ULTRASONOGRAPHY LECTURE II_2

ULTRASONOGRAPHY LECTURE II

OBJECTIVES:

• Explain the concept of key wave properties and its importance in sonography

• Define ultrasound and describe its behavior

• To know and understand the physics of ultrasound

SOUND

ACOUSTICS

• A branch of physics that deals with sound and sound waves

TWO WAYS TO DEFINE SOUND:

1. Experiential:

   • What we hear through the human auditory system.

2. Physical:

   • Propagation of pressure variations through a medium from a sound source to a listener.

SOUND

• A form of energy which causes a mechanical disturbance in the form of vibration of molecules within a medium

• Sound is a mechanical wave that travels in a medium in a LONGITUDINAL WAVE and STRAIGHT-LINE FASHION

• In order to be transmitted, sound requires a medium containing molecules, and therefore cannot travel through a vacuum.

• The production of sound requires a vibrating object.

TAKE NOTE:

• Sound is a travelling variation of acoustic variables

• Acoustic variable include pressure, density, and particle motion

ULTRASOUND WAVE

• Travels in a sinusoidal form

TYPES OF ULTRASOUND WAVE

1. LONGITUDINAL OR COMPRESSED WAVE

1. TRANSVERSE WAVE OR SHEAR WAVE

WHEN A SOUND TRAVELS THROUGH A MEDIUM

• The molecules of that medium are alternately compressed and rarefied.

Compression

• Regions of high pressure

Rarefaction

• Regions of low pressure

“The phase of the wave when the molecules are pushed together is called compression, and when apart, rarefaction”

FREQUENCY

• Number of cycles per second

• Units: Hertz (Hz)

• 1 Hz = 1 cycle/second

• Ultrasound imaging frequency range: 2-20 MHz

• Determines pitch and is crucial for image resolution and penetration.

The lower the frequency, the higher the penetration, and lower of resolution

PERIOD (T)

• The time for a sound wave to complete one cycle

• Unit: sec, ms, µs

“FREQUENCY INCREASES PERIOD DECREASES”

WAVELENGTH / CYCLE LENGTH

• Distance over which one cycle occurs

• Length of cycle from front to back

PROPAGATION VELOCITY

• Speed at which a sound wave travels through a medium (cm/sec)

• Determined by density and stiffness of medium

1. Slowest in air/gas

2. Fastest in solids

• Velocity = wavelength x frequency

• Largely determined by the resistance of the medium to compression, which in turn is influenced by the density of the medium and its stiffness or elasticity.

AMPLITUDE

• The strength/intensity of a sound wave at a given time

• Represented as hight of the wave

• Decreases with increasing depth

• Defines the brightness of the image

• The higher the amplitude the brighter the image

POWER

• Rate at which the energy is transmitted into the body

• It is not contant, the acoustic power varies over time and space

• Unit: W, mW

INTENSITY

• The amount of energy per second that passes through specified area

• Intensity of sound measures in bel or decibel

• Intensity of wave is proportional to the amplitude of the wave square

PRESSURE

• Amount of force over a specific area ‣ Unit: Pascal, Megapascal

• Proportional to the amount of force and volume of the sound wave

• Inversely proportional to the area covered