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Why must patients fast in preparation for CT scans?

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Patients scheduled for a CT scan, or computed tomography scan, fast to avoid having food in their digestive system; this helps doctors see scan images more clearly and make accurate diagnoses, as stated by Johns Hopkins Medicine. Doctors order two types of CT scans: those with contrast dyes and those without. Although requiring fasting prior to the scan, some patients receiving CT scans of their digestive organs drink beverages before their scans; these beverages contain contrast agents, which illuminate organs during the scanning process, according to the University of Arkansas for Medical Sciences.

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CT scans produce images of various parts of the body, including the digestive system, extremities and head. Typically, only patients scheduled for scans of the digestive system fast beforehand, according to Johns Hopkins Medicine. Doctors encourage patients to not consume solid foods at least two hours before their procedures, but encourage consumption of certain fluids and beverages. Being hydrated expands the organs, improving their visibility on CT scans. Consuming a barium fluid also improves CT scan quality and accuracy by coating and illuminating the digestive organs, as stated by Massachusetts General Hospital.

CT procedures offer prescribing physicians valuable insights to patients' conditions, but present some risks too, according to Johns Hopkins Medicine. These scans use radiation for producing images, which may pose threats to pregnant women. Before the scan, technicians ask patients about allergies, as some allergies give patients a higher risk of having adverse reactions to contrast dyes, as stated by the University of Arkansas for Medical Sciences.

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