Advanced MRI Techniques

1. Diffusion-Weighted Imaging (DWI)

DWI is a short technique used to detect acute ischemia in the brain parenchyma and is an integral part of neuroimaging. DWI data processing helps in obtaining an Apparent Diffusion Coefficient (ADC) maps. The ADC values have an inverse relation to the cellularity and grade of glioma. ADC values are also low in tumours such as primitive neuroectodermal tumours and lymphomas. DWI can be diagnostic in non-neoplastic conditions such as epidermoids and cholesteatomas as well as pyogenic abscesses. DWI can be used for preoperative grading of brain tumours, predicting cellularity, directing the site of biopsy, differentiating between recurrent disease and radiation effects, and in the assessment of treatment response. In the postoperative imaging, DWI can delineate residual tumour tissue with high cellularity as well as the ischemia resulting from the intervention.

2. Diffusion Tensor Imaging (DTI)

DTI is an advancement of Diffusion-Weighted Imaging and provides information of the tracts in the brain. It can provide important preoperative information regarding contiguous white matter fibers. This information provided by DTI enables the surgeon to decide the safest surgical approach, particularly in tumors involving the brainstem and those in proximity to the internal capsules. A fusion of data obtained on fMRI and DTI is advantageous for pre-operative planning of tumour excision in proximity to the eloquent cortex. DTI of the spinal cord is also performed to assess the integrity of the spinal tracts.

3. MRI Spectroscopy

MRI spectroscopy relies on the MRI phenomena of chemical shift and spin-spin coupling effects to provide important biochemical information. The main metabolites of interest in brain tumours include:

• N-acetyl aspartate (NAA), a marker of neuronal integrity;
• Choline (Cho), a marker of cellular membrane turnover;
• Creatine (Cr), a marker of bioenergy stores;
• Lactate (Lac), a product of anaerobic glycolysis;
• Lipids (Lip), by-products of necrosis;
• Glutamate-glutamine, and Gamma-aminobutyric acid (Glx and GABA), neurotransmitters;
• Myoinositol (mI), a glial cell marker.

Creatine is used as an internal reference marker for cellular metabolism. And, if present, lactate may be interpreted as a marker for hypoxia, while the presence of mobile lipids within a tumour indicates necrosis. Both represent additional characteristic features of malignancy. The presence of certain metabolites may be detected in particular tumour subtypes, such as alanine in meningioma, taurine in medulloblastomas or amino acids in pyogenic abscesses. In tuberculomas, an important differential diagnosis of ring-enhancing lesions in tropical countries, the lipid-lactate peak is tall and reflects the caseation within. Choline peak may be slightly increased in chronic lesions due to the cellularity caused by aggregated macrophages.

In addition to helping in confirming the diagnosis, MRI spectroscopy can be used in the grading of lesions. Higher choline and lipid-lactate levels indicate a higher grade and a preserved spectrum; a slight increase in choline and absent/minimal lipid-lactate levels indicate a lower grade. In surveillance imaging of high-grade gliomas, the height of the choline peak is expected to decrease as a response to the treatment. In recurrent glial tumours, choline increases, while as radiation effect, choline can be expected to be low, though there is a considerable overlap of these features. The interpretation then depends on DSC and 3D ASL perfusion studies, in addition to the morphology of the abnormality on the conventional images.

4. Perfusion Weighted Imaging (PWI)

PWI is an MRI technique, which detects variations in the hemodynamics of the blood flowing in the brain. PWI can be performed with or without injecting an exogenous contrast. Perfusion techniques involving the injection of contrast include Dynamic Susceptibility Contrast (DSC), and Dynamic Contrast-Enhanced (DCE).

In the commonly used DSC perfusion technique, a contrast agent is injected rapidly as a bolus into the venous system. To assess the passage of the agent across the blood-brain barrier, echoplanar imaging is used. BV (Blood Volume), BF (Blood Flow), MTT (Mean Transit Time), and TTP (Time To Peak) maps are obtained. rCBV ratios have a direct relation to the grade of glial tumours, even in non-enhancing high-grade tumours and can indicate the site of biopsy by outlining the aggressive areas within a heterogeneous tumour bed. Effects of radio and chemotherapy treatment, including pseudoprogression or necrosis, demonstrate lower CBV values. Steroids and anti-angiogenic drugs such as Bevacizumab may lower the rCBV ratios, despite lack of response. The knowledge of these eventualities helps in better interpretation.

5. Arterial Spin Labelling (ASL)

ASL is a novel technique. It can be used as an alternative or as an addition to DSC in the majority of cases, especially on our new generation 3 Tesla Magnetic Resonance (MRI) scanner. Three-dimensional pseudo-continuous ASL (3D pcASL) is the technique found to be most sturdy by experts. The technique uses an endogenous tracer, namely the water in the flowing arterial blood, and hence does not require the use of contrast. This is beneficial for patients with compromised renal function, repetitive follow-ups of post-chemotherapy patients, and in children, for whom fast injection for DSC is difficult. ASL can measure absolute values and is not affected by permeability, along with being less sensitive to variations caused by hemorrhage. The utilization of 3D ASL in routine imaging of brain tumours, with accurate post labelling delay, which varies according to the age of the patient is encouraged.

ASL has been very useful in brain tumour assessment, in both pre and post-operative setting – to grade the tumours, to localize the site of biopsy, to assess post-operative residual neoplastic activity, differentiate recurrence and post-treatment changes, and to assess the efficacy of newer treatment modalities, like anti-angiogenic drugs.

6. Functional MRI (fMRI)

fMRI is a technique that measures alterations in the oxygenated and deoxygenated blood during neuronal activation while performing a specific task, and at rest. When a task is performed, there is increased blood flow in the region of the neuronal activity, which results in a decrease in the deoxygenated hemoglobin, as oxygen brought in is in excess. Deoxyhaemoglobin has paramagnetic properties, and the resultant distortion of the magnetic field can be measured as a small signal. This technique is called Blood Oxygenation Level-Dependent (BOLD) contrast. The fMRI data obtained is then registered onto high-resolution MRI anatomical images acquired without moving the patient. This data, along with DTI, can be transferred to the neuro-navigating system in the operative suite.

The role of functional MRI in neuroimaging includes the mapping of the eloquent areas and establishing their relationship to the margins of tumours or other intracranial lesions, and assessing the side of cerebral dominance.