Automated alignment of perioperative MRI scans: A technical note and application in pediatric epilepsy surgery

Intentional Insertion of Air to Predict the Diagnostic Accuracy of Stereotactic Biopsy and a Uniform Grading System for Reporting

BACKGROUND AND OBJECTIVES

Stereotactic biopsies are a relatively safe and reliable way of tissue diagnosis and characterization of eloquent area lesions/neoplasm. However, predicting the accuracy of the site of biopsy with the desired/planned site is not always possible. We describe a technique to identify the precise location of the biopsy site in the post-operative computed tomography (CT) scan using the injection of a low volume of air into the biopsy cannula.

METHODS

Hundred consecutive biopsies were performed in 80 adults/20 children (59 males/41 females, median age 51 years) over 3 years, consisting of 75 frameless and 25 frame-based stereotactic biopsies. After the biopsy specimens had been collected, a small volume of air (median 1 cc) was injected into the site. Post-operative CT was done within 4 hours of the biopsy to see the site of the air bubble, and the same was correlated with the histopathological accuracy.

RESULTS

Intra-cranial air in the selected target was present in 95 patients (Grade 1 and 2), while the air was seen in the track (Grade 3) in 3% and at an unrelated site (Grade 4) in 2% of cases. Both Grade 4 biopsies were negative on histopathology (diagnostic yield = 98%). Two negative biopsies were reported, which were both predicted with the Grade 4 biopsy. The grading allowed uniform reporting across series and eliminated the chance of upgrading/downgrading the report due to wrong site sampling within the lesion/neoplasm.

CONCLUSION

The air-injection manoeuvre proposed for use in stereotactic biopsies of intra-cranial mass lesions is a safe and reliable technique that allows the exact biopsy site to be located without any related complications.

Shift in electrocorticography electrode locations after surgical implantation in children

Interpreting electrocorticography (ECoG) in the context of neuroimaging requires that multimodal information be integrated accurately. However, the implantation of ECoG electrodes can shift the brain impacting the spatial interpretation of electrode locations in the context of pre-implant imaging. We characterized the amount of shift in ECoG electrode locations immediately after implant in a pediatric population. Electrode-shift was quantified as the difference in the electrode locations immediately after surgery (via post-operation CT) compared to the brain surface before the operation (pre-implant T1 MRI). A total of 1140 ECoG contracts were assessed across 18 patients ranging from 3 to 19 (12.1 ± 4.8) years of age who underwent intracranial monitoring in preparation for epilepsy resection surgery. Patients had an average of 63 channels assessed with an average of 5.64 ± 3.27 mm shift from the pre-implant brain surface within 24 h of implant. This shift significantly increased with estimated intracranial volume, but not age. Shift also varied significantly depending of the lobe the contact was over; where contacts on the temporal and frontal lobe had less shift than the parietal. Furthermore, contacts on strips had significantly less shift than those on grids. The shift in the brain surface due to ECoG implantation could lead to a misinterpretation of contact location particularly in patients with larger intracranial volume and for grid contacts over the parietal lobes.

Changing concepts in presurgical assessment for epilepsy surgery

Candidates for epilepsy surgery must undergo presurgical evaluation to establish whether and how surgical treatment can stop seizures without causing neurological deficits. Various techniques, including MRI, PET, single-photon emission CT, video-EEG, magnetoencephalography and invasive EEG, aim to identify the diseased brain tissue and the involved network. Recent technical and methodological developments, encompassing both advances in existing techniques and new combinations of technologies, are enhancing the ability to define the optimal resection strategy. Multimodal interpretation and predictive computer models are expected to aid surgical planning and patient counselling, and multimodal intraoperative guidance is likely to increase surgical precision. In this Review, we discuss how the knowledge derived from these new approaches is challenging our way of thinking about surgery to stop focal seizures. In particular, we highlight the importance of looking beyond the EEG seizure onset zone and considering focal epilepsy as a brain network disease in which long-range connections need to be taken into account. We also explore how new diagnostic techniques are revealing essential information in the brain that was previously hidden from view.

A systematic evaluation of intraoperative white matter tract shift in pediatric epilepsy surgery using high-field MRI and probabilistic high angular resolution diffusion imaging tractography

OBJECTIVE Characterization of intraoperative white matter tract (WMT) shift has the potential to compensate for neuronavigation inaccuracies using preoperative brain imaging. This study aimed to quantify and characterize intraoperative WMT shift from the global hemispheric to the regional tract-based scale and to investigate the impact of intraoperative factors (IOFs). METHODS High angular resolution diffusion imaging (HARDI) diffusion-weighted data were acquired over 5 consecutive perioperative time points (MR₁ to MR₅) in 16 epilepsy patients (8 male; mean age 9.8 years, range 3.8-15.8 years) using diagnostic and intraoperative 3-T MRI scanners. MR₁ was the preoperative planning scan. MR₂ was the first intraoperative scan acquired with the patient's head fixed in the surgical position. MR₃ was the second intraoperative scan acquired following craniotomy and durotomy, prior to lesion resection. MR₄ was the last intraoperative scan acquired following lesion resection, prior to wound closure. MR₅ was a postoperative scan acquired at the 3-month follow-up visit. Ten association WMT/WMT segments and 1 projection WMT were generated via a probabilistic tractography algorithm from each MRI scan. Image registration was performed through pairwise MRI alignments using the skull segmentation. The MR₁ and MR₂ pairing represented the first surgical stage. The MR₂ and MR₃ pairing represented the second surgical stage. The MR₃ and MR₄ (or MR₅) pairing represented the third surgical stage. The WMT shift was quantified by measuring displacements between a pair of WMT centerlines. Linear mixed-effects regression analyses were carried out for 6 IOFs: head rotation, craniotomy size, durotomy size, resected lesion volume, presence of brain edema, and CSF loss via ventricular penetration. RESULTS The average WMT shift in the operative hemisphere was 2.37 mm (range 1.92-3.03 mm) during the first surgical stage, 2.19 mm (range 1.90-3.65 mm) during the second surgical stage, and 2.92 mm (range 2.19-4.32 mm) during the third surgical stage. Greater WMT shift occurred in the operative than the nonoperative hemisphere, in the WMTs adjacent to the surgical lesion rather than those remote to it, and in the superficial rather than the deep segment of the pyramidal tract. Durotomy size and resection size were significant, independent IOFs affecting WMT shift. The presence of brain edema was a marginally significant IOF. Craniotomy size, degree of head rotation, and ventricular penetration were not significant IOFs affecting WMT shift. CONCLUSIONS WMT shift occurs noticeably in tracts adjacent to the surgical lesions, and those motor tracts superficially placed in the operative hemisphere. Intraoperative probabilistic HARDI tractography following craniotomy, durotomy, and lesion resection may compensate for intraoperative WMT shift and improve neuronavigation accuracy.