The prime determinant of the strength of ultrasonic echoes is the impedance mismatch (Z) between adjacent tissue components. The larger the mismatch, the stronger the echo, so that interfaces between soft tissues and bone, for example, give very strong echoes and, within soft tissues, the most significant components are fibrous tissue (often in the form of the perivascular microskeleton) and fatty tissue. Thus, while uniform regions of fibre or fat are echo poor (subcutaneous fat and retroperitoneal fibrosis are examples), admixtures between them and watery tissues give stronger echoes.

A second important factor is the concentration of the scatterers: for a given impedance mismatch, a region that contains a large number of scatterers is more echogenic than one where they are spread out. Commonly the 'dilution' of scatterers is caused by an increase in water content. The low reflectivity of the congested liver in right heart failure is an example (Fig. 1). Malignant tumours are a general case: until they grow large enough to undergo necrosis or calcification (which produce new reflectors), they tend to be echo poor. Similarly, the oedematous tissues in acute inflammation give lowlevel echoes; examples include the echo-poor pancreas in acute pancreatitis and the 'dark liver' in acute hepatitis. On the other hand, the high concentration of reflectors is the cause of the echogenic kidneys in recessive (infantile) polycystic renal disease (the interfaces between the innumerable cysts cause strong echoes), and strong echoes are obtained from the multiple interfaces of the vessel walls of haemangiomas and even more so in angiomyolipomas, where there is also admixed fatty tissue. Fatty infiltration in the liver (steatosis) is a common example (Fig. 2).

FIGURE 1 ■ Congested liver. In heart failure, the liver may become congested with extra fluid. The separation of the reflectors reduces the liver echoes so that it becomes less echogenic than the kidney. In addition, the vascular markings are accentuated because they are not affected. GB = gallbladder, K = kidney.

FIGURE 2 ■ Fatty liver. The multiple interfaces between fatladen liver lobules and the surrounding watery tissues give these fatty liver high-intensity echoes which can be seen as increased contrast with the adjacent renal cortex (compare Fig1). K = kidney, L = liver.

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