Ultrasound Scan — Purpose, Procedure & Normal Values | MyMedicPlus

An ultrasound (sonography) scan uses high-frequency sound waves — typically in the range of 2–18 MHz, far above the 20 kHz upper limit of human hearing — to produce real-time, dynamic images of the body's internal structures. A hand-held transducer (probe) containing piezoelectric crystals simultaneously emits brief ultrasound pulses and detects the reflected echoes (sound waves bounced back from tissue interfaces). The time taken for each echo to return and its amplitude are processed by a computer to construct a greyscale image on the monitor. Denser structures that strongly reflect sound — bone, gallstones, kidney stones — appear white (hyperechoic); fluid-filled structures that transmit sound easily without reflection appear black (anechoic); soft tissues produce intermediate shades of grey (isoechoic or hypoechoic relative to surrounding structures). Doppler ultrasound — colour Doppler and spectral Doppler — detects and measures blood flow velocity and direction in arteries and veins, enabling assessment of vascular obstruction, stenosis, and regurgitation. Ultrasound produces no ionising radiation and is universally accepted as the safest imaging modality, extensively used in obstetrics, abdominal imaging, musculoskeletal medicine, cardiology, and emergency medicine.

Ultrasound is one of the most versatile diagnostic imaging modalities, ordered across a wide range of clinical specialties. Abdominal and hepatobiliary: evaluating the liver (fatty liver disease, cirrhosis, hepatitis, liver masses, haemangiomata), gallbladder (gallstones, cholecystitis, polyps, wall thickening), bile ducts (choledocholithiasis, biliary obstruction), pancreas (pancreatitis, masses), spleen (splenomegaly, haematoma, infarction), and kidneys (hydronephrosis, renal stones, cysts, masses, renal artery stenosis). Obstetric ultrasound is the global standard for fetal monitoring: dating and nuchal translucency scan (11–13 weeks), anomaly scan (18–20 weeks), growth and placenta assessment in the third trimester, and fetal Doppler flow studies. Gynaecological: uterine fibroids, endometrial thickness, ovarian cysts and masses, polycystic ovarian syndrome (PCOS), free pelvic fluid, and endometriosis. Scrotal and testicular: testicular torsion (surgical emergency), epididymo-orchitis, hydrocele, varicocele, testicular masses. Thyroid nodule assessment and guidance for fine needle aspiration biopsy. Musculoskeletal: rotator cuff tears, tendinopathy, plantar fasciitis, joint effusions. Vascular: deep vein thrombosis (DVT), carotid artery stenosis, peripheral arterial disease, aortic aneurysm measurement.

Preparation requirements vary substantially depending on which organ system is being examined. For an abdominal ultrasound (liver, gallbladder, bile ducts, pancreas, spleen): fast for 4–6 hours before the examination. Fasting is essential to ensure the gallbladder is fully distended with bile and visible — eating or drinking contracts the gallbladder. Gas in the bowel from recent eating also degrades image quality. Water and essential medications with small sips of water are usually acceptable. For a pelvic ultrasound (uterus, ovaries, bladder): drink 500–750 mL (three to four large glasses) of plain water approximately 1 hour before the appointment and do not urinate — a comfortably full bladder is essential, as it displaces air-filled bowel loops out of the pelvis and creates an acoustic window through which the pelvic organs can be clearly visualised. For an endovaginal (transvaginal) pelvic ultrasound: no preparation needed — an empty bladder is actually preferred. For a renal tract ultrasound (kidneys and bladder): drink normally and attend with a moderately full bladder. For thyroid, musculoskeletal, or superficial structure ultrasound: no preparation is required. Wear comfortable, loose-fitting clothing with easy access to the area being scanned.

You lie on an examination couch in a private room or cubicle. The sonographer or radiologist exposes the area of the body to be scanned. A water-based, hypoallergenic coupling gel is applied generously to the skin surface — the gel is at room temperature in most modern departments and eliminates air gaps between the probe and skin that would completely block ultrasound transmission. The transducer is pressed gently against the gel-covered skin and moved systematically across the area, tilting, rotating, and applying variable degrees of pressure to obtain images from multiple angles. The sonographer takes screenshots and video clips, measures organ dimensions, and records any abnormal findings. For a transvaginal (endovaginal) pelvic ultrasound, a thin, elongated transducer covered with a disposable sterile sheath and lubricating gel is gently inserted into the vagina to obtain closer views of the uterus and ovaries — this is a standard, routine procedure that typically causes only mild pressure. A transrectal ultrasound (TRUS) of the prostate is performed with a small probe inserted into the rectum. Doppler assessment requires the patient to remain still during flow measurements. The examination typically takes 20–45 minutes.

A radiologist or specialist sonographer reviews the images and produces a written report, typically available within 24–48 hours for routine non-urgent scans. Urgent scans such as suspected ectopic pregnancy, testicular torsion, acute appendicitis, biliary colic with cholangitis, or aortic aneurysm are reported immediately and findings communicated directly to the referring clinician. The report describes the size, shape, echogenicity (texture), margins, and vascularity of each organ examined, and notes any focal lesions, free fluid, or Doppler flow abnormalities. Normal organ size reference ranges: right liver lobe — up to 15 cm (craniocaudal); spleen — up to 12 cm; right kidney — 9–12 cm (10–12 cm left kidney); gallbladder wall — less than 3 mm; common bile duct — up to 6 mm (up to 8 mm post-cholecystectomy); aorta — less than 3 cm diameter. Echoic descriptors: a simple cyst is anechoic (black) with posterior enhancement, thin walls, and no internal structure — a uniformly benign finding. Complex lesions with internal echoes, thick walls, or vascularity require further evaluation with CT, MRI, or biopsy.

Ultrasound is entirely safe — it uses sound waves, produces no ionising radiation, and has no known adverse biological effects at the diagnostic intensities and durations used in clinical practice. It is the imaging modality of choice throughout pregnancy, including in the first trimester. The coupling gel occasionally causes mild skin irritation in sensitive individuals but is generally hypoallergenic. Ultrasound's main diagnostic limitations stem from its physical properties: poor penetration through gas (bowel gas, lung air, surgical emphysema) and bone (skull, ribs, vertebrae) severely limits imaging of these areas and structures behind them. Image quality is significantly degraded in obese patients with a thick abdominal wall and in individuals with excessive bowel gas — both common situations. Small lesions below approximately 3–5 mm may not be consistently detected. Ultrasound is notably operator-dependent — image quality, thoroughness, and diagnostic accuracy vary considerably with the experience and skill of the individual sonographer. CT and MRI provide complementary whole-organ survey, characterisation of complex lesions, and superior depth penetration when ultrasound findings are inconclusive or additional anatomical detail is required.