1
Q
pulse echo imaging
A
- Overview
- Ultrasound beam is intermittently transmitted with pulse typically 2-3 cycles long
- Majority of time occupied by listening for echoes
- Many repetitions of the pulse echo sequence are necessary to construct an image
- Key steps
- Emission
- Voltage applied to piezoelectric crystal produces a pulse of certain length (spatial pulse length/spatial pulse width)
- Number of times voltage is fed is the pulse repetitions frequency (PRF), typically 1000-5000
- Accounts for 0.1% of operating time
- Transmission
- Coupling medium required to transmit pulses
- Includes matching layer and silicon gel
- Reception
- Crystal is deformed by returning echo, producing a voltage
- Emission
2
Q
Amplitude is dependent upon
A
- Amplitude of emitted pulse (Varies with voltage applied)
- Distance of reflector from transducer (due to attenuation)
- Acoustic impedance at the interface
- Type of interface (Specular/non-specular)
- Angle of incidence of beam to interface
- Frequency of transducer
- Amplification by the transducer
3
Q
Major components of transducer design
A
- Piezoelectric crystal
- Converts electrical energy into mechanical (sound) energy by physical deformation of the crystal structure. Conversely, mechanical pressure applied to its surface creates electrical energy
- Sensor electrodes
- Allow voltage to be applied to crystal
- Matching layer
- Provides interface between transducer element and the tissue and minimises the acoustic impedance differences between the transducer and the patient
- Backing block/damping layer
- Made of tungsten/rubber in an epoxy resin
- Absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing
- Damping broadens the bandwidth and shortens the pulse
- Acoustic absorber
- Insulating cover
- Transducer housing
4
Q
Temporal characteristics of ultrasound pulse
A
- Frequency of the pulse – 3-10MHz
- Spatial pulse width
- Number of wavelengths per pulse
- Usual range is 2-5
- Pulse repetition frequency
- Number of time voltage spike is fed to crystal per second
- Range: 1000-5000 timers per second
5
Q
Image reception
A
- Amplifier
- Digital scan converter
6
Q
Image amplification process
A
- Pre-amplifier: avoids loss of low level signals
- RF amplifier: increases small voltages to make them suitable for further processing
- Time Gain Compensation: compensates for attenuation of ultrasound beam by selectively amplifying echoes from deeper structures. Results in artefacts including posterior acoustic enhancement and shadowing
- Compression (“Dynamic range compression”)
- Allows you to tell the ultrasound machine how you want the echo intensity displayed as shades of gray. A broad/wide range will display more shades of gray and an overall smoother image. A smaller/narrow range will display fewer shades of gray and appear as a higher contrast with a more black-and-white image.
- Compression is logarithmic: low level signals amplified much more than high level signals since the signal levels to be differentiated are the low magnitude signals from small internal interfaces
- Demodulation – shape of ultrasound pulse is changed e.g. smoothed
- Rejection (suppression) – low level signals are rejected
7
Q
A
8
Q
Digital scan converter
A
- Action: stores the echo data (position & amplitude) and converts it into a format suitable for display
- Functions
- Digitisation: using Analogue to Digital Converter (ADC) echo is converted to binary number
- Pre-processing
- Control of TGC (described above in amplification) and compression
- ‘Write’ zoom – region of interest is re-scanned every time zoom is changed
- Post-processing
- ‘Read zoom’- simply magnifies the stored image
- Windowing
9
Q
Doppler
A
- Objects moving toward detector reflect sound with higher frequency
- Objects moving away reflect sound with lower frequency
- Shift in frequency is proportional to cos(θ) which is the angle between ultrasound beam and the moving object
- Maximum frequency shift when the object is perpendicular to the detector (or moving away), i.e. angle is 0 or 180 degrees. There is no frequency shift at 90 degrees
10
Q
Doppler and blood flow
A
- Based on shift of frequency in an ultrasound wave caused by a moving reflector (RBC’s are the reflector)
By comparing the incident ultrasound frequency with the reflected ultrasound frequency from the blood cells, it is possible to discern the velocity of the blood
11
Q
Doppler values:
- F =
- v =
- θ =
- c =
A
- F = 2-10 MHz
- v = 0-5m/sec
- θ = 0-60 degrees
- c = 1540m/sec
12
Q
Continuous doppler
A
- Crystal is lightly damped to produce a continuous wave with a narrow bandwidth approaching a single frequency
- A second crystal is required to receive the signal (due to continuous output from the first one)
- Advantage
- Continuous frequency provides no limit on the maximum velocity that can be assessed
- Disadvantage
- There is no depth resolution
13
Q
Pulsed doppler
A
- Pulses repeatedly directed along same scan line to obtain signals
- Longer pulse lengths than traditional B-mode USS are used to improve accuracy of frequency shift (which reduces axial resolution) due to more narrow bandwidth
- Advantage
- Offers depth resolution to only sample echoes that return at a particular time (depth)
- Disadvantage
- Limited by the pulse repetition frequency
- Maximum Doppler shift that can be measured is half the pulse repetition frequency (Nyquist theorem)
- When the blood velocity is too high it creates ambiguity in the Doppler signal (referred to as aliasing as the blood appears to be flowing backwards)
- Maximum Doppler shift that can be measured is half the pulse repetition frequency (Nyquist theorem)
- Limited by the pulse repetition frequency
14
Q
Spectral Doppler
A
- This refers to the display of all the detected frequencies versus time and allows calculation of flow parameters such as Resistive Index (RI)
- High Resistance bed :
- RI 0.6-0.9
- Peripheral arteries
- Low Resistance beds
- RI 0.2-0.4
- Renal And Cerebral vessels
- High Resistance bed :
15
Q
Continuous wave
A
- Use: continuous wave doppler
- Continuous alternative voltage of the desired frequency is applied to crystal
- No damping
- Very narrow bandwidth approaching a single frequency
16
Q
Long Pulse
A
- Use: pulsed Doppler
- Narrow range of frequencies
- Is a tradeoff between the very narrow frequencies needed for Doppler, but gives a spatial pulse length short enough to sample depth
- Light damping
17
Q
Short pulse
A
- Use: conventional imaging
- Wide range of frequencies centred on a resonant frequency (broad-band) (similar to ringing a bell with a hammer)
- Heavy damping (similar to holding bell when it is struck)
Short SPL = good axial resolution
18
Q
Focussing: Rationale
A
- Unfocussed transducer is reliant on beam width for lateral resolution. Narrow beam diameter is better but this creates a short near zone (Fresnel) and a more divergent far zone (Fraunhofer)
- 2 types of focussing – mechanical and electronic
19
Q
Mechanical Focussing
A
- Internal focussing via a curved transducer
- External focussing via using an acoustic lens/mirror
20
Q
Electronic Focussing
A
- Multi-element array
- Instead of a single crystal, several elements are used to form the beam and they can be fired in sequence, from outer to inner, to change the focus
- 2 types exist: linear and phased
- Linear fires a sub-set of elements one after the other (produces a rectangular image)
- Phased uses all of the elements every time but they fire after a variable time delay. Advantage is a smaller footprint
21
Q
Axial Resolution
A
- Axial (linear) resolution (SPL and frequency)
- Ability to discern two closely spaced objects in the direction of the beam
- Determined by the pulse length and frequency
- Higher frequency = better axial resolution
- Short SPL = better axial resolution
- Resolution = ½ SPL
- Constant at all depths
- Trade-off with high frequency is poor penetration
22
Q
Lateral resolution
A
- The ability to discern as separate two closely spaced objects perpendicular to the beam direction
- Determined by beamdiameter
- Narrow beam diameter is better
- On average 4x worse than axial resolution
- The best lateral resolution occurs at the near field-far field interface
- Increasing number of lines per frame increases lateral resolution (at expense of reduced frame rate)
23
Q
Elevational Resolution
A
- Determined by the ‘height’ of the transducer
- Usually the worse measure of resolution for a transducer
24
Q
Harmonic Imaging
A
- Exploits non-linear propagation of ultrasound through the body tissues
- High pressure portion of the wave travels faster than low pressure resulting in distortion of the shape of the wave
- Change in waveform leads to generation of harmonics (multiples of the fundamental or transmitted frequency) from tissue
- By only listening to 2nd order harmonics, low level noise is eliminated
- Improves SNR and resolution
25
Improving spatial resolution
* Use the highest frequency possible (downside is depth of penetration)
* Place the probe as close as possible to the region of interest
* Decrease FOV reduction in pixel size
26
Probe choice
* Size of probe:determined by the size of the acoustic window availabe
* Small footprint if the acoustic window intothe body is small (will be phased array)
* Larger footprint where this limitation doesnot occur
* Invasive or non-invasive
* Invasive for TV or prostate USS
* Shape
* Linear gives best near field
* Curved gives larger field at depth e.g. abdominal applications
27
Ultrasound Assumptions
* Transmitted beam is a narrow straight line
* Ultrasound travels along this line and does not deviate
* Ultrasound travels directly from transducer to reflector then back to the transducer
* Propagation speed is 1540m/s in all tissues
* Attenuation coefficient is uniform in all tissues
* All echoes are from the most recent transmit pulse
28
Attenuation artefact
* Shadowing
* Assumption: that attenuation is uniform at all tissues
* Hypo-intense signal area distal to an object or interface
* Low-signal distal to objects of a very high ultrasound attenuation
* Caused by objects with high attenuation or reflection of the incident beam
* Enhancement
* Enhancement occurs distal to objects having very low ultrasound attenuation, such as fluid-filled cavities (e.g., a filled bladder or cysts).
Hyperintense signals arise from increased transmission of sound by these structures
29
Reverberation artefact
* Multiple echoes generated between two closely spaced interfaces reflecting ultrasound energy back and forth during the acquisition of the signal and before the next pulse
30
Thermal Effects
* A temperature rise of up to 1.5°C above normal body temperature is considered safe for an unlimited time.For sensitive tissues (e.g. fetal) higher temperature increases may be safe for short periods of time.
* Most machines display a Thermal Index (TI) which indicates the likely temperature increase in °C.
* 3 types of TI are given
* **TIS – for soft tissue**
* TIB – for bone
* TIC – for the cranium
31
Mechanical Effects
* The non-thermal bioeffect likely to cause harm iscavitation
* small gas bubbles oscillate in the ultrasound field - under some conditions they may grow in size and thencollapse, creating extremely high energy in theadjacent tissues (equivalent to temperatures greaterthan 1000°C).
Machines therefore display a Mechanical Index (MI) indicating the likelihood of cavitation. Values up to 1.9 are generally considered safe
32
Ultrasound Contrast
* Used for vascular/perfusion imaging
* Gas bubbles are injected intravenously
* Produce significant scattering due to extreme difference in impedance between gas and soft tissue. This way very small capillary beds can be imaged
* For contrast agents, the vibration modes of the encapsulated gas reemit higher-order harmonics due to the small size of the micro bubbles and the resultant contraction/expansion from the acoustic pressure variations
* Harmonic imaging enhances contrast agent imaging by using a low-frequency incident pulse and tuning the receiver (using a multifrequency transducer) to higher-frequency harmonics.
33
Time gain compensation
* Pre-processing technique which amplifies the echoes from deeper structures to create a uniform appearance regardless of depth. Results in artefacts, namely acoustic enhancement and shadowing due to assumption of uniform attenuation through soft tissue of 1540m/s.
34
Dynamic range compression
* What is dynamic range?: the effective operational range of a device from the threshold signal to saturation level
* Why is it needed? Some of the reflected echoes contain very high voltage signals which cannot be processed (will cause saturation) so they must be compressed into a more useable range
* Dynamic range varies with the component
* Memory Has a dynamic range of 40-45db
* Receiver - 100dB or more
* Transducer - 100dB or more
* Display - 20 to 30dB
* Dynamic Range (also known as Compression) allows you to tell the ultrasound machine how you want the echo intensity displayed as shades of gray. A broad/wide range will display more shades of gray and an overall smoother image. A smaller/narrow range will display fewer shades of gray and appear as a higher contrast with a more black-and-white image.
35
Side lobe artefact
* Assumption: that beam travels in straight line
* Side lobes are low-frequency pulses sent out from the side of the transducer
36
Mirror artefact
* Assumption: that the sound only goes from transducer-to reflector-to transducer
* Where sound is reflected off a large interface (e.g. diaphragm) causing parts of the image to be in the wrong location
37
Misregistration artefact
* Assumption: that sound travels at uniform speed AND sound travels travel in a straight line
That deep structures can be seen as closer
38
Radiology Physics
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