Ultrasound 1 & 2 Flashcards
What is sound ?
A disturbance travelling through a medium; a series of interconnected particles.
Sound can be a longitudinal wave as well as a pressure wave
Longitudinal wave
Sound is a longitudinal wave that causes particles to vibrate (the particle motion is parallel to the direction of the energy).
Pressure wave
Sound is also, a type of pressure wave – molecules move close and further away, providing areas of higher and lower pressure.
Compression
High density region of particles.
Peaks
Rarefaction
Low density region of particles.
Troughs
Amplitude
Magnitude of the pressure change between peaks (compression) and trough (rarefaction).
Higher amplitude —> louder noise; related to power, measured in decibels (Db).
Wavelength
Distance between successive compressions/rarefactions.
Frequency
Number of sound waves per second; Hertz (Hz).
1Hz
1 wave per second
What is frequency in relation to wavelength ?
Inversely proportional to wavelength (λ) i.e. if wavelength decreases then frequency increases and vice versa.
What is frequency in relation to speed ?
Frequency is proportional to speed (c).
Audible human range
20 Hz to 20 kHz
Speed of a vibration
Speed refers the distance travelled of a vibration wave per unit time (i.e. how fast).
What does speed depend on ?
Depends on the properties of the medium (gas < liquid < solid).
What does speed relate to ?
Acoustic impedance
Frequency of a vibration
The number of vibrations an individual particle makes per unit time (i.e. how often)
Ultrasound
Ultrasound refers to frequencies >20kHz
(above human audible range).
MHz
Megahertz
10^6 Hz
GHz
Gigahertz
10^9 Hz
What are the medical ultrasound operating values ?
Medical ultrasound typically operates at 1 to 15MHz .
(~50 to 750x higher frequency than the maximal human hearing range)
What are some considerations for ultrasound ?
Resolution
Penetration
What is resolution ?
Sharpness of image
What is penetration ?
Depth of image
What does resolution relate to ?
Related to frequency.
i.e. when wavelengths of 1mm used, structures smaller than 1mm appear blurred (the waves don’t hit the target structure)
Therefore, higher frequency = higher resolution