Physics of Ultrasound Flashcards
1
Q
Define: sound
A
A mechanical wave that propagates through a medium
2
Q
Discuss the interaction of sound with matter
A
- Energy at the sound source produces vibrations which are carried by a longitudinal wave through matter to a receptor
- Can NOT propagate through a vacuum
- Amplitude, power, and intensity of sound wave all decrease as it travels through a medium as time and distance increase
- Propagation speed - velocity of sound through a medium
- all travels the same (regardless of frequency or source) through a specific medium
- Density increases → speed decreases
- Soft tissue = 1540 M/sec
- all travels the same (regardless of frequency or source) through a specific medium
3
Q
Define: sound wave
A
- Caused by particle vibration
- Energy movement, not molecule movement
- Requires a medium (can’t happen in vacuum)
- Mechanical wave
- specifically, a longitudinal wave - particles of the medium move parallel (in the same direction) as the motion or energy
- Movement of the wave is caused by a series of compressions (areas of high pressure) and rarefactions (areas of low pressure) which creates a rhythmic and cyclic wave
4
Q
Define: frequency
A
Number of cycles per 1 second
- Determined by sound source
- Inversely related with period (time it takes for one cycle to occur)
- Unit: cycles/second or Hz
- 1 cycle/sec = 1 Hz
- 1000 cycles/sec = 1kHz
- 1,000,000 cycles/sec = 1 MHz (megahertz)
5
Q
7 parameters of a sound wave
A
- Wavelength- one compression + one rarefaction
- Period- the time it takes for one cycle to occur
- Frequency- the number of cycles per second, inversely related with period
- Amplitude*- loudness
- Power*
- Intensity*
- Propagation Speed- is the speed at which a sound wave travels through a medium
* refers to size/strength of wave
6
Q
Define: amplitude
A
- Strength of a sound wave by describing the magnitude of the wave
- Measured by the difference between the maximum value of an acoustic variable and the baseline (average)
- Initially determined by the source, but decreases as a wave propagates through tissue
- Varies due to temp, pressure, and medium characteristics
- Units: most commonly measured in decibels (dB) but can be reported in any unit of an acoustic variable
7
Q
Define: acoustic impedance
A
- Differences in transmissions due to tissue types
- Greater difference → higher reflection → lower transmission through interface
- Ie Air:Tissue = 100% reflected
- Use coupling agent to reduce acoustic impedancebetween room air and skin surface
8
Q
Define: attenuation
A
- Decrease in amplitude/power/intensity with increase in travel distance
- Can cause artifact
- 4 mechanisms
- Absorption - converted to heat (greatest contributer to attenuation)
- Reflection - return to source (transducer), perpendicular incidence, two media have different impedances
- Refraction - bends or changes angles as it passes through tissue, not perpendicular to interface
- Scattering - increases with higher frequencies, permit imaging of parenchyma, sound strikes scatterers, scatter waves interact with each other, send back to transducer
- Example:
- High attenuation:
- Bone
- No thru transmission, shadows
- Low attenuation
- Water
- High attenuation:
9
Q
Define: transducer
A
- Anything that converts one type of energy into another
- Examples:
- Lightbulb - converts electrical energy to heat/light
- Muscles - converts chemical energy into mechanical energy
- Headphones- converts electrical energy to sound
- Examples:
10
Q
Define: piezoelectric crystal
A
- Special crystal in ultrasound transducer: when force (high frequency vibration) is applied to a crystal →excitation of molecules → electric charge
- The charge can be detected, and amplified, and interpreted by the system
11
Q
List and describe the components of an ultrasound transducer
A
- Electric connector - transmits electric signals to and from the unit
- Housing- protects the sonographer and patient from electrical shock and houses the transducer components
- Damping material - absorbs extra energy to reduce the pulse length (crystal ringing or noise) → improves image resolution
- Electrode - captures/exerts the electric charge either stimulating or produced by the crystal
- Piezoelectric crystal - thickness and material propagation speed determines operating frequency of the transducer
- Matching layer - decreases the amount of impedance mismatch between the transducer and the patient’s skin
12
Q
Types of transducers
(electronic)
A
LINEAR
- Produces rectangular shaped image
- Elements fired in small groups in sequence
- Often used in vascular and small parts, high-resolution OB
- Tend to have mid range- higher frequencies
- PEARLS: high resolution, short focal depth, great for superficial structures
- PITFALLS: poor penetration as we look at structures deeper in tissue
CURVED
- Curved image shape
- Firing is sequenced
- Electronically focused
- Used for Abdominal, OB, Gyn
- Tends to have lower frequencies
- PEARLS: Great penetration, good “general use” probe
- PITFALLS: Not as much detail resolution of some structures
ENDOCAVITY ARRAY
- Curved image shape
- Used for GYN, OB (especially early OB), and endo-rectal imaging
- Tends to have highest frequencies
- PEARLS: Great resolution, ability to visualize structures that cannot be seen transabdominally
- PITFALLS: Slightly more invasive, limited range of motion, limited penetration on some patients
- PROBES MUST ALWAYS BE COVERED WHILE IN USE AND DISINFECTED WITH HIGH LEVEL DISINFECTANT
13
Q
Define: refraction
A
Bends or changes angles as it passes through tissue, not perpendicular to interface
14
Q
Define: resolution
A
- Ability to delineate 2 closely related objects
- 2 types
- Lateral - perpendicular to main axis of beam. Varies by depth and focal length.
- Axial - along main axis of beam. Independent of depth, but related to the pulse length of wave
- Low resolution → artifact (loss of detail) and two separate structures may be displayed as one
15
Q
Define: dynamic range
A
aka contrast resolution?
- the difference between the largest and smallest amplitudes within a signal
- Is the distribution of the grayscale of the reflected signal
- Low dynamic range image appears “black and white” with only a few “in-between” grayscale levels
- Too low and image compression occures
- High dynamic range image has more shades of grey, are often “softer,” and have more preserved subtleties in the image presentation
- Too high and image lacks contrast needed for diagnosis
19
Q
Doppler effect
A
- Effect of differing frequencies with motion.
- Commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer.
- Received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession
