Ultrasound in Medicine and Imaging

Ultrasound in Medicine

• Ultrasound uses sound waves with frequencies from 20 kHz to 1 GHz (for medical applications).
• This frequency range is above the upper limit of human hearing.
• SONAR (SOund Navigation and Ranging) is a technology that uses sound waves to detect objects underwater.

Ultrasound Transducers

• Transducers convert electrical energy to mechanical energy (ultrasound) and vice versa.
• Various types exist, differing in frequency and size, including curvilinear, phased array, linear, and hockey stick transducers.

Basic Principle of Ultrasound Imaging

• Ultrasound pulses are transmitted into the body, often using a gel to eliminate air and improve contact.
• Echoes reflected back from tissues are detected and amplified on an oscilloscope.
• The time it takes for the echo to return is proportional to the depth of the tissue.

Ultrasound Generation

• Ultrasound signals are produced and detected using piezoelectric crystals.
• Applying an alternating current (AC voltage) to the crystal causes it to vibrate, generating ultrasound waves.
• The vibration of the crystal creates a mechanical wave.

Ultrasound Image Production

• Focal Zone: For best image quality, the object should be at the focal point of the transducer (near-field).
• Acoustic Impedance: This property of a material relates to how much it will reflect or transmit ultrasound. Different tissues have different acoustic impedances. A sudden change in impedance results in reflection/scattering.
  • High impedance differences (e.g., bone/tissue) result in increased reflection.
  • Low impedance differences result in less reflection.
• Refraction: A change in the direction of the sound wave as it passes from one tissue to another with a different velocity, creates image artifacts.

Quality of Ultrasound Imaging

• Spatial Resolution: Quality is determined by the interaction of the ultrasonic wave with tissue.
  • Spatial resolution is limited by the wavelength of sound.
    • Shorter wavelengths lead to better spatial resolution and vice versa.
• Attenuation: The reduction in intensity of an ultrasound wave as it passes through tissue.
  • Higher frequencies and higher attenuation are associated with tissues with poorer image quality.
• The appropriate frequency is chosen to balance good resolution with sufficient penetration depth.

Image Quality

• Frequency choice and compromises between good resolution and deep penetration are important.
• Higher frequency = better resolution but limited penetration.
• Lower frequency = better penetration but lower resolution.
• Different frequency ranges are best suited for various applications:
  • 3-5 MHz for large organs.
  • 4-10 MHz for small organs.

Reflection

• Perpendicular reflection produces a strong echo.
• Non-perpendicular reflection causes echo signal loss.
  • Smooth surfaces result in low scattering and good image quality, while rough surfaces result in high scattering and poor image quality.

Ultrasound Image Modes

• A-Mode (1D): Measures the depth of tissues by detecting the time for echoes to return, obtaining a one-dimensional image. Used in ophthalmology for locating tumors or foreign bodies
• B-Mode (2D): Creates two-dimensional images by measuring variations in reflected intensity as different tissue interfaces are encountered. Used for creating an image that shows the internal structure of a body section.
• M-Mode (2D+Motion): Combines 2D image with the ability to observe movement (e.g. heart/valves). The transducer stays stationary so motion is displayed as dots.
• D-Mode (3D + Motion, or 4D): Combines 3D imaging with the ability to observe movement. Used for fetal imaging.

Physiological Effects of Ultrasound Therapy

• Low Intensity Ultrasound: Used for diagnostic purposes with minimal impact, minimal or no harmful effects.
• Medium Intensity: Causes heating effects (diathermy).
• High Intensity Ultrasound: Used to destroy tissues.

Ultrasound Applications

Specific applications in different medical fields were highlighted, including brain tumor detection, ophthalmic procedures (eye lens, retina), general internal organ imaging.