A2 Physics Medical Imaging

when does a piezo-electric crystal change shape

when a potential difference is applied across it

what happens if a piezo-electric crystal changes shape

the crystal generates an e.m.f.

how are ultrasound waves created

• they are generated by a piezo-electric transducer which converts electric energy to ultrasound energy through the use of a piezo-electric crystal such as quartz

how are ultrasound waves detected by a piezo-electric transducer

• when receiving it coverts sound waves to alternating p.d.

• when transmitting it converts alternating pd to sound waves

how is the reflection of pulses of ultrasound used to obtain diagnostic information

• sound waves are reflected at the boundaries to the transducer causing it to vibrate

• vibration generates electrical signals

• using speed of sound waves and time for it to reach back, the distance can be known

• ultrasound tells us about depth and nature of the organ/bones

what is specific acoustic impedance of a medium

Z=pc, where c is speed of sound in the medium and p is the density

formula for intensity reflection coefficient of a boundary between two media

formula for attenuation of ultrasound in matter

how are X-rays produced

• electrons are accelerated by an applied p.d.

• electrons hit the target

• x-rays are produced when electrons decelerate

formula for minimum wavelength of X-rays produces from accelerating pd

use of x-rays in imaging internal body structures

• reduce exposure to radiation —> aluminum filters absorb long wavelengths which are more harmful to the body

• improve contrast and sharpness of photos —> hard x-ray for bones, soft x-ray for tissues

what is contrast in x-ray imaging

difference in the degree of blackening between structures

how does computed tomography (CT) scanning produce a 3D image

• it combines multiple x-ray images taken in the same section from different angles to obtain a 2D image of the section

• repeating this process along an axis then combining 2D images of multiple sections

what is a tracer

a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied

what particles are decayed in a tracer used in positron emission tomography (PET scanning)

beta plus decay

what is annihilation

a particle interacting with its antiparticle so that mass is converted into energy

what particles are involved in the annihilation process

electrons and positrons

what is conserved in the annihilation process

mass, energy and momentum

in PET scanning, what happens when positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue

it produces a pair of gamma-ray photons traveling in opposite directions

formula for energy of gamma-ray photons emitted during annihilation

how are gamma photons used to produce an image

• two gamma photons travel in opposite directions

• gamma photons are detected and arrive at different times

• this helps determine the location of production of gamma

• the image of tracer concentration in tissue is then produced