Ultrasound Imaging

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• Ultrasound waves are longitudinal mechanical waves, with a frequency of between 1000 to 20000 Hz.
  • This is above the human hearing range.
• Ultrasound imaging is a form of medical imaging that uses ultrasound waves to create an image of internal body structures, e.g. organs.
  • The most common use of this is for pregnancy, to have an ultrasound image of the baby.
• Ultrasound waves are used in medical imaging for a few reasons.
  • One reason is that since they are outside the audible range, they don’t cause discomfort to the patient.
  • Another reason is that ultrasound waves have very high frequencies, which produce a better resolution as they diffract less. Therefore, their intensity is maintained for longer distances, and more superficial structures can be imaged.
  • Finally, it’s a good alternative to x-ray, as it doesn’t have a possibility of causing cancer, and some structures in x-ray scans aren’t shown, such as soft tissue.
• However, since ultrasound waves don’t travel well through air or bone, it is not effective at rendering bones or parts filled with gas.
• Furthermore, ultrasound imaging may not be able to identify objects deep within the body.

Process

1. A transducer generates pulses as it moves along the surface of the tissue. The pulses travel through a conductive gel(which enables direct transmission of the wave from the transducer to the tissue) and into the tissue, and 3 things happen.
2. Wave’s Journey
   1. Due to absorption, the waves lose intensity and energy as they travel further into the tissue. Therefore their amplitude decreases as they travel into the tissue.
   2. The ultrasound waves reflect at the intermedia between 2 tissue
      1. When the wave hits a smooth surface, and the wavelength is smaller than the imaged surface, specular reflection occurs . Thus, a single, reflected wave is formed, moving in one direction
      2. When the wave hits a rough surface, diffuse reflection occurs. The wave is reflected in multiple directions with lower amplitudes, and thus intensities.
      3. The ultrasound waves have a possibility to refract, causing a change in the ultrasound wave speed and wavelength
      4. Acoustic impedance (ai) is a property that measures the resistance ultrasound waves encounter in tissue. A higher density of tissue means a greater acoustic impedance, which means that the ultrasound waves are deflected with a smaller decrease in intensity. Thus, at the surface of dense tissue, e.g. bones, the reflection, and thus the intensity, is greater. The larger the difference in acoustic impedance between the 2 mediums, the greater the fraction of the wave that is reflected, and vice versa.
3. The reflected waves arrive at the transducer
4. The distance between interfaces is calculated using the angle of return and time take to return
   1. This is because of $s=vt$, and the law of reflection
5. A grey scale image is produced, using their intensity
   1. Greater intensity of reflected light means a whiter image, thus denser objects (e.g. bone) appear brighter than rarer objects
6. An image that shows the boundaries of various structures and their densities is created, and is known as a sonogram