What is an MRI scan? What metal to remove, implants and claustrophobia, and how long it takes in Hong Kong

What is an MRI scan, and how is it different from CT?

The Cancer Online Resource Hub describes MRI as a safe, accurate imaging test with no radiation or pain: it uses a strong magnetic field, radio waves and sophisticated computer technology to show the body's internal tissues clearly, helping diagnose disease in different parts of the body (accessed October 2026).

St. Paul's Hospital compares the two: CT uses X-rays, while MRI involves no ionising radiation; CT is better at quickly showing bone, the lungs and acute bleeding, while MRI stands out in distinguishing soft tissue, and the hospital gives stroke, tumours and torn tendons and ligaments as examples (accessed October 2026).

For CT see CT scan, and for the English and Chinese names see MRI in Chinese and English.

How do I prepare for an MRI scan? Do I need to fast?

The Cancer Online Resource Hub lists (accessed October 2026): unless the appointment notice says otherwise, you can usually eat and drink as normal; unless told otherwise, keep taking medicines prescribed by your doctor; change into a patient gown and remove all metal items such as glasses and watches, do not take in items affected by the magnetic field such as credit cards and stored-value travel cards, and keep clothes and belongings in the patient locker; avoid eye make-up.

St. Paul's Hospital says most MRI scans (for example of the head, spine or joints) need no fasting, and states that make-up and all jewellery must be removed before entering the scan room (accessed October 2026). Whether other areas need fasting follows the instructions of the hospital where you book.

Can I have an MRI with metal or an implant in my body?

The Cancer Online Resource Hub says the conditions in the table may endanger the patient or interfere with the images, and you should tell the radiology staff before the scan if any applies (accessed October 2026); the page also warns that an incompatible metal device in the body may move, heat up and cause burns, malfunction, or be pulled onto the MRI machine.

What do hospitals say about implants?

St. Paul's Hospital states (accessed October 2026): some newer pacemakers are "MRI-compatible", but a cardiologist must adjust the settings before the scan and it is done under close monitoring; cochlear implants and clips on blood vessels in the brain need their model checked against MRI safety standards; people who do metal grinding work, or who have had metal fragments enter the eye or body, need an X-ray first to confirm safety.

Union Hospital says anyone with a metal implant must tell staff before the scan, and the hospital decides from the implant's MRI safety labelling (MRI Conditional / MRI Safe) whether you can be scanned and which machine and settings to use; fixed dentures generally do not matter, but some removable dentures or metal items may interfere with the images and it recommends removing them before the scan (accessed October 2026).

What happens during the scan, and how long does it take?

The Cancer Online Resource Hub says that, depending on what is needed, MRI takes about thirty minutes to an hour; you need to cooperate and keep still throughout, and may need a sedative prescribed by the treating doctor, taken by mouth or injected, or a general anaesthetic; the machine is loud at times during the scan, and earplugs or headphones are provided; the scan room has an intercom and staff give you an emergency button to hold, which you can press to reach the control room; depending on the case, you may need an intravenous gadolinium-based contrast agent (accessed October 2026).

Hospitals describe the time differently: St. Paul's Hospital lists 30 minutes to 2 hours; Union Hospital lists about 30 to 60 minutes, longer if contrast is injected (both accessed October 2026).

The Cancer Online Resource Hub says the scan generally causes no discomfort and you can return to normal activities and work afterwards; a radiologist reviews the images and writes a report for the treating doctor (accessed October 2026). For when reports are usually ready see when will my test report be ready.

Can I have an MRI if I have claustrophobia? How loud is it?

The Cancer Online Resource Hub lists claustrophobia among the conditions to tell the radiology staff about before the scan, and lists anxiety, excessive fear or panic attacks caused by the enclosed space as possible during the scan. The page also notes that the noise during the scan (such as knocking and beeping) cannot be avoided and can be as loud as a hundred and thirty decibels or more; if nerves are stimulated, you may briefly feel numbness, muscle twitching, warmth or pain (accessed October 2026).

Union Hospital says most patients can complete the scan with staff help: you can reach staff through the intercom or a call bell during the scan, a family member can accompany you if needed, or a safe dose of sedative can be used after a doctor's assessment (accessed October 2026). If you have claustrophobia, tell the radiology staff before the scan and follow the hospital's arrangements.

Will I need contrast? Can I have an MRI if I am pregnant?

As for gadolinium-based MRI contrast, the Cancer Online Resource Hub describes it as very safe (accessed October 2026): about one patient in a hundred has a mild reaction such as headache, dizziness, nausea or vomiting; if contrast leaks at the injection site, the area may be uncomfortable, swollen or painful, settling within one to two days; severe reactions (such as seizures or severe anaphylactic shock) are rare but can be fatal, with a risk of death of one in four hundred thousand; for patients who already have kidney failure, the chance of nephrogenic systemic fibrosis after the injection is one to seven per cent. St. Paul's Hospital says high-risk patients, such as those with severely reduced kidney function, have a blood test to assess kidney function before the injection (accessed October 2026). Whether contrast is needed depends on the area scanned and the doctor's clinical judgement (Union Hospital, accessed October 2026).

Sources differ on pregnancy and are listed separately: the Cancer Online Resource Hub says the long-term effects of MRI on the body or a foetus are not yet known with certainty, and anyone who is or may be pregnant should tell the radiology staff before the scan; St. Paul's Hospital states the scan should be avoided in the first 3 months of pregnancy; Union Hospital, citing international radiology safety guidance, says MRI without contrast is regarded as safe in pregnancy, but the final decision is made jointly by the treating doctor, the radiologist and the pregnant woman (all accessed October 2026). Whether to have a scan during pregnancy should be decided in discussion with the treating doctor.

Do I need a referral for an MRI scan? Public or private?

A radiographer may not perform diagnostic imaging without a referral from a registered medical practitioner, dentist, Chinese medicine practitioner, chiropractor or similar, except in an emergency, and referrals from dentists, Chinese medicine practitioners outside the Chinese Medicine Hospital of Hong Kong and chiropractors are limited to specified plain X-ray tests (Radiographers Board Code of Practice, December 2025); the code binds only registered radiographers, and whether a referral is needed follows the booking provider's requirement.

To book a new case at a public hospital specialist outpatient clinic, a referral letter issued by a locally registered doctor within the last three months is required (Hospital Authority, January 2018). The Cancer Online Resource Hub describes the public-private radiological collaboration programme (Radi Collaboration) for Hospital Authority patients with specified cancers (for example colorectal, breast and nasopharyngeal cancer) whom a doctor clinically assesses as eligible: the doctor may invite the patient to choose voluntarily to have CT or MRI at a private provider, fully subsidised by the Hospital Authority, with no application needed from the patient; in general the private provider serves booked patients within five working days (accessed October 2026).

For public waiting times see public CT and MRI waiting times, for public and private fees see MRI fees, and for booking private imaging see how to book private imaging.

Who decides whether I need an MRI scan, and who explains the report?

Whether to have an MRI scan, of which area, and whether contrast is used, is decided by the doctor according to the condition. Union Hospital states that a specialist radiologist analyses the images and writes the report, and the treating doctor explains the results and next steps to the patient (accessed October 2026); individual cases are decided by the treating doctor. For what radiologists do see what does a radiologist do.

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