Part 1: L1, X-rays Overview

Question 1

Common imaging techniques:

Answer 1

• X-ray (including CT and fluoroscopy)
• Ultrasound
• MRI
• Endoscopy
• Thermography
• Encephalography
• PET
• Gamma camera
• Microscopy

Question 2

Key considerations for imaging agent design:

Answer 2

• Toxicity
• Stability
• Uptake and localisation/distribution
• Circulation/residence time
• Route of excretion (whole body will typically be renal or hepatic)
• Penetration of signal

Question 3

Bremsstrahlung X-rays: How do they occur?

Answer 3

• High energy electron comes very close the nucleus and electromagnetic interaction causes a deviation of the trajectory where the electron loses energy and an X-ray photon is emitted
• Energy of emitted photon can take any value up to a maximum corresponding to the energy of the incident photon
• Collision between electrons results in ejected electrons from the relevant shell

Question 4

Development in X-ray detection:

Answer 4

• Initially obtained by silver based photographic plate (not very sensitive due to low quantum efficiency; 2% of X-rays interact)
• Replaced by an ‘X-ray cassette containing film’ and rare earth elements that converted X-rays into lower energy photons which could interact with the film
• e.g. Gd2O2S (up to 25% QE)

Question 5

Fluoroscopy- What is it? Give one relevant technology

Answer 5

• Real time detection of X-rays
• Required innovations including image intensifiers

Question 6

X-ray interaction with matter: (Give the 4 modes)

Answer 6

X-ray beam hits an atom:
1. Photon absorbed, excites electron which escapes in same direction as incoming photon -> Photoelectric absorption
2. Photon transfers some of its energy into an electron, lower energy photon is emitted, and direction changes, electron can also escape in different direction -> Compton Scattering
3. If photon is 1.02 MeV, photon can transfer into an electron and positron -> Pair Production
4. Nuclear reactions can occur at even higher energies

Question 7

Relevance of size of atom to X-ray application:

Answer 7

• The absorption of X-rays follows and exponential decay with thickness
• mu denotes the attenuation coefficient for an element, which increases atomic number (z) (not linear; photoelectric absorption increases more rapidly than compton scattering)
• Bigger atoms -> stronger X-ray absorption (greater scattering)

Question 8

3 types of contrast in medical applications: (Give contributing factors for each)

Answer 8

1. Object contrast (Density/thickness, atomic number)
2. Subject contrast (energy of X-ray background)
3. Displayed image contrast (detector characteristics, image processing, display quality)

Question 9

Computed tomography:

Answer 9

• CT scans produce axial sections of the body
• Scan is made up of multiple X-ray attenuation measurements around an object’s periphery

Question 10

5 generations of CT imaging:

Answer 10

1. Columnated pencil beams (issue of high dose)
2. Spread beams
3. Fan beam (wider), one set of detectors
4. Fixed detector (ring of detectors)
5. Electron gun creating cone of X-rays -> move coil around, focussing onto target ring

Question 11

When may softer X-rays be used? How are X-rays typically generated?

Answer 11

• Softer low energy X-rays used to image soft tissue such as breast screening generated by Mo (issue of low penetrance prevents wider applications)
• Anode composition typically W, or a 5% Re in W alloy

Question 12

Solid state detectors and zinc detectors: makeup and function

Answer 12

• Solid state detectors: Si doped with Li or Ge - X-ray photons are converted to electron-hole pairs in the s/c and collected to detect the X-rays - REAL TIME!
• CdTe and its alloy with Zn detectors have an increased sensitivity, which allows lower doses of X-rays to be used