Imaging Studies for Traumatic Brain Injuries
Medical providers now have a wide array of imaging tools at their disposal when it comes to diagnosing and treating Traumatic Brain Injuries. The purpose of this article is to briefly describe the different types of imaging studies now available.
MRI and CAT Scans:
The MRI and CAT scan slice the brain radiographically into slabs. The MRI does this with magnetic fields; the CAT scan uses x-rays. The MRI provides more detail than the CAT scan. Hence, brain damage seen on an MRI – as small as 1-2mm in size — may escape detection by a CAT scan. The CAT scan is superior to the MRI in detecting fresh blood in and around the brain, while the MRI is better at detecting the remnants of old hemorrhaged blood, called hemosiderin. CAT scans are often repeated to insure that a brain injury is not becoming more extensive, usually in the early stages of ER treatment. The “strength” of an MRI is measured in terms of Teslas. A standard MRI found in most facilities is a 1.5 Tesla MRI. Facilities more focused on diagnosing brain injuries typically will have a 3.0 Tesla MRI. The more powerful the scan, the greater the resolution and the greater the chance of discovering smaller lesions associated with Diffuse Axonal Injury (DAI).
Diffusion Tensor Imaging (DTI):
Diffusion Tensor Imaging is a type of MRI which uses special software to view parts of the brain a normal MRI cannot. The interesting premise of this new technology is that it measures the movement of water molecules in relation to the white track fibers of the white matter of the brain. If the fibers are healthy and untorn, then the water molecules will show parallel movement along those tracks as they slide along them. Torn or missing white matter fiber will allow perpendicular movement of the water molecules.
This technology allows for visualization of natural damage to the white matter. Very few medical centers have this technology. Because this study shows minute damage not picked up by standard MRI scans, it is very useful in diagnosing DAI. The most common causes of DAI are high speed car accidents, falls from a height, sports injuries and blast injuries.
A PET scan (Positron Emission Tomography) is performed by injecting a small amount of radioactive chemical into a vein. As the chemical travels through the body, it is absorbed by the organs and tissues. During the test, a scanner records the energy produced by the cells. A computer converts the recording into three-dimensional pictures of an area of the body and any cells that are changing show up at a brighter contrast to any surrounding, normal cells.
PET scanning for brain injuries involves the tracking of glucose in the brain. The brain uses glucose for energy. By labeling a glucose molecule with a radioactive “tag,” and then inhaling radioactive glucose and placing the patient’s head under a large geiger counter, one can identify abnormal areas of the brain that are underutilizing glucose. Because cyclotrons are needed to generate the radioactive gas, PET scanning is not widely available.
SPECT scanning (single photon emission computed tomography) is similar to PET scanning in that a radioactive chemical is administered intravenously to the patient, but the radioactive chemical remains in the bloodstream and does not enter the brain. As a result, the SPECT scan maps the brain’s vascular supply. Because damaged brain tissue normally shuts down its own blood supply, focal vascular defects on a SPECT scan are circumstantial evidence of brain damage. The advantage of a SPECT scan over a PET scan is its ready availability and relatively cheap cost. Recent studies have demonstrated abnormal SPECT scans after head trauma when the CAT and MRI were normal, suggesting that the SPECT scan is more sensitive to brain injury then either CT or MRI scans. Because the radioactive chemicals used in SPECT and PET scans are carried to all parts of the body by vascular tree, SPECT scans and PET scans are used sparingly in patients of reproductive age.
MRA stands for Magnetic Resonance Angiography. It is an MRI technique that specifically evaluates vessels such as arteries. Brain MRAs evaluate the vessels of the brain to look for aneurysms, vascular malformations such as AVMs, narrowing and blockage of the vessels of the brain, among others. Brain MRAs are typically ordered for many different symptoms to exclude an aneurysm or vascular malformation, or search for a source of bleed. They are also used during the evaluation of stroke to detect blockages and narrowing of the arteries that feed the brain.
Functional MRI or functional Magnetic Resonance Imaging (fMRI) is a type of specialized MRI scan. It measures the hemodynamic response (change in blood flow) related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging. Since the early 1990s, fMRI has come to dominate the brain mapping field due to its relatively low invasiveness, absence of radiation exposure, and relatively wide availability.
Since the 1890s it has been known that changes in blood flow and blood oxygenation in the brain (collectively known as hemodynamics) are closely linked to neural activity. When nerve cells are active they increase their consumption of oxygen, switching to less energetically effective, but more rapid anaerobic glycolysis.The local response to this oxygen utilization is to increase blood flow to regions of increased neural activity, which occurs after a delay of approximately 1–5 seconds. This hemodynamic response rises to a peak over 4–5 seconds, before falling back to baseline (and typically undershooting slightly). This leads to local changes in the relative concentration of oxyhemoglobin and deoxyhemoglobin and changes in local cerebral blood volume in addition to this change in local cerebral blood flow.