How to Prepare for Your Scan: Fasting, Bladder and What to Bring

It depends on the scan. Most MRIs need no preparation at all. Abdominal ultrasound usually means around six hours without food. Pelvic ultrasound usually means a full bladder. CT with contrast often means a short fast and sometimes a blood test. X-rays and DEXA generally need nothing.

This is general information, not medical advice. If you have symptoms that are worrying you, speak to your GP or call 111.

The rule that beats every checklist

Your clinic's own instructions win. Every time.

Preparation varies between providers because their scanners, protocols and radiologists vary. One centre asks for four hours without food before an abdominal ultrasound, another asks for six. One asks you to arrive with a full bladder, another gives you water on arrival. Neither is wrong, and the one that applies to you is the one on your appointment letter or in your confirmation email.

Use the checklist below to know what to expect and what to ask about. Use your letter to know what to do.

Preparation by scan type

Why fasting is asked for at all

It is not a formality. For an abdominal ultrasound, food makes the gallbladder empty and contract, which hides exactly the thing the sonographer is often looking for, and digestion fills the bowel with gas, which blocks the sound waves. A gassy abdomen genuinely produces an inferior scan.

Most UK clinics ask for around six hours, often meaning no breakfast for a morning appointment or a light early breakfast and nothing after for an afternoon one. Water is usually allowed and often encouraged. Chewing gum and smoking make you swallow air, so both are typically discouraged.

Our guide to abdominal ultrasound explains what the scan can and cannot show, which is worth reading before you book one.

For CT with contrast, the short fast is about reducing nausea, not about the pictures.

Why a full bladder is asked for

For a transabdominal pelvic ultrasound, a full bladder does two useful things: it pushes gas-filled bowel out of the way, and it gives the sound waves a clear fluid window through to the uterus, ovaries or prostate behind it.

A common instruction is to drink around a litre of water an hour before and not to go to the toilet after that. Aim for comfortably full. Painfully full does not improve the image and makes lying still harder. If you genuinely cannot hold it, tell the receptionist rather than the scanner room, because they can usually rearrange the order of the appointment.

Note the reversal for transvaginal ultrasound, where an empty bladder is usually wanted. If you are booked for both, the clinic will normally do the transabdominal part first and ask you to empty afterwards.

Contrast: what changes

Contrast is a dye that makes certain tissues and blood vessels stand out. MRI uses gadolinium-based agents; CT uses iodine-based ones. Around a quarter of private scans of the abdomen and many cancer-staging scans use it, and it typically adds around £125 to an MRI at UK list prices checked in September 2026.

Practically, contrast means: a cannula in your arm, sometimes a blood test for kidney function beforehand, a warm flushing sensation as it goes in, and usually a short wait afterwards before you leave.

Tell the clinic in advance about kidney problems, previous reactions to contrast, asthma, and any diabetes medication. The Royal College of Radiologists publishes guidance on gadolinium-based contrast agent administration that UK centres work to, and screening kidney function in the right patients is part of it. Whether you need contrast at all is a decision for the referrer and the radiologist, not something to choose from a menu.

What to bring to any scan appointment

• Photo ID and your booking confirmation or referral letter
• A list of your current medicines and doses
• Your implant card, if you have any implanted device
• Previous scan reports and images, on disc or via a link, if you have had related imaging before
• The payment card you booked with, if you are self-paying and anything is outstanding

Previous imaging is the one people leave behind and the one radiologists most want. Comparison with an older scan often changes the report, and it is far easier to arrange before the appointment than after.

What to wear

Soft clothes with no metal: no zips, studs, hooks, underwiring or metallic thread. This matters most for MRI, where you may be given a gown regardless, and for X-ray and CT of the area concerned.

Leave jewellery and watches at home rather than removing them in a cubicle. For MRI specifically, our guide to what to expect from an MRI scan covers the full list, including the safety questionnaire you will complete before you go in.

Medication, diabetes and blood thinners

Do not stop anything on your own initiative because of a scan.

Most medicines can be taken as normal, including on a fasting morning with a sip of water. The ones worth a phone call in advance are insulin and other diabetes medicines, where a fasting instruction may need the timing adjusted, and anticoagulants, where a biopsy or injection is planned as part of the appointment.

If you wear a medication patch, a glucose monitor or an insulin pump, say so when you book rather than on the day.

If you are pregnant or might be

Tell the clinic before the appointment, not in the scanner room. This applies to every type of scan, including MRI, and it applies if you are unsure.

Nobody on a website can tell you whether a particular scan is appropriate in pregnancy. That decision sits with the referring clinician and the radiologist or radiographer, weighing what the scan would show against what is known about the exposure. What you can do is give them the information early enough to make it properly.

Frequently asked questions

How long do I need to fast before an abdominal ultrasound? Most UK clinics ask for around six hours without food, and some allow small sips of water. The reason is practical: food makes the gallbladder contract and fills the bowel with gas, and both make the pictures worse. Follow the exact instruction on your appointment letter.

Do I need to fast before an MRI? Usually not. Most MRI scans need no food or drink preparation at all. The exceptions are scans of the abdomen, the small bowel and the bile ducts, and some scans given with contrast, where clinics commonly ask for a few hours without food. Your letter will say.

How full does my bladder need to be for a pelvic ultrasound? Comfortably full, not painfully. A common instruction is to finish about a litre of water an hour before the appointment and not empty your bladder after that. A full bladder pushes the bowel out of the way and gives the sonographer a clear window onto the uterus, ovaries or prostate.

What should I bring to a scan appointment? Photo ID, your referral or booking confirmation, a list of your medicines, your implant card if you have one, and any previous scan reports or images. If you are self-paying, bring the payment method you booked with. Wear clothes with no metal.

Can I take my usual medication before a scan? In most cases yes, including on a fasting day with a small sip of water. Do not stop anything on your own. Ring the clinic in advance if you take insulin or other diabetes medication, blood thinners, or anything that has to be taken with food, because the timing may need adjusting.

If you have not booked yet, you can compare scans and clinics across the UK and check each centre's own preparation instructions before you commit to a date.