12. Elements of Image Quality
13. Transducer Technologies
• Mechanical Sector Transducer
• Linear and Curved Transducer
• Phased Array Transducer
1. What is Ultrasound?
• Sound waves with frequencies grater than 20.000 Hz
• Clinically useful frequencies range from 1.0 – 30.0 MHz
• Sound requires a media for propagation
2. Ultrasound Transducer
• A piezoelectric device called a transducer is used to convert electrical energy to mechanical energy and vise versa.
• This means that a single element can be used to transmit and receive ultrasound signals.
3. Ultrasound Speed in Soft Tissues
Sound, regardless of frequency, travels at the same speed in like mediums.
Wavelength determines resolution. Shorter wavelength produces high resolution
Speed or Velocity = Frequency X Wavelength
Frequency and its wavelength in soft tissue
4. Ultrasound Propagation
• Ultrasound requires a media for propagation.
• Various mediums propagate sound at different speeds.
Speed or Velocity = Wavelength X Frequency
5. Acoustic Transmission and Reception
Sound waves from transducer will travel in straight line, axial to the transducer. As we know, the body is mad up of an infinite variety of tissues densities (skin, muscles, and organs).Sound waves will react differently to density changes and boundary layers by reflecting and refracting in different directions.
6. Reflection and Refraction due to Interfaces
Sound reacts in a particular manner when incident on a boundary between tissue layers. Energy returned to transducer is called “reflected energy” or echo.
Sound changes direction when crossing boundaries unequal to incident angle. Propagation speed (due to medium density) changes incident and reflection angles.
Irregular boundaries redirect sound in different directions
When reflectors are smaller than the ultrasound wavelength, energy is divided in all directions.
7. Ultrasound Attenuation
• The major causes for attenuation of sound in tissue is due to “Friction”caused by the moving molecules.
• Amplitude and intensity decrease as sound travels through a medium.
Attenuation is linear
• The depth of penetration in tissue is determined by attenuation.
• As the frequency increases the attenuation increases.
• As the frequency decreases the attenuation decreases resulting in increased penetration.
For soft tissue Attenuation 0.5 – 0.7 dB/cm/MHz
8. Echo Position and image formation
By knowing the speed of sound in soft tissue (1540 m/s) we can calculate
the depth of all insonated targets and plot their position.The graph shows, each vector in the
sector is sampled with short pulses of energy (PRF) .
As targets are insonated at different depths, their returning echo’s are plotted by determining
the elapsed time between the transmission and reception. The system computer plots the
position of echo’s according to the speed and frequency of sound pulses.
• Echo positions are determined along multiple vectors (scan lines) in a predetermined pathway.
• Short pulses of energy are used.
• Echo positions are determined by the elapsed time between the transmitted pulse and the returned echo at a transmission velocity of 1 cm./13 micro sec.
• Length of pulses determines axial resolution (PRF).
• Scan lines are combined into a two-dimensional picture.
9. Image Resolution
Focusing is necessary for the returned energy to be reproduced into the clear, sharp pictures that are needed for diagnosis.
There are basically four types of resolution, three have some control over and one is determined by the quality of the system electronics.
• Axial Resolution is the resolution along the path of the beam in the axial dimension. It is directly related to the frequency of the transducer.
• The higher frequency (shorter pulses) the better the axial resolution.
• Approximately equal to one-half the length of the ultrasound pulse.
• Axial Resolution improves by damping and increasing frequency.
• Higher frequencies produce sound waves with limited penetration but good resolutions.
• Lower frequencies produce strong sound waves which penetrate to grater depths, but have less resolution because of the grater distances between the sound waves.
• Lateral Resolution is directly related to the vector density of the system and the focusing (narrow pulse) characteristics of the transducer.
• Approximately equal to beam diameter – varies with depth.
• Typical value : 2 – 10 mm – improved by focusing.
• A transducer with a large aperture will have better lateral resolution.
• As the graph shows, lateral resolution resolves and separates targets in the lateral dimension.
• Both axial and lateral resolution works together to receive and focus the smallest dots of data for the best image resolution.
• Temporal resolution is the ability to resolve the position of moving targets at any instant in time.
• It is controlled by the frame rate, ( image per sec), the higher frame rate results in better temporal resolution.
• Doppler functions slow frame rates considerably, system capabilities maintain high frame rates in all scanning situations.
• Contrast resolution is the differentiation of similar and dissimilar tissues.
• It is directly related to the quality of system electronics, transducers and the gray scale that the system uses to define tissue densities.
• More shades or gray will define subtle tissue differences than limited gray shades.
• Gray scale is the level of black or white intensity of the detected returned echo.
• A digital scan converter converts images into numbers and data is displayed in pixels which contain bits.
• More bits per pixels = more shades of gray.
• Ultrasound systems typically display 256 shades of gray in all scan modes
10. Image Processing
• The signal from the transducer is converted from analog to digital signal.
• Digitization gives us signal processing power and control
• Once data has been manipulated it is then routed to either B-scan or Doppler processors for that particular processing.
• Finally, all signals are then re-converted for display on an analog video monitor.
An ultrasound signal path
11. Ultrasound Parameters
Propagation - Power - Intensity – Amplitude – Frequency –Wavelength -Time
12. Elements of Image Quality
1. Uniform image throughout the field of view
2. Like structures look alike
3. Cystic/fluid filed structures are clear and free from artifacts.
4. All Structures are anatomically correct and are in the correct display orientation.
5. All Structures should appear to be the correct size, shape, and thickness
6. Sufficient penetration to demonstrate the desired anatomy