A novel method for automatic evaluation of the effective dynamic range of medical ultrasound scanners

Abstract

Quality of imaging in medical ultrasound system can be described by means of parameters such as spatial resolution, accuracy in distance measurements and depth of penetration: despite of their importance, it must be pointed out that the information which can be obtained from an ultrasound image is determined also by the dynamic range of signal levels displayed (gray scale characteristic) and by the minimum detectable changes in echo signal amplitude. Moreover, the knowledge the relationship between echo amplitude and gray level on the image (Gray Scale Mapping Function or GSMF) allows to evaluate the other parameters, like low contrast spatial resolution and depth of penetration, referring to the echo strength and so to the sensitivity of the diagnostic system. If system control settings are carefully selected, the echo range between lower and upper saturation of the GSMF determines the effective dynamic Range (EDR), that is the echo range (in dB) that corresponds to the “useful” gray levels spanned on the image: also EDR can be used as a performance indicator of ultrasound systems. Tissue equivalent test objects (ultrasound phantoms) are today available for direct determination of the gray scale characteristic and minimum detectable echo signal changes: they are usually embedded with large targets of different size and image contrast (contrast target in a background matrix). Contrast target provide a gray scale reference with uncertainty on echo level difference with respect to the background usually over ±1dB and a few points of contrast, moreover they are very expensive. The aim of the present study is the implementation of a novel and less expensive method for the automatic determination of the GSMF and EDR on medical ultrasound scanners: by means of a general purpose ultrasound phantom, the method allows the evaluation of the GSMF by a least squares estimation on the results of the analysis and processing of a series of uncompressed bi-dimensional ultrasound images directly produced by the scanner. Finally EDR has been investigated on 3 medical ultrasound scanners of different technologies and results are proposed and explained.KeywordsDynamic RangeMeasurementUltrasound Scanner