US Physics Unit 3

a pulse

a pulse

short emission of sound (a few cycles)

pulses are separated in time with ____ of no emission (gaps/listening time)

periods

What is pulsed ultrasound based on

principle of echolocation

echolocation

this distance between structures and the ultrasound probe can be calculated by using this formula

2 way we look for

echo, round trip, time to reflector and back

1 way we look for

time for reflector

equation for 2 way

d=c(t/2)

equation for 1 way

d=ct

for imaging each pulse is ______ long for imaging

2-3 cycles

for doppler each pulse is ____ long

5-30

Determined by source of sound in PW

PRF, PRP, DF, PD, BW, QF

Determined by the source and the medium in PW

SPL

Pulse Repetition Frequency (PRF)

is the number of pulses per seconds (kHz)

The PRF for imaging

4-15 kHz

the PRF for Doppler

5-30 kHz

equation for PRF in kHz

# of pulses/s

Pulse Repetition Period (PRP)

is the time from the beginning of one pulse to the beginning of the next pulse (ms)

PRF is _____ related to PRP, just like frequency and period

inversely

A _____ PRF is desirable for image quality and Doppler sampling

high

PRF is ______ by the speed of sound in soft tissues

speed

Range ambiguity

when a pulse is sent out before all the echoes from the previous pulse have been received.

If PRF is increased, penetration or depth must

decrease

Pulse Duration (PD)

the time it takes for a single pulse to occur

equation for PD

T (# of cycles/pulse)

if frequency is increased, pulse duration is ______

decreased

for most diagnostic medical ultrasound, there are short periods when the machine is sending pulses into the body and _____ times when the machine is waiting for echoes

longer

Duty Factor (DF)

the percent of time that the sound is on (unitless)

For imaging the DF ranges from

0.1-0.1%

DF ranges from _____ in doppler

0.5-5%

DF equation

PD/PRP

Spatial Pulse Length (SPL)

the distance covered by a single pulse

SPL equation

wavelength (# of cycles/pulse)

Bandwidth (BW)

the range of frequencies contained in a pulse

Equation for BW

highest f- lowest f

CW Bandwidth

1

fractional bandwidth

is equal to the bandwidth divided by the operating frequency

equation for fractional bandwidth

BW/f0

Shorter pulses have ________ bandwidths

wider/broader

the _____ the pulse, the fewer number of cycles

shorter

having a broader bandwidth will result in ______ frequencies present

more

Quality Factor (QF)

refers to the purity of the beam or how close it is to the operating frequency

Equation for QF

f0/BW

we want _____ pulses so we can have a broad BW-improved resolution

short

PRF is the number of ____ occurring in 1 s

pulses

pulse-repetition ______ is the time from the beginning of one pulse to the beginning of the next

period

the PRP ______ while PRF increases

decreases

PD is the _____ it takes for a pulse to occur

time

SPL is the _____ of _____ that a pulse occupies while it travels

length, space

______ is the fraction of time that pulsed ultrasound is actually on

duty factor

PD equals the number of cycles in the pulse multiplied by

period

SPL equals the number of cycles in the pulse multiplied by _____

wavelength

the DF of continuous wave sound is

1

if the wavelength is 2 mm, the SPL for a 3 cycles pulse is _____mm

6

How many cycles are there in 1s of continuous wave 5MHZ ultrasound

5,000,000

Amplitude

difference between max value and equilibrium value of an acoustic variable

Power equation

P= Energy/ Time

Intensity equation

I=P/Area

The Intensity of a sound beam is _____ at its center than on the periphery

higher

the intensity of pulsed ultrasound also decreased toward the ____ of the pulse

end

Spatial Peak (SP)

is the greatest intensity in a pulse in reference to space

Spatial Average (SA)

is the average intensity across the pulse

PW is ____ uniform in space or time in terms of Intensity

not

The Beam Uniformity Ratio (BUR)

shows how uniform the intensity is in space

BUR equation

BUR= SP/SA

BUR will always be >1 because the peak will always be higher than the ____

average

Temporal Peak (TP)

is the greatest intensity measured as the pulse passes by

Pulse Average (PA)

is the average of all intensities measured as the pulse passes by

Temporal Average (TA)

includes all the time between the pulse being measured and the next pulse

SPTA

Spatial Peak/Temporal Average

Thermal interactions within tissue

FDA Limit on SPTA

<720 mW/cm2

Attenuation

the weakening of sound as it propagates

gain

an increase in strength of the sound wave

dynamic range

the ration of largest to smallest power in dB

Intensity Ratio

I/Io

I/I0 < 1

I<Io

I/Io>1

I>Io

I/Io=1

I=Io

Equation for dB

10 log (I/Io)

If I/Io <

=-dBi

If I/Io>1

+dB

If I/Io=1

=0 dB

How do we get gain?

Constructive interference, and amplifiers

Time Gain Compensation (TGC)

How to change this to make the picture stronger

-3dB represents a reduction in intensity or power by a factor of

2

-10dB represents a reduction in intensity or power by

10

-6dB represents a reduction in amplitude by a factor of

2

Amplitude is the maximum _____ that occurs in an acoustic variable

variation

Intensity is the ___ in a wave divided by _____

power, area

A unit for intensity is ______

mW/cm2

Intensity is proportional to ______ squared

amplitude

if power is doubled and area remains unchanged, intensity is ________

doubled

if area is doubled and power remains unchanged, intensity is ______

halved

if both power and area are doubled, intensity is ______

unchanged

if amplitude is doubled, intensity is _______

quadrupled

Attenuation is the reduction in ___ and ___ as a wave travels through a medium

amplitude, intensity