TAs: Süleyman Yasin Peker
Topics covered:
Introduction to Verasonics Vantage 32LE Ultrasound Research System.
Introduction to ultrasound transducers.
Introduction to ultrasound imaging modes.
Real-time data acquisition using Verasonics.
Time-of-Flight measurement of common carotid artery (cca) using ultrasound and image processing.
Flow measurement and Colour Doppler and Power Doppler imaging using ultrasound.
Experiment details:
Introduction
This experiment involves the use of the Verasonics Vantage 32LE, a sophisticated ultrasound research system, to explore various aspects of ultrasound imaging and data acquisition. Participants will gain hands-on experience with different ultrasound imaging modes, including A-Mode, B-Mode, Colour Doppler, and Power Doppler, while understanding the applications of linear array, curvilinear/convex, and phased array transducers. The experiment will also cover real-time data acquisition using the Verasonics system, focusing on measuring Time-of-Flight values for echoes and analyzing the power spectrum of ultrasound signals in Doppler mode. This comprehensive approach will provide a deeper understanding of both the technical and practical aspects of ultrasound technology.
Theoretical Background
Ultrasound Transducers
A linear transducer is a straight transducer. The linear transducer’s length determines the image’s sector width and shape. A linear transducer offers detailed resolution at superficial depths. Linear transducers are most frequently used with MSK, nerve, small parts, vascular, and pediatric applications.
A convex transducer is a curved transducer. The shape of a convex transducer determines the sector width and shape of the image. A convex transducer offers a wider field of view for larger or deeper structures. Convex transducers are most frequently used for abdomen, obstetrics/gynecology (OB/GYN), urology, and some musculoskeletal (MSK) applications.
A phased array transducer has a small footprint with a sector image shape and features high temporal resolution and penetration. This allows clinicians to image structures that are moving in real-time. Phased array transducers are most frequently used with applications ranging from cardiac, transcranial, abdomen, and pediatrics.
Ultrasound Imaging Modes
The earliest ultrasound mode showed returning echoes in a one dimensional, graphical format (Figure 6). Known as amplitude or A-mode, the information could be used to measure the distance between, or thickness of, tissues. An example is examining the cornea, lens and chambers of the eye.
“B” or Brightness mode represents the amplitude peaks seen in A-mode as dots or pixels of varying brightness (Figure 6).
Using B-mode, ultrasound systems can send sequential ultrasound pulses out in different directions to form multiple image lines. This process is completed quickly and repeatedly, creating the typical ultrasound image seen on all systems.
In this display mode, Doppler data received by the ultrasound system is displayed as coloured pixels within the sample box shown on the B-mode image (Figure 7). This gives information about direction and a semi-quantitative assessment of blood flow velocities.
A variation of Colour Doppler available on most systems is a Power Doppler display. This mode ignores the directional information provided by Doppler shift and displays the total Doppler signal strength as shades of one colour (Figure 8). Whilst it does not display any data on flow direction, Power Doppler is a useful tool for examining low velocity blood flow and is more sensitive to flow than Colour Doppler.
Time-of-Flight Measurement
Time-of-Fight (TOF) in ultrasound refers to the time it takes for an ultrasound pulse to travel from the transducer to a target (such as tissue or a vessel wall) and back to the transducer after being reflected. It is a critical measurement used to determine the depth of structures within the body, as the distance traveled by the ultrasound wave is directly related to the time it takes for the wave to return to the transducer.
The formula to calculate the Time-of-Flight for a single echo is:
where:
d is the depth of the target (the distance from the transducer to the reflecting surface),
c is the speed of sound in the medium (typically c≈1540 m/s in soft tissue),
The factor of 2 accounts for the round-trip distance (to the target and back).
Calculating Time-of-Flight for Two Echoes
When considering two echoes, the TOF for each can be calculated separately if the depths d1 and d2 are known:
To find the difference in Time-of-Flight between the two echoes:
Procedure
Introduction to Verasonics Vantage 32LE Ultrasound Research System
Begin with an overview of the Verasonics Vantage 32LE system, including the main console, transducers, and the user interface.
Discuss the system's key features, such as its high-performance computing capabilities, customizable imaging modes, and real-time data acquisition.
Demonstrate how to power on the system, navigate the user interface, and set up the system for imaging.
Introduction to Ultrasound Transducers
Present the various transducer types available with the Verasonics system, including linear array, curvilinear, and phased array transducers.
Explain the physical principles behind each transducer type, focusing on beam formation and the impact of transducer geometry on imaging.
Describe the common applications of each transducer type:
Linear Array: High-resolution imaging of superficial structures (e.g., vascular studies).
Curvilinear Array: Deeper imaging, commonly used in abdominal imaging.
Phased Array: Cardiac imaging, due to its ability to scan through narrow acoustic windows.
Introduction to Ultrasound Imaging Modes
A-Mode (Amplitude Mode):
Explain the concept of A-mode imaging and its significance.
Demonstrate the use of A-mode to measure distances within tissues.
B-Mode (Brightness Mode):
Describe B-mode as the most common imaging mode for real-time 2D imaging.
Show how to set up and acquire B-mode images of the common carotid artery (CCA).
Discuss the interpretation of B-mode images, focusing on anatomical structures.
Colour Doppler Imaging:
Introduce the principles of Doppler ultrasound and how it measures blood flow velocity.
Demonstrate the setup and acquisition of Colour Doppler images, focusing on blood flow within the CCA.
Discuss the interpretation of Colour Doppler images, with emphasis on flow direction and velocity.
Spectral Doppler Imaging:
Explain the concept of Spectral Doppler and its use in quantifying blood flow velocity.
Demonstrate how to acquire a Spectral Doppler waveform and interpret the results, focusing on the CCA.
Real-Time Data Acquisition Using Verasonics
Set up the Verasonics system for real-time data acquisition in B-mode.
Position the selected transducer on the subject’s neck, over the CCA.
Adjust imaging parameters (e.g., depth, gain, focus) to optimize image quality.
Acquire and save real-time B-mode images of the CCA.
Acquire raw RF echo signal data.
Transition to Colour Doppler mode and acquire real-time flow images.
Save all acquired data for further analysis.
Time-of-Flight Measurement of Common Carotid Artery (CCA) Using Ultrasound and Image Processing
Use the B-mode images acquired previously to identify the CCA and measure the depth of the anterior and posterior walls.
Calculate the TOF for ultrasound pulses reflecting from these walls using the equations described in the theoretical background section.
Use image processing software to analyze the B-mode images and extract TOF data.
Discuss the significance of TOF measurements in clinical practice, such as assessing arterial wall thickness.
Flow Measurement and Colour Doppler Imaging Using Ultrasound
Set up the Verasonics system for Colour Doppler imaging of the CCA.
Acquire Colour Doppler images, ensuring that the flow is visualized across the vessel.
Use Spectral Doppler to obtain a waveform of the blood flow velocity.
Measure key parameters from the Doppler waveform, such as peak systolic velocity (PSV) and end-diastolic velocity (EDV).
Analyze the flow patterns, and discuss potential clinical implications, such as identifying stenosis or other vascular conditions.
Report requirements:
The report for this part is expected to include:
Abstract and Introduction
Procedures and Steps
Experiment Related (Plots & Calculations)
Discussion and Conclusion
Experiment Related (Plots & Calculations)
Using the acquired raw RF echo signals, calculate the TOF values of the CCA walls and artery diameter of the CCA.
Plot the raw RF echo signals of three different channels and discuss the differences between the acquired echo signals.
Using the acquired B-Mode image, calculate the artery diameter and calculate the TOF values of the CCA walls and compare the results with the calculations of the raw RF echo signals.
Using the acquired B-Mode images, select the frames that could be the possible systolic and diastolic blood pressure frames. Calculate the artery diameter change and the discuss the health situation of the subject by giving references to the literature.
Using the raw RF signals acquired during the Colour Doppler Imaging, calculate the spectrogram of a single channel data and observe the flow pattern. If flow pattern is not visible, change the processed channel and calculate again. Plot the spectrogram of the signal.
Submit the Matlab codes with the experiment report.