PET SCAN  [ back to Neuroimaging ]
PET SCAN or positron emission tomography scan uses a $6,000,000 gamma camera with a double ring of crystals. The patient is given an intravenous drip of glucose tagged with a radioactive isotope having a half life of just 20 minutes. He is then challenged with a variety of cognitive tasks which will "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 when the patient’s head is placed under the gamma camera.

If those parts of the brain are damaged, dying or dead they will absorb very little, if any, glucose, and will show up an icy blue or purple on the finished scan. Yellows and greens are in between the extremes. The colors have no intrinsic significance, and are merely a way of creating visual contrast to enable the radiologist to discriminate and distinguish varying levels of metabolic activity in various parts of the brain.

There is no pattern of color distribution and color hue which is uniquely distinctive to traumatic brain injury. Stroke, tumor, psychological depression, schizophrenia, chronic alcohol abuse and other conditions which depress regional brain metabolism can each produce a picture of purples and blues in similar, overlapping parts of the brain. How can the radiologist tell one from the other? This takes a lot of training. The method involves analyzing the patient’s clinical history for the presence of these and other conditions, so the color fingerprint they leave can be subtracted out. By gradually excluding other diagnostic possibilities, the radiologist can reach the opinion that the abnormalities visualized by PET are most consistent with a TBI. To make his opinion reliable, the radiologist must ensure that the patient has been weaned off any prescription medications which might depress brain metabolism during the testing procedure. The radiologist must also compare the PET image of the brain to all other image studies of the patient's skull and brain to see how tight a correlation exists between the trauma and the location and severity of the lesion areas. Finally the radiologist looks for a relationship between the lesions visible on PET with the deficits identified through neuropsychological testing and their neuroanatomical correlates.

The level of confidence he will enjoy that a color coded PET image is consistent with or compatible with TBI will vary with the patient’s clinical history and neuropsychological test results. Concluding that a patient with chronic severe depression predating the traumatic incident also has a frontal lobe injury from trauma is not impossible, for there may be pockets of blue and purple in other areas of the brain, which would not be affected by depression. However, reaching the conclusion that the patient suffered a traumatic injury to his frontal lobes is much easier if he had no prior history of mental illness, and functioned effectively in a job with great demands on his intellectual and decision making capacity, until slamming his forehead in an automobile accident, after which he exhibited characteristic signs and symptoms of a frontal lobe injury, such as inability to make a decision, forgetfulness, distractibility, apathy, etc.

How does the technique work? When taken up into the cells of the patient’s brain during cognitive processing, the radioactive glucose molecules give off a positively charged particle the size of an electron called a positron. When positrons come into contact with electrons they physically annihilate each other and cause an energy release in the form of gamma rays traveling in exactly the opposite direction. The sodium iodide crystals in the gamma camera imprint the simultaneous arrival of gamma rays from each collision. The data is digitized and fed into a computer which then back calculates the physical location of each collision in the brain, and generates a 3 dimensional map of metabolic activity. High metabolic activity is associated with the greatest number and density of collisions, whereas low metabolic activity reflects fewer collisions, because the radioactive glucose was not being taken up into that part of the brain. One thing PET scans have demonstrated is that a small cortical lesion at the surface of the brain visible on MRI can be the proverbial tip of the iceberg with regard to traumatic disruption of brain function.

In TBI litigation there is an inevitable battle of experts over the "clinical utility and clinical validity" of PET scans. The plaintiff wants the PET scan admitted into evidence to confirm the diagnosis of TBI, while the defense wants the PET scan excluded from evidence as a "research toy" of pointy-headed, academic neuroscientists which may have a place in the lab but has never been demonstrated to have clinical validity. Who is right? Time has changed the debate. The defense point was much stronger 20 years ago. Today it is very weak and should not be heeded. PET scans are used routinely to diagnosis heart disease, metastatic cancer, stroke and a variety of brain disorders. It is now being used to map the "penumbra" of damaged but potentially salvageable brain tissue just hours after stroke. Brain 124 Part I, Jan. 2000. Health insurance companies pay for them on a doctor's prescription. All the big drug manufacturers use them when applying for FDA approval of a new drug application, to show how the drug affects the metabolism and function of the human brain. On 5/16/00 Gary Small of the UCLA School of Medicine reported in the Proceedings of the NAS that PET scans of adults at peak risk of developing Alzheimer's Disease (those with the APOE-e4 gene) could detect diminished function of the temporal and parietal lobes (the earliest manifestation of Alzheimer's) in time to initiate preventive therapies. The Alzheimer's Association called this "a valuable advance."

PET is clearly being used clinically to detect brain disease. The question on PET is not whether it gives us an accurate picture of the state of brain metabolism and brain function, but the limits of interpretation of that picture. Reputable experts will not say "this PET scan proves Mr. Jones has a traumatic brain injury." Rather, they will say, "the pattern of metabolic disturbance on this PET is consistent with a traumatic brain injury and is more consistent with a TBI than other brain conditions suggested as possibilities by the this patient's medical history." Just as some people continued to insist the world was flat after Columbus, the "experts" hired by liability insurance companies continue to this day to challenge the validity of PET as a scientific tool to assist in the diagnosis of TBI. Hence any attorney representing a plaintiff with a TBI who has had a PET scan must know the medical literature on PET and must retain a top expert (e.g. a nuclear medicine physician) to defend the technique.