A cardiac PET scan is a diagnostic examination that involves acquiring images of the heart based on the detection of radiation from the emission of positrons. A PET scan has been ordered by your physician to help with your plan of care. To ensure the most accurate results from your cardiac PET scan, careful preparation, including specific dietary instructions, is essential. This article provides a detailed guide on how to prepare for a cardiac PET scan, focusing on dietary guidelines and other important considerations.
Understanding Cardiac PET Scans
Positron emission tomography (PET), also called PET imaging or a PET scan, is a diagnostic examination that involves getting images of the body based on the detection of radiation from the emission of positrons. During the test, your healthcare provider injects a small amount of radioactive tracer into an IV in your vein. The tracer isn’t a dye or contrast. Depending on the tracer, it can stay in your body for a couple of minutes or a few hours. A special camera, called a PET scanner, detects the radiation energy the tracer releases. The PET scanner creates 3D computer images of your heart based on the energy it detects.
Pre-Scan Dietary Instructions
How do I prepare for a cardiac PET scan? Your healthcare provider will give you specific instructions. Be sure to follow them for the most accurate results. In general, you can expect the following to prepare for your heart PET scan:
Fasting and Hydration
Do not eat or drink anything, except water, for 6 hours before the exam. Don’t eat or drink anything except water for four hours before the test. It is important that you stay well hydrated.
Caffeine Restriction
Don’t have anything that contains caffeine for 24 hours before the test. This includes coffee, tea, cola and other sodas, chocolate and strawberries (these contain a small amount of caffeine). It also includes decaffeinated and caffeine-free products (these contain small amounts of caffeine). Caffeine affects test results.
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High-Fat, Low-Carbohydrate Diet
Eat foods high in fat and low in carbohydrates for 24 to 48 hours before your heart PET scan if your provider is checking for cardiac sarcoidosis or infection. These are rare tests. People who get a cardiac PET scan for other reasons won’t need to follow this diet. Dietary carbohydrate intake normally triggers insulin secretion, which activates the predominantly expressed glucose transporter GLUT4 in normal myocardium and allows glucose to enter cells.
The Rationale Behind Dietary Preparation
Because normal myocardium has a variable avidity for glucose, and hence 18F-FDG, preparation to promote free fatty acid metabolism and suppress physiologic glucose metabolism is necessary for successful PET 18F-FDG imaging of pathologic processes in the heart. Dietary carbohydrate intake normally triggers insulin secretion, which activates the predominantly expressed glucose transporter GLUT4 in normal myocardium and allows glucose to enter cells.
Specific Dietary Recommendations
Current SNMMI/ASNC/SCCT guidelines recommend preparation with a diet rich in fat and devoid of carbohydrates for 12-24 hours prior to the scan, a 12-18 hour fast, and/or the use of intravenous heparin approximately 15 min prior to 18F-FDG injection. The reviewed data suggest that a HFNC diet should be implemented for one or two meals with a subsequent fast of at least 4 hours, although an even longer fast may be preferred for optimal suppression of myocardial glucose uptake.
Importance of Myocardial Suppression
Because normal myocardium has a variable avidity for glucose, and hence 18F-FDG, preparation to promote free fatty acid metabolism and suppress physiologic glucose metabolism is necessary for successful PET 18F-FDG imaging of pathologic processes in the heart. Dietary carbohydrate intake normally triggers insulin secretion, which activates the predominantly expressed glucose transporter GLUT4 in normal myocardium and allows glucose to enter cells.
Meta-Analysis Findings
A recent meta-analysis of 16 studies and 559 patients referred for cardiac 18F-FDG PET for suspected or known cardiac sarcoidosis by Tang et al. sought to examine how different patient preparations may impact the sensitivity and specificity of 18F-FDG PET for the diagnosis of cardiac sarcoidosis. This study and its findings are limited by the fact that there is no reliable reference standard on which to base these diagnostic accuracy parameters for cardiac sarcoidosis. Tang et al. used the Japanese Ministry of Health and Welfare Criteria in their analysis; however, many prior studies have shown that imaging is more sensitive and specific than these clinical criteria. Furthermore, because most studies were small and preparation strategies were heterogeneous, there was insufficient power to determine if one specific technique results in superior diagnostic accuracy.
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Examples of Dietary Preparation Protocols
Several methods of patient preparation have been described and evaluated in the literature. One well-studied strategy is to utilize a high-fat and low- or no-carbohydrate (HFLC or HFNC) diet in preparation for myocardial 18F-FDG PET imaging. In these studies, patients are instructed to eat at least one fatty meal prior to the exam with minimal or no carbohydrates with a subsequent fast prior to 18F-FDG administration. Alternatively, other studies have described the use of fasting as a means of shifting cardiac metabolism toward free fatty acids and away from glucose prior to 18F-FDG administration. This strategy also reduces insulin release and promotes systemic lipolysis, both of which suppress myocardial glucose utilization. Pharmacologic strategies have also been employed with the aim of manipulating myocardial metabolism. Most studies have combined two or more of these strategies to achieve optimal myocardial suppression.
Outcomes of Various Dietary Strategies
Three randomized studies have been performed. Cheng et al. compared preparation in groups of 21 patients with an unrestricted diet, a HFLC meal and subsequent fast of approximately 12 hours, and a HFLC meal with a high-fat drink 1 hour prior to the exam with findings of significantly improved suppression with a HFLC diet and fast over the other strategies (57% vs 81% vs 62% success). Demeure et al. compared the effectiveness of four different approaches: a HFLC meal followed by a 12 hour fast, HFLC meal followed by a high-fat drink or olive oil 1 hour prior to imaging, and a HFLC meal followed by 12 hour fast and oral verapamil. Their study included groups of nine patients. It found improved suppression in the HFLC meal followed by 12 hour fast group over the HFLC meal together with a drink or olive oil group (89% vs 50%) and similar suppression with the addition of verapamil to the HFLC and 12 hour fast preparation (89% vs 89%).
In one study, 60 patients underwent preparation with two HFLC meals and a 4 hour fast, and 50 patients were prepared with a greater than 12 hour fast with significantly improved suppression in the HFLC group (90% vs 54% success). Manabe et al. assessed a brief fast of more than 6 hours followed by intravenous heparin prior to imaging in comparison to a LC meal with an 18 hour fast and heparin administration and found improved efficacy in the latter (72% vs 100%). Kobayashi et al. demonstrated that a LC diet bolstered by a high-fat drink led to complete suppression in 71% of patients as compared to 29% of patients prepared with a greater than 6 hour fast alone. In this study, patients prepared with a diet who had focal uptake were frequently noted to have uptake in the papillary muscles. A 24 hour dietary preparation with a fat-allowed, low-carbohydrate diet was shown to be superior to unrestricted preparation with 68% vs 15% suppression. Likewise, a 24 hour dietary preparation with <3 g of carbohydrates and >8 hour fast (98%) led to improved myocardial suppression over a fast alone (67%) in oncologic patients. A long-term fast of approximately 18 hours was compared to a 12 hour fast with adjunctive heparin administration in subgroups of 18 and 19 healthy patients, respectively, with a higher rate of suppression in the group with a longer duration of fasting (67% vs 12%).
Common Patterns of Non-Specific Uptake
Common patterns of non-specific 18F-FDG uptake involved the basal ring and lateral wall. A recent study reviewed 150 cardiac 18F-FDG PET images at a single institution with three groups of 50 patients prepared with either a 6 hour fast, a low carbohydrate diet with a 12 hour fast, or a low carbohydrate diet followed by a 12 hour fast and heparin bolus had successful suppression in 28%, 54%, and 88% of those in the respective groups. Significantly improved suppression was noted in the final group as compared to fasting alone.
Standardized Protocols and Suppression Rates
Blankstein et al. employed a preparation consisting of HFNC dinner and breakfast followed by a fast of at least 4 hours in a population of 118 patients undergoing evaluation for sarcoidosis with successful suppression in 103 patients (87%). In a study utilizing cardiac 18F-FDG PET imaging to detect metabolically active coronary plaques, a protocol of a HFLC meal followed by a high-fat drink was effective in 20 of 32 patients (63%). In another publication, 30 control patients were prepared with a HFLC diet for two meals with a 4 hour fast preceding their imaging with adequate suppression seen in 28 patients (93%), though focal papillary muscle uptake was noted in five of the subjects with adequate suppression.
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Medication Guidelines
Routine medications may be taken, unless you have been instructed otherwise. Any non-diabetic medications may be taken the day of your scan.
Diabetic Medications
If you are diabetic, do not take diabetic medications for 12 hours before your scan. Take your usual medications except diabetic medications, cough drops, or gum. If you take insulin, ask your provider how much you should take on the day of your cardiac PET scan. You may need to take 50% of your usual morning dose and eat a light meal four hours before the test. If you take pills to manage your blood sugar, your provider will likely tell you not to take your medication before the test. Bring your diabetes medication with you so you can take it after you finish the test. Don’t take your diabetes medication and skip a meal before your heart PET scan. If you own a glucose monitor, bring it with you to check your blood sugar levels before and after your test. If you think your blood sugar is low, tell your provider right away. Plan to eat and take your medication after your heart PET scan.
Heart Medications
If you take heart medications: Ask your provider about changes you need to make to your medications before your cardiac PET scan. Don’t stop taking any medication unless your provider tells you to. You should only take these medications on the day of the test if you have chest pain.
Asthma Medications
If you have asthma: Your provider will likely tell you not to take theophyllinefor 48 hours before your heart PET scan. Bring your asthma inhaler with you to the test.
Medications with Caffeine
Medications with caffeine: Don’t take any over-the-counter medication that contains caffeine (such as pain relievers and diet pills) for 24 hours before your cardiac PET scan. Talk to your provider if you have questions about other medications that may contain caffeine.
General Medication List
Bring a list of all medications and supplements you take, including over-the-counter products you buy without a prescription.
Day of the Scan
What to Wear
If possible, do not wear clothing with belts, zippers, snaps or metal of any kind to your appointment. Try to stay warm on the day of your scan.
Blood Glucose Level
The team will test your blood glucose level. We are looking for your blood glucose level to be between 70mg/dL and 200mg/dL.
During the Test
After replacing your shirt with a hospital gown and having a blood pressure check, you’ll:
- Have an electrocardiogram (EKG) before and during the test to keep track of your heart’s electrical activity. Painless, sticky patches on your body collect this information.
- Get an IV in your arm or hand so you can receive medication and a radioactive tracer during the test.
- Lie still on the exam table with a camera above you in the middle of a large, donut-shaped machine.
- Receive a small amount of radioactive tracer through your IV so your provider can scan to check your blood flow while you rest.
- Receive medication through your IV to make your heart react as if you were exercising. You may feel warm or cold and may have a mild headache.
- Get another small amount of tracer so your provider can scan to check your blood flow while you’re “active.” Your provider will compare the two scans (resting and active).
Depending on the disease your provider is checking for, the steps above may differ. Your healthcare team will ask you how you’re feeling several times during the test. Tell them if you feel pain or discomfort in your chest, arm or jaw, short of breath, dizzy, or lightheaded. After the injection you may be given one cup of an oral barium contrast to drink. You will rest quietly in a reclining chair for 75 minutes.
Scan Duration
Your appointment will take about 2 hours total (scan time: approx. How long does a cardiac PET scan take? The appointment will take about one to three hours. However, the machine may only scan you for a total of 30 minutes.
Post-Scan Instructions
After the Test
After your test, you will be considered “radioactive” due to the injection you receive. A provider will remove the IV from your arm or hand at the end of the test. You can change out of your hospital gown and go home. Someone will need to drive you home if you had a sedative. Be sure to drink a lot of water over the next 24 hours so you can flush the tracer out of your system.
Potential Side Effects
Side effects may include some temporary redness and pain on the skin where you had your IV. An allergic reaction to the radiotracers’ radiation is very rare and the amount of radiation exposure is low.
When to Seek Medical Attention
If you feel any symptoms such as nasal congestion, itchy eyes, hives, rashes, sneezing, restlessness, tremors, pain, nausea, vomiting, dizziness, please notify the staff immediately. The staff is prepared to handle these situations. If you feel any of these symptoms after you have left the department, please contact your doctor or go to the nearest emergency room.
Special Considerations
Pregnancy and Nursing
A cardiac PET scan can harm a fetus or infant, so let your provider know if you’re pregnant or nursing.
Claustrophobia
People who feel anxious in small spaces may be uncomfortable during the scan.
Optimizing Myocardial Suppression
An ideal preparation strategy for cardiac 18F-FDG PET imaging would facilitate complete suppression of physiologic 18F-FDG uptake by normal myocardium. Nevertheless, the included studies did not always report complete suppression. In fact, multiple studies determined adequacy of suppression by semi-quantitative grading comparing cardiac to hepatic uptake or by quantifying the intensity of myocardial uptake in relation to a threshold value. In the current review, each individual study had a unique definition for adequacy, which rarely required complete suppression of 18F-FDG uptake (i.e., the optimal definition of adequate).
Recommendations for Optimal Preparation
Based on the reviewed literature, we advocate for the use of at least two HFNC meals followed by a fast of at least four hours as the most effective preparation for cardiac 18F-FDG PET imaging of inflammation. Intravenous heparin, when administered just prior to imaging following at least one HFLC meal and overnight fast, appears to be part of a regimen that is as efficacious as two HFNC meals with a fast but requires exposure to an intravenous medication with possible adverse reactions. We do not recommend preparation plans that utilize fasting alone; however, when such an option has to be employed, i.e., a patient who cannot eat or has dietary restrictions precluding the recommend diet, fasting for at least 18 hours is suggested. We also do not advocate the use of calcium channel blockers or unrestricted diets. Likewise, patients should be instructed to avoid eating or drinking anything within at least 4 hours of their test, and a high-fat supplement given during this interval immediately prior to the exam does not appear to have additive benefit. Exercise should be avoided for 24 hours prior to the exam. Special caution should be taken in inpatient studies. It is critical to avoid the use of intravenous medications that contain dextrose within 12 hours of imaging. Likewise, there must be communication between nurses, physicians, nutritionists, kitchen staff, and the patient to avoid inadvertent errors in preparation.
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