Ultrasound Physics Chapter 1 pt. 1

sonus

means sound in Latin

graphien

means to write in Greek

Image

reproduction or imitation of the physical form of a person or thing

- ultrasound is a visible counterpart of an invisible object

Sound can travel through all of the following except:

-bone

-steel

-air

-vacuum

vacuum

round trip time

the time it takes the pulse to travel through soft tissue and return to the transducer

What is the formula for round trip time?

13 usec x depth

- 6.5 usec to travel one way

What is the relationship between depth and round trip time?

direct

Audible sound

20Hz to 20,000Hz or 20KHz

Infrasound

below human hearing range

- below 20Hz

Ultrasound

above human hearing

- >20,000Hz or 20KHz

Diagnostic Medical Ultrasound

1MHz to 20MHz

The brightness of an echo, as presented on the display, represents the ___________ of the echo.

amplitude

Wave

cyclical transfer of energy

Waves transfer ___________ from one location to another.

energy

Sound is a ________ wave

Mechanical

- needs a medium (air)

Mechanical Wave

- requires physical interaction

-require a medium in which they must travel

-for ultrasound, waves interact with the tissue of the patient

Is ultrasound a transverse wave?

NO

- ultrasound is a longitudinal mechanical wave

How do longitudinal waves propagate?

by particle compression and rarefaction that results in particle motion that is parallel or in the same direction as the wave propagation direction

What is required for sound propagation or sound transmission?

Molecular motion/ interaction

Compression

refers to areas of high pressure or high particle density

Rarefaction

refers to areas of low pressure, low density or when the particles are relaxed back to their natural state

As sound propagates, it causes mechanical changes in the medium. These changes can be measured in four physical quantities called the acoustic variables which are:

- pressure

-density

- temperature

-particle motion

Frequency

how many cycles of compression and rarefaction occur per second

Cycle

one episode of a repeating event; one compression and one rarefaction

What is the formula for Frequency?

# of cycles/sec

What is the unit for Frequency?

Hz

Higher frequency= _____ penetration and ______ resolution

less; better

Period

the time it takes for one cycle to occur

What is the formula for Period?

1/frequency

What is the unit for Period?

usec

Frequency and period in ultrasound are determined by?

sound source (transducer)

What is the relationship between frequency and period?

indirect

Propagation speed

speed at which a wave moves through a medium

Propagation speed depends on?

physical properties/ composition of the medium ONLY

- NOT dependent on the frequency of the sound wave

What are the four properties that determine propagation speed?

-elasticity

-density

-compressibility

-stiffness (bulk modulus)

Elasticity

the ability of an object to return to its original shape and volume after a force is no longer acting upon it

What is the relationship between prop speed and elasticity?

indirect

Density

Mass per unit volume

what is the relationship between the prop speed and density

indirect

Compressibility

indicates the fractional decrease in volume when pressure is applied to the material

- the higher the _____, the easier it is for the medium to be reduced in volume

What is the relationship between prop speed and compressibility?

Indirect

Stiffness

opposite of compressibility

the _____ the medium, the less its compressibility

the greater the ______, the greater the prop speed

What is the relationship between stiffness and prop speed

direct

What is prop speed in air?

330 m/sec

What is prop speed in fat?

1440 m/sec

What is prop speed in soft tissue?

1540 m/sec

What is prop speed in muscle?

1588 m/sec

What is prop speed in bone?

4080 m/sec

Wavelength

the physical distance from one compression to the next compression; the length of one cycle

What determines wavelength?

frequency of the wave and the medium in which it travels

What is the formula for wavelength?

prop speed/ frequency of transducer

What is the unit for wavelength?

mm

What is the relationship between wavelength and prop speed?

direct

What is the relationship between wavelength and frequency?

indirect

Higher frequency transducers have _____ wavelengths and _____ periods

shorter

Do shorter wavelengths give better or worse image resolution?

Better

Amplitude

the distance representing the strength or height of the wave from the mean or baseline

What is the unit for amplitude?

volt

Power is proportional to _____, which is proportional to _____

intensity; amplitude squared

What is amplitude usually related to?

the voltage or strength of the electrical signal that will come from the electrical outlet to stimulate the transducer element

If amplitude is doubled, the power will be changed by?

4

When you turn down power, you are turning down_____?

amplitude

What is the formula for Intensity?

I=P/A

What is the relationship between intensity and power?

direct

What is the relationship between intensity and area?

indirect

What is the unit for intensity?

watts/cm2

What is the relationship between intensity and frequency?

NONE