Speckle Suppression in Ultrasonic Images Based on Undecimated Wavelets
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Speckle Suppression in Ultrasonic Images Based on Undecimated Wavelets Fabrizio Argenti Dipartimento di Elettronica e Telecomunicazioni, Universit`a di Firenze, Via di Santa Marta 3, 50139 Firenze, Italy Email: [email protected]
Gionatan Torricelli Dipartimento di Elettronica e Telecomunicazioni, Universit`a di Firenze, Via di Santa Marta, 3, 50139 Firenze, Italy Email: [email protected] Received 11 February 2002 and in revised form 29 October 2002 An original method to denoise ultrasonic images affected by speckle is presented. Speckle is modeled as a signal-dependent noise corrupting the image. Noise reduction is approached as a Wiener-like filtering performed in a shift-invariant wavelet domain by means of an adaptive rescaling of the coefficients of an undecimated octave decomposition. The scaling factor of each coefficient is calculated from local statistics of the degraded image, the parameters of the noise model, and the wavelet filters. Experimental results demonstrate that excellent background smoothing as well as preservation of edge sharpness and fine details can be obtained. Keywords and phrases: ultrasound image denoising, speckle filtering, linear minimum mean square error filtering, undecimated discrete wavelet transform.
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INTRODUCTION
Since the introduction of first coherent imaging systems, speckle noise has been widely studied. Speckle makes a homogeneous object to assume a granular appearance, and consequently, the contrast of the image is drastically reduced. The presence of a speckle pattern in a coherently formed image is due to the received backscatter signal from unresolvable particles constituting the inspected mean. Particular attention has been reserved to speckle noise in ultrasonic images since the degradation in the acquired image implies strong uncertainties in the detection of pathologies performed by an expert human observer. The texture of the speckle pattern tends also to hide fine details useful for computer-aided diagnosis. Moreover, it severely decreases the effectiveness of image postprocessing algorithms. The theoretical foundations of speckle were given in optics, where laser holographic image formation has been studied [1]. By using a laser as a monochromatic coherent radiation, it was possible to reconstruct the inspected object by using the backscattered signal. The signal statistical properties obtained by theoretical analysis have been validated in many other imaging systems using coherent radiation, like radar and ultrasound, even if, in these cases, the representation of the image obtained by envelope detection is poorer due to the propagation of the radiation through an inhomogeneous
medium. Ultrasound images represent the worst case since the ultrasonic wave encounters multiple interfaces that implies a masking effect for those reflectors laying farther from the probe. Both phase and amplitude (speckle) noise degrade the backscattered signal. Phase aberration may occur because of the imperfections of the focusing system that is realized by means of a de
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