Basic Principles of Ultrasound - Physics 1

sound

Mechanical wave in which particles in the medium move. Classified according to frequency.

infrasound

below range of human hearing (elephant)

vibrations occur at a frequency of <20 Hertz

audible sound

sound that can be heard

vibrations occur at a frequency of 20-20,000 Hertz

ultrasound

beyond range of human hearing

vibrations occur at a frequency of >20,000 Hertz

diagnostic ultrasound frequency

vibrations occur at 1-30 MHz

sonography

specialized imaging technique to visualize soft tissue structures in the body

sonus (greek)

sound

graphien (greek)

to write

echocardiology

ultrasound exams of only cardiac structures

sonographer

member of the allied health profession who has recived specialized education in sonography and has completd national board exams successfully

sonologist

a physician who has received specialized training in ultrasound

piezoelectric crystals

crystals in the transducer that are electrically stimulated to produce vibrations; vibrations produce sound

what are acousic variables

help to distinguish and identify sound waves

quantities that vary in a sound wave

one of the variables must have rhythmic oscillation to be a sound wave

3 acoustic variables

pressure, density, distance (PDD)

Pressure

concentration of force in an area (units = Pascals (PA))

Density

the amount of mass in an volume (units = kg/cm3)

Distance

measure of particle motion (cm, ft, etc)

Sound wave characteristics

mechanical wave

require medium (tissue) for travel

areas of compression and refraction

Logitutional pressure waves - always travel in a straight line (parallel)

compression

areas of high pressure and density (crest)

rarefraction

areas of low pressure and density (trough)

sound waves require a medium to travel through

no medium = no sound

soundwaves used in ultrasound imaging are:

mechanical longitutional waves

mechanical longitutional waves

sound waves travel parallel or in one direction; allows for waves to be reflected and analyzed

What are 7 acoustic parameters that describe a sound wave?

period, frequency, wavelength, propagation speed, power, amplitude, intensity

period

time it takes for a wave to repeat itself (wave consists of 1 hill and 1 valley aka a cycle)

unit of measurement = microseconds

Period = time

frequency

number of cycles any of the acoustic variabls go through in a specific duration of time

Frequency (f) = cycles/ second = Hertz

frequency quiz:

if there are five cycles in 1 second, what is the frequency (in Hertz)?

5 Hertz (Good job!!)

frequency quiz:

if there are 15 cycles in 3 seconds, what is the frequency (in Hertz)?

5 Hertz (Good job!!)

frequency for ultrasound

1,000,000 cycles/ seconds = 1 MegaHertz (MHz)

kilo-

1,000

hecto-

100

deca-

10

deci-

10^-1

centi-

10^-2

milli-

10^-3

metric conversions

the multiplication or division of 10s to a base number

low frequency scanning anatomy

abdominal, cardiac

high frequency scanning anatomy

breast, OB, blood vessels (more superficial structures)

increase frequency (f) = increase resolution (quality of image) = decrease penetration = increase cycle = decrease distance = decrease attenuation = decrease wavelength (lamda) = decrease period = decrease power (Watts) = decrease intensity

increase frequency (f) = decrease period

Period is a measurement of time. When the frequency increases (more hills and valleys in a second), there is less time between the start and end of each cycle (hills and valleys get closer together)

increase frequency (f) = decrease wavelength (lamda)

Wavelength is a measuement of distance. When the frequency increases (more hills and valleys in a second), the distance between each hill and valley decreases.

wavelength

horizontal distance of one cycle of a wave (1 hill, 1 valley)

unit = millimeters (mm) in ultrasound

what is the wavelength determined by?

determined by source (transducer) and medium (tissue) through which it is traveling

relationship between wavelength and period

directly related

Doth refer to same area on a wave (1 hill, 1 valley)

Wavelength is a measurement of distance

Period is a measurement of time

relationship between wavelength and frequency to ultrasound

shorter wavelengths (higher frequency) = higher resolution (quality of images)

longer wavelengths (lower frequency) = lower resolution

quiz:

Given the frequency, which soundwave has the longest wavelength?

A. 10 Hertz

B. 8 Hertz

C. 4 Hertz

D. 20 Hertz

*Hertz = cycles /sec

C

Explaination: The longest soundwave will have the lowest frequency. The lower the frequency, the more horizontal distance there is between one wave cycle (1 hill and one valley).

quiz:

Given the wavelengths, which soundwave has the greatest frequency?

A. 20 cm

B. 2000 mm

C. 0.2 km

D. 20 dm

A

Explaination: In order to solve this question, you need to convert the measuements to meters (base unit in the metric system). In this case 20 cm = 0.2 m, 2000 mm = 2 m, 0.2 km = 200 m, and 20 dm = 2 m. The greatest frequency will be the soundwave with the shortest wavelength (horitzontal distance between 1 hill and valley).

quiz:

Which frequency will give the best resolution (best image quality)?

A. 30 Hz

B. 20 Hz

C. 5 Hz

D. 10 Hz

A.

Explaination: The higher the frequency (cycles/ sec) the greater the resolution.

Cycle

a complete repetition of a wave (1 hill, 1 valley) or one complete pattern of compression (crest) and rarefraction (trough)

propogation speed

the rate at which a sound wave travels through a medium in 1 second

what determines propogation speed?

Determined by medium (tissue) that is characterized by stiffness and density

regardless of frequency, sound travels at the same speed through any specific medium

propogation speed units of measurement

any distance divided by time

Ultrasound:

meters per sec (m/s)

millimeters per microsecond

typical propogation speeds in the body

500 m/s to 4000 m/s

propogation speed magic number for soft tissue

1540 m/s

speed of sound in different mediums

travels fastest in solids, slower in liquids, and slowest in gases

2 determining characteristics for propogation speed

1. stiffness, 2. density

stiffness

ability to resist compression (ex: golf ball, bone)

relationshsip between stiffness and propogation speed

direct relationship

increase stiffness = increase speed = brighter images

density

relative weight of a material

in two materials of equal volume, the dense material will weigh more (ex: air and concrete)

relationship between density and propogation speed

inverse relationship

Increase density (heavier) = decrease speed = darker images

what characteristic results in the fastest speed

stiff but not dense

what characteristics result in the slowest speed

not stiff but very dense

what happens when there is an euqla density and stiffness

denser medium will have slower speed

Formulas

Wavelength (lamda) = propogation speed (c)/ frequency (f)

Propogation speed (c) = frequency (f) x Wavelength (lamda)

magnitude of the sound wave

size, strength, or magnitude of sound wave is described by 3 parameters: Power, Intensity, Ampplitude

attributes of amplitude, power, and intensity

Determine by sound source

Is adjustable

Decrease as sound travels

amplitude "Bigness"

Measure of the height of the sound wave. From the center of wave to either the crest or trough.

Can have units of any of the acoustic variables (PA, kg/cm3, or distance measurement)

power

Rate at which energy is transferred over the entire sound beam

Unit = Watts (W) or milliwatts (mW)

relationship between power and amplitude

power = amplitude^2

(power is proportional to amplitude^2)

intensity

Concentration of energy in a sound beam

Units = W/cm^2

relationship between power and intensity

directly related

When the power increases the intensity increases.

intensity in sound beam

sound beam has different intensity at different depths/side to side locations within the sound beam

attenuation

The weakening of sound waves as it travels in a meadium

Increase attenuation (lose more sound waves)

Decrease attenuation (lose less sound waves)

relationship between attenuation, power, and intensity

directly related

decrease in power = decrease in intensity = decrease in attenuation

If you decrease the power, the intensity of the sound beam decreases, resulting in a decrease in the lost of sound waves as it travels through a medium.

relationship between distance and attenuation

directly related

increase attenuation = increase distance

The further you travel through a medium, the more sound waves you lose.

attenuation is determined by 2 factors

1. path length

2. frequency of sound

relationship between attenuation and frequency

directly related

increase attenuation = increase frequency

"Atteniation is greater in higher frequency sound than in lower frequency sound." - Edelman

3 process contribute to attenuation

1. absorption

2. scattering

3. reflection

absorption

conversion of sound to heat

relationship between absorption and frequency

directly related

increase absorption = increase frequency

relationship between absoprtion, frequency, and attenuation

"As a result of absorption, higher frequency waves attenuate more than lower frequency waves." - Edelman

scattering

random redirection of sound in many directions

Why does scattering happen?

"Sound scatters when tissue border is small; that is equal to or less than the wavelength of the incident sound beam." - Edelman

relationship between frequency and scatter

directly related

increase frequency = increase scattering

"Higher frequency beams scatter much more than lower frequency beams." - Edelman

reflection

as sound strikes a boundary (tissue), a portion of the waves energy is "redirected" back to the sound source

relationship between reflection and attenuation

reflection weakens sound waves that continue forward

When does reflection occur?

likely to occur when the boundary is large; more than few wavelengths of the sound

2 forms of reflection in soft tissue

1. specular

2. diffuse

specular reflection

a reflection produced by a smooth surface that return in one direction

specular relfection characteristics

angle dependent and not affected by transducer frequency

"once the wave is slightly off axis, the reflection does not return to the transducer" - Edelman

diffuse reflection

reflection that occurs when parallel rays of light hit a rough surface and all reflect at different angles

diffuse reflection characteristics

create texture of organs and increase with frequency, results in higher resolution images

Incident beam

beam from transducer

reflected beam

A beam reflected off a surface that comes back to the transducer. Returns in the same direction it came from.

transmitted beam

portion of the beam that is not reflected and continues on in a parallel direction.

Law of Conservation

reflected + transmitted = incident

2 criteria for reflection to occur

1. difference in acoustic impedance between 2 tissues

2. sound beam strikes a boundary between two media at 90 degrees

acoustic impedance

Tissue property that influences the amount of reflection or acoustic resistanc to sound traveling in a medium. It is a calculated number per specific type of medium (tissue).

Unit = rayls (Z)

reflection with normal incidence

1. no reflection occurs if 2 media have identical impedances

2. small reflection occur if impendances are slightly different

3. large reflection occurs if impendances are substantially different

quiz: Edelman pg 25

Which of the following waves is infrasonic?

A. 4MHz

B. 4 MHz

C. 28 Hz

D. 2 Hz

D

Explaination: A wave with a frequency less than 20 Hz cannot be heard because its frequency is less than the lower limit of human hearing.