What is an ultrasound scan? Fasting for a liver or upper abdominal scan, drinking water for a lower abdominal scan, the procedure and referral in Hong Kong
An ultrasound scan uses high-frequency sound waves to produce images of organs such as the liver, gallbladder, pancreas, kidneys, uterus, ovaries and breasts; under normal circumstances it involves no radiation and causes no pain; before an upper abdominal scan patients need to fast for four to six hours, and before a lower abdominal scan they need to drink plenty of water to fill the bladder (Cancer Online Resource Hub, accessed October 2026). Requirements differ; follow the instructions of the hospital where you book.
What is an ultrasound scan and what can it look at?
Ultrasound is high-frequency sound far above the range of human hearing that can pass through skin and tissue; an ultrasound scan uses it to produce high-resolution images of major organs such as the liver, gallbladder, pancreas, kidneys, uterus, ovaries and breasts, to diagnose conditions such as cirrhosis, gallstones, pancreatitis, kidney stones, tumours, and narrowed or blocked blood vessels, and it can also check the internal organs and growth of a baby in pregnancy; under normal circumstances an ultrasound scan involves no radiation, is harmless and causes no pain (Cancer Online Resource Hub, accessed October 2026).
For ultrasound of the heart see echocardiogram, for the X-ray test of the breast see mammogram, and for the English and Chinese names see ultrasound in Chinese and English.
How do I prepare for an ultrasound scan? Do I need to fast for a liver scan?
The Cancer Online Resource Hub lists different preparation by body area (accessed October 2026), shown in the table; the page also advises wearing simple, loose clothing and lifting clothing or changing into the gown provided for the scan.
| Area scanned | Preparation |
|---|---|
| Upper abdomen (for example liver, gallbladder) | Fast for four to six hours |
| Lower abdomen (for example kidneys, reproductive organs) | Drink plenty of water to fill the bladder |
| Other areas (for example breast, thyroid) | No special preparation |
What happens during the scan, and how long does it take?
You usually lie on the examination bed and may be asked to turn or lie on your side; a layer of water-based gel is put on the skin so the sound waves can pass into the body, then the probe is moved over the area and the picture appears on the screen straight away; you may be asked to breathe in or hold your breath for a clearer picture; the Cancer Online Resource Hub says the whole scan takes about 20 minutes to half an hour, longer for some areas or harder scans (accessed October 2026).
Afterwards the gel is wiped away and you can leave, and you can usually eat and go about your day as normal; the radiologist writes the report and passes it to the treating doctor (Cancer Online Resource Hub, accessed October 2026). For when reports are usually ready see when will my test report be ready.
Do I need a referral for an ultrasound 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, so an examination done by a doctor is not covered by it, and whether a referral is needed follows the booking provider's requirement; the referral documents each hospital needs differ.
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 ultrasound fees see ultrasound fees, for booking private imaging see how to book private imaging, and for what radiologists do see what does a radiologist do.
Who decides whether I need an ultrasound scan, and who explains the report?
Whether to have an ultrasound scan, and of which area, is decided by the doctor according to the condition; which specialty follows up an abnormal finding (such as a shadow or cyst in the liver) is decided by the treating doctor after reading the report, and individual cases are decided by the treating doctor.
Frequently asked questions
Do I need to fast for a liver ultrasound?
The Cancer Online Resource Hub states that patients need to fast for four to six hours before an upper abdominal scan (for example liver or gallbladder) (accessed October 2026); requirements differ, so follow the instructions of the hospital where you book.
Does an ultrasound scan involve radiation?
Under normal circumstances an ultrasound scan involves no radiation, is harmless and causes no pain (Cancer Online Resource Hub, accessed October 2026).
How long does an ultrasound scan take?
The whole scan takes 20 minutes to half an hour or longer, depending on the area and how difficult it is (Cancer Online Resource Hub, accessed October 2026).
Do I need a doctor's referral for an ultrasound scan?
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, so an examination done by a doctor is not covered by it, and the booking provider's requirement applies; whether to have a scan is decided by the doctor according to the condition.