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Physics And Instrumentation Flashcards

1
Q

Amplitude

A

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

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2
Q

Wavelength

A

Distance between two successive waves

Measured in length (m\mm)

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3
Q

Frequency

A

Number of wave cycles (oscillations) per second
Measured in Hz
100 oscillations per sec = 100 Hz

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4
Q

Audible sound

A

20 - 20000 Hz

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5
Q

Infrasound

A

<20 Hz

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6
Q

Ultrasound

A

> 20KHz

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7
Q

Frequency of Ultrasound for echo

A

1.5 - 7 MHz

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8
Q

Propagation Velocity

A

speed at which the wave propagates through the medium.

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9
Q

Propagation velocity of various tissues

A 
Lowest to highest
Air
Fat
Soft tissue (I.e heart)
Blood
Muscle 
Bone
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10
Q

Propagation velocity equation

A

Propagation velocity =frequency x wavelength

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11
Q

Relationship between frequency and resolution and penetration

A

Higher frequency = better resolution = poorer penetration

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12
Q

Acoustic impedance

A

Resistance to ultrasound transmission

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13
Q

Two types of reflection

A

Speculator (mirror-like) reflection

Backscatter

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14
Q

Specular reflection

A

‘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

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15
Q

Backscatter reflection

A

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.

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16
Q

Rayleigh scatterers

A

Scatter equal in all directions (e.g RBCs)

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17
Q

Attenuation

A

As an ultrasound pulse travels through tissue, it will gradually lose energy

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18
Q

Define HID

A

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.

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19
Q

HID Equation

A

HID (soft tissue) = 6/f

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20
Q

Refraction

A

change in direction of an ultrasound pulse as it passes across a boundary between two tissues (or materials) of different acoustic impedance.

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21
Q

What is a transducer?

A

Probe that generates ultrasound and works as transmitter and receiver of ultrasound waves

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22
Q

Piezoelectric effect

A

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.

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23
Q

Phased array transducer

A

consist of an array of piezoelectric elements (typically 128 for a 2D echo probe, several thousand for a 3D probe).

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24
Q

Structure of an ultrasound transducer

A 
Cable 
Backing layer
Piezoelectric elements 
Matching layer
Acoustic lens
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25
Backing Layer
Has high impedance and is designed to absorb ultrasound and ‘damp down’ reverberation (‘ringing’) of the piezoelectric elements.
26
Matching layer
improves the impedance matching between the elements and the body.
27
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.
28
Far field | Fraunhofer zone
Ultrasound diverges after near field
29
Acoustic lens
Helps to focus ultrasound beam
30
Fundamental frequency
Original frequency of transmitted signal from transducer
31
Harmonics
Multiples of original frequency of signal returning to transducer
32
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.
33
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.
34
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.
35
B-mode
Brightness mode | represented the amplitude of the returning signal by the brightness of a dot.
36
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
37
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
38
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
39
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
40
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.
41
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
43
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.
44
Sector width
determines the field of view across which the ultrasound beam sweeps.
45
Grey scale compensation
Dynamic range Adjusts number of shades of grey displayed in image Chooses degree of contrast
46
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)
47
Spatial resolution
2 components : Axial resolution Lateral resolution = azimuthal resolution
48
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
49
Lateral resolution
Azimuthal resolution Relates to objects perpendicular to us beam Varies according to sector width/depth/gain Higher gain = worse lateral resolution
50
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
51
Imaging artefacts
'Structures' and/or distortions seen on echo that are not actually present in the heart or in that location
52
Acoustic shadowing
Highly echo reflective structure (e.g. Mechanical valve prosthesis) blocks us from penetrating further causing echo dropout in far field
53
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.
54
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
55
Ring down artefact
Resonance phenomenon from collection of gas bubbles | Appears similar to comet tail
56
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.
57
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
58
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’.
59
Unit of measurement of intensity of exposure
Watts/cm2 Power per unit area Maximum intensity within the ultrasound beam
60
Spatial peak temporal average (SPTA)
Spatial peak averaged over the duration of exposure (temporal average) SPTA = Spatial peak avg / temporal avg
61
Biological effects of ultrasound
``` Thermal effects (heating) Mechanical effects ```
62
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 ```
63
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
64
Focus
Fine tuned with phased array transducers | Concentrates on area of interest
65
Speed of sound in tissue
1540m/s
66
Equation for propagation speed
Propagation speed = frequency x wavelength
67
Period
Time taken for one cycle or wavelength to occur
68
Period equation
Period = 1/frequency
69
Basis of continuity equation
Law of conservation of mass
70
EROA equation
EROA = regurgitant flow rate / flow velocity

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