Ultrasound Basics Chapter 4

Chapter 4: Ultrasound Basics

• Ultrasound science blends modern electronics and acoustics, widely used since 1945, particularly in industrial and medical fields.
• Ultrasound is crucial in industrial and medical contexts due to the increased number of ultrasonic devices.
• This chapter explores ultrasound applications, principles, physical characteristics of acoustic waves, biological uses, and propagation in biological environments.

Ultrasound Applications

• Industries use it in metal, plastics, and composites.
• Medical uses include ultrasound and focused ultrasound thermotherapy.
• In agriculture, it's used to vibrate water, creating an aerosol that supplies root systems with oxygen.
• Remote sensing (sonar) and telemetry (distance measurement) are other applications.
• Automotive industry uses it for obstacle avoidance.
• Non-destructive control is another area of use.

Physical Acoustics and Definition of Sound

• Acoustics is the study of sound waves, their production, propagation, and effects.
• A wave is energy that disturbs a medium, categorizing into mechanical (needs a medium) and electromagnetic (no medium).
• Sound is a mechanical wave resulting from a vibrating body in a medium (solid, liquid, or gas.)
• Sound propagation requires three elements:
  • A source producing vibration.
  • A medium to transmit the vibration.
  • A receiver to detect the vibration.

Mode of Propagation

• Sound waves are longitudinal waves, propagating via compression and decompression of medium molecules.
• Longitudinal waves have the same direction of vibration and displacement.

Sound Propagation Mode

• No sound occurs when a medium is undisturbed; particles are evenly spaced with no vibration.
•  Oscillation and mechanical vibrations are absent, thus no sound is generated or transmitted.

Classification of Sound Waves

• Audible sounds for humans are within the range of 20 Hz to 20,000 Hz (20kHz).
• Frequencies below 20 Hz are infrasound; frequencies above 20 kHz are ultrasound.

Physical Parameters of Sound: Frequency

• Frequency represents the number of periodic oscillations per second, measured in Hertz (Hz).
• It's inversely related to the period (T), with the formula: f = 1/T

Physical Parameters of Sound: Wavelength

• Wavelength (λ) is the distance travelled by an acoustic wave during a period.
• It relates to the speed (C or V) and frequency (f) of the wave: λ = C/f = V/f, where C or V is the speed in m/s.

Physical Parameters of Sound: Velocity (Speed) of Sound

• Sound speed depends on the medium (solid, liquid, or gas),
• Temperature and pressure also influence the sound's speed.
• Examples include air (340 m/s at 20°C), water (1480 m/s at 20°C), and steel (6000 m/s).

Sound Intensity

• Sound intensity quantifies the distribution of sound energy in space.
• Loudness is related to the amplitude (sound pressure) of sound waves, with a large amplitude indicating a high intensity.

Pure Sound

• A pure sound consists of a single frequency.
• Pure sounds are not naturally occurring; they are electronically generated.

Complex Sound

• A complex sound is the sum of several pure sounds. This is the nature of most sounds heard in everyday life.

Production of Ultrasonic Waves

• An ultrasonic transducer converts electrical energy into mechanical energy (ultrasound) and vice versa, using the piezoelectric effect.

Principle of Ultrasound (Echo)

• Ultrasound utilizes the reflection of sound waves (echoes) to form images.
• The time delay between the emitted signal and the reflected echo determines the target's distance, allowing for the creation of images that show different structures based on internal differences.

Ultrasound Frequency, Image, and Organ Depth

• Ultrasound uses frequency between 1 MHz to 20 MHz (or up to 50MHz in certain applications).
• The higher frequency, the smaller structures can be visualized, but they're less efficient for deep imaging.

Why is there an echo?

• Echoes arise due to changes in the propagation medium (different acoustic impedance values). Acoustic impedance is a measure of how much a medium resists sound propagation.

Acoustic Impedance

• Acoustic impedance (Z) is a property of a medium that characterizes its reflection and transmission of sound waves.
• Z = ρ * v where ρ is the density of the medium and v is the speed of sound in the medium.

Ultrasound Interaction With Matter

• When ultrasound encounters an object, several phenomena occur, most notably reflection and refraction.
• These interactions are key in forming ultrasound images by providing information about both the medium through which the waves travel, and the nature/type of any encountered objects.
•  Information about the intensity (amplitude) and the time taken for the waves to return (delay).

Echography Image Formation

• Applying gel to a patient's skin facilitates contact, minimizing interference.
• The probe transmits and receives ultrasound waves, reflected by structures in the body to produce images.
• Two essential parameters for ultrasound image quality are the distance between the probe and the tissue structure, and the intensity of the reflected echo.

Types of Echography Images

• Anechogenic images correspond to media that transmit ultrasound without significant reflection (e.g., water, fluids). These appear dark.

• Echogenic images arise from media that reflect ultrasound with moderate regularity, appearing as shades of grey.

• Hyperechogenic images result from highly reflective media appearing as bright white.