MR Hardware invivo NMR course 2014
32
Bart van de Bank [email protected] Ingmar Voogt [email protected] Michel [email protected] MR Hardware RF Coils
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Transcript of MR Hardware invivo NMR course 2014
Bart van de Bank [email protected]
Ingmar Voogt [email protected] Italiaander [email protected]
MR HardwareRF Coils
Program:2
13:30 – 14:15 MR Hardware
14:15 – 15:00 RF Coils
15:00 – 17:00 RF Practice
Learning objectives3
After this lecture you should be able to mention:
The main components present in a MR system
The function of each component
The properties of each component
MR Hardware:
4
Magnet systems
Magnet shim system
Gradient system
RF system & coils
Computer system
Bmagnetic field
Icurrent
Main Magnet (B0) Create the main magnetic field
static
Solenoid coil
Superconductive wire
Superconductive switch
Liquid He cryostat
Shielding
5
Passive shielded magnet Heavy Fe-construction
Bo affected by
ambient temperature asymmetry
6
Active shielded magnet Reverse field coil
Superconductive
Coupled to main coil
7
Gradients Variate the main magnetic
field
Gradient coils (X,Y,Z)
Switching rate ~ kHz
Current through gradients several Amperes
Air or water cooling
8
Radio Frequency Coils (RF-coils) Generate and receive
electromagnetic fields
Radio Frequency coils (MHz)
B1-field perpendicular to B0-field
9
Shim system Optimize magnetic field
homogeneity
Passive shim (Fe)
Superconductive shim
Gradient coils (X,Y,Z) 0th order
High order shim coils Up to 3rd (clinical systems) Up to 5th (research systems)
X2, Z(X2+Y2), etc..
10
11
12
Gradient system Basic gradient design
X,Y,Z, versus Slice, Phase, Readout (Orientation)
Gradient properties/limitations
Eddy currents
Acoustic noise
Peripheral nerve stimulation
13
Basic gradient design14
Basic gradient design15
Orientation systems Gradients:
x, y, z
Patient: left, head, anterior
Image: slice, phase encoding,
readout
Matrix conversions
16
Gradient properties/limitations shape --> current
conversion Rectangular, trapezoid
or sine
Amplitude [mT/m]
Rise/fall time/slewrate [µs] [T/m/s]
Duty cycle
17
Eddy currents Eddy currents in
conductors
stronger magnetic field
faster changing field
thicker material
lower resistivity material
B Et
18
Minimize conductive loops!
Eddy currents19
Perpheral nerve stimulation21
RF system Radio Frequency ~ MHz (
= γB0)
Receive channel
Transmit channel
Multi channel
Multi nuclei
Faraday cage
22
Radio Frequency (RF) Irradiate spins with
electromagnetic energy to start resonance phenomenon:
Larmor frequency = γ * B0 i.e. 63 MHz @ 1.5T for
proton 52 MHZ @ 3T for
phosphorous 280 MHz @ 7T for
fluorine
Nucleus: Gyromagnetic ratio (γ)
1H 42.5762H 6.53613C 10.70519F 40.05323Na 11.26231P 17.235
23
Transmit channel24
Receive channel25
Multi receive channels26
27
Multi nuclei28
Faraday cage29
Practical issueSpecific Absorption Ratio (SAR)
B1-field limited by SAR
Rate absorbed RF power per mass of tissue
Heating of tissue, due to electric fields
ρ = sample density
total body weight
local tissue weight
1 gram of local tissue
power integral
30
2
2ESAR
Computer system and data flow31
Have a short break 5 minutes
After break RF Coils
32
