Metrology and Quality Assurance Flashcards
What quantities can be measured in an ultrasound?
Acoustic output quantities:
Acoustic pressure
Intensity
Power
Heating
Image quantities:
Monitor brightness and grey levels
Element drop out
Distance measurement
Axial and lateral resolution
Penetration/noise
Why are quantities measured?
Safety
Acceptance testing (checking against a specification)
Performance monitoring / quality assurance
To comply with regulations
Research, e.g., safety and exposure, image quality, model comparison
What is a pressure measurement device?
Hydrophone: underwater microphones
Piezoelectric device made from PVDF (high receiving constant so more sensitive than PZT)
What are the different types of hydrophones?
Needle : needle-shaped hydrophones with the small active element mounted on the tip of the needle
Membrane: the membranes are sufficiently thin to be acoustically transparent at the frequencies of interest
Lipstick: With a curved tip to improve the frequency response by reducing the likelihood of interface waves propagating across it
Fibre-optic: based on an optical fibre so have small active area
What does every hydrophone need and what does it provide?
Calibration certificate
calibration gives acoustic pressure from measured voltage
corrects the measured time series so that it corresponds more closely to the actual field that the hydrophone was measuring
When is it difficult to make calibration measurements?
beyond 40 MHz
How is pressure measured?
Transducer and hydrophone are aligned in degassed, deionised water
Scanning system used to move hydrophone and measure pressure
What is secondary calibration?
a hydrophone is calibrated by comparing its response to that of a previously-referenced hydrophone - the ‘reference’ hydrophone
This is done by placing them, one after the other, in a known acoustic field
What is primary calibration?
Refers to the calibration of the reference hydrophone which itself is calibrated using NPL’s ‘primary calibration standard’
How are two acoustic pulses compared?
Possible to compare the calibrated time series directly, but that is not always very informative
A number of well-defined measures that are commonly used to characterised both pulses and then compare the,:
p+ :peak positive pressure (max pulse)
p - : peak negative pressure (max negative pulse)
p_i : pressure squared integral (total integral of the squared signal)
t_d: pulse duration (t_d = 1.25 (t_2 - t_1))
What other measurements can be used to compare single acoustic pulses?
Measures relating to acoustic intensity
I = p^2 / ρ0 c0
I_tp: temporal peak (max intensity pulse)
I_pa: pulse average (average over t_d)
I_ta: temporal average (average over an integer number of pulse repetitions)
What measurements can be used to compare acoustic fields?
When there are many waveforms, spatial peak value is used to characterise an entire scan
I_sptp: spatial peak temporal peak
I_sppa: spatial peak pulse average
I_spta: spatial peak temporal average
I_sata: spatial average temporal average intensity = (Σ I_ta for all points where I_ta > 0.25 I_spta) / number of points counted
Why is deionised and degassed water used to make quantitative measurements?
Making the measurements in such a well-characterised fluid allows them to be repeated at a later date
What is a set back of using deionised and degassed water?
They are not representation of tissue: the absorption is very much greater in tissue than in water
the measurement will be of lower amplitude when the transducer is used with the same settings on soft tissue
What is used to overcome the difference in measurements of soft tissue and water?
Derating: a standard way to artificially add the effect of some absorption to the measurements
output measurements are often derated by 0.3 dB MHz-1 cm-1
Attenuation value is typical of lower loss tissue
Quantities that have been derated are indicated with a subscript ‘3’
