Ultrasound Physics

Front 1

High vs low temporal resolution

-focusing

-sector size

-line density

-depth

Back 1

Back (Definition)

Front 2

Acoustic parameters

Back 2

Back (Definition)

Front 3

Direction of Sounds waves

Back 3

Longitudinal

Compression and rarefaction

Front 4

Speed of sound affected by

Back 4

Density and stiffness

More stiff, faster propagation (More compressible, slower propagation)

I.e Gas < liquid < solid

More dense, slower propagation

Density does not affect attenuation

Front 5

At what frequency is imaging performed

Back 5

1-30 MHz

Front 6

Formula for velocity

Back 6

Velocity = frequency x wavelength

Front 7

Ultrasound

Back 7

Frequency > 20 kHz

Front 8

Audible sound

Back 8

20Hz to 20,000Hz

Front 9

As sound travels deeper in the body amplitude

Back 9

Decreases

Amplitude is the strength of a sound beam

Front 10

As sound propagate through body, power and intensity (power/beam area)

Back 10

Decreases

Front 11

Safety of ultrasound determined by

Back 11

Intensity

Front 12

What is the relationship between period and frequency

Back 12

Period = time required to complete one cycle

Period and frequency are reciprocals of each other

Front 13

Pulse repetition frequency

-definition

-formula

-relation to depth

Back 13

Back (Definition)

High PRF means less depth

Front 14

Pulse

Back 14

Stream of cycles emitted by the ultrasound that’s inherent to the property of the ultrasound. It cannot be changed. Usually lasts for 2-4 cycles

Front 15

Pulse duration

Back 15

Back (Definition)

Front 16

PRP x PRF = ?

Back 16

1

Reciprocals

Front 17

Duty factor

Back 17

Machine spends most time listening and less time transmitting pulse

Normal .001 to .01

Front 18

Attenuation

Back 18

Directly Related to distance and frequency

Ultrasound intensity decreases with distance and higher frequency

Front 19

How does Frequency relate to depth

Back 19

Frequency = cycles/second

Frequency = velocity/wavelength

The velocity of sound through tissue remains constant, so increasing frequency means decrease in wavelength. So less distance travelled > decrease in depth

Front 20

Rayleigh scattering

Back 20

Back (Definition)

Front 21

In an organized image, reflection and scattering is:

Back 21

Specular and Rayleigh

Front 22

If transmission speed is less than incident speed, then transmitted angle is

Back 22

Lower

Front 23

13 microsecond rule

Back 23

Back (Definition)

Front 24

Axial Resolution

-formula

-relation to frequency and wavelength

Back 24

Improves with decreased wavelength and increased frequency

Front 25

Lateral resolution

Back 25

Best at beam focus.

Usually not as good as axial resolution because pulses are wide but short in length

Front 26

Impedance within ultrasound structure

Back 26

Impedance = density x propagation speed

Front 27

What is the difference between period and pulse duration

Back 27

Period is the time of 1 cycle.

Period = 1/frequency

Pulse duration is the time for all the cycles in 1 pulse

Pulse duration + signal receiving time = pulse repetition period

Front 28

Difference between period and wavelength

Back 28

Period is TIME of 1 cycle

Wavelength is DISTANCE travelled in 1 cycle

Front 29

Ways of increasing frame rate

Back 29

1) narrowing the imaging sector, which decreases the time it takes to scan one frame

2) decreasing the depth which decreases the PRP

3) decreasing the line density, which requires fewer lines to scan one frame (at the cost of spatial resolution)

4) turning off multifocus, which decreases the number of pulses needed per line.

Front 30

Frame rate formula

Back 30

1/time to scan one frame

1/(PRP x number of scan lines per frame)

1/(PRP x #pulses)

Front 31

Pulse Repetition Period

Back 31

Time for sending and receiving 1 pulse.

Only related to depth

PRP= Imaging depth x 13us/cm

Front 32

Backing / Damping material

Back 32

Absorbs sound and causes the emitted pulse to be short in duration and length

Leads to:

1. decreased sensitivity - can not convert low level reflections in to electrical signals

2. Wide bandwidth - many diff frequencies of sound above and below main frequency

3. Low quality factor,

QF = main frequency/ Bandwidth

Front 33

Frequency and thickness of PZT crystal

Back 33

Inversely related, thinner material produces higher frequency signal

Front 34

To increase the focal depth

Back 34

1. Increase transducer frequency

2. Increase diameter of PZT crystal

Front 35

To decrease beam divergence

Back 35

1. Increase beam frequency

2. Increases beam diameter