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Pulse-Echo Imaging Instrument

Pulse-Echo Imaging Instrument

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Pulse-Echo Imaging Instrument

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  1. Pulse-Echo ImagingInstrument

  2. Pulse-Echo Instrumentation • Voltage activation of the PE crystal • Ultrasound formation • Propagation • Reflection • Charge formation of crystal • Processing • Display

  3. Pulse-Echo Instrumentation Transmitter Display Receiver Amplifier Detector Scan Converter TRX TGC

  4. Pulse-Echo Instrumentation Pulser Components • HV pulse generator • The clock generator • The transducer

  5. Pulse-Echo Instrumentation Generated Wave Applied Voltage + + V P TIME TIME - -

  6. Pulse-Echo Instrumentation The Pulser rate is known as the pulse repetition frequency (PRF). Typical PRF 3,000 – 5,000. PRF automatically adjusted as a function of imaging depth.

  7. Pulse-Echo Instrumentation Switch that controls the output power of the HV generator is the attenuator.

  8. Pulse-Echo Instrumentation ATTENUATOR TRX PULSER

  9. Pulse-Echo Instrumentation • CLOCK GENERATOR • Controls the actual number of pulses which • activate the crystal. • Responsible for sending timing signal to the • Pulse generator • TGC circuitry • Memory

  10. Pulse-Echo Instrumentation TGC UNIT CLOCK GENERATOR HV GENERATOR MEMORY TRS CRT DISPLAY TRX

  11. Pulse-Echo Instrumentation • Sensitivity refers to the weakest echo • signal that the instrument is capable of • detecting and displaying. • Factors that determine sensitivity are • Transducer frequency • Overall and TGC receiver gain • Reject control • Variable focal zone on array real-time • instruments.

  12. Pulse-Echo Instrumentation • Increasing the voltage causes • Greater amplitude – greater penetration • Longer pulses – degrades axial resolution • Increase exposure

  13. Pulse-Echo Instrumentation Transducer has dual roles; transmitting and receiving signals. The transducer is capable of handling a wide range of voltage amplitude. The Receiver is capable of handling only smaller signals Therefore it is desirable to isolate the pulser circuit from the receiver circuit.

  14. Pulse-Echo Instrumentation The Transmit Receive Switch TRS – positioned at the input of the receiver and is designed to pass only voltages signals originating at the transducer by the returning echoes.

  15. Pulse-Echo Instrumentation • The Receiver Unit consist of • Radiofrequency Amplifier • Time gain compensation unit • Demodulation Circuit • Detector Circuit • Video Amplifier

  16. Pulse-Echo Instrumentation MEMORY PULSER TGC UNIT RF RECEIVER TRX TRS CRT DISPLAY DEMODULATOR DETECTOR VIDEO AMPLIFIER

  17. Pulse-Echo Instrumentation • Radio-Frequency Amplifier • Amplify weak voltage signals. • This is called GAIN

  18. Pulse-Echo Instrumentation Electric signals generated by the transducer are weak and needs amplification. The gain is the ratio of the output to input Voltage or Power. Gain = Voltage Out Voltage In

  19. Pulse-Echo Instrumentation • The Imaging effect of adjusting gain are: • Increasing the gain - increased sensitivity, better penetration • Decreasing the gain – decreased sensitivity, less penetration • Too high a gain – overloads the display, loss or spatial resolution

  20. Pulse-Echo Instrumentation Normal Gain Saturation Level Amplitude Distance

  21. Pulse-Echo Instrumentation Excess Gain Saturation Level Amplitude Distance

  22. Pulse-Echo Instrumentation Primary objective of grayscale pulse-echo imaging is to make all like reflectors appear the same in the Image regardless where they are located in the sound beam.

  23. Pulse-Echo Instrumentation Time Gain Compensation TGC - electronic process of adjusting the overall system gain as a function of the transmit time.

  24. Pulse-Echo Instrumentation • TGC Controls • Near Gain • Slope Delay • Slope • Knee • Far Gain • Body Wall

  25. Pulse-Echo Instrumentation MAX GAIN KNEE Gain dB NEAR GAIN SLOPE DELAY Depth cm

  26. Pulse-Echo Instrumentation MAX GAIN KNEE NEAR GAIN Gain dB SLOPE Depth cm Body wall

  27. Pulse-Echo Instrumentation KNEE Gain dB SLOPE CUT-OFF DELAY Depth cm

  28. Pulse-Echo Instrumentation The slide potentiometer allows adjustment of receiver gain for small discrete depth increments.

  29. Pulse-Echo Instrumentation Slide Potentiometer Gain dB Depth (Time)

  30. Pulse-Echo Instrumentation Frequency Tuning of the Receiver The frequency band width of the receiver refers to the range of ultrasound signal frequencies that the receiver can amplify with a maximum gain.

  31. Pulse-Echo Instrumentation • Types of Amplifiers • Wide-Band • Narrow-Band

  32. Pulse-Echo Instrumentation Wide-band amplifier Narrow-band amplifier Gain Gain Frequency MHz Frequency MHz

  33. Pulse-Echo Instrumentation Receiver Unit Receiver A Receiver B Output To System TRX Receiver C Receiver D Frequency Selector Switch

  34. Pulse-Echo Instrumentation DYNAMIC RANGE The dynamic range is a measure of the range of echo signal amplitudes. The dynamic range can be measured at any point. The dynamic range decreases from transducer, to receiver to scan converter and finally to display.

  35. Pulse-Echo Instrumentation • Large range in signal amplitudes is due to: • Normal variation in the reflection amplitude. • Frequency dependent tissue attenuation.

  36. Pulse-Echo Instrumentation RF amplifier can handle a wide range of signal amplitude at its input – but cannot accommodate the corresponding output using linear amplification.

  37. Pulse-Echo Instrumentation Linear amplification - all voltages amplitudes, regardless of size at the point of input are amplified with the same gain factor.

  38. Pulse-Echo Instrumentation LOGARITHMIC AMPLIFICATION In Logarithmic amplification weak echoes amplitudes are amplified more than strong echoes. This can reduced the dynamic range by as much as 50%. The process of reducing the signal DR by electronic means is called COMPRESSION

  39. Pulse-Echo Instrumentation Linear Amplification A Gain B Logarithmic Amplification Input signal

  40. Pulse-Echo Instrumentation R-F amplifier can also set the electronic level in the machine. S-N level – compares real echo signals the system can handle versus the non-echo signals presents (Noise). The Higher the SN ratio – better the operation of the system.

  41. Pulse-Echo Instrumentation Pre-amplification is a technique to reduce system noise. Positioning of part of the amplifier circuitry in the transducer housing reduces system noise.

  42. Pulse-Echo Instrumentation REJECTION Rejection is the receiver function that enables the operator to systematically increase or decrease the minimum echo signal amplitude which can be displayed. Alternate names = Threshold, Suppression.

  43. Pulse-Echo Instrumentation Saturation Level Dynamic Range Rejection Level Noise Level Zero Signal Level

  44. Pulse-Echo Instrumentation SIGNAL PROCESSING RF waveform – oscillating type of voltage signal (AC) First Step in processing the signal is Demodulation. Demodulation is the process of converting the electric signal from one form to another.

  45. Pulse-Echo Instrumentation • DEMODULATION • Rectification • Detection

  46. Pulse-Echo Instrumentation • RECTIFICATION • Rectification results in the elimination of the negative portion of the RF signals • Half Wave Rectification • Full wave Rectification

  47. Pulse-Echo Instrumentation Half-Wave Rectification

  48. Pulse-Echo Instrumentation Full-Wave Rectification

  49. Pulse-Echo Instrumentation DETECTION The main effect of detecting the rectified RF signal is to round out or smooth the signal as to have a single broad peak. The rectified RF signal following detection is referred to as a Video Signal.

  50. Pulse-Echo Instrumentation Smoothing