1TT Liu, BE280A, UCSD, Fall 2005Bioengineering 280APrinciples of Biomedical ImagingFall Quarter 2005Ultrasound Lecture 3TT Liu, BE280A, UCSD, Fall 2005 Doppler Effecthttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm2TT Liu, BE280A, UCSD, Fall 2005 Doppler Effect € Δf =2vf0c − v≈2vf0cExamplev = 50 cm/sc = 1500 m/sf0 = 5 MHz2vf0c= 3333 HzTT Liu, BE280A, UCSD, Fall 2005Dopplerhttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm3TT Liu, BE280A, UCSD, Fall 2005CW Dopplerhttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htmTT Liu, BE280A, UCSD, Fall 2005CW Dopplerf0f0+Δf-f0-f0-Δf*Effective width = 1/Tobs4TT Liu, BE280A, UCSD, Fall 2005CW Doppler € Resolution Δf =1/TobsΔv =cΔf2 f0=c2Tobsf0ExampleDesign goal : Δv = 5 cm /s; f0= 5 MHzTobs=c2Δvf0=1500m /s2 0.05m / s( )5 ×106( )= 3msNote that for a depth of 15 cm, it takes only200 usec for echos to return.TT Liu, BE280A, UCSD, Fall 2005PW Dopplerhttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm5TT Liu, BE280A, UCSD, Fall 2005PW Dopplerhttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htmTT Liu, BE280A, UCSD, Fall 2005Siemens Medical Systems; jnormal common carotid artery6TT Liu, BE280A, UCSD, Fall 2005Aliasing€ Measure Doppler shifts at a specified rangeFor unambiguous range, one pulse at a time.TPR=2rmaxc (e.g. 200 usec for 15 cm depth)To avoid aliasing require1TPR> 2ΔfmaxΔf-Δf*1/TPR=Δf-ΔfAliasedΔf-ΔfTT Liu, BE280A, UCSD, Fall 2005Aliasinghttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm7TT Liu, BE280A, UCSD, Fall 2005Aliasinghttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htmTT Liu, BE280A, UCSD, Fall 2005Aliasinghttp://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos-html/chapter_01.htm8TT Liu, BE280A, UCSD, Fall 2005PW Doppler€ Velocity Resolution (same as with CW) Tobs>1Δf=c2Δvf0Range ResolutionWant to interrogate velocities from a small region Δz =cTpulse2We also need to make sure that particles remain within this regionover the observation time TobsvmaxTobs< Δz ⇒ Tobs<Δzvmax=cTpulse2vmaxTT Liu, BE280A, UCSD, Fall 2005PW Doppler€ Design ExampleRmax= 6 cm ⇒ TPR=2(0.06m)1500m /s= 80 µsec1TPR> 2Δfmax=4vmaxf0cc4TPRf0> vmax⇒ for f0= 5MHz we find that vmax< 93.75cm /sIf we choose Δv = 1cm / s then Tobs=c2Δvmaxf0= 15msRange resolution : Δz > vmaxTobs= 1.4cmTpulse=2Δzc=
View Full Document
Unlocking...