MR

Recent Advances in MR Neuro-Imaging

Over the past few decades, as novel therapies for patients with neurological and neurosurgical disorders are being developed, we are witnessing a shift in imaging from merely providing anatomical information towards providing information about physiology

"Intraoperative MRI allows the surgeon to view the brain at all times during surgery and helps him remove tumours without damaging adjacent brain structures" - Dr Darshana Sanghvi-Purandare Consultant Radiologist Kokilaben Dhirubhai Ambani Hospital Mumbai

Historically, from the discovery of X-rays by Roentgen in 1895 , to the introduction of MRI by Damadian in 1969 , radiological advances have revolutionised the practice of modern medicine. The last few decades have witnessed dramatic innovations and improvisations in MR imaging technology. Newer advances in MR, neuroimaging include MR spectroscopy, diffusion and diffusion tensor imaging with tractography, perfusion imaging and functional imaging using the bold technique. The advent of intra-operative MRI has made a significant contribution to neurosurgery. Neuroimaging plays a central role in the management of all diseases of the central nervous system including epiplepsy, stroke, infections, dementia, movement disorders and neuro-oncology, by improving diagnostic accuracy, affecting patient care and monitoring dynamic changes within the brain during therapy.

Over the past few decades, as novel therapies for patients with neurological and neurosurgical disorders are being developed, we are witnessing a shift in imaging from merely providing anatomical information towards providing information about physiology. Neuroimaging is presently utilised in clinical practice for initial diagnosis and mapping of disease extent and distribution, pre-operative grading of tumours, biopsy planning, surgery and radiation portal planning for tumours , judging response to therapy and finally prognostication. The next decade will witness further sophistication of these techniques and with data available from larger studies, it is expected that imaging will continue to provide new and unique insights in neurology and neurosurgery which should hopefully contribute to the better management of patients with diseases of the central nervous system.

Diffusion & Diffusion Tensor Imaging

Diffusion-weighted MR imaging is a technique sensitised to the random motion of water molecules (Brownian motion) in biological tissues. Certain pathologies (like a stroke) constrain the normal random motion of water molecules in brain tissue and this is referred to as restricted diffusion. Diffusion-weighted MR imaging can detect an acute infarct (stroke) in less than 30 minutes after the occurrence of the clinical event. This enables neurologists to treat and reverse the effects of a stroke before significant damage can occur.

A more sophisticated extension of diffusion imaging is diffusion tensor imaging or DTI. DTI is a non-invasive in-vivo method for mapping white matter fibre tract trajectories in the human brain and spinal cord. The demonstration of white matter tracts by DTI can provide critical information to the neurosurgeon in cases of brain tumours by displaying the relation of a tumour to an adjacent white matter tract.

Perfusion Imaging

Perfusion imaging with MRI is an exciting new radiologic technique for non invasive evaluation of cerebral haemodynamics in certain definite clinical settings. Cerebral perfusion imaging describes the passage of blood through the brain's vascular network. It involves the dynamic injection of an intravenous contrast agent that is tracked by serial MR imaging during its first pass circulation through the brain tissue capillary bed. In this technique, approximately 700 images of the brain are obtained in about a minute and 20 seconds. Perfusion imaging allows for calculation of haemodynamic parameters in the brain such as cerebral blood volume and cerebral blood flow.

Perfusion imaging, especially with MRI, has become an integral component of the complete radiological assessment of brain tumours. At the Kokilaben Dhirubhai Ambani Hospital, MR perfusion studies in combination with diffusion images are used in the setting of acute ischemic stroke to establish the presence of brain tissue at risk (penumbra) for dying if ischemia continues without recanalisation of an intravascular thrombus. Reversal of penumbra is associated with a significant decrease in morbidity and mortality in patients of stroke who have been treated at the hospital. Stroke remains a leading cause of mortality and morbidity in India and the world.

MR Spectroscopy

MR Spectroscopy (MRS) is the only non-invasive technique capable of measuring chemicals within the human brain. MR spectroscopy equipment can be tuned (just like a radio receiver) to pick up signals from different chemical nuclei within the body.The metabolic information received is displayed as peaks in a graph or a visually appealing colour map showing concentrations of various chemicals in diseased tissues. These colour maps are overlapped or fused with conventional MR techniques to improve anatomical localisation. This helps to diagnose various pathologies and distinguish tumours from other mass lesions such as infections. A common example is elevated choline in a mass suggests the diagnosis of an aggressive tumour, whereas the presence of lipid often leads to the diagnosis of a tuberculous abscess.

Functional MRI ( F MRI )

The evolution of Magnetic Resonance Imaging (MRI) as a technique for assessment of brain function, as opposed to its more conventional role as a tool for studying brain anatomy and pathology, has been quite remarkable over the past decade. Functional MRI has contributed to improve our insight into how the human brain works, both in the normal and diseased states. Functional MRI refers to the demonstration of brain function with neuro - anatomic localisation on a real time basis. The vast majority of these studies are performed using 'Blood Oxygen Level Dependent' contrast or BOLD which requires the detection of very small signal intensity changes - zero to three per cent at 1.5 Tesla and up to six per cent at 3Tesla for voxel volumes as small as 3 x 3 x 5 mm. The principle of the BOLD technique of F-MRI is that performing a pre-defined cognitive task leads to regionally increased neuronal activity and localised haemodynamic changes that produce a MR signal response.

Intraoperative MRI

So far, the planning of neurosurgical procedures and their intraoperative performance has been based on images obtained from a preoperative MR examination.

The dynamic changes occurring in the brain (brain shift) during the course of surgery cannot be predicted from preoperative imaging. The development of intra-operative MRI techniques provides the neurosurgeon with online information that is required to perform accurate intraoperative image guided surgery. At the Kokilaben Dhiruvhai Ambani Hospital, intraoperative MRI allows the surgeon to view the brain at all times during surgery and helps him remove tumours without damaging adjacent brain structures. It also tells him whether the entire tumour has been resected without having to wait for post operative imaging.

In the operating room at Kokilaben Hospital, a standard high strength MRI scanner is completely integrated with a high-tech operative environment, incorporating a dedicated surgical suite with a state-of-the-art neuro-navigation and digitised image transfer and projection system. During surgery the patient is placed outside a 5 Gauss line so that neurosurgical procedures can be performed with standard instruments. The patient is placed on a rotatable surgical table and anytime during the operation, the surgery can be interrupted and the patient placed in the MRI by simple rotation of the table. Due to the high strength of the MR scanner high resolution images can be obtained.

Conclusions

Functional neuroimaging methods which comprise F-MRI and perfusion-MRI have the potential to complement existing and newer structural imaging techniques in the sequential and objective assessment of both global and local changes in neuronal tissues. Neuroimaging plays a pivotal role in various degenerative and neoplastic diseases,improving diagnostic accuracy, affecting patient care, monitoring dynamic changes within brain during therapy, and establishing them as the arbiter of novel therapy that may one day prove cure of various brain diseases a reality. Reflecting the fundamental importance and applicability of MRI in the medical field, Paul Lauterbur of the University of Illinois at Urbana-Champaign and Sir Peter Mansfield of the University of Nottingham were awarded the 2003 Nobel Prize in Physiology or Medicine for their 'discoveries concerning magnetic resonance imaging'.

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