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Display of Motion & Doppler Ultrasound. Resident Class. Hemodynamics. Plug Laminar Disturbed Turbulent. Blood Flow Characterization. Plug Flow. Type of normal flow Constant fluid speed across tube Occurs near entrance of flow into tube. Laminar Flow. also called parabolic flow
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Display of Motion &Doppler Ultrasound • Resident Class
Hemodynamics • Plug • Laminar • Disturbed • Turbulent Blood Flow Characterization
Plug Flow • Type of normal flow • Constant fluid speed across tube • Occurs near entrance of flow into tube
Laminar Flow • also called parabolic flow • fluid layers slide over one another • occurs further from entrance to tube • central portion of fluid moves at maximum speed • flow near vessel wall hardly moves at all • friction with wall
Flow • Disturbed Flow • Normal parallel stream lines disturbed • primarily forward particles still flow • Turbulent Flow • random & chaotic • individual particles flow in all directions • net flow is forward • Often occurs beyond obstructionsuch as plaque on vessel wall
Flow, Pressure & Resistance • Pressure • pressure difference between ends of tube drives fluid flow • Resistance • more resistance = lower flow rate • resistance affected by • fluid’s viscosity • vessel length • vessel diameter • flow for a given pressure determined by resistance
Flow Variations • pressure & flow in arteries fluctuate with pulse • pressure & flow in veins much more constant • pulse variations dampened by arterial system
Flow Rate Measurements • Volume flow rate • Volume of liquid passing a point per unit time • Example • 100 ml / second
Flow Rate Measurements • Linear flow rate • Distance liquid moves past a point per unit time • Example • 10 cm / second
Flow Rate Measurements Volume Flow Rate = Linear flow rate X Cross Sectional Area
Flow Rate Measurements Volume Flow Rate = Linear flow rate X Cross-sectional Area High Velocity Small Cross-section Low Velocity Large Cross-section Same Volume Flow Rate
Volume Flow Rates • constant volume flow rate in all parts of closed system Sure! Any change in flow rate would mean you’re gaining or losing fluid.
Stenosis • narrowing in a vessel • fluid must speed up in stenosis to maintain constant flow volume • no net gain or loss of flow • turbulent flow common downstream of stenosis
Stenosis • If narrowing is short in length • Little increase in overall resistance to flow • Little effect on volume flow rate • If narrowing is long • Resistance to flow increased • Volume flow rate decreased
Doppler Shift • difference between received & transmitted frequency • caused by relative motion between sound source & receiver • Frequency shift indicative of reflector speed OUT IN
Doppler Examples • change in pitch of as object approaches & leaves observer • train • Ambulance siren • moving blood cells • motion can be presented as sound or as an image
q Doppler Angle • angle between sound travel & flow • 0 degrees • flow in direction of sound travel • 90 degrees • flow perpendicular to sound travel
Flow Components • Flow vector can be separated into two vectors Flow parallel to sound Flow perpendicular to sound
Doppler Sensing • Only flow parallel to sound sensed by scanner!!! Flow parallel to sound Flow perpendicular to sound
Doppler Sensing • Sensed flow always < actual flow Actual flow Sensed flow
Doppler Sensing • cos(q) = SF / AF Actual flow (AF) q Sensed flow (SF) q
Doppler Equation 2 X fo X v X cosq f D = fe - fo = ------------------------- c • where fD =Doppler Shift in MHz fe = echo of reflected frequency (MHz) fo = operating frequency (MHz) v = reflector speed (m/s) q = angle between flow & sound propagation c = speed of sound in soft tissue (m/s) q
Relationships 2 X fo X v X cosq f D = fe - fo = ------------------------- c • positive shift when reflector moving toward transducer • echoed frequency > operating frequency • negative shift when reflector moving away from transducer • echoed frequency < operating frequency q q
cosq q Relationships 2 X fo X v X cosq f D = fe - fo = ------------------------- c • Doppler angle affects measured Doppler shift q
Simplified (?) Equation 2 X fo X v X cosq f D = fe - fo = ------------------------- c • Solve for reflector velocity • Insert speed of sound for soft tissue • Stick in some units 77 X fD (kHz) v (cm/s) = -------------------------- fo (MHz) X cosq Simplified:
Doppler Relationships • higher reflector speed results in greater Doppler shift • higher operating frequency results in greater Doppler shift • larger Doppler angle results in lower Doppler shift 77 X fD (kHz) v (cm/s) = -------------------------- fo (MHz) X cos
Continuous Wave Doppler • Audio presentation only • No image • Useful as fetal dose monitor
Continuous Wave Doppler • 2 transducers used • one continuously transmits • voltage frequency = transducer’s operating frequency • typically 2-10 MHz • one continuously receives • Reception Area • flow detected within overlap of transmit & receive sound beams
- = Continuous Wave Doppler:Receiver Function • receives reflected sound waves • Subtract signals • detects frequency shift • typical shift ~ 1/1000 th of source frequency • usually in audible sound range • Amplify subtracted signal • Play directly on speaker
Doppler Pulses • short pulses required for imaging • minimizes spatial pulse length • optimizes axial resolution • longer pulses required for Doppler analysis • reduces bandwidth • provide purer transmitted frequency • important for accurate measurement of frequency differences needed to calculate speed
Color-Flow Display Features • Imaged electronically scanned twice • imaging scan processes echo intensity • Doppler scan calculates Doppler shifts • Reduced frame rates • only 1 pulse required for imaging • additional pulses required when multiple focuses used • several pulses may be required along a scan line to determine Doppler shift
Gate Duplex Doppler Gates • operator indicates active Doppler region on display • regions are called gates • only sound in gate analyzed for frequency shift • can be isolated based on delay time after pulse
Spectral Display • shows range of frequencies received • amplitude of each frequency indicated by gray shade • can be displayed real time • fast Fourier Transform (FFT) technique frequency range Frequency Elapsed Time
Spectral Broadening • display indicates range of frequencies • corresponds to range of speeds of blood cells • range indicative of type of flow • laminar, disturbed, turbulent frequency range Frequency Time
Pulse Wave Doppler • Allows range selectivity • monitor Doppler shift (frequency difference) at only selected depth(s) • ability to separate flow from >1 vessel or localize flow within vessel
Spectral vs. Color-Flow • spectral Display shows frequency range directly • Color Doppler’s color represents complete spectrum at each pixel frequency range Frequency Elapsed Time
Power Doppler • AKA • Energy Doppler • Amplitude Doppler • Doppler angiography • Magnitude of color flow output displayed rather than Doppler frequency signal • flow direction or different velocities not displayed "Color Power Angio" of the Circle of Willis