What is an X-ray? How much radiation one X-ray gives, X-rays in pregnancy, the procedure and referral in Hong Kong

What is an X-ray and what can it show?

The Cancer Online Resource Hub explains that X-rays are electromagnetic radiation invisible to the eye, and the patient feels nothing as they pass through the body; after passing through, the X-rays strike a sensitive screen on the other side to form the image, and they are usually used to examine the outline of the body part concerned, lumps or abnormalities (accessed October 2026).

St. Paul's Hospital explains that the most common use is diagnosing bone and lung disease, for example a chest X-ray can diagnose pneumonia, lung cancer and fluid on the lungs, and an abdominal X-ray can diagnose bowel obstruction; some X-ray examinations need an injection of contrast to tell different tissues apart, and the hospital says contrast is generally a safe drug but may sometimes cause side effects (accessed October 2026).

For CT, which also uses X-rays, see CT scan; for tests that do not use X-rays see MRI scan and ultrasound scan; for the X-ray test of the breast see mammogram.

What happens during the X-ray, and how long does it take?

Hong Kong Baptist Hospital's Ambulatory Medical Centre states (accessed October 2026): X-ray examinations are performed by registered radiographers; you lie on the examination table or stand against a flat panel, and the radiographer helps position you so the part to be examined is between the X-ray machine and the image receptor; keep still, and hold your breath when asked if needed; the radiographer may ask you to change position to take more images from other angles; each exposure usually lasts only seconds, and you can eat and drink as usual afterwards.

Special X-ray examinations that use contrast need their own preparation; follow the instructions of the hospital where you book. For when reports are usually ready see when will my test report be ready.

How much radiation does an X-ray give?

The table is compiled from the Department of Health's Radiation Health Division and Hong Kong Baptist Hospital's Ambulatory Medical Centre; the sources' figures differ slightly and have not been merged.

Is the radiation from an X-ray safe?

St. Paul's Hospital says most X-ray examinations involve very low radiation doses, and used carefully their benefits far outweigh the possible harm from radiation (accessed October 2026); Hong Kong Baptist Hospital's Ambulatory Medical Centre also says most diagnostic X-ray doses are low, about 0.02 to 0.04 mSv for a chest X-ray for example, a level generally regarded as safe (accessed October 2026).

The Radiation Health Division's Guidance Notes on radiation protection in diagnostic radiology (revised August 2026), written for people who use X-ray equipment, say referring doctors should avoid asking for X-rays routinely without a clinical indication, and should order one only when the information needed (for example from the patient's previous radiological examinations) is lacking; where two or more imaging procedures could give the diagnostic information needed, radiologists should choose the one with the least overall risk to the patient. For bringing earlier tests to a new doctor see what earlier tests to bring to a new doctor. Whether to have an X-ray, and of which area, is decided by the doctor according to the condition.

Can I have an X-ray if I am pregnant?

The Radiation Health Division's Guidance Notes on radiation protection in diagnostic radiology (revised August 2026, written for people who use X-ray equipment) say: unless there is a sufficient clinical reason, examinations that expose the abdomen should be avoided for women who may be pregnant; X-rays of areas away from the foetus (for example the chest, skull or hands and feet) can be done safely at any time in pregnancy, provided the equipment has suitable shielding and the beam is properly limited; the guidance also recommends notices in prominent places in X-ray departments asking anyone who may be pregnant to tell the doctor or radiographer before the examination.

St. Paul's Hospital's information sheet on X-ray radiation safety (July 2020) states that if a woman having an abdominal X-ray cannot be sure she is not pregnant, the examination is usually postponed: low-dose abdominal examinations are done within the first 28 days from the start of the menstrual cycle (the "28-day rule"), while higher-dose examinations such as barium enema or abdominal or pelvic CT are done within the first 10 days (the "10-day rule"), and patients are asked to give staff the date of their last period. Hong Kong Baptist Hospital's Ambulatory Medical Centre says that if a woman having an abdominal X-ray cannot be sure she is not pregnant, the examination is usually postponed; low-dose abdominal examinations are done within the first 28 days from the start of the period, and patients are asked to give the date of their last period (accessed October 2026). Whether to have an X-ray during pregnancy is decided by the doctor.

Do I need a referral for an X-ray?

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 (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. The code also limits radiographers to the plain X-ray items in the table when the referral comes from one of these three groups.

Public or private? Who decides whether I need an X-ray?

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). For public and private X-ray fees see X-ray fees, for booking private imaging see how to book private imaging, and for what radiologists do see what does a radiologist do.

Whether to have an X-ray, and of which area, is decided by the doctor according to the condition; follow-up of the report in individual cases is decided by the treating doctor.

Frequently asked questions