The vast majority of traumatic brain injuries are closed head, and thus inaccessible to direct visual inspection in the absence of a surgery to open a flap of skull and peer inside at the wounded brain. Before modern neuroimaging, diagnosis of closed head TBI was made by clinical inference rather than looking at the patient’s physical brain. Neurologists inferred the existence of brain swelling with increased intra-cranial pressure by means of symptoms such as loss of consciousness, drowsiness, vomiting, confusion, unequal pupils, tunnel vision, lost or diminished reflexes, and the like. If they were quite sure the patient had brain swelling or a blood clot compressing his brain, they would refer him for neurosurgery. Otherwise they would refer him to a neuropsychologist for testing to see if pen and paper tests could shed light on the probable existence and location of “subtle” brain damage.
Today CT scans are routinely used in emergency rooms to search the brain, the space between its membranes, and the space between its membranes and the skull for active bleeding or clots. CT is very helpful, but has limits. It will not detect “mild” TBI consisting of diffuse shear injury to axons without rupture of blood vessels (statistically the most common form of TBI). Further, if a “slow leak” of intra-cranial blood develops 48-72 hours after head trauma, the CT done in the emergency room will miss it and mislead the doctor into thinking the patient will be OK. After the acute phase of injury, follow-up neuroimaging may be done with any one of a variety of techniques including MRI, quantitative MRI, functional MRI, MRI Spectroscopy, MRA, SPECT, PET, transcranial MEG or Diffusion Tensor. For more information on these, follow the links below.
The brain is not just a thing (a physical structure with 3 dimensions occupying space), but a living organ which performs cognitive, visual, motor, emotional, psycho-social, and behavioral functions through burning oxygen and glucose, firing electric impulses down axons and secreting excitatory or inhibitory neurotransmitters and neuromodulators. Although occupying only 2% of the volume of the body, the brain burns 20% of the body’s oxygen and 20% of its glucose to do this mental work. During the 1980s and more so during the 1990s, newer forms of “functional” neuroimaging came into regular use. These PET Scans, SPECT Scans (and most recently MEG) have the capacity to detect minute functional disturbance of normal brain metabolism following traumatic damage and to represent it visually in the form of reduced glucose uptake, reduced cerebral blood flow and reduced electrical discharge (nerve cell firing).
Objective visualization of functional damage is more meaningful in evaluating a TBI than having a static picture or snapshot of the physical appearance of brain structure. Structures which appear intact on CT/MRI may be badly malfunctioning due to subtle damage, visible only on neuroimaging which measures abnormalities in rate of cerebral blood flow and neuronal consumption of oxygen or glucose. A car battery that looks normal on CT or MRI may be dead and unable to start a car engine.
An excellent example of this is schizophrenia. People with schizophrenia exhibit very dramatic and distinctive cognitive problems (such as auditory or visual hallucinations, flight of ideas and disordered thinking) as well as gross behavioral problems (apathy, impoverished emotional expressiveness and a tendency to sit mutely watching TV and smoking). Yet CT/MRI scans of brain structure show no obvious or diagnostically specific brain damage in these patients. See, Harvard Mental Health Letter Feb. 2001 “How Schizophrenia Develops: New Evidence and Ideas.” Scientists who compare structural images of 1000s of schizophrenics vs. “normals,” have noticed that people with schizophrenia have slightly smaller overall brain volume and larger ventricles than “normals,” yet this would not even be noticeable on one scan.
A radiologist who read a CT/MRI of a schizophrenic patient following a concussion with no evidence of bleeding or swelling, would write “normal scan” if he was not given the patient’s history of schizophrenia. The same is true of TBI. Functional brain disturbance which causes disorders of behavior does not show up on structural imaging. A review of older structural imaging techniques and newer functional imaging techniques follows below.
( links coming soon )
- CAT Scan
- MRI Spectroscopy
- PET Scan
- Quantitative MRI
- Functional MRI
- SPECT Scan
- 3-Dimensional Neuroimaging
- Diffusion Tensor
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