The gallbladder and bile ducts are usually assessed initially with conventional gray-scale ultrasound (US). Contrast enhanced US (CEUS) is used when a diagnosis cannot be reached with conventional US. CEUS is easy to learn and perform. US contrast agents can be safely administered in patients with renal function impairment. In this pictorial essay the physics, examination technique and indications of CEUS for examining the gallbladder and bile ducts are reviewed. Gallbladder indications include elucidating normal variants, differentiating sludge from neoplastic lesions, benign and malignant pathology, infection, wall rupture and hemobilia. In the biliary tree CEUS is used for studying benign and malignant tumors, including metastases and cholangiocarcinoma, as well as intrabiliary injection. K E Y W O R D Sbile ducts, CEUS, contrast agents, gallbladder, ultrasound | I N TR ODU C TI ONThe gallbladder and bile ducts are usually adequately imaged with grayscale sonography (US) and often no additional imaging is required. In cases when further imaging is needed to elucidate US findings, contrast-enhanced ultrasound (CEUS) overcomes the limitations of color Doppler imaging and can be used immediately after grayscale US examination. | PHYSICS OF US CONTRAST AGENTSContrast agents are composed of microbubbles of gas contained in a shell of protein, lipid, or polymer. 1 They have no renal or cardiac toxicity and can be safely administered in patients with renal insufficiency. CEUS examination requires dedicated contrast agent-specific techniques. 5,6 One of these techniques is phase inversion, where 2 different US pulses in opposite phases are emitted by the scanner sequentially. This achieves optimal contrast resolution and minimal stationary tissue signal. 7 A split-screen setting of the machine's monitor (dual view mode) allows simultaneous display of contrast-enhanced and conventional gray-scale images. However, a common artifact results from strongly reflective structures (eg, the diaphragm, vessel walls, the abdominal wall, or gas-filled structures) appearing as contrast enhancement on the CEUS part of the screen. 8

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