Physics exam 1 (ch 1-4)

Directly related

Two items that are associated such that when one item INCREASES, the other INCREASES

Inversely related

Two items are associated such that when one item INCREASES, the other DECREASES

Reciprocals

Special type of INVERSE relationship. When two numbers with a reciprocal relationship are multiplied together, the result is ONE. 2 and ½ are reciprocals.

Describe a sound wave

Sound is a LONGITUDINAL wave that travels in a STRAIGHT line. Sound MUST travel through a medium where molecules are alternately COMPRESSED (squeezed together) and RAREFIED (stretched apart)

Acoustic propagation properties

The effects of the medium upon the sound wave

Biologic effects

The effects of the sound wave upon the biologic tissue that it passes through

How are sound waves identified?

Oscillations in ACOUSTIC VARIABLES- pressure, density, distance

Pressure

Concentration of force in an area. Units: pascals

Density

Concentration of mass in a volume. Units: kg/cubic cm

Distance

Measure of particle motion. Units: cm, ft, miles

What are used to describe the features of continuous sound waves?

ACOUSTIC PARAMETERS- period, frequency, amplitude, power, intensity, wavelength, propagation speed

Transverse wave

Particles move in a direction that is PERPENDICULAR to the direction that the wave propagates

Longitudinal wave

Particles moves in the SAME direction that the wave propagates

In-phase waves

A pair of waves are considered in-phase when their PEAKS occur at the SAME TIME and at the SAME LOCATION

Out-of-phase

When two waves are out of-phase, their PEAKS occur at DIFFERENT TIMES, and so do their troughs

Interference

Occurs when 2 waves overlap each other at the same instant and at the same location, creating a NEW SINGLE wave

Constructive interference

The interference of a pair of IN-PHASE WAVES results in the formation of a SINGLE wave of GREATER AMPLITUDE than either of its components

Destructive interference

The interference of a pair of OUT-OF-PHASE waves results in the formation of a SINGLE WAVE of LESSER AMPLITUDE than at least one of its components

Complete destructive interference

When two out-of-phase waves are of equal amplitude, they may cancel each other out

A sound wave is also described as a _ wave

PRESSURE wave

What does a transverse wave represent if it is not a sound wave?

It represents the areas of high and low PRESSURE during the compressions and rarefactions; aka pressure wave

period

the TIME it takes to complete a single cycle. units: microseconds

Is period determined by the sound source or the medium?

sound source only

Can period be changed by the sonographer?

NO. It cannot be changed while using a basic ultrasound system with a particular transducer.

Will the period change if we switch from sound system a to sound system b?

yes, for old systems with only one transducer

frequency

the number of cycles that occur in 1 second. units: Hz (aka per second). 1 cycle/second = 1 Hz, 1000 cycles/second = 1kHz, 1,000,000 cycles/second = 1 MHz

what is the typical value of period in diagnostic ultrasound?

0.06 to 0.5 microseconds

what is the typical frequency in clinical imaging?

2 MHz to 15 MHz (2 million to 15 million cycles per second)

is frequency determined by the sound source or medium?

sound source ONLY

can frequency be changed by the sonographer?

NO

infrasound

has frequency less than 20 Hz. Too low to hear

audible sound

has frequency between 20 Hz to 20 kHz

ultrasound

has frequency greater than 20 kHz. Too high to hear

what is the relationship between period and frequency?

they have an INVERSE relationship. They are RECIPROCALS (period x frequency = 1)

what is the relationship between frequency and penetration?

INVERSE relationship. High frequency = low penetration; Low frequency = high penetration

what is the relationship between frequency and image resolution?

DIRECT relationship. High frequency = high resolution; Low frequency = low resolution

what are the 3 parameters that describe the MAGNITUDE of a sound wave?

amplitude, power, intensity

amplitude

relates to the STRENGTH or BIGNESS of a wave. Defined as the difference between the avg value and the MAX value of an ACOUSTIC VARIABLE. It is also defined as the difference between the avg value and the MIN value of an acoustic variable.

what are the units for amplitude?

It has the units of any of the acoustic variables. pressure = pascals; density = g/cubic cm; distance = cm, in; sound wave = decibels dB

Is amplitude determined by the sound source or the medium?

SOUND SOURCE initially. Amplitude does decrease as the sound wave propagates through the body, and the rate at which it decreases depends on the characteristics of both the sound wave and the medium

Can the sonographer control the amplitude?

YES, but we shouldn’t!

peak-to-peak amplitude

the difference between maximum and minimum values of an acoustic variable. The peak-to-peak amplitude is TWICE the value of amplitude

power

the rate of energy transfer or the rate at which work is performed. Units = watts

How are amplitude and power related?

DIRECTLY related. Power is proportional to the wave’s AMPLITUDE SQUARED

Is power determined by the sound source or the medium?

SOUND SOURCE

Can power be changed by the sonographer?

YES, but we shouldn’t!

if the amplitude is 2 dB, what is the power?

4 watts. power = amplitude squared

if the amplitude is increased to 4 dB, what is the new power?

16 watts

if a sonographer INCREASES the amplitude of a wave by a factor of 2, what happens to the power?

power increases by a factor of 4 (4 times its original value)

if a sonographer DECREASES the amplitude of a wave by a factor of 1/2, what happens to the power?

power decreases to ¼ of its original value or 25%

intensity

the CONCENTRATION of energy in a sound beam. units = watts/cm²

what is the formula for intensity?

intensity = power (watts) / beam area (cm²)

what is the relationship between intensity and the beam area?

bigger area = lower intensity; smaller area = higher intensity (think of a flashlight)

Is intensity affected by the sound source or the medium?

SOUND SOURCE

Can the sonographer change the intensity?

YES, but we shouldn’t!

how are amplitude, power, and intensity related?

DIRECTLY related

what is the relationship between intensity and power?

intensity is proportional to power. if a wave’s power is doubled, then the intensity is doubled

what is the relationship between intensity and amplitude?

intensity is proportional to amplitude². when a waves amplitude is doubled, the intensity increases to 4 times its original value

wavelength

the LENGTH or DISTANCE of one complete cycle. Units = mm, meters, any unit of length

what is the typical wavelength in soft tissue?

0.1 to 0.8 mm

is wavelength determined by the sound source or the medium?

BOTH

can the sonographer change the wavelength?

NO

what is the relationship between wavelength and frequency?

INVERSELY related, as long as the wave stays within one medium (like period!)

what is the wavelength of 1 MHz sound in soft tissue?

1.54 mm

what is the rule that defines the relationship between frequency and wavelength of sound in soft tissue?

wavelength (mm) = 1.54 (mm/microsec) / frequency (MHz)

when traveling through the same medium, if the frequency DOUBLES, the wavelength is _ ?

halved. This will not work if the medium changes because the wavelength is determined by both the medium and the sound source

propagation speed

the rate that sound travels through a medium. Units: m/sec, mm/microsec, any units of distance/time

is the propagation speed determined by the sound source or the medium?

MEDIUM ONLY

can the sonographer change the propagation speed?

NO. Sound speed will only change when sound travels to a different medium!!!

What is the average speed of sound in biologic or SOFT TISSUE (blood)?

1540 m/s or 1.54 km/s or 1.54 mm/microsec

what is the speed of sound in FAT?

1450 m/s

what is the speed of sound in the LUNGS?

300 m/s

what is the speed of sound in BONE?

3000 m/s

The speed of sound for a continuous wave from a 7.5 MHz transducer is 1850 m/sec. The wave is then pulsed with a duty factor of 50%. What is the new speed?

The speed stays the same, 1850 m/sec because the medium did NOT change.

what characteristics of a medium determine the speed of sound in that medium?

stiffness and density

how does stiffness affect speed?

Same letters, same direction! DIRECTLY related. high stiffness = high speed; low stiffness = low speed

bulk modulus

SAME AS STIFFNESS

what are compressibility and elasticity?

the OPPOSITE of stiffness. low elasticity/compressibility = high stiffness = high speed. high elasticity/compressibility = low stiffness = low speed

how does density affect speed?

Different letters, opposite directions! INVERSELY related. high density = low speed, low density = high speed

what type of materials will have the fastest speed?

Very stiff but not dense materials like bone will have the fastest speed!

what type of materials will have the slowest speed?

Not stiff and very dense materials will have the slowest speed

True or false. Propagation speed increases as frequency increases.

FALSE. propagation speed is determined by the medium only

what is pulsed sound?

a collection of cycles that travel together; MUST have beginning and end

is ultrasound continuous wave or pulsed wave?

pulsed wave

what are the 2 components of pulsed ultrasound?

transmit/talking/on time and receive/listening/off time

can continuous sound waves produce an anatomical image?

NO, they have no “off” time

pulsed ultrasound

short bursts of pulses of acoustic energy

Does a pulse ever change?

NO, a pulse never changes- only the dead time

pulse duration

the actual time from the start of a pulse to the end of that pulse. “transmit” time. Units- microseconds

what is the typical value of pulse duration in diagnostic ultrasound?

0.3 to 2 microsecs

is the pulse duration determined by the sound source or the medium?

SOUND SOURCE

can the sonographer change pulse duration?

NO

In clinical imaging, how many cycles make up a pulse?

2-4 cycles

what is the formula for pulse duration?

pulse duration (microsec) = # of cycles x period (microsec)

How is pulse duration related to the number of cycles and the period?

DIRECTLY proportional

How is pulse duration related to frequency?

INVERSELY proportional. pulse duration = # of cycles / frequency (MHz)

which type of pulse is more desirable in diagnostic imaging and why?

SHORTER duration pulses are desirable because they create images of greater accuracy

spatial pulse length (SPL)

The distance from the start of a pulse to the end of a pulse. Units: mm

Is spatial pulse length determined by the sound source or medium?

BOTH sound source and medium! Like wavelength!!

can the sonographer change spatial pulse length?

NO