In vivo study of the human retino-choroidal layers by RF-Signal analysis. II. Automated digital image analysis on the M-scan

Abstract

A computer-assisted interactive method has been developed for the quantitative determination of small tissue movements in the posterior ocular wall. An ultrasonic M-scan has been reconstructed from a sequence of digitized RF-signals obtained from a region in the fundus of the eye. The repetition rate is four signals/sec. The RF-representation of the M-scan is transformed into an image with 256 grey levels and is displayed on a color screen. Axial shifts of each signal caused by relative movements of the transducer and patient are corrected by digital phase-correction, thus preserving axial changes in the layers. After phase-correction, layers of interest may be defined interactively in one RF-line. Dynamic changes and tissue movement caused by the pulsation of internal blood vessels are determined by changes of layer distances as a function of time. The detection and registration of the dynamics in all RF-lines is performed by an adapted image processing software. To prove the dependence of tissue movements on the heart rate, the correlation between changes of distances and the finger-pulse is calculated. The finger-pulse signal is digitized and stored synchronously with each RF-signal.

A special high resolution RF-data acquisition system is necessary for these measurements since detected changes in distances of the layers are usually smaller than 15 µ. The signal processing procedure and results of the analysis of M-scans taken from the normal posterior ocular wall will be presented.