Chapter 6: Level 1

1

Companion Presentation

Frank R. MielePegasus Lectures, Inc.

Ultrasound Physics & Instrumentation5th Edition

License Agreement

This presentation is the sole property of

Pegasus Lectures, Inc.

No part of this presentation may be copied or used for any purpose other than as part of the partnership program as described in the license agreement.

Materials within this presentation may not be used in any part or form outside of the partnership program. Failure to follow the license agreement is a violation

of Federal Copyright Law.

All Copyright Laws Apply.

Chapter Outline

Chapter 1: Math

Chapter 2: Waves

Chapter 3: Attenuation

Chapter 4: Pulsed Wave

Chapter 5: Transducers 

Chapter 6: System Operations  ‐ Level 1

Chapter 7: Doppler

Chapter 8: Artifacts

Chapter 9: Bioeffects

Chapter 10: Contrast and Harmonics

Chapter 11: Quality Assurance

Chapter 12: Fluid Dynamics

Chapter 13: Hemodynamics

Chapter 14: MSK

Chapter 15: HIFU

Chapter 16: Elastography

Chapter 17: IMT Ultrasound Imaging

Chapter 18: Strain Imaging

Chapter 19: Patient Care

Chapter 6: Level 1

2

Chapter 6: System Operation - Level 1

System operation deals with both the processing of the returning echoes and the system controls.

Level 1 focuses on:

general signal processing of the received radio frequency (RF) echoes

the basic controls of receiver gain

time gain compensation

the concept of signal to noise ratio (SNR).

Level 2 focuses on overall system design, with more in depth discussion about compensation, scan conversion, compression, measurements, and display.

System Major Subsystems

There are many functions that an ultrasound system performs. The system is commonly subdivided into two major sub-systems:

– The Front End (often referred to as the receiver)

– The Back End (often referred to as the scan converter)

In reality, there are many ways in which the system functions could be partitioned including separating out the display monitor and the data storage systems.

Basic Processes of Real-Time Imaging

There are six core functions that an ultrasound system must perform:

– Transmit beams (Front end)

– Receive beams (Front end)

– Process the returned data (Front end and Back end)

– Perform measurements in the processed data (Back end)

– Display the processed data (Back end)

– Store the processed data (Back end)

Chapter 6: Level 1

3

Basic System Functions

Fig. 27: (Pg. 158)

Transmitter (Pulser)

In Chapter 4 we learned about pulsed wave. In Chapter 5 we learned about steering and focusing by phasing, and sequencing. The transmit beamformer is responsible for creating all of the timing, phase delays, and transmit signals which generate each individual beam and, over time, a scan.

The “Receiver”

The receiver of the front end of the ultrasound system performs many functions such as:

– Amplification

– Compensation

– Compression

– Demodulation

– Rejection

(These are the five functions that are generally included on credentialing exams. In reality there are many major functions not included in this list.)

Chapter 6: Level 1

4

The Receiver

In reality there are many more functions performed by the receivers such as:

A/D conversion

beamforming (phasing and adding together each channel signal)

frequency filtering

parallel processing

The Concept of Signal to Noise Ratio

One of the most important concepts to learn is that of signal to noise ratio (SNR).

The signal to noise ratio is a measure of how strong a signal is relative to the background noise, or: the ratio of the signal amplitude to the noise amplitude.

Note that a large signal does not guarantee a high quality image, since even a large signal could be masked by noise. In essence poor SNR can result from a low amplitude signal, a high noise level, or both.

The Concept of Apparent SNR

A distinction should be made between true SNR and apparent SNR.

If the system adjustments are set inappropriately, the SNR may appear to be poor, even when the true SNR is relatively good.

Chapter 6: Level 1

5

Signal to Noise Ratio

Notice that in two of the three cases, the SNR is poor. In the middle picture, the SNR is low because of high amplitude noise. In the picture on the right, the SNR is poor because the signal has a low amplitude.

Fig. 1: (Pg. 149)

Apparent SNR

Although the SNR is the same in both of these cases, the first case results in poor apparent SNR, whereas the second case appears as good SNR.

Fig. 2: (Pg. 150)

(A) Signal appears weak (B) Signal appears strong

Both of the images were produced using the same transmit power but different receive gain. The lower gain of Figure 3a results in poor apparent SNR, whereas the higher receive gain of 3b appears as good SNR.

Fig. 3a: Signal Appears Weak Fig. 3b: Signal Appears Strong

Fig. 3: (Pg. 150)

Varying Receive Gain & Apparent SNR

Chapter 6: Level 1

6

Figure 4a has poor SNR because of low transmit power. Figure 4b shows good SNR as a result of higher transmit power. Both images used the same receive gain.

Fig. 4a: Poor SNR (Weak Signal) Fig. 4b: Good SNR

Fig 4: (Pg. 151)

Varying Transmit Power & SNR

Too Much Transmit vs. Too Much Gain

Notice that Figure 5a appears bright but has good SNR as a result of high transmit power. Figure 5b, appears too bright but has poor SNR as a result of lower transmit and excessive receive gain.

Fig. 5a: Transmit too High Fig. 5b: Receive Gain too High,

but Good SNR SNR Good but Apparent SNR Worse

Fig. 5: (Pg. 151)

SNR (from Online Image and Animation Library)

(Pg. 151)

A: Poor SNR B: Good SNR C: Poor Apparent SNR

D: Poor Apparent SNR E: Good SNR F: Poor Apparent SNR

Chapter 6: Level 1

7

Analog Signals and Digital Conversion

Signals measured coming from the patient are analog signals. Forease in processing and simplification of electronics, these analog signals are converted to digital signals.

Analog signal are continuous in time.

Digital signals are created by sampling an analog signal at discrete time

intervals.

The electronic device used for conversion is referred to as an analog to

digital (A/D) converter.

The rate at which the sampling is performed can affect whether the digital

signal accurately represents the original analog signal.

Faster signals require faster sampling.

Low Frequency Analog Signal

Fig. 6: Slowly Varying Analog Signal (Pg. 153)

Higher Frequency Analog Signal

Fig. 7: Quickly Varying Analog Signal (Pg. 153)

Chapter 6: Level 1

8

Analog to Digital Converter (A/D)

Fig. 10: An 8-bit A/D Converter (Pg. 153)

Sampling an Analog Signal

Fig. 21: Graphical Representation of Sampling (Pg. 67)

Sampling Clock

Analog to Digital Conversion

“Sampling of a Slowly Varying Analog Signal”

“The Sampling Clock”

Every time the clock “ticks” the A/D converter samples the

signal and outputs a digital value representing the amplitude of the signal.

Chapter 6: Level 1

9

Sampled (Digital) Signal

Fig. 24: Graphical Representation of a Digital Signal (Pg. 68)

Reconstructing from a Digital Signal

Fig. 25: Reconstructed Signal (Pg. 68)

Sampling a Higher Frequency Signal

Fig. 26: Sampling a Quickly Varying Analog Signal (Pg. 69)

Chapter 6: Level 1

10

Digital Signal Representation

Fig. 27: Graphical Representation of the Digital Signal (Pg. 69)

Reconstructing from the Digital Signal

Fig. 28: Reconstructed Signal (Pg. 70)

Original versus Reconstructed Signal

Fig. 29: Reconstructed Versus Original Signal (Pg. 70)

Chapter 6: Level 1

11

Nyquist Criterion

The Nyquist Criterion states that to avoid aliasing, the sample frequency must be at least twice as fast as the highest frequency in the signal you want to detect.

2sampling signalf (minimum) f (maximum)

Determining Nyquist (2 Hz Signal)

Fig. 30: Analog Signal (Pg. 72)

Sampling at 25 Hz

Fig. 31: Sampled Signal (Pg. 72)

Chapter 6: Level 1

12

Reconstruction (No Aliasing)

Fig. 32: Reconstructed Signal (Pg. 72)

Sampling Too Slowly

Fig. 33: Analog Signal (Pg. 73)

Digital Signal From Sampling at 2 Hz

Fig. 34: Sampled Signal (Pg. 73)

Chapter 6: Level 1

13

Reconstructed Signal is Aliased

Fig. 35: Reconstructed Signal (Pg. 73)

Reconstructed Signal Not Aliased

Fig. 37: Sampled Signal (Pg. 74)

Sampling at 4 Hz

Fig. 36: Analog Signal (Pg. 74)

Chapter 6: Level 1

14

Nyquist Limit: Sample Twice as Fast

Fig. 38: Reconstructed Signal (Pg. 74)

Violation of Nyquist: Aliasing

Fig. 39: Aliasing (Pg. 75)

Improving SNR

There are many ways of improving the signal strength.

Increase transmit power

Use a lower frequency transducer (for deeper imaging depths)

Use a different imaging plane

Maneuvers to remove attenuators such as lung and gas

Move transmit focus deeper

Use a larger aperture transducer (allows for deeper focus)

Use semi-invasive techniques (“endo-probes” and transesophageal)

Note that increasing the receiver gain does not improve the SNR – it increases both the signal and the noise proportionally.

Chapter 6: Level 1

15

Raw RF Signal

The returning RF signals are changed (modulated) by the mechanical interaction of the wave with the body. Larger acoustic impedance mismatches result in higher amplitude signals. Signals from deeper depths (later in time) are also attenuated more than reflections from shallower depths (earlier in time).

Fig. 28: (Pg. 160)

Amplified Raw RF Signal

Amplification is the process of multiplying the received signal to make the signal larger. Amplification results in an increase of the amplitude for signals from all depths uniformly.

Fig. 29: (Pg.160)

Amplification Visualized

The image on the left provides a reference for the amplified image on the right.

Chapter 6: Level 1

16

Varying Receive Gain (Amplification)

Fig. 30: (Pg. 161)

30a: Severely Undergained 30b: Badly Undergained 30c: Undergained

30d: Appropriately Gained 30e: Optimally Gained 30f: Slightly Overgained

Amplification (Animation)

(Pg. 161)

Miele Enterprises, LLC

Compensated RF Signal

More amplification is required for signals from deeper depths to compensate for the increased attenuation. Notice in the image below that the signals later in time are amplified more than the signals earlier in time (compare with image of Figure 29).

Fig. 31: (Pg. 162)

Chapter 6: Level 1

17

Compensation Visualized

The image on the left provides a reference for the compensated image on the right.

Compensation is performed by the system TGCs.

TimeAmplified Raw RF Signal

Am

plitu

de

Am

plitu

de

Compensated RF SignalTime

Receive Gain and TGC

Fig. 32: (Pg. 162) Fig. 33: (Pg. 162)

Inappropriate TGC settings can result in regions appearing too light or too dark as seen in the two images below.

Incorrect TGC Settings

Fig. 34a: (Mid-range TGCs Too Low) Fig. 34b: (Mid-range TGCs Too High)

(Pg. 162 - 163)

Chapter 6: Level 1

18

Simple Compression Map

Compression is a method to reduce the dynamic range by mapping a larger range of signals into a smaller range of signals. The following simplistic map shows a 5 to 1 reduction in signal dynamic range.

Fig. 35: (Pg. 163)

Log Compressed RF Signal

Compression is needed since the dynamic range of the returning echoes is much greater than the dynamic range visible to the human eye. Through compression, the ratio of the maximum to the minimum signal is significantly reduced.

Fig. 36: (Pg. 164)

Compression Visualized

The image on the left provides a reference for the amplified image on the right.

Compensated RF Signal

Am

plitu

de

Time Log Compressed RF Signal

Am

plitu

de

Time

Compression in the receiver is not under user control. However there is more signal compression which takes place in the back end of the system that is under user control.

Chapter 6: Level 1

19

Various Compression Maps

37a:Most Contrast 37c

37d 37e 37f: Least Contrast

37b

(Pg. 164)

Compression (Animation)

(Pg. 165)

Rectification of RF Signal

The process of signal detection (demodulation) is actually comprised of two, more fundamental steps: rectification and envelope detection. The following image demonstrates the process of rectification, converting the signal from being bi-polar to uni-polar.

Fig. 38: (Pg. 165)

Chapter 6: Level 1

20

Envelope Detection of RF Signal

The second stage of signal detection is envelope detection. The output of this stage is basically the early form of ultrasound referred to as amplitude mode (A-mode). Notice how the height of the amplitude corresponds to the amplitude of the signal.

Fig. 39: (Pg. 165)

Demodulated Signal (A-mode)

In A-mode, the horizontal axis corresponds to time (which is related to depth) and the vertical axis corresponds to the signal strength (amplitude).

Fig. 40: (Pg. 165)

Demodulation Visualized

The image on the left provides a reference for the demodulated image on the right.

Log Compressed RF Signal

Am

plitu

de

TimeDemodulated Signal (A-Mode)

Am

plitu

de

Time

Chapter 6: Level 1

21

Demodulated Signal after Rejection

The premise of rejection is that signals below a “threshold” are eliminated as too weak to be of value. The reality is that “rejection” is more a natural limit that results from noise in the image. Signals below the noise floor are masked by the noise in the image.

Fig. 41: (Pg. 166)

Rejection Visualized

The image on the left provides a reference for the application of rejection to the image on the right.

Demodulated Signal (A-Mode)

Am

plitu

de

TimeDemodulated Signal After Rejection

Time

Am

plitu

de

A-mode from a Radial Artery

Fig. 42: (Pg. 166)

Chapter 6: Level 1

22

Add Title

Blank Slide:

This blank slide is here to help facilitate adding new content. If you would like to add material to this presentation, copy this slide and place in the correct location.