Patient Preparation For FDG PET Imaging Studies

Patient preparation for FDG PET studies is perhaps more critical and more complex than for any other commonly performed imaging study. This is due to the fundamental basis of the imaging which depends upon the in vivo distribution of the glucose analog, F-18 Fluorodeoxy-D-glucose into normal tissues and on the strikingly increased glucose metabolism of neoplastic tissue.

Even Normal FDG Distribution is Complex

As glucose is one of the key energy substrates for nearly every aspect of cellular function, the distribution of any tracer analog of glucose is complex. As the brain uses glucose as its sole energy source, it is not surprising that maximal FDG activity after IV injection is present in cerebral gray matter. In contrast, the myocardium is omnivorous. Under fasting conditions, fatty acids usually comprise the “preferred” substrate for myocardial energy metabolism but in many patients, glucose is utilized under such conditions. Most other metabolically active organs utilize glucose to a substantially lesser degree than brain and heart. Hence, imaging after FDG injection reveals a complex FDG distribution pattern, which, however, is useful in localizing sites of abnormal uptake, providing an “anatomic backdrop” in normal organs.

FDG as an Engineered Glucose Analog

There are several differences between the uptake and metabolism of FDG compared with unlabeled glucose. Both molecules are transported into cells by the same glucose transporter proteins and both are phosphorylated by hexokinase. FDG, however, cannot be further metabolized and is trapped within the cell as a consequence of the phosphorylation, whereas glucose is either stored as glycogen or rapidly metabolized. Further, unlike glucose, FDG is not reabsorbed by the renal tubule, and is excreted, accounting for the presence of activity in the renal collecting system and bladder. Because of the renal excretion, we prefer patients to be well hydrated prior to the study so that collecting system activity is minimized.

Hyperglycemia and FDG PET

As a glucose analog, FDG competes with glucose for transport into cells. Consequently, hyperglycemia will competitively inhibit FDG uptake into both normal tissues and tumor. Accordingly, hyperglycemia in diabetics or patients on steroids may significantly decrease tumor uptake of FDG. In general, blood glucose levels below 200 mg/dl are not felt to affect overall study sensitivity but it is possible that in patients with subtle or small lesions, even levels below 200 mg/dl may have some deleterious effect. Above 200 mg/dl, we will not go forward with the test and will refer the patient back to the referring physician for better diabetic control prior to imaging.

Insulin Level and FDG Uptake

Insulin has a profound effect on FDG uptake. In patients with elevated insulin levels either due to failure to fast or to recent administration of short acting insulin or even earlier administration of long acting insulin, FDG is shunted to skeletal muscle and a lesser extent fat. FDG uptake in tumor may be dramatically reduced or even abolished. Insulin-dependent diabetics should hold insulin on the morning of the test where possible, or be injected with FDG at least 90 min after short acting insulin injection (preferably IV to prevent delayed absorption from a subcutaneous site). Long acting insulin should be held at least overnight. As sulfonylureas may cause hypoglycemia in fasting patients with “mild” diabetes, it is generally preferable to hold them on the morning of the test. Other medications used to treat diabetes do not have this effect. One of our physicians will contact any diabetic patient to discuss their therapeutic regimen and recent control status to assure a safe but effective study.

Other Factors Impacting FDG Distribution

Other activities that may affect FDG distribution include exercise (increases glucose metabolism in affected muscles), tobacco use on the day of the test, and caffeine (for brain studies). While there is variable FDG uptake in stomach and colon particularly (but occasionally in small bowel), it is not clearly understood how this uptake can be minimized. False positive FDG uptake can occur in a variety of inflammatory and infectious lesions as well as in cellular benign tumors and it is important that these factors, if known, be communicated to us at the time of the test scheduling.

PET and Pregnancy

While the radiation dose from FDG PET is relatively low (due largely to the short half life of the F-18 of 110 min), patients who are known to be, or might be pregnant should not be studied except in very unusual circumstances. Women who are breast feeding may not do so for approximately 8 hrs after the procedure, but breast milk stored for that time may be used as very little tracer is excreted in breast milk. The main potential for radiation dose to the infant is from close contact with the patient who will be radioactive for a number of hours after the test.

Handling Claustrophobia or Anxiousness

FDG PET imaging is an easily tolerated test for patients albeit a relatively lengthy one. Patients should wear comfortable clothes. If they are anxious or claustrophobic, benzodiazepines may be helpful. We maintain a stock of short acting benzodiazepines for such patients. These drugs also are useful in anxious patients to prevent the often confounding FDG uptake in brown fat that may be seen in anxious or shivering patients.

At the time of scheduling, our staff discusses the following instructions with your patients but it is also important that you understand how these factors may affect test results. They are also outlined on the reverse side of our new Request forms. As always, if you have any questions, don’t hesitate to call us.

Need to check this out and make sure it is consistent with what what we have already in the site.- or else link to another area on the site where the patient prep is enumerated…

Summary:

1. Patients should be fasting for at least 4 – 6 hrs. This must include chewing gum, candies and even glucose syrup based medications such as cough preparations.
2. Patients should be well hydrated with water only on the day of the test.
3. No glucose-containing IV fluids or enteral or parenteral nutrition for inpatients.
4. Patient should not exercise strenuously on the day before or on the day of the test.
5. No smoking on the day of the test.
6. In diabetics, moderate to good control is imperative such that blood glucose at the time of the test is below 200 mg/dl.
   
   1. No short acting insulin, if possible, on the day of the test. If required for glucose >200 mg/dl, it must be given IV at least 90 min prior to the study time.
   2. No long acting insulin for at least 12 hrs prior to the study.
   3. Hold sulfonylureas on morning of the study unless patient is poorly controlled without it and hypoglycemia is unlikely with fasting.
      
7. Differentiated thyroid cancer is a special case. There is increasing evidence that in the assessment of patients with rising thyroglobulin levels but negative I-131 scans, FDG PET is best performed when the patient is maximally TSH stimulated, either by thyroid hormone withdrawal or using recombinant human TSH. PET scans should thus be scheduled immediately after the I-131 scan, before the patient is restarted on thyroid replacement therapy.