PET Scans

Positron Emission Tomography (”PET”) uses radioactive tracers to obtain images of organs and internal body structures. The PET scanner resembles a CT scanner but instead of measuring X-rays passing through the body, it measures gamma rays emanating from tissues where tracer elements have accumulated.

For brain imaging, different types of tracers are ingested or injected in the bloodstream depending on the type of information the doctor is looking for. Based on the tracer selected, images can focus on showing microscopic blood flow, oxygen or glucose consumption associated with brain activity, or variations in dopamine concentrations that can signal damaged areas.

Because the resulting brightly colored images show where biochemical processes are abnormal, the physician can more clearly see the degree and extent of an injury.

Though the images from PET scans are not as detailed as CT or MRI, new techniques are allowing doctors to fuse PET and CT images together. Newer machines perform a CT scan immediately after the PET scan while the patient is still on the table and in the same position. When the PET and CT images are combined, the anatomical and biochemical data are shown together so the doctor can see both what is wrong and precisely where.

Note that some insurance companies consider PET tests “experimental,” and will not cover their cost.

SPECT Scans

Single Photon Emission Computed Tomography (”SPECT”) is very similar to PET. It is a simpler technology, though, and is limited to showing blood flow. Still, it can be very effective in identifying damaged areas of the brain, and is less expensive than PET.

SPECT also requires a radioactive tracer based on the specific type of imaging desired, but the machinery is different from PET and CT scanners. Instead of the patent’s head being inside a donut-shaped ring, the SPECT camera rotates around the patient.

Preparation

The radioactive tracer is typically injected about one hour before the scan so that the targeted tissues can absorb it. The amount of radioactivity involved is so low that there is no risk to the patient even from having several scans done in a short period. On very rare occasions, someone may have an allergic reaction to the tracer compound.

Both PET and SPECT scans require the patient remain very still throughout the 30 to 90 minutes of the procedure. This can be uncomfortable and some patients may receive a sedative to help them relax.

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Physicians use the test not only for assessing consciousness at a given moment, but also for determining whether a patient is improving or declining by comparing scores over a period of time.

How the Glasgow Coma Scale Works

The test has three parts with different scores assigned to each. A score is recorded for the highest level of response in each test, and the total of the three scores determines the result.

• Eye Response (score of 1 to 4)
• Verbal Response (score of 1 to 5)
• Motor Response (score of 1 to 6)

Eye Response

4 = Eyes open spontaneously without requiring stimulation
3 = Eyes open in response to voice (does not apply to awakening a sleeping patient)
2 = Eyes open in response to pain (usually applied by pinching the fingernail)
1 = Eyes do not open

Verbal Response

5 = Responds coherently when asked questions (name, age, day and month, etc.)
4 = Speaks coherently but exhibits confusion or disorientation
3 = Speaks random words or thoughts, or doesn’t respond coherently to questions
2 = Makes sounds but not understandable words
1 = Makes no sounds

Motor Response

6 = Can perform simple movements when asked
5 = Responds to pain or pressure being applied by trying to reach for or remove the source
4 = Attempts to pull away or turn away when pain or pressure is applied
3 = Responds to pain by bringing forearms to the chest and clenching fists
2 = Responds to pain by rigidly extending arms, arching head back, and/or extending the legs and toes
1 = Does not respond to pain

What the Glasgow Coma Scale Score Means

Adding the scores from each of the three tests yields the Glasgow Coma Scale score, which can indicate the severity of a traumatic brain injury.

15 – 13: Mild injury with good prognosis for recovery. Some symptoms (emotional, mild cognitive impairment, headaches) may persist for an extended period. (See Understanding Post-Concussion Syndrome.)

12 – 9: Moderate injury with probable lasting cognitive disability that can benefit from rehabilitation.

8 – 3: Comatose. Survivors are likely to have severe physical and mental disabilities.

Limitations in Using the Glasgow Coma Scale

Because younger children may have limited verbal capabilities, the Glasgow Coma Scale is primarily used for older children and adults. Younger patients are evaluated with the Pediatric Glasgow Coma Scale, a slightly different test.

When other injuries or medical interventions interfere with the patient’s ability to open his eyes (eye injuries or bandages), speak (facial injury or breathing tube), or move (injury to limbs), physicians record the individual scores for only the testable responses. Obviously, the total will not fit the evaluation criteria above. In these cases, it is up to the physician to interpret the partial score.

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• Concussion Symptoms
• MRI vs. CT Scan in Determining Brain Injuries
• PET and SPECT Scans of Brain Function

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CT Scans

If the doctor’s initial evaluations indicate a skull fracture or intracranial bleeding, a CT (or CAT) scan is usually ordered. CT stands for Computerized Tomography. CAT stands for Computerized Axial Tomography. They’re both essentially the same thing, a way to obtain detailed X-ray pictures of cross-sections through the body.

A CT scan is preferred over standard X-rays because of its greater detail. Testing is fast and results are quick; making it exceptionally valuable when prompt diagnosis and treatment are critical. Unlike some other scanning methods, the CT scan can be taken while the patient is hooked up to IV’s or other medical equipment.

Though the CT scanner uses ordinary X-rays, the rays are focused from many different angles to yield a picture of a very narrow “slice” of the body. The scanner’s computer assembles the X-ray data into the final image or “tomogram.” A series of tomograms provide a detailed picture of internal body structures.

The CT scans can reveal hematomas, hemorrhages, and skull fractures giving the neurologist exactly the information necessary for deciding if emergency treatment is needed and precisely where.

MRI

Magnetic Resonance Imaging (”MRI”) is not often used in acute head injury cases. After the acute phase has passed, the doctor may want an MRI to evaluate the location and extent of brain injury to determine further treatment and rehabilitation options.




Although MRI images yield finer detail than CT scans, MRI’s drawbacks include:

• Longer to perform
• Not as readily available as a CT scanner in most hospitals
• Is not practical for patients hooked up to medical equipment
• Cannot be used if patient has metal embedded anywhere in the body
• Is not tolerated well by some patients because of the confined space inside the MRI machine

MRI uses powerful magnetic fields and the magnetic reaction of the body’s cells to construct cross-sectional images similar to CT scans. Because it doesn’t use X-rays, it can be safer than CT if multiple imaging sessions are expected.

Variations of MRI technology can also examine brain functioning and identify injuries not visible in CT scans. But even the detail available using MRI cannot detect mild concussions.

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