Patient characteristics
From February 2018 to September 2020, we prospectively evaluated 121 pretreated glioma patients (73 males, 48 females; mean age, 51.6 ± 11.6 years) who had undergone multimodal therapy, including resection, radiotherapy, alkylating chemotherapy, or combinations thereof (Table S1 (Online Resource 1)). Patients were referred for 3T hybrid PET/MR imaging using the radiolabeled amino acid O-(2-[18F]fluoroethyl)-L-tyrosine (18F-FET) at different time points after completion of first-line therapy (median time, 14 months; range, 1-214 months). The referrals were made to assess residual metabolic active tumor [19] after surgery, evaluate response to adjuvant chemotherapy, or to differentiate tumor relapse from treatment-related changes.
The inclusion criteria comprised an ECOG (Eastern Cooperative Oncology Group) [20] performance score of 0–1, absence of major depression, no seizures at presentation, and fluency in the German language. Patients were screened and registered for the study by phone calls, reviewed on the day of imaging, and were included in the study after providing informed written consent according to the Declaration of Helsinki. The local ethics committee approved the protocol (17–365). As detailed in Table S1 (Online Resource 1), patients had either received tumor resection (n = 108) or biopsy (n = 13), and the majority (n = 100) had undergone at least one series of local radiotherapy (60 ± 2 Gy in 92%) at a median interval of 13 months (range, 2-213 months) between the start of radiotherapy and imaging. Fourteen patients had two radiotherapy series. According to the 2016 WHO classification [1], n = 81 patients had a CNS WHO grade 4 glioblastoma, n = 28 patients a CNS WHO grade 3 astrocytoma, and n = 12 patients a CNS WHO grade 3 oligodendroglioma. A total of 81 patients (67%) had mild neurological (48%) or other symptoms (19%) without requiring assistance for personal needs. All patients except one were right-handed. Based on clinical status, serial MRI findings, and 18F-FET PET results, the diagnosis of a glioma residual or relapse was made in 58 of 121 patients.
Hybrid PET/MR imaging
Simultaneous PET/MR imaging was performed on a 3T hybrid PET/MR scanner (Siemens Trim-TRIO/BrainPET, Siemens Medical Systems, Erlangen, Germany) equipped with a couch-mounted birdcage-like quadrature transmitter head coil and an 8-channel receiver coil. The PET insert consisted of 72 rings (axial field-of-view, 19.2 cm; center spatial resolution, 3 mm FWHM). The PET data were corrected for random and scatter coincidences, dead time, attenuation (based on a T1-weighted anatomical MRI scan), as well as motion artifacts and reconstructed by OPOSEM (ordered Poisson ordinary subset expectation maximization) reconstruction (2 subsets, 32 iterations) [21]. All patients underwent a dynamic PET scan from 0 to 50 min post-injection of 3 MBq of 18F-FET per kg of body. For clinical purposes, the summed activity from 20–40 min post-injection and the time-activity curves were evaluated according to established protocols [14].
The MRI protocol [13] comprised a 3D high-resolution T1-weighted magnetization-prepared rapid acquisition gradient echo (MPRAGE) native scan, a contrast-enhanced MPRAGE scan recorded after injection of gadolinium-based contrast agent, a T2-weighted sampling perfection with application-optimized contrasts using different flip angle evolution (SPACE) scan and a T2-weighted fluid-attenuated inversion recovery (T2/FLAIR) scan. High-angular-resolution diffusion imaging (HARDI) measurements were acquired using a diffusion-weighted double-echo echo-planar imaging (EPI) sequence (55 slices; TR = 8000 ms; TE = 112 ms; b-values = 0, 13 interleaved volumes and b = 2700 s/mm2, 120 gradient directions; voxel size = 2.4×2.4×2.4 mm3). Afterward, a non-diffusion weighted (b = 0) volume was acquired with a reversed phase-encoding direction needed for the EPI distortion correction.
Cognitive performance
Cognitive performance was assessed on the day of imaging and based on ten cognitive tests selected from a more extensive test battery developed for the 1000BRAINS study of healthy subjects [13]. This set of tests could be finished within 20–30 minutes by most of the patients.
As shown in Table S2 (Online Resource 1), the Trail-Making tests A and B were used to asses processing speed/attention and executive function/concept shifting [22]. Semantic word fluency (imagined shopping tour), language processing (number transcoding test), verbal working memory (digit span forward/backward) and verbal episodic memory (word list immediate/delayed recall) were tested by the DemTect dementia screening instrument [23]. Visual-spatial working memory was tested with the Corsi Block-Tapping test (forward/backward) [24]. All scores were compared to those of a group of n = 121 healthy subjects [13] that were matched for age, gender, and educational level (http://uis.unesco.org/sites/default/files/documents/international-standard-classification-of-education-1997-en_0.pdf).
Whole-brain structural connectome
The main steps for determining the whole-brain structural connectome and prediction modeling of cognitive performance are depicted in Fig. 1. We used the tractography imaging pipeline based on the GitHub-fork MRtrix3Tissue (https://3tissue.github.io), a recently developed modification of the fiber-tracking software MRtrix3 (https://www.mrtrix.org) based on constrained spherical deconvolution (CSD) [25]. CSD-based fiber mapping assumes that the diffusion-weighted MRI signal results from the spherical convolution of a response function with the underlying FOD function [26]. The novel single-shell 3-tissue constrained spherical deconvolution (SS3T-CSD) method generates estimates of white matter fiber orientation distribution functions (FODs) as bias-free as possible, even within different compartments infiltrated by the tumor [16, 27].
The HARDI data underwent image preprocessing following published recommendations (https://osf.io/ht7zv) and comprised corrections for EPI distortion, eddy current, motion distortion, and bias field. Fiber tracking was based on Anatomically-Constrained Tractography (ACT), which poses physiological restrictions on the behavior of healthy neuronal fibers in terms of their propagation and termination [17]. These assumptions were lifted in the area of pathologic tissue by masking out the entire lesioned areas in order to allow detection of any residual fiber connections in the structurally altered brain tissue. Resection cavities were manually contoured by a radiation oncologist (M.K.), the T1-contrast-enhancing lesions and T2/FLAIR hyperintensities were automatically segmented using the deep-learning-based software HD-GLIO-AUTO (https://github.com/NeuroAI-HD/HD-GLIO-AUTO), and 18F-FET PET segmentation was implemented by an FSL (https://fsl.fmrib.ox.ac.uk) custom script using a tumor-to-brain ratio (TBR) of 1.6 (20–40 minutes summed activity) as the lower threshold [28]. All segmentations were visually inspected, manually corrected, and added to form a composite lesion mask.
Functional cortical regions (= nodes) were defined using the Schaefer-Yeo Atlas [15] which comprises 100 nodes (50 in each hemisphere) belonging to seven networks: visual, somatomotor, dorsal attention, ventral attention, limbic, frontal control, and default mode network. The number of fibers connecting any two cortical regions (= weighted edge) were entered into a 100x100 structural connectivity matrix, resulting in 4950 possible edges (symmetric matrix, no self-connection of nodes), see Figs. 1 and 2.
Connectome-based predictive modeling
The relationship between structural brain connectivity and cognitive functions was analyzed by an established method for connectome-based predictive modeling (CPM) [18]. All steps were performed in Matlab (Matlab R2022a, MathWorks, Natick, MA, USA). The CPM protocol (Fig. 1) comprises four parts. For feature selection (i), each fiber count (edge) in the connectivity matrix was related to any of the cognitive test scores using Spearman's rank correlation, and only significant (p < 0.001) edges were selected. Next, summary connectivity values (ii) were calculated from the selected edges by separately summing the edge values of negative and positive signs.
For model construction (iii), linear regressions between the cognitive scores and the summary connectivity scores were computed. Finally, the model’s generalizability and predictive power (iv) were evaluated by leave-one-out cross-validation and permutation analysis. In addition, binary matrices were constructed for each test, labelling the predictive edges present in both 90% of the cross-validation models and in those models with significant (p < 0.05) predictive power (validated edges). From these binary matrices, the critical network nodes for each cognitive test were identified from their degree, i.e. the number of adjacent validated edges.