Physics and Instrumentation

Pulse-Echo Technique

Pulses are sent into tissues and bounce back creating echoes that provide anatomic information.

What does the strength of an echo correspond to?

The brightness of the dot on the display screen.

What does the brightness of a dot correspond to?

The strength of the echo.

What does the echo return time correspond to?

The location of the reflecting tissue.

What does the location of the dot correspond to?

Echo Return time

Ultrasound machines measure…

Time

Ultrasound machines calculate…

Distance

Displacement Concept

Distance = Speed x Time

Distance = Speed x Time

Displacement Concept

Distance ____ when time Increases

increases

Distance ____ when time decreases

decreases

Average speed of sound used in diagnostic ultrasound

1540 m/s or 1.54 mm/μs

The denser the medium the ____ the speed

faster

Speed of sound equation

C = √(B/P)

where C is the speed of sound, B is the bulk modulus, and P is the density of the medium.

Operating Principle 1

One-to-one correspondence where echoes from one pulse appear as one scan line

Operating Principle 2

Retrospective computational beamforming determines echo information

Two-Dimensional scan formats

Cross sectional images called B scans or gray-scale scans

Linear Format

Closely spaced parallel vertical scan lines which all travel in the same direction with each pulse having a different starting point.

Rectangular Display

Format?

Linear Format

Sector Format

Each pulse has the same starting point and each pulse travels in slightly different directions.

Pie/Fan Display

Format?

Sector Format

Combination / Curvilinear Format

Each pulse has slightly different starting points and each pulse travels in slightly different directions.

Curved Display

Format?

Combination / Curvilinear

Volume Imaging

Three-dimensional echo data acquired by many two-dimensional sections placed together.

Type of imaging

Volume Imaging / 3D

Doppler Effect

Change in frequency caused by a moving object

Used to detect and measure tissue motion and blood flow

Can be audible, colour or spectral

Colour Doppler

colour-coded representations of information placed on a gray scale image.

Type of doppler

Colour doppler

Type of doppler

Power doppler

Spectral Doppler

Information is applied to a speaker for audible evaluation and to a display for analysis.

Type of doppler

Spectral doppler

What is sound?

Traveling variation of acoustic variables.

Compressions

High localized pressure

High localized pressure

Compressions

Rarefactions

Low localized pressure

Low localized pressure

Rarefactions

Wavelength (λ)

Is a measure of distance between compressions (a peak and a peak) or rarefactions (a through and a through)

Is a measure of distance between compressions (a peak and a peak) or rarefactions (a through and a through)

Wavelength (λ)

Frequency (F)

How many cycles of a wave pass a particular point in a given period of time.

Units: Hz, kHz, MHz

How many cycles of a wave pass a particular point in a given period of time.

Units: Hz, kHz, MHz

Frequency (F)

Propagation Speed (C)

Determined by the medium it is traveling through

1540 m/s or 1.54 mm/μs

Determined by the medium it is traveling through

1540 m/s or 1.54 mm/μs

Propagation speed (C)

Period (T)

Time it takes for one cycle to occur

Equations for F, C and λ

C = Fλ

F = C/λ

λ = C/F

Equations for T and F

T = 1/F

F = 1/T

When frequency increases the period…

Decreases

When frequency decreases the period…

Increases

When frequency increases the wavelength…

Decreases

When frequency decreases the wavelength…

Increases

Harmonics

High pressure portions of the wave travel faster than low pressure portions.

Results in higher and better resolution.

Name

Harmonics

Continuous Wave

A wave that is on 100% of the time.

Type of wave

Continuous

Pulsed Wave

A type of wave that is only on for a portion of the time, alternating between wave and listening time

Most commonly used in diagnostic u/s

Type of wave

Pulsed wave

Terms for …

Continuous wave

Terms for …

Pulsed Wave

If frequency increases, period ___ and pulse duration ___

decreases, decreases

If frequency decreases, period ___ and pulse duration ___

increases, increases

If the number of cycles in a pulse decreases, pulse duration ___

decreases.

If the number of cycles in a pulse increases, pulse duration ___

increases.

Shorter pulses = ___ image quality

better

Longer pulses = ___ image quality

worse

if pulse duration increases, duty factor ___

increases

If pulse duration decreases, duty factor ___

decreases

If PRP decreases, duty factor ___

increases

If PRP increases, duty factor ___

decreases

If PRF increases, duty factor

increases

If PRF decreases, duty factor ___

decreases

The shorter the pulse, the ___ the bandwidth

greater

The longer the pulse, the ___ the bandwidth

smaller

If frequency increases, attenuation ___

increases

If frequency decreases, attenuation ___

decreases

Pulse Repetition Frequency (PRF)

The number of pulses occurring in one second

The number of pulses occurring in one second

Pulse Repetition Frequency (PRF)

Pulse Repetition Period (PRP)

The time from the beginning of one pulse to the beginning of the next

The time from the beginning of one pulse to the beginning of the next

Pulse Repetition Period (PRP)

Pulse Repetition ___

Frequency

Pulse Repetition ___

Period

PRF equation

PRF = 1 / PRP

PRP equation

PRP = 1 / PRF

Pulse Duration (PD)

The time for a pulse to occur

Arrow

Pulse Duration

Pulse Duration Equation

PD = T x # cycles

T = PD / # cycles

# cycles = PD / T

Sonographic pulses are typically ___ cycles long

Doppler pulses are typically ___ cycles long

2-3 , 5-30

Duty Factor (DF)

Fraction of time that pulsed ultrasound is on

DF for 2D u/s is ___% - ___%

DF for doppler u/s is ___% - ___%

DF for continuous wave is ___%

0.1% - 1%

0.5% - 5%

100%

Duty Factor equation

DF = PD / PRP

PD = DF x PRP

PRP = PD / DF

Spatial Pulse Length (SPL)

The length of space that a pulse takes up

The length of space that a pulse takes up

Spatial Pulse Length (SPL)

Arrow

Spatial Pulse Length

Spatial Pulse Length equation

SPL = λ x # cycles

λ = SPL / # cycles

# cycles = SPL / λ

Bandwidth

The range of frequencies contained in a pulse

The range of frequencies contained in a pulse

Bandwidth

Fractional Bandwidth

How large the bandwidth is compared with operating frequency

How large the bandwidth is compared with operating frequency

Fractional Bandwidth

Arrow

Fractional Bandwidth

Fractional Bandwidth equation

Fractional bandwidth = bandwidth / operating frequency

Operating frequency = bandwidth / fractional bandwidth

Bandwidth = fractional bandwidth x operating frequency

Amplitude

The maximum variation that occurs in an acoustic variable

The maximum variation that occurs in an acoustic variable

Amplitude