Augment low-field intra-operative MRI with preoperative MRI using a hybrid non-rigid registration method

Background

Preoperatively acquired diffusion tensor image (DTI) and blood oxygen level dependent (BOLD) have been proved to be effective in providing more anatomical and functional information; however, the brain deformation induced by brain shift and tumor resection severely impairs the correspondence between the image space and the patient space in image-guided neurosurgery.

Method

To address the brain deformation, we developed a hybrid non-rigid registration method to register high-field preoperative MRI with low-field intra-operative MRI in order to recover the deformation induced by brain shift and tumor resection. The registered DTI and BOLD are fused with low-field intra-operative MRI for image-guided neurosurgery.

Results

The proposed hybrid registration method was evaluated by comparing the landmarks predicted by the hybrid registration method with the landmarks identified in the low-field intra-operative MRI for 10 patients. The prediction error of the hybrid method is 1.92 ± 0.54 mm, and the compensation accuracy is 74.3 ± 5.0%. Compared to the landmarks far from the resection region, those near the resection region demonstrated a higher compensation accuracy (P-value = .003) although these landmarks had larger initial displacements.

Conclusions

The proposed hybrid registration method is able to bring preoperatively acquired BOLD and DTI into the operating room and compensate for the deformation to augment low-field intra-operative MRI with rich anatomical and functional information.     

Labelling accuracy in Tasmanian seafood: An investigation using DNA barcoding

Mislabelling of seafood products has been documented in numerous countries for over three-quarters of a century. With a trend towards increased consumption of seafood, the informed consumer demands accurately labelled products that provide full disclosure of composition. DNA barcoding can be used to accurately identify a seafood product to species based on its genetic signature, and so provides a means to test the authenticity and accuracy of seafood labelling. This can be especially useful for products such as fillets which have few or no unambiguous identifying characters, and can easily be mislabelled. We investigated labelling accuracy in seafood retailers in Tasmania, Australia. Thirty-eight seafood products were obtained from seafood retailers, sequenced for the barcoding gene region cytochrome oxidase subunit 1(CO1), and subsequently identified to species level by querying GenBank and Barcode of Life Data Systems (BOLD) DNA sequence records. Results were compared with standard fish names (SFN) prescribed under the Australian Fish Names Standard (AFNS) and FishBase. Of the 38 samples, none were deemed to have been mislabelled under Australian regulation, although in some cases naming discrepancies and ambiguity may cause confusion for some consumers. Our work, while reflecting high standards in Tasmanian seafood, highlights the need for mandatory standard labelling across all seafood products so as to eliminate any possible misrepresentation.     

Measurement of activation-related changes in cerebral blood volume: VASO with single-shot HASTE acquisition

Object The recently developed vascular space occupancy (VASO) fMRI technique is gaining popularity as it facilitates the measurement of cerebral blood volume (CBV) changes concomitant with brain activation, without the use of contrast agents. Thus far, VASO fMRI has only been used in conjunction with a GE-EPI (gradient-echo echo planar imaging) sequence, which is proceeded by an inversion recovery (IR) experiment to selectively null the blood signal. The use of GE-EPI has potential disadvantages: (a) the non-zero TE may lead to BOLD contamination and (b) images suffer from the EPI-typical inhomogeneity artefacts. Materials and methods Here, we propose the use of VASO based on an IR-HASTE (inversion recovery half-Fourier acquisition single-shot turbo spin echo) sequence. Results Results from a visual stimulation study (n = 8) show a 43% higher functional contrast-to-noise (CNR) of HASTE compared to EPI, with a strongly increased count of active voxels at the same significance threshold. Sensitivity to inflow effects was investigated and found to be similar for both methods. Conclusion As HASTE VASO yields essentially artefact-free images, it appears to be the method of choice for measuring relative CBV changes with VASO.     

基于时间自动机的脑网络时空演化建模方法

人类大脑本质上是不断变化的整体,但基于静息态功能磁共振成像重构技术的人脑网络动态特性研究尚在起步阶段,并且大多采用定性的方法描述。采用时间自动机理论对脑网络在时间和空间上的系统动态特征和演化过程展开了建模方法研究,首先通过对采样时间区间上血氧依赖水平信号的处理得到全脑脑区在单个采样点上的状态描述,然后通过无监督聚类获取其状态集,研究其状态随时间转换的可观测性,最后在此基础上结合时间自动机理论对脑网络状态的演化过程进行建模,从而达到对脑网络动态特性定量描述的目的。实验结果显示该方法可定量描述人脑网络的状态转换规律和演变进程,对不同被试数据具有普适性,并且可辨识出被试的异常演化过程,为脑网络动态特性的深入研究提供了理论基础。     

基于fMRI瞬时功率的独立成分分析

独立成分分析(independent component analysis,ICA)采用一种统计隐变量模型,假设信号是由各信源线性叠加构成.为了解决功能磁共振数据(functional magnetic resonance imaging,fMRI)中由于信源非线性叠加造成的ICA检测误差,提出了基于瞬时功率的ICA方法.首先,由电流能量形式将fMRI数据推广为fMRI能量信号;然后,由血氧水平依赖(blood oxygenation level dependent,BOLD)信号与T2*信号的关系,给出了两种反映BOLD能量变化的瞬时功率fMRI信号;最后,采用空间ICA分析fMRI瞬时功率信号,得到与各脑部活跃区域能量相关的独立成分.从理论和仿真试验两个方面阐明了新方法的合理性和优越性,同时应用于实际癫痫fMRI数据,经与传统ICA方法比较,该方法能够在静息态下鲁棒地检测脑部能量异常区域.     

Effect of fMRI acoustic noise on non-auditory working memory task: comparison between continuous and pulsed sound emitting EPI

Conventional blood oxygenation level-dependent (BOLD) based functional magnetic resonance imaging (fMRI) is accompanied by substantial acoustic gradient noise. This noise can influence the performance as well as neuronal activations. Conventional fMRI typically has a pulsed noise component, which is a particularly efficient auditory stimulus. We investigated whether the elimination of this pulsed noise component in a recent modification of continuous-sound fMRI modifies neuronal activations in a cognitively demanding non-auditory working memory task. Sixteen normal subjects performed a letter variant n-back task. Brain activity and psychomotor performance was examined during fMRI with continuous-sound fMRI and conventional fMRI. We found greater BOLD responses in bilateral medial frontal gyrus, left middle frontal gyrus, left middle temporal gyrus, left hippocampus, right superior frontal gyrus, right precuneus and right cingulate gyrus with continuous-sound compared to conventional fMRI. Conversely, BOLD responses were greater in bilateral cingulate gyrus, left middle and superior frontal gyrus and right lingual gyrus with conventional compared to continuous-sound fMRI. There were no differences in psychomotor performance between both scanning protocols. Although behavioral performance was not affected, acoustic gradient noise interferes with neuronal activations in non-auditory cognitive tasks and represents a putative systematic confound     

Activelets: Wavelets for sparse representation of hemodynamic responses

We propose a new framework to extract the activity-related component in the BOLD functional magnetic resonance imaging (fMRI) signal. As opposed to traditional fMRI signal analysis techniques, we do not impose any prior knowledge of the event timing. Instead, our basic assumption is that the activation pattern is a sequence of short and sparsely distributed stimuli, as is the case in slow event-related fMRI.We introduce new wavelet bases, termed “activelets”, which sparsify the activity-related BOLD signal. These wavelets mimic the behavior of the differential operator underlying the hemodynamic system. To recover the sparse representation, we deploy a sparse-solution search algorithm.The feasibility of the method is evaluated using both synthetic and experimental fMRI data. The importance of the activelet basis and the non-linear sparse recovery algorithm is demonstrated by comparison against classical B-spline wavelets and linear regularization, respectively.     

High resolution SE-fMRI in humans at 3 and 7 T using a motor task

Object The sensitivity of spin echo (SE) experiments to blood oxygenation level dependent (BOLD) contrast was explored in a study of the same six subjects carried out at 3 and 7 T. Materials and methods Multi-slice, single shot, spin echo, echo planar images with a voxel size of 1 × 1 × 3 mm³ were acquired at three different echo times, during execution of a simple motor task. Results Significant activation was observed at all echo times at both field strengths. Analysis of the fractional signal change as a function of echo time indicated that the change in relaxation rate, ΔR ₂, at 7 T was −0.51 ± 0.14 s ⁻¹, which was 1.3 times larger than the value found at 3 T. Measurements of the percentage signal change on activation and temporal signal to noise ratio showed that there was an increase in the BOLD contrast to noise ratio (CNR) at 7 versus 3 T by a factor of 1.9. There was no overlap of areas of significant activation in the SE data acquired at either field strength with the site of large veins. Conclusion SE-BOLD CNR in motor cortex was found to increase significantly at 7 T compared with 3 T.     

fMR; methodology

Changes of myocardial oxygenation can be studied by measurements of the apparent transverse relaxation timeT ₂ ^* , which is correlated with the oxygenation state of hemoglobin. In this study, ten patients with coronary artery disease (CAD) underwent blood oxygenation level dependent (BOLD)T ₂ ^* measurements using a segmented gradient echo pulse sequence with ten echoes.T ₂ ^* measurements were performed in a single short-axis slice of the heart at rest and under pharmacological stress with dipyridamole (DIP), which increases myocardial blood flow. For comparison, all patients underwent X-ray angiography and stress-echocardiography within 4 days after the MR exam. In one patient, MR examination was repeated 10 weeks after percutaneous transluminal coronary angioplasty (PTA). In the differentialT ₂ ^* maps, expected ischemic areas of myocardium were identified in six patients. In these regions,T ₂ ^* values (30±8 ms) were significantly reduced when compared to the remaining myocardium (48±9 ms,P<0.01). In four patients, the myocardial region of interest could not be assessed owing to severe susceptibility artifacts in the ischemic region. The success of the PTA treatment could be visualized from a more homogeneous DIP induced increase inT ₂ ^* within the ischemic myocardium (from 26±1 to 29±1 ms before PTA versus 26±1 to 31±4 ms after PTA,P<0.001. This work was presented in part at the 15th Annual Meeting of the European Society for Magnetic Resonance in Medicine and Biology (Geneva 1998), at the 6th Scientific Meeting of the International Society for Magnetic Resonance in Medicine (Sydney 1998, Book of Abstracts, p. 897), and at the Annual Meeting of the German Society of Cardiology (Mannheim 1998).     

Exploring human brain neuronal currents with phase MRI

Purpose: Brain activity‐associated neuronal currents produce weak transient magnetic fields that would affect both magnitude and phase of the local MRI signal, but these very small signal changes are not reliably detectable with conventional fMRI methodologies. A recent simulation study, using a realistic model specifically for human cerebral cortex, indicates that the phase signal change induced by spontaneous activity may reach a detectable level (up to 0.2°) in favorable conditions. This study aimed to investigate neuronal current‐induced signal changes in human visual cortex with phase MRI. Materials and Methods: Six healthy subjects participated in a phase fMRI study using a temporally well‐controlled visual stimulation paradigm with a known neuronal firing pattern in visual cortex. The precise timing of the paradigm provides a means of detecting and testing the neuronal current‐induced phase signal changes, and placing a series of acquisition windows to fully cover the entire response duration enables a thorough detection of any detectable phase signal changes induced by the stimulus‐evoked neuronal currents. Results: The presented phase MRI method demonstrated to be reliable, and the improved phase measure has achieved a sensitivity level of 0.2° for detecting any significant phase signal changes under a practical length of fMRI session. The test found no sign of any significant neuronal current‐induced phase signal changes in any subject and study. Conclusions: Under the experimental condition, the upper limit of the neuronal current‐induced phase signal changes was found to be less than 0.2° in the human visual cortex, consistent with the model prediction.