Ultrasound PCS 335

Ultrasound PCS 335

Display of Motion & Doppler Ultrasound Resident Class Hemodynamics Blood Flow Characterization Plug Laminar Disturbed Turbulent 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 obstruction such 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 fluids 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 Same Volume Flow Rate Low Velocity Large Cross-section Volume Flow Rates constant volume flow rate in all parts of closed system

Sure! Any change in flow rate would mean youre 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 I OUT

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 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() = SF / AF Actual flow (AF)

Sensed flow (SF) Doppler Equation 2 X fo X v X cos 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) = angle between flow & sound propagation c = speed of sound in soft tissue (m/s) Relationships 2 X fo X v X cos 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

Relationships 2 X fo X v X cos f D = fe - fo = ------------------------c Doppler angle affects measured Doppler shift cos

Simplified (?) Equation 2 X fo X v X cos f D = fe - fo = ------------------------c 77 X fD (kHz) v (cm/s) = -------------------------Simplified: fo (MHz) X cos Solve for reflector velocity Insert speed of sound for soft tissue Stick in some units Doppler Relationships

77 X fD (kHz) v (cm/s) = -------------------------fo (MHz) X cos higher reflector speed results in greater Doppler shift higher operating frequency results in greater Doppler shift larger Doppler angle results in lower Doppler shift

Continuous Wave Doppler Audio presentation only No image Useful as fetal dose monitor Continuous Wave Doppler 2 transducers used one continuously transmits voltage frequency = transducers 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 - =

Pulse Wave vs. Continuous Wave Doppler Continuous Wave Pulse Wave No Image Image Sound on continuously

Both imaging & Doppler sound pulses generated 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 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 Gate Spectral Display shows range of frequencies received amplitude of each frequency indicated by

gray shade Frequency frequency range can be displayed real time fast Fourier Transform (FFT) technique

Elapsed Time Spectral Broadening display indicates range of frequencies corresponds to range of speeds of blood cells range indicative of type of flow

Frequency frequency range laminar, disturbed, turbulent 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 Dopplers color represents complete spectrum at each pixel Frequency

frequency range 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

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