What is a CT scan? How long to fast, what to know about contrast, and whether you need a referral in Hong Kong

What is a CT scan?

The Cancer Online Resource Hub describes CT as a safe imaging test: computer technology combined with a rotating X-ray unit draws precise cross-sectional pictures of the body to diagnose disease inside it, and the body is exposed to X-rays during the scan (accessed October 2026).

St. Paul's Hospital explains that the computer can also rebuild the cross-sections into three-dimensional images of organs; the uses it lists include diagnosing head injury, blood vessel disease, stroke, tumours, inflammation, infection, stones and fractures (accessed October 2026).

For MRI, which does not use X-rays, see MRI scan; for plain X-rays see X-ray examination; for the English and Chinese names see CT scan in Chinese and English.

How do I prepare for a CT scan? How long should I fast?

The Cancer Online Resource Hub reminds patients (accessed October 2026): tell the doctor first if you have an allergy-related history such as asthma or eczema, or have reacted to foods, drugs or X-ray contrast; if you think you may be, or are, pregnant, tell the radiology staff first; people with diabetes adjust their medicine as the doctor instructs; abdominal scans mostly need contrast or water to drink a few hours ahead; if contrast is to be injected into a vein, you sign a consent form first.

Sources differ on fasting, as shown in the table, and the three have not been merged; follow the instructions of the hospital where you book.

What else do individual hospitals ask?

St. Paul's Hospital notes that patients taking diabetes medicine containing metformin may need, following the doctor's guidance, to stop it from the day before the scan, to help the injected contrast leave the body through the kidneys (accessed October 2026).

Union Hospital asks patients over 65 or with kidney disease or diabetes to provide a serum creatinine result from within 6 months, and patients having a coronary angiography scan not to drink caffeinated or other stimulating drinks within 12 hours before (accessed October 2026).

These are individual hospitals' practices; how to adjust any medicine follows the doctor's instructions.

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

You lie on the scanner bed, which moves you into the ring-shaped opening; X-rays pass through you and a computer produces cross-sectional images; keep as still as you can and follow the radiology staff's instructions; the whole procedure is supervised by a doctor, who may inject intravenous contrast if needed to make the diagnosis more accurate, in which case you sign a consent form; children may need sedation so the scan can be completed (Cancer Online Resource Hub, accessed October 2026). Union Hospital says some scans need you to hold your breath briefly, and the radiographer will tell you when (accessed October 2026).

Sources describe the time differently: the Cancer Online Resource Hub says the whole process depends on the individual case and generally takes most of a day; St. Paul's Hospital lists about ten to fifteen minutes for a general scan and twenty to thirty minutes for a heart and blood vessel scan; Union Hospital lists about 5 to 15 minutes of scanning, depending on the type (all accessed October 2026).

Afterwards you can usually eat and go about your day as normal, and the result is sent to the treating doctor (Cancer Online Resource Hub, accessed October 2026); Union Hospital advises patients who had contrast to drink more water to help clear it (accessed October 2026). For when reports are usually ready see when will my test report be ready.

What are the risks of contrast?

The Cancer Online Resource Hub says contrast is generally very safe but complications occasionally occur (accessed October 2026): reactions are mild, moderate or severe; mild ones are mostly brief discomfort, and in some people appear later, within 24 hours; moderate ones need treatment and monitoring; severe ones need immediate treatment; contrast may also leak out of the vein during injection, causing discomfort at the site; the chance of death from sensitivity to contrast is about one in two hundred and fifty thousand.

St. Paul's Hospital's CT procedure information sheet (July 2020) gives a chance of about four in ten thousand for more serious side effects and about two point one in a million for death, and says that injecting contrast raises the chance of lactic acidosis in people with diabetes on metformin whose kidney function is reduced; these figures differ from the Cancer Online Resource Hub's and are listed separately here.

Whether contrast is used is decided by the doctor according to clinical need (Union Hospital, accessed October 2026); tell the doctor beforehand about any history of allergy.

Can I have a CT scan if I am pregnant?

The Cancer Online Resource Hub reminds patients who think they may be, or are, pregnant to tell the radiology staff before the scan (accessed October 2026). Union Hospital says anyone who is or may be pregnant should tell the treating doctor and staff in advance, because CT uses X-rays and its suitability in pregnancy needs particularly careful assessment (accessed October 2026).

St. Paul's Hospital's information sheet states that if a woman cannot be sure she is not pregnant the scan is usually postponed: a low-dose CT scan can be done within the first 28 days from the start of the menstrual cycle (the "28-day rule"), while higher-dose scans such as abdominal or pelvic CT are done within the first 10 days (the "10-day rule") (July 2020). Whether to have a scan during pregnancy is decided by the doctor.

Do I need a referral for a CT 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. Union Hospital states that CT scans are scheduled by the radiology department after referral by a clinician (accessed October 2026).

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 CT scan fees and CT and MRI scans: government hospital or private, and for booking private imaging see how to book private imaging.

Who decides whether I need a CT scan, and who explains the report?

Whether to have a CT scan, of which area, and whether contrast is used, is decided by the doctor according to the condition. Union Hospital states that a radiologist analyses the images and writes the report, which goes back to the treating doctor to explain (accessed October 2026); follow-up in individual cases is decided by the treating doctor. For what radiologists do see what does a radiologist do.

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