Recanalization and reversal of diffusion abnormalities and markedly reduced cerebral blood volume in a patient with transient ischemic attack

Resolution of diffusion abnormalities is a finding that is rarely seen in hyperacute cerebral ischemia. We report a serial study of diffusion and perfusion magnetic resonance imaging (MRI) in a case of transient ischemic attack (TIA). In the current case, marked decrease of cerebral blood volume (CBV) as well as reduced diffusion in the affected middle cerebral arterial territory was shown by the initial diffusion and perfusion MRI at 1.5 h after onset. The initial MRI findings as well as neurological symptoms recovered probably due to prompt and spontaneous recanalization of thrombosed vessel.     

Physiological origin of low‐frequency drift in blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI)

We investigated the biophysical mechanism of low‐frequency drift in blood‐oxygen‐level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) (0.00–0.01 Hz), by exploring its spatial distribution, dependence on imaging parameters, and relationship with task‐induced brain activation. Cardiac and respiratory signals were concurrently recorded during MRI scanning and subsequently removed from MRI data. It was found that the spatial distribution of low‐frequency drifts in human brain followed a tissue‐specific pattern, with greater drift magnitude in the gray matter than in white matter. In gray matter, the dependence of drift magnitudes on TE was similar to that of task‐induced BOLD signal changes, i.e., the absolute drift magnitude reached the maximum when TE approached T whereas relative drift magnitude increased linearly with TE. By systematically varying the flip angle, it was found that drift magnitudes possessed a positive dependence on image intensity. In phantom experiments, the observed drift was not only much smaller than that of human brain, but also showed different dependence on TE and flip angle. In fMRI studies with visual stimulation, a strong positive correlation between drift effects at baseline and task‐induced BOLD signal changes was observed both across subjects and across activated pixels within individual participants. We further demonstrated that intrinsic, physiological drift effects are a major component of the spontaneous fluctuations of BOLD fMRI signal within the frequency range of 0.0–0.1 Hz. Our study supports brain physiology, as opposed to scanner instabilities or cardiac/respiratory pulsations, as the main source of low‐frequency drifts in BOLD fMRI. Magn Reson Med 61, 2009. © 2009 Wiley‐Liss, Inc.     

Detection of hyperacute parenchymal hemorrhage of the brain using echo-planar T2*-weighted and diffusion-weighted MRI

We investigated the usefulness of echo-planar imaging (EPI) as well as T2^*-weighted and diffusion-weighted MRI (DWI) to identify hyperacute hemorrhage (within 24 h after ictus) in the brain. Seven patients were examined 3.5 to 24 h after onset of symptoms using a whole-body 1.5-T MR system. Two diffusion-weighted sequences were run to obtain isotropic and anisotropic diffusion images. Apparent diffusion coefficients (ADC) were calculated from the isotropic diffusion images. All DWI images as well as the T2*-weighted EPI images showed the hematomas as either discrete, deeply hypointense homogeneous lesions, or as lesions of mixed signal intensity containing hypointense areas. We conclude that even in the early phase after hemorrhage, sufficient amounts of paramagnetic deoxyhemoglobin are present in intracerebral hemorrhages to cause hypointensity on EPI T2^*-weighted and DWI images; thus, use of ultrafast EPI allows identification of intracerebral hemorrhage. Received: 21 March 2000 Revised: 26 July 2000 Accepted: 27 July 2000     

Real-time autoshimming for echo planar timecourse imaging.

Head motion within an applied magnetic field alters the effective shim within the brain, causing geometric distortions in echo planar imaging (EPI). Even if subtle, change in shim can lead to artifactual signal changes in timecourse EPI acquisitions, which are typically performed for functional MRI (fMRI) or diffusion tensor imaging. Magnetic field maps acquired before and after head motions of clinically realistic magnitude indicate that motion-induced changes in magnetic field may cause translations exceeding 3 mm in the phase-encoding direction of the EPI images. The field maps also demonstrate a trend toward linear variations in shim changes as a function of position within the head, suggesting that a real-time, first-order correction may compensate for motion-induced changes in magnetic field. This article presents a navigator pulse sequence and processing method, termed a "shim NAV," for real-time detection of linear shim changes, and a shim-compensated EPI pulse sequence for dynamic correction of linear shim changes. In vivo and phantom experiments demonstrate the detection accuracy of shim NAVs in the presence of applied gradient shims. Phantom experiments demonstrate reduction of geometric distortion and image artifact using shim-compensated EPI in the presence of applied gradient shims. In vivo experiments with intentional interimage subject motion demonstrate improved alignment of timecourse EPI images when using the shim NAV-detected values to update the shim-compensated EPI acquisition in real time.     

Current techniques in MR imaging of cerebral infarction]

The imaging of cerebral infarction is a current topic now that the usefulness of both thrombolysis for hyperacute infarction and carotid endarterectomy has been established. Diffusion-weighted imaging (DWI) by EPI is a highly sensitive method of detecting hyperacute infarction. Further, the combination of DWI and perfusion MR makes it possible to detect hyperacute infarction as well as predict the final size of infarction. Contrast-enhanced MRA can demonstrate carotid bifurcation without flow-related signal loss within 30 seconds and can visualize major cervical arteries from the aortic arch to the circle of Willis.     

Disparity of activation onset in sensory cortex from simultaneous auditory and visual stimulation: Differences between perfusion and blood oxygenation level-dependent functional magnetic resonance imaging

PURPOSE: To compare the temporal behaviors of perfusion and blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in the detection of timing differences between distinct brain areas, and determine potential latency differences between stimulus onset and measurable fMRI signal in sensory cortices. MATERIALS AND METHODS: Inversion recovery (IR) spin-echo echo-planar imaging (EPI) and T2*-weighted gradient-echo EPI sequences were used for perfusion- and BOLD-weighted experiments, respectively. Simultaneous auditory and visual stimulations were employed in an event-related (ER) paradigm. Signal time courses were averaged across 40 repeated trials to evaluate the onset of activation and to determine potential differences of activation latency between auditory and visual cortices and between these scanning methods. RESULTS: Temporal differences between visual and auditory areas ranged from 90-200 msec (root-mean-square (RMS) = 134 msec) and from -80 to 930 msec (RMS = 604 msec) in perfusion and BOLD measurements, respectively. The temporal variability detected with BOLD sequences was larger between subjects and was significantly greater than that in the perfusion response (P < 0.04). The measured time to half maximum (TTHM) values for perfusion imaging (visual, 3260 +/- 710 msec; auditory, 3130 +/- 700 msec) were earlier than those in BOLD responses (visual, 3770 +/- 430 msec; auditory, 3360 +/- 460 msec). CONCLUSION: The greater temporal variability between brain areas detected with BOLD could result from differences in the venous contributions to the signal. The results suggest that perfusion methods may provide more accurate timing information of neuronal activities than BOLD-based imaging.     

Benefits of perfusion MR imaging relative to diffusion MR imaging in the diagnosis and treatment of hyperacute stroke

BACKGROUND AND PURPOSE: The development of thrombolytic agents for use with compromised cerebral blood flow has made it critical to quickly identify those patients to best treat. We hypothesized that combined diffusion and perfusion MR imaging adds vital diagnostic value for patients for whom the greatest potential benefits exist and far exceeds the diagnostic value of diffusion MR imaging alone. METHODS: The cases of patients with neurologic symptoms of acute ischemic stroke who underwent ultra-fast emergent MR imaging within 6 hours were reviewed. In all cases, automatic processing yielded isotropic diffusion images and perfusion time-to-peak maps. Images with large vessel distribution ischemia and with mismatched perfusion abnormalities were correlated with patient records. All follow-up images were reviewed and compared with outcomes resulting from hyperacute therapies. RESULTS: For 16 (26%) of 62 patients, hypoperfusion was the best MR imaging evidence of disease distribution, and for 15 of the 16, hypoperfusion (not abnormal diffusion) comprised the only imaging evidence for disease involving large vessels. For seven patients, diffusion imaging findings were entirely normal, and for nine, diffusion imaging delineated abnormal signal in either small vessel distributions or in a notably smaller cortical branch in one case. In all cases, perfusion maps were predictive of eventual lesions, as confirmed by angiography, CT, or subsequent MR imaging. CONCLUSION: If only diffusion MR imaging is used in assessing patients with hyperacute stroke, nearly one quarter of the cases may be incorrectly categorized with respect to the distribution of ischemic at-risk tissue. Addition of perfusion information further enables better categorizing of vascular distribution to allow the best selection among therapeutic options and to improve patient outcomes.     

Cocaine administration decreases functional connectivity in human primary visual and motor cortex as detected by functional MRI.

Functional magnetic resonance imaging (fMRI) was conducted to observe the effects of cocaine administration on the physiological fluctuations of fMRI signal in two brain regions. Seven long-term cocaine users with an average age of 32 years and 8 years of cocaine use history were recruited for the study. A T2*-weighted fast echo-planar imaging (EPI) pulse sequence was employed at 1.5 T to acquire three sets of brain images for each subject under three conditions (at rest, after saline injection, and after cocaine injection [0.57 mg/kg]). Cross-correlation maps were constructed using the synchronous, low frequency signal from voxel time courses after filtering respiratory, cardiac, and other physiological noise. A quantitative evaluation of the changes in functional connectivity was made using spatial correlation coefficient (SCC) analysis. A marked 50% reduction in SCC values in the region of primary visual cortex and 43% reduction in SCC values in the region of primary motor cortex were observed after cocaine administration. This significant reduction in SCC values in these cortical regions is a reflection of changes in neuronal activity. It is suggested that the observed changes in low frequency components after acute cocaine administration during a resting, no-task situation may be used as a baseline reference source when assessing the effects of cocaine on task-driven activation or on mesolimbic dopamine pathways.     

Multiphasic perfusion computed tomography in hyperacute ischemic stroke: comparison with diffusion and perfusion magnetic resonance imaging

PURPOSE: The purpose of this study was to compare multiphasic perfusion computed tomography (CT) with diffusion and perfusion magnetic resonance imaging (MRI) in predicting final infarct volume, infarct growth, and clinical severity in patients with hyperacute ischemia untreated by thrombolytic therapy. METHOD: Multiphasic perfusion CT was performed in 19 patients with ischemic stroke within 6 hours of symptom onset. Two CT maps of peak and total perfusion were generated from CT data. Diffusion-weighted imaging (DWI) and perfusion MRI were obtained within 150 minutes after CT. Lesion volumes on CT and MRI were compared with final infarct volume and clinical scores, and mismatch on CT or MRI was compared with infarct growth. RESULTS: The lesion volume on the CT total perfusion map strongly correlated with MRI relative cerebral blood volume (rCBV), and that on the CT peak perfusion map strongly correlated with MRI relative cerebral blood flow (rCBF) and rCBV (P < 0.001). The lesion volume on unenhanced CT or DWI moderately correlated with final infarct volume, but only lesion volume on unenhanced CT weakly correlated with baseline clinical scores (P = 0.024). The lesion volumes on the CT peak perfusion map and MRI rCBF similarly correlated with final infarct volume and clinical scores and more strongly than those on mean transit time (MTT) or time to peak (TTP). DWI-rCBF or CT mismatch was more predictive of infarct growth than DWI-MTT or DWI-TTP mismatch. CONCLUSION: Multiphasic perfusion CT is useful and of comparable utility to diffusion and perfusion MRI for predicting final infarct volume, infarct growth, and clinical severity in acute ischemic stroke.     

Relationship between caffeine-induced changes in resting cerebral perfusion and blood oxygenation level-dependent signal

BACKGROUND AND PURPOSE: Recent interest has emerged in the use of pharmacologic methods to maximize blood oxygenation level-dependent (BOLD) signal intensity changes in functional MR imaging (fMRI). Adenosine antagonists, such as caffeine and theophylline, have been identified as potential agents for this purpose. The present study was designed to determine whether caffeine-induced decreases in cerebral perfusion result in enhanced BOLD responses to visual and auditory stimuli. METHODS: MR imaging was used to measure resting cerebral perfusion and stimulus-induced BOLD signal intensity changes in 19 patients. We evaluated the relationship between resting cerebral perfusion and the magnitude of BOLD signal intensity induced by visual and auditory stimulation under caffeine and placebo conditions. RESULTS: The data showed that changes in resting cerebral perfusion produced by caffeine are not a consistent predictor of BOLD signal intensity magnitude. Although all cerebral perfusion was reduced in all study participants in response to caffeine, only 47% of the participants experienced BOLD signal intensity increase. This finding was independent of the participants' usual caffeine consumption. CONCLUSION: The data presented herein show that the relationship between resting cerebral perfusion and the magnitude of BOLD signal intensity is complex. It is not possible to consistently enhance BOLD signal intensity magnitude by decreasing resting perfusion with caffeine. Future studies aimed at evaluating the relationship between perfusion and BOLD signal intensity changes should seek a means to selectively modulate known components of the neural and vascular responses independently.     

Echo planar imaging at 4 Tesla with minimum acoustic noise

PURPOSE: To minimize the acoustic sound pressure levels of single-shot echo planar imaging (EPI) acquisitions on high magnetic field MRI scanners. MATERIALS AND METHODS: The resonance frequencies of gradient coil vibrations, which depend on the coil length and the elastic properties of the materials in the coil assembly, were measured using piezoelectric transducers. The frequency of the EPI-readout train was adjusted to avoid the frequency ranges of mechanical resonances. RESULTS: Our MRI system exhibited two sharp mechanical resonances (at 720 and 1220 Hz) that can increase vibrational amplitudes up to six-fold. A small adjustment of the EPI-readout frequency made it possible to reduce the sound pressure level of EPI-based perfusion and functional MRI scans by 12 dB. CONCLUSION: Normal vibrational modes of MRI gradient coils can dramatically increase the sound pressure levels during echo planar imaging (EPI) scans. To minimize acoustic noise, the frequency of EPI-readout trains and the resonance frequencies of gradient coil vibrations need to be different.     

Retrospective estimation and correction of physiological artifacts in fMRI by direct extraction of physiological activity from MR data

A physiological artifact reduction method based on extracting respiratory motion and cardiac pulsation directly from functional MR data is described. In fast low angle shot (FLASH), respiratory cycles are derived utilizing the phase of the center of a navigator echo, in echo-planar imaging (EPI) from the phase of the center κ-space point. Cardiac cycles are determined from projections obtained from the navigator echo (FLASH) and the center κ-space line (EPI). Because direct extraction of physiological parameters eliminates the need for external monitoring, the method can be more readily implemented. Experimental results illustrate that the technique provides effective compensation for physiology-related signal fluctuations in functional MRI and performs as well as the retrospective technique using external physiological monitoring. Key words: fMRI; motion artifacts; physiological motion; image processing.     

Multi-section magnetic susceptibility perfusion echo-planar imaging of the breast

PURPOSE: To examine the diagnostic value of multi-section magnetic susceptibility perfusion echo-planar imaging (perfusion EPI) in patients with breast tumors prior to T1-weighted dynamic MRI. MATERIALS AND METHODS: MR imaging was performed in 75 patients with pathologically proved breast tumors using a 1.5 Tesla MR unit (MAGNETOM Symphony, Siemens Medical Solutions, Erlangen, Germany). Perfusion EPI was carried out before, during, and after the bolus injection of 0.1 mmol Gd-DTPA/kg. Two patients had two carcinomas in the same breast, one patient had both a benign and a malignant lesion in the same breast, and two patients had lesions in both breasts. Histopathological diagnosis was non-invasive ductal carcinoma in 9, invasive carcinoma in 49, and benign lesion in 22. The first-pass signal intensity loss of the lesions was calculated by perfusion EPI. RESULTS: Fifty-one of 58 carcinomas but only 4 of 22 benign lesions had a signal intensity loss of 20% or more during the first pass, for a sensitivity of 88% and specificity of 82% CONCLUSION: Perfusion EPI can be used as a useful diagnostic tool for differentiation between benign and malignant lesions. It is also thought to be a promising method for diagnosing multifocal breast lesions.     

Diffusion-prepared fast imaging with steady-state free precession (DP-FISP): a rapid diffusion MRI technique at 7 T

Diffusion MRI is a useful imaging technique with many clinical applications. Many diffusion MRI studies have utilized echo-planar imaging (EPI) acquisition techniques. In this study, we have developed a rapid diffusion-prepared fast imaging with steady-state free precession MRI acquisition for a preclinical 7T scanner providing diffusion-weighted images in less than 500 ms and diffusion tensor imaging assessments in ～1 min with minimal image artifacts in comparison with EPI. Phantom apparent diffusion coefficient (ADC) and fractional anisotropy (FA) assessments obtained from the diffusion-prepared fast imaging with steady-state free precession (DP-FISP) acquisition resulted in good agreement with EPI and spin echo diffusion methods. The mean apparent diffusion coefficient was 2.0 × 10(-3) mm(2) /s, 1.90 × 10(-3) mm(2) /s, and 1.97 × 10(-3) mm(2) /s for DP-FISP, diffusion-weighted spin echo, and diffusion-weighted EPI, respectively. The mean fractional anisotropy was 0.073, 0.072, and 0.070 for diffusion-prepared fast imaging with steady-state free precession, diffusion-weighted spin echo, and diffusion-weighted EPI, respectively. Initial in vivo studies show reasonable ADC values in a normal mouse brain and polycystic rat kidneys.     

磁共振脑功能成像方法的初步研究

探讨磁共振功能成像的方法，本文应用血氧合水平磁共振成像法，通过单次激发回波平面成像技术检测脑功能活动时的功能区信号变化。对８例右利手健康男性进行右手指随意运动的磁共振功能成像实验。实验采用静止－运动－静止三个阶段，运动又有简单复杂之分。此外，亦将梯度回波与平面回波技术同时应用在磁共振功能成像中以便进行比较。结果表明功能图像上可见左侧初级运动皮层区的信号变化，而在本实验中梯度回波图像上结果为阴性。平面回波成像技术在平面回波磁共振功能成像中磁敏感性高，稳定性好，有良好的时间分辨率，而梯度回波序列则较为不敏感。     

磁共振扩散加权成像在甲状腺良恶性病变鉴别诊断中的价值

目的：探讨MR扩散加权成像在甲状腺良恶性病变鉴别诊断中的价值。方法：采用平面回波成像（EPI）技术。对50例甲状腺局灶性病变（良性组36例,恶性组14例）行DWI检查,分别测量b值取0、150、300和500s/mm2时良恶性病灶的信号强度和ADC值,并比较其差异。所有患者均经手术（49例）或活检（1例）病理证实。结果：良性与恶性病变的信号强度在b值为0、150、300和500s/mm2时差异均无统计学意义（P〉0.05）。b值为150、300和500s/mm2时良性组与恶性组之间ADC值的差异均有统计学意义（P〈0.01）。以恶性组ADC值95%可信区间上限作为诊断阈值,则b值为150、300和500s/mm2时,诊断阈值分别为1.992×10^-3、1.582×10-3和1.410×10-3mm2/s。应用ROC曲线分析不同b值时ADC阈值诊断恶性病变的作用,显示b值为150s/mm2时价值最大,其诊断敏感度、特异度、符合率、阳性和阴性预测值分别为78.57%、84.62%、82.50%、73.33%和88.00%。结论：甲状腺恶性病变的ADC值低于良性病变,b值取150s/mm2、阈值设为1.992×10-3mm2/s有助于良恶性病变的鉴别。     

EPI image reconstruction with correction of distortion and signal losses

PURPOSE: To derive and implement a method for correcting geometric distortions and recovering magnetic resonance imaging (MRI) signal losses caused by susceptibility-induced magnetic field gradients (SFGs) in regions with large static field inhomogeneities in echo-planar imaging (EPI). MATERIALS AND METHODS: Factors to account for field inhomogeneities and SFGs were added in a traditional EPI equation that was a simple Fourier transform (FT) for expressing the actual k-space data of an EPI scan. The inverse calculation of this "distorted EPI" equation was used as a kernel to correct geometric distortions and reductions in intensity during reconstruction. A step-by-step EPI reconstruction method was developed to prevent complicated phase unwrapping problems. Some EPI images of phantom and human brains were reconstructed from standard EPI k-spaces. RESULTS: All images were reconstructed using the proposed multistep method. Geometric distortions were corrected and SFG-induced MRI signal losses were recovered. CONCLUSION: Results suggest that applying our method for reconstructing EPI images to reduce distortions and MRI signal losses is feasible.     

Diffusion weighted fMRI at 1.5 T

Functional magnetic resonance imaging (fMRI) is capable of detecting task-induced blood oxygenation changes using susceptibility sensitive pulse sequences such as gradient-recalled echo-planar imaging (EPI). The local signal increases seen in the time course are believed to be due to an increase in oxygen delivery that is incommensurate with oxygen demands. To help isolate the sources of functional signal changes, the authors have incorporated various forms of diffusion weighting Into EPI pulse sequences to characterize the apparent mobility of the functionally modulated protons. Results suggest that the majority of the functional signal at 1.5 T arises from protons that have apparent diffusion coefficients that are approximately four or five times higher than that of brain tissue. This implies that significant functional signal sources are either protons within the vascular space or protons from the perivascular space that is occupied by cerebro-spinal fluid.