Imaging Tools to Assess Traumatic Brain Injury
By Richard H. Adler
June 16, 2005
A variety of test are available to assess and diagnosis traumatic brain injury. As technology progresses, new tools are being developed to better assess the structure and function of the brain. The following is a concise discussion on various tools available with their strengths and limitations. For purposes of this article we start by recognizing that imaging of the brain can be grouped into two main categories:
- Tests that show the structure of the brain; and
- Tests that show the function of the brain.
This is a very important distinction to keep in mind. For example, things can look normal in structure but function abnormally. It is also true that the brain’s structure can look abnormal but function pretty well. As a result, misinterpretations of the meaning of both normal and abnormal imaging tests are fairly common.
CT scans, for example, show brain structure. They show the brain at a moment in time but cannot show brain functioning taking place at the molecular level. A normal CT scan does not necessarily mean that the person is normal. The brain can look normal on a CT scan but be functioning abnormally. Did you know that a recently dead person can have a normal CT scan? Therefore, it is important to remember what each test is meant to do and not meant to do.
Plain x-rays films are best for showing bone and are helpful in the assessing injury to the skull. X-rays, however, have limited value in evaluating brain injury since the brain itself can not be seen by x-ray. Also, x-rays only visualize the skull from the outside and do not produce clear images of the inside or the undersurface of the skull. As a result, it may be difficult to see fractures in those areas on x-rays. Because of these limitations x-rays of the head are not frequently used situations to assess closed head injury but may be used to assess significant skull fractures or penetration of objects through the skull.
II. Computerized Tomography (CT)
CT stands for Computerized Tomography. The images produced by CT scans are computer generated rather than formed directly by electromagnetic radiation as in X-rays. CT imaging uses a computer to digitally construct an image based upon the measurements of the absorption of x-rays through the brain. CT scans give us pictures of the brain that look as though the top of the person’s head had been sliced off and one could look down into the skull of the brain. During CT imaging, the x-ray source rotates around the patient’s head and each rotation produces a single cross sectional imaging “slice” like slices in a loaf of bread. CT allow physicians to see a horizontal piece of the body, just as if you were taking a slice of bread out of the loaf. CT scans are often the preferred imaging test in the emergency room because (1) it can be administrated quickly (and even when the patient is hooked up to an IV or other ICU equipment), and (2) it can identify intra cranial bleeding and clotting.
CT scanning does have limitations. For example, CT imaging largely shows only structural damage and accumulation of fluid, and has a poorer resolution than an MRI. A brain lesion must be generally (1.0 to 1.5 centimeters) to be visualized by CT scan. There can be significant brain injury with no visible change on CT scan if the injury occurs at the microscopic level. For example, in a “diffuse axonal injury” multiple individual neurons can be injured at the microscopic level with no visible evidence on CT scan.
CT scans can also be misleading and inferior to other imaging test when used immediately after head trauma. A CT will not visualize little acute bleeds or provide information on whether intra cranial pressure events have resolved. This is because CT scans can only detect lesion of a certain density. CTs can miss a slow leak of blood from a partially torn blood vessel which may take days or weeks to grow into a clot large enough to show up on the scan. Also, if CT is taken too long after the traumatic brain injury, it may miss even a large clot. Once active bleeding has stopped and the clot begins to undergo hardening and reabsorption, it will appear very similar to the surface of the brain and can not be visually distinguished from it. Therefore, CT works best after head trauma with a large active bleeding lesion but not in other situations.
A negative CT scan does not mean there is no brain injury. Despite this, insurers will most definitely argue that a negative CT scan means there is no evidence of traumatic brain injury. Understanding the limitations of CT scans allows one to effectively rebut such an assertion by the insurance company.
MRI stands for Magnetic Resonance Imaging. A MRI is a neuro-imaging technique used to show abnormalities of soft tissues of the brain in finer detail and greater clarity than CT scans. A large magnet or radio waves are used instead of x-rays to take pictures of the body’s tissues. A patient is placed on his back with his head inside a large donut shaped scanning device. The device uses super cooled electric coils made of titanium alloys to generate a powerful, vertical magnetic field which is held at a constant strength measured in “Tesla” units. The brain responds to the magnetic field because it is composed of 90% water, which in turn is made up of molecules of hydrogen and oxygen which carry a single proton in each atom of hydrogen. The radio frequency of emission data from the spinning water protons in the patient’s brain are gathered, digitized and used by the computer to reconstruct a three dimensional image of the location and appearance of the soft tissues and fluids within the brain.
Though the MRI is more sensitive than the CT scan in picking up bleeds on the brain, it is important to understand that even the most powerful MRI scanners can not penetrate to the cellular level of the brain or show diffuse cellular brain damage from a sheer-strain injury. Even today’s MRI machines can only visualize brain tissue of 1.5 millimeters. At this level of sensitivity there remains millions of brain cells that can not be seen. As a result, a great majority of patients diagnosed with mild traumatic brain injury continue to have negative MRIs. It is important to remember that a negative MRI does not by itself rule traumatic brain injury. Unfortunately, we still sees insurance adjusters, insurance hired doctors, and even ER personnel and family doctors claiming that a negative MRI is incompatible with a traumatic brain injury.
Stands for functional Magnetic Resonance Imaging. Technological advances allows the MRI to be used to map changes in brain blood flow that correspond to brain activity related to a specific task or sensory process. Observing both the structures and which structure participate in specific functions and provides high resolution, noninvasive reports of brain activity detected by blood oxygen level dependant signal. This new ability to directly observe brain function opens an array of new opportunities to advance our understanding of brain organization and assessing brain structure and function.
PET stands for Positron Emission Tomography. This is a nuclear medicine imaging technique that provides information about certain neurological conditions. PET utilizes metabolism and physiology to demonstrate problem spots in the brain. Unlike x-ray, CT or MRI, PET does not rely upon anatomy or structural abnormalities.
The ability of PET scans to show chemical function (metabolism) of organs and tissues is a reason for its outstanding accuracy at detecting disease. When getting a PET scan, the patient will be give a small blood sample to check his/her blood sugar or glucose level. After the IV catheter is in place, the radiologist injects a small amount of radioactive glucose into his/her blood stream. This glucose is called a tracer and will be distributed throughout the body. After the injection the patient is asked to relax and remain quit for about an hour. The patient will then be laid on the scanning bed that moves slowly through the scanner detecting the injected tracer. The scanner sends the resulting information to a computer. The computer generates several images for the doctor.
During the scanning the patient will be challenged with a variety of cognitive tasks which stress certain parts of the brain by making them perform mental work. If those parts of the brain are intact, healthy, and fully functional, they will absorb a lot of the radioactively-tagged glucose which will light up as a nice bright orange or red color. However, if those parts of the brain are damaged, dying, or dead, they will absorb very little, if any, glucose. These portions will appear as an icy blue or purple on the finished scan. Yellows and greens are in between the extremes. The colors themselves have no real value other than a way of creating a visual contrast to enable doctors to discriminate and distinguish different levels of metabolic activity in various parts of the brain. There is no pattern of color distribution or color hue which is uniquely distinctive to traumatic brain injury. As a result, doctors and expert witnesses will not say that a PET scan by itself proves conclusively that the patient had a traumatic brain injury. Rather, doctors and experts will say that that pattern of metabolic disturbance on the PET scan is consistent with traumatic brain injury and is more consistent with TBI than other brain conditions. Consequently, doctors and expert witnesses will look at the relationship between the lesion visible on the PET scan with the deficits identified through neuropsychological testing and neurological examination.
SPECT stands for Single Photon Emission Computed Tomography. This technology uses a rotating gamma camera to image the brain in slices which can be viewed individually or stacked together to give a three dimensional view of the brain. During a SPECT scan, the patient is administrated an IV dose of radioactive isotope and is challenged with mental tasks, then placed under the camera. The radioactive dye compound spontaneously releases gamma rays as it travels through the blood vessels in the brain. The data is processed by a computer which generates a map of regional cerebral blood flow.
The rate of blood flow in the brain over time is called cerebral perfusion. A traumatic brain injury tends to create a picture of distinguishably unequal cerebral perfusion, high in healthy parts of the brain and low in the damaged ones. As a result, SPECT scan is good at detecting localized disturbances and cerebral blood flow in patients with traumatic brain injury. The SPECT scan, however, has less clarity than a PET scan.
EEG stands for Electroencephalograph. This test measures the electrical activity in the brain. Special patches called electrodes are applied to the head to measure the activity. The test is painless and can be done at bedside or in the EEG department of a medical center. Generally, EEGs do not pick up brain deficits for mild traumatic brain injured patients because EEG is not good at picking up diffuse damage in the white cell matter of the brain. However, in severe head injury cases the EEG will be useful in understanding brain pathology in great detail. For example, EEG provides valuable information in the assessment and treatment of epilepsy.
If any of your patients have symptoms such as headaches with nausea, memory loss, double vision, loss of concentration, decreased attention span, increased sensitivity to distractions, etc, they may have sustained a traumatic brain injury. It is important to record these symptoms in your chart notes. If these symptoms do not resolve within four to six weeks, a referral to a neurologist and/or a neuropsychologist for evaluating and testing to explore the condition and rule out a more significant injury is called for. The results of this evaluation and testing may necessitate close monitoring and/or cognitive re-training and assistance to the patient. When there is a traumatic event, it is prudent to advise your patient to consult with an attorney specializing in personal injury law, insurance matters as closed-head injuries and traumatic brain injuries often present unique challenging issues in the successful resolution of a claim.