Module 6 - 6.5.2 Diagnostic Methods in Medicine

Question 1

A Tracer (definition)

Answer 1

A radioactive substance either ingested by or injected into a patient

Question 2

Tracers can either (2)

Answer 2

• diagnose an illness
• treat an illness

Question 3

Considerations for Tracers (5)

Answer 3

• gamma must be used (least ionising, most penetrating)
• source must not be toxic
• half life must be long enough to carry out investigation but not long enough to damage cells
• must be able to get source to part of body where its needed
• must be possible to monitor tracer once inside the body

Question 4

A Gamma Camera

Answer 4

detects gamma photons emitted by a tracer

Question 5

A Gamma Camera is used to

Answer 5

produce real time image of path of tracer through the body

Question 6

How a Gamma Camera Works (6)

Answer 6

• gamma rays are emitted from tracer inside patient
• rays hit collimator (stops non perpendicular rays ) (obtains exact location of emission of gamma rays)
• rays pass through large scintillating crystal (fluorescent material that absorbs high energy photons to produce many photons of visible light)
• photons produced hit photomultiplier tubes (photoelectric effect: 1 photon = 1 electron emitted)
• emitted electrons create electric signal
• connected to computer to form an image

Question 7

How a PET Scan is Carried Out (11)

Answer 7

• tracer chemically combined with biologically active molecule (sugar) is introduced to bloodstream
• the tracer is a radioactive isotope that emits positrons
• wait an hour for molecules to become concentrated in certain areas
• cancer cells are more active and so need more energy (glucose) so tracer is more concentrated in those areas
• ring of detectors containing gamma cameras surround patient
• each positron emitted from tracer travels a short distance before interacting with an electron in one of patient’s atoms
• annihilation occurs producing 2 gamma rays travelling in opposite directions
• rays are detected by scintillator in gamma cameras
• if detected at opposite sides of ring within fraction of a nanosecond, software uses difference in arrival time to calculate origin of rays
• a 3D image showing the concentration of the tracer around the body is produced
• cancer cells appear brighter

Question 8

PET vs CAT Scans (4)

Answer 8

• PET = more expensive
• PET = 2-4 hours, CAT = 30 mins
• CAT = higher dose of radiation
• CAT = very detailed tissue and bone structure images, whereas PET shows biological functions of a part of the body