Unit 3: Pulsed Wave Ultrasound

Echo-Ranging Principle

forms the basis for most ultrasonic modes; the distance between structures and the ultrasound probe can be calculated using the echolocation formula

Distance Formula

v = d/t —> d = vt

Echolocation Formula

d = c x (t/2) —> 2 way

How many cycles does each pulse typically have?

2-3 for imaging

5-30 for Doppler

PW Properties Determined by SOURCE:

PAPIF, PRF, PRP, PD, DF, BW + QF

PW Properties Determined by SOURCE + MEDIUM:

wavelength, SPL

PW Properties Determined by MEDIUM:

acoustic velocity (c)

Pulse Repetition Frequency (PRF)

the number of pulses per second (kHz)

What is the typical PRF?

4-15 kHz for imaging

5-30 kHz for Doppler

PRF Equation

PRF (kHz) = # of pulses / s

Pulse Repetition Period (PRP)

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

PRP Equation

PRP (ms) = 1 / PRF (kHz)

PRF and PRP Relationship

inversely related

Is a high or low PRF desirable for image quality and Doppler Sampling?

high PRF, but is limited by speed of sound in soft tissues

Range Ambiguity

occurs when a pulse is sent before echoes from the previous pulse have been received

-the last echoes from the first pulse appear as if they are the first echoes from the second pulse

If PRF is increased, penetration or depth must decrease

PRF x depth_max ≤ c/2

Pulse Duration (PD)

the time it takes for a single pulse to occur

PD Equation

PD (μs) = T x (# of cycles/pulse)

T = 1/f

f and PD Relationship

inversely related; if f increased, PD is decreased

Duty Factor (DF)

the percent or fraction of time that sound is on

DF_PW < 1

DF_CW = 1

DF Equation

DF = “on”/ “on” + “off”

DF = PD/PRP = TA/PA

DF Ranges

Imaging 0.1-1%

Doppler 0.5-5%

Spatial Pulse Length (SPL)

the distance covered by a single pulse

SPL Equation

SPL = λ x (# of cycles/pulse)

SPL = (c/f) x (# of cycles/pulse)

Bandwidth (BW)

the range of frequencies contained in a pulse; representative of PW

-CW only has 1 frequency; PW has a range of them

BW Equation

BW = highest f - lowest f

Fractional Bandwidth

eequal to bandwidth divided by operating frequency (BW/f_o)

Why do shorter pulses have wider/broader BWs?

because they have fewer cycles; having a broader BW will result in more frequencies present

Quality Factor

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

QF Equation

QF = f_o/BW

PW Intensity

intensity varies in terms of space and time; measured across the beam

-space —> the intensity is higher at its center than periphery

-time —> the intensity decreases toward the end of the pulse

How is intensity measured across the beam?

space or time

Spatial Peak (SP)

the greatest intensity in a pulse reference to space

Spatial Average (SA)

the average intensity across the pulse

Beam Uniformity Ratio (BUR)

shows how uniform the intensity is in space

-CW is relatively uniform in terms of time

-PW is not uniform in space or time

BUR Equation

BUR = SP/SA

-will always be >1 because the peak is higher than average

Temporal Peak (TP)

greatest intensity measured as the pulse passes by

Pulse Average (PA)

average of all intensities measured as the pulse passes by

Temporal Average (TA)

like PA but it includes all time between the pulse being measured an the next pulse

What if there is no time between pulses, then:

TA = PA because sound is always on (CW)

How many ways can an ultrasound beam be expressed to take account that the I changes over time and space?

6

What are the 6 ways that intensity is measured, from lowest to highest?

SATA, SPTA, SAPA, SPPA, SATP, SPTP

SPTA

the most used way to measure Intensity because it relates well to thermal interactions within tissue

-SP because it is highest I in space

-TA because it includes all time

I_SPTA < 720 mW/cm²

Decibel (dB)

relative unit of measure for changes in sound intensity

What do Decibels do?

compare two values of the ultrasound strength (amplitude, power, or intensity)

-found by using a log of a ration between 2 powers, intensities, or amplitudes

Attentuation

the weakening of sound as it propagates

What is gain and dynamic range?

Gain: an increase in strength of sound wave

Dynamic Range: the ratio of largest to smallest power

Intensity Ratio

I/I_o

I = new intensity (final)

I_o = output intensity (initial)

Decibel Equation

dB = 10log(I/I_o)

-if I < I_o = -dB

-if I > I_o = +dB

-if I = I_o = 0 dB

DMU Decibel Values

-3 dB represents a reduction in intensity/power by a factor of 2, 1/2, or 50%

-10 dB represents a reduction in intensity/power by a factor of 10, 1/10; producing a 90% reduction

-6 dB represents a reduction in amplitude by a factor of 2, 1/2, or 50%

-because I = Amp², when reducing Amp by 1/2, I is reduced by 1/4

Decibel vs. Ratio

-decibels give us the ratio of change in strength, but not the value of change

-we can calculate dB from ratio, vice versa

-without one of the values of strength, we cannot calculate the other