Resting state fMRI brain mapping in pediatric supratentorial brain tumors

Functional mapping of eloquent brain areas is crucial for preoperative planning in patients with brain tumors. Resting state functional MRI (rs-fMRI) allows the localization of functional brain areas without the need for task performance, making it well-suited for the pediatric population. In this study the independent component analysis (ICA) rs-fMRI functional mapping results are reported in a group of 22 pediatric patients with supratentorial brain tumors. Additionally, the functional connectivity (FC) maps of the sensori-motor network (SMN) obtained using ICA and seed-based analysis (SBA) are compared. Different resting state networks (RSNs) were extracted using ICA with varying levels of sensitivity, notably, the SMN was identified in 100% of patients, followed by the Default mode network (DMN) (91%) and Language networks (80%). Additionally, FC maps of the SMN extracted by SBA were more extensive (mean volume = 25,288.36 mm3, standard deviation = 13,364.36 mm3) than those found on ICA (mean volume = 13,403.27 mm3, standard deviation = 9755.661 mm3). This was confirmed by statistical analysis using a Wilcoxon signed rank t test at p < 0.01. Results clearly demonstrate the successful applicability of rs-fMRI for localizing different functional brain networks in the preoperative assessment of brain areas, and thus represent a further step in the integration of computational radiology research in a clinical setting.


Background
Central nervous system (CNS) tumors are among the most common malignancies in childhood and are a leading cause of mortality in this age group [1]. As surgery is the only definitive course of treatment for many of these patients [2], an accurate presurgical functional mapping is crucial to ensure the best possible functional outcome after the surgery. Since the first functional mapping experiments using blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) were published in 1990 [3], BOLD task-based fMRI has been increasingly used in the clinical setting for presurgical functional mapping. Task-based fMRI (tb-fMRI) entails the performance of a task during scanning; this is typically alternated with a control condition in a symmetrical block design paradigm. Tb-fMRI requires the patient's understanding and cooperation during the examination to acquire useful results [4][5][6].
Resting-state fMRI (rs-fMRI) was introduced in 1995 by Biswal and colleagues in their seminal work [7]. Rs-fMRI assesses low-frequency fluctuations in the BOLD signal while the subject is at rest, enabling the detection of functionally correlated cortical areas even if they are anatomically remote. This lead to the discovery of the so-called resting-state networks (RSNs) [8][9][10]. The two most commonly used techniques for rs-fMRI analysis are seed-based analysis (SBA) and independent component analysis (ICA). SBA is performed by placing seeds corresponding to the target RSN. It depends on a priori knowledge of expected location of seeds comprising the network, typically a priori atlases or parcellation maps. On the other hand, ICA is a data driven approach which depends on the analysis of signal alterations along the time-course of each voxel and the extraction of correlated activation areas [11]. The main advantages of rs-fMRI over tb-fMRI are less time needed for whole brain mapping, as multiple networks can be extracted from one scan, and less cooperation is required from the patient [11].
In this study, the feasibility of functional mapping using ICA rs-fMRI in pediatric patients with supratentorial brain tumors are reported as well as the feasibility of extracting the RSNs most relevant for presurgical mapping, including sensory-motor network (SMN), language network, default mode network (DMN), dorsal attention network (DAN), medial visual and lateral visual networks. Additionally, SBA functional connectivity (FC) maps of the SMN were extracted and compared to the SMN ICA FC maps in the lesioned hemisphere.

Methods
The present descriptive study was conducted during a period of 18 months, from February 2018 to July 2019. Institutional review board (IRB) approval, in accordance with the Declaration of Helsinki was acquired. A signed written consent form was acquired from the legal guardians of the patients before scanning.

Participants
This study was carried out on 22 patients; (15 males), age range 2-18 years (mean age 8.6 years). The study included patients with supratentorial tumors with no history of previous surgical intervention, radiotherapy or chemotherapy or other CNS diseases. Patients were excluded from the study if they had an absolute contraindication to MRI scanning, a degenerative or metabolic disease that may involve the CNS or a multicentric or multifocal brain tumor. Table 1 summarizes the clinical and demographic data of the patients. The most common pathology was Ganglioglioma (n = 4) followed by pleomorphic Xanthoastrocytoma (n = 3), meningioma (n = 3) and dysembryoblastic neuroepithelial tumor (DNET, n = 3). The most common location of the tumor was in the frontal lobe (n = 9) followed by the temporal lobe (n = 7).

Clinical assessment
Patients were assessed using 5 components of the pediatric NIH stroke scale (NIHSS) [12] the visual, motor, and sensory domains were used without any modification, while the language and dysarthria components were used after translation to Arabic. This modified pediatric NIH stroke scale graded performance on each domain between 0 (normal) and 4 (worst performance), with higher scores indicating worse disability (Additional file 1: Table S1). Patients were assessed preoperatively and at 3 months post-operatively. When functional deficits covered by the Pediatric NIH stroke scale were absent, signs of increased intracranial tension (ICT) or seizures, were recorded.

MR imaging protocol
MR imaging used a 3 Tesla (T) Ingenia MRI scanner (Philips, Eindhoven, the Netherlands) equipped with a 16-channel receive phased-array head coil.

Data processing and analysis
All structural and fMRI images were preprocessed using the CONN toolbox [13]. The default CONN preprocessing pipeline was used, this included the following steps: functional realignment and unwarping; slice timing correction; structural and functional segmentation and normalization to Montreal National Institute (MNI) template, outlier rejection, and functional smoothing. Spatial smoothing was performed using a Gaussian kernel with full width at half maximum of 6.0 mm. Denoising was done using linear regression to remove signal changes related to white matter, cerebrospinal fluid, motion, breathing and cardiac pulsations. Finally, temporal band-pass filtering at (0-0.9 Hz) and linear detrending were performed. Preprocessed and denoised data generated by CONN was analyzed using ICA and SBA. The fMRI Software Library's (FSL) [14] Multivariate Exploratory Linear Optimized Decomposition (MELODIC) [15] was used to generate spatio-temporally independent components (ICs) using single-subject ICA. 20 components were generated from each dataset, FSL generated z-maps were converted to uncorrected p-maps using fslmaths. Seed-to-voxel analysis using CONN's first-level analysis was used to generate seed-based functional connectivity uncorrected p-maps.

ICA
Independent components were examined by two neuroradiologists with 6 and 4 years of experience, while being blinded to the SBA results. The radiologists independently aimed to identify all target networks possible from the data. All networks were identified by the radiologists through their experience and in comparison to published literature on the morphology and the anatomical location of the RSNs [16,17]. When disagreement emerged, a final decision was reached by discussion. The language network was defined by the presence of at least 2 of 4 functionally connected components, namely Wernicke's area and/or its right sided homologue (posterior superior temporal gyrus) and Broca's area and/or its right sided homologue (inferior frontal gyrus-pars opercularis and pars triangularis), connectivity in at least 2 of the 4 areas was required to identify the candidate RSN as a language network. When disagreement occurred, the raters reviewed the data together and consensus was achieved.

SBA
Seeds were defined using the contralesional hemisphere labels of the SMN resting-state network atlas included with CONN derived from Human connectome project (HCP) rs-fMRI data [18,19].

Comparing SMN ICA and SBA functional connectivity in the lesioned hemisphere
ICA and seed-based FC maps for SMN were generated with a p value of less than 0.01-thus we avoided using direct thresholding-and the resulting images were binarized. Next we applied a mask of the SMN region of interest (ROI) of the diseased hemisphere from the CONN networks atlas after 3-pass dilatation to account for intersubject variability as well as for possible displacement caused by the tumor.

Clinical outcome
Pre and 3 months post-operative clinical evaluation of the included patients was performed (Table 2). Nine patients presented with signs of increased ICT or generalized seizures, of whom 8 resolved at 3 months postoperatively, 2 other patients presented with attacks of localized motor seizures without deficits and developed motor deficits post-operatively. Ten patients presented with neurological deficits according to modified NIHSS and one patient died postoperatively. Re-assessment of 9 remaining patients at 3 months postoperatively showed complete resolution or improvement in 6 patients, stationary course in 2 patients and one patient showed worsening scale. One patient presented with a mass at the back of his head felt by the mother that turned out to be Ewing Sarcoma of the occipital bone, he did not develop deficits post-operatively.

ICA versus SBA
SBA maps showed statistically significant difference compared to ICA maps (Wilcoxon signed rank test p < 0.01), with SBA showing larger maps of activation (Table 3) (Fig. 7).

Discussion
Throughout the past decade, rs-fMRI has been validated in multiple studies as a technique for presurgical brain mapping, where results were highly concordant with functional maps extracted through tb-fMRI, intraoperative electro-cortical stimulation (ECS) and even direct visual identification of the motor cortex based on anatomical landmarks [20][21][22].
Multiple studies compared the reliability of rs-fmri analysis to tb-fMRI, ECS or both. Zhang el al. [23] conducted a study on 17 healthy controls and 4 patients, using SBA rs-fMRI. They were able to localize sensorimotor cortex compared to ECS and performed as well or better than tb-fMRI. In a study on 17 supratentorial glioma patients, SBA rs-fMRI was performed using a seed placed on the hand knob area of the healthy hemisphere. Compared to ECS, the overall sensitivity and specificity values of rs-fMRI were 90.91 and 89.41%, respectively. In the 15 patients who were able to perform tb-fMRI, the sensitivity and specificity values for the hand motor area were 78.57 and 84.76%, respectively. For these 15 patients who underwent both rs-fMRI and tb-fMRI, there was no statistical difference in sensitivity or specificity between these two methods [24]. Another study conducted on 10 healthy subjects and 25 patients with left sided brain tumors compared tb-fMRI, SBA rs-fMRI and visual anatomical localization of the hand motor found important differences in the locations (i.e., ˃ 20 mm) of the determined hand motor voxels by these three MR imaging methods [20]. In the current study, the sensorimotor cortex was detected using SBA and ICA maps in all patients, the pattern of activation was correlated to the anatomical structure regarding the expected activation area and the two networks showed similar activation areas in the lesional hemisphere. Several studies used ICA rs-fMRI to extract the SMN, Kokkonen et al. [25] compared tb-fMRI and rs-fMRI using ICA in a group of patients with brain tumors of heterogeneous histopathology as well as a group of healthy controls, they found successfully localized the SMN in both groups, i.e. the brain tumors did not hamper visualization of the SMN in the cases. In the resting state group ICA, both the cases and the controls presented sensorimotor areas similar to the motor task group ICAs and to each other. Schnieider et al. [26] compared ICA rs-fMRI to tb-fMRI in a group of 19 brain tumor patients. They successfully and consistently identified motor cortex by using rs-fMRI and visual assessment by 3 radiologists for all patients. Even when subdividing the motor cortex into 3 segments (hand, face and foot), the sensitivities of  rs-fMRI and tb-fMRI were comparable. Another study compared both SBA and ICA RSNs to ECS and found significant correlation between resting-state low frequency fluctuations of BOLD signal and the functional unity of distinct cortical areas confirmed by ECS. Additionally, the ICA method succeeded in detection of the sensori-motor and language networks in 90 (92%) and 73 (75%) cases respectively [27]. In the current study, using ICA, the sensorimotor network was identified in 100% of cases and the 4 components of language network were identified in 41% of cases. When comparing the SMN FC maps extracted using SBA and ICA, SBA maps were larger than ICA maps at a p value of ≤ 0.01.
Tie et al. [28] tried detecting ICAs related to language networks in preoperative patients, with 5 experts rating the language networks in ICAs generated from rs-fMRI data of 17 patients. They found moderate agreement between experts, moreover they found that 5 experts agreed upon 1 component most representative of the language network in 10 of the patients, which jumped to 14 upon consensus of only 4 experts. In the present study, using ICA, at least two of the components the language network was detected in 17 out of 22 patients whilst the 4 components of the language network were identified in 9 out of 22 patients. The two main methods for ICA RSN recognition are subjective identification of these networks by experts [23,25] and using automated software for detection [29], however the latter may be limited by structural changes caused by tumoral infiltration and mass effect of the tumor on target regions of interest [29]. Additionally, some of the RSNs are more readily reproducible than others, for example, in our study we were able to detect the SMN and DMN more frequently than other RSNs e.g. lateral visual, and DAN.
Thresholding FC maps to segregate actual activity from noise is a debatable subject, especially due to inter-individual differences in activation intensity [30]. A common practice in clinical pre-surgical fMRI is manual thresholding, which is operator dependent [30][31][32]. Some authors suggested methods to standardize the thresholding of the extracted FC maps including setting a percentage of the maximum intensity detected below which activations are discarded [30]. Even with manual thresholding, radiologists should be aware of the differences in intensity values of FC maps generated by their analysis pipeline. Even with manual thresholding, radiologists should be aware of the difference in the extent of FC maps generated by their analysis pipeline. In our study, the SBA pipeline generated larger FC maps, consequently, more thresholding parameters would be required on these maps for proper delineation of RNSs required for clinical pre-surgical mapping. A recent meta-analysis of published preoperative tb-fMRI studies showed that preoperative fMRI planning -in addition to other advanced imaging techniques-reduced post-operative morbidity [33]. It is safe to project the same conclusions to rs-fMRI, since its maps are closely correlated to those of tb-fMRI, as previously mentioned.  Nevertheless, the correlation between preoperative rs-fMRI and post-operative outcome -especially in pediatric patients-is a difficult subject due to several factors. First, there is no consensus on a clinical functional assessment method for pediatric brain tumors; in fact, many clinical assessment techniques for brain neoplastic lesions rely more on the day-to-day well being rather than gross and fine motor, sensory and language skills e.g. Karnofsky Performance Score, European Organisation for Research and Treatment of Cancer Quality of Life Core Questionnaire..etc. [34]. Other factors include different localization of the neoplastic lesions in relation to the canonical sensori-motor and language eloquent brain areas and their effect on these areas (e,g, displacement Vs infiltration) and surgical approach to the lesions (e.g. debulking Vs excision). The correlation of the tumoral effect on other RSNs e.g. DMN and post-operative cognitive outcome is similarly challenging as in-addition to the previous limitations-the role these networks play in cognitive functions is not yet fully understood [35]. One of the promising methods for better understanding these global and local cerebral RSNs is graph theory analysis, which may also play an important role in the future in the preoperative planning and preservation brain network connectomics through critical node planning [36]. This study was not without limitations, the heterogeneous pathology and location of the tumors -even though reflects actual daily practice-falls short of accounting for their effect on the detection and extent of the extracted RSNs. The definition of the language network did not include some of the ROIs that may play a role in language processing e.g. premotor cortex. Finally, detection of language lateralization using rs-fMRI is still a challenging research subject with promising published data [37,38].

Conclusions
In summary, rs-fMRI is a promising tool for pre-surgical planning and predicting the functional prognosis in supratentorial brain tumors. Its importance is specifically higher in the pediatric population. Integration of rs-fMRI in different institutions requires good understanding of the basic concepts and technical aspects of the examination as well as adequate knowledge on how to analyze and interpret the results.