Physics And Instrumentation Flashcards
Amplitude
Indicates strength of sound wave
Measured in dB
measured as the difference between the peak pressure in the medium and the average pressure.
Adjusted by changing transmit power
Wavelength
Distance between two successive waves
Measured in length (m\mm)
Frequency
Number of wave cycles (oscillations) per second
Measured in Hz
100 oscillations per sec = 100 Hz
Audible sound
20 - 20000 Hz
Infrasound
<20 Hz
Ultrasound
> 20KHz
Frequency of Ultrasound for echo
1.5 - 7 MHz
Propagation Velocity
speed at which the wave propagates through the medium.
Propagation velocity of various tissues
Lowest to highest Air Fat Soft tissue (I.e heart) Blood Muscle Bone
Propagation velocity equation
Propagation velocity =frequency x wavelength
Relationship between frequency and resolution and penetration
Higher frequency = better resolution = poorer penetration
Acoustic impedance
Resistance to ultrasound transmission
Two types of reflection
Speculator (mirror-like) reflection
Backscatter
Specular reflection
‘Mirror-like’
occurs at tissue boundaries where the reflector is relatively large (at least two wavelengths in diameter) and smooth
To maximise energy reflected, incoming beam should be perpendicular to the reflector as possible
Backscatter reflection
occurs with small and/or rough-surfaced structures, where the reflected ultrasound will scatter in many different directions. the returning signal will be weaker than from a specular reflector, but will not be dependent on the angle of the incident (incoming) ultrasound beam.
Rayleigh scatterers
Scatter equal in all directions (e.g RBCs)
Attenuation
As an ultrasound pulse travels through tissue, it will gradually lose energy
Define HID
depth (in cm) in soft tissue in which the intensity of the ultrasound is reduced by 50 per cent, and depends upon the frequency (f) of the ultrasound emitted by the transducer, measured in MHz.
HID Equation
HID (soft tissue) = 6/f
Refraction
change in direction of an ultrasound pulse as it passes across a boundary between two tissues (or materials) of different acoustic impedance.
What is a transducer?
Probe that generates ultrasound and works as transmitter and receiver of ultrasound waves
Piezoelectric effect
Voltage applied –> piezoelectric crystals oscillate –> change shape –> generate ultrasound
Ultrasound sound wave returns –> crystals oscillate and change shape –>creates electrical voltage later used for image analysis.
Phased array transducer
consist of an array of piezoelectric elements (typically 128 for a 2D echo probe, several thousand for a 3D probe).
Structure of an ultrasound transducer
Cable Backing layer Piezoelectric elements Matching layer Acoustic lens
Backing Layer
Has high impedance and is designed to absorb ultrasound and ‘damp down’ reverberation (‘ringing’) of the piezoelectric elements.
Matching layer
improves the impedance matching between the elements and the body.
Near field
‘Fresnel zone’
US beam remains cylindrical for a short distance after leaving transducer
Imaging quality best within the near field and maximising depth of near field is important for image optimisation
length of the near field is greater at higher transducer frequencies and wider transducer diameters.
Far field
Fraunhofer zone
Ultrasound diverges after near field
Acoustic lens
Helps to focus ultrasound beam
Fundamental frequency
Original frequency of transmitted signal from transducer
Harmonics
Multiples of original frequency of signal returning to transducer
Second harmonic imaging
filters the returning signal to remove the fundamental frequency and build up an image using the second harmonic components of the signal. In so doing, the image resolution improves (because of the higher frequency), particularly for far field structures.
Disadvantages of second harmonic imaging
requires a higher power output, and it does slightly alter the appearance of myocardial texture and also the apparent thickness of structures such as valve leaflets compared with fundamental imaging.
A-mode
Amplitude mode
plotted the amplitude of the reflected ultrasound (as a ‘spike’ with a certain amplitude) versus the distance of the reflected signal from the transducer.
B-mode
Brightness mode
represented the amplitude of the returning signal by the brightness of a dot.
M-mode
Motion mode
records motion along a single ‘line of sight’, selected by careful positioning of the on-screen cursor across a region of interest
2D imaging
ultrasound probe sweeps a beam across the heart around 20–30 times per second, creating a series of scan lines (usually around 120) each time it makes a sweep, in order to build up a 2D image
Frame rate
Number of image frames generated each second
reducing the sector width and/or depth will reduce the time taken to generate an image frame, increasing the frame rate
Transmit Power
controls the amount of ultrasound energy delivered to the patient, and to minimize the risk of adverse mechanical or thermal effects it is important to use the lowest setting possible
Gain
refers to the amplification of the received signal to increase the brightness of the displayed images. Gain can be adjusted for the whole image (overall gain) or for part of the image (see time-gain compensation (tGC) below). While a high gain setting can be useful for detecting weaker signals that might otherwise not be visible, it reduces lateral resolution and also increases noise.
Time gain compensation
known as depth compensation, and corrects for the attenuation of the ultrasound signal that occurs with increasing distance from the transducer. tGC boosts the gain of the signals returning from the far field to ensure an even ‘echo brightness’ across the whole depth of the image.
Slider bars
Depth
determines how far the ultrasound beam ‘looks’ into the patient and is an important determinant of frame rate. the greater the depth setting, the longer the transducer will have to wait for the ultrasound pulse to make its round trip before repeating the pulse, and so the lower the frame rate.
Sector width
determines the field of view across which the ultrasound beam sweeps.
Grey scale compensation
Dynamic range
Adjusts number of shades of grey displayed in image
Chooses degree of contrast
Resolution
Ability to discriminate between 2 objects that are close together in space (spatial resolution) or two events that occur close together in time (temporal resolution)
Spatial resolution
2 components :
Axial resolution
Lateral resolution = azimuthal resolution
Axial resolution
Relates to objects along axis of us beam
Determined by transducer frequency and pulse length
Higher frequency + shorter pulse length = better axial resolution
Typical axial resolution = 3mm
Lateral resolution
Azimuthal resolution
Relates to objects perpendicular to us beam
Varies according to sector width/depth/gain
Higher gain = worse lateral resolution
Temporal resolution
=frame rate
Frame rate depends on time taken to collect data required to take one image
M mode higher frame rate compared to 2d imaging
Imaging artefacts
‘Structures’ and/or distortions seen on echo that are not actually present in the heart or in that location
Acoustic shadowing
Highly echo reflective structure (e.g. Mechanical valve prosthesis) blocks us from penetrating further causing echo dropout in far field
Reverberation
Sound encounters 2 highly reflective layers. Sound is bounced back and forth between two layers before travelling back therefore takes longer and presumed to be farther and display additional ‘reverberated’ images in a deeper tissue layer.
Recognised to be artefact as moves in tandem with original structure.
Comet tail
Similar to reverberation
Produced by front and back of strong reflector
Reverberation space is very narrow and blends into a small band to look like a comet tail
Ring down artefact
Resonance phenomenon from collection of gas bubbles
Appears similar to comet tail
Beam width artefact
When us beam is wider than diameter of refractor being scanned. Therefore normal tissues that lie adjacent to lesion are included in the beam width and their echo texture is averaged to be that of the lesion.
Can be reduced by focusing us beam to minimise its diameter.
Side lobe artefact
Arises from unwanted/unavoidable side beams. Therefore signals returning from side lobe beams are interpreted as coming from main beam, resulting in display of blurred structure some distance away from original
Ultrasound processing method
returning echo signal at the transducer undergoes a series of initial processing steps which include amplification, tGC and filtering. the video signal is then sent to a scan converter, which converts the signal into a ‘rectangular’ format suitable for display. the resulting data undergo further processing (‘post-processing’) and can then be stored in a digital format and/or can undergo digital-to-analogue conversion to create a video signal for display on a monitor (and/or archiving onto videotape). this process occurs so rapidly that the acquired data can be displayed on a monitor almost in ‘real time’.
Unit of measurement of intensity of exposure
Watts/cm2
Power per unit area
Maximum intensity within the ultrasound beam
Spatial peak temporal average (SPTA)
Spatial peak averaged over the duration of exposure (temporal average)
SPTA = Spatial peak avg / temporal avg
Biological effects of ultrasound
Thermal effects (heating) Mechanical effects
Thermal effects of ultrasound
Most notable in TOE caused by conversion of mechanical energy of us into heat energy as it passes through the tissues Amount of heating is related to: Transducer frequency Transmit power Focus Depth
Mechanical effects
Mechanical effects of ultrasound can also be measured by mechanical index (MI), which is the peak negative (rarefactional) pressure divided by the square root of the transducer frequency. An MI of <1 is considered safe.
include cavitation, in which gas bubbles are created as ultrasound passes through the tissues.
When bubble contrast is used it can cause resonance and bubble disruption
Focus
Fine tuned with phased array transducers
Concentrates on area of interest
Speed of sound in tissue
1540m/s
Equation for propagation speed
Propagation speed = frequency x wavelength
Period
Time taken for one cycle or wavelength to occur
Period equation
Period = 1/frequency
Basis of continuity equation
Law of conservation of mass
EROA equation
EROA = regurgitant flow rate / flow velocity
