Evaluation of Alzheimer’s Disease using F-18 FDG

Hollie Gibbs

Senior NMT Student

Medical College of Georgia

Deborah Gibbs

CNMT

PET

NCT

MCGHI

Jonathan Rohe

CNMT

PET

MCGHI

chart

Patient History

The patient is a 72-year old female who has a history of a cerebrovascular accident and dementia. An MRI performed on 12/04/08 revealed a watershed infarct in portions of the right frontal and parietal lobes. A PET scan was performed 01/28/09 to determine whether or not the patient’s dementia was due to this infarct or if Alzheimer’s disease was also a factor.

Patient Prep

The patient was telephoned the day before the exam and instructed to refrain from eating or drinking anything except water after midnight. Upon arrival, the patient identification was verified and the applicable patient history along with height and weight were obtained. The patient’s blood sugar was tested and found to be 117 mg/dL, which is within the acceptable range of 60-120 mg/dL. Intravenous access was obtained and a 250 mL saline drip was connected. The patient was reclined with pillows, a warm blanket, low-lighting, and made as comfortable as possible to minimize movement during the 30 to 60 minute uptake phase

Radiopharmacy

The patient was then injected with 15.22 mCi of F-18 fluorodeoxyglucose (FDG). The dose is determined by patient weight as well as the capability or limitations of the PET scanner on which imaging is performed. FDG is a glucose analog that is taken up by nearly all cells in the body, as glucose is the primary energy source for most cells. The brain is certainly no exception, with 95% of its energy coming from glucose utilization. Since the brain is so glucose-avid, adequate uptake can be obtained in as little as 30 minutes post-injection.

Patient Positioning

The patient was monitored during the uptake period using a camera system allowing technologists to maintain a wise distance. At about 30 minutes into the uptake phase the patient’s IV was removed and the patient was instructed to empty her bladder as much as possible. This helps ensure that the patient will be comfortable while on the imaging table and also minimizes both patient and technologist dose by eliminating the FDG that has been cleared through the urinary system during the uptake phase.

The patient was then positioned supine on the imaging table with arms by her side and secured with a Velcro strap. A knee cushion and blanket were provided for comfort, and a U-shaped cushion was placed underneath the patient’s head to cradle it and help ensure that it remained still. The patient was advanced into the gantry until the internal lasers sat just below the chin for the Surview and the lasers were also lined up along the Z-axis with the external auditory meatus. The Surview was then used to set the single 10-minute bed position to include the entire brain.

Instrumentation and Processing

Using the Philips Gemini TF PET/CT scanner, a CT Surview image was taken from the point of the lasers up to ensure proper positioning and that all potential artifacts were removed, if possible. At the acquisition station, the area of the scan is determined by dragging and lengthening a box over the Surview so that the brain is centered in both the PET and CT acquisitions. The CT image is acquired first and reconstruction is performed by the computer before the PET portion begins. Once the PET acquisition is complete, the images can be pulled up and previewed before the PET reconstruction and attenuation correction automatically begin. The box drawn for the CT field of view must be slightly longer than the PET one to ensure proper reconstruction.

Images and Findings

The two arrows on the transverse fused PET/CT image above point to areas of little to no uptake of FDG, meaning that these portions of the brain are not very metabolically active. According to the radiologist’s report, the dramatic decrease in activity in the anterior portion of the right frontal lobe as well as the mid- and posterior parietal lobe correlates with the known infarct shown on the MRI. The left frontal lobe exhibits normal uptake, and the difference between the left and right occipital lobes is minimal. Also noted, but not well seen on this slice is a fair decrease of activity in both temporal lobes, with the left side showing less activity and including a portion of the parietal lobe. It is this decreased activity in the parietal and temporal lobes that confirms Alzheimer’s disease.

Discussion

Dementia in most cases is caused by one of three factors: cerebrovascular disease (30%), treatable conditions such as drug intoxication, thyroid disorder, or a vitamin B12 deficiency (20%), or Alzheimer’s disease (50%). In diagnosing Alzheimer’s, it is important to rule out the other two factors first. It is interesting that in this particular case it appears that both the infarct from the CVA and Alzheimer’s disease contribute to the patient’s dementia. On the images, decreased activity due to both causes can be seen. The areas of decreased activity that correspond with the infarct shown on MRI are obviously due to the CVA, while the decreased activity in the parietal and temporal lobes is what indicates Alzheimer’s disease. In some patients with Alzheimer’s disease there may also be a decrease in activity in the hippocampus and slightly in the frontal lobe. The disease process of Alzheimer’s involves a heightened rate of neuronal death in these areas of the brain, leading to a decrease in the blood and oxygen supply that, in turn, inhibits the metabolism of glucose. Another characteristic of the defect seen with Alzheimer’s disease is that it is a symmetric pattern affecting the pair of lobes, whereas dementia due to infarct will show only in the areas supplied by the affected artery. The nature of this pattern and the ability of PET to demonstrate it make it instrumental in diagnosing Alzheimer’s even in the presence of an infarct.