Physics 'AS' & 'A' Level: Medical Imaging

Physics 'AS' & 'A' Level: Medical Imaging

1. MAGNETIC RESONANCE IMAGING (MRI)

2. Ultrasound

   • Piezo-electric effect (of a crystal)
     • Ultrasound transducer also acts as the detector of reflected ultrasound waves.
       • It can do this because the piezo-electric effect works in reverse: a varying stress applied to the crystal produces a varying e.m.f. across the crystal.
     • Ultrasound transducers use the piezo-electric effect to generate ultrasound waves
       • An alternating voltage is applied across the crystal, which then acts as the vibrating source of ultrasound waves.
   • Ultrasound's Frequency
     • 2.0 MHz ultrasound waves will be able to distinguish detailed features
     • More higher-frequency ultrasound waves can be use to detect smaller features inside the body
       • Unfortunately, higher frequency waves are absorbed more strongly and so a more intense beam must be used.
   • The principle of an ultrasound scan
     • Method is to direct ultrasound waves into the body. Passing through various tissues and are partially reflected at each boundary where the wave speed changes. The reflected waves are then detected and used to construct an internal image of the body.
       • Fraction of the incident intensity of ultrasound that is reflected at the boundary
         • This depends on the acoustic impedance Z of both materials.
           • The acoustic impedance Z of a material is the product of the material's density ρ and the speed of sound c travel in the material.
             • Z = ρc
         • The greater the difference in acoustic impedances, the greater the reflected fraction of the ultrasound waves.
           • For e.g. Between Bone rib and Lungs. This means that bone shows up well in an ultrasound scan, but it is difficult to see different soft tissues.
           • For e.g. Between patient’s skin is in contact with air, and 99.95% of the ultrasound will be reflected before it has entered the body
             • To overcome this, the transducer must be ‘coupled’ to the skin using a gel whose impedance matches that of the skin. This process of impedance matching explains why the patient’s skin is smeared with gel before a scan.
         • Reflected Intensities = Reflected Intensity(Ir) / Incident Intensity(Io)
           • Ir / Io = [ (Z2 − Z1)/(Z2 + Z1) ](square)
       • Ultrasound waves are gradually attenuated as they pass through the body
         • Ultrasound's energy is absorbed, hence their amplitude and intensity decrease
           • The intensity I decreases with distance x
             • I = Io expo(−αx)
         • Echoes from tissues deeper in the body are weaker and must be amplified.