PET Scans as a Diagnostic Procedure for Alzheimer's Disease

As the population in the United States ages, many families are faced with the prospect of a loved one developing a frightening, confusing, and emotionally overwhelming diagnosis: Alzheimer's disease (AD). While many adults have some short-term memory loss in their lifetime, especially in their later years, an individual developing the onset of AD will experience a heightened sense of disarray. Indications such as drastic mood swings, decreased judgment with familiar tasks, difficulty with speech or motor skills, and disorientation to time and place can be early warning signs to a family that their loved one may be developing AD and prompt a physician referral for a PET scan (Alzheimer's Association, 2009). These same symptoms, however, could also be an indication of other significant medical concerns, ranging from dementia or depression to hyperthyroidism.

Fortunately, modern medicine offers tests and procedures to help distinguish AD from other conditions. Nationwide studies have shown that patients who have a positron emission tomography (PET) scan as part of their neurological workup are far less likely to be misdiagnosed, leading to more effective treatment protocols (Schmidt, 2002). In fact, a UCLA medical team concluded that of every 100 potential AD patients examined using traditional methods (i.e., without the benefit of a PET scan), as many as 23 would be incorrectly diagnosed and up to eight cases would be missed entirely (Schmidt, 2002). For a family devastated by watching their loved one spiral into dark confusion, this error rate is not acceptable. "Previous studies show that PET examinations in AD predict the clinical progression and the autopsy diagnosis with high accuracy, especially at the early stages of dementia when the clinical impressions are less certain" (Mosconi et al., 2006). In 1999, PET scans were shown to improve correct diagnoses of AD by up to 14% and more recent studies have shown up to a 30% improvement ("PET scan," 1999; "Nuclear medicine," 2004). It is vital that the most conclusive diagnosis possible be reached, for the sake of patients and their families. PET scans are making this possible.

A PET scan to diagnose AD is a non-invasive procedure that requires no anesthesia. "Patient preparation for FDG-PET imaging varies among institutions. However, some generalizations can be made" (Fischbach & Dunning III, 2009). Beforehand, the patient will be asked to dress warmly (to prevent shivering that would affect the scan results), refrain from tobacco, alcohol, and caffeine use for 24 hours and eating or drinking for at least four hours (six hours for a diabetic), and, if medication is necessary, to ingest it with only a small sip of water ("Nuclear medicine," 2004; Straight A's, 2008). At the imaging center, the patient will have their blood glucose measured and be taken to a quiet, darkened scanning room or separate uptake room depending on the institution where they will be asked to lie supine on the scanning table (Von Schulthess, 2007). A head holder and careful positioning will be used to minimize body movement, especially of the head (Von Schulthess, 2007). The table is attached to a PET scanner, which resembles a large donut surrounding one end of the table (American College of Radiography [ACR] & Radiological Society of North America [RSNA], 2009). An injection of a radioactive tracer called Fluorodeoxyglucose (FDG) is administered. FDG is a combination of a radioactive compound and glucose that the PET scanner is able to "see" as the body uses the sugar for energy ("Nuclear medicine," 2004). Although FDG is the most common tracer, some larger facilities may use other radiopharmaceuticals. This dose of radiation, which is equivalent to approximately two chest x-rays ("Nuclear medicine," 2004), is a safe amount and should not worry the patient. After receiving the FDG injection the patient will rest quietly while the radioactive compound is absorbed and distributed throughout the tissues (Von Schulthess, 2007). After the tracer has been properly absorbed, the scanning begins. The patient will be instructed to stay awake and lie as still as possible while the table slowly moves through the scanner "donut," taking images (Fischbach & Dunning III, 2009). The patient should expect to spend two hours at the imaging center; however, the entire procedure from arrival to conclusion can range from 15 minutes to 2 hours. After the scanning is completed the patient returns home.

"The only contraindication to a nuclear medicine scan is pregnancy, and even this is only a relative indication" (Yudofsky & Hales, 2008). Diabetes is not a contraindication; however, diabetics who take insulin shots will need to consult their doctors beforehand. Another diagnostic procedure may be recommended for morbidly obese patients who cannot access the scanner or if intravenous (IV) access cannot be obtained for injection of the FDG. Severely claustrophobic, agitated, uncooperative, or confused patients, or individuals unable to stay still for the duration of the test may require sedation (Barker, 2008). Depending on what they are looking for, some tests will preclude use of tranquilizers because they affect the metabolism of glucose (Fischbach & Dunning III, 2009).

"FDG PET has been approved by the FDA for the diagnosis of AD" (Pupi & Nobili, 2005) and typically there are no side effects. Slight redness or discomfort at the injection site, although rare, is comparable to what might be felt during routine blood work (E. Duncan, personal communication, June 16, 2009). FDG is used in such a small quantity that there are no clinical side effects from the radiation (ACR & RSNA, 2009). According to the ACR and RSNA: "There are no known long-term adverse effects from such low-dose exposure" and although a small number of people have allergic reactions to radioactive tracers, this occurrence is extremely rare and symptoms are generally mild (2009).

A major benefit of a PET scan to diagnose AD is that it eliminates uncertainty and increases reliability of the results. "In their reviews, Silverman and Mosconi described the superiority of PET to SPECT in diagnosing early AD, and PET indeed offers many advantages for detecting functional abnormalities in the AD brain" (Ishii & Minoshima, 2005). It can document the degree to which specific areas of brain activity are compromised by AD. "PET is a test that may diagnose Alzheimer's early enough to make full use of drug therapies, reassure fearful patients who might not have the disease or redirect treatment for other conditions" ("Alzheimer's: Early detection," 2009). The Radiology-Info (2004) website states: "Alzheimer's disease has a metabolic abnormality (bilateral temporoparietal hypometabolism) that is significantly different from metabolic abnormalities found in other forms of dementia" (PET and Alzheimer's Disease Detection section). PET scans are the only imaging modality that can measure and monitor neurological function and activity. As such, "PET can theoretically aid in the diagnosis of dementia, even in the absence of the gross structural damage detected by other imaging techniques, such as CT or magnetic resonance imaging" (Gill, Rochon, Guttman, & Laupacis, 2003).


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