Ultrasound Tissue Characterization of the Normal Kidney

Aim: Ultrasound tissue characterization (USTC) is a precursor of ultrasound virtual histology (USVH), already applied to B-mode images of coronary, carotid, and peripheral arteries, as well as venous thrombosis. Elevated echogenicity has been described for a rejected transplanted kidney. We analyzed data from healthy young adults as reference for further renal USTC. Methods: Ultrasound kidney images of 10 volunteers were analyzed. Pixel brightness in the 0-to-255 range was rescaled to zero for black and 200 for fascia brightness before automatic classification into 14 ranges, including ‘‘blood-like’’ (0Y4), ‘‘fat-like’’ (8Y26), ‘‘hypoechoic muscle-like’’ (41Y60), ‘‘hyperechoic muscle-like’’ (61Y76), 4 ranges of ‘‘fiber-like’’ (112Y196), ‘‘calcium-like’’ (211Y255) and intermediary intervals. Nomenclature was readapted using nonechoic, hypoechoic I to IV, echoic I to IV, hyperechoic I to IV, and saturated echoes to avoid inference to actual kidney tissue. Descriptive and comparative statistics were based on percentages of pixels in specific brightness ranges. Sample Population: Eight women and 2 men, 26 T 4 years (range, 22Y34 years) old, were studied. Kidney length was 10.5 T 0.9 cm (9.0Y12.0 cm). Doppler US resistivity index was 0.67T 0.03 (0.62Y0.71). Results: Original fascia brightness converted to 200 value had a mean T SD of 206 T 16 (range, 181Y236). Kidney grayscale median averaged 37 T 6 (27Y48). Most pixels were hypoechoic II to IV (8Y60), averaging 78% T 6% (66%Y87%). Percentages for fat-like, intermediary fat/muscle-like, and hypoechoic muscle-like intervals averaged 25%, 28%, and 25%, respectively. Conclusions: A reference database for USTC/USVH of normal young kidneys was created for future comparisons with transplanted and abnormal kidneys. Normal renal echoes have low brightness. Hyperechoic pixels may represent abnormalities.