Pulse Echo Imaging Systems

1. Pulse Echo Imaging Systems

2. Now we are going to learn what must happen to the echoes returning to the transducer to give us a visual display To date, we have discussed: • Behavior of sound waves • Interaction with tissue

3. Pulse Echo Imaging Systems • Integrated components that permit the reception & image display of the echo voltages from the transducer • Use the returning echoes • Strength • Direction • Arrival time

4. DISPLAY PROCESSOR DAC I MAGE POST PROCESSOR IMAGE MEMORY PROCESSOR PRE-PROCESSOR SCAN CONVERTER S I GNAL PROCESSOR COMPRESSION DETECTION FILTER FORMER SUMMER BEAM ECHO DELAYS PULSER ADCS PULSE DELAYS AMPS T/R

5. Pulse Echo Imaging Systems • beam former • signal processor • image processor • display

6. Beam Former Responsible for: • Electronic beam scanning • Steering, focusing • Apodization • Aperture functions with arrays

7. Functions of a Beam Former • generate voltage that drive the transducer • determine PRF, coding, frequency, and intensity • scan, focus and apodize the transmitted beam • amplify the returning echo voltages • compensate the attenuation • digitize the echo voltage stream • direct, focus and apodize the reception beam • sends the digitized echo voltages to the signal processor

8. Beam Former consists of: • Pulser • Pulse delays • Transmit/receive switch • Amplifiers • Analog-to-digital converters • Echo delays  • Summer

9. Pulser • Produces electric voltages to drive the transducer forming the beam • Transducer then produces US pulsesthat travel into the patient • Initial voltages are 1-2 cycles of electric pulses

10. Pulser • Voltage pulse frequency determines resulting US transducer pulse frequency • Pulse range: 4 - 15 kHz • PRF is the # of voltage pulses/sec sent to the transducer an = # US pulses/sec • US PRF = voltage PRF • 1 US pulse is produced from each voltage pulse

11. High PRF is wanted to receive display information at a rapid rate, however, it must be restricted enough to provide proper display of returning echoes to avoid echo misplacement. Echoes for deeper structures take longer to return & that all the returning echoes from 1 pulse must be received before the next pulse is emitted if echo misplacement is to be avoided. SO… Reducing the PRF & the # of images generated each second is required

12. Output Power Control • Transducer’s intensity is increased by  the voltage to the transducer with the output power control (aka – overall power, power, output, input) •  the output power increases the amplitude of the transmitted sound beam & the received echoes •  the output by 3 dBs, doubles the intensity of the sound beam & increases the ultrasonic exposure to the patient!

13. INPUT

14. Output Power Control • The greater the voltage amplitude produced by the pulser = the greater the US pulse intensity produced by the transducer & ranges up to 100 V • Output level shown on the display in a percentage or decibels relative to maximum (100% or 0 dB) output

15. Output Power Control  output by 3 dBs = ½ the sound beam intensity • Lower output reduces received echo amplitude. •  receiver gain will compensate for this •  intensity =  patient bioeffects

16. Example Reducing output by 50% (-3 dB) with a 5-MHz transducer = a reduction in penetration by 5% (i.e. 12.0 to 11.4 cm) or the difference shown by the 2 arrows

17. Changing the output power changes the biological effects that occur in tissue For this reason, use minimum output power & maximum receiver gain

18. Beam Former consists of: • Pulser • Pulse delays • Amplifiers • Transmit/receive switch • Analog-to-digital converters • Echo delays  • Summer

19. Pulse Delays - performs the sequencing & phasing for beam scanning, steering, focusing, aperture, and apodization of array transducers

20. Pulse Delays • Receives 1 input from the pulser but provides multiple outputs to the transducer elements • Each elements in the array needs a different delay to form the ultrasound beam • Each delay & element combination is called a transmission channel •  the # of channels allows more precise control of beam characteristics

21. DISPLAY PROCESSOR DAC I MAGE POST PROCESSOR IMAGE MEMORY PROCESSOR PRE-PROCESSOR SCAN CONVERTER S I GNAL PROCESSOR COMPRESSION DETECTION FILTER FORMER SUMMER BEAM ECHO DELAYS PULSER ADC PULSE DELAYS AMP T/R

22. Beam Former consists of: • Pulser • Pulse delays • Transmit/receive switch • Amplifiers • Analog-to-digital converters • Echo delays  • Summer

23. T/R Switch (transmit/receive) • Directs the driving voltages from the pulser & pulse delays to the transducer during transmission • Directs the returning echo voltages from the transducer to the amplifiers during reception • Protects sensitive amplifiers input components from the large driving voltages of the pulser

24. Beam Former consists of: • Pulser • Pulse delays • Transmit/receive switch • Amplifiers • Analog-to-digital converters • Echo delays  • Summer

25. Amplifiers -increases amplitude • Converts small voltages received from the transducer elements to larger ones usable for processing and storage • This amplification is called gain • 1 amplifier for each channel in the beam former

26. Gain - ratio of amplifier output power to input power (output power ÷ input power) Power ratio = voltage ratio2 & is in dB Example: - input voltage amplitude is 2 mV & output voltage amplitude is 200 mV voltage ratio = 200/2, or 100. The power ratio is (100)2 = 10,000 = a gain of 40 dB

27. Gain Control • Determines amount of amplification that occurs in the amplifier • Too little gain - weak echoes are not imaged • Too much gain, saturation occurs; i.e. - echoes appear bright & differences in echo strength are not seen

28. GAIN

29. GAIN

30. Amplifiers • Have 60 - 100 dB range of gain • Transducer voltages range from a few microvolts (μV) for blood to a few hundred millivolts (mV) from bone or gas • I.E. - 60 dB gain amplifier - output power is 1,000,000 X the input power & the output voltage is 1,000 X the input. with a 10-μV voltage input, the output voltage is 10 mV. If the gain is increased to 100 dB, the output voltage increases to 1 V

31. Recall: 3 dB = a power gain of X 2 10 dB = X 10. Values can also be combined; 13 dB corresponds to (X 2 X 10) or X 20

32. Compensation • Amplifier compensates for the weaker echoes to return from a distance due to the attenuation that limits the depth we can image • time gain compensation [TGC] & depth gain compensation [DGC] equalizes differences in received echo amplitudes caused by different reflector depths

33. Compensation • If reflectors with equal reflection coefficients vary in distance from the transducer, then the reflectors will not result in echoes of equal amplitude arriving at the transducer due to attenuation • Longer path lengths result in greater attenuation & later arrival times

34. Compensation • These amplitudes must be adjusted to compensate for differences in attenuation so we can display echoes from similar reflectors at a similar brightness • Voltages from echoes arriving later are amplified to a greater degree than earlier ones. The later an echo arrives, the farther it traveled, the weaker it is and the more it needs to be amplified.

35. TGC • When properly used, it appears as if there had been no attenuation • Increase of gain with depth is commonly called the TGC slope • TGC slope is in decibels of gain per centimeter of depth

36. TGC • Sonographer adjusts the TGC to compensate for the attenuation of the tissues being imaged to achieve, an average uniform brightness throughout the image • Typical TGC amplifiers compensate for about 60 dB of attenuation

37. TGC

38. Beam Former consists of: • Pulser • Pulse delays • Transmit/receive switch • Amplifiers • Analog-to-digital converters • Echo delays  • Summer

39. Analog-to-Digital Converters (Digitizers) • Analog echo signal is in a foreign language to the digital scan converter that only reads numbers or digits • ADCs convert the analog voltages representing echoes to numbers for digital signal processing and storage

40. Beam Former consists of: • Pulser • Pulse delays • Transmit/receive switch • Amplifiers • Analog-to-digital converters • Echo delays  • Summer

41. Echo Delays Digitized echo voltages pass through digital delay lines to accomplish reception dynamic focus & steering functions

42. Beam Former consists of: • Pulser • Pulse delays • Transmit/receive switch • Amplifiers • Analog-to-digital converters • Echo delays  • Summer

43. Summer • The properly delayed channel signal components are added together (summed) to produce the scan line • Reception apodization & dynamic aperture functions are also performed

44. Pulse Echo Imaging Systems • beam former • signal processor • image processor • display

45. Signal Processor • Filter • Detection • Compression

46. Signal Processor • Bandpass filtering-digital filtering • Amplitude detection (RF to video) – detection of the amplified, compensated, summed, returning echoes • Compression (dynamic range reduction)compression of the amplified, compensated, summed, returning echoes

47. Signal Processor • Filter • Detection • Compression

48. Filtering • aka reject, threshold, suppression • Amplifiers with an electronic filter, called a bandpass filter, are used to reduce noise in the electronics. • Bandpass filter permits a range of frequencies (its bandwidth) & rejects those above & below the bandwidth

49. Signal Processor • Filter • Detection • Compression

50. Detection - Conversion of echo voltages from radio frequency (RF) form to video form while retaining the echo voltage amplitudes (AKA - demodulation)