MRI Hardware: magnet,
gradient, and RF coils
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital
1 of 96
The Main Magnet
• What does MRI need ?
– A strong magnet, with sufficient
size to put a person in
• Permanent made of iron ?
• Electromagnet using currents ?
2 of 96
MRI using Iron-core Permanent Magnet
As large as 0.3 Tesla has been reported
3 of 96
Permanent MRI with Rectangular Bore
Hitachi System
4 of 96
C-shaped Permanent Magnet MRI
Siemens Magnetom Open
5 of 96
Pros and Cons
• No electricity required
• Magnetic field restricted within the
iron core (least fringing field)
• Iron-core magnet too heavy (30 tons !)
for the floor
6 of 96
Another Type of Permanent Magnet
Open space MRI (reduced volume and weight)
7 of 96
Open MRI with Reduced Weight Design
Toshiba MRI System
8 of 96
Resistive Electromagnet MRI
As large as 0.3 Tesla has been reported
9 of 96
Electromagnet MRI Photo
Bruker Electromagnet MRI
10 of 96
Pros and Cons
• Somewhat lighter and cheaper (perhaps
the only advantages)
• Strong electromagnet needs huge
electrical power: Expensive electricity bills,
strong fringing field, poor stability
• Maximal current limited in copper wires
11 of 96
Iron-core Electromagnet MRI
Less iron than permanent magnet MRI
12 of 96
To me, it looks similar to this …
Toshiba System
13 of 96
An Example (Still Not Too Light)
Early Siemens open system
14 of 96
Combine the Advantages
• Iron-core electromagnet : Few
• Electromagnet without electricity
– Somewhat lighter with no electricity
bills from the magnet
– Superconducting electromagnet :
currently the dominant type for MRI
15 of 96
Structure of Superconducting Magnet
vacuum
liquid helium
liquid nitrogen
As large as 8 Tesla has been reported
16 of 96
Superconducting Magnet
• Disappearance of resistance of metal/alloy
under very low temperature
• Wire immersed in liquid helium (boiling
point –269 0C)
• Vacuum for thermal isolation
• Liquid nitrogen buffer (boiling point –1960)
17 of 96
Photo for Superconducting Magnet
Before decorating packaging
18 of 96
After Beautiful Packaging …
GE Signa Horizon
Siemens Magnetom
19 of 96
Double-donut MRI
Open design for simultaneous surgery
20 of 96
Other Open MRI (Fonar)
Patient standing
Table can be rotated
21 of 96
Symptom may be Gesture Dependent
Lying (recumbent)
Standing (weight bearing)
22 of 96
I Can Image Whatever Position !
Useful for musculoskeletal examinations
23 of 96
Back to Supercond MRI
• Strong field with good stability
• No electricity required once charged up
• Lighter than permanent magnet (6~7 tons)
• Fringing field is also strong
• High price for the instrument and cryogen
24 of 96
Improved Performance
• Magnetic shielding
– Iron placed outside to restrict the
fringing field (passive)
– Outer coil with reversed current to
cancel the fringing field (active)
25 of 96
Schematic Passive Shielding
~50% reduction per cm of iron plate
26 of 96
Passive Shielding
~50% reduction per cm of iron plate
27 of 96
Schematic Active Shielding
Major coil (e.g., 2.5T)
Reversed current (e.g., 1.0T)
Inner field gets partially cancelled too
28 of 96
Improved Performance
• Field homogeneity : shim coils
– Individual single wire loops added
to the main magnet
– Independent current sources
– Locally adjusting the field strength
29 of 96
Shim Coil
Power
sources
Independent power for current adjustment
30 of 96
Why Supercon Popular ?
• Strong magnetic field = large signal
= good images
– Signal at 0.3 T ~ 20% at 1.5 T
• Good stability for the magnetic field
• The market needs push a great deal
of research and developments
31 of 96
Spatial Encoding Hardware
• Gradient coils
– Gradient = locally varying
magnetic field
• Still the principle of electromagnet
• One in each of the x, y, z directions
32 of 96
Locally Varying Magnetic Field
The Maxwell pair
33 of 96
The z Gradient
Bo
y
z
x
Local field along z, variation also along z
34 of 96
Photo of z Gradient Coil
35 of 96
The More Precise z Gradient
Schematic z gradient
36 of 96
Photo of MRI z Gradient Coil
z gradient coil
37 of 96
Direction of the Gradient
• Local field always along z
• “Direction of the variation”
• “x gradient” means that the magnetic
field along z direction varies as a
function of x
38 of 96
y Gradient
Bo
y
z
x
Local field along z, variation along y
39 of 96
y Gradient Coil
Golay-type gradient coil (four saddles)
40 of 96
x Gradient
Bo
y
z
x
Local field along z, variation along x
41 of 96
x Gradient Coil
Golay coil (y gradient rotated for 900)
42 of 96
Photo for x or y Gradient Coil
43 of 96
More Precise x(y) Gradient Coil
Fold as a cylinder
Photo of the real object
44 of 96
Combing the x, y, z Gradinet Coils
z gradient
y gradient
x gradient
45 of 96
Winding of Gradient Coil
• Just like a solenoid electromagnet
• Coil resists changes in current
– Lenz’s Law
• Gradient change is slower than
alteration of the driving current !
46 of 96
Inductance & Gradient Rise Time
rise time
Gradient
(less winding)
Gradient
(more winding)
rise time
47 of 96
The Gradient Echo Sequence
t
RF
Changes are not immediate
Gs
t
Gp
t
Gr
t
48 of 96
Rise Time
• Certainly related to gradient strength
• 0.5 ~ 1.0 msec easily achievable
• Not a severe issue, unless for very
fast imaging
• Will be mentioned in future classes
49 of 96
Other Gradient Criteria
• 3-axis ability
• Eddy current
• Linearity
• Duty cycle
• Acoustic noise, coil cooling, torque
balance ...
50 of 96
Generation of Eddy Current
Magnetic flux lines of the z gradient
z gradient coil
Current induced in (outer) cyrostat metal
51 of 96
Restricting the Outer Flux Lines
Magnetic flux lines of the z gradient
z gradient coil
Gradient shielding to reduce eddy current
52 of 96
Radiofrequency Coils
• Radio-frequency (RF) coils
• Responsible for signal excitation
and detection near the Larmor
frequency
53 of 96
For Excitation
• Needs to generate high-frequency rotating
magnetic fields
• Requirements
– High efficiency near Larmor frequency
– B1 should be perpendicular to Bo
54 of 96
For Receiving
• Needs to receive high-frequency signals
effectively
• Requirements
– High efficiency near Larmor frequency
– Winding orientation should be
perpendicular to Bo
55 of 96
Analogy of Radio Broadcasting
MRI RF excitation
proton
Radio broadcasting
and receiving
Similar although strictly not exactly the same
56 of 96
The Resonant Circuit
• High efficiency around a specified
frequency range
• Covering human tissue of interest
• Inductance-capacitance resonant
circuits
57 of 96
Principles of Resonant Circuits
Capacitor
Inductor
electric field <--> magnetic field
energy exchange at fixed frequency
potential energy <--> kinetic energy
exchange at fixed frequency
Somewhat similar to simple pendulum
58 of 96
Schematic Resonant Circuit
capacitor added
capacitor
inductor
inductor (inherent value)
59 of 96
Surface Coils in MRI
60 of 96
Don’t Use Solenoid for RF Coil !
B1 has to be perpendicular to Bo !
61 of 96
Solenoid: No Excitation Nor Receiving
z
Bo
Bo
RF coil
y
x
B1 is exactly parallel with Bo !
62 of 96
Solenoid RF in Permanent Magnet MRI
Bo perpendicular to human body axis
63 of 96
Saddle Coil and Helmholtz Coil
Saddle coil
Helmholtz pair
64 of 96
Alderman-Grant Coil
Volume coverage
65 of 96
Knee Coil
66 of 96
Another Possibility for the Knee Coil
Slotted tube resonator
67 of 96
Extension of Simple Resonant Circuit
68 of 96
Birdcage coil
69 of 96
Head Coil
“Struts” typical of birdcage type
70 of 96
Image Comparison from Different Coils
Body coil
Head coil
3-in surface coil
71 of 96
Improving the RF Coil
• Circularly polarized coil
– or Quadrature coil (GE)
• Extra 40% gain in SNR compared
with linearly polarized coil
72 of 96
Linearly & Circularly Polarized Coil
y
y
B1
B1
x
x
Linear polarized
Circularly polarized
73 of 96
CP Alderman-Grant Coil
Linear polarized
Circularly polarized
74 of 96
CP Birdcage Coil
Note two pairs of driving current entrance
75 of 96
CP Head Coil
No visual difference from outer package
76 of 96
Excitation & Receiving
• Excitation coil
– B1 preferably homogeneous
• Receiving coil
– wire close to human subject
77 of 96
Comparison of Coil
• Volume coil
– Wide coverage, good B1 spatial
homogeneity
• Surface coil
– Can be placed near tissue of interest,
good receiving sensitivity
78 of 96
Usage of RF Coils
• Large coverage :
– Use smallest volume coil for both
excitation and receiving
• Local area :
– Volume (body) coil for excitation,
surface coil for receiving
79 of 96
Endorectal Coil
Soft wire in balloon, extended naturally after inflation
80 of 96
Soft Foldable Coil
Wrist imaging
Shoulder imaging
81 of 96
Combining Advantages
• Large field-of-view (FOV)
• Strong signal, high-quality images
• Assemble many surface coils
– Surface coil phased array
82 of 96
Phased Array Coil
• Ensemble of many surface coils
• Avoid interference among antennas
• Proper geometric arrangement to
eliminate mutual inductance
• Signals received separately and
simultaneously (multi-channel = $)
83 of 96
Phased Array Coil
84 of 96
Photo of Phased Array Coil
Spine phased array
85 of 96
Images from Spine Phased Array
Large FOV from combining many images
86 of 96
Other Phased Array Coils
8-channel head coil array
87 of 96
Other Phased Array Coils
8-channel torso coil array
88 of 96
Large-N Phased Array Coils
32 channels (aggressively continued R&D)
89 of 96
Excitation Phased Array ?
• Yes, it is possible !
• But needs multi-channel matched RF
power amplifier
• Not used at 3T or lower
• Useful at >7T and large coverage (out of
scope for this semester)
90 of 96
Other RF Coil Criteria
• B1 homogeneity
• Output power requirement
• Linearity of RF power amplifier
• Number of receiver channels
• Automatic arc detection ...
91 of 96
RF Shielding
• Voltage detected in MRI ~ uV level
• Frequency ~ FM radio (everywhere)
• All MRI systems need RF shielding to
isolate radiofrequency waves from
outside of scanner room
92 of 96
Copper RF Shielding for MRI Room
from Nelco
93 of 96
Major MRI Hardware
RF coil
Magnet
Shim coil
Gradient coil
94 of 96
The Entire MRI System Block Diagram
Magnet & Cryogen
Host
Computer
Pulse
Controller
Array
Processor
Console,
Monitor,
Disk, ...
Grad Amp
Gx
Grad Amp
Gy
Grad Amp
Gz
RF Amp
T/R
DAQ
RF
Coil
RF Xcvr
95 of 96
MRI Hardware: magnet,
gradient, and RF coils
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital
96 of 96