Preparing for Your F-18 FDG PET Scan: A Complete Patient Guide

Understanding the F-18 FDG PET Scan and Its Importance

An f-18 fdg pet scan is a highly sensitive imaging technique that uses a radioactive tracer called fluorodeoxyglucose (FDG) to visualize metabolic activity in the body. Because cancer cells, inflammation, and infections often consume glucose at a higher rate than normal tissues, this scan plays a critical role in oncology, cardiology, and neurology. In Hong Kong, where the incidence of lung, colorectal, and liver cancers ranks among the highest in Asia, the f18 fdg pet scan has become an indispensable tool for staging, restaging, and monitoring treatment response. According to the Hong Kong Cancer Registry, over 35,000 new cancer cases were diagnosed in 2022, and a significant proportion of these patients undergo an fdg pet scan at some point during their care. However, the accuracy of the scan heavily depends on how well you prepare. A single missed dietary instruction or an unmanaged blood sugar spike can compromise image quality and lead to misleading results. This guide will walk you through every step of preparation, ensuring your experience is smooth and your results are as diagnostically valuable as possible.

General Preparation Checklist

Informing Your Physician About Medical History, Allergies, and Current Medications

Before scheduling your f-18 fdg pet scan, you must have a thorough discussion with your referring physician and the nuclear medicine team. Provide a complete list of any chronic conditions, including diabetes, kidney disease, or heart problems. Allergies, especially to contrast agents or latex, should be documented. Your current medications, both prescription and over-the-counter, as well as supplements and herbal remedies, must be reviewed. For example, metformin, a common diabetes drug, should often be paused for 48 hours before the scan to avoid interfering with the tracer uptake in the bowel. Similarly, patients on corticosteroids may need dose adjustments because these drugs can suppress normal FDG uptake in lymphoid tissues, creating a false impression of reduced metabolic activity. In Hong Kong, where traditional Chinese medicine is widely used, it is especially important to inform your doctor about any herbal tonics or supplements, as some ingredients (e.g., ginseng or astragalus) can influence glucose metabolism and confound scan interpretation.

Arranging Transportation to and from the Scan Facility

Due to the injection of a radioactive tracer, you will not be able to drive yourself home after the f18 fdg pet scan. The tracer can cause drowsiness or dizziness in some individuals, and the uptake period requires complete rest without distraction. Most nuclear medicine centers in Hong Kong, such as those at Queen Mary Hospital or the Hong Kong Sanatorium & Hospital, advise patients to arrange for a family member or friend to provide transportation. Public transport like the MTR is generally acceptable if you are accompanied, but taxi or private car is preferable. You should also plan for potential delays: the entire process, from check-in to post-scan instructions, can take 3 to 4 hours. If you are coming from the New Territories or outlying islands, allow extra travel time to avoid rushing, as stress can elevate blood sugar and affect scan quality.

What to Bring: Photo ID, Insurance Card, Physician's Order

On the day of the scan, bring your valid photo identification (Hong Kong Identity Card or passport), your insurance card (if applicable), and the original physician's order or referral form. Some facilities may also require a previous imaging report (e.g., CT or MRI) for comparison. It is wise to carry a list of all current medications and a small snack for after the scan, as you will be fasting for several hours. In Hong Kong, private hospitals often mandate upfront deposit payments, so be prepared with a credit card or cheque. Keeping these documents in a single folder reduces check-in anxiety and ensures a seamless workflow.

Dietary Restrictions (24-48 Hours Before)

Fasting Guidelines: Duration and Allowed Liquids

Strict fasting for at least 6 to 8 hours before the injection is mandatory. Many centers recommend an overnight fast of 10 to 12 hours for morning appointments. Only plain, unflavored water is permitted during this period. Black coffee, tea (even without sugar), chewing gum, and hard candies are strictly forbidden because they can stimulate insulin release or provide a source of sugar that competes with the FDG tracer. For afternoon appointments, you may be allowed a light breakfast of plain water only, but this should be confirmed with your specific center. The goal is to achieve a fasting state that keeps your insulin levels low and allows the tracer to be taken up by metabolically active tissues rather than being diverted to muscles or the liver.

Foods and Beverages to Strictly Avoid

For a full 24 to 48 hours before the fdg pet scan, avoid all forms of sugar: table sugar, honey, syrup, pastries, candies, and sweetened beverages (including soft drinks and fruit juices). Caffeine and alcohol can cause fluctuations in blood glucose and interfere with tracer distribution. High-carbohydrate meals, such as rice, noodles, bread, and pasta—staples of many Hong Kong diets—should be eliminated during this window. Even small amounts of hidden sugars in sauces (e.g., soy sauce, ketchup) or processed foods can raise your blood glucose level above the acceptable threshold (typically below 150–200 mg/dL). Patients who inadvertently consume carbohydrates may be asked to reschedule, as elevated glucose directly competes with FDG for cellular uptake, reducing the scan's sensitivity.

Recommended Low-Carb, High-Protein Diet Leading Up to Fasting

In the two days preceding the fast, adopt a low-carbohydrate, high-protein diet to stabilize your blood sugar and deplete glycogen stores. Excellent choices include lean meats (chicken, beef, pork), fish, eggs, tofu, green leafy vegetables, and nuts. Healthy fats like avocado and olive oil are encouraged. In Hong Kong, you might select a steamed fish with vegetables for lunch or a char siu (barbecue pork) without the sweet glaze, paired with a green salad. Avoid starchy vegetables such as potatoes, corn, and peas. This pre-fasting regimen helps ensure that your body is primed for the fast and that your baseline glucose is low. Do not attempt to compensate by eating extra carbohydrates the night before the fast; this will only raise your starting point and make it harder to achieve the desired fasting state.

Medication Management

Review All Medications with Your Doctor and the Scan Center

Every drug you take, from prescription antihypertensives to over-the-counter pain relievers, must be reviewed. Some medications can alter FDG biodistribution. For instance, benzodiazepines and muscle relaxants can reduce brain activity and skew neurological scans. Iron supplements or bismuth-containing antacids may cause artifacts. The scan center should provide you with a medication checklist at least one week prior. In Hong Kong, private centers often send an email or SMS reminder with attached instructions. If you are unsure about a specific supplement—such as glucosamine or vitamin D—call the nuclear medicine department for clarification. Never assume a product is safe simply because it is labeled "natural."

Specific Instructions for Diabetes Medications (Insulin, Oral Agents)

Diabetic patients require special attention. For type 1 diabetics on insulin, the timing of the last dose and meal must be carefully aligned with the scan. Often, a short-acting insulin is given the night before, and the morning dose is withheld until after the scan. Type 2 diabetics on oral agents like metformin are usually instructed to stop the medication 48 hours before the scan to avoid bowel uptake of FDG, which can mask abdominal pathology. Sulfonylureas (e.g., glipizide) may need to be paused for 24 hours to prevent hypoglycemia during fasting. Blood sugar should be monitored at home; if it drops below 70 mg/dL, contact your doctor immediately. The target range is typically between 100 and 150 mg/dL. For patients with poorly controlled diabetes, the scan may need to be rescheduled to ensure reliable results.

Considerations for Steroids and Other Affecting Drugs

Corticosteroids (e.g., prednisone, dexamethasone) can suppress inflammatory cells and reduce FDG uptake in areas of infection or malignancy, leading to false negatives. If you are on long-term steroids for autoimmune disease or post-transplant immunosuppression, your specialist may recommend a temporary dose reduction—but only if medically safe. Other drugs that may interfere include chemotherapy agents (within 3 days of the scan), granulocyte colony-stimulating factors (G-CSF), and certain anticonvulsants. Always disclose the timing of your last chemotherapy cycle, as recent treatment can cause a "flare" uptake that mimics disease progression. The interaction between drug timing and scan interpretation is complex, so full disclosure is non-negotiable.

Do Not Discontinue Medications Without Medical Advice

It cannot be overstated: never stop taking a prescribed medication without explicit guidance from your healthcare team. Abruptly withdrawing beta-blockers, anticoagulants, or antidepressants can have serious consequences. The goal of medication management is not to eliminate all drugs but to optimize the scan window while maintaining your health. For example, antihypertensives should be taken as usual unless directed otherwise, as uncontrolled high blood pressure can cause anxiety and increase glucose levels. When in doubt, consult your referring physician and the nuclear medicine specialist to create a customized plan.

Physical Activity and Dress Code

Avoiding Strenuous Exercise 24 Hours Prior to the Scan

Intense physical activity within 24 hours before the f-18 fdg pet scan can cause increased FDG uptake in muscles, leading to image artifacts and potentially masking lesions. Activities to avoid include running, weight lifting, cycling, swimming, dancing, and even heavy gardening or house cleaning. This is because exercise increases glucose consumption in skeletal muscles, and the tracer will accumulate in those areas, making them appear "hot" on the scan. For neurological scans, you should also avoid vigorous sexual activity or excessive talking, as these can activate specific brain regions. Light walking is acceptable but keep it brief and gentle. In Hong Kong, many patients are tempted to walk up the Mid-Levels escalator or hike on the trails; resist these urges and rest instead.

Wearing Comfortable, Loose-Fitting Clothing Free of Metal

On the day of the scan, wear comfortable clothes without metal zippers, buttons, hooks, or underwire bras. Metal can cause significant attenuation artifacts on the CT component of the PET/CT scanner. Opt for sweatpants or loose trousers, a cotton t-shirt or jumper, and rubber-soled shoes. Remove all jewelry, including rings, earrings, necklaces, and watches. If you have dental braces or surgical clips, inform the technologist. They will note these items and compensate during image reconstruction. Some facilities provide disposable gowns, but wearing your own comfortable clothing can help you relax during the uptake period. In Hong Kong's temperate climate, bring a light jacket as the scanning room is often air-conditioned.

Blood Sugar Control

Why Stable Blood Glucose Is Crucial for Accurate Results

The principle behind the fdg pet scan is that FDG, a glucose analog, is transported into cells via glucose transporters (GLUTs). Cancer cells typically have upregulated GLUTs, leading to increased FDG uptake. If your blood glucose is high, the abundant native glucose competes with FDG for transport into cells, reducing the amount of tracer that reaches the tumor. This can result in a false negative—a dangerous outcome. Conversely, very low glucose can cause increased physiological uptake in the brain and heart, obscuring nearby lesions. A stable, fasting glucose level between 70 and 150 mg/dL (approximately 3.9 to 8.3 mmol/L) is the global standard. In Hong Kong, the Hospital Authority recommends that fasting glucose be below 140 mg/dL (7.8 mmol/L) for optimal PET imaging.

Target Blood Sugar Range for the Scan

The ideal range is generally 80–120 mg/dL (4.4–6.7 mmol/L). If your pre-scan glucose exceeds 200 mg/dL (11.1 mmol/L), the scan will likely be postponed. For diabetic patients, the target may be adjusted slightly higher (up to 150 mg/dL) to avoid hypoglycemia. Each imaging center has a specific cutoff; be sure to ask yours. For instance, a study at Queen Elizabeth Hospital showed that patients with blood glucose above 180 mg/dL had a 30% higher false-negative rate for liver metastases. Checking your glucose at home before leaving for the appointment can save you a wasted trip. If the reading is borderline high, you may be allowed to walk gently to utilize some glucose, but never take extra insulin without medical oversight.

Tips for Managing Blood Sugar, Especially for Diabetic Patients

Begin the low-carb diet 48 hours before the scan and monitor your glucose at least three times daily. On the day of the scan, do not take your morning insulin or oral hypoglycemic agents unless specifically directed. Plan your scan early in the morning to minimize the fasting window. Stay hydrated with water; dehydration can cause glucose levels to rise. If you feel symptomatic (shaky, sweaty, confused) due to low blood sugar, carry a small emergency supply of glucose tablets or juice, but only consume them under supervision. Inform the technologist immediately upon arrival if you experience any symptoms. The nuclear medicine staff is trained to handle diabetic emergencies and may have protocols for administering small amounts of glucose if necessary.

On the Day of the Scan

Arrival and Check-In Procedures

Arrive at the imaging center 30 minutes before your scheduled appointment. After registering, a technologist will verify your identity, confirm your fasting status, and measure your blood glucose using a finger-stick test. You will be asked to empty your bladder because a full bladder can interfere with pelvic imaging. Then, you will change into a hospital gown if needed. All metal objects must be removed. The technologist will explain the procedure and answer any last-minute questions. In Hong Kong, many centers offer multilingual staff (Cantonese, Mandarin, English), so don't hesitate to ask for clarification. If your blood glucose is within range, you will proceed to the injection room.

Tracer Injection and Quiet Uptake Period

A small IV cannula will be inserted into a vein in your arm or hand, and the f18 fdg pet scan tracer (F-18 FDG) will be injected slowly. You may feel a cold sensation as the tracer enters. After injection, a strict "quiet uptake period" of 45–90 minutes begins. During this time, you must remain still in a reclined chair or bed, avoiding all physical and mental stimulation: no reading, no phone use, no talking, no chewing gum, and no sleeping if possible (for neurological scans). The goal is to allow the tracer to distribute evenly and be taken up by tissues at rest, minimizing background noise. You will be covered with a warm blanket, as shivering can increase muscle uptake. The room will be dimly lit and quiet. Many patients find this period relaxing, but some become restless. Try breathing exercises or gentle meditation.

The Scanning Process: What to Expect in the Scanner

After the uptake period, you will be taken to the scanning suite. You will lie on a narrow, padded table that moves through a large, doughnut-shaped scanner (PET/CT). The scan itself takes 20–40 minutes. You must remain perfectly still; movement even by a few millimeters can blur the images. The machine may make whirring and clicking sounds. You will be able to communicate with the technologist via an intercom. If you feel claustrophobic or anxious, inform them—they can offer a mild sedative in some cases. The CT portion takes only a few seconds, but the PET acquisition is longer. For a whole-body scan, the table slides through the scanner in segments. You may be asked to hold your breath at intervals to reduce diaphragm motion. The entire procedure is painless, though if you have arthritis or back pain, a small pillow under your knees can provide comfort.

After Your Scan

Post-Scan Hydration and Returning to Normal Activities

Immediately after the scan, you are encouraged to drink plenty of water to help flush the tracer out of your system. The radioactive dose is very low (equivalent to a few years of natural background radiation), and it decays completely within 6–12 hours. You can resume your normal diet and medications as instructed by your doctor. If you paused metformin or insulin, follow the resumption schedule carefully. Most people feel fine and can return to work or daily activities. However, you should avoid close, prolonged contact (more than 1 hour at arm's length) with pregnant women and infants for the remainder of the day, as a precaution. Public transportation is safe, but you may want to minimize time in crowded spaces. In Hong Kong, where density is high, this is practical: simply sit at a distance or wear a mask.

When and How to Expect Your Results

The images must be reconstructed and interpreted by a nuclear medicine radiologist. This process typically takes 24 to 72 hours. In some private hospitals, you may receive preliminary verbal results within a day. A final written report will be sent to your referring physician, who will discuss the findings with you during a follow-up appointment. Do not ask the technologist or nurse for results at the time of scan; they are not authorized to give them. If you have not heard back within a week, contact your doctor's office. The report will include a description of all areas of abnormal FDG uptake, measured as Standardized Uptake Value (SUVmax). Your doctor will explain what these values mean in the context of your clinical history.

Frequently Asked Questions (FAQs)

Can I brush my teeth on the morning of the scan?

Yes, you can brush your teeth with water, but do not swallow any toothpaste. Rinse thoroughly with water. Avoid mouthwash that contains alcohol.

What if I accidentally ate a piece of candy or bread before the fast?

Call your imaging center immediately. Depending on the time of intake, your scan may need to be rescheduled. It is better to be honest than to proceed with a compromised scan.

Is the radioactive tracer dangerous?

No. The dose is extremely low and has no known short-term or long-term side effects. The benefits of accurate diagnosis far outweigh the minuscule radiation risk. For context, one f-18 fdg pet scan delivers about 7–10 mSv of radiation, similar to the radiation received from natural background over 2–3 years in Hong Kong.

Can I breastfeed after the scan?

If you are breastfeeding, stop for 4–6 hours after the injection and pump and discard the milk. The tracer can pass into breast milk in trace amounts.

What about claustrophobia?

Let the staff know in advance. Some centers offer open-type PET scanners or mild anxiolytics. The uptake period is spent out of the machine, so the scanning time is relatively short.

Emphasizing Adherence to Instructions for the Best Scan Outcome

Your f-18 fdg pet scan is a sophisticated tool that can guide critical treatment decisions. Every preparation step—from diet and medication management to blood sugar control and physical rest—serves to maximize the diagnostic accuracy of the f18 fdg pet scan. A minor slip, such as drinking a cup of tea or going for a morning jog, can undermine the entire investment of time and medical resources. By following this comprehensive guide, you are not only ensuring the best possible image quality but also respecting the time and expertise of your healthcare team. Remember, the key to a successful fdg pet scan lies in the details. Stay disciplined, stay informed, and ask questions whenever you are uncertain. Your health journey deserves nothing less than the most reliable results.