In this article, a model for the amplitude statistics of the backscatt
ered ultrasonic signal is presented, We propose to view a tissue as be
ing composed of a large number of small units, each having slightly di
fferent characteristics. This variability within the tissue is express
ed by fluctuations in the parameters of the local probability distribu
tion function (PDF), Based an analogous expressions derived for radio
propagation and optical scintillations, the local PDF is modulated by
a lognormal distribution of the local standard deviation, Integrating
the local contributions yields the amplitude PDF for the entire tissue
, We show that the local entropy is a normal variable, since for four
different local PDFs it is linearly related to the logarithm of the lo
cal standard deviation, When the local entropy histogram exhibits dist
inct and multiple peaks, the local entropy distribution can be used fo
r region segmentation, This fact is demonstrated for ultrasonic images
of ovarian cysts.