[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 affectimage 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.
Introduction
Ultrasound
Medical sonography is a soft-tissue imaging modality that utilises ultrasound energy and the acoustic properties of the body to produce an image. This is achieved by using a transducer to transmit pulses of ultrasound into the body tissue and recording a collection of echo amplitudes returning to the receiver over time.
The universal wave equation is an important formula to remember:
c = f x λ
Where c = propagation speed (1540 m.s-1 for soft tissue), f = frequency (rate of oscillation), λ = wavelength (length of a single cycle e.g. 0.8 – 0.08 mm)
Causes pressure oscillations in medium through which it is travelling
Short pulse transmitted into body
echoes are received back form tissue structures in the path of the ultrasound
Interaction of ultrasound with tissue
There are 4 main mechanisms by which ultrasound interacts with tissue.
Attenuation
As ultrasound frequency increases, the rate of attenuation or absorption increases exponentially. Thus high frequencies can be used only for superficial areas (breast, thyroid, testes, MSK, peripheral vessels) or with invasive probes (transvaginal, transrectal, transoseophageal, laparoscopic).
Attenuation = 10 log10 (I1 / I2)
measured in decibels, where I = intensities at differing depths
Attenuation = α x L x f x dB
where α = attenuation coefficient of tissue (dB/cm/MHz), L = total distance traveled by ultrasound, f = ultrasound frequency (MHz)
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Worked Example Depth = 10cm, frequency = 5 MHz attenuation of transmitted pulse = 0.5 x 10 x 5 = 25 dB attenuation of returning echo = 0.5 x 10 x 5 = 25 dB round path (total) attenuation = 50 dB ∴ 50 = 10 log10 (I1 / I2) ∴ 50/10 = log10 (I1 / I2) 1050/10 = I1 / I2 = 100,000 This means the intensity is reduced by a factor of 100,000 due to the attenuation |
The absorbed sound wave energy is converted into heat. Little absorption and almost no scatter or reflection occurs in fluids
Reflection
Occurs at interfaces between tissues of differing acoustic impedance (Z) (i.e. density). Acoutic impedance of a material is defined as:
Z = ρ ⋅ c
measured in kg/(m2s), where ρ = density (kg/m3 ), c = speed of sound (m/s).
When ultrasound encounters an interface between two tissues, some of the energy is reflected back as an echo, the rest passes through the interface
The percentage of ultrasound intensity reflected depends in part on the angle of incidence of the beam. Increasing the angle of incidence decreases the likeliness of the reflected sound reaching the transducer.
The amount of energy reflected depends on how different the two tissues are:
◦ the difference between the acoustic impedance (Z) of the tissues at an interface
◦ if they are very different, almost all the energy will be reflected
If the tissue interface is at right angles to the ultrasound beam
(“perpendicular incidence”) the reflected energy will travel back to
the probe as an echo
At other angles of incidence the reflected energy will not return to
the probe
Highly reflective interfaces:
Tissues – gas bone plastic
Refraction
When ultrasound passes through an interface between two tissues with different propagation speeds, the direction of travel is altered. The change in direction depends on both the difference in the propagation speeds and the incidence angle. Alteration of the direction of travel due to differences in ultrasound propagation speeds describes the change in direction of the transmitted ultrasound energy with nonperpendicular incidence.
Refraction is described by Snell’s law:
sin θi/sin θt = v1/v2
where θi = angle of incidence, θt = transmitted angle, v1 = velocity in medium 1, v2 = velocity in medium 2.
- If the velocity of sound in medium 2 is greater than that of medium 1, the transmission
angle is greater than the angle of incidence. - If the velocity of sound in medium 2 is less than that of medium 1, the transmission
angle is less than the angle of incidence.
Scattering
Occurs with small tissue structures (such as vessels), where a small fraction of energy is scattered in all directions due to reflection or refraction. Most of the information in the ultrasound image is the result of scattering. The echos from individual scatterers add together randomly, producing a granular texture in the image called speckle. Speckle is a statistical interference pattern.
Echo arrival time
t = 2d / c
or d = ct/2
Pulse Repetition Frequency (PRF)
The PRF is the number of pulses transmitted per second. The higher it is the more quickly the machine can gather the information for each image and so the more images there will be per second.
The machine must not transmit again until all the echoes produced by the most recent pulse have been received. If this rule is violated range ambiguity occurs.
The deeper the ultrasound penetrates the longer the machine must wait for echoes and the lower the Pulse Repetition Frequency (PRF) and Frame Rate will be.
In B-mode ultrasound, the PRF affects
(A) Pulses per second
(B) Frame rate
(C) Number of lines per frame
(D) Maximum penetration depth
It does not affect ultrasound frequency
The ultrasound beam
Ideally the transmitted pulse would travel along a narrow well defined line as this would allow each echo to be correctly positioned in the image. In reality it travels along a beam, the shape and size of which depends on the frequency and the aperture (the area on the probe face used to create the beam).
The higher the frequency and larger the aperture the narrower the beam will be. The beam will be narrowest at focus and wider at other depths. The depth of focus is set by the user.
The beam is best at focus, worse at other depths, degrading the lateral resolution of the image i.e. as the beam scans across a given object it is imaged several times in different lateral positions, creating a lateral smearing effect.
Transducer
An oscillating electricl signal is applied to the transduce and it transmits an ultrasound pulse
The transduce then becomes a receive, detecting the returning echos and converting them into electrical signals
The transducer is divided into a large number of narrow strips (elements) interspaced with filler materials
Focussing
Statistical inteference patterns, hence the average echo is more meaningful
Gel reduces air
Echos process to form an image or give info on blood flow
Quiz
Attenuation = 10 log10 (I1 / I2) ∴ 10 log10 (99) = 19.95