Does Eating Between PET/CT Scans Affect Accuracy of Results?

Positron Emission Tomography/Computed Tomography (PET/CT) scan, is fasting required before the examination?

Hello, Director Zheng.
I have undergone several PET scans due to my breast cancer.
I would like to inquire about the fasting requirement of 6 hours prior to the scan, which is not an issue for me.
However, I have questions regarding the interval between the first and second scans.
Will eating or not eating during the break between the first and second scans affect the accuracy of the report? If the physician says one scan is sufficient, then there is no issue regarding eating.
However, I have experienced a second scan, and during the break, I found varying information from several hospitals regarding their PET scan QA or procedural guidelines—some mentioned eating or drinking, while others did not.
Therefore, I would like to know if fasting before the scan is necessary, and if there is a significant impact on the results if I eat during the break between the first and second scans.
Or is it sufficient that the physician feels it is okay for me to go home after the second scan? My concern is that even though the physician said I could go home after the second scan, I am unsure if eating or not would affect the accuracy of the report.
Thank you for your assistance, Director Zheng.

Xia Xiaojie, 60~69 year old female. Ask Date: 2022/11/09

Dr. Zheng Xiaoyi reply Nuclear Medicine

Hello, Ms.
Hsia.
I apologize for the delayed response due to the busy clinical schedule.
The radiopharmaceutical used in positron emission tomography (PET) imaging, FDG, is a compound similar to glucose.
If blood sugar levels are too high, it can compete with the radiotracer, resulting in reduced image quality and affecting the physician's interpretation.
Therefore, fasting is required before the examination to avoid hyperglycemia, which can degrade image quality.
Typically, blood sugar levels peak about one hour after eating, which coincides with the interval between the first and second imaging sessions.
If a patient eats immediately after the first scan, they may experience elevated blood sugar levels during the second scan.
Theoretically, this could impact the quality of the second imaging session, but I have not found substantial literature discussing this issue.
Based on my experience, not fasting before the first scan has a more significant impact on image quality, while the effect before the second scan is relatively minor.
Additionally, sometimes we prefer patients to eat to distend the stomach, which helps in assessing for any gastric lesions.
I apologize for not being able to provide a more definitive answer.
I recommend discussing your situation further with your specialist.
Wishing you good health and all the best.

Reply Date: 2022/11/09

More Info

When it comes to PET/CT scans, particularly in the context of cancer diagnosis and monitoring, the preparation and conditions surrounding the scans can significantly influence the accuracy of the results. Your question about whether eating between scans affects the accuracy of the results is quite pertinent, especially for patients undergoing multiple scans for conditions like breast cancer.

Firstly, it is essential to understand the purpose of fasting before a PET scan. The primary reason for fasting for at least six hours before the scan is to minimize the uptake of glucose by normal tissues. PET scans utilize a radioactive glucose analog (usually F-18 fluorodeoxyglucose, or FDG) to visualize metabolic activity in tissues. Cancer cells typically have a higher metabolic rate and thus absorb more of this tracer than normal cells. If a patient eats before the scan, especially foods high in carbohydrates, it can lead to increased glucose levels in the bloodstream, which may result in non-specific uptake of the tracer in normal tissues. This can obscure the images and potentially lead to false positives or misinterpretation of the results.

Regarding your specific concern about eating between the first and second scans, it is generally advisable to maintain the fasting state throughout the entire imaging process, especially if the scans are done in close succession. If the scans are separated by a significant amount of time, and the physician has not provided specific instructions regarding eating or drinking, it is best to err on the side of caution. Consuming food or beverages (other than water) during the interval could potentially alter the distribution of the tracer in your body, leading to variability in the results.

In practice, many facilities may have different protocols regarding hydration and minimal intake of clear liquids between scans. Some may allow water, while others may recommend complete fasting. It is crucial to follow the specific instructions given by your healthcare provider or the imaging center. If there is any uncertainty, it is always a good idea to ask the medical staff for clarification before the procedure.

In summary, while the initial fasting before the PET scan is critical for accurate results, maintaining a fasting state between scans is also advisable to ensure that the results are not compromised. If you have any concerns about your specific situation, discussing them with your healthcare provider can provide personalized guidance tailored to your health needs and the protocols of the imaging facility. This will help ensure that the scans yield the most accurate and useful information for your ongoing care and treatment.

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