Ultrasound Imaging and Physics

Ultrasound Imaging

• The ultrasound machine transmits high-frequency sound pulses into the body using a probe.
• Ultrasound waves are similar to acoustic and seismic waves.
• X-rays and gamma rays can pass through a vacuum, while sound waves require a medium for transmission.

Ultrasound vs. X-rays

• Diagnostic ultrasound imaging uses high-frequency sound waves to create images of the inside of the body.
• X-rays use electromagnetic radiation to create images within the body.

Ultrasound Physics

• Ultrasound is the term used to describe sound waves with frequencies beyond the range of human hearing.
• Medical ultrasound uses sound waves to create images of stationary and moving tissues within the body.
• Phased or Steered Array Transducers are used for real-time scanning and operate at a frequency of 2-3MHz.
• Convex transducers are used for general-purpose ultrasound, and are ideal for thin adults or children.
• The ultrasound beam produced by the transducer has a variable intensity along its length due to the superposition of sound waves from piezoelectric crystals.
• The intensity of the ultrasound beam decreases as the wavefront travels through the medium.

Doppler Shift Equation

• Doppler Shift: The difference between the incident frequency and the reflected frequency of ultrasound waves.
• Doppler shift can be used to measure the velocity of blood flow, as the frequency of the reflected waves changes based on the velocity of the blood.
• Doppler Shift Equation:

•   2vs 
     𝛿V = ------------ cos θ
     v
     ∆v = Frequency change (doppler shift in Hz)
     vs --- velocity of blood (m/s)
     v = velocity of sound (1540 m/s)
     θ = angle between sound beam and direction of blood flow (Doppler angle)
  

• The Doppler shift falls into the audible range for most physiologic motions between 2 and 10 MHz (2-10 million cycles per second).

Components of Ultrasound Machines

• Separate controls to alter the surface (near) echoes and the deep (distant) echoes. These are known as near gain and far gain controls.
• Frame freeze control: Holds the image on the screen for extended viewing.
• Calliper control: Measures the distance between two points shown on the image.
• Recording options include:
• X-ray recording: Expensive but high quality image, requires darkroom and single emulsion X-ray film.
• Self-processing camera and film: Attached to ultrasound unit, expensive.
• Image recording units: Prints on special paper, less expensive, but images need to be protected from heat and light.

Choosing the Appropriate Transducer

• A convex transducer at 3.5 MHz, focused at 7-9 cm, is generally best for ultrasound examinations.
• Linear and sector transducers at 3.5 MHz may also be needed.
• A 5.0 MHz transducer focused at 5-7 cm is recommended for children or thin adults.
• Obstetric Ultrasound: (3.5 MHz recommended):
• Linear or convex transducers are used.
• 5MHz is most effective during early pregnancy, 3.5 MHz is better during later pregnancy.
• General Purpose ultrasound (3.5 MHz recommended):
• Sector or convex transducer used.
• Suitable for upper abdomen, pelvis, and obstetric examinations.

Types of Transducers

• Linear Transducer: Creates a rectangular image, useful for superficial structures.
• Sector Transducer: Produces a fan-shaped image, useful for scanning through limited acoustic windows.
• Convex Transducer: Produces a curved or trapezoidal image, useful for abdominal and pelvic scans.

Scattering of Ultrasound by Blood

• Ultrasound does not reflect from blood, instead it is scattered in all directions.

Test your knowledge on ultrasound imaging and its principles. This quiz covers how ultrasound machines work, the differences between ultrasound and X-rays, and the physics behind sound waves used in medical imaging. Gain insights into various types of transducers and their applications in diagnosing conditions.