5: Imaging systems

Question 1

Types of x-ray machines

Answer 1

• Diagnostic still pictures- examine bones, organs and tissue structure
• Continuous picture (fluoroscopy)- examine systems as they function
• Motion picture (angiography)- examining circulatory system as it functions
• Diagnostic scans (tomography)- 3D info
• Therapeutic- destruction of cancerous tissues and tumours

Question 2

X-ray generation

Answer 2

• Characteristic radiation- electrons collide, resulting in the production of an x-ray as electron in anode slows down and loses energy
• Bremsstrahlung- radiation generated by rapid deceleration of electrons during close approach to a nucleus

Question 3

X-ray detection

Answer 3

• X-rays passed through Bucky grid to remove scattered rays
• Images received on photographic film or fluoroscopic screen
• Light/dark areas on film represent high/low tissue penetration

Question 4

Fluoroscopy

Answer 4

• Continuous x-ray
• Utilises:
o X-ray tube
o Image intensifier- light detected, amplified and converted to light
o TV camera- intensified light detected by TV camera and transferred to monitor

Question 5

Nuclear medicine imaging

Answer 5

• Radiopharmaceuticals injected into body
• Radiopharms absorbed by organs and emit radiation
• Detected externally and localised
• Info about organ size, tissue structure, biochemical activity
• Static or changing time
• Nuclear medicine imaging therefore images organ function

Question 6

Sodium iodide (NaI) detector

Answer 6

• NaI crystal, when hit by incoming gamma rays, scintillates with visible light
• Photocathode turns the light into electrons
• Photomultiplier tube amplifies these electrons into a proportional electronic signal
• Support electronics amplify and shape the electrical signal

Question 7

Scintillation counter

Answer 7

measures gamma ray intensity- placed close to patient skin

Question 8

Gamma ray/Anger camera

Answer 8

• Static system
• Radiation detected by single large NaI crystal, with an array of photomultiplier tubes
• Relative fraction of light seen by each tube determines where gamma ray entered the crystal

Question 9

SPECT (single-photon emission computed tomography)

Answer 9

• Essentially a gamma camera mounted on a rotation gantry
• Enables projections from different angles to be collected and combined using image reconstruction techniques
• Determines 3D distribution of radiotracer

Question 10

PET (positron emission tomography)

Answer 10

• Radioactive tracer injected
• As tracer decays, positron emitted
• Pair of photons emitted when positron collides with electron
• Line connecting interactions of both photons = line of response
• 3D image can be generated based on distribution of line of responses

Question 11

Modes of ultrasound scanning

Answer 11

• A mode = echo intensity at a single line against echo time
• M mode = A-mode recordings over time to demonstrate tissue motion
• B mode = echo signals from multiple directions obtained- brighter pictures produced