Back-end Scan Mode Subsystem in Ultrasound Technique

In the back-end scan mode subsystem, clinically useful information is extracted from echo signal. For B-mode imaging, an envelope detector, a logarithmic (LOG) compressor , an image processor and a digital scan converter (DSC) are involved. Typically, clinical meaningful information about tissues is contained in the envelope variation of echo signal arising from different tissues. An envelope detector performs the function of removing the carries signal are logarithmically compressed in a LOG compressor foe efficient visualization. Transducers and the analog receiver block respond to a large range of amplitude of echo signals, which is usually over 100dB. The function before a LOG compressor should have the capability to deal with this large dynamic range in order to receive very weak signals attenuated from objects positioned at a depth of an imaging plane. On the order hand, the dynamic range of display is around 40 dB. However , the clinically meaningful amplitude variations of echo signals are least 60 dB, so that it cannot be directly displayed on a monitor without information loss(Zagsebski, 1996). A Log compressor is want to overcome this problem. Small amplitude signals are raised by reducing the large dynamic range the LOG compressor, thus being accentuated on a display device allowing the retention of clinically useful information.

 After the log compression, an image processor carries out focal zone blending, edge enhancement, auto gain control(AGC), black hole/noise spike filling, lateral filtering and persistence in order to achiever high image quality.(Phelp et al,2004). These method are employed in high-end ultrasound imaging system ti generate the best possible images with superior contras , spatial resolution and image uniformity (Szabo, 2004). The manipulated echosignals are mapped onto pixels of a monitor following echo signals cannot be directly mapped onto pixel of monitor following echo amplitude vs gray scale conversion. However each samples point of echo signals cannot be directly mapped each pixel sometimes because its spacial location does not corresponded to a pixel. This mismatching problem is especially serous in the sector scanning since samples are acquired in a polar coordinate system. Contrary to pixels which arrangement in a Cartesian coordinate system. Under this circumstances, wherefores, scan conversion processing is necessary to find appropriate pixel values from echo samples through coordinate transforamtion and data interpolation.