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TABLE 16-6. TYPICAL PRF, PRp, AND DUTY CYCLE VALUES FORULTRASOUND OPERATION MODESPRF (Hz)PRP (f!sec)M-modeReal-timePulsed Doppler5002,000-4,0004,000-12,0002,000500-250250-830.050.2-0.40.4-1.2A 500 Ilsec PRP corresponds to a PRF of 2 kHz and a maximal range of 38.5cm. For a PRP of250 Ilsec (PRF of 4 kHz), the maximum depth is halved to 19.3cm. Higher ultrasound frequency operation has limited penetration depth, allowinghigh PRFs. Conversely, lower frequency operation requires lower PRFs becauseechoes can return from greater depths. Ultrasound transducer frequency should notbe confused with pulse repetition frequency, and the period of the sound wave (I If)should not be confused with the pulse repetition period (I/PRF). The ultrasoundfrequency is calibrated in MHz, whereas PRF is in kHz, and the period of the ultrasoundis measured in microseconds compared to milliseconds for the PRP.Pulse duration is the ratio of the number of cycles in the pulse to the transducerfrequency, and is equal to the instantaneous "on" time. A pulse consisting of twocycles with a center frequency of 2 MHz has a duration of 1 IlseC. Duty cycle, thefraction of "on" time, is equal to the pulse duration divided by the pulse repetitionperiod. For real-time imaging applications, the duty cycle is typically 0.2% to 0.4%,indicating that greater than 99.5% of the scan time is spent "listening" to echoes asopposed to producing acoustic energy. Intensity levels in medical ultrasonographyare very low when averaged over time, as is the intensity when averaged over spacedue to the collimation of the beam.For clinical data acquisition, a typical range of PRF, PRP, and duty cycle valuesare listed in Table 16-6.In multielement array transducers, all preprocessing steps are performed in parallel.Each transducer element produces a small voltage proportional to the pressureamplitude of the returning echoes. An initial preamplification increases the detectedvoltages to useful signal levels. This is combined with a fixed swept gain (Figs. 16-26 and 16-28), to compensate for the exponential attenuation occurring with distancetraveled. Large variations in echo amplitude (voltage produced in the piezoelectricelement) with time are reduced from ~1,000,000:1 or 120 dB, to about1,000: 1 or 60 dB with these preprocessing steps.Early ultrasound units used analog electronic circuits for all functions, whichwere susceptible to drift and instability. Even today, the initial stages of the receiveroften use analog electronic circuits. Digital electronics were first introduced inultrasound for functions such as image formation and display. Since then, there hasbeen a tendency to implement more and more of the signal preprocessing functionsin digital circuitry, particularly in high-end ultrasound systems.