Ultrasound

Ultrasound

1. energy transmission

   • sound waves are attenuated (decrease in energy intensity) as they travel through a biological medium
     • reflection, refraction, absorption
       • absorbed in skin/muscle (if it gets that deep)
       • reflected off skin
       • refracted in fat

2. as waves penetrate/absorb they cause fluid motion which causes

   • molecules (cells) to stream, rotate, and bounce off one another (vibration = friction = thermal effects)
   • molecules (cells) may change shape or undergo other mechanical effects (non-thermal)

3. Frequency determines penetration and absorption

   • ultrasonographic spectrum
   • law of Grotthus-Draper
   • penetration is inversely related to frequency
     • 1 MHz (low frequency) = deep penetration (up to 5 cm)
     • 3 MHz (high frequency) = shallow penetration (up to 2.5 cm)

4. Laws of energy

   • the more energy absorbed by the superficial tissues, the less remains to be transmitted to underlying tissues (Grotthus-Draper Law)
   • ultrasound penetrates through tissues higher in water content
     • needs a medium to get it through skin (Gel/water)
       • Why need a medium?
         • soft tissue - air: reflects 99.9% of ultrasonic waves
         • soft tissue - bone: reflects 15-40%
         • soft tissue: fat: reflects 1%
         • water - soft tissue: reflects 0.2%
         • ultrasonic wave cannot pass through air - reverses direction
           • never let US head just be on and not in contact w/ water/gel
             • will burn out and crystal replacements are expensive
         • the greater the US head the greater the columniation
   • absorbed in tissues high in protein
   • refracted at joints
   • reflects or bounced off bone

5. Energy

   • amplitude or intensity (watts or W/cm2
   • sound waves affect tissue by creating vibrations of different intensities
   • heating happens when the body cannot clear the developing heat as fast as it is created

6. Ardnt-Schultz principle

   • temp changes are related to
     • the amount and rate of vibrations/friction (W/cm2)
       • duration (time) will also affect temperature changes
         • low intensities for long times can heat tissue as the energy is cumulative

7. Anatomy of US machine

   • generator
   • transducer
     • converts electrical energy into acoustic energy
     • houses the crystal
   • crystal
     • piezoelectric crystal when shocked with electricity will change shape; creating vibrations which are then transitioned through the sound head through the medium and then into the skin
     • synthetic
       • barium titanate
       • lead zirconate titanate
       • produces more uniform beam and costs less than quartz
     • quartz
       • no longer used in most machines
     • crystal is mounted to inside of transducer head