Two people can both be having an ultrasound or CT and receive different instructions about eating. That does not necessarily mean one centre is wrong. The exact examination and the question it needs to answer may be different.
Why an ultrasound may require fasting
Food can make the gallbladder contract and can increase bowel gas. For some upper abdominal ultrasounds, fasting may help the sonographer assess the gallbladder, liver, pancreas, or nearby structures.
An ultrasound of the thyroid, breast, muscles, or many other body parts usually does not need an empty stomach. Pelvic ultrasound preparation is more often about bladder filling than food.
Why CT instructions vary
Many CT examinations do not require fasting. A centre may still ask you not to eat for a period when certain oral contrast is planned, when the scan involves the abdomen, or when local protocols call for it.
Routine intravenous contrast alone does not always require fasting. Sedation or anaesthesia is different and comes with its own safety instructions.
Medicines and health conditions matter
Do not stop regular medicine unless the referring clinician or imaging centre tells you to. If a medicine is normally taken with food, ask how to manage it during fasting.
Tell the centre if you have diabetes, are pregnant, have fluid restrictions, or become unwell when you miss meals. Children, older adults, and people using insulin may need an individual plan.
If you ate after being told to fast, tell the centre. Do not hide it or assume the scan is automatically cancelled. The team may continue, adjust the plan, delay the examination, or rearrange it depending on why fasting was required.
Confirm the whole instruction
Ask whether you may eat, drink water, and take your regular medicines. Also confirm when the fasting period begins. "Nothing by mouth" and "no solid food" do not mean exactly the same thing.
The bottom line
Fasting is tied to the exact scan, not simply to the words CT or ultrasound. Follow the instruction from the centre performing the examination and ask before making assumptions.