• Demonstrate knowledge of the basic physical nature of ultrasound waves and the interactions that occur as it traverses through tissues and other media.
• Demonstrate knowledge of the various types of ultrasound transducers which are available, and to be able to choose a transducer on the basis of its physical characteristics and suitability for a given application.
• Demonstrate knowledge of the basic principles of ultrasound imaging and how various technical factors affectimage quality.
• Describe how real-time systems work, and be aware of the interplay between temporal resolution, spatial resolution and depth of penetration.
• Describe the basic physical principles underlying the use of the Doppler effect in ultrasound imaging.
• Explain how choice of frequency affects attenuation, spatial resolution, and the maximum flow rate that can be detected.
• Describe the operation of a simple duplex transducer.
– Recognise simple ultrasound artefacts and explain how they are formed.
– Discuss the main mechanisms by which ultrasound could damage tissue. Have a knowledge of safe levels of exposure for imaging and safety recommendations.
Category 2
• Demonstrate knowledge of some of the basic parameters which characterise a sound wave. Conduct simple calculations relating to frequency, wavelength and relative intensity in decibels. Demonstrate working knowledge of the relative magnitudes of sound velocity, acoustic impedance and attenuation in various biological media, and their implications for imaging.
• Describe details of the main physical parameters that characterise transducers, and their effect on the image.
• Describe the basic principles of B-mode pulse-echo imaging.
Understand parameters such as pulse length, frequency, pulse repetition frequency and TGC affect the image.
• Perform simple calculations using the Doppler shift equation and understand the concepts underlying spectral analysis colour Doppler and power Doppler.
• Describe the basic principles of compound imaging.
• Describe the basic principles of panoramic imaging.
• Explain the factors which produce more complex artefacts such as aliasing and side lobes.
Category 3
• Demonstrate a working (although not necessarily detailed) knowledge of more complex technology involving:
– Special transducers
– Harmonic imaging, 3D imaging and ultrasound contrast agents.
Image resolution
Each tissue structure is smeared laterally by an amount equal to the
beamwidth
The lateral resolution is therefore determined by the beamwidth, and it
varies with depth
The narrower the beam the better the resolution
The axial resolution is determined by the pulse duration and is the same
at all depths
The shorter the pulse the better the resolution
Axial resolution is substantially better than lateral resolution
Tissue interfaces should therefore be scanned at perpendicular
incidence as far as possible, since this will maximise resolution
Measurement protocols often specify perpendicular incidence
Spatial Resolution
Each tissue structure is smeared laterally by an amount equal to the beamwidth
Axial resolution
Axial resolution (longitudinal, depth or linear resolution) is resolution in the direction parallel to the ultrasound beam. The resolution at any point along the beam is the same; therefore axial resolution is not affected by the depth of imaging.
Axial resolution = Spatial pulse length / 2
Where SPL = number of cycles in the pulse × wavelength
Hence any measure that shortens the length of the ultrasound pulse will improve axial resolution:
- decreasing the number of cycles in the pulse
- damped transducers – produces a shorter pulse
- increasing the frequency – reduces the wavelength
Axial resolution is substantially better than lateral resolution. Axial resolution is a component of pulse length, while the azimuthal resolution is a component of beamwidth. As the beam width is larger than the pulse
length, the azimuthal resolution is always worse. Thus tissue interfaces should therefore be scanned at perpendicular incidence. Measurement protocols often specify perpendicular incidence.
Lateral resolution
The lateral resolution (azimuthal resolution) is therefore determined by the beam width and it varies with depth due to divergence. The narrower the beam the better the resolution
Lateral resolution is defined as the ability of the system to distinguish two points in the direction perpendicular to the direction of the ultrasound beam. It is also known as azimuthal resolution. Lateral resolution is affected by the width of the beam and the depth of imaging. Wider beams typically diverge further in the far field and any ultrasound beam diverges at greater depth, decreasing lateral resolution. Therefore, lateral resolution is best at shallow depths and worse with deeper imaging.
Temporal resolution
Temporal resolution is the ability to detect that an object has moved over time. For the purposes of medical ultrasound, temporal resolution is synonymous with frame rate. Typical frame rates in echo imaging systems are 30-100 Hz. The temporal resolution or frame rate = 1/(time to scan 1 frame). The time to scan one frame is equal to the pulse repetition period x number of scan lines per frame.
Common means of improving frame rate include 1) narrowing the imaging sector, which decreases the time it takes to scan one frame 2) decreasing the depth which decreases the PRP 3) decreasing the line density, which requires fewer lines to scan one frame (at the cost of spatial resolution) 4) turning of multifocus, which decreases the number of pulses needed per line. See some examples below:
Time gain compensation (TGC)
Ultrasound echoes grow progressively weaker as they come from deeper in the body, due to attenuation. The TGC function corrects for this by progressively increasing the gain (i.e. the amount of amplification) applied to the echoes. The machine estimates the required “slope” (i.e. the rate at which the gain increases with time) based on the ultrasound frequency and assuming average tissue. A set of controls is provided to allow fine-tuning of the TGC to compensate for tissue variations
Dynamic range
The term “dynamic range” refers to the range of echo strengths used to create the image
◦ it may be as much as much as 60 dB (i.e. the strongest echo has an intensity 1,000,000 times greater than the weakest echo)
The display (and the human eye) can only deal with a range of around 30 dB (a ratio of 1000:1)
The compression (or dynamic range) function compresses the dynamic range of the echo signal to fit what can be displayed
Stronger echoes are compressed more than weaker ones
◦ variations in the strength of strong echoes are not generally
significant
◦ variations in the “echogenicity” (echo strength) of soft tissue
are clinically important
Ultrasound artifacts
To determine whether a given echo (or structure) is an artifact, and to reduce or eliminate it, move the probe or the patient
Speckle
A textured appearance that results from small, closely spaced structures that are too small to resolve as seen on images of solid organs.
-Image noise is the result of random signals produced in the electronic preamplifier of the transducer.
-Noise reject controls can be adjusted to filter out weak noise, but this also eliminates weak echo signals.
Reverberation
- Occurs when there are two or more parallel reflective tissue interfaces. Ultrasound reflects back and forth between these, generating multiple equally spaced echoes
- Echoes are the result of multiple reflections occurring from two adjacent interfaces. Produces delayed echoes that are incorrectly localized as a more distant interface. The number of reverberations is limited by the power of the beam and sensitivity of the detector.
- Reverberation artifact is produced by the beam bouncing off the posterior interface, then off the anterior interface, then back off the posterior interface, and finally being recorded as a faint image further away from the actual structure.
Acoustic shadowing
is the reduced echo intensity behind a highly attenuating or reflecting object, such as a stone creating a “shadow.”
Acoustic enhancement
The increased echo intensity behind a minimally attenuating object such as a fluid filled cyst.
Refraction
causes artifacts in the form of spatial distortions.
Speed displacement
artifacts are caused by the variability of the speed of sound in different tissues.
Side lobes
emissions of ultrasound energy off axis and give rise to artifacts when echoes arise “off axis” but are placed as having occurred on the central axis.
Anything that falls within the volume that is scanned appears in the image
Often this causes echoes from adjacent tissues to appear to be within an otherwise echo-free region (a blood vessel, the urinary bladder etc.)
Grating lobes
result from the division of a smooth transducer into a large number of small elements in multielement transducer arrays.
Axial resolution improved by:
• Shorter wavelength (higher frequency)
• Shorter length of pulse (lower Q-value of backing material)
Lateral resolution improved by:
• Smaller beam width
Temporal resolution worsened by:
• Smaller PRF
• Deeper structures
• More focal zones
• Doppler
• Larger sector width