Current Medical Imaging - Volume 6, Issue 4, 2010

Forty Years of Technological Evolution in Image-Guided Neurosurgery: Past, Present and Future Aspects Although initial descriptions of neurosurgical procedures date back as early as into the 17th century BC in the ancient Egypt [1], modern neurosurgery - the juvenile medical field - changed tremendously within the past 40 years and nothing other than the fact of the craniotomy itself is similar to these initial reports. Neurosurgery ever since has been guided by anatomical structures such as the coronal suture as a reliable bony landmark or the Sylvian fissure. Surgical indications in those days were based on the patient's symptoms only and the approach was driven by neuroanatomical knowledge often based on observations of single cases suffering from strokes and postmortem analysis of patient's brains respectively Prior to the advent of imaging modalities, the first stereotactic procedures had been introduced in 1873 by Dittmar [2], however; the modern era of stereotaxy began in 1908 with the description of a stereotacic device for animal purposes only by Horsely and Clarke [3]. In 1918, Mussen introduced the first stereotacic device for human application [4]. This stereotactic apparatus, however, did not succeed into clinical application as surgeons did not believe in this technology. Spiegel and Wycis [5] were the first to use a stereotactic device in a patient in 1947, but this model was far from being applied in daily clinical routine as it was made out of a custom made plaster cast to fit the patient's individual head combined with a frame on top of it. Two years later, Talairach and Leksell introduced a stereotacic device [6], which modified “descendant” is still used today. Stereotactic procedures became very popular for biopsies as well as for the treatment of Parkinson's disease, the later becoming less frequent with the discovery of L-Dopa. First imaging modalities like ventriculography or pneumoencephalography [7, 8], which appear inconceivable nowadays, helped in the topographical orientation, but a milestone in the evolution was the invention of cranial computer tomography in 1973 by Hounsfield [9]. Since then, the entire neurological field changed and pathology in the brain became visible preoperatively. Procedures could be planned based on an individual anatomy. In 1979, the combination of cranial computer tomography and a stereotactic frame was introduced by Brown [10], while another imaging modality evolved contemporaneously. Based on the work of Bloch and Purcell [11, 12], magnetic resonance imaging was invented, but it was not before 1971, when Damadian [13] and Lauterbur [14] enabled the generation of first MR images. Clinical widespread took some time, but from today's perspective, MR imaging represents the Gold standard of high resolution medical imaging. Computer-based image-guided technology intraoperatively was invented by Kelly and colleagues in 1982 [15]. They were the first to use a computer-assisted stereotactic laser in the treatment of an intracranial lesion. Thus, the century of modern image- guided neurosurgery was born. In 1987, Watanabe and colleagues invented the principle of a frameless computer-guided neuronavigation based on the patient's individual images [16]. The improvement of MR imaging, newer neuronavigation technology and more recently the invention of intraoperative MRI [17] have further assisted the evolution of image-guided neurosurgery as we know it today. Within the past 40 years there was a tremendous jump technology-wise and neurosurgical procedures became more precise and safer for the patient. We're still not at the end of the rope. Higher field strengths, more detailed image resolution, more precise image-to-patient registration and the combination of various technologies like image fusion, integration of functional data, the use of intraoperative MRI and advanced technologies intraoperatively improve this field every day. This issue gives an overview on current state of the art treatment options starting with today's knowledge on the intracranial pathologies, preoperative mapping methods, imaging modalities, recent intraoperative technologies and long-term follow-up treatment options. After so many developments within the last 40 years, one can barely think of any further ground breaking technology to further improve image-guided neurosurgery, but now - looking back to the ancient Egypt and being astonished what was possible then - we might be amused in the near future how it was back then in the 21st century.

The 4th edition of the WHO (World Health Organization) classification of tumours of the central nervous system published in 2007 provides several new entities, new variants of established entities as well as changes in the tumour grading system. In addition, new molecular markers are introduced and information about the genetic bases of brain tumour development is extended. This review gives an overview about the current WHO classification system, discusses remaining controversies and provides an introduction of the standard diagnostic procedures.

Neurosurgical resection of brain lesions aims to maximize excision while minimizing the risk of permanent injury to the surrounding intact brain tissue and resulting neurological deficits. While direct electrical cortical stimulation at the time of surgery allows the precise identification of essential cortex, it cannot provide information preoperatively for surgical planning. Brain imaging techniques such as functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS) are increasingly being used to localize functionally critical cortical areas before brain surgery. The use of multimodal preoperative information improves the accuracy of preoperative planning and facilitates decision-making regarding the extent and exact location of surgical resections. This manuscript reviews how fMRI and TMS can be used in presurgical settings to map motor as well as higher cognitive functions (i.e. language). Pre-operative fMRI can be used to identify the brain regions that are activated during specific sensorimotor or language tasks. TMS is able to disrupt neuronal processing in the targeted brain area which in turn may affect task performance, if the stimulated cortex makes a critical contribution to the brain functions subserving the task. While the relationship between task and functional activation as revealed by fMRI is correlative in nature, the neurodisruptive effect of TMS reflects a causal effect on brain activity. The use of preoperative fMRI is well established although the number of studies on presurgical language fMRI is still limited. In contrast, the reliability and accuracy of preoperative TMS remains to be determined.

While static contrast-enhanced T1-weighted MR imaging remains the current standard imaging method to diagnose and evaluate therapeutic of brain tumors, advanced MR techniques such as perfusion MR imaging can allow for more sophisticated analysis of dynamic hemodynamic processes of these lesions. This article will highlight the key physical concepts, technical parameters, pitfalls, limitations, as well review important clinical applications of MR perfusion in neuro-oncology.

Surgery is currently the first treatment for low-grade gliomas (LGG). However, LGG often involves eloquent areas, in patients with no or mild preoperative deficit. Therefore, cortical and subcortical structures, essential, for brain functions must be preserved. Presurgical functional neuroimaging and tractography can show the relationships between eloquent regions and the tumor, but they have several limitations. Consequently, intraoperative electrical mapping is more and more used by neurosurgeons, to tailor the resection according to individual functional boundaries. Nonetheless, due to the invasive feature of LGG, the glioma removal is regularly incomplete to avoid postsurgical permanent deficit. The goal of this review is to provide new insights into cerebral plasticity, that is, the brains ability to reorganize its functional maps consecutively to slow-growing lesions like LGG. Longitudinal studies combining pre-, intra- and post-operative brain mapping methods may enable to analyze functional redistribution over time at the individual scale. Such plastic potential can open the door to multiple surgeries spaced by several months or years, with the aim to optimize the benefit/risk ratio of surgery, i.e. to increase the extent of resection of LGG before anaplastic transformation - thus to increase the overall survival - while preserving and even improving the quality of life.

Gross total resection, though proven to be effective, is often hindered by the notoriously challenging differentiation between malignant glioma and the surrounding edematous brain when using standard white light microscopy. Intraoperative imaging or neuronavigation by preoperative datasets deliver inconsistent results as they often fail to demonstrate tumor or resection borders reliably. With the introduction of the 5-aminolevulinic acid (5-ALA, Gliolan®) fluorescence microscopy, neurosurgeons command a highly specific biological tumor marker for malignant glioma resection, to help distinguish the vital tumor tissue from normal brain and improve the extent of tumor removal.

Neuronavigation has become a useful tool in neurosurgical operating-theatres, displaying tumor outlines in the navigation-monitor (segmented preoperatively in the 3D-MRI dataset) and the operating-microscope. Furthermore, preoperatively assessed functional data can also be integrated: Functional MRI (fMRI) and magnetoencephalography (MEG) enable the visualization of eloquent cortical areas. MR-Spectroscopy (MRS), positron emission computed tomography (PET) and single-photon-emission-computed-tomography (SPECT) demonstrate metabolic data. Fiber-tractography, today mostly based on a diffusion-weighted image data set (DTI), enables us to outline subcortical major white matter tracts, for example the corticospinal tract or the speech-associated tracts. A combination of these methods is today known as multimodal imaging. Multimodal imaging contributes to maximum resection of intracerebral lesions such as malignant gliomas without an aggravation of postoperative morbidity, especially in combination with intraoperative MRI to overcome the problem of the so called “brain-shift”.

Neurosurgeons rely on image-guidance to perform safe, successful surgery efficiently and cost-effectively. Neuronavigation involves rigid frame-based or fiducial-based frameless stereotactic guidance that derives its accuracy from computed tomography (CT) or magnetic resonance imaging (MRI). This imaging can be obtained days or immediately before surgery. Fundamentally, however these systems do not address the brain shift that results from the loss of cerebrospinal fluid once the cranium is opened. Intraoperative MRI (ioMRI) systems were developed in part to compensate for such brain shift during surgery. Like its predecessors, ioMRI affords excellent soft tissue discrimination and the ability to view the surgical site in three dimensions. Moreover, as all of these approaches use CT or MR imaging, being able to view the tumor being biopsied or resected allows the surgeon to preoperatively determine a surgical corridor that avoids critical structures. However, unlike its predecessor, ioMRI also allows the surgeon to see in real-time what the surgery has accomplished, allowing the surgeon to revisit the surgical field to exclude the presence of intraoperative hemorrhage. Although all ioMRI systems offer T1- and T2-weighted imaging, only high-field (≥ 1.5 Tesla) systems are routinely capable of MR spectroscopy (MRS), MR angiography (MRA), MR venography (MRV), diffusion- and susceptibility- weighted imaging, as well as ultrastructural (diffusion and fiber tracking) and physiologic imaging by means of MR perfusion and brain activation (fMRI) studies. Identifying vascular structures with MRA and MRV may prevent their injury during surgery. Biopsying areas of elevated phosphocholine on MRS may improve the diagnostic yield for brain biopsy. Locating brain function may affect the surgical path chosen to biopsy or resect a tumor. Despite these advantages, however, there remains debate over the optimal field strength and configuration for an ioMRI-guided system.

Soon after the invention of intraoperative MR imaging in 1997, various applications have been presented. Besides being an early resection control, intraoperative MRI was used to update neuro-navigation and replace frame based stereotactic systems. Recently, advanced MR imaging techniques (including MR spectroscopy (MRS), diffusion tensor imaging (DTI), functional MRI (fMRI), and dynamic susceptibility weighted MR imaging (DSC-MRI)) were introduced, applied in an intraoperative environment. Feasibility and clinical applications have previously been presented. This review summarizes all these advanced techniques and discusses advantages and disadvantages during the resection of a lesion using intraoperative MRI.

Survival time probably is the most important factor for patients suffering from GBM. However, survival time is also of interest when analyzing efficacy of new therapeutic approaches. Several factors, including the degree of tumor resection, have been reported to influence survival time in patients with GBM. Postoperative imaging modalities including MRI, CT, as well as TCS in this setting play a crucial role to exactly determine the degree of tumor removal. In this review we discuss the advantages and disadvantages of these imaging modalities and the associated pitfalls. Newer modalities like PET-CT and new therapeutic strategies including locally applied chemotherapy (Gliadel) or convection enhanced delivery (CED) and related imaging findings are also discussed.

Gliomas are diverse and challenging primary brain tumors that affect an increasing number of adults annually. Although not curable, recent advances in diagnosis and treatment are making a significant impact on length and quality of life. This paper will address the recent advancements, modern management, and current opportunities in glioma diagnosis, imaging and treatment.