MRI is superior to other imaging modalities in the imaging of DeMyelinating Diseases such as MS and other White Matter disorders of the Brain and Spinal Cord.
It is possible to visualize 2 - 5 mm White Matter lesions and watch their progress over time.
Contrast enhancement may make it possible to determine the acute PeriVascular involvement of new active lesions in MS.
Focal HyperIntensities similar to those of MS are seen frequently in elderly patients and are usually due to Ischemic DeMyelination or to multiple small Infarcts.
Similar lesions also can follow radiation therapy, Lyme Disease and occasionally severe recurrent Migraine headaches.
Small disseminated metastases may have a similar appearance. Contrast enhanced MRI differentiates acute plaques from old lesions.
Lesions are identified by MRI in about 95% of the patients with Clinically Definite MS, whereas CT demonstrates lesions in only about 25% of such patients.
Lesions of the Spinal Cord including areas of DeMyelination are best shown by MRI and detected in 75% of MS patients.
Prior to 1983 the diagnostic criteria for MS were based entirely on clinical evaluations.
Technological developments in Brain imaging in MS became a significant contributing factor to changing the MRI Criteria used for the diagnosis of MS.
In 1986 the National Multiple Sclerosis Society issued a statement recommending the optional use of MRI in the diagnosis of MS.
MRI is increasingly being used as a measure of pathological disease activity in monitoring the efficiency of potential new treatments for MS.
A major advantage of MRI over clinical monitoring is that it detects a large amount of SubClinical disease activity.
The two main approaches to MRI are detecting active lesions, and measuring total Lesion Load.
In Relapsing/Remitting and Secondary/Progressive MS, Gadolinium enhancement increases the number of detectable active lesions and also probably correlates with pathological activity.
An important limitation of MRI monitoring is that conventional Brain MRI abnormalities often show little or no relationship with clinical disability.
One explanation for this may be pathological Heterogeneity of lesions that all look the same on a conventional image.
Two MRI techniques most widely used:
- Measuring Total Brain Load
- Counting The Number of Active Brain Lesions
Brain lesion load can be measured in a crude fashion by grading the size of the lesion by their largest diameter.
Lesion area can be measured more precisely by outlining them manually on a computerized image.
More fully automated lesion area measurement techniques, such as Thresholding and Edge Detection have been developed.
The accuracy of Lesion Load Measurement will also be increased by thinner slices, which reduce partial volume errors.
Active Brain lesions are defined by a series of criteria, but basically: ANY New, Enlarging or Enhancing Lesion is defined as Active. MRI monitoring helps make this distinction.
The Pulse Sequences used in MS assessment vary, but Fast Spin Echo (FSE) Sequences are the current method of choice. Minimizing scan time is important for both reducing cost and improving patient comfort.
Generally, 4 to 8 Echoes are used, thus reducing scan time to one quarter or one eighth of standard Spin Echo (SE) Sequences.
The appearances of SE and FSE sequences are very similar, and they are equally sensitive in detecting MS Lesions.
The FSE sequence takes 1 - 3 minutes, and thus can be obtained during the 5 minutes required between injecting Gadolinium and obtaining optimal contrast - Enhanced Images.
Other MR techniques are needed that specifically identify the pathological features, most likely to result in disability, namely DeMyelination and Axonal Loss.
Magnetization Transfer Imaging and Proton MR Spectroscopy are two techniques that show promise in this regard.
Magnetization Transfer (MT) is a technique that has the potential for detecting changes in Myelin.
The macromolecular structure of Myelin is the physical basis of MT, by using off-resonance irradiation, macromolecular protons can be saturated.
These protons then exchange with free-water protons and produce a decrease in signal intensity of the free water protons.
This is quantified by using a Magnetization Transfer Ratio (MTR) of signal intensities.
This method had been sensitive to changes in a spectrum of White Matter lesions, including regions of apparent Myelin loss in patients with MS.
Furthermore, MTR's may be abnormal in patients with Normal-Appearing White Matter, demonstrated by standard MRI imaging.
As stated, conventional MRI is a very sensitive technique for detecting abnormalities in the White Matter.
However, its major problem is that it lacks specificity. A lesion that is just edematous without Myelin loss, could appear identical on standard T2 weighted images.
Also, only macroscopic DeMyelination can be seen. We have little insight into the Normal-Appearing White Matter in MS patients.
MTR may enable subcatagorization of MS lesions into lesions with low MTR (presumed to be DeMyelinating lesions) and lesions with higher values (primarily edematous lesions).
Another important use of MT is, in conjunction with Gadolinium, to increase the number and extent of enhancing lesions.
This can improve the detection of Blood-Brain Barrier abnormalities in patients with MS.
As with any type of imaging technique, motion, producing artifacts and invalidating the transfer measurement, is a limiting factor for MT imaging.
Another important source of inaccuracy is partial volume averaging, leading to an over or underestimation of the Transfer Ratio.
MT technique is basically achieved by obtaining two sets of image, and through digital subtraction, results in transfer contrast images. The ultimate role of MT in MS is unknown.
It can be a powerful probe to subcategorize lesions and Normal-Appearing White Matter in MS patients. It could be extremely useful in following MS patients, particularly those involved in drug treatment protocols.
It may, in the future, because of its increased sensitivity to White Matter changes, enable earlier, more accurate diagnosis and perhaps have prognostic value as well.