T employing stimulated echo acquisition mode sequence and a long diffusion time (Δ = 245 ms). Time‐resolved series of metabolite apparent diffusion coefficient values were derived.

Results:

Significant increases in apparent diffusion coefficient of 3.3 ± 1.4% (p = 0.05) and 3.9 ± 0.9% (p = 0.002) for total n‐acetyl aspartate and total creatine, respectively, and 8.1 ± 2.5% (p = 0.03) for total choline were observed in response to visual stimulation.

Conclusion:

The increase in apparent diffusion coefficient for these metabolites is a potential indication for microstructural changes in neurons during neural activation and/or for an increase in the energy‐dependent cytoplasmic streaming associated with enhanced metabolism during visual stimulation. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Functional magnetic resonance imaging of the spinal cord during sensory stimulation in diabetic rats

Purpose

To determine if differences exist between control and diabetic rats in functional magnetic resonance imaging (fMRI) activity of the spinal cord and if fMRI can provide a means of early detection of diabetic neuropathy.

Materials and Methods

fMRI of the spinal cord, using noxious electrical stimulation (15 V (～8 mA), 0.3 msec, 3 Hz) of the hind paw, was performed in groups of control and streptozotocin (STZ)‐induced type 1 diabetic rats.

Results

Diabetic rats were lighter, hyperglycemic, and had lower blood pH than controls. fMRI activity at the lumbar enlargement of the spinal cord was identified in the dorsal horn ipsilateral to stimulus of all animals. Signal intensity changes across the lumbar spinal cord during periods of activity were not significantly different between control and diabetic rats, with a trend toward greater signal changes in controls. When specific regions of the spinal cord were analyzed, control rats exhibited significantly increased blood–oxygen level‐dependent (BOLD) fMRI activity in both ipsilateral and contralateral dorsal horn compared to diabetic rats.

Conclusion

The results of this study are consistent with reports that primary afferent input to the spinal cord is diminished by diabetes, and suggest that BOLD fMRI may be useful in early detection of diabetic neuropathy. J. Magn. Reson. Imaging 2009;30:271–276. © 2009 Wiley‐Liss, Inc.     

青少年抑郁症的功能磁共振研究进展

近年来功能磁共振成像在成人抑郁症中的研究较为深入,而对于青少年抑郁症的研究则相对有限,故本文就目前功能性磁共振在青少年抑郁症中的研究进展作一综述。     

3.0T MR DTI及DTT对正常青年人腕管内正中神经的定量分析

目的 运用扩散张量成像(DTI)定量分析健康青年人腕管内正中神经(MN)部分功能参数值特点及扩散张量纤维束成像(DTT)重建MN纤维束.方法 采用3.0T MR对25名健康志愿者行腕部常规MR及DTI序列扫描,测量DTI各参数值并重建MN纤维束.结果 ①MN各向异性分数(FA)值=0.686±0.089、MN轴向扩散系数(AD)值=(2.085±0.263)mm2/s、MN径向扩散系数(RD)值=(0.568±0.151)mm2/s、MN平均扩散率(ADC)值=(1.073±0.140)mm2/s.②各层面FA、RD值之间存在显著差异(P<0.05),各层面AD、ADC及各本征值E1、E2、E3之间差异无统计学意义(P>0.05).③腕管内MN从近心端到远心端DTI各部分功能参数均有不同的变化趋势.④MN FA、MN AD、MN RD、MN ADC的95%置信区间上下限分别为(0.675,0.698)、(2.052,2.117)、(0.549,0.587)、(1.056,1.091).⑤利用DTT可以完整地重建显示MN纤维束,远端分支亦能显示.结论 应用DTI及DTT技术可以定量描述MN的FA及直观显示重建的MN纤维束,可为诊断MN病变提供定量的观察指标.     

Multiple-region gradient arrays for extended field of view, increased performance, and reduced nerve stimulation in magnetic resonance imaging.

This article presents a novel design for magnetic resonance imaging (MRI) gradient systems. This design may allow the development of MRI scanners that are capable of imaging large regions with high performance while minimizing the potential for nerve stimulation. The general concept of the gradient system is that spatial oscillation is incorporated such that each gradient coil creates multiple, approximately linear gradient regions that oscillate in gradient polarity. Separate radiofrequency (RF) coil arrays are designed to be sensitive to the signals within each linear region and thus allow signal measurements to be obtained separately from each region. Enabling image acquisition in the transition region that separates each pair of adjacent linear regions requires a second gradient system with imaging regions that overlap and coincide with the transition regions of the first gradient system. Imaging the extended field of view (FOV) is accomplished by interleaved operation of the two gradient systems. Simulated annealing is used to create designs for both longitudinal and transverse gradient systems with two imaging regions.     

Surgical evaluation of magnetic resonance imaging findings in piriformis muscle syndrome

Objective  The objective of this study was to evaluate the accuracy of magnetic resonance imaging (MRI) in the diagnosis of the piriformis muscle syndrome (PMS). Materials and Methods  In ten patients, seven female and three male, with a long history of clinical symptoms of the PMS, an MRI was performed as the last preoperative diagnostic tool. All patients were imaged using 2T MR system (Elscint, Haifa, Israel). Axial and coronal spin-echo, fast spin-echo (FSE), and fat-suppressed FSE-weighted images were made through the pelvic region with 3-mm section thickness and a 0.5-mm gap to show the whole piriformis muscle and the course of sciatic nerve on its way out of the pelvis. A routine examination also included axial fast spin-echo T2, three-dimensional gradient echo. Results  In seven cases, an MRI abnormality for the PMS was found. In two women, the MRI demonstrated a bigastric appearance of the piriformis muscle with a tendinous portion between the muscle heads and the course of the common peroneal nerve through the muscle between the tendinous portions of the muscle. In one female patient, the common peroneal nerve passed through the hypertrophied piriformis muscle. In four patients, the MRI showed a hypertrophied aspect of the piriformis muscle and an anteriorly displaced sciatic nerve. All MRI findings were confirmed surgically. In three patients, no apparent abnormalities could be observed, but after a surgical treatment, i.e., a tenotomy of the piriformis muscle and neurolysis of the sciatic nerve, all symptoms disappeared. Conclusion  In piriformis muscle syndrome, MRI may demonstrate signal abnormalities of the sciatic nerve as well as its relationship with the normal and abnormal piriformis muscle.     

MR neurography of the median nerve at 3.0T: optimization of diffusion tensor imaging and fiber tractography

Objectives

The purpose of this study was to systematically assess the optimal b-value and reconstruction parameters for DTI and fiber tractography of the median nerve at 3.0 T.

Methods

Local ethical board approved study with 45 healthy volunteers (15 men, 30 women; mean age, 41 ± 3.4 years) who underwent DTI of the right wrist at 3.0 T. A single-shot echo-planar-imaging sequence (TR/TE 10123/40 ms) was acquired at four different b-values (800, 1000, 1200, and 1400 s/mm²). Two independent readers performed post processing and fiber-tractography. Fractional anisotropy (FA) maps were calculated. Fiber tracts of the median nerve were generated using four different algorithms containing different FA thresholds and different angulation tolerances. Data were evaluated quantitatively and qualitatively.

Results

Tracking algorithms using a minimum FA threshold of 0.2 and a maximum angulation of 10° were significantly better than other algorithms. Fiber tractography generated significantly longer fibers in DTI acquisitions with higher b-values (1200 and 1400 s/mm² versus 800 s/mm²; p < 0.001). The overall quality of fiber tractography was best at a b-value of 1200 s/mm² (p < 0.001).

Conclusions

In conclusion, our results indicate use of b-values between 1000 and 1400 s/mm² for DTI of the median nerve at 3.0 T. Optimal reconstruction parameters for fiber tractography should encompass a minimum FA threshold of 0.2 and a maximum angulation tolerance of 10.     

Brain mapping of median nerve somatosensory evoked potentials with combined99mTc-ECD single-photon emission tomography and magnetic resonance imaging

Single-photon emission tomography (SPET) was performed during electrical median nerve stimulation and used to detect focal neuronal activation in the somatosensory pathways. Intravenously administered technetium-99m ethyl cysteinate dimer (ECD) was used as a blood flow tracer to obtain baseline and activated images in each of three subjects. After image registration, baseline images were compared voxel by voxel with the activation images. In addition, the mean summation of the activated-state images of the subjects was compared with the mean summation of the baseline-state images of ten normal subjects. Discrete brain regions occupying 0.9%–1.6% of total brain volume showed an increase in signal from 33.6% to 35.0%. For further anatomical localization of regional increases in signal, the MRI scan of each subject was registered and superimposed on the activated-state SPET image. This method may be used to localize lesions in various disorders of the central nervous system.     

A novel multi-segment surface coil for neuro-functional magnetic resonance imaging

The construction of two novel mult-segment (MS) resonators are described. The signal-to-noise ratio (SNR) and B₁ homogeneity of the coils are compared with that of a surface coil and a standard quadrature head coil. The images obtained with the MS designs revealed a surface coil-like fall-off in signal with depth. The SNR offered by MS coils was found to be better than the head coil at depths less than approximately 6 cm. Bilateral motor cortex activation on normal subjects performing finger tapping tasks is demonstrated.     

Functional magnetic resonance imaging with intermolecular double-quantum coherences at 3 T.

Functional magnetic resonance imaging (fMRI) based on the selection of intermolecular double-quantum coherences (iDQC) was performed with a standard birdcage coil at 3 T in a group of normal human volunteers. Suppression of spurious signal contributions from unwanted coherence-transfer pathways was achieved by combining a two-step phase cycle and a long repetition time of 5 s. A gradient-recalled echo iDQC sequence (echo time, T(E) = 80 ms) yielded robust activation with a visual paradigm. Maximum z-scores were about half of those observed with conventional blood-oxygen level dependent fMRI, whereas the functional signal change increased by more than a factor of 5. No activation was obtained with a spin-echo iDQC sequence (T(E) = 160 ms), in which dephasing accumulated during the evolution period was fully rephased by an appropriate delay time. It is hypothesized that substantial inherent diffusion weighting of the iDQC technique efficiently suppresses intravascular contributions to the functional contrast. A consistent quantitative explanation of the observed amount of signal change currently remains speculative.     

NMR properties of human median nerve at 3 T: Proton density,T1, T2, and magnetization transfer

Purpose

To measure the proton density (PD), the T1 and T2 relaxation time, and magnetization transfer (MT) effects in human median nerve at 3 T and to compare them with the corresponding values in muscle.

Materials and Methods

Measurements of the T1 and T2 relaxation time were performed with an inversion recovery and a Carr‐Purcell‐Meiboom‐Gill (CPMG) imaging sequence, respectively. The MT ratio was measured by acquiring two sets of 3D spoiled gradient‐echo images, with and without a Gaussian saturation pulse.

Results

The median nerve T1 was 1410 ± 70 msec. The T2 decay consisted of two components, with average T2 values of 26 ± 2 msec and 96 ± 3 msec and normalized amplitudes of 78 ± 4% and 22 ± 4%, respectively. The dominant component is likely to reflect myelin water and connective tissue, and the less abundant component originates possibly from intra‐axonal water protons. The value of proton density of MRI‐visible protons in median nerve was 81 ± 3% that of muscle. The MT ratio in median nerve (40.3 ± 2.0%) was smaller than in muscle (45.4 ± 0.5%).

Conclusion

MRI‐relevant properties, such as PD, T1 and T2 relaxation time, and MT ratio were measured in human median nerve at 3 T and were in many respects similar to those of muscle. J. Magn. Reson. Imaging 2009;29:982–986. © 2009 Wiley‐Liss, Inc.