Magnetic Resonance Imaging (MRI)

Diffusion weighted imaging

What is it?

Diffusion-weighted imaging is based on the random or Brownian motion of water molecules. By acquiring an image with little diffusion weighting and another image with substantial diffusion weighting, the apparent diffusion coefficient (ADC) can be calculated on a voxel-by-voxel basis. Some pathologic processes appear to change the characteristic of the brain microenvironment, which in turn results in alteration of the apparent diffusion coefficient. The ADC in brain tissue is principally determined by tissue cellularity, as measured by the intracellular volume fraction and extracellular volume fractions.

Potential applications?

• tumours
• stroke

Diffusion tensor imaging

What is it?

DTI is a relatively new MR modality and has increasingly been used over the last 5 years. The DTI technique, which is sensitive to the Brownian motion of water, enables the measurement of restricted and/or hindered movement of water molecules as they diffuse in the brain. Due to the highly organized nature of the white matter, the main diffusion orientation will generally coincide with the orientation of the axons in this tissue. Therefore, DTI can characterize the orientation and integrity of white matter fibers in vivo and in a non-invasive manner (Basser94). Because interpreting a tensor representation can be non-intuitive, scalar metrics have been proposed to simplify DTI data. In this context, fractional anisotropy (FA), which is a normalized measure of the degree of diffusion anisotropy, and mean diffusivity (MD), i.e. the average amount of diffusion , are generally examined and have been related to the integrity of white matter bundles.

Potential applications?

• tumours
• multiple sclerosis
• amyotrophic lateral sclerosis
• demyelination
• schizophrenia
• bipolar disorder
• leukemia
• effect of chemo- and/or radiation therapy
• depression

MR spectroscopy

What is it?

Magnetic resonance spectroscopy allows noninvasive and in vivo exploration of the molecular composition of tissue. It identifies certain molecular constituents - the metabolites - involved in physiological or pathological processes. Only a limited number of molecules with protons are observable in MRS. In brain MRS, the principle molecules that can be analyzed are:

• N-acetyl-asparate (NAA): molecule present in healthy neurones
• Creatine/phosphocreatine (Cr): energy metabolism molecules
• Choline compounds (Cho): marker in the synthesis and breakdown of cell membranes
• Myo-inositol (mI): only found in glial tissue
• Glutamine-Glutamate-GABA complex (Glx): neurotransmitters
• Lactate (Lac): anaerobic metabolism

Potential applications?

• tumours
• radiation injury
• epilepsy
• demyelination
• infection

Perfusion weighted imaging

What is it?

If MR imaging is performed during administration of a bolus of a Gd (gadolinium)-chelated contrast agent, a transient decrease in signal intensity is present. Specifically, if a high concentration of a Gd contrast agent is confined to the vasculature, as is the case under bolus conditions, a difference in susceptibility between the contrast-containing vessel and tissue occurs. The signal change can be converted into a relaxation rate change, which is proportional to the fraction of blood volume within each image voxel. In addition to measuring cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) can also be determined from the same dynamic data.

Potential applications?

• tumours
• stroke

Susceptibility weighted imaging

What is it?

Susceptibility-weighted imaging (SWI) is a 3D, flow-compensated, radiofrequency spoiled gradient- recalled echo sequence that takes advantage of susceptibility variations between tissues. SWI combines magnitude and phase images to detect these differences. The technique was introduced in 1997 and was originally called “high-resolution blood oxygen level–dependent venography”, but it was renamed “susceptibility-weighted imaging” in 2004 because the sequence provides much more information beyond mere venography. The venography application relies on the paramagnetic effects of deoxyhemoglobin, which serves as an intrinsic contrast agent, for cerebral veins (which contain deoxygenated blood). SWI also allows visualization of other extravascular blood degradation products with paramagnetic properties, such as methemoglobin and hemosiderin. Thanks to the unique combination of phase and magnitude information, SWI also shows some diamagnetic substances, such as calcium. The use of SWI in clinical MRI of the brain has become widespread, and many new indications have emerged.

Potential applications?

• patients with a history of craniocerebral trauma
• dementia
• neurodegenerative disorders
• brain infections
• vascular malformations
• brain tumors
• stroke
• epilepsy

Arterial spin labelling

Functional MRI

What is it?

Since functional MRI (fMRI) maps activation patterns in the brain, it is an important technique for better understanding the function of different brain regions of healthy subjects and subjects with various pathologies. fMRI is based on the idea that, when a brain region is activated, new energy must be transported to this region, which leads to an increased blood flow in this part of the brain. This can be imaged by repetitive MR scans and detected by appropriate signal processing methods. Advantages such as minimal invasiveness, no radioactivity, widespread availability, and virtually unlimited study repetitions make fMRI ideally suited to the study of in vivo brain function.

Potential applications?

• dementia
• neurodegenerative disorders
• brain infections
• multiple sclerosis
• brain tumors
• stroke
• epilepsy
• depression
• obsessive compulsive disorder
• schizophrenia
• biopolar disorder

Resting state functional MRI

What is it?

As in fMRI, the brain function is analyzed with resting state fMRI. However, in contrast to conventional fMRI, no task must be performed by the patient in the scanner. As a result, the functional networks in the resting brain are revealed. The advantage of resting state fMRI is thus that the results will not depend on the performance of the task, which can be a limiting factor in the conventional fMRI analysis of some patient groups.

Pharmacological MRI

What is it?

Pharmacological MRI is a general name for the combination of MRI studies with the application of drugs. This broad field encompasses examinations of a range of effects that a drug may have on the brain, going from structural changes to functional or metabolic alterations. The potential of pharmacological MRI lies in investigating mechanisms of drug action at the brain system level and in establishing novel pharmacodynamic measures of regional drug action.