Ultrasound tissue characterization of the normal kidney

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.

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" (0-4), "fat-like" (8-26), "hypoechoic muscle-like" (41-60), "hyperechoic muscle-like" (61-76), 4 ranges of "fiber-like" (112-196), "calcium-like" (211-255) 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.

Eight women and 2 men, 26 ± 4 years (range, 22-34 years) old, were studied. Kidney length was 10.5 ± 0.9 cm (9.0-12.0 cm). Doppler US resistivity index was 0.67 ± 0.03 (0.62-0.71).

Original fascia brightness converted to 200 value had a mean ± SD of 206 ± 16 (range, 181-236). Kidney grayscale median averaged 37 ± 6 (27-48). Most pixels were hypoechoic II to IV (8-60), averaging 78% ± 6% (66%-87%). Percentages for fat-like, intermediary fat/muscle-like, and hypoechoic muscle-like intervals averaged 25%, 28%, and 25%, respectively.

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.