1. Ultrasound Physics Image Formation ‘97

2. Real-time Scanning Each pulse generates one line Except for multiple focal zones frame one frame consists of many individual scan lines lines frames PRF (Hz) = ------------ X -------------- frame sec. One pulse = one line

3. Multiple Focal Zones Multiple pulses to generates one line Each pulse generates portion of line Beam focused to that portion 1 st focal zone 2nd focal zone 3rd focal zone

4. M Mode Multiple pulses in same location New lines added to right horizontal axis elapsed time (not time within a pulse) vertical axis time delay between pulse & echo indicates distance of reflector from transducer Elapsed Time Each vertical line is one pulse Echo Delay Time

5. M-Mode (left ventricle)

6. Scanner Processing of Echoes Amplification Compensation Compression Demodulation Rejection Amplification Compensation Compression Demodulation Rejection

7. Amplification Increases small voltage signals from transducer incoming voltage signal 10’s of millivolts larger voltage required for processing & storage Amplifier

8. Compensation Amplification Compensation Compression Demodulation Rejection Amplification Compensation Compression Demodulation Rejection

9. Need for Compensation equal intensity reflections from different depths return with different intensities different travel distances attenuation is function of path length Display without compensation time since pulse echo intensity

10. Equal Echoes Voltage Amplification Voltage Amplitude after Amplification Equal echoes, equal voltages Later Echoes Early Echoes Voltage before Compensation Time within a pulse

11. Compensation (TGC) Body attenuation varies from 0.5 dB/cm/MHz TGC allows manual fine tuning of compensation vs. delay TGC curve often displayed graphically

12. Compensation (TGC) TGC adjustment affects all echoes at a specific distance range from transducer

13. Compression Amplification Compensation Compression Demodulation Rejection Amplification Compensation Compression Demodulation Rejection

14. Challenge Design scale that can weigh both feather & elephant

15. Challenge Re-Stated Find a scale that can tell which feather weighs more & which elephant weighs more

16. Compression 1,000 1101001000 3 = log 1000 1101001000 2 =log 100 1 = log 10 0 = log 10 100,000 10,000 1,000 100 10 1 5 4 3 2 1 0 InputLogarithm Can’t easily distinguish between 1 & 10 here Difference between 1 & 10 the same as between 100 & 1000 Logarithms stretch low end of scale; compress high end

17. Demodulation Amplification Compensation Compression Demodulation Rejection Amplification Compensation Compression Demodulation Rejection

18. Demodulation & Radio Any station (frequency) can carry any format

19. Demodulation Intensity information carried on “envelope” of operating frequency’s sine wave varying amplitude of sine wave demodulation separates intensity information from sine wave

20. Demodulation Sub- steps rectify turn negative signals positive smooth follow peaks

21. Rejection Amplification Compensation Compression Demodulation Rejection Amplification Compensation Compression Demodulation Rejection

22. also known as suppression threshold object eliminate small amplitude voltage pulses reason reduce noise electronic noise acoustic noise noise contributes no useful information to image Amplitudes below dotted line reset to zero

23. Image Resolution Detail Resolution spatial resolution separation required to produce separate reflections Detail Resolution types Axial Lateral

24. Resolution & Reflector Size minimum imaged size of a reflector in each dimension is equal to resolution Objects never imaged smaller than system’s resolution

25. Axial Resolution minimum reflector separation in direction of sound travel which produces separate reflections spatial pulse length depends on spatial pulse length Distance in space covered by a pulse HEY H.......E.......Y Spatial Pulse Length

26. Axial Resolution Separation just greater than half the spatial pulse length Gap; Separate Echoes Axial Resolution = Spatial Pulse Length / 2

27. Axial Resolution Separation just less than half the spatial pulse length Overlap; No Gap; No Separate Echoes Axial Resolution = Spatial Pulse Length / 2

28. Improve Axial Resolution by Reducing Spatial Pulse Length increase frequency Decreases wavelength decreases penetration; limits imaging depth Reduce cycles per pulse requires damping reduces intensity increases bandwidth Spat. Pulse Length = # cycles per pulse X wavelength Speed = Wavelength X Frequency

29. Lateral Resolution Definition minimum separation between reflectors in direction perpendicular to beam travel which produces separate reflections when the beam is scanned across them Lateral Resolution = Beam Diameter

30. Lateral Resolution if separation is greater than beam diameter, objects can be resolved as two reflectors

31. Lateral Resolution Complication: beam diameter varies with distance from transducer Near zone length varies with Frequency transducer diameter Near zone length Near zone Far zone

32. Contrast Resolution

33. difference in echo intensity between 2 echoes for them to be assigned different digital values 89 88

34. Pre-Processing Assigning of specific values to analog echo intensities analog to digital (A/D) converter converts output signal from receiver (after rejection) to a value 89

35. Gray Scale the more candidate values for a pixel the more shades of gray image can be stored in digital image The less difference between echo intensity required to guarantee different pixel values See next slide

36. 1 2 3 4 5 6 7 1 2 6644 53237764 25 52 1 2 3 4 5 6 7 2 4 11 78 10636 14 116 48 124 8 9 10 11 12 13 14

37. Display Limitations not possible to display all shades of gray simultaneously window & level controls determine how pixel values are mapped to gray shades numbers (pixel values) do not change; window & level only change gray shade mapping 17 = 65 = Change window / level 17 = 65 =

38. Presentation of Brightness Levels pixel values assigned brightness levels pre-processing manipulating brightness levels does not affect image data post-processing window level 125 25311111182222176 199192 85 69133149112 77103118139154125120 145301256223287256225 178322325299353333300