Diagnostic Ultrasound - Abdomen and Pelvis
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Diagnostic Ultrasound - Abdomen and Pelvis

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09.07.2019 Views

Liver Transplant Hepatic Artery Stenosis/Thrombosis 274 Liver Transplant Portal Vein Stenosis/Thrombosis 275 Liver Transplant Hepatic Venous Stenosis/Thrombosis 276 Liver Transplant Biliary Stricture 277

Approach to Hepatic Sonography Diagnoses: Liver Patient Preparation Although no specific patient preparation is generally required to visualize the liver, the other organs of the hepatobiliary system, such as the gallbladder, are ordinarily examined as part of a complete sonographic liver assessment. Therefore, for a comprehensive hepatobiliary assessment, patients should be instructed to fast for 6-8 hours prior to the ultrasound examination. If dehydration is a concern, water may be given, but other activities that increase stomach and intestinal gas production should be limited. Imaging Protocols The liver is the largest organ in the abdomen and must be scanned systematically and carefully to ensure a thorough examination. The patient should be positioned supine initially, but a variety of positions, including decubitus scanning, may be needed depending on organ size and orientation and the presence of overlying bowel gas. Imaging from the subcostal approach with cranial angulation of the transducer in normal and deep suspended respiration typically provides optimal visualization. If the liver is high in position, or shrunken and cirrhotic, an intercostal approach may be necessary. The scan protocol ordinarily begins by visualizing the left lobe in the mid sagittal plane beneath the sternum and xiphoid process and proceeds with the sonographer scanning laterally to visualize first the left lobe and then the right lobe. The transverse plane is scanned next, followed by oblique planes along the long and short axes of specific anatomic structures, such as the gallbladder and common bile duct. The sonographer must visualize the entire liver in real time from the most lateral portions of the left lobe through the right lobe, and then capture a series of standard sagittal and parasagittal views for documentation purposes. Similarly, in the transverse plane, the liver must be examined from the dome to the inferior tip. Standard views to be documented include the following: • Longitudinal views: Lateral segments of left lobe, aorta, inferior vena cava, caudate lobe and ligamentum venosum, porta hepatis, gallbladder fossa, right lobe segments, right lobe with longitudinal view of right adrenal fossa and kidney • Transverse views: Left lobe dome, left portal vein, caudate lobe and ligamentum venosum, right lobe dome, hepatic venous confluence and individual right, middle, and left hepatic veins, gallbladder, and liver with right kidney • Oblique views: Common bile duct in long axis, main portal vein in long axis, gallbladder in long axis, short axis, and decubitus views Transducer Selection and Technical Factors The sonographer should select the highest frequency transducer that provides sufficient penetration to visualize the entire depth of the liver. As a visual check, in a normal liver the diaphragm should be clearly visible on longitudinal and transverse images of the right lobe. With modern ultrasound technology, this usually implies broad-bandwidth 3-5 MHz transducers capable of imaging in both harmonic and fundamental modes and with multiple focal zones. Curved linear transducers generally provide the best compromise between good near-field imaging and wide field of view. For images of the hepatic capsule, a high-frequency linear transducer should be used. Time-gain compensation and the overall receiver gain settings should be set so that the liver has a homogeneous and uniform echo texture from the near field to the far field. Speckle reduction techniques, such as spatial and frequency compounding, and adaptive filtering techniques can work synergistically with harmonic imaging modes to reduce image noise. Color, spectral, and power Doppler modes should be part of every dedicated assessment of the liver. Color Doppler mode should be used to document patency and direction of flow in the portal and hepatic veins as well as the hepatic artery. If flow appears absent on color Doppler, power Doppler should be used since it is less dependent on insonation angle. Power Doppler is also useful for minimizing artifacts resulting from background motion, and for distinguishing dilated bile ducts from vascular structures within the liver. The Doppler signatures of the hepatic vessels obtained in spectral Doppler mode are often quite useful in the identification and characterization of hepatic pathology. The normal portal vein shows hepatopetal, minimally undulating flow. By contrast, the normal hepatic veins have a triphasic velocity waveform similar to the appearance of the central venous pressure waveforms. The hepatic artery interrogated in the hepatic hilum shows peak velocity ranging from 30-60 cm/sec, and a low-resistance waveform with continuous antegrade flow during diastole. In liver transplants, careful evaluation of the hepatic artery, portal vein, and hepatic veins are important for detection of potential complications including stenosis or thrombosis. The hepatic duct should be carefully evaluated for developing strictures at the anastomosis. Equipment parameters in Doppler modes, such as the wall filter and velocity scale (pulse repetition frequency), should be set so as to correctly display the expected range of velocities in the vessels being interrogated. For quantitative assessment of flow velocity, angle correction should be performed in spectral Doppler mode in order to calibrate the machine to calculate an accurate velocity from the frequency shift information that is detected at the transducer. Anatomy-Based Imaging Issues The normal liver should display relatively uniform, intermediate-level echogenicity slightly higher than that of the renal cortex. Its length should typically not extend below the inferior pole of the right kidney unless a Reidellobe is present. Portal triads ramifying within the liver typically display echogenic walls, whereas the hepatic venous walls will not be echogenic unless insonated at exactly ninety degrees. Certain normal hepatic structures can simulate pathology. For example, the fibrous ligamentum teres and ligamentum venosum may cause acoustic shadowing and create the appearance of mass lesions or abnormal echogenicity of the caudate lobe. Overlying ribs may also cause shadowing, particularly during intercostal scanning. Selected References 1. Heller MT et al: The role of ultrasonography in the evaluation of diffuse liver disease. Radiol Clin North Am. 52(6):1163-75, 2014 2. McNaughton DA et al: Doppler US of the liver made simple. Radiographics. 31(1):161-88, 2011 3. Kruskal JB et al: Optimizing Doppler and color flow US: application to hepatic sonography. Radiographics. 24(3):657-75, 2004 174

Liver Transplant Hepatic Artery Stenosis/Thrombosis 274<br />

Liver Transplant Portal Vein Stenosis/Thrombosis 275<br />

Liver Transplant Hepatic Venous Stenosis/Thrombosis 276<br />

Liver Transplant Biliary Stricture 277

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