Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

(3) Demodulation and"Envelope" detectionFIGURE 16-29. The receiver processes thedata streaming from the beam former. Stepsinclude time gain compensation (TGC),dynamic range compression, rectification,demodulation, and noise rejection. The userhas the ability to adjust the TGC and thenoise rejection level.1. Gain adjustments and dynamic frequency tuning. Time gain compensation(TGC) is a user-adjustable amplification of the returning echo signals as a functionof time, to further compensate for beam attenuation. TGC (also known astime varied gain, depth gain compensation, and variable swept gain) can bechanged to meet the needs of a specific imaging application. The ideal TGCcurve makes all equally reflective boundaries equal in signal amplitude, regardlessof the depth of the boundary (Fig. 16-30). Variations in the output signalsare thus indicative of the acoustic impedance differences between tissue boundaries.User adjustment is typically achieved by multiple slider potentiometers,where each slider represents a given depth in the image, or by a three-knob TGCcontrol, which controls the initial gain, slope, and far gain of the echo signals.For multielement transducers, TGC is applied simultaneously to the signal fromeach of the individual elements. The TGC amplification further reduces themaximum to minimum range of the echo voltages as a function of time toapproximately 50 dB (300: 1) from the ~60dB range after preprocessing.Dynamic frequency tuning is a feature of some broadband receivers thatchanges the sensitivity of the tuner bandwidth with time, so that echoes fromshallow depths are tuned to a higher frequency range, while echoes from deeperstructures are tuned to lower frequencies. The purpose of this is to accommodatefor beam softening, where increased attenuation of higher frequencies in a broadbandwidth pulse occur as a function of depth. Dynamic frequency tuning allowsthe receiver to make the most efficient use of the ultrasound frequencies incidenton the transducer.2. Dynamic range compression. Dynamic range defines the effective operationalrange of an electronic device from the threshold signal level to the saturationlevel. Key components in the ultrasound detection and display that are mostaffected by a wide dynamic range include the ADC and the display. For receiver