Análisis filogenético

4.6.3.1

Postoperative diffusion changes

Initial studies in epilepsy used DTI as a marker for Wallerian degeneration or plasticity. Following surgery resulting in a VFD, diffusion parameters of the optic radiation alter consistent with Wallerian degeneration (reduced FA, increased MD) (Wieshmann et al. 1999). Region-of-interest analysis of the sagittal stratum (comprising optic radiation and ILF) in 14 patients shows significantly lowered FA on the operated side by 4 weeks with a greater difference in those with more severe deficits (Taoka et al. 2005). Similarly change in FA of the ipsilateral ILF at 2 months post-surgery correlates with the VFD (McDonald et al. 2010).

Paper Subjects Aim Algorithm, Software, Parameters Regions-of-interest Ciccarelli et al. 2003a,

Ciccarelli et al. 2003b

11 controls 21 controls

Reproducibility assessment Producing group maps

D (FMT) Seed voxel at apex of Meyer’s loop, restrict to

hemisphere posterior to third ventricle

Behrens et al. 2003a 8 controls Thalamo-cortical connections P (FSL - Bayesian) Seed voxel lateral to LGN

Catani et al. 2003 11 controls Occipito-temporal connections D (custom), FA 0.2 Seeds in LGN + occipital white matter Kamada et al. 2005 2 patients undergoing

surgery

Surgical monitoring with VEP D (dTV), FA 0.15 Seed voxel at apex of Meyer’s loop

Ciccarelli et al. 2005 10 controls 7 optic neuritis

Group maps after optic neuritis D (FMT), FA 0.1, ADC 0.002 Single voxel at apex of Meyer’s loop with FA>0.5, restricted to hemisphere posterior to genu

Powell et al. 2005 2 patients undergoing ATLR

Predicting visual outcome P (Camino - PICo) Single voxel anterior to LGN (eigenvector in anteromedial-posterolateral direction) Yu et al. 2005 16 patients

(3 with tumours)

Surgical planning D (Siemens Leonardo), FA 0.2, angle 20 Seeds in LGN (sagittal) and occipital

Yamamoto et al. 2005 5 controls Depicting 3 layers of optic radiation

D (Philips PRIDE) Seeds in LGN (sagittal) and near visual cortex (upper, between, lower calcarine fissure)

Kikuta et al. 2006 10 patients with AVM Correlation with visual outcome D (DTIStudio - FACT), FA 0.2, angle 41 Seeds in LGN and occipital Yamamoto et al. 2007 12 controls Comparison of acquisition

schemes

D (DTIStudio - FACT), FA 0.25, angle 70 Seeds in LGN (sagittal) and occipital (coronal)

Okada et al. 2007 34 patients with AVM Quantitative analysis by lesion D (DTIStudio - FACT), FA 0.2, angle 70 Seeds in temporal stem including Meyer’s loop (sagittal), calcarine cortex (coronal), exclusion zones medial and lateral to Meyer’s loop (coronal)

Romano et al. 2007 25 patients with tumours Integration into neuronavigation D (BrainLab), FA 0.2, angle 35 Seed in LGN Maruyama et al. 2007 10 patients with AVM Integration into gamma knife

planning

D (dTV), FA 0.18 Seed voxels in apex of Meyer’s loop, with target in calcarine fissure

Nilsson et al. 2007 7 controls, 2 patients undergoing ATLR

Assess variability in Meyer’s loop D (Philips PRIDE), FA 0.25 Seeds anterior to LGN (coronal) and stratum sagittale (coronal)

Govindan et al. 2008 10 patients with occipital lobe epilepsy

Plasticity following occipital lobectomy

D (FACT), FA 0.2, angle 60 Seeds posterior thalamus (coronal) and medial Meyer’s loop (sagittal), exclusion zones to remove artefactual connections

Bassi et al. 2008 37 preterm infants Correlation with visual function P (FSL - Bayesian), threshold 10% Seed white matter lateral to LGN (9 voxels), restrict to hemisphere posterior to third ventricle

Wang et al. 2008 10 controls Define landmarks for tracts D (dTV), FA 0.18, 160 steps Seed occipital (coronal), target LGN (sagittal) Taoka et al. 2008 14 patients undergoing

ATLR/SAH

Correlation between Meyer’s loop damage and VFD

D (dTV), FA 0.18, no angular threshold Seed anterior to LGN (coronal), target stratum sagittale (coronal)

Sherbondy et al. 2008 8 controls Apply ConTrack algorithm P (ConTrack), angle 130, length <300mm Seeds 4mm sphere in LGN (determined by streamline from optic chiasm) + calcarine sulcus

Chen et al. 2009 48 patients undergoing ATLR

Correlate Meyer’s loop damage to VFD

D (BrainLab), FA 0.15, length >50mm Seeds in LGN and occipital (lower, mid, upper)

Yogarajah et al. 2009 20 controls, 21 patients undergoing ATLR

Correlate Meyer’s loop location and VFD

P (Camino - PICo), FA 0.1, no angular threshold

Seed anterolateral to LGN (15 voxels), way stratum sagittale posterior to splenium (coronal), exclusion midline + iterative frontal exclusion mask

Romano et al. 2009 28 patients undergoing tumour surgery

Presurgical planning D (BrainLab), FA 0.15, angle 55 Seed in LGN

Clatworthy et al. 2010 20 controls Automation of seed point selection

P (FSL - Bayesian) Seed encompassing anterior optic radiation (coronal) + target in primary visual cortex, exclusion masks in midline + frontal

Hofer et al. 2010 6 controls Reconstruct entire visual pathway D (custom), FA 0.1, angle 30-70 Seed in LGN, target in white matter near visual cortex White & Zhang 2010 10 controls Evaluate Meyer’s loop D (GE Functool), FA 0.01, ADC 0.01, steps

160

Thudium et al. 2010 12 patients undergoing SAH

Preoperative planning, intraoperative guidance

D (BrainLab), FA 0.2, length >50mm Seeds in roof of temporal horn and primary visual cortex

Wang et al. 2010b 16 controls Compare measurements with different software

D (BrainLab), FA 0.2, angle 70, length >50mm

D (Philips FiberTrak), FA 0.15, angle 90, length >50mm

Seeds in LGN/adjacent temporal lobe and occipital lobe

Sun et al. 2011 44 patients with lesions near OR

Intraoperative guidance D (BrainLab), FA 0.15, angle 20, length >50mm

Seeds in LGN and occipital (lower, mid, upper)

Henze et al. 2012 13 controls, 13 schizophrenia

Determine if visual pathways affected in disease

D (NeuroQLab - TEND), FA 0.02, angle 107, length <400mm

Seed in LGN and Meyer’s loop (coronal), target stratum sagittale posterior to splenium (coronal)

Wu et al. 2012 10 controls Characterise relationship to other tracts

D (Slicer) Multiple slices of optic tract near LGN

Benjamin et al. 2012 13 controls Investigate optimal seed region D (CRKit), FA 0.1, angle 30 (6 steps per voxel)

Optimum is broad seed next to LGN, way point in stratum sagittale (coronal), exclusion midline + frontal

Table 4.4 - Summary of regions-of-interest and parameters used for optic radiation tractography

Algorithm: D = deterministic, P = probabilistic, aFM = advanced fast marching tractography, FACT = fibre assignment by continuous tracking, FMT = fast marching tractography, PICo = probabilistic

4.6.3.2

Tractography and postoperative visual outcome

Determining the location of the optic radiation by tractography for surgery is critical for three reasons. Firstly, the optic radiation shows high anatomical variability (Ebeling & Reulen 1988). Secondly, the optic radiation cannot be delineated on conventional MRI sequences, and finally it cannot be identified visually during surgery.

Tractography can be used to measure the distance from the temporal pole to Meyer’s loop (TP-ML). Using deterministic tractography in healthy controls a range of 34-51mm (mean 44mm) was found but Meyer's loop appeared posterior to the temporal horn in all cases, which does not agree with dissectional studies (Nilsson et al. 2007). Nevertheless, preoperative tractography superimposed on postoperative imaging showed disruption in the patient with a VFD, but an intact Meyer's loop in the unaffected patient (Powell et al. 2005).

Preoperative tractography measurements were related to outcome in 14 patients undergoing ATLR or SAH (Taoka et al. 2008). In those without a postoperative VFD, Meyer's loop was on average 5.0mm behind the resection margin whilst in those developing a complete quadrantanopia the resection involved on average 7.5mm of Meyer's loop. However in the 3 patients with the most minor deficits, tractography placed Meyer's loop posterior to the resection margin in all cases, possibly as a result of the deterministic algorithm used.

Further work established that tractography was indeed predictive of the visual deficit. 48 patients undergoing ATLR were studied with pre- and intraoperative imaging including DTI (Chen et al. 2009). Damage to Meyer's loop and postoperative VFD were each classified into 5 groups on the basis of preoperative/intraoperative tractography and Goldmann perimetry respectively. A significant correlation between the two was found.

This study was notable in that tractography was performed using intraoperative imaging, but Meyer's loop could not always be delineated due to the damage (using a deterministic algorithm). Further, rigid registration of the pre- and intraoperative tractography confirmed significant brain shift during surgery (horizontal shift 0.0-11.1mm, mean 3.75mm, either inward or outward; vertical shift 0.0-7.8mm, mean 2.46mm, either up or down) such that preoperative tractography would no longer be valid without compensation.

In view of the limitations of deterministic tractography, preoperative probabilistic tractography and postoperative Goldmann perimetry were used in 21 patients undergoing ATLR (Yogarajah et al. 2009). Linear regression showed that both TP-ML distance and resection size were predictive of the post- operative VFD, with the former having a greater effect.

4.6.3.3

Comparing deterministic and probabilistic tractography

The difference between deterministic and probabilistic algorithms is critical. Studies that place Meyer's loop significantly more posterior than dissection-derived data use deterministic tractography (Table 4.5). Whilst anatomical dissections may provide a small underestimate due to the difficulties in distinguishing the optic radiation from other adjacent fibres, it is more likely that deterministic tractography techniques are giving an overestimate. A direct comparison of deterministic and probabilistic approaches in 11 controls and 7 patients gave a TP-ML distance of 32-51mm (mean 41mm) for the deterministic algorithm and 17-42mm (mean 30mm) for the probabilistic algorithm (Nilsson et al. 2010). It is noteworthy that all but one study employing deterministic tractography places Meyer’s loop posterior to the temporal horn in all subjects, whereas all studies employing probabilistic tractography place Meyer’s loop either anterior or posterior to the temporal horn (Table 4.5).

4.6.3.4

Using tractography real-time during surgery

The next step is to use these data for intraoperative guidance. The first study in epilepsy surgery superimposed preoperative deterministic tractography of Meyer's loop on the head-up surgical display to guide entry in the temporal horn in patients undergoing transcortical or subtemporal SAH (Thudium et al. 2010). On entry to the temporal horn, CSF leakage led to unacceptable brain shift so image guidance was no longer possible. Three out of 12 (25%) developed a post-operative VFD (two out of ten using the subtemporal approach), which is no better than a series of subtemporal SAH performed without image guidance (Hori et al. 2007). Preoperative tractography has also been used to guide extratemporal epilepsy surgery in an intraoperative MRI environment (Sommer et al. 2013). This is discussed in detail in the next chapter.