Keywords

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

7.1 Introduction

18F-FDG PET is a frequently used imaging modality in the evaluation of cancer patients. A high-quality study performed 18F-FDG PET study should be repeatable (same result produced if imaged on the same system) and reproducible (similar result if imaged at different sites). An essential component of this is adequate patient preparation to ensure study reproducibility and technical quality. Rigorous instructions should be followed regarding patient procedure. In addition, adequate referral information is important so that the correct timing of study and imaging protocol can be followed, e.g. lung gating for a base of lung lesion. This section addresses some of these issues, and summaries of required clinical information, patient preparation, procedure and imaging parameters are shown in Tables 7.1, 7.2 and 7.3.

Table 7.1 Contents of PET/CT request [1–5]
Full size table
Table 7.2 General instructions for an 18F-FDG PET scan [1–5]
Full size table
Table 7.3 18F-FDG PET/CT imaging parameters [1–5]
Full size table

FDG is a glucose analogue and is transferred intracellularly by glucose transporters. Many tumour cells overexpress glucose transporter proteins and hexokinase intracellularly, which allows FDG to be used to image these tumours.

7.2 Patient Preparation

One of the main aims in patient preparation is to reduce the hyperinsulinemic state, which occurs with recent glucose ingestion. Increased glucose levels cause competitive inhibition of 18F-FDG uptake by the cells leading to decreased tumour (or other active process) to background ratio. Also increased insulin secondary to elevated blood glucose increases translocation of GLUT4, thereby shunting 18F-FDG to organs with high density of insulin receptors (e.g. skeletal muscles). Patients should thus fast for at least 6 h prior to the study to ensure low insulin levels. Recent EANM guidelines suggest that patients with blood glucose <11 mmol/l can have FDG administered, whilst patients with glucose >11 mmol/l need to be rescheduled. Patients with diabetes (particularly with insulin-based treatments) need to be carefully scheduled to avoid a hyperinsulinemic state. (An example of scheduling includes a late morning appointment with an early breakfast and insulin injection.)

If glucose control is not achieved, then the PET scan can be rescheduled.

Other patient preparations also aim to reduce tracer uptake in normal tissues, thus increasing target and nontarget uptake. Patients should be hydrated adequately to decrease the concentration of FDG in the urine, decreasing artefacts and potentially reducing radiation dose. Drinking water is permitted; however, flavoured water contains sugar and cannot be consumed prior to the PET scan. Patients should be advised to dress warmly on the way to the PET suite and should be kept in a warm room prior to the administration of FDG. This is to avoid accumulation of FDG in activated brown fat. In some cases with no contraindications to oral beta-blockers, propranolol (1 mg/kg, maximum 40 mg) should be given at least 90 min before FDG injection to reduce FDG uptake in brown adipose tissue. This is especially important in young patients. Strenuous physical activity should be avoided for at least 6 h prior to the scan to avoid excessive skeletal uptake. During the uptake period, the patient should not talk and avoid reading or chewing, to minimise uptake in these respective muscles.

If a lesion near the myocardium or the myocardium itself is being evaluated for suspected disease, careful patient preparation is required to limit cardiac uptake. A low-carbohydrate, high-fat, high-protein diet for at least 24 h before the scan (Table 7.4) and extended fasting for 18 h before the scan are recommended to switch the myocardial energy substrate from glucose to fatty acids. This is coupled with one or two intravenous bolus of heparin (50 IU/kg) given 90 min prior to18F-FDG injection for suppression of myocardial FDG uptake.

Table 7.4 Example of low-carbohydrate high-fat diet provided to patients prior to FDG PET cardiac imaging
Full size table

Review of patients’ medication should be performed, e.g. steroids in high doses may cause hyperglycaemic states and, in patients with suspected vasculitis, may reduce the sensitivity of the test; metformin may cause diffuse large bowel uptake due to increased glucose utilisation of the intestinal mucosa. If intravenous contrast is going to be administered, metformin needs to be withheld on the day of the test and for a further 48 h.

7.3 Timing of FDG PET Scan After Treatment

When the PET scan is being protocolled, adequate information about previous treatments should be available to the authoriser to ensure accurate timing, e.g. in chemotherapy response assessments in lymphoma, the FDG PET scan should not be performed too early to avoid false negatives due to tumour stunning or false positives due to inflammatory uptake. An interval of at least 10 days should be allowed post chemotherapy (interim PET) or at least 3 weeks at the end of chemotherapy to allow evaluation of response to chemotherapy. If patients are undergoing radiotherapy, the recommended post-therapy interval is 2–3 months.

Key Points

  • Rigorous instructions should be followed regarding patient procedure.

  • Adequate referral information is important so that the correct timing of study and imaging protocol can be followed.

  • Increased glucose levels cause competitive inhibition of 18F-FDG uptake.

  • Increased insulin secondary to elevated blood glucose increases translocation of GLUT4.

  • Patients should thus fast for at least 6 h prior to the study to ensure low insulin levels.

  • Patients with blood glucose <11 mmol/l can have FDG administered, whilst patients with glucose >11 mmol/l need to be rescheduled (EANM guidelines).

  • Patients with diabetes (particularly with insulin-based treatments) need to be carefully scheduled to avoid a hyperinsulinemic state.

  • Patients should be hydrated adequately to decrease the concentration of FDG in the urine, decreasing artefacts and potentially reducing radiation dose.

  • Keep the patient in a warm room for 30–60 min before the FDG injection.

  • Strenuous physical activity should be avoided for at least 6 h prior to the scan.

  • An interval of at least 10 days should be allowed post chemotherapy (interim PET) or at least 3 weeks at the end of chemotherapy to allow evaluation of response to chemotherapy.

  • In patients undergoing radiotherapy, the recommended post-therapy interval is 3 months.