RFisshortforRadioFrequency.Incommunicationengineering,Theterm“RF-signals”isusedtodenotesignalscontainingfrequencyinformationinthefrequencybandsusedforradiocommunication.ThetermRFhasbeenadoptedbytheultrasoundindustry,whereitisusedasastandardnotationforunprocesseddata,wherethefrequencyinformationisintact.Inultrasoundimaging,thereceivedRFsignalistheoutputofthebeamformer:
Modernultrasoundprobes(linear-andphasedarraytransducers)consistofseveralrectangularelementsofpiezoelectricmaterial.Thepiezoelectricelementsarecapableofconvertingvaryingpressuretoelectricalsignals.
Atreceive,theanalogsignalfromeachindividualprobeelementisfirstamplified(AnalogGain)toensureoptimaluseofthedynamicrangeoftheAnalogtoDigital(A/D)converters.Theanaloggainfactorvarieswithdepthtoamplifysignalsfromdeepregionsmost(TGC,TimeGainCompensation).
Thesampledsignalsaredelayedindividuallytofocusthebeamtoacertaindepthanddirection.Thedelayedsignalsareweightedtoobtainthedesiredapodizationandbeamprofile.Finally,theweightedanddelayedsignalsaresummedinphase,andthisistheRFsignal.
ThesamplingrateoftheRFsignalattheoutputofthebeamformeris20MHz,andtheresolutionis20bits.ThesamplingrateoftheA/Dconvertersis40MHzwitharesolutionof12bits.7bitsareaddedbysumming128channels(128=2^7)andthelastbitisaddedwhenthesamplingrateisreducedfrom40to20MHz.
ItisalsoimportanttonotethatwhenacquiringRF-dataontheSystemFive,theAnalogGainandweightsappliedtotheindividualchannelsarenotaffectedbytheGainknobandTGCslidersonthefrontpanel.Buttheoverallgainisadjustediftransmitpowerischanged.