Bart van de Bank B.vandeBank@rad.umcn.nl

Ingmar Voogt I.J.Voogt@umcutrecht.nlMichel Italiaander M.Italiaander@umcutrecht.nl

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

Acoustic noise20

Lorentz force

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