1.8 MRI

Question 1

What is the principle behind the working of MRi?

Answer 1

1. • Unpaired protons in the body
     (mostly H+ atom in water—abundant)
     align randomly and act as bar magnets
2. • When placed in an external static magnetic field (A),
     the protons align
3. • When another magnetic field (B) is applied,
     the protons are turned out of alignment
4. • When this magnetic field is intermittently turned off and on,
     the radiofrequency energy taken up by the protons
     are released before the realignment takes place.
     Also there is some ‘Precession’—a wobbling
     motion that occurs when a spinning object
     is subject to an external force.
5. • This energy released is measured by a set of 3-dimensional
     orthogonal gradient coils in the MRI machine
6. • The energy released by protons in different tissues is different,
     and hence a 3-dimensional image with varying intensity is formed

Question 2

Some numbers

Answer 2

1. • Earth’s magnetic field = 0.5–1.0 Gauss
2. • 10,000 Gauss = 1 Tesla
3. • MRI requires magnetic fields between 0.2–3 Tesla
     (30,000 times the earth’s magnetic field)
4. • MRI Safe zone < 5 Gauss
5. • > 5 Gauss—
     pacemakers will malfunction and all personnel need screening
6. • MRI Conditional zone—50 Gauss
7. • > 50 gauss—ferro magnetic objects
     become projectiles and monitors malfunction

Question 3

What do you understand by T1 + T2 weighted images?

Answer 3

1. • ‘T’ is the relaxation time constant.

• T1 weighted (early image)—
  few milliseconds after the electro magnetic field is removed
• T2 weighted (later image)—
  later than T1
• Protons in hydrogen take a long time to decay to original position,

so fluid will appear dark (minimal signal) in T1

but white (better signal) in T2

Question 4

can you list the problems posed to the anaesthetists when taking an anaesthetised patient to MRi?

Answer 4

1. • Patient factors
2. • MRI factors
3. • Anaesthetic equipment factors
4. • Anaesthetic monitoring factors
5. • Location factors

Question 5

• Patient factors

Answer 5

° Patients needing anaesthesia are usually ITU patients,
paediatric patients,
patients with learning difficulties,
seizures,
or movement disorders

° Pregnancy:

Currently recommended that pregnant women should
ideally not be scanned during the first trimester of pregnancy

due to magnetic field problems, noise,
and also unscavenged anaesthetic gas issues

° Patients with implants:
Pacemakers, cochlear implants, intraocular foreign body,
and ferromagnetic aneurysm clips are absolute contraindications.

Modern implants such as joint prosthesis,
surgical clips,
heart valves,
and sternal wires are deemed safe.

All patients and staff need screening

Question 6

• MRI factors

Answer 6

° Presence of strong magnetic field

• Exert large forces on any ferromagnetic materials in the proximity
• Induce currents in metallic objects and cause local heating
• Interfere with monitoring
• Magnetic field in the vicinity can also derange the
  quality of images produced
• < 5 Gauss is the safe zone

_________________________________

° Noise such as that due to the gradient coils switching on and off

• > 85 dB (above safe level)
• Patient and staff should be protected
• Can mask the monitor alarms

___________________________________

° Heat:
That produced by the radiofrequency radiation is absorbed by patient

Question 7

• Anaesthetic equipment factors

Answer 7

1. ° MRI safe:
   Equipment will not pose a danger to patients and
   staff but does not guarantee that it will function correctly
2. ° MRI compatible:
   Equipment is both safe to enter the MR examination
   room and will operate normally without interference to the MR scanner

3 ° Anaesthetic machines, cylinders, circuits, ventilators, vaporisers and
scavenging are now available as MRI-compatible

4. ° Infusion pumps fail if field strength is > 50 Gauss

Question 8

• Anaesthetic monitoring factors

Answer 8

° MRI-compatible short (15 cms) braided
ECG leads and insulated pulse
oximetry cables are necessary

° NIBP—plastic connectors;

IBP—pressure transducer cabling is passed through
the wave guides or use MRI-compatible pressure transducers

° Capnography and airway pressure monitoring
requires longer sample
lines with a 20-second delay

° Monitoring screens should be in the control room
and carbon fibre cables passed via the wave guide port

Question 9

• Location factors

Answer 9

° Usually remote

° Difficult to access in case of emergency

Question 10

What is Faraday’s cage?

Answer 10

Faraday’s cage is a radiofrequency shield

built into the fabric of the MR room.

To allow infusion lines or monitoring cables to enter the MR room,
a hollow brass tube or ‘waveguide’ is built into the Faraday cage passing
through into the control room.

Mechanism:
An external static electrical field causes the electric charges
within the cage’s conducting material to be distributed
such that they cancel
the field’s effect in the cage’s interior.

Examples: Microwave oven, MR room.

Question 11

What is quenching?

Answer 11

The coils used in MR magnets need to be kept cold (liquid helium)
in order to maintain superconductivity.

Quenching is a process involving the rapid boil-off
of the cryogen that causes an immediate loss of superconductivity.

This can happen spontaneously,
due to error on installation,
or deliberately such as in order to switch the scanner off.

This produces a large volume of helium gas,
which is vented to the outside atmosphere through a quench pipe.

In the event of damage to the quench pipe,
the buildup of helium could potentially lead to asphyxiation.