Modeling Speckles for Ultrasound Imaging
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Ultrasound has been used in a variety of clinical settings, including obstetrics and gynecology, cardiology and cancer detection. The main advantage of ultrasound is that, unlike x-ray imaging, it does not require ionizing radiation, which may increase the risk of getting cancer. In addition, using ultrasound imaging as a medical diagnostic tool is more cost effective compared to other radiation free diagnostic techniques, such as the MRI. However, a difficult and crucial challenge in ultrasound medical imaging is the necessity of reducing the appearance of speckles in B-scan images. Speckles are caused by coherent interference of reflected ultrasound waves by structures smaller than the Rayleigh scattering limit. Appearance of speckles tend to reduce the perception of small structures, and ultimately limit diagnostic accuracy of medical imaging system. Since it is difficult to model speckles using architectural structure of tissues, modeling techniques involve a more constructive approach: predicting the speckle pattern of an arbitrary scattering medium for a given transducer geometry; such a model was developed by Foster et al. (1983). Investigation of the origin and nature of speckles involve creating a mathematical model to predict the RF output signal waveform of a particular transducer geometry, and then extending that model to incorporate the effects of varying sizes of scatterers on the RF voltage trace. The complete model will then be verified by comparing theoretical results with experimental results obtained from pulse-echo experiments. For this project, we have focused more on the experimental aspect of the investigation by producing B-mode images of scatterer phantoms. These phantoms are standard samples made using the discrete scatterer model, where scatterers of uniform shape and size are embedded in a homogeneous medium. Before imaging scatterers smaller than the Rayleigh scattering limit, it is important to ensure that scatterers much larger than the Rayleigh limit (discrete scatterers) can be successfully imaged. We conducted pulse-echo experiments using a 5 MHz single element focused transducer, and imaged phantoms containing discrete scatterers of the following sizes: 4.76 mm metal beads, 2.85 mm 3.45mm glass beads and 1.00 mm 1.03 mm glass beads. The RF voltage traces from the pulse-echo experiments were then converted to B-mode images using a Matlab program.