Color Doppler Ultrasound - Basic physics

Color Doppler Ultrasound - Basic physics - Easy to Learn

1. Color Doppler Ultrasound (Basic/Physics ) by Syed Yousaf Farooq

2. PRINCIPLE OF DOPPLER ULTRASOUND BASIS – ‘ DOPPLER EFFECT’ • DISCOVERY – Physicist Christian Johann Doppler in 1842. • ‘When a sound source and the reflector are moving toward each other, the sound waves are spaced closer together and reach the reciever at a higher frequency than they were originally emitted.

3. DOPPLER SHIFT • Doppler shift is the difference between the transmitted and received frequencies. • Transmitted and received Frequencies are in the MHz range. Doppler shift frequencies often in audible range.

4. DOPPLER EQUATION • Relationship between Doppler shift frequency (FD) and reflector velocity (v). f o is the ultrasound frequency, or the transmitted beam frequency. v is the reflector velocity (m/s; cm/s). q is the Doppler angle . c is the speed of sound.

5. DOPPLER ULTRASOUND MODES/INSTRUMENTATIONS Continuous wave Doppler. Pulsed wave Doppler. Spectral doppler. Colour doppler. Power doppler.

6. CONTINUOUS WAVE DOPPLER

7. PULSED-WAVE (PW) DOPPLER Uses brief pulses of ultrasound energy using only one crystal.

8. DUPLEX ULTRASOUND SCANNING Duplex ultrasound instruments are real-time B-mode scanners with built-in Doppler capabilities. ● B-mode Imager ( Outline anatomic structures ). ● Pulsed-Doppler ( Flow and movement patterns ).

9. PULSE REPETITION FREQUENCY ( PRF ) No. of pulses transmitted per second. • With increasing scanning depth PRF decreases as more time is needed for the echoes to return. • At a minimum, the PRF must be at least twice the frequency of the Doppler signal to construct the signal successfully. • This sets upper limit to the flow velocities to be accurately recorded • Use higher PRF setting for high flow velocities, low PRF for slow venous flow.

10. A (PRF = 700 Hz) B (PRF = 4500 Hz)

11. Continuous wave (CW) Doppler Pulsed wave (PW) Doppler

12. COLOUR DOPPLER • Based on pulsed doppler technique . • Doppler shifts - converted to colour and the moving blood is displayed in colors that correspond to its velocity and direction. Positive Doppler shifts are encoded as red and negative shifts are encoded as blue. Velocity of the flow is represented in shades of color - faster - brighter

13. COLOR DOPPLER B A

14. • • • • COLOR BOX Operator adjustable area within the US image . Affects Image resolution & quality ( change in box size )- Frame rate decreases with increasing box size . As small & superficial as possible Deep color box – slower PRF results aliasing . COLOR DOPPLER.

15. COLOR BOX

16. COLOR DOPPLER.

17. COLOUR DOPPLER Advantages It provides an overall view of flow in organ or structure . Provides directional information about flow, Provides velocity information about flow and shows turbulent type flow . Limitations • • • • • SEMI QUANTITATIVE Angle dependence Aliasing Artifacts caused by the noise Poor temporal resolution

18. Doppler frequency spectrum display B A, Color Doppler maging B, Doppler frequency spectrum waveform A

19. SPECTRAL DOPPLER • DEPICTION OF DOPPLER SHIFT INFORMATION IN WAVEFORM COLOUR DOPPLER • UTILIZE DOPPLER SHIFT INFORMATION TO SHOW BLOOD FLOW IN COLOR

20. S P E C T R A L D O P P L E R ADVANTAGES depicts quantitaive flow at one site allows calculations of velocity and indices good temporal resolution COLOR DOPPLER ADVANTAGES Overall view of flow Directional information about flow Averaged velocity information about flow

21. OPTIMISATION OF COLOR FLOW DOPPLER EXAMINATION 1. Transducer Frequency.   For superficial structures 7-10MHZ is used. For deep abdominal structures – 3MHZ – 5MHZ is optimal. Choice of transducer frequency is paramount because the intensity of the scattered sound varies in proportion to the 4th power of the Doppler frequency.

22. OPTIMISATION OF COLOR FLOW DOPPLER EXAMINATION 2. Doppler Angle. Strongest signals of Doppler results when the motion is parallel to beam. A Doppler angle of 90 does not display flow because no doppler shift detected.

23. OPTIMISATION OF COLOR FLOW DOPPLER EXAMINATION 3. Sample Volume. • • Ideal sample volume – 2/3rd of the vessel width positioned in the center of the vessel. If sample volume is more: - Spectral broadening If sample volume is less: Measured velocity is too low.

24. OPTIMISATION OF COLOR FLOW DOPPLER EXAMINATION 6. Velocity scale. • • • n Controls the range of frequencies displayed Too high scale- dynamic range is too large and low velocity signals are missed simulating an area of thrombosis If scale is too low the dynamic range is too small to display the high velocity signals accurately resulting i aliasing.

25. REFERENCES Diagnostic Ultrasound, Carol M Rumack and others, 4 th ed. Grainger & Allison’ s Diagnostic Radiology- A Textbook of Medical Imaging, 6 th ed. Teaching Manual of Color Duplex Sonography,2 nd ed. Frederick W. Kremkau, Diagnostic Ultrasound - Principles and Instruments, 7th Edition, St. Louis,-Missouri, Saunders Elsevier, 2006 Rubin JM ,Bude RO CarsonPL et al ;Power Doppler US; A potentiallyuseful alternative to to mean frequency based color Doppler US.

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