Quantitative MRI of the Brain
Neuroradiology is the part of radiology that focuses on the diagnosis and characterization of abnormalities of the central nervous system consisting primarily of the brain and the spinal cord. Magnetic resonance imaging (MRI) is a common modality in neuroradiology.
The Methods of qMRI
When talking about quantitative MRI (qMRI) we mean methods that measures the relaxation values. In contrast, to weighted images which are dependent on many more factors than relaxation such as patient size, scanner imperfections.
This gives us measurable values and help the radiologist to understand if the suspected tumor for example really is a tumor, which can be hard to tell by just looking at the image. Anders Tisell, PI for this project and adjunct senior lecturer is involved in many of the qMRI research projects at CMIV.
Previously we have worked with a technique which was based on the two-dimensional technique of thick slices of the whole brain. This has now been developed into the three-dimensional technique we call QALAS, where the information is gathered as a volume with one-millimetre isotropic resolution of the whole brain.
After we have measured, we can still create weighted MR images by calculating them from the qMRI and this is called synthetic MRI.
From the qMRI we can also calculate the amount of myelin, which is the fatty insulating layer formed around nerves in the brain and spinal cord, Anders explains. The myelin helps speeding up the nerve impulse. Multipel Sclerosis (MS) breaks down the myelin and we have a technique to measure these damages or scarrings, so called MS lesions. We can see if the amount of myelin starts to decrease or remain stable. By following the patients, we can discover early signs of MS lesions so that the treatment might be changed, if necessary, Anders continues. With new drugs coming, we can help monitoring to see if they can maintain the amount of myelin.
Another project also using the same technique with 3D QALAS is monitoring brain tumors.
The radiologist sees the tumor since the tumor loads contrast agents. By using the quantitative technique mentioned above, it is possible to see that outside the part of the tumor that the radiologist sees we can see that there are more tumors. Tumors in the brain are often very aggressive and even if the neurosurgeon removes everything that we can see, we know that there is still tumor tissue left.
- With the new 3D-technique we get better resolution and hope that we in that way will be able to see where the tumor will continue to grow, that we will be able to detect the real tumor border, Anders continues.
Yet another example of how this technique is used is in radiation therapy, where patients who are in the middle of radiation therapy or who have finished it are followed. Due to the radiation that is given during therapy it looks as if there are contrast agents loaded. This is a reaction to radiation, so called pseudo progress. Due to this pseudo progress MRI is not performed one month after surgery but only after three months, when the pseudo progress is decreased. With these patients you must wait even though the diagnosis is not so good, so it would be much more preferred if you did not have to wait.
- With the qMRI, both 2D and 3D, we hope to see already after one month if it is pseudo progress or tumor progress. We are right now evaluating this new technique, Anders says.
It is well known that radiation therapy causes tissue damages. Another aspect of the project is also to investigate these damages and see how the tissue is affected by radiation. If it is possible to see damaged tissue, there might be a possibility to change the dose plan and treat the tumor from another angle.
Example images of a 33 years old MS patient. All images are calculated from a single 6 minute QALAS scan. In the top row quantitative parameter maps, in the middle row synthetic contrast images and in the bottom row the myelin volume.
Still with the same the same technique there are ongoing research with Parkinson’s disease which is a progressive nervous system disorder that effects movement. Today there are no MRI examination that can specifically diagnose Parkinson’s disease. The symptoms starts gradually and by looking at the substantia nigra there are hopes of early detection of changes. Is there a difference between healthy volunteers and patients? If the project turns out well this examination method could be implemented clinically.
Patients suffering from hydrocephalus i.e., when there is excessive cerebrospinal fluid (CSF) in the brain which can cause gait difficulties and mild dementia, are often treated surgically by inserting a shunt to relieve pressure on the brain. There are problems with monitoring the pressure to find the optimal state in the ventricles. With QMRI it is possible to see CSF as well as white and gray matter and hopefully also the ventricular volume to help deciding the optimal pressure state.
There is so much going on and still much to do, Anders conclude.