Noise power spectrum of ultrasound uniformity images

Abstract

Background The Noise Power Spectrum (NPS) is an effective tool for providing information on how the total variance of uniformity images is distributed in frequency space. The NPS is a tool which can widen the understanding of noise in ultrasound as noise in this imaging modality includes both electronic noise and static structural noise. However, surprisingly, very little research is found in the literature and unlike projection X-Ray Imaging and CT, NPS is not included in ultrasound QC protocols. Objectives (1) To establish a scan protocol to measure the NPS for ultrasound scanners in the case of linear transducers; (2) To specify an IQWorks analysis tree for the calculation of All Noise, Stochastic and Static NPS for Regions of Interest (ROIs) at near field, focal region and far field; (3) To Investigate the short term reliability of the methodology. Research Design The protocol was tested on a Siemens ACUSON Sequoia using a linear transducer and two phantoms (Gammex RMI404GS LE, CIRS Model 551). An IQWorks analysis tree was developed for the calculation of NPS of ultrasound uniformity images. Three ROIs were selected at near field, focal region and far field to evaluate the mean pixel value and image variance of the ROIs, Stochastic Integral NPS, Static Integral NPS, All Noise Integral NPS and the shape of one-dimensional NPS at different depths. The short term reliability of the methodology was checked by measuring the coefficient of variation of the mean pixel value, image variance and NPS values over repeat acquisitions. Results The NPS in ultrasound is mainly of a low-frequency component. There are interesting changes of the NPS at different depths. The NPS tool can be used as part of an ultrasound QC programme using the NPS IQWorks analysis tree specified in this project.