The aim of the study is to develop a non-invasive method for quantifying cerebral glucose metabolism by PET scans in humans. This method will be used in subsequent studies, where the effect of the pancreatic hormone glucagon on cerebral glucose metabolism will be studied.

The golden standard for quantifying cerebral glucose metabolism by PET scans is based on arterial blood sampling, which complicates research setup. This study will investigate if image derived measurements instead of arterial blood samples can be used to quantify cerebral glucose metabolism.

We will compare the calculations of cerebral glucose metabolism based on arterial blood samples and image derived measurements and hopefully these will correlate.

Healthy participants will be included, and each participant will participate in one study day, which includes intravenous administration of radioactively labelled glucose (18-FDG), arterial blood sampling and PET scans.

Participants will arrive after a four hour fast. An arterial cannulation will be placed in the radial or brachial artery for blood sampling. A venous catheter will be placed in the antecubital vein for administration of 18-FDG. Blood glucose is measured.

Before administration of 18-FDG a "low dose" CT scan of the heart and brain will be performed.

At time 0 minutes 20 MBq 18-FDG will be administered, 10 minutes automatic blood sampling (from the arterial cannulation) will be started and a 10 minutes three part dynamic PET scan of the heart will be performed.

Approximately at time 20 minutes 180 MBq 18-FDG will be administered and a 40 minutes three part dynamic PET scan of the brain will be performed. During the first 10 minutes of the brain scan automatic blood sampling (from the arterial cannulation) will be performed and thereafter manual blood sampling will be performed every 5 minutes.

After the 40 minutes brain PET scan a 5 min post scan of the heart will be performed.

Radioactivity will be measured in the arterial blood samples and the results will be used for an Arterial Input Function (AIF).

Radioactivity measurements derived from the heart scans will be used for an Image Derived Input Function (IDIF).

Inclusion Criteria:

• Capable of understanding the participant information and signing the consent form
• Between 25 and 70 years of age
• Body mass index <= 25 kg/m^2

Exclusion Criteria:

• Enrolment in other research project that might interfere with the study

• Diabetes diagnosis

• Pregnancy or breastfeeding

• Use of medications which, in the opinion of the investigator, may jeopardise participant's safety or compliance with the protocol

• Diagnosis of psychiatric disorders, dementias, or any other neurological disorders that in the opinion of the investigator precludes compliance with the study protocol, evaluation of the results or represents an unacceptable risk for the participants safety

• Severe claustrophobia

• Cardiac problems including any of the following:

  1. Classified as being in New York Heart Association (NYHA) class III or IV
  2. Angina pectoris (chest pain) within the last 6 months
  3. Acute myocardial infarction (heart attack) within last 2 years

• Inadequately treated blood pressure at screening defined as repeated resting blood pressure outside the range 90-150 mmHg for systolic and 50-100 mmHg for diastolic.

• Active or recent malignant disease

• Current or history of severe alcohol use or drug/chemical abuse as per investigator's judgement

• Any chronic disorders or severe diseases which, in the opinion of the investigator, might jeopardise participant's safety or compliance with the protocol