Understanding Ultrasound Physics CH.3

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Sidney K. Edelman, THIRD EDITION

Parameters

DESCRIBE features of a sound wave:(7)
period, frequency, amplitude, power, intensity, wavelength and propagation speed

Parameters

Can be determined by the SOUND SOURCE or the MEDIUM

Sound Source

Is the ULTRASOUND SYSTEM and TRANSDUCER
Some parameters are set by the machine
Some parameters can be adjusted by the sonographer, while others can not.

Medium

The tissue through which the sound is traveling/
Some parameters are determined by the medium

1. Period

The TIME it takes a wave to vibrate a single CYCLE or
The TIME from the START of a cycle to the START of the next cycle.

UNITS:
any unit of TIME, such as seconds, hours, days, ms(milliseconds)

Period

Determined by: SOUND SOURCE
Adjustable: NOT ADJUSTABLE

2. Frequency

The number of CYCLES that occurs in ONE SECOND or
The number of particular EVENTS that occurs in a specific DURATION of time

UNITS: Hertz(Hz)=per second
1 cycle/second=1 hertz

Frequency

Determined by: SOUND SOURCE
Adjustable: NOT ADJUSTABLE

Frequency affects penetration and image quality

The relationship between PERIOD and FREQUENCY

INVERSELY RELATED:
-As frequency INCREASES, period DECREASES
-As frequency DECREASES, period INCREASES
and
RECIPROCAL: Period x Frequency = 1

Frequency Spectrum of Sound

1. INFRASOUND- less than 20 Hz
2. AUDIBLE SOUND - between 20Hz-20,000 Hz (20kHz)
3. ULTRASOUND - greater than 20,000 Hz (20kHz)

Three "Bigness" Parameters

1. Amplitude
2. Power
3. Intensity
Describe the SIZE, MAGNITUDE, or STRENGTH of a sound wave

3. Amplitude

The "bigness" of wave- measured from the middle/undisturbed value to the maximum value in either direction- up (peak) or down (trough)

UNITS: Can have units of any of the acoustic variables (pressure- pascals, density- g/cm3, distance- cm, inches etc and decibels, dB)

Amplitude

Determined by: SOUND SOURCE
Adjustable: YES, a control on the machine allows the sonographer to alter amplitude

Peak-to-Peak Amplitude

The total measurement between the MAXIMUM and MINIMUM values of the wave
It is 2x the value of amplitude

4. Power

RATE of ENERGY TRANSFER or the RATE at which WORK is performed

UNITS: Watts (w)

Power

Determined by: SOUND SOURCE
Adjustable: YES

The relationship between POWER and AMPLITUDE

DIRECTLY RELATED- When power increases, so does the amplitude

Power is proportional to the wave's AMPLITUDE SQUARED

5. Intensity

Concentration of energy in a sound beam. Depends on the POWER and the AREA over which the power is applied

UNITS: Power (w)/ Area (cm2)

Intensity

Determined by: SOUND SOURCE
Adjustable: YES

Amplitude, Power, and Intensity

are DIRECTLY RELATED
When intensity increases, so does amplitude and power etc

Relationship between INTENSITY AND POWER

PROPORTIONAL-
If the wave's power is doubled, the intensity is doubled

Relationship between INTENSITY and AMPLITUDE

Intensity is proportional to the waves AMPLITUDE SQUARED
If a wave's amplitude is doubled, the intensity is increased to 4x its original value

6. Wavelength

The DISTANCE or LENGTH of one complete CYCLE

UNITS: any unit of distance or length (mm, meters, etc.

Wavelength

Determined by: BOTH the SOURCE and MEDIUM
Adjustable: NOT ADJUSTABLE

Relationship between WAVELENGTH and FREQUENCY

INVERSELY RELATED
As frequency increases, wavelength decreases, (as long as the wave remains in one medium)

-the lower the frequency, the longer the wavelength

7. Propagation Speed

DISTANCE that a sound wave travels through a MEDIUM in ONE SECOND

UNITS: m/s meters per second, mm/micro second, or any distance divided by time

Propagation Speed

Determined by: ONLY the MEDIUM
Adjustable: NOT ADJUSTABLE

-Regardless of the frequency, all sound travels at the same speed through any specific medium

Slowest to Fastest propagation

A (air) FAT (fat) WAITER (water) SERVED (soft tissue) BLOODY (blood) MASHED (muscle) BONE (bone)
Sound travels fastest in solids, slower in liquids, and slowest in gases

Speed of Sound in Soft Tissue

1540 m/s or 1.54 mm/micro seconds

speed(m/s) = frequency(Hz) x wavelength (m)

What characteristics of a medium affect the speed of sound in that medium?

1. Stiffness
2. Density

Stiffness

Ability of an object to RESIST COMPRESSION:

a stiff material will retain its shape, whereas a non-stiff material will change its shape

Relationship between STIFFNESS and SPEED

DIRECTLY RELATED:
The stiffer it is, the faster it goes
The opposite of stiff is elastic and compressible

Density

Describes the relative weight of a material.

-The higher the density, the higher the weight

Relationship between DENSITY and SPEED

INVERSELY RELATED:
As materials become more dense (heavier) the speed of sound in the material decreases

When density increases, speed decreases

Materials that are STIFF, but NOT DENSE will have the fastest speed.

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