6.5 (medical imaging)

medical imaging

Describe the basic structure of an X-ray tube and how X-ray photons are produced

• metal target, anode, cathode, high voltage supply, heater filament

• high voltage supplied between cathode and anode

• the incoming electrons hit the metal target and the KE is transferred into X-ray photons

Name all the types of X-ray interaction

• Simple scattering

• Photoelectric effect

• Compton effect

• Pair production

Describe simple scattering in terms in X-rays

• X-ray photon is scattered by an atomic electron

Describe the photoelectric effect in terms of X-rays

• an X-ray photon interacts with an electron leading to the photoelectron being emitted

Describe pair production in terms of X-rays

• the X-ray photon transforms into an electron and positron pair

Describe the Compton scattering

• electron ejected from atom

• photon then has increased wavelength

Why are contrast media used in X-rays

• they have a large attenuation coefficient so absorbs more X-rays

• increasing contrast between surrounding flesh

Describe how a computerised axial tomography (CAT) scan can produce high quality images

• X- ray moves around patient

• a thin fan-shaped X-ray beam

• cross sections through the patient taken

• 3D image produced

• soft tissues identified

Explain the advantages of using technetium-99m for imaging organs

• emits gamma photons

• which are detected by gamma cameras

• has a small half life

Name and describe the three main components of the gamma camera

• scintillator, crystal of sodium iodine when hit by a photon emits many photons of visible light

• photomultiplier tubes, photoelectric effect, electrical pulse produced for every incident photon of light

• collimator, gamma photons parallel to axis of the lead tubes (collimator) are detected, therefore excluding scattered photons

Describe how the medical tracer fluorine -18 nuclei are located using a PET scanner

• fluorine -18 gives of a positron, which interact with an electron and annihilate

• leading to the emission of two gamma photons in opposite directions

• detected by gamma cameras, time delay between the gamma photons

• the difference in time between gamma photons used to locate the point of annihilation

Explain how an ultrasound transducer can emit ultrasound

• piezoelectric effect

• an alternating p.d. applied

• crystals produce vibrations

Explain how ultrasound is used to measure the speed of blood flow in artery

• pulses are aimed at blood cells

• the detected wavelength of returning wave is different to emitted waves

• ultrasound emitted at an angle

Explain impedance acoustic matching

• reduction of reflection

• by using similar acoustic impedance

Explain how an A- scan (ultrasound) could be used to measure the thickness of a patients eye lens

• pulses of ultrasound sent to eye

• reflections from front and back of lens

• thickness found from speed of ultrasound

Describe a B-scan

• 2D or 3D image built up from many returning echoes

• transducer is moved to different angles and positions

• The brighter the dot the greater the amplitude of the reflected pulse

State what is meant by the piezoelectric effect

• the material contracts or expands when a p.d. is applied across it