MRI EQUIPMENT Essentials

MRI EQUIPMENT
SG 2002
• Room
Essentials
– Radiofrequency shielding (Faraday Cage)
– Fringe Field Containment
• Static magnetic field (0.15 - 2.0 Tesla)
• Variable magnetic field - gradients
– field coils and amplifiers
• Radiofrequency transmitter/receiver
coils and amplifiers
• Computer processing and display
1

Virtually Essentials
• Magnet shielding
– passive or active
•Shim coils
• MR compatible trolley
2

Not essential but nice to have
• MR compatible GA and monitoring
equipment
• ECG leads and expansion bellows for
cardiac and respiratory gating
3

Room requirements
• Large enough to contain fringe field up
to 5 Gauss (0.5mT) line
– pacemakers
5 Gauss
Line
• Completely enclosed in wire mesh -
excludes radio waves from outside -
zipper artefact.
• Obvious practical considerations
– room for trolleys etc
4

Static magnetic field
• Main field, B o responsible for lining
up magnetic moment hydrogen
protons, with (parallel) or against
field (antiparallel)
• Slightly greater than half are lined up with the field.
Magnetic moments almost cancel out. The slight excess
are the ones we can image (7 in 10million)
• Proportion increases with increased field strength
• Improved SNR
• Is bigger always better
– Increase in some artefacts and T1 acquisition times
Types of magnet
• Permanent (up to 0.35 T)
– uses magnetised core - like horseshoe magnet
–permanently on
– field strength limited by weight of magnet
– 0.2T field strength
• iron 23 tons
• neodymium alloy 9000 lbs
5

Electromagnet
• Resistive (up to 0.5 T)
– normal electromagnet (solenoid)
– windings have resistance therefore lots of heat
generated. Limits the amount of current possible and
consequently the strength of the magnet is less.
– Cooling required.
– may be switched on and off
– use lots of electricity - expensive to run
– Power =I 2 R
Example: 0.15T magnet
Requires 200 Amps
Equivalent to 50kW
6

• Superconducting (

Gradients
• Frequency of precession w o dependent
on field strength
–w o is given by the Larmor equation w o = Bo x g
where Bo is the magnetic field strength in Tesla and g is the Gyromagnetic ratio
for hydrogen (42.57 MHz/T)
• Gradients impose additional graded
magnetic field on main field
8

Magnet isocentre
0.98T 1.0T 1.02T
Gradients
• This means we can define where an MR
signal comes from in space because its
frequency will depend on the local field
strength
• Gradient strength measured in mT/m
– 20mT/m average
• Very rapid switching
–

• Three sets
required for
x,y,z directions
• z - direction of
main field
•Helmholtz
configuration
Gradient Coils
• x and y direction
• Golay or saddle
coils
Gradient Coils
10

Radiofrequency (RF) coils
• Essential to put radiowaves into patient
and to detect signal from precessing
protons
• Should only excite protons within
volume of interest
• Ideally in close proximity to maximise
signal detection
• May be transmit/receive or receive only
• RF amplifiers - 20kW
Radiofrequency (RF) coils
• Built-in body coil
• Anatomy specific coils - better SNR, less
heating
–Head
–Neck
–Extremity
–Spine
–Breast
11

Surface coils
• Aerial loops placed next to region if
interest
– High SNR next to coil
– Small field of view
– Best for superficial anatomy - TMJ, eye
Volume Coils
• Better RF homogeneity
•Even SNR
• Saddle, Birdcage configurations
12

Phased Array Coils
• Array of surface coils joined together
• Avoids problem of poor coverage
• Maintains SNR
• Spine coils
Open low field or closed high field
• Closed - advantages
– Usually high field
– Better field
homogeneity
– Larger field of view
– Better SNR
– Better image quality
– Faster scan times
– Thinner slices improved
spatial resolution
– Spectroscopy, fMRI,
EPI
• Closed - disadvantages
– Patient unfriendly
–Claustrophobic
– Limited access makes
interventional techniques
difficult
– Positioning can be difficult
– Usually noisy
– Tissue heating (SAR limits)
– Cryogen refills
– cooling of gradient power
supply
13

• Open - advantages
– less claustrophobic
– Patient friendly
– easier monitoring
– easier positioning
– intervention e.g.
biopsy, tumour
ablation possible
–quiet
– low tissue heating
• Open - disadvantages
–poor SNR
– longer scan times
– thicker slices
– small FOV
– not capable of some
techniques
– expensive on electricity
– cooling of magnet power
supply
14

Field homogeneity
• Uniformity of magnetic field
– important for accurate spatial registration
of MR signal
• Better than 1ppm
• Achieved by addition of pieces of iron
(passive shimming) or solenoids (active
shimming)
Peripherals
• Life sign monitoring
• Anaesthetic trolley
•ECG gating leads
• Respiratory gating bellows
15

Cardiac Gating
Non ECG gated
ECG gated
Respiratory Gating
Non respiratory gated
Respiratory gated
16

References
• Boddy J (1999) High Field Closed MRI Scanners
Clinical MRI 8(4) p124-126
• Dunne S (1999) Low Field Open MRI Scanners
Clinical MRI 8(4) p117-123
• Keen M (1999) Lecture Notes on the Physics of MRI
Clinical MRI 9(3) p70-81
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