Neuroimaging for preoperative epilepsy surgery planning has the objectives of: (i) finding the underlying abnormal anatomical structure linked to the epileptic activity; (ii) describing in detail the anatomy of the individual patient who will undergo surgery; and, (iii) serving as an intraoperative guide during the neurosurgical procedure. The number of tests is chosen according to the patient, the lesion location(s) and the availability of imaging. The most widely-used technique is MRI, with the choice of protocols depending of the hospital and the patient. At Great Ormond Street Hospital, a range of sequences, including various T1 and T2-weighted protocols, are chosen according to the needs of the patient. Adding a DTI protocol and performing tractography could assist with patient care in two important ways:
1. Identifying critical white matter tracts so surgical procedures can be altered to minimise the risk of tract damage.
2. Providing an additional visual aid for the clinician and parents to help them better understand the underlying causes and impacts of epilepsy.
Each patient has a different anatomy, so it is not practical to use anatomical atlases for surgical planning. Identifying tracts is particularly important for patients with distorted anatomies. The value of DTI tractography in supporting surgical procedures in this way is demonstrated in Section 8.3 for brain tumour patients.
Based on testing of a range of DTI protocols, the new 60-direction protocol that was adopted by this study was the most suitable for paediatric preoperative planning, as it produced higher-quality images in a shorter scanning time compared to other protocols. This protocol is fully described in Table 3.1 and Section 3.2.
7.6 Conclusions
Epilepsy is the most common neurological condition in childhood, with a prevalence of 1%, while 2–4% of children under 5 years old suffer an episode of febrile convulsions. Little is known about how seizures affect the white matter pathways of the brain that are not involved in the epileptic focus. This chapter has examined the impact of seizures on the optic radiations in the paediatric population using DTI tractography.
A cohort of 20 children with long-term epileptic activity have been compared with age-matched controls. The epilepsy cohort presented a decreased mean FA for the front of both optic radiations compared with the control cohort, despite the patients having neither visual epileptic activity nor visual impairment. Several hypotheses are proposed that could explain this conclusion, including the poor functioning of the thalamus or neurotransmitters, or structural damage caused by repeated seizure episodes.
A cohort of 21 children who had suffered a single episode of prolonged febrile convulsions were also compared with age-matched controls. This cohort did not present lower mean FA in either optic radiation, suggesting that a single seizure episode might not be sufficient to cause widespread damage to the optic radiations. It is possible that small-scale damage was not detected using the clinical tractography protocol, or that the patients recovered to some extent between the seizure and the MRI scan. Other motor or cognitive white matter pathways near the epileptic focus could have been more affected by the seizure.
The new 60-direction DTI tractography protocol adopted by this study was identified as the most suitable for improving paediatric preoperative planning, as it produced higher-quality images in a shorter scanning time than other protocols that were tested.
8 T
RACTOGRAPHY
APPLICATIONS IN
NEUROSURGERY
Understanding the relationship between brain structure, and three-dimensional mapping of the brain, are foundations of successful neurosurgery. The living brain is not susceptible to dissections and manipulations without causing extreme damage. The aim of this chapter is to examine how tractography imaging can help neurosurgeons to perform safer and better informed surgical procedures, by providing safe and non-invasive descriptions of individual white matter tracts that account for variations between patients and the impacts of pathologies.
The potential benefits of tractography are examined in two case studies. The first case study demonstrates the benefits of DTI tractography for patients with a brain tumour and with tuberous sclerosis. The second case study examines hydrocephalus patients, and demonstrates the value of VEP-enhanced tractography, which was introduced in Chapter 5.
8.1 Introduction
Neurosurgery is constantly evolving as new technologies and techniques become available and increasingly complex cases are operated on. Technology has been and continues to be an important tool for diagnosis and treatment of several brain
pathologies, and improvements have contributed to improving outcomes and reducing complications by minimising unintended damage.
Tractography is a new imaging technique that describes individual white matter tracts and it is starting to be included in several hospital protocols for preoperative imaging, for example at Great Ormond Street Hospital where this study took place. For many pathologies, for example hydrocephalus, few or no case reports have been published using tractography, and there are no standard DTI or tractography analysis protocols for this purpose (Liasis et al., 2009).
8.2 Methodology
Children were recruited following referrals from the lead clinician. All four cases were inpatients at the Neurosurgery department at GOSH. The standard recruitment protocol and consent described in Section 3.1 was used in all cases. The patient and family were approached by the author and written and verbal information about the study was provided. The patient was included in the study when consent was granted. DTI was added to the clinical scan MRI protocol and analysed as described in Section 3.4. DTI and tractography maps of the white matter were produced in all cases. The tractography analysis method from Section 3.4 was used for both brain tumour cases. For the hydrocephalus patients, visual function was tested using VEPs and analysed using the NeuroScan software, as described in Section 5.3. The aim of using a three- dimensional display of visual electric activity was to produce maps that could be co- registered with the DTI maps so ROIs could be located and used as an aid to produce tractography maps of the optic radiations. Since the brain anatomy was so distorted in both cases, it was difficult to identify anatomical structures related to the primary visual cortex so it was not possible to use the tractography method developed in Section 3.4. Instead, the hydrocephalus ROI seeding points were identified using functional information from the VEPs. The tractography maps were produced for the optic radiations and given to the clinician as an aid to the surgery. In one case, a third ventriculostomy was performed, while a VP shunt was the chosen procedure in the other.
The original aim was to repeat the MRI and functional VEP procedure following surgery. However, this was not possible in either case, because one patient had
complications during the surgery and the second woke up during the MRI scan and could not be reassessed before discharge. The comparison of preoperative and postoperative scans for hydrocephalus patients is a future research ambition.