Lecture 5: MPI

Question 1

Explain the indications for MPI and why it is used? (6 marks)

Answer 1

• Indications are chest pain (angina) when exercising and used for assessment of CAD due to build-up of plaque.
• Carry out stress and rest measurements as stenosis or narrowing of the arteries may not be noticable when blood flow is low.
• It only becomes evident during exercise when need for oxygen increases and increases in blood flow occur, results in defects which may or may not be reversible.

Question 2

Briefly compare MPI with other modalities which may be used (MRI, CT, US, coronary angio and MPI) (10 marks)

Answer 2

• MPI -> perfusion and function (high cost and dose 7 mSv)
• CT -> structure and function (dose and expensive)
• Coronary angiography -> visualize arteries, little functional data and invasive (dose and expensive)
• US -> structure and function but not ideal for CAD (no dose and cheap)
• MRI -> everything and no dose but expensive

Question 3

What two tracers are commonly used in MPI and what are the differences between them? (14 marks)

Answer 3

• Tc-99m sestamibi or testrafosmin, HL 6hrs, 140 keV energy, LEHR collimator used due to reasonable penetration,
  - 7 mSv (lower than Thallium)
  - tracer accumulates in myocardium and stays there, stress and rest carried out separately
• Thallium-201, HL 73 hrs, 70-80 keV energy, LEAP collimation required due to lack of penetration
  - 15 mSv dose (higher than Tc-99m) due to increased HL and lower energy
  - sodium analogue exchanged in and out of cells, means can carry out stress and rest in one examination

Question 4

How can CAD be quantified? (4 marks)

Answer 4

• wall motion
• any ischemia (defects) and reversibility
• ECG changes
• visualisation of blood flow

Question 5

Outline the MPI protocol. (4 marks)

Answer 5

• can be two day, 2 x 400 MBq, stress then rest.
• if the stress if fine, then no rest required
• can be one day, 250 MBq then 750 MBq to flood earlier activity
• Pharmacological stress agents may be used if patients cannot or won’t exercise, may be more reliable, do not take with caffiene

Question 6

What is gating and why is it used in MPI? (3 marks)

Answer 6

• Gating means link up images to points in the cardiac cycle
• generally use around 7-8 gates and acquire multiple images at these points
• can combine them to increases number of counts and improve the image quality

Question 7

What are the pros of cons of using PET compared with MPI? (6 marks)

Answer 7

Pros:

• More pathology detectable
• FDG used for metabolism
• Rb82 Cl for blood flow

Cons:

• cost
• availability
• increases in staff doses due to preparation of two tracers and methods used