components of mri scanner - university of cape...
TRANSCRIPT
AttheendofthistalkIhopeyoucan:1. Identifythemajorhardwarecomponentsofan
MRIsystem2. Haveafeelfortheunderlyingphysicsrelevant
tothehardware3. Appreciatethelimitationsandtrade-offs
betweenequipmentoptions4. Beabletoasktherightquestions
Objectives
OverviewSchematicRoomDropinMagnetCryogenBath/StaticFieldCoil
GradientCoils
RFCoil(s)
RFShield
OverviewSchematic
T/R
Rec
OverviewSchematic
T/R
Rec
• Magnet• StaticField,B0
• Gradientcoilsandamplifiers• Spatio-temporalvariationofB0
• RFpath• OscillatingMagneticfield,B1
T/R
Rec
The MagnetFunction: Create a large, relatively homogeneous static magnetic field, B0
Physics refresher
Ampere’s Law
The Magnet
X
Y
ZZ
X
The Magnet
Superconducting Coil Liquid Helium
- B0 = 1.5 T- ≈ 30 000x Earth’s field- requires 100s Amps!- not practical in
normal copper wire- would require
≈ 5 MW!
B0
• 5Gaussline≈50mfromanunshielded1.5T
• passiveshielding• activeshielding
The Magnet
The Fringe Field
5 G
50 m
N SS N
The Magnet
otherOptions/Trade-offs
• Field Strength- magnet- site- associated
hardware• Size and Shape vs
Homogeneity- bore diameter- bore length- open geometry- shimming
• Safety → no metal
↑Tesla = ↑$
T/R
Rec
The Gradient System
Function: Create linear variations in B0, in X-, Y-, and Z-directions
Maxwell Coils
The Gradient Coils
Z-direction gradients
The Gradients
X- and Y- direction gradients
Y
Z
Golay Coils
• Max Gradient Amplitude10 - 80 mT/m
• induces“Eddy”currentsintotheconductivematerials
• Eddycurrents,in-turn,generategradients• artefacts
• pre-emphasis• activeshielding
The Fringe Field
The Gradients
Physics refresher
Faraday’s Law
Gapplied Gactual
t
• Gradientcoilshavefiniteinductance
• Gradientcoilshavefiniteresistance
Inductance and Resistance
The Gradients
can’t instantaneously change current in an inductor
GappliedGactual ∆G/∆t ≡ slew rate
t
• Gradient Slew Rate• 100-400 T/m/s• e.g. ramp time 100 µs to 15 mT/m
• Gradientcoilshavefiniteinductance
• Gradientcoilshavefiniteresistance
Inductance and Resistance
The Gradients
current in a resistor dissipates heat
• Gradient coils are usually water cooled• Heating may limit max current (gradient
amplitude) and duty cycle
• High Power Audio Amps• go to 11
• May be combined• parallel for ↑current
• e.g. 600 A• series for ↑faster
switching • e.g. 2000 V
• Safety considerations• fast switching → PNS
The Gradient Amplifiers
The Gradients
T/R
Rec
The RF Path
Function: Create and detect a rotating magnetic field, B1, that is perpendicular to B0
The RF Coil(s)T/R
Rec
The RF Coil(s)T/R
Rec
Resonant Circuit
Surface Coi l
X
Y
Z
The RF Coil(s)T/R
Rec
Surface Coi l
X
Y
Z
X
- inhomogeneous- lownoise
30°90°270°
The RF Coil(s)T/R
Rec
Volume Coi l
X
Y
The RF Coil(s)T/R
Rec
Volume Coi l
X
Y
The RF Coil(s)T/R
Rec
Volume Coil
X
Y
Thelargerthecoil,- ↑ homogeneity- ↑ noise- ↑ powerrequirements
Many Coil geometries
RF Transmission and DetectionT/R
Rec
T/RRF Coil
Rec
T/RT/RRF Coil
Volume Coi l
Surface Coi l
T/R
Rec
RF AmplifierT/R
Rec
T/R
Rec
• HighFrequency(≈100MHz)• matchLarmorFrequency
• HighPower(≈10kW)• ↑ power=↓ RFpulseduration
• HighLinearity• Safetyconsiderations:tissueheating(SAR)
Signal ReceptionT/R
Rec
T/R
Rec
• Gainthroughapre-amp• DemodulationandDigitization• Storage
Review
T/R
Rec
credit
• Images taken from various websites
• http://anthrosarah.blogspot.com/2007/09/this-post-was-inspired-by-discussion-on.html
• http://members.wri.com/jeffb/visualization/solenoid.shtml• http://www.openmriofcanada.com/• http://www.tesla.co.uk/products.htm• http://www.magnex.com/products/gradient_coils.htm• http://www.dotynmr.com/mri/mri_litzcagepg.htm• http://www.medicalimagingmag.com/issues/articles/images/20
06-01_09/2006-01_09-1.jpg• http://www.dotynmr.com/mri/mri_litzcagepg.htm