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.