107. MRI: VARYING MAGNETIC FIELDS

Question 1

1. What are Varying Magnetic Fields?

Answer 1

• they are magnetic fields
• they vary their strength with time
• they are caused by Gradient Switching

NB:
- they are of a greater safety concern than Static
Magnetic fields

Question 2

2. Why are Varying Magnetic fields a greater concern?

Answer 2

• they are an example of an Oscillating Magnetic Field
• they cause the electrical current flow in conductors

Question 3

3. What does this image show?

Answer 3

• these are Visual Phosphenes
• they are caused when an individual is exposed to
  extremely high levels of magnetic field variation
• this usually happens when we use a magnetic field that
  is greater than 4 Teslas

NB:
- this happens because of the induced currents in the
nerves and tissues

Question 4

4. What are some other consequences of time Varying Magnetic Fields?

Answer 4

• alterations in the Biochemistry of cells
• alterations in the Biochemistry of Fracture Unions
• mild cutaneous sensations
• involuntary muscle contractions
• cardiac arrhythmias

Question 5

5. What is the most common Bio-effect of MR systems?

Answer 5

• tissue heating

Question 6

6. What usually causes the tissue heating?

Answer 6

• the Radio Frequency Coils
• the Antennas

THEY CAN CAUSE:
- burn hazards

Question 7

7. What kind of Insulation do Radio Frequency Coils and Antennas need?

Answer 7

• they need electrical insulation
• they need thermal insulation

Question 8

8. How can we describe Energy Dissipation?

Answer 8

• it can be described in terms of the
  Specific Absorption Rate (SAR)

Question 9

9. How do we measure the Specific Absorption Rate?

Answer 9

• it is expressed as Watts per kilogram (W/kg)

Question 10

10. What effects the quantity of the Specific Absorption Rate?

Answer 10

• induced electrical field
• tissue density
• patient size
• pulse duty cycle
• conductivity

Question 11

11. What do we use the Specific Absorption Rate to calculate?

Answer 11

• we use it to calculate an expected increase in the Body
  Temperature
• this happens during an average examination

Question 12

12. What kind of Specific Absorption Rate is best for the patient?

Answer 12

• a low one
• this is what is safest

Question 13

13. What happens when a patient is exposed to Specific Absorptions that are three times greater than the recommended levels?

Answer 13

• there are no serious harmful effects
• the body temperature will increase
• this increases the heart rate

Question 14

14. Which kinds of patients can not be considered as candidates for an MRI?

Answer 14

• patients that have compromised thermoregulatory
  systems