Mersey US Flashcards
- Pressure varies along an US wave
- Pressure is highest at a compression & lowest at a rarefaction
- Wavelength = distance between corresponding compressions
- Frequency = number of wavelengths emitted each second
- Amplitude = increase in pressure at a compression
Acoustic Impedance (Z)
- A measure of how a tissue resists the passage of the US wave
- Z is affected by the density & compressibility of tissue
- For any tissue: Z = density x wave speed
- Z is measured in units of rayl
- NB – Z is independent of the US frequency
- Bone and PZT crystals have high Z
- Air & lung have low Z
- Differences in Z between tissues are important for
creating reflections (echoes)
- Wavelength range 0.1-1.0mm in clinical images
- Audible range of the human ear is 20Hz - 20kHz
- US imaging uses a frequency range 2-15 MHz
- In US, frequency is inversely proportional to wavelength
➢ At 1.5MHz: wavelength = 1.0mm in soft tissue
➢ At 15MHz: wavelength = 0.1mm in soft tissue
Speed of Propagation (Wave Speed)
- Waves speed depends on the stiffness or
compressibility of the tissue or medium - Wave speed, c = frequency x wavelength
- US machines assume the waves travel at 1540m/s in the patient
- Speed of propagation varies for different tissues
➢ Bone 4000 ms-1
➢ Fat 1450 ms-1
➢ Lung 600 ms-1
Wave speed is independent of the US frequency
NEED TO KNOW FOR EXAM
Interactions of US with Tissue
There are 4 basic interactions of US radiation with tissue:
* Reflection – partial reflection can occur at boundaries of 2 dissimilar tissues.
* Scattering – structures smaller than the wavelength of the beam scatter US radiation in all directions
- Refraction – change in direction of US wave when striking a boundary of 2 tissues obliquely
- Absorption – US energy is converted into heat as it passes through tissue
Specular Reflection
- Specular reflection occurs at large, smooth boundaries between
tissues where there is a change in acoustic impedance - Incident wave is partially reflected
- The reflected ‘echo’ is weaker than the incident wave
- The incident wave is transmitted with reduced amplitude
- The transmitted wave can create further echoes
- When the incident beam strikes the boundary obliquely specular
reflection still occurs - The angle of reflection = angle of incidence
- The intensity of the reflected wave is still reduced
Scattering
- Structures < wavelength cause scattering
- Very weak echoes produced in all directions
- Wavelength depends on frequency, so scattering will also depend on frequency
- At 1.5MHz, scattering occurs from objects < 1mm
- At 15MHz, scattering occurs from objects < 0.1mm
Specular reflection – echo strength
- The intensity of the reflected wave (with respect to the incident wave) depends on the difference in acoustic impedance between the 2 tissues (Z1 & Z2)
- Relative echo strength is independent of US frequency
Diffuse Reflection
- Diffuse reflection occurs if the reflecting surface interface isrough or has undulations that are smaller than the wavelength of the radiation
- Multiple reflected waves are generated & emitted in different
directions
Refraction
- Refraction is the change in direction of a wave passing between 2 tissues where there is a change of wave speed
- It only occurs when the incident beam strikes the tissue boundary obliquely (there is no refraction for normal incidence)
- The transmitted wave moves in a different direction from the incident wave
- The magnitude of the change in direction depends on the ratio of the wave speeds in the 2 tissues
- The reflected wave is unaffected
US Beam Attenuation
- As ultrasound pulses (and reflected echoes) travel
through tissue, their intensity is reduced or
attenuated.
- Attenuation of the US beam is due to:
➢ Reflection and scattering removes/redirects energy
from the US beam
➢ Beam divergence – energy is spread over a wider area thus reducing intensity
➢ Frictional losses – results in absorption of US energy by tissue and is converted into heat.
US Beam Attenuation
* The intensity of any US beam decreases exponentially
with depth in tissue.
* Attenuation increases with US frequency
US Attenuation Coefficients
NEED TO KNOW
- US attenuation coefficients are quoted in units of dB/cm/MHz
- Soft tissue attenuation coefficient ~0.8dB/cm/MHz:
– 0.8 dB/cm at 1 MHz (17% reduction in intensity per cm)
– 4 dB/cm at 5MHz (60% reduction in intensity per cm)
- Water/blood attenuation coefficient
~0.15dB/cm/MHz:
– 0.15 dB/cm at 1 MHz (3% reduction in intensity per cm)
– 0.45 dB/cm at 5MHz (10% reduction in intensity per cm) - Bone attenuation coefficient ~15dB/cm/MHz:
– 15 dB/cm at 1 MHz (97% reduction in intensity per cm) - Lung attenuation coefficient >30dB/cm/MHz
Ultrasound Transducer
- An ultrasound transducer converts electrical energy into ultrasound waves (mechanical energy) & vice versa
- Transducer acts as both the source & detector of US waves using the Piezoelectric effect
- US production - alternating voltage applied to the end faces of the transducer crystal causes it to vibrate
- US detection – waves arriving at the crystal face cause it to vibrate & generate an alternating voltage
- PZT (lead zirconate titanate) often used as the transducer crystal
Ultrasound Transducer
- The crystal is backed with a damping material to prevent the crystal from vibrating
- This is important for the production of short pulses
- An “acoustic window” is fitted to the front surface of the transducer to protect the crystal from damage and transmit as much of the US radiation as possible into the patient
ACOUSTIC FIELDS
- US imaging is performed in the near field as far as the focal zone.
- The shape of the beam is affected by the frequency of the US radiation and
the diameter of the transducer crystal. - The near field is narrower at higher frequencies
Unfocused beam - parallel - then diverges
Focused converges and then diverges
Basis for US Imaging
- All US imaging derives spatial information about the location of tissue
boundaries using the “pulse echo” principle - A short pulse of US (typically ~1µ sec in duration) is transmitted into the body
& the time taken for it to return to the transducer is measured - If the velocity of the pulse in tissue is known (or assumed) then the depth of
the reflecting structure relative to the transducer can be determined
US Pulses
- A pulse is a short burst of US radiation
- Each pulse has finite duration and size (length)
- Length is typically 2-3 wavelengths
- Duration is typically ~1s
- The number of pulses emitted each second by the transducer is called the
pulse repetition frequency (PRF) - In clinical US imaging the PRF is typically 3-6kHz
US Scanning
* There are various ways data can be recorded
➢ A-mode
➢ B-mode
➢ M-mode
➢ CW Doppler
➢ Pulsed Doppler
➢ Harmonic imaging
A-Mode Scan
- In A-mode (“A” = “amplitude”) pulses are sent down a single scan line and
produces a graphical display, not an image - Depth is represented by the horizontal axis of the display
- The amplitude of each echo is represented on the vertical axis
- A-scans can accurately depict the depth of structures in the patient
- Used in opthalmology to measure the axial length of the eye & in obstetrics to
measure the size of the fetal head - A-mode uses high US frequency (typically 10MHz) to improve accuracy
B-Mode Scan
Brightness Mode
We fire a scan line into the patient, we’re going to get partial reflection at these two positions. And in the image, we see two dots which match the depth of the reflecting surfaces. The brightness of the dots reflects the magnitude of the echo at those places, and then we do that for subsequent lines across the image.
M Mode Scan
stands for motion. With this, it’s like a B mold scan, except you only scanned on a single line, so there’s a single scan line.
Time Gain Compensation
- Depth of echo = Return time x wave speed / 2
- Echoes arriving later are from a greater depth and have suffered greater attenuation
- In TGC pulses are amplified according to when they arrive at the
transducer - With TGC all echoes from the same tissue boundary will have the same
brightness – regardless of depth
US Transducers / Probes
- Used to generate & detect US pulses
- Modern transducers contain an array of 64 to 512 small piezoelectric crystals
(usually PZT) - The crystals have a thickness = 1⁄2 wavelength of emitted US waves
- High frequency probes have thin crystals (0.1mm at 7.5MHz)
- Low frequency prob
es have thick crystals (0.5mm at 1.5MHz) - There are 3 main types of transducers/probes
Linear Array Transducer
- A linear array probe consists of 256 to 512 discrete transducer elements.
- A small group of adjacent elements are simultaneously activ
ated to create a scan
line - Sequential activation of adjacent groups of crystals creates a series of scan lines
across the transducer surface. - All scan lines are emitted at 90 degrees to the face of the probe
Linear Array Transducer
- These probes generate a limited FoV which is rectangular
- Tend to operate at higher frequencies (>5MHz) to improve resolution but
only image superficial structures (1-4cm depth of view)
- Useful for scanning superficial structures such as the neck, breast, scrotum and extremities, or paediatric studies
Curvilinear Array Transducer
- These probes have a wider FoV than linear array probes
- Tend to operate at lower frequencies (2-5MHz) to allow the beam to penetrate to greater depth (typically 4-8cm)
- Used for abdominal, fetal, and obstetric studies
- Diverging scan lines with depth results in reduced lateral resolution
Phased Array Transducer
- Phased arrays generate sector FoVs
- Tend to operate at low frequencies (1-
5MHz) - Useful for cardiac imaging (small
footprint means the probe can scan
between the ribs)
Lateral Resolution
Lateral resolution in B-Mode scans depends on:
Beam width
* Best resolution is in the focal zone
Number of scan lines per frame
* More scan lines provides better resolution,
but each frame takes longer to acquire
Frequency of the US radiation
* Resolution is better (beam width is
narrower) at higher frequency
Lateral resolution is typically:
2.5mm at 3MHz
1mm at 10MHz
Lateral Resolution is DEPTH DEPENDANT
In the near and far fields, the beam
is wider than it is at the focal
distance.
Two point reflectors separated by
the same distance will appear as a
single structure in the image in the
near & far field, but will be resolved
at the focal distance.
Axial Resolution
Axial resolution in B-Mode scans is determined by the spatial pulse length (SPL)
Axial resolution = 1/2 x SPL
Axial resolution improves with increasing frequency
Axial resolution is not depth dependent
Doppler Effect
Doppler Effect
Continuous Wave (CW) Doppler
- CW Doppler requires 2 transducers
- 1 transmits continuously, the other
receives continuously - Doppler signal arises from where the
beams overlap - Because the pulse-echo principle is not
used there is NO DEPTH information
- CW probes emit narrow bandwidth US
- Can measure very high flow velocities
- Doppler frequency shift can be
converted into an audible signal and
also as a velocity spectrum graph - Often used in cardiac scanners to
investigate the high velocities in the
aorta
Shift in KHz = audible range
Colour Flow Doppler
- Colour coded flow information is superimposed on a B-mode image
- Live image, updated in real time displaying overall view of flow in a region
- Displays direction & magnitude of blood flow
- Red & blue indicate flow in different directions
- BART – Blue Away, Red Towards the probe
- Aliasing occurs if Doppler frequency > 1⁄2 PRF
Power Doppler
- A form of colour flow imaging
- Displays magnitude of blood flow only – no directional
information - No aliasing artifacts
- Good for detecting low flow rates
Harmonic Imaging
- The profile of an US wave becomes distorted as it travels through tissue.
- The wave speed is actually dependent on pressure, which is different during
the compression & rarefaction parts of the cycle. - The wave travels faster during compression & slower during rarefaction
- As a result the wave profile changes and this becomes more evident the
further the wave travels. - The change in wave profile enhances frequency content at multiples of the
transmit frequency (harmonic frequencies).
Spectral Doppler Display
- A display of the spectrum of Doppler frequencies (flow velocities) on the vertical axis vs time on the horizontal axis
- Examines detailed flow information along a single scan line
- Allows detailed analysis of distribution of flow
- Good temporal resolution – can examine flow waveform
- Allows calculations of velocity and indices
- Can be used for CW or PW Doppler
Harmonic Imaging
- In Harmonic Imaging, the returning signal (C) is actually a combination of
frequencies. - It contains not only the fundamental signal that was originally
transmitted (A), but also the harmonic signal (B), which is twice A’s
frequency.
Relative changes in the fundamental and harmonic signal with depth
The production of harmonic waves is greatest where the beam intensity is
highest - along the central axis of the beam.
Little or no harmonics are produced by weaker waves, such as those from
the edges of the US beam, scattered echoes, side lobes and reverberations.
Reverberation Artifact
Echoes generated from two parallel
highly reflective surfaces may be
repeatedly reflected back and forth
before returning to the transducer for
detection. When this occurs, multiple
echoes are recorded and displayed.
The echo that returns to the
transducer after a single reflection will
be displayed in the proper location.
The sequential echoes will take longer
to return & will be erroneously placed
at increased distances from the
transducer.
Comet Tail Artifact
- Comet tail artifact is a form of
reverberation, where the two reflective
interfaces (& echoes) are very closely
spaced. - On the display, the sequential echoes may be so close together that individual signals are not perceivable. Also, the later echoes will have decreased amplitude due to attenuation, resulting in decreased width. The result is an artifact with a triangular, tapered shape.
- This artifact is often caused by small
calcifications and metal objects (foreign
bodies and surgical clips).
Ring Down Artifact
* Ring down artifacts resemble comet tail artifacts in appearance.
- The artifact is caused by an interaction of the US beam with clusters of gas bubbles. When the US beam reaches the gas bubbles, it is capable of exciting the liquid trapped between the bubbles, which causes the liquid to resonate. These vibrations create a continuous sound wave which is transmitted back to the receiver.
- Because the resonant wave is emitted continuously (not as a pulse) it is portrayed as a streak extending posteriorly from the gas cluster.
Speed of Sound Artifact
When sound travels through material
with a velocity significantly slower than
the assumed 1540 m/sec, the returning
echo will take longer to return to the
transducer.
The image processor assumes that the
length of time for a single round trip of
an echo is related only to the distance
travelled by the echo.
The echoes are thus displayed deeper on
the image than they really are.
Refraction Artifact
When an US beam is obliquely incident on
the boundary between two tissues with
different speeds of sound (eg. fat and muscle) refraction occurs. The direction of the US beam is altered as it crosses the boundary.
The image processor assumes all emitted
pulses travel in straight lines from the
transducer.
Hence the position of the reflector (black
circle) will be incorrectly placed in the image
(dotted circle).
Thermal Index (TI)
- TI provides an onscreen indication of the relative potential for a tissue temperature rise produced after long exposure
- TI = W / Wdeg where W is the acoustic power of the beam at a specified
point, and Wdeg is the power required for a maximum temperature rise of
1°C anywhere in the beam.
(TI is estimated by the machine and can be in error by as much as x2)
- Different values for soft tissue (TIS) & bone (TIB)
- Pulsed Doppler techniques generally involve greater temporal
average intensities and powers than B-mode, and hence greater
heating potential.
LEARN:
- No restriction on exposure time for TIS ≤ 0.7
- Embryo/fetal scanning not recommended for TIS > 3.0
