[Cat 1] 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.
[Cat 1] Demonstrate knowledge of the basic principles of ultrasound imaging and how various technical factors affect image quality.
[Cat 1] Describe how real-time systems work, and be aware of the interplay between temporal resolution, spatial resolution and depth of penetration.
[Cat 1] Describe the basic physical principles underlying the use of the Doppler effect in ultrasound imaging.
[Cat 1] Explain how choice of frequency affects attenuation, spatial resolution, and the maximum flow rate that can be detected.
[Cat 1] 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.
[Cat 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.
[Cat 2] Describe details of the main physical parameters that characterise transducers, and their effect on the image.
[Cat 2] 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.
[Cat 2] Perform simple calculations using the Doppler shift equation and understand the concepts underlying spectral analysis colour Doppler and power Doppler.
[Cat 2] Describe the basic principles of compound imaging.
[Cat 2] Describe the basic principles of panoramic imaging.
[Cat 2] Explain the factors which produce more complex artefacts such as aliasing and side lobes.
[Cat 3] Demonstrate a working (although not necessarily detailed) knowledge of more complex technology involving:
• Special transducers
• Harmonic imaging, 3D imaging and ultrasound contrast agents.
Imaging Assumptions
The transmitted beam is a narrow straight line
Ultrasound travels along this line and does not deviate
Ultrasound travels directly from transducer to reflector (or scatterer) then back to the transducer
The propagation speed is 1540 m/s in all tissues
The attenuation coefficient is uniform in all tissues
All echoes are from the most recent transmit pulse
Probe Selection
- Frequency
- dictated by penetration depth required
- high frequencies give best resolution, lower frequencies give greater penetration
- Size
- determined by acoustic window available
- e.g. large probe for upper abdomen, compact probe for heart
- Type
- determined by field of view required
- linear gives best near field, curved gives larger field at depth
Beam formation
Pulse Echo Operation
Display modes
A-mode
Amplitude mode is the display of the processed information from the receiver versus time. As echoes return from tissue boundaries and scatterers (a function of the acoustic impedance
differences in the tissues), a digital signal proportional to echo amplitude is produced as a function of time. One “A-line” of data per PRP is the result. As the speed of
sound equates to depth (round-trip time), the tissue interfaces along the path of the
ultrasound beam are localized by distance from the transducer. The earliest uses of
ultrasound in medicine used A-mode information to determine the midline position
of the brain for revealing possible mass effect of brain tumors. A-mode and A-line
information is currently used in ophthalmology applications for precise distance
measurements of the eye. Otherwise, A-mode display by itself is seldom used.
In A-mode, the echoes are depicted as vertical lines with an amplitude proportional to the echo strength on a time scale.
Usage
- Uncommonly used
- Mainly ophthalmic sonography for precise intraocular measurements
B-mode
Brightness mode produce the conventional 2D image, where intensity/brightness at each point represents the signal amplitude of the corresponding echo.
In B-mode, the echos are formed from the electronic conversion of the A-mode and A-line information into brightness-modulated dots along the A-line trajectory.
- White for maximum echo strength
- Grey for mid-range echoes
- Black for echo-free areas
The B-mode display is used for M-mode and 2D grey-scale imaging.
M-mode
Motion mode is a technique that uses B-mode information to display the echoes from a moving organ, such as the myocardium and valve leaflets, from a fixed transducer position and beam direction on the patient.
The echo data from a single ultrasound beam passing through moving anatomy are acquired and displayed as a function of time, represented by reflector depth on the vertical axis (beam path direction) and time on the horizontal axis.
In the M-mode, the echoes are displayed as lines of bright dots on a time line.
It provides excellent temporal resolution of motion patterns, allowing the evaluation of the function
of heart valves, myocardium and other cardiac anatomy. Only one anatomical dimension is represented by the M-mode technique, and with advances in real-time 2D echocardiography,
Doppler, and color flow imaging, this display mode is of much less importance than in the past.
Usage
- Used in echocardiography to measure mitral valve excursion
Harmonic imaging
Harmonic imaging is a technique in ultrasonography that provides images of better quality as compared with conventional ultrasound technique.
Physics
Harmonic imaging exploits the non-linear propagation of ultrasound through the body tissues. The high-pressure portion of the wave travels faster than the low-pressure portion resulting in distortion of the shape of the wave. This change in waveform leads to the generation of harmonics (multiples of the fundamental or transmitted frequency) from a tissue. At present, the 2nd harmonic is being used to produce the image because the subsequent harmonics are of decreasing amplitude and insufficient to generate a proper image.
These harmonic waves that are generated within the tissue, increase with depth to a point of maximum intensity and then decrease with further depth due to attenuation. Hence there is an optimum depth below the surface at which maximum intensity is achieved.
Advantages over conventional ultrasound
- decreased reverberation and side lobe artifacts
- increased axial and lateral resolution
- cyst clearing
- increased signal to noise ratio
- improved resolution in patients with large body habitus