Doppler

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Doppler and Duplex:

Dr. Varun Babu MD Resident Dept. of Radiology AIMS, Cochin Doppler and Duplex June 7, 2011

Doppler Effect:

Change in the perceived frequency of sound emitted by a moving source Christian Doppler, 1843 Doppler Effect

Doppler Shift:

Doppler Shift

Doppler Shift:

Doppler Shift

Doppler Effect:

Employs an audio mode Used to study motion, primarily of the circulatory system 2 transducers used – a transmitter and a receiver Doppler Effect

Continuous Wave Doppler:

2 piezoelectric crystals 3 – 8 Mhz Doppler shift : frequency of initial signal algebraically subtracted from that of the returning echoes Returning signals – cellular elements of blood Doppler shift is greatest when beam strikes a vessel at an acute angle Δ ν = 2 ν s cos θ /v where ν = freq. of initial beam s = blood velocity v = sound velocity Δ ν = Doppler shift θ = angle btw sound beam & direction of blood flow Continuous Wave Doppler

Continuous Wave Doppler:

Continuous Wave Doppler

Continuous Wave Doppler - Transducers:

Continuous Wave Doppler - Transducers

Continuous Wave Doppler - Transducers:

Continuous Wave Doppler - Transducers High Q material As much of generated power must be transmitted to patient Backing: done using Air Quarter wave matching Why use high frequency? Scatter principle More sharply defined Larger doppler shift Superficial vessels (5-8 MHz); carotids (5 MHz) Low frequency Decreases amplitude attenuation of the beam in body tissues Deep arteries and veins

Continuous Wave Doppler:

Flow of blood Towards the transducer: Δ ν is positive Away from the transducer: Δ ν is negative Doppler shift (Δ ν ) depends on Velocity of blood flow Frequency of the transmitter ultrasound beam Ultrasound frequency range of 2 – 10 Mhz range, Δ ν will range from 0 – 10 KHz for velocities ranging 0 – 100 cm/s Continuous Wave Doppler

Pulsed Doppler:

Detects the depth at which a returning signal has originated Depth determined by ‘Time of Flight’ By ‘gating’ on the receiving transducer for a short period at a specified time following transmission of the burst, only Doppler signals originating from a specific depth are recorded Pulsed Doppler

Pulsed Doppler:

Pulsed Doppler Speed of sound in tissues = 1540 m/s = 154,000 cm/s Sound travels 1 cm in soft tissue in 6.5 μ sec. A round trip = 13 μ sec i.e., the required time between successive pulses is at least 13 μ sec per cm of range in the sample volume

Pulsed Doppler and ‘Duplex’:

Detection of rapidly moving structures (in heart) High pulse repetition frequency Low ultrasound frequency Steeper θ angle Duplex Combining pulsed doppler with real time imaging or any other display mode A continuous wave doppler – identifies the blood flow in the vessel and the pulsed doppler unit images blood flow through various levels of the vessel Pulsed Doppler and ‘Duplex’

Interpreting a Doppler Signal:

Frequency on the vertical axis and time on the horizontal axis By convention, flow towards the transducer is displayed above the baseline Simple observations permit diagnosis and grading of disease Interpreting a Doppler Signal

Pulsatility Measurements:

Independent of beam/vessel angle Resistance Index(RI) Max. velocity – min. velocity /max. velocity Sensitive to changes in downstream flow resistance Diastolic flow in low resistance vascular beds Pulsatility Index (PI) Max. velocity – min. velocity/time – avg. max. velocity over a cardiac cycle Devised to characterize triphasic flows seen in femoral artery Pulsatility Measurements

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