Real-time 3T fMRI data of brain tumour patients for intra-operative localization of primary motor areas

The utilisation of resting-state fMRI as a pre-operative mapping tool in patients with brain tumours in comparison to task-based fMRI and intraoperative mapping: A systematic review

PURPOSE

Resting-state functional Magnetic Resonance Imaging (rs-fMRI) is suggested to be a viable option for pre-operative mapping for patients with brain tumours. However, it remains an open issue whether the tool is useful in the clinical setting compared to task-based fMRI (T-fMRI) and intraoperative mapping. Thus, a systematic review was conducted to investigate the usefulness of this technique.

METHODS

A systematic literature search of rs-fMRI methods applied as a pre-operative mapping tool was conducted using the PubMed/MEDLINE and Cochrane Library electronic databases following PRISMA guidelines.

RESULTS

Results demonstrated that 50% (six out of twelve) of the studies comparing rs-fMRI and T-fMRI showed good concordance for both language and sensorimotor networks. In comparison to intraoperative mapping, 86% (six out of seven) studies found a good agreement to rs-fMRI. Finally, 87% (twenty out of twenty-three) studies agreed that rs-fMRI is a suitable and useful pre-operative mapping tool.

CONCLUSIONS

rs-fMRI is a promising technique for pre-operative mapping in assessing the functional brain areas. However, the agreement between rs-fMRI with other techniques, including T-fMRI and intraoperative maps, is not yet optimal. Studies to ascertain and improve the sophistication in pre-processing of rs-fMRI imaging data are needed.

Blood oxygen level dependent functional magnetic resonance imaging for presurgical planning

Functional magnetic resonance imaging (fMRI) has become a common tool for presurgical sensorimotor mapping, and is a significant preoperative asset for tumors located adjacent to the central sulcus. fMRI has changed surgical options for many patients. This noninvasive tool allows for easy display and integration with other neuroimaging techniques. Although fMRI is a useful preoperative tool, it is not perfect. Tumors that affect the normal vascular coupling of neuronal activity will affect fMRI measurements. This article discusses the usefulness of blood oxygen level dependent (BOLD) fMRI with regard to preoperative motor mapping.

Online spatial normalization for real-time FMRI

Real-time functional magnetic resonance imaging (rtfMRI) is a recently emerged technique that demands fast data processing within a single repetition time (TR), such as a TR of 2 seconds. Data preprocessing in rtfMRI has rarely involved spatial normalization, which can not be accomplished in a short time period. However, spatial normalization may be critical for accurate functional localization in a stereotactic space and is an essential procedure for some emerging applications of rtfMRI. In this study, we introduced an online spatial normalization method that adopts a novel affine registration (AFR) procedure based on principal axes registration (PA) and Gauss-Newton optimization (GN) using the self-adaptive β parameter, termed PA-GN(β) AFR and nonlinear registration (NLR) based on discrete cosine transform (DCT). In AFR, PA provides an appropriate initial estimate of GN to induce the rapid convergence of GN. In addition, the β parameter, which relies on the change rate of cost function, is employed to self-adaptively adjust the iteration step of GN. The accuracy and performance of PA-GN(β) AFR were confirmed using both simulation and real data and compared with the traditional AFR. The appropriate cutoff frequency of the DCT basis function in NLR was determined to balance the accuracy and calculation load of the online spatial normalization. Finally, the validity of the online spatial normalization method was further demonstrated by brain activation in the rtfMRI data.

Safe resection of arteriovenous malformations in eloquent motor areas aided by functional imaging and intraoperative monitoring

BACKGROUND

Arteriovenous malformations (AVMs) proximal to motor cortical areas or motor projection systems are challenging to manage because of the risk of severe sensory and motor impairment. Surgical indication in these cases therefore remains controversial.

OBJECTIVE

To propose a standardized approach for centrally situated AVMs based on functional imaging and intraoperative electrophysiological evaluation.

METHODS

We conducted a retrospective analysis of 15 patients who underwent surgical treatment for AVMs in motor cortical areas or proximal to motor projections. Preoperative assessment included functional magnetic resonance and 3-dimensional tractography. Operations were performed under continuous electrophysiological monitoring aided by direct brain stimulation. We identified critical bloody supply to the motor areas by temporary occluding the feeding vessels under electrophysiological monitoring. Clinical outcome was evaluated with the modified Rankin Scale.

RESULTS

Total resection was achieved in 12 cases, whereas electrophysiology limited total extirpation in 3 cases. A significant reduction of motor evoked potentials by up to 15% of the initial values was associated with good recovery of motor function; in contrast, the disappearance of potentials correlated with long-term impairment. The mean follow-up time was 13 months, and clinical assessments revealed overall functional improvement (P < .05). After surgery, 11 patients were asymptomatic or presented with only minor neurological deficits.

CONCLUSION

Surgical resection of AVMs in eloquent motor areas can be considered a safe option for selected cases when performed in conjunction with a detailed functional assessment. Possible selection criteria for surgical treatment are discussed in light of the presented clinical data.

Special surgical considerations for functional brain mapping

The development of functional mapping techniques gives neurosurgeons many options for preoperative planning. Integrating functional and anatomic data can inform patient selection and surgical planning and makes functional mapping more accessible than when only invasive studies were available. However, the applications of functional mapping to neurosurgical patients are still evolving. Functional imaging remains complex and requires an understanding of the underlying physiologic and imaging characteristics. Neurosurgeons must be accustomed to interpreting highly processed data. Successful implementation of functional image-guided procedures requires efficient interactions between neurosurgeon, neurologist, radiologist, neuropsychologist, and others, but promises to enhance the care of patients.

Real-time fMRI: a tool for local brain regulation

Real-time fMRI permits simultaneous measurement and observation of brain activity during an ongoing task. One of the most challenging applications of real-time fMRI in neuroscientific and clinical research is the possibility of acquiring volitional control of localized brain activity using real-time fMRI-based neurofeedback protocols. Real-time fMRI allows the experimenter to noninvasively manipulate brain activity as an independent variable to observe the effects on behavior. Real-time fMRI neurofeedback studies demonstrated that learned control of the local brain activity leads to specific changes in behavior. Here, the authors describe the implementation and application of real-time fMRI with particular emphasis on the self-regulation of local brain activity and the investigation of brain-function relationships. Real-time fMRI represents a promising new approach to cognitive neuroscience that could complement traditional neuroimaging techniques by providing more causal insights into the functional role of circumscribed brain regions in behavior.

A joint acquisition-reconstruction paradigm for correcting inhomogeneity artifacts in MR echo planar imaging

One of the main sources of signal degradation in rapid MR acquisitions, such as Echo Planar Imaging (EPI), is magnetic field variations caused by field inhomogeneities and susceptibility gradients. If unaccounted for during the reconstruction process, this spatially-varying field can cause severe image artifacts. In this paper, we show that correcting for the resulting degradations can be formulated as a blind image deconvolution problem. We propose a novel joint acquisition-processing paradigm to solve this problem. We describe a practical implementation of this paradigm using a multi-image acquisition strategy and a corresponding joint estimation-reconstruction algorithm. The estimation step computes the spatial distribution of the field maps, while the reconstruction step yields a Minimum Mean Squared Error (MMSE) estimate of the imaged slice. Our simulations show that this proposed joint acquisition-reconstruction method is robust and efficient, offering factors of improvement in the quality of the reconstructed image as compared to other traditional methods.