Use of phantoms and test objects for local dynamic range evaluation in medical ultrasounds: A preliminary study

In the ultrasound image the relationship between echo amplitudes and gray levels is expressed by means of the Grayscale Mapping Function (GMF), that is the greyscale transfer function associated with the echo displayed. The GMF allows the determination of some image quality parameters and quantities, among which the Local Dynamic Range (LDR) is relevant, since it is defined as the 20·log10 of the ratio of the minimum echo amplitude that yields the maximum grey level in the digitized image to that of the echo that yields the lowest grey level at the same location in the image and the same settings. This study reports the implementation of a method for the automatic determination of the LDR on medical ultrasound scanners and its application by means of a commercial grayscale ultrasound phantom, nevertheless it can be used also with general purpose phantoms: the LDR is obtained from the estimation of the GMF, based on processing of a sequence of uncompressed bidimensional ultrasound images provided by the scanner. In the manuscript, some theoretical considerations have been done to determine the GMF and its fitting model, as well as the LDR values, after that an experimental setup is described and some results are shown for an ultrasound system equipped with two different probes.