30 Nov, -0001

State of Art Imaging of Brain tumours : Looking beyond Morphology

Neuroimaging has evolved tremendously over last few decades. Imaging now not only defines anatomy but can provides an insight to molecular biology of the tumour. With techniques like perfusion we can assess features like neoangiogenesis and predict tumour grade. MR spectroscopy is a non invasive technique to decipher the constituent metabolites which further help characterise lesion aggressiveness. Diffusion Tensor Imaging can depict the relationship of the white matter tracts with the lesion. This helps in pre-operative planning as well as in predicting clinical outcomes. FMRI is one of the most recently developed forms of neuroimaging providing detailed maps of brain activity derived from hemodynamic responses to neuronal activity. The brain activation maps obtained by fMRI can help the surgeon to identify regions involved in motor control and language function ,including high-risk areas that, if damaged during surgery, would likely result in a significant neurological deficit. Furthermore, such fMRI data may directly influence the surgical management of patients in many possible ways, such as to help determine the most feasible treatment option (e.g. craniotomy performed with or without intraoperative mapping); the necessary extent of brain exposure; the safest surgical entry point and corridor. With Neuronavigation protocols the surgeon can direct the craniotomies to exact anatomical correlates significantly reducing the size of craniotomy defects and increasing the surgical precision.

Perfusion Imaging : Initially predicting tumour grade was based mainly on pattern of contrast enhancement which relies on disruption of blood brain barrier. Perfusion in turn is based on neoangiogensis which correlates with tumour grade better. High gradetumours may have only subtle enhancement but due to underlying leaky fragile neoangiogensis will have a higher perfusion. Perfusion also reliably differentiates true tumour progression from pseudo progression. It can differentiate tumour recurrence from radiation necrosis. DCE ( Dynamic contrast enhanced), DSC ( Dynamic susceptibility Contrast ) and Arterial Spin Labelling are three basic perfusion techniques which are commonly used. Arterial spin labelling is relatively newer and novel method of perfusion based on magnetic tagging of blood protons. It is a non contrast yet quantitative perfusion technique. It obviates the need of an intravenous access, can be used in patients with compromised renal functions and is not altered by regional intralesionalhaemorrhage.

Spectroscopy : MRS provides insight into the biochemical profile of brain tissue. There are two classes of spatial localization techniques for MR spectroscopy; single-voxel (SV) techniques (commonly used methods includes ‘PRESS’and‘STEAM’ which record spectra from one region of the brain at a time, or multi-voxel techniques (‘MR spectroscopic imaging’ (MRSI), also called ‘Chemical Shift Imaging’ (CSI) which simultaneously record spectra from multiple regions and thereby map out the spatial distribution of metabolites within the brain. Brain tumors have decreased N-acetyl aspartate (NAA) signals, and often also have increased levels of Choline (Cho), leading to increased Cho/NAA ratios. The decrease in NAA is widely interpreted as the loss, dysfunction or displacement of normal neuronal tissue since NAA is believed to be primarily of neuronal and axonal origin. The ‘Cho’ signal actually contains contributions from several different choline-containing compounds, which are involved in membrane synthesis and degradation; it has been suggested that it is increased in brain tumors due to increased membrane turnover. Other relatively common metabolic changes in human brain tumors are elevated signals in the lactate and lipid region of the spectrum, and also sometime increased levels of myo-inositol (mI) in short echo time (TE) spectra. The increase in lactate is most likely the result of anaerobic glycolysis, although it could also be due to insufficient blood flow leading to ischemia, or possibly also due to necrosis. Increased levels of mI are believed to reflect increased numbers of glial cells, which have been reported to contain high levels of mI, and in particular have been reported to be high in grade II gliomas. Within regions of non-enhancing signal abnormality, elevated Cho/NAA and Cho/Cr ratios have been observed in infiltrative edema compared to vasogenic edema reflecting the increased cellularity underlying the signal abnormality.

Diffusion Tensor Imaging : An advanced application of diffusion imaging is DTI, which utilises both diffusivity (eigenvalues) and direction (eigenvectors). In presurgical planning, DTI-based tractography is used to guide surgical resection by analyzing the integrity of white matter fiber trajectory in order to determine whether there is tumor invasion or tumor displacement of the adjacent white matter tracts. Often, tumor boundaries are not clearly delineated by conventional imaging, and DTI tractography may improve border characterization leading to greater resection and improved outcomes.The identification and preservation of white matter tracts is also important in preserving the neurological functional integrity of patients undergoing resection of lesions near eloquent cortex.

Functional MRI : fMRI indirectly measures neuronal activity using the ratio of deoxyhemoglobin to oxyhemoglobin as a contrast mechanism, known as blood oxygen level dependent (BOLD) signal. Haemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated. This difference in magnetic properties leads to small differences in the MR signal of blood depending on the degree of oxygenation. Since blood oxygenation varies according to the levels of neural activity these differences can be used to detect brain activity.

fMRI can be used to for sensory motor, language, and memory mapping—all of which have important implications for pre-surgical planning and intra-operative navigation.

In task-based fMRI, the patient alternates between a passive resting state and task performance, usually motor or language function, while relative changes in BOLD signal are measured and used to infer areas of cortical activation. Apart from localizing eloquent cortex, task-based fMRI can be used to characterize tumors. Decreased BOLD signal is noted in cortex involved by tumor and differences are also seen between high- and low-grade tumor suggesting alterations in cerebral blood volume of the tumor affected area.fMRI can also be applied to guide DTI by delineating a seed region for fiber tractography.

To conclude, with current state of art neuroimaging techniques it is virtually possible to preoperatively assess the morphology, biochemical constitution and functional characteristics of a tumour, to plan appropriate surgical trajectory with exact anatomic correlates and to follow up for treatment response.