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SOE 482: Ultrasound Principles 2

The following questions relate to the use of ultrasound…

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Question No. 2

Q: What is the piezoelectric effect?

Answer No. 2

  • An effect exhibited by certain quartz crystals (such as lead zirconate titanates) known as 'piezoelectric crystals'
  • The piezoelectric crystal will change shape when an electrical potential is applied across it:
    • Dimensions change slightly depending upon the polarity of the applied voltage
    • Change in shape causes local changes in pressure, leading to mechanical waves (ultrasonic radiation) being generated at the exact frequency of the applied voltage
  • The converse effect occurs when pressure is applied to a piezoelectric crystal:
    • Compressive stress causes a change in dimensions of the crystal
    • Generates an output voltage at the frequency of the compressive waves
  • Allows piezoelectric crystals to act as transducers:
    • Direct effect converts mechanical energy into electrical energy
    • Converse effect converts electrical energy into mechanical energy

Question No. 3

Q: How are ultrasound waves and images generated?

Answer No. 3

  • Ultrasound waves are produced by an ultrasound probe:
    • An electric current is passed through a piezoelectric crystal:
    • Act as transducers converting electrical energy into mechanical vibrations at high frequencies
    • Generates ultrasound waves at the frequency of the voltage applied
  • Ultrasound waves travel through a given medium
  • When a structure is encountered some waves are reflected back towards the probe
  • The probe detects returning waves:
    • The pressure effect distorts the piezoelectric crystals producing an output voltage
    • Converts the mechanical energy into electrical energy
  • Output signals from the probe are then converted into an image:
    • The time between the waves being sent out and returning is calculated
    • Depth of a structure is determined by the time taken for a wave to return
    • Pixels are created at the appropriate depth for returning waves
    • The brightness of the pixel correlates with the strength of returning wave
  • Multiple crystals are located within a probe:
    • Positioned adjacent to each other in an "array"
    • Connected electrically to generate a 2D image
  • As the ultrasound probe both emits and receives the signal they are known as 'transceivers'.

Question No. 4

Q: Does the transducer produce ultrasound waves continuously?

Answer No. 4

  • The transducer alternates through phases of generating and receiving ultrasound waves
  • The 'Transmission Phase' is the period of wave generation:
    • Usually very brief (0.5–3 μs)
  • Followed by a 'Receiver Phase' for wave detection:
    • Much longer (up to 1 ms) than the transmission phase
    • Allows echoes from a range of depths to be detected
  • The combined duration of the transmission and receiver phases is the pulse repetition period:
    • Determines the frame rate of an ultrasound generated image
    • Shallower depths allows for a shortened receiver phase and thus a higher frame rate

Question No. 5

Q: What is the frequency of ultrasound waves?

Answer No. 5

High-frequency sound waves greater than 20 kHz

  • Medical ultrasound uses waves of very high frequency, in the range of 2-20 MHz
  • The human audible range is 20-20 000 Hz

Question No. 6

Q: What is the structure of an ultrasound transducer?

Answer No. 6

Question No. 7

Q: What types of ultrasound probes are available?

Answer No. 7

  • Probes can be characterized according to their:
    • Shape
    • Arrangement of the piezoelectric crystals
    • Frequency range
    • Footprint
  • In general they are classified as:
Type
Description
Uses
Examples
Linear-Array
  • Crystals arranged in a linear fashion
  • Flat rectangular footprint which produces a rectangular image
  • Generally of high frequency (5-10 MHz) providing good resolution for shallow structures
  • A "hockey stick" probe is a linear array probe with a small footprint for use in paediatrics or smaller anatomical areas
  • Vascular access
  • Superficial nerve blocks (supraclavicular and axillary brachial plexus, forearm, femoral etc.)
  • Musculoskeletal imaging
  • Pleural imaging
Curvilinear (Convex) Array
  • Crystals arranged alongside each other across a curved face
  • Produce a sector shaped image with a curved top and bottom
  • Generally of lower frequencies (2–5 MHz), allowing for better imaging of deeper structures
  • Abdominal and pelvic imaging
  • Lung Imaging
  • Deep nerve blocks (sciatic, infraclavicular brachial plexus etc.)
Phased Array
  • Crystals arranged in a very small cluster
  • Has a flat footprint which produces a pie-shaped image
  • Generally of lower-frequency (2–8 MHz) allowing imaging of deeper structures
  • Small footprint makes it useful for cardiac imaging between ribs
  • Echocardiography
Endocavity
  • Specialist probes with long handles to scan inside body cavities
  • Common examples include: vaginal, anorectal, laparoscopic and oesophageal probes
  • Internal imaging (transvaginal, transrectal etc.)

Question No. 8

Q: What are the benefits and uses of high and low-frequency probes?

Answer No. 8

Type
High-Frequency Probes
Low-Frequency Probes
Description
  • 6-15 MHz range
  • Good resolution (0.5 mm axial and 1.0 mm lateral)
  • Reduced penetration (5-6 cm depth of field)
  • 2-5 MHz typical range
  • Good penetration (5-18 cm depth of field)
  • Reduced resolution (2.0 mm axial and 3.0 mm lateral)
Use
  • Vascular access
  • Superficial nerve blocks (supraclavicular and axillary brachial plexus, forearm, femoral etc)
  • Musculoskeletal imaging
  • Pleural evaluation
  • Abdominal and pelvic imaging
  • Lung Imaging
  • Deep nerve blocks (sciatic, infraclavicular brachial plexus etc)
  • Neuraxial structures

Question No. 9

Q: Which imaging modes can be used with ultrasound?

Answer No. 9

A-Mode
(Amplitude Mode)
  • Simplest form of imaging using a single wave emitted from the probe
  • Scans a line through the body with echoes plotted as a function of depth
  • Rarely used in intensive care or anaesthesia
  • Used by ophthalmologists to measure the diameter of the eyeball
B-Mode
(Brightness Mode)
  • The most commonly used mode in anaesthesia and intensive care
  • Uses a linear array of transducers to produce a line of ultrasound waves
  • Ultrasound scans through a section of tissue and is reflected back producing a two-dimensional view
  • The intensity of the image generated is proportional to the intensity of reflected echoes received
  • Best image is produced when the reflector is at 90° to the ultrasound beam
M-Mode
(Motion Mode)
  • Used to show how a structure moves temporally across a single beam of ultrasound
  • Plots a rapid sequence of a-mode images over time
  • Often used in cardiac or thoracic scanning to show the movement of structures (e.g. heart valve motion)
Doppler Mode
  • Utilizes the doppler effect to detect the direction and velocity of flow
  • Colour Flow Doppler: displays the direction and magnitude of flow with colour. Blue indicates flow Away from the ultrasound probe and Red indicates flow Towards the probe (BART)
  • Duplex Doppler: combines real time color doppler superimposed onto real-time grey-scale b-mode image

Question No. 10

Q: What is the doppler effect?

Answer No. 10

  • Describes the change in the transmitted frequency of a sound wave (ft) compared to the reflected (observed) frequency of the wave (fr) that occurs due to the relative motion between the observer and the source
    • If the source and the observer are stationary any waves reflected back will be of the same frequency (fr = ft)
    • If the source is moving toward the observer it causes waves to be more closely packed together, so the observer witnesses a higher frequency wave (fr > ft)
    • Conversely, if the source is moving away from the observer it causes waves to be more loosely packed together, so the observer witnesses a lower frequency wave (fr < ft)
  • The change in the observed frequency from that originally transmitted is known as the 'Doppler shift' (fd = fr - ft)
    • It is proportional to the relative velocity between the source and the observer
  • It does not matter whether it is the source or the observer which is moving
  • An example in everyday life is the change in pitch of a siren as a police car passes by

Question No. 11

Q: How is the velocity or magnitude of movement calculated using the doppler effect?

Answer No. 11

  • The detected Doppler shift frequency (fd) can be used to calculate the velocity of an object the waves reflect off using the 'doppler equation':
v = flow velocity (m/s)

c = the speed of sound in tissues (m/s)

fd = Doppler frequency shift that is received (Hz)

cos θ = cosine of the angle between the sound beam and moving fluid (45°)

fT = frequency of the transmitted ultrasound from the transducer (Hz)

Question No. 12

Q: Which features on an ultrasound scanner can be changed to optimise the image?

Answer No. 12

Gain
  • Describes the degree of amplification of the reflected signal that is received
  • If low gain is used, tissues which are poor reflectors will not be visualised
  • If high gain is used noise is added to the image and it can become difficult to delineate different structures
Focus
  • The beam possesses a focal point where resolution is highest, much like a camera lens
  • This is usually set to an area 2/3rds of the total depth
  • The structure of interest should be positioned within the area of greatest focus
Depth
  • Optimum depth should always be selected to allow visualisation of the structures without additional areas
  • Shallower depth allows for a faster frame rate
Dynamic Range
  • Describes the range between the minimum low and maximum high signal intensity that is displayed
  • This equates to the contrast that is viewed within the image upon the screen
  • Generally a high dynamic range is used to improve the image quality although it may need to be reduced in the presence of "noise"
Sector Width
  • Selecting a narrower sector width allows a faster frame rate and increased resolution but with reduced field of view
Frequency
  • Can often be changed to increase the resolution depending upon the structures being imaged

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