Describe the nature of X-rays
- Electromagnetic radiation
- Travel at speed of light in a vacuum
- Short wavlength: 10-8 to 10-13
- High frequency
How are X-rays produced
Fast moving electrons are rapidly decelerated; as the electrons slow down their k.e. transformed to photons of e.m. radiation
Name 3 ways X-rays interact w. matter
- Photoelectric effect
- Compton scattering
- Pair production
Describe photoelectric absorption mechanism
- Energy of X-ray photon < 0.1 MeV
- X ph absorbed by e- in target atom
- e- gains enough energy to escape from atom
- μ ∝ Z3 ; μ ∝ 1/E3
Describe Compton scattering absorption mechanism
- Energy of X ph 0.5 - 5.0 MeV
- X ph loses part of its energy to e-in target atom
- Inelastic collision between e- and ph
- Scattered X ph w. lower energy; Compton e- ejected in diff. direction so momentum conserved
- µ independent of Z; µ decreases with E
Describe pair production absorption mechanism
- Energy of X ph high: > 1.02 MeV
- X ph passes through eletric field of nucleus, produces electron, positron pair
- Mass comes from E of X ph
- e+ soon annihilated
- µ ∝ Z2; µ rises slowly with E
Describe how an X-ray tube works
See page 228 for diagram
- Cathode: Filament gets so hot, electrons are emitted (-ve electrode)
- Electron beam: electrons are accelerated across vacuum by 50-200kV voltage to anode
- Anode: made of molybdenum or tungsten (+ve electrode); electrons strike it and their k.e. is converted to X phs
- X-rays are collimated (parallel beam)
How is an X-ray tube used in medical imaging
The X-ray tube is above the patient who is lying on the detection system
The X-rays pass throught the patient’s body
Draw an X-ray spectrum
See page 228
What is the braking radiation on an X-ray spectrum
The “hump” on the graph
The electron loses its energy as it interacts with the electric field of the anode nuclei
Single (or more often several) X phs emitted
What is the characteristic radiation on an X-ray spectrum
Sharp lines on graph
Electron causes rearrangement of electrons in an anode atom
Anode electron drops from a high energy to a lower energy level
Single X ph emitted w. energy equal to diff. in energy levels
Energy levels diff. for diff. materials so will have diff. characteristic spectral lines
Describe how an intensifier screen works
- It’s a sheet containing phosphor -> substance that emits vis. light when hit by X ph
- The X-ray film is sandwiched between 2 intensifier screens
- Each X ph releases ~ 100s vis. light photons
- Reduces dose of X-rays incident to patient ~100-500x
Draw an image intensifier
See page 233
Describe how an image intensifier works
- Collimator lines up X-rays
- Input phosphor converts X into many vis. light ph
- Photocathode releases electrons from vis. light ph by photoelectric effect
- Focusing electrodes accel. and focus electrons across vacuum towards anode
- Output phosphor converts electrons to vis. light
- This image on the computer screen can be viewed or stored
Describe how contrast media works
Is a good absorber of X-rays
Patient swallows barium containing liquid (barium meal) or has it injected into tissue of interest
The tissue becomes a better absorber of X-rays so shows up more clearly on final image
They have high Z numbers: more electrons for X to interact with so are avbsorbed more easily
Attenuation mechanism mostly photoelectric effect where µ ∝ Z3
Describe how a CAT scan works
See page 234 for diagram
- Patient lies in vertical ring of detectors
- X-ray tube rotates around ring, exposing patient to a fan-shaped beam of X-rays from all directions
- Detectors opposite send electronic records to computer
- Computer software builds up 3D model of patient in slices
Advantages of CAT scans
- Images are 3D and show relationship between diff. tissues i.e. depth
- Improves contrast between tissues of similar density
Disadvantages of CAT scans
Increased exposure (150-1000x) to ionising radiation compared to X-ray -> cell damage and cancer
