Chapter 1: Physical Principles Flashcards
Absorption
The conversion of sound energy to heat
Acoustic speckle
The interference pattern caused by scatters that produces the granular appearance of tissue on a sonographic image
Acoustic variables
Changes that occur within a medium as a result of sound traveling through that medium.
Ex: pressure, density, & particle motion
Amplitude
The maximum or minimum deviation of an acoustic variable from the average value of that variable; the strength of the reflector.
Measured in Pascals (Pa)
Attenuation definition
What are the 3 mechanisms of attenuation?
A decrease in the amplitude and intensity of the sound beam as sound travels through tissue.
ABSORPTION, Reflection, and Scattering
Attenuation coefficient
The rate at which sound is attenuated per unit depth
frequency/2
Axial resolution
The ability to accurately identify reflectors that are arranged parallel to the ultrasound beam
longitudinal, axial, radial, range, depth
Backscatter
Scattered sound waves that make their way back to the transducer and produce an image on the display
Beam uniformity ratio
SP/SA factor
Beam uniformity coefficient
The ratio of the center intensity to the average spatial intensity
BUR = Spatial peak / Spatial average
Capacitive macromachined ultrasound transducers
Technology used to create comparable transducer technology to piezoelectric materials
Compression
The part of the cycle of movement when molecules are pushed closed together. An area in the sound wave of high pressure and density. Opposite of rarefaction when molecules are spread further apart.
Damping
The process of reducing the number of cycles of each pulse in order to improve axial resolution
Decibel
A unit that establishes a relationship of comparison between two values of power, intensity, or amplitude
Density
Mass per unit volume
Duty factor
The percentage of time that sound is actually being produced
Elasticity
Stiffness
The ability of an object to resist compression and relates to the hardness of a medium
Frequency
The number of cycles per second
Frequency = Propagation speed / Wavelength
Half-intensity depth
Half-value layer depth
The depth at which sound has lost half of its intensity
Hydrophone
A device used to measure the output intensity of the transducer
Impedance definition
What factors influence impedance
The resistance to the propagation of sound through a medium (z). Measured in Rayls
The density and the propagation speed of the medium.
Inertia
Newton’s principle that states that an object at rest stays at rest, and an object in motion stays in motion, unless acted on by an outside force.
Intensity
The power of the wave divided by the area of which it is spread; the energy per unit area
Watts per square centimeter (W/cm2)
Intensity reflection coefficient (IRC)
The percentage of sound reflected at an interface
Intensity transmission coefficient (ITC)
The percentage of sound transmitted at an interface
ITC + IRC = 100%
Longitudinal waves
Waves in which the molecules of the medium vibrate back and forth in the same direction that the waves are traveling. (Gust of wind blows through grass causing the stalks to bend back and forth.)
Nonspecular reflectors
Reflectors that are smaller than the wavelength of the incident beam
Normal incidence
Angle of incidence is 90 degrees to the interface
Oblique incidence
Angle of incidence is less than or more than 90 degrees to the interface
Path length
Distance to the reflector
Period
The time it takes for one cycle to occur
Piezoelectric materials
Element
Crystal
A material that generates electricity when pressure is applied to it, and one that changes shape when electricity is applied to it
Power
The rate at which work is performed or energy is transmitted.
Watts (W) or milliwatts (mW)
Pressure
Force per unit area
The concentration of force
Propagation speed
The speed at which the sound wave travels through a medium. All sound travels at the same speed through a medium despite its frequency. Affected by medium elasticity and density.
Pulse duration
The time during which the sound is actually being transmitted
The “on” time
Pulse repetition frequency
The NUMBER of pulses of sound produced in 1 second. (kilohertz)
Inversely related to repetition period.
Pulse repetition period
The TIME taken for 1 pulse to occur. (Start of 1 to start of next)
Inversely related to pulse repetition frequency. Includes ‘on’ and ‘off’ time.
Rarefaction
Part of a cycle of movement consisting of the area in the sound wave of low pressure and density. Opposite of compression. Molecules will show differences in density as rarefaction and compression occur.
Rayleigh scatterers
Small scattering reflectors
Reflection
The echo; the portion of sound that returns from an interface
Refraction
The change in the direction of the transmitted sound beam that occurs with oblique incidence angles and dissimilar propagation speeds
Scattering
The phenomenon that occurs when sound waves are dispersed into different directions because of the small reflector size compared with the incident wavelength.
Snell’s law
Law used to describe the angle of transmission at an interface based on the angle of incidence and the propagation speeds of the two media.
Spatial pulse length
The length of a pulse as calculated by the number of cycles multiplied by the wavelength.
Specular reflections
Reflections that occur when the sound impinges upon a large, smooth, reflector at a 90 degree angle.
Total attenuation
The total amount of sound (in dB) that has been attenuated at a given depth
Transverse waves
Type of wave in which the molecules in a medium vibrate at 90 degrees to the direction of travel. (Doing the wave in a stadium.)
Wavelength
The length of a single cycle of sound. (lambda)
Giga Mega Kilo Hecto Deca
Billion 1,000,000,000 Million 1,000,000 Thousand 1,000 Hundred 100 Ten 10
Nano Micro Milli Centi Deci
Billionth 0.000,000,001 Millionth 0.000,001 Thousandth 0.001 Hundredth 0.01 Tenth 0.1
What are the different parameters of sound?
Amplitude, power, intensity, propagation speed, and wavelength
What 2 properties influence the propagation of sound in a medium?
Stiffness (elasticity) and Density (inertia)
Stiffness is (directly/inversely) related to propagation speed. Density is (directly/inversely) related to propagation speed.
Directly
Inversely
Higher frequencies travel (faster/slower) through a medium.
Trick question. All sound, regardless of its frequency, travels at the same speed through any particular medium.
Power and intensity are proportional to amplitude squared.
If amplitude doubles, power quadruples.
If amplitude triples, power increases 9x.
If amplitude is halved, power decreases by 1/4
What are the 5 specific parameters of pulsed wave sound?
Pulse repetition frequency (PRF) Pulse repetition period (PRP) Pulse duration (PD) Duty factor (DF) Spatial pulse length (SPL)
Pulse repetition frequency is (directly/inversely) related to imaging depth.
Inversely. As imaging depth increases, the longer for echos to return, the fewer pulses per second.
Frequency is (directly/inversely) proportional to attenuation. Path length is (directly/inversely) proportional to attenuation.
Directly
Directly
What is the typical range of frequency for diagnostic ultrasound imaging?
1 to 20 MHz
What is the typical wavelength used in clinical imaging?
0.1 - 0.8 mm
What is the speed of sound in soft tissue?
1540 m/s
At what frequency do sound waves become ultrasonic?
What range is used for diagnostic ultrasound?
> 20,000 Hz (20 KHz)
2 - 15 MHz
