**1. Introduction**

The goals of neurosurgical resection are best described by the statement: "*maximal safe resection with minimal morbidity*". In oncological neurosurgery, this often encompasses achieving a gross total resection (GTR), which can be challenging depending on the location (i.e. eloquent cortex), ease of accessibility (i.e. superficial *vs.* deep), and the presence of vital *en passant* fiber tracks. Pioneering work by Dr. Wilder Penfield at the Montreal Neurologic Institute by performing

craniotomies and resections allowed for safe resection of epileptogenic foci in the brain, resulting in seizure control in his patients (superbly outlined in his recent biography) [1]. This concept was soon applied by Penfield and his team to other pathological entities affecting the cortex and subcortical areas and was subsequently adopted by many academic neurosurgical centers worldwide.

Penfield's pioneering brain mapping techniques have since laid the groundwork for the development of intraoperative cortical stimulation techniques to guide maximal safe extent of resection (EOR). However, we now understand the limitations of this technique, including the progressive decline in the spatial resolution of bipolar stimulation throughout the course of surgical resection; iatrogenic edema caused by tissue retraction and/or resection altering the resistance and conductance of brain tissue requiring adjusted increases in applied stimulus strength at the price of decreased spatial resolution; and finally, adjustment of stimulation parameters which could lead to spurious spread of excitation to areas not immediately next to the point of stimulation, which could introduce further uncertainty in determining the EOR. Another caveat is the fact that awake surgery also requires a larger craniotomy than the actual size of the lesion or area to be removed due to the fact that greater cortical access is often needed to place multiple electrodes over the hemispheric surface to localize both the lesional site as well as the topography of adjacent possibly eloquent brain areas.

Various emerging, invasive monitoring techniques have expanded the scope and utility of this approach such as: Extracellular cortical stimulation via implanted on-lay grids for epilepsy patients; Foramen ovale electrodes inserted via transbuccal access for patients with mesial temporal sclerosis; And/or intraoperative cortical surface stimulation with bipolar Ojemann-type electrodes. The latter is sometimes used in combination with monopolar fiber-track-stimulation via pointy tip electrodes for patients with deep seated intra-axial lesions, amongst other modalities. Another option is the application of transcranial (scalp) stimulation techniques for evoked motor or sensory potentials (MEPs or SEPs) to monitor the integrity of functional pathways. However, not all patients are suitable to undergo awake procedures or these invasive types of monitoring nor are all neurosurgeons trained to perform surgeries using these methods. Another disadvantage of these awake monitoring techniques is that a surgical procedure itself is required before valuable functional information can be obtained. As a result, important patient management decisions must be made upfront without complete knowledge of the anatomic relationships between the lesion borders and functionally eloquent cortex [2]. For these reasons, we consider it beneficial to obtain comprehensive preoperative imaging, in particular, fMRI and DTI, to identify eloquent cortex controlling movement, primary sensory perception, vision, and speech, and to understand the spatial relationships between critical *en passant* fiber tracts and these functionally eloquent cortical regions to allow for surgical planning and determining the EOR.

#### **1.1 A historical perspective on functional magnetic resonance imaging**

In 1890, Sir Charles Sherrington and Dr. Charles Roy at Cambridge University were amongst the first neuroscientists to experimentally demonstrate a link of brain function to cerebral blood flow [3]. In 1963, Drs. Linus Pauling and Charles Coryell reported differences in the magnetic properties of blood based on the oxygenation status of hemoglobin. Oxygen-carrying hemoglobin (oxy-Hgb) was weakly repelled by magnetic fields, whereas blood with de-oxygenated hemoglobin (deoxy-Hgb) was attracted by a magnetic field. However, it was not until the 1990's when two American researchers at Bell Laboratories in Murray Hill, New Jersey, recognized the utility of this phenomenon to study the oxygenation state of the brain using

*Pre-Surgical and Surgical Planning in Neurosurgical Oncology - A Case-Based Approach… DOI: http://dx.doi.org/10.5772/intechopen.99155*

MRI and clearly demonstrated that the metabolic effect of neuronal activation in brain tissue yielded distinct magnetic properties which correlated with deoxy-Hgb and oxy-Hgb concentrations. Ogawa and colleagues then demonstrated that blood oxygen level derived (BOLD) signals could be used to generate intrinsic MRI contrast which could be further augmented by gradient-echo techniques [4]. Kwong and colleagues then followed with the use of gradient-echo and inversion recovery echo planar imaging sequences to map signal changes within the human primary motor and visual cortices [5]. These studies laid the groundwork for the development of distinct protocols that are used in modern day fMRI studies.

#### **1.2 The utilities of diffusion weighted and diffusion tensor imaging**

*Diffusion weighted imaging* (DWI) assesses the restricted diffusion of intracellular water molecules in the brain and is routinely used for stroke assessment in hypoxic and metabolically compromised regions of the brain. Hypoxia-induced breakdown of the energy-dependent transmembrane potential can be demonstrated early on in the ischemic process by applying three gradient-directions to DWI sequences to estimate the "average diffusivity" allowing for very early radiographic detection (within minutes of the ischemic insult).

*Diffusion tensor imaging* (DTI) takes advantage of the fact that there is directionally restricted diffusion of molecules in certain tissues depending on the observer's viewing angle (i.e. along *vs.* perpendicular to nerve fibers) [6]. In DTI, each voxel has one or more pairs of parameters: a rate of diffusion and a preferred direction of diffusion, described in terms of three-dimensional space, for which that parameter is valid [7]. The properties of each voxel of a single DTI image are usually calculated by vector or tensor math from six or more different diffusion weighted acquisitions, each obtained with a different orientation (or viewing angle) of the diffusion sensitizing gradients [8]. The diffusion tensor model is a rather simple model of the diffusion process, assuming homogeneity and linearity of the diffusion within each image voxel. In order to measure the tissue's complete diffusion profile, one needs to repeat the MR scans and apply different directions (and possibly strengths) of the diffusion gradient for each scan. The high information which is contained by a DTI voxel makes it extremely sensitive to subtle pathologies in the brain. In addition, the directional information can be exploited at a higher level of structure to select and follow neural tracts through the brain — a process called *tractography*. The underlying molecular directional restriction is also called *anisotropic diffusion***.** Such directionality can be color coded in three dimensions (anterior/posterior, superior/ inferior, and lateral/medial) which is useful to visualize the axonal tract organizations of the brain. Fiber tractography is therefore an added three-dimensional reconstruction technique based on DTI data to assess axonal directions using the collected primary diffusion restriction data. DTI can therefore provide additional structural information about the organization of the white matter in and around primary and secondary brain lesions which is useful to the surgeon in procedural planning.

#### **1.3 Current status of the field**

Modern imaging technologies such as BOLD fMRI and DTI, as briefly outlined above, have allowed for significant improvements in the surgical team's ability to minimize perioperative neurosurgical morbidity. The complementary use of other non-invasive imaging modalities such as CT angiography or MR perfusion scans, MR spectroscopy, 3D single-molecule super-resolution microscopy, and more recently transcranial magnetic brain stimulation [9], further permits the

surgical team to gain significant insight into the access and resectability of certain lesions and to reliably predict the maximally safe EOR. Furthermore, the ability to use these imaging modalities to engage patients is crucial in the obtained consent process.

One of the hindrances to such technology-driven and more transparent surgical disease management strategies remains the fact that not all these highly informative technologies are widely available. Hospital funding for subspecialty-trained MRI physicists and MRI technicians may be limited and there is hesitancy to implement these technologies due to several factors, including: 1) The absence of large scale randomized clinical trials to support the routine integration of fMRI and DTI for pre-operative surgical planning; 2) The problems encountered in some earlier fMRI studies with respect to precise spatial location of a lesion; 3) The inability to correlate imaging features to electrical activity surrounding the tumor in some earlier studies; 4) The inability to use fMRI for distinguishing brain regions that are considered not primary eloquent sites, yet appear to be essential areas for circuit functions *vs* those areas that may be sacrificed without causing a lasting major neurological deficit; and 5) High interobserver variability in fMRI threshold determinations and DTI segmentation algorithms, which require specialty training and experience. The situation is further complicated by the fact that many ancillary health care practitioners (including medical-, neuro-, and radiation oncologists) are not familiar enough with the potential that fMRI and DTI can bring to presurgical planning and the roles they can play for improving surgical outcomes.
