The brain can be the victim of numerous pathologies, including malignant tumors, strokes, infection, head injuries, disease, and dystonias. We will discuss the more common pathologies, as well as those that are uncommon, but may prove to be interesting.
Diseases of the Brain As the space in this section will not allow for an all-inclusive report on brain diseases, we will focus on diseases that may be more commonly seen in MRI. We will discuss Parkinson’s disease, Alzheimer’s disease, dementias other than Alzheimer’s disease, multiple sclerosis, and dystonia.
Dystonia Dystonia is a disorder characterized by involuntary muscle contractions that cause slow repetitive movements or abnormal postures. Individuals may have a tremor or other neurologic features. Dystonia may affect one muscle, groups of muscles, or muscles throughout the body.
Some forms of dystonia are genetic, but the cause for the majority of cases is unknown. Researchers believe that dystonia results from an abnormality in, or damage to, the basal ganglia or other brain regions that control movement. There may be abnormalities in the brain’s ability to process neurotransmitters, which are the chemicals that help neurons communicate. There also may be abnormalities in the way the brain processes information and generates commands to move.
Dystonia can be divided into three groups, which are idiopathic, genetic, and acquired. Idiopathic dystonia does not have a clear cause, but this grouping includes many dystonias that occur.
Genetic dystonia can be inherited in a dominant manner, with widely varying symptoms and severity.
Acquired dystonia, or secondary dystonia, can result from environmental or other damage to the brain, or from exposure to certain types of medications.
Birth injuries, infections, trauma, TBI, concussions and strokes can all cause acquired dystonia. Dystonia can occur at any age, but are typically classified as early onset or adult onset. It can progress through various stages, with the patient eventually displaying dystonic postures and movements even when relaxed.
Dystonia is also classified based on the regions of the body that are affected. Generalized dystonia affects most or all of the body.
Focal dystonia is localized to a specific part of the body. The most common focal dystonia is cervical dystonia, or spasmodic torticollis, which involves the muscles in the neck causing the head to turn to one side or be pulled forward or backward.
The second most common focal dystonia is blepharospasm, which is the involuntary, forcible contraction of the muscles controlling eye blinks.
Multifocal dystonia involves two or more unrelated body parts. Segmental dystonia affects two or more adjacent parts of the body. Hemidystonia involves the arm and leg on the same side of the body. A variety of dystonias have been identified that have a genetic cause, and mutations in specific genes have been linked to specific dystonic syndromes.
There are no medications to prevent dystonia or slow its progression, but treatment options exist that can ease some of the symptoms. Botulinum toxin injections into affected muscles prevent muscle contractions and provide temporary improvement in the abnormal postures and movements that characterize dystonia. This toxin injection blocks the release of the neurotransmitter acetylcholine, which causes muscle contraction. Off-label usage of certain classes of medications can block or regulate various neurotransmitters.
Deep brain stimulation (DBS) can be used if medications do not sufficiently alleviate symptoms. DBS involves the use of controlled amounts of electricity that are sent into the region of the brain that is generating the dystonic symptoms to interfere with and block the electrical signals that are causing the symptoms.
Surgery may be performed to interrupt the pathways responsible for the abnormal movements.
Small regions of the thalamus, globus pallidus, or other deep centers in the brain can be purposely damaged to reduce symptoms of dystonia.
Dystonia abnormalities are typically not visible on MRI. However, dystonia that is associated with neurodegeneration with brain iron accumulation (NBIA) displays in a specific manner on MRI. NBIA is a group of rare genetic disorders characterized by abnormal accumulations of iron in the basal ganglia, which is the region of the brain that assists in regulating movements. The high brain iron is typically seen in the part of the basal ganglia called the globus pallidus, as well as the substantia nigra. On a T2- weighted MRI image of a patient with NBIA, the center of the globus pallidus will display high signal MRI Anatomy and Positioning Series | Module 7: Neuro Imaging 60 intensity. Low signal intensity is seen in the surrounding region due to the abnormal accumulation of iron. This is termed the “eye of the tiger” sign in MRI. Figure 72 Eye-of-the-tiger sign indicated by white arrows on center T2 image; T1-weighted image appears normal, but high signal intensity can be seen in the globus pallidus on proton density, T2 FLAIR, and both axial and coronal T2- weighted images; as T2-weighting increases, so does conspicuity of T2 hypoint
[MRI Anatomy and Positioning Series | Module 7: Neuro Imaging Hitachi Medical Systems America, Inc. MRI Anatomy and Positioning Series. We offer teaching modules to allow users of Hitachi MRI scanners to review anatomy that will be seen on various MRI exams, and to enhance their positioning skills