Chapter 2 Sound part 1

sonographic physics

speed of sound in soft tissue

1540 m/sec or 1.54 mm/µsec

What is sound

Is a mechanical, longitudinal wave that travels in a straight line, can’t travel in a vacuum & carries energy

what is longitudinal wave

particles of a medium that move (vibrate) in the same direction as the wave travels

Sound waves cause areas of compression (______/______) and rarefractions (______/______) in the molecules of the medium

high pressure / high density ; low pressure / low density

A transverse wave (shear wave) are …

particles of the medium move in a direction perpendicular to that of the wave front

Sound is best described as …

a traveling pressure variation; pressure starts at a normal value, increases to a maximum value, decreases to a minimum value & returns to normal

What are the 4 acoustic variables of sound?

Pressure, density, temperature, & distance

What is pressure & its units?

Concentration of force ; lb/sq in, pascals (pa)

What is density & its units?

concentration of mass of weight, mass per unit of volume ; kg/cubic cm

What is temperature & its units?

Concentration of heat energy ; degrees

What is distance & its units?

measures of particle motion ; cm, feet, miles

What does the sound source (XDCR) determine?

Frequency, period, intensity, amplitude, wavelength

What does the medium determine?

Propagation speed, wavelength

What is frequency?

the numbers of cycles of an acoustic variable that occur in 1 sec

Equation of frequency

Cycles/sec= hertz

Frequency of audible sound is…

between 20 Hz & 20,000 Hz (20Hz to 20 kHz)

Frequency of Infrasound is …

less than 20 Hz (too low to be heard by humans)

Frequency of ultrasound is ….

20,000 Hz or 20kHz, exceeding the upper limit of human hearing

Imaging frequency range is…

2 to 20 MHz

what does frequency effect?

penetration (depth) & resolution (image quality)

Low frequency is for ____ & high frequency is for _____

depth ; better image

What is period?

The length of time it takes to complete a single cycle, the time from the start of one cycle to the start of the next one

T/F Period & frequency cannot be changed by sonographer

True

Period & frequency are ____ of each other, making them _____ related.

Reciprocals ; inversely

Formula for Period

T or P = 1/frequency sec

What is wavelength

the distance or length that one complete cycle occupies; the cycle length

Wavelength is _______ related to frequency

inversely

T/F Higher (increased) frequency waves have longer wavelengths

False; higher frequency waves have shorter wavelengths

What wavelengths generally produce higher quality images?

Shorter wavelengths

Wavelengths are determined by ….

both the medium and source (XDCR)

If a new medium is encountered, the wavelength & ______ will be affected, but not the ______.

speed ; frequency

What is Propagation speed?

the speed at which sound moves through a medium

In which state of matter does sound travel from fastest to slowest?

solids, liquids, gases

What is the equation for wavelength?

λ= C (prop speed)/F

Equation for propagation speed

Prop speed (m/s)= frequency (Hz) x wavelength (m)

Speed is determined only by…

the characteristics of the medium (density & stiffness)

T/F Prop speed is not affected by the sound source & cannot be changed by the sonographer

True

What is density?

is the concentration of mas per unit volume

Density ____ & speed _____

increases ; decreases (opposite letters D&S)

Stiffness _____ & speed _____

increases ; increases (same letters S&S)

what is stiffness?

a material’s ability to maintain shape, even when pressure is applied

Velocities are slower in _____, higher in _____, and highest in _____

gases, liquids, solids

What is another name for stiffness?

Bulk Modulus

Hardness is the _____ of a material to compression

resistance

Hardness is the ____ of compressibility/ elasticity

inverse

Compressible material has a ___ hardness & a hard material has a ____ compressibility

low ; low

What is the dependence of prop speed on pressure that causes sound (pressure) waves to change shape

Harmonics

What are even and odd multiples called?

Even harmonics & odd harmonics

_____ pressure portions of the wave travel faster than the _____ pressure portions

Higher ; lower

What is nonlinear propagation?

Which speed depends on pressure & the wave shape changes

What is fundamental frequency?

Original frequency

What is sinusoidal characterized by?

a single frequency equal to the number of cycles per second

What causes a wave that is originally in a smooth-curve form (sinusoidal) to progress toward a different shape?

Sawtooth

What does sawtooth contain?

additional frequencies that are even & odd multiples of the original frequency

As the wave gets less sinusoidal, the harmonics get ….

stronger

What does using harmonic frequency echoes improve?

the quality of the sonographic images

We send in a ____ of sound, wait for the _____ & then display the ______

pulse ; echo ; information

Brighter dots on the screen are

stronger echoes

darker dots on the screen are

weaker echoes

What is pulsed wave operation?

major benefit associated with pulsing wave on & off

Most of our techniques are ….

pulsed

What is applied to the cycles within the pulse & pulse parameter to the entire pulse or set of pulses

Wave parameters

A continuous wave cannot create

an image

______ or pulses of acoustic energy are produced & then received by the ____ to construct an image

Short bursts ; XDCR

What is the pulse of an ultrasound?

A collection of cycles that travel together

What are the five key descriptors of a pulsed ultrasound?

pulse duration, spatial length, pulse repetition period, pulse repetition frequency, & duty factor

What is pulse duration?

The time from the start of a pulse to the end of that pulse

What is pulse duration only determined by?

The source

Pulse duration is determined by the number of …

cycles in the pulse & the period of each cycle

Equation for Pulse duration

#cycles in pulse X period / #of cycles in pulse/frequency

PD when the period of the cycles ______, requiring a lower ____

increase ; frequency

PD increases when # of cycles in the pulse _____

increases

PD decreases with the ______ # of cycles or ______ period

decrease ; decrease

PD decreases with ______ frequency

increase