Recent Advances in Magnetic Resonance Imaging Making Neurosurgery Safe in the 21st Century
Aadil S. Chagla
Advances in Magnetic Resonance Imaging
With the advent of the 21st century, neurosurgical mortality and morbidity has reduced in India. Some of the factors that have contributed to this are improved micro neurosurgical skills, instrumentation and training, modern anaesthesia drugs and techniques, and advances in neuroimaging. We can now study not only the anatomy but also the functional and the metabolic processes of the brain. We would like to highlight these advances in Magnetic Resonance Imaging (MRI) technology that have made neurosurgery safer today.
High field strength MRI and parallel acquisition techniques
There have been many advances particularly in Magnetic Resonance (MR) hardware, software and pulse sequences. With the use of higher field strengths like 3 Tesla (T) and 7 T MRI, imaging time has reduced and signal to noise ratio has improved. Scanning time has significantly reduced by the use of parallel acquisition techniques. Such techniques as Sensitivity Encoding (SENSE) and Generalised Autocalibrating Partial Parallel Acquisition (GRAPPA), wherein different channels scan different subparts of the entire field of view (FOV) and the entire data is gathered which is then converted to an image by a reconstruction algorithm.
Gradient echo MR pulse sequences
Volumetric assessment of the brain, including volumetry of hippocampus in epilepsy patients, is feasible by gradient echo MR pulse sequences such as Magnetization Prepared Rapid Acquisition Gradient Echo (MPRAGE). Other special techniques in the evaluation of epilepsy patients include T2 Relaxometry, Volumetric FLAIR images, diffusion imaging and evaluation of ADC. Gradient Echo Imaging (GRE) has also been helpful in functional brain imaging and in better assessment of the brain’s anatomy. Steady state GRE sequences have helped in development of special MR techniques such as MR myelography and MR cisternography.
Constructive interference in steady state (CISS)
Another gradient echo sequence useful for evaluation of cranial nerves, inner ear anatomy, intraventricular lesions, cystic lesions and cerebellopontine angle lesions is a CISS sequence (Figure1 and 2). It has a different nomenclature in different MRI scanners such as FIESTA (fast imaging employing steady-state acquisition) by General Electric, True FISP (true fast imaging with steady-state precision) by Siemens and balanced FFE (fast field echo) by Philips. CISS sequence is widely used in assessment of cranial nerves especially in cases of trigeminal neuralgia and acoustic schwannomas.
Susceptibility weighted imaging (SWI)
Subarachnoid haemorrhage is one of the life threatening emergencies in neurosurgery. SWI, based on distortion of magnetic field by tissue of higher susceptibility leading to loss of signal on magnitude images, has become an important technique for its early detection. The technique has also been used in evaluation of vascular malformations, blood disorders, cerebral infarction, traumatic brain injury, neoplasms, metabolic and neurodegenerative disorders.
Diffusion weighted imaging (DWI)
DWI, based on detection of movement of water molecules in the tissues, has emerged as one of the important means of early detection of stroke, even before it can be seen on T2 weighted MR images or on Computed Tomography (CT) scan (Figure 3). DWI has helped in revolutionising the treatment of stroke from just patient care to complete cure. It has also helped in differentiating between vasogenic edema and cytotoxic edema, in evaluation of hypoxic injuries, traumatic brain injuries, infectious lesions such as abscess & encephalitis, epidermoid cyst, and in the evaluation of various hypercellular tumors such as lymphoma and meningioma. It helps in differentiation of tumor recurrence from radiation necrosis, in grading of tumors and in identifying the areas within the tumors from where a biopsy can be performed.
Diffusion tensor imaging (DTI)
Based on the same principle of molecular diffusion, the white matter tracts can be mapped spatially, using the technique known as DTI. It utilizes the property of anisotropy of diffusion of water molecules. Directionally colour coded maps are generated which depicts the tracts. It helps a neurosurgeon find out the relation of the intra-axial tumor such as gliomas with the white matter tracts and to know whether the tract is merely displaced or infiltrated by the lesion and therefore provides the neurosurgeon the confidence in planning his approach while tackling complex tumors involving eloquent brain. DTI is also helpful in assessment of the damage of the white matter tracts due to prior ischemic event (stroke), demyelinating disease such as multiple sclerosis and in patients with traumatic brain injury.
MR spectroscopy (MRS)
Measurement of various metabolites present in the tissues can be done by a non-invasive technique called MRS. The metabolites, their peak position and their concentration or spectral pat- tern are assessed. In proton MRS, major metabolites studied are N-acetylaspartate (NAA), choline (Cho), creatine (Cr), lipid, lactate, alanine, myoinositol, glutamate and glutamine (Figure 4). The concentration of various metabolites changes in abnormal tissues and thus helps in preoperative and postoperative evaluation of neoplastic disorders, diagnosis of infectious lesions, hypoxic ischemic encephalopathy, metabolic and neurodegenerative disorders, epilepsy, multiple sclerosis and traumatic brain injuries. In cases of brain tumors, MRS helps in diagnosis, assessment of the extent of infiltration, grading of the tumors as either high or low grade, and in differentiating between tumor recurrence and radiation necrosis.
MR angiography (MRA)
Vascular structures, both intracranial and ex- tracranial, can be evaluated non-invasively by MRA. It can be contrast enhanced (CE-MRA) or without the use of intravenous contrast namely TOF (Time-Of-Flight) and PC (Phase Contrast) angiography (Figure 5). It helps in assessing the luminal narrowing of the vessels, quantification and extent of stenosis, flow in vessels distal to stenosis and in screening and post-management assessment of the intracranial aneurysms. Atherosclerotic plaque morphology can be studied to assess irregularity and ulceration in the plaque as predictors of stroke. MRA is a non-invasive method of assessment of the vessels, however it may miss small and slow flow vascular malformations, which can be more accurately assessed by digital subtraction angiography (DSA).
MR venography (MRV)
Cerebral venous sinus thrombosis is a crucial cause of neurosurgical morbidity and mortality. MRV is now established as the non-invasive imaging technique of choice for the initial evaluation and follow-up of cerebral venous thrombosis. Although contrast enhanced MR venography can be performed, conventional time-of-flight (TOF) and phase contrast (PC) MR techniques, which do not require the use of a paramagnetic contrast agent, are now the commonly advised and clinically accepted methods for assessment of the venous system. MR venography is also helpful assessing whether a venous sinus adjacent to a tumor such as meningioma is simply displaced or rather infiltrated by the lesion.
Functional MRI (fMRI)
Assessment of the brain function with neuroanatomic localisation on a real time basis is now possible with fMRI. Simple tasks such as finger tapping and verb generation cause regional blood flow changes in the brain, which in turn causes signal changes that are detected by fMRI. fMRI maps the eloquent cortex that can get displaced or disorganised by pathologic processes. It is used in neurosurgical planning, dementia, seizures and in psychiatric disorders. It helps a neurosurgeon orient himself to the anatomy and function of the neuroparenchyma during surgery.
Vascularity of a suspected abnormal area can be assessed by Perfusion MRI. Changes in perfusion is evaluated by the help of relative cerebral blood volume (rCBV), cerebral blood flow (CBF), mean transit time (MTT) and time to peak (TTP). It is used in patients with stroke, in cases of space occupying lesions such as solid or cystic appearing neoplasms, in atypical infective lesions such as giant tuberculomas and in tumefactive demyelination. Ischemic penumbra is the viable tissue at risk where there is functional impairment. Timely detection of penumbra as perfusion-diffusion mismatch helps in stopping further damage by providing appropriate management.
3-dimensional (3D) MRI
Many MR sequences can now be acquired in three dimensions such as MR angiography, MPRAGE / FSPGR, CISS and volumetric FLAIR (Fluid attenuated inversion recovery) sequences. These sequences help complete evaluation of the pathology in all the planes. FSPGR (fast spoiled gradient-echo) sequence is widely used in assessing ring enhancing lesions of brain such as neurocysticercosis and tuberculomas. 3D FLAIR is used in the assessment of infiltration of neuroparenchyma by tumor, vascular white matter changes, demyelinating plaques, and in mesial temporal sclerosis (MTS). It is used to assess hippocampus and adjacent gray matter to rule out MTS and associated focal cortical dysplasia.
Magnetization Transfer (MT) Imaging
MT Imaging is another technique for evaluation of various brain disorders. It depends on the concentration of the macromolecules such as myelin whereas MT contrast depends on the transfer of magnetization between macromolecular pool and water pool. This transfer of magnetisation from macromolecules to water leads to the loss of signal. It has been used to differentiate tuberculoma, which shows low MT ratio (MTR), from pyogenic abscess and tumors. It has also been used to study plaques in multiple sclerosis, studying progress in myelination, in neurodegenerative disorders and in psychiatric ailments. Recently it has also been used in evaluating pituitary adenomas in patients with hyperprolactinemia, as prolactin secreting adenomas show higher MTR value and thus higher signal as compared to MTR value of normal pituitary gland. The reverse is true for nonsecreting adenomas. It can also be used in postoperative follow up as residual or recurrent adenomatous tis- sue will show high MTR values.
Dynamic contrast MRI and GRASP (Golden- angle radial sparse parallel MR imaging)
Other advancements in the field of MRI which has helped in better evaluation of pituitary lesions are high resolution MR imaging at 3T, 3D volumetric assessment of pituitary volume, dynamic contrast MRI, DWI, MRS, Volume interpolated breath-hold imaging (VIBE), and intraoperative MRI. High field strength 3T MRI improves signal to noise ratio and spatial resolution and thus helps to easily differentiate between normal and abnormal tissue. Dynamic contrast MRI is done using fast turbo spin echo (TSE). After a bolus injection of intravenous gadolinium, six consecutive sets of three images are obtained in coronal section at intervals of every 10 seconds. After about 30-60 seconds, there is maximum homogenous contrast enhancement of the pituitary gland with no uptake in the microadenoma. Therefore, the microadenoma is seen as a relatively non enhancing lesion; however, microadenomas less than 3 millimeters are difficult to assess, differentiation between a microadenoma and a pituitary cyst is also difficult and may require another delayed (30-60 minute) scan. MR evaluation of posterior pituitary is difficult due to inherent high signal on these T1 weighted scans. In order to overcome such limitations, recently GRASP, based on 3D gradient-echo sequence, has come up with a new volumetric dynamic imaging technique. It acquires the whole dynamic study into a single continuous scan during which contrast is injected and image reconstruction is done. It has the advantage of high spatial and temporal resolution and improved fat suppression. Therefore, coronal radial volumetric interpolated brain examination (VIBE) with a GRASP acquisition has emerged as a useful and feasible technique even on 1.5 T MRI. It helps in both qualitative and quantitative evaluation of temporal change in signal enhancement pattern of the normal and abnormal areas of pituitary gland[7,8].
Cerebrospinal fluid (CSF) flow imaging
CSF flow dynamics can now be studied both qualitatively and quantitatively by CSF flow imaging, by using phase-contrast MR technique and cardiac gating. It has been used in evaluating patients suspected of normal pressure hydrocephalus, communicating and non-communicating hydrocephalus, arachnoid cyst, Chiari I malformation, syringomyelia, and in assessing patency of ventriculostomy and ventriculo-peritoneal shunts.
Imaging of spine
MRI has a significant role in imaging of spine. Although bony changes are better detected on CT scan, MRI is the modality of choice for imaging of soft tissue structures. Gradient echo based and fast spin echo based imaging, along with high field MR scanners have been successful in demonstrating spinal pathologies with greater clarity, sensitivity and accuracy. Advanced imaging techniques such as DTI and MRS have shown higher sensitivity in diagnosis and subsequent management of cervical spondylotic myelopathy and spinal cord injury. Short tau inversion recovery (STIR) imaging, which suppresses fat, depicts marrow edema in cases of trauma and infectious diseases involving spine. It also has high sensitivity in demonstrating cord edema. DWI, although difficult in evaluation of spinal cord pathologies, have been useful in assessment of patients with spinal cord injury and in diagnosis of spinal epidermoid cysts[10,11].
Intraoperative MRI (iMRI)
Intraoperative MRI is a procedure used to create real time images of the brain during neurosurgery. This is particularly important when tumors are thought to have been removed completely but have significant residue, thus preventing inadequate excision of tumor and reoperation. Therefore, with this facility, there is a greater chance for tumors to be removed completely. There are two ways via which this may be achieved (a) a portable MR device may be moved on to the operating room to get the images or (b) an iMRI device may be placed in an adjacent room so that the doctors can safely shift the patient for intra-operative MRI during surgery. In certain cases when image guiding modalities are used to navigate inside the brain and brain shifts occur with the release of cerebrospinal fluid from cisternal and subarachnoid spaces and from ventricles, then presurgical imaging is no longer precise in targeting the tumor. It therefore becomes mandatory to have an MRI which will allow precise intraoperative localisation of the tumor. It requires an entire operative room that is conducive to the huge magnetic field, which is necessary for the imaging. Therefore, not only neurosurgical instrumentation but also anaesthesia equipment needs to be MR compatible. This is a huge cost to a neurosurgical facility and, in turn, the patient. It is also more time consuming and therefore fewer cases are done as compared to when this technology is not used.
In conclusion, we have presented some of the recent advances in MR technology. However, we must weigh the benefits and the costs involved to the patient before we advise these recent innovations in MR technology. We must realise that the decision to use expensive tests must be carried out in the best interest of the patient and not to recover investment costs or unfairly favour technology merely to prove a point! Proper wisdom and good ethical practice on the part of the clinician is of paramount importance to our patient population. We must keep in mind the economic conditions in our country and use all our skills and knowledge to practice medicine which still continues to be the most noble of all professions. We will end by stating that just because there is a new technology available it should not become a protocol in the management plan for a patient. This will escalate medical costs and even small medical centres and hospitals with good track records in treating patients will not be able to compete with multinational or corporate hospitals with plenty of resources and funding, thus creating a monopoly in medical practice.
Disclosure: The authors have no conflict of interests to declare.
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