The standard MRI techniques incorporated in the MS protocol are T1 +/ Contrast, T2, PD and FLAIR. Apart from these sequences, more advanced ones (DIR, SWI, DWI, ADC, DTI and fMRI) can help for better characterization of the plaques
The T1 sequence gives a general view of the brain’s anatomical structure, correlating with the anatomical colors for white and grey matter. Therefore it’s helpful for the observation of cerebral atrophy which is the strongest imaging correlation with disability in patients with advanced chronic MS. It cannot define the activity of the lesions, but is used for counting the number of lesions and identifying their location. The permanent axonal loss lesions are depicted as hypointense signals, known as "black holes".
T1 with contrast
Repetition of T1 sequence after intravenous application of Gadolinium contrast makes possible the visualization of acute, active lesions, due to the disrupted blood barrier from the inflammation, surrounding the lesion. Using T1W imaging with contrast in the protocol for examining the MS patients helps in the diagnosis of the disease and its activity. This makes possible the covering of the diagnostic criteria for dissemination in time (DIT) in a single baseline MRI.
The contrast-enhanced lesions (CELs) are depicted as hyperintense and based upon shape may be defined as nodular (nCEL) or ring (rCEL) lesions. Several short-term studies pointed towards the assumption that rCELs represent areas of a more aggressive inflammatory process. [1]
T2 weighted images
On T2 weighted images signal intensity of the hydrogen and water makes the liquids (CSF) and grey matter appear hyperintense (H+ rich) and the white matter hypointense (H+ poor).
The MS plaques are depicted as hyperintensities against the low signal background of white matter. This is one of the main techniques, incorporated in the worldwide used McDonald criteria for defining the number of lesions.
It is used for visualizing plaques located deep in the white matter and periventricular. Some smaller lesions, located borderline of the ventricle walls sometimes could be harder to determine due to the high signal of the CSF.
The Proton Density (PD)
PD weighted image visualizes the number of protons per volume (the Proton density of the tissue). Tissues with few protons have low signal intensity (hypointense white matter), while tissues with many protons have high signal intensity (hyperintense grey matter).
This sequence is the investigation of choice for infratentorial lesions where usually artifacts from bones and the perivascular CSF spaces (Virchow-Robin spaces) may appear on T2 as white hyperintense signals.
Fluid Attenuated Inversion Recovery (FLAIR)
FLAIR maintains the heavy T2 weighting and at the same time suppresses the signal from the ventricular CSF. Because of its very long Time to Echo (TE) and Repetition Time (TR) abnormalities remain bright but normal CSF fluid is attenuated and appears dark.
Compared to the PD- sequences the suppression of CSF on Flair is greater, which increases the contrast between periventricular lesions and CSF and enhances the detection of the lesions and the determination of their borderline, size and shape.
The disadvantage of the sequence is inferior quality lesion detection in the posterior fossa and spinal cord where PD is preferred.
Double Inversion Recovery (DIR)
DIR is an inversion recovery MS pulse sequence that uses two different inversion pulses. The technique can be used to suppress signal from two different tissues or to suppress signal that moved between the two pulses. In neuroimaging, two inversion times are selected to suppress signal from cerebrospinal fluid and white matter.
This improves the sensitivity as it suppresses both the CSF and the white matter, while the MS lesions remain hyperintense. The sequence is showing delineation of the cortical and juxtacortical lesions.
Many recent researches have proven the positive correlation between the DIR-detectable grey matter (GM) lesions, GM atrophy overlap in the brain and the clinical disability of the patients. [2]
Susceptibility Weighted Imaging (SWI)
Susceptibility Weighted Imaging (SWI) exploits the magnetic susceptibility differences of various compounds including deoxygenated blood, blood products, iron and calcium, thus enabling a new source of contrast in MR.
In neurodegenerative diseases, the ability to measure the amount of ferritin in the brain may help predict prognosis, disease progression and treatment outcomes.
The units with a higher magnetic field (7 Tesla and more) and the SWI sequence made possible the detailed visualization of the perivenular distribution of the demyelinating lesions by showing the MS plaque surrounding the small veins. [3]
Diffusion Weighted Imaging (DWI)
Diffusion-Weighted Imaging uses the motion of water molecules within tissues to evaluate diffusion restriction and depicts these areas as white signals on images.
Free water usually moves equally unrestricted in all directions, an effect knows as isotropic diffusion. In areas where the water is restricted inside or by tissues, preferential directions are taken and movement consequently becomes anisotropic. Therefore, water mobility in the brain is markedly reduced in compact tissue, such as white matter (WM), is reduced to a lesser extent in the grey matter (GM), and is almost free in the cerebrospinal fluid (CSF). Pathological processes that alter the normal brain structure may affect water motion and diffusion.
Diffusion restriction might be detected in areas of axonal or myelin loss, in the periphery of the lesion where inflammation occurs in the active stage. In those cases, the MS plaques appear as hyperintense and this method could be used for the determination of active lesions when applying CE T1WI is not appropriate for the patient. [4]
Apparent diffusion coefficient (ADC)
In the DW Imaging some T2- shine through signals are visible (because of the long Te). To differentiate an artifact from a true pathology ADC maps are being used. They are independent of the T2 effects and reflect only the effect of the diffusion itself.
Diffusion tensor imaging (DTI)
The Diffusion tensor imaging is based on the reconstruction of large fiber bundles using three-dimensional tractography. The different colors represent different directions of the fibers. This method delimits major tracts of WM in vivo: after the selection of one, or more than one Regions of interest (ROI), nervous pathways are reconstructed by tracking along the principal direction of the fibers passing through the ROI. In MS patients this technique can be used to analyze the displacement of fibers as well as to detect Wallerian degeneration. [5]
Functional MRI (fMRI)
Functional Magnetic Resonance Imaging (fMRI) detects task or sensory triggered regional brain activity. In MS patients with fMRI techniques could be visualized:
- functional cortical changes in patients with MS, that aren’t visible in healthy patients.
- different patterns of cortical activities according to the disease phenotype
- dynamic functional cortical changes over the course of the disease
- correlation between the extent of functional activity and the extent of global and regional MS-related tissue damage [6]