Physics principles
process whereby sound energy is dissipated in a medium, primarily in the form of heat.
resistance of sound as it propagates through a medium.
effects on the sound beam caused by the medium; includes pressure, density, and particle motion (distance and temperature).
relating to the strength of the compression wave; maximum variation of an acoustic variable.
amount of space within a specific boundary.
weakening of sound as it propagates through a medium.
attenuation occurring with each centimeter that sound travels.
range of frequencies found in pulse ultrasound.
distance around the perimeter of an object.
region of high pressure or density in a compression wave.
a nonpulsed wave in which cycles repeat indefinitely.
one complete variation in pressure or other acoustic variable.
a unit used to compare the ratio of intensities or amplitudes of two sound waves or two points along the wave.
concentration of mass, weight, or matter per unit volume.
dependence of velocity or other physical parameters on frequency.
amount of space from one object to another.
fraction of time that pulse ultrasound is on.
comparison of range of frequencies (bandwidth) with operating frequency.
number of cycles in a wave occurring in 1 second.
thickness of tissue required to reduce the intensity of the sound beam by one-half; also known as depth of penetration, half boundary layer, or penetration depth.
echoes of twice the frequency transmitted into the body that reflect back to the transducer, which improves image quality.
one cycle per second; unit of frequency.
determines how much of an incident sound wave is reflected back from the first medium and how much is transmitted into the second medium.
direction of incident beam with respect to the media boundary.
rate at which energy transmits over a specific area.
one thousand cycles per second.
wave traveling in a straight line.
incident ultrasound traveling at an oblique angle to the media boundary.
incident ultrasound traveling at an angle perpendicular to the media boundary.
speed at which a wave moves through a medium.
a collection of a number of cycles that travel together.
portion of time from the beginning to the end of a pulse; sonography generally uses 2 to 3 cycles whereas Doppler uses 5 to 30 cycles per pulse.
time between the beginning of one cycle and the beginning of the next cycle.
a few pulses of ultrasound followed by a longer pause of no ultrasound. During this “silence,” returning echoes are received and processed.
for short pulses, the Q factor is equal to the number of cycles in a pulse; the lower the Q factor, the better the image quality.
regions of low pressure or density in a compression wave.
occurs when the reflector is much smaller than the wavelength of the sound beam.
the beam redirected back to the transducer after striking a media boundary.
redirection (return) of a portion of the sound beam back to the transducer.
angle between the reflected sound and a line perpendicular to the media boundary.
change in direction of the sound wave after passing from one medium to another.
redirection of sound in several directions on encountering a rough surface; also known as nonspecular reflections.
a traveling variation of acoustic variables.
distance over which a pulse occurs.
multiple echoes received at the same time generating interference in the sound wave, resulting in a grainy appearance of the sonogram.
these comprise the boundaries of organs and reflect sound in only one direction; specular reflections are angle dependent.
resistance of a material to compression.
the sound beam continuing on to the next media boundary.
amount of occupied space of an object in three dimensions.
Sound waves
• A traveling variation of acoustic variables (pressure, density, and particle motion).
• Longitudinal, mechanical, pressure waves.
• Matter must be present for sound to travel; it cannot travel through a vacuum.
• Sound waves carry energy—not matter—from one place to another.
• Vibrations from one molecule carry to the next molecule along the same axis. These oscillations continue until friction causes the vibrations to cease.
• Contain regions of compression (high pressure) and rarefaction (low pressure).
| METRIC PREFIX | VALUE | SYMBOL |
| Tetra | 1012 (trillion) | T |
| Pico | 10−12 (trillionth) | p |
| Giga | 109 (billion) | G |
| Nano | 10−9 (billionth) | n |
| Mega | 106 (million) | M |
| Micro | 10−6 (millionth) | μ |
| Kilo | 103 (thousand) | k |
| Milli | 10−3 (thousandth) | m |
| Hecto | 102 (hundred) | h |
| Centi | 10−2 (hundredth) | c |
| Deca | 101 (ten) | Da |
| Deci | 10−1 (tenth) | d |
Wave Variables Wavelength (λ) = Propagation Speed (c)/Frequency (ƒ)
| WAVE VARIABLE | DEFINITION | UNITS | DETERMINED BY | RELATIONSHIP |
| Frequency (ƒ) | Number of cycles in 1 s | HzkHzMHz | Transducer | Proportional to image quality and attenuationInversely proportional to the wavelength, period, and penetration depth |
| Period (T) | Time to complete one cycle | smsμs | Transducer | Proportional to the wavelengthInversely proportional to frequency |
| Propagation speed (c) | Speed with which a wave travels through a medium | smsμs | Stiffness and density of the medium | Proportional to the stiffness of the mediumInversely proportional to the density of the mediumDense structures or pathologies decrease propagation speedStiff structures increase the propagation speed (bone)Soft tissue—propagation speed is equal to 1.54 mm/μs13 μs for sound to travel 1 cm in soft tissue round-trip |
| Wavelength (λ) | Distance it takes to complete one cycle | mmm | TransducerMedium | Proportional to the period and penetration depthInversely proportional to frequency |
| PROPERTY | DEFINITION | UNITS | DETERMINED BY | RELATIONSHIP |
| Amplitude | Maximum variation that occurs in an acoustic variableMagnitude from the neutral value to the maximum extent in an oscillationRelates to sound strength | Depends on the acoustic variable | Ultrasound systemOperator-adjustable using output or power control | Proportional to powerDecreases as the wave propagates through tissue |
| Intensity | Relates to the strength of the sound beamRate at which energy passes through unit areaEqual to the total power of the beam divided by the area over which the power is spread | W/cm2mW/cm2 | Ultrasound systemOperator-adjustable using output or power control | Proportional to powerInversely proportional to the beam areaProportional to amplitude of the wave squared |
| Power | Rate at which energy is transmitted into the bodyRate at which work is done | WmW | Ultrasound systemOperator-controlled using output or power control | Proportional to intensity |
| Pressure | Amount of force over a specific areaAcoustic variable | Pascal (Pa)MPa | Operator-adjustable using output or power control | Proportional to amount of force and volume of the sound waveInversely proportional to the area covered |
Pulse ultrasound
• Electrical energy applied to the transducer produces short bursts of acoustic energy.
• A pulse must have a beginning and an end.
• There are two components to a pulse: transmitting (on) and receiving (off).
Properties of Pulse Ultrasound
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| PROPERTY | DEFINITION | UNITS | DETERMINED BY | RELATIONSHIP |
| Bandwidth | Range of frequencies contained in a pulse | MHz | TransducerUltrasound systemCannot be adjusted by the operator | Inversely proportional to the length of the pulse (SPL) and Q factorPortion of the bandwidth used is adjusted with the multi-Hertz or harmonic control |
| Duty factor (DF) | Percentage of time that pulsed ultrasound is transmitting (on-time) | None | TransducerOperator-adjustable with depth control | Proportional to PRF and PDInversely proportional to PRP |
| Pulse duration (PD) |