La taxonomía de tomate

Several studies have examined the use of tractography in patients with arteriovenous malformations (AVM). Preoperative DTI in 10 patients with AVM near the optic radiation showed a disrupted optic radiation in the three patients with significant preoperative visual loss (Kikuta et al. 2006). Similar findings were seen with post-operative DTI and visual fields, although one patient with an intact optic radiation on imaging had developed a quadrantanopia. A subsequent study of 34 patients confirmed that the optic radiation and pyramidal tract were less well visualised in patients when the nidus was close to the lesion or with associated neurological symptoms (Okada et al. 2007). Incorporation of tractography into the planning stage for gamma knife surgery for AVM to ensure the dose to the optic radiation does not exceed a safe limit has been suggested (Maruyama et al. 2007). However, findings were based on retrospective analysis of 10 patients, only one of whom developed a VFD.

Paper Subjects Tractography method TP-ML distance (mean) ML-TH distance (mean) ML/TH relationship

Yamamoto et al. 2005 5 controls D (Philips PRIDE) 33.1-40.0mm (37.3mm) -4.3mm to -3.7mm (-4.0mm) ML posterior

Nilsson et al. 2007 7 controls D (Philips PRIDE) 34-51mm (44mm) -21mm to -8mm (-16mm) ML posterior

Taoka et al. 2008 14 patients D (dTV II) 30.0-43.2mm (36.6mm)

Sherbondy et al. 2008 8 controls P (ConTrack) 24-34mm (28mm) -1mm to +8mm (3mm) ML anterior or posterior

Chen et al. 2009 48 patients D (BrainLab iPlan) 20.9-51.5mm (32.1mm)

Yogarajah et al. 2009 20 controls P (Camino) 24-47mm (35mm) -11 to +9mm (0mm) ML anterior or posterior

21 patients 24-43mm (34mm) -15 to +9mm (0mm)

Mori et al. 2009 10 controls D (DTI Studio) 26.1-58.0mm (40.3mm)

Wang et al. 2010b 16 controls D (BrainLab iPlan) (Operator A) 26.6-48.9mm (36.3mm)

D (BrainLab iPlan) (Operator B) 26.8-48.2mm (36.3mm)

Nilsson et al. 2010 11 controls

7 patients

D (Philips) 32-51mm (41mm) -21mm to -8mm (-14mm) ML posterior

P (FSL) 17-42mm (30mm) -15mm to +10mm (-2mm) ML anterior or posterior

White & Zhang 2010 10 controls D (GE Functool) Left 26.3-33.5mm (30.5mm)

Right 28.8-34.0mm (31.1mm)

-2.6mm to +3.8mm (+0.6mm)

-4.5mm to +2.1mm (-1.0mm)

ML anterior or posterior

Wu et al. 2012 10 controls D (Slicer) Left 34.8-49.5mm (39.9mm)

Right 34.6-53.6mm (40.7mm)

-11.1mm to -7.0mm

-14.7mm to -6.2mm

ML posterior

Benjamin et al. 2012 7 controls measured

P (CRKit) Left -1mm to +5mm (+1.1mm)

Right -4mm to +3mm (-0.7mm)

ML anterior or posterior

Table 4.5 - Anatomical variability of the optic radiation as shown by tractography

Tractography method: D = deterministic, P = probabilistic; TP-ML distance: temporal pole to Meyer’s loop distance. ML-TH distance: Meyer’s loop to temporal horn distance (+ = ML anterior to TH, -

Nevertheless tractography aids surgical planning. In a study of two patients with lesions near the optic radiation, preoperative deterministic tractography was used to delineate the optic radiation and resective surgery was undertaken with continuous VEP monitoring (Kamada et al. 2005). In the first patient, the resection was achieved with stable intraoperative VEP and no post-operative VFD. In the second patient, surgery resulted in hemianopia. Tractography data were incorporated into the neuronavigation system and the intraoperative VEP remained stable until the resection reached the optic radiation according to the neuronavigation. The authors were however fortunate that minimal brain shift occurred intraoperatively, so that the simple affine registration used remained accurate.

In 16 patients undergoing surgery near the pyramidal tract or optic radiation (only two of the latter), a three-dimensional object in anatomical space could be created within 15 minutes, including DTI acquisition time (Nimsky et al. 2006b). A further anatomical MRI acquisition after head fixation but prior to any surgical intervention allowed the object to be displayed on the operating display. However, diffusion and anatomical data were rigidly registered thus not allowing for the distortion in diffusion images and the two anatomical scans were rigidly registered without further intraoperative scans so no allowance for brain shift was made limiting the application.

Despite these shortcomings, tractography data do lead to a change in the neurosurgery performed in the majority of cases (Romano et al. 2007). Twenty-five patients undergoing tumour resection underwent preoperative tractography to delineate the pyramidal tract, arcuate fasciculus and optic radiation for preoperative planning. Images first without and then with tractography were displayed to the surgeon, resulting in a changed surgical approach in 4 (16%). The data were then used for neuronavigation updated by a preoperative anatomical scan. The surgeons reported retrospectively that extent of resection was affected in 17 (68%), so overall surgery was altered in 20 (80%) of patients. It is unclear how much benefit this conferred, but only a single patient developed a new neurological deficit (dysphasia).

4.7 Conclusions

Meyer’s loop of the optic radiation is at risk during temporal lobe surgery with many patients experiencing a postoperative VFD and a lesser proportion not meeting visual criteria for driving. Epilepsy surgery has yielded much information about the nature and extent of these deficits and anatomical dissection has characterised the variability in the location of Meyer’s loop. Meyer’s loop frequently passes anterior to the temporal horn and the degree of the characteristically pseudowedge-shaped partial superior quadrantanopia is related to the degree of damage to this structure. Alternative approaches to surgery, including several techniques for SAH, may pose a lesser risk to vision but have not been systematically studied.

Diffusion tensor imaging tractography has the potential to delineate the optic radiation but probabilistic tractography is necessary to accurate delineate Meyer’s loop. Most studies have concentrated on preoperative planning but tractography also has the potential for real-time guidance of neurosurgery. However, numerous challenges including brain shift must be addressed to maximise utility. Intraoperative

5 INTRAOPERATIVEMAGNETICRESONANCEIMAGING

5.1 Introduction

Image-guidance has the potential to improve the outcome of surgery by increasing the extent of resection of the lesion whilst minimising the surgical morbidity from damage to critical white matter tracts and eloquent cortex. In this chapter, I summarise the approaches to intraoperative imaging and then discuss the design, safety and clinical utility of intraoperative MRI (iMRI) systems. Two key technical challenges of iMRI are compensation for brain shift and geometric image distortion. Previous approaches to these problems are summarised and a novel approach is employed in this thesis (Chapters 6 and 10).