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1、Doppler PrinciplesColor PrinciplesThe DifferencesIn Grayscale,strong echo reflections cause pixels to be displayed.In Color and Doppler,changes in frequency cause pixels to be colored.A Doppler ProcessorThe Doppler processor puts a dot on the“spectrum”based on the velocity of a moving object.Doppler
2、 ProcessorWhen there are objects moving at different speeds,the Doppler processor displays a dot for each object.Doppler ProcessorThe vertical position represents the objects velocity.Doppler ProcessorWhen several objects move at the same velocity,the corresponding dot on the spectrum is brighter.Do
3、ppler ProcessorDoppler InformationVelocityTurbulenceIntensityThere are 3 key pieces of information in a Doppler signal In general,flow toward the transducer will be above the baseline,and flowaway from the transducer will be below the baseline.Pop Quiz1.Describe 3 key differences between the two Dop
4、pler Waveforms below.2.Match the waveform with the sample volume position.AB21The Angle ProblemIf the flow isnt parallel to the ultrasound beam,the ultrasound system only sees a portion of the velocity.Actual VelocityApparent VelocityThe Angle ProblemThe transducer only“sees”a portion of the total v
5、elocity.The Angle ProblemOn the other side,the apparent direction appears to be opposite,even though the flow direction is the same!The Angle ProblemAnd in the center.there is NO apparent flow.Doppler processors cannot see flow perpendicular to the ultrasound beam.The Doppler EquationDont memorize t
6、his.But remember a couple of rules of thumb:WHATSO WHATThe apparent velocity is You wont get much flow whendependent on the angle()the angle is near 90The higher the transmit Lower frequency probes are frequency(ft),the lower the used to measure higherdetected velocityvelocitiesv=fd c2 ft cos The Al
7、iasing ProblemWhat if you checked your clock every 5 seconds.what would you see on the second hand?1234The Aliasing ProblemWhat happens if you look at the same clock every 55 seconds?111098Pop QuizHow often do you have to look at the clock to correctly determine the direction of the second hand?12Po
8、p QuizHow often do you have to look at the clock to correctly determine the direction of the second hand?126Every 29.99999 secondsPop QuizHow fast can the clock be,if youre going to accurately demonstrate its direction when you look at it every 15 seconds?Pop QuizHow fast can the clock be,if youre g
9、oing to accurately demonstrate its direction when you look at it every 15 seconds?60 sec.clockPop QuizHow fast can the clock be,if youre going to accurately demonstrate its direction when you look at it every 15 seconds?60 sec.clock50 sec.clockPop QuizHow fast can the clock be,if youre going to accu
10、rately demonstrate its direction when you look at it every 15 seconds?60 sec.clock50 sec.clock40 sec.clockPop QuizHow fast can the clock be,if youre going to accurately demonstrate its direction when you look at it every 15 seconds?60 sec.clock50 sec.clock40 sec.clock30 sec.clock?Pop QuizHow fast ca
11、n the clock be,if youre going to accurately demonstrate its direction when you look at it every 15 seconds?No faster than one rotation every 30 seconds.60 sec.clock50 sec.clock40 sec.clock30 sec.clock20 sec.clock?Pop QuizWhat if you looked at the clock every 10 seconds?Pop QuizWhat if you looked at
12、the clock every 10 seconds?No faster than one rotation every 20 seconds.The faster you sample,the faster the clock can be.50 sec.clock40 sec.clock30 sec.clock20 sec.clock15 sec.clock?The Aliasing ProblemPulsed Doppler has the limitation that the maximum velocity is determined by the“sampling rate.”T
13、he sampling rate is known as the PRF,pulse repetition frequency.If the velocity is too high,the flow will appear to go backwards,just like the clock.Doppler SummaryThree Key Pieces of Information in a Doppler SignalVelocityIntensityTurbulenceIn general,flow toward the transducer is above the baselin
14、e,flow away is below the baseline.Major limitation is maximum velocity,which is based on frequency and sampling rate.2nd limitation is the angle of flow relative to the transducer.Color DopplerColor Doppler uses the same principals.The system acquires a Doppler Spectrum from many points in the image
15、.For each point,the system determines the average velocity.The average velocity is converted to a color.Generally RED toward the transducer and BLUE away.Anatomy of a Color Flow ImageThe result is an image with colors representing the average velocities.General directionof ultrasound beamAnatomy of
16、a Color Flow ImageThe result is an image with colors representing the average velocities.General directionof ultrasound beamThis area is blue,soflow is generally awayfrom the beam,left to rightRABTAnatomy of a Color Flow ImageThe result is an image with colors representing the average velocities.Gen
17、eral directionof ultrasound beamThis area is red,so flow is away the beam,right to leftThis area is blue,soflow is generally towardsfrom the beam,left to rightRABTDealing with the Doppler ProblemsWhat is going on here?Why is the blood flow multi-colored,rather than all red or all blue?Color Flow has
18、 Dopplers LimitationsOf the three pieces of information from a Doppler signal,CFM uses only the VELOCITY information.VelocityIntensityTurbulenceAnd has the same limitations:Aliasing:Maximum velocity limited by PRFDependent on angle of flowFrame Rates are a problem too!Since Color Flow requires lots
19、of Doppler signals,the frame rates can be slow.Your only choices are:Do less color flow(make the color box smaller)CheatL500:Parallel ProcessingL400:Automatic Control TechnologyDealing with the Doppler ProblemsWhat is going on here?Where is the flow velocity the highest?A?B?C?ABCDealing with the Dop
20、pler ProblemsThe flow velocity is the same in the vessel,but the angle makesit look like the blood is accelerating from A to B to C.What is going on here?Where is the flow velocity the highest?A?B?C?ABCDealing with the Doppler ProblemsWhat is going on here?Why is the blood flow multi-colored,rather
21、than all red or all blue?Dealing with the Doppler ProblemsWhat is going on here?Why is the blood flow multi-colored,rather than all red or all blue?Its a combination of changing angles and aliasing.Dealing with the Doppler ProblemsRemember this?What if we used Intensity instead of Velocity to map th
22、e flow information?VelocityTurbulenceIntensityDealing with the Doppler ProblemsVelocity asks“how fast is it going?”Velocity is subject to angle problems.Apparent velocity changes with angle.Velocity is subject to aliasing.Depends on PRF.Intensity asks“how many are going?”Intensity is not subject to
23、angle problems.How many doesnt change with angle.Intensity is not subject to aliasing.How many doesnt change with sampling rate.Dealing with the Doppler ProblemsThe result is color flow that is not angle dependent,and not subject to aliasing.We call it Power Doppler Imaging(PDI).It appears to be mor
24、e sensitive(no zero angle to worry about).SummaryDoppler provides 3 key types of informationVelocityIntensityTurbulenceVelocity information is subject to angle and aliasing problems.Color Doppler maps average velocity to a color,so it has aliasing and angle problems too.Power Doppler maps intensity to a color,so it does not have aliasing nor angle problems.Frame Rate will be a problem for CFM&PDI unless you have a smart color processor.