bme1450 intro to mri february 2002 the basics the details – physics the details – imaging
TRANSCRIPT
BME1450 Intro to MRIFebruary 2002
The BasicsThe Details – PhysicsThe Details – Imaging
February, 2002BME 1450 Introduction to MRI2
Example of MRI Images of the Head
Bone and air are invisible.
Fat and marrow are bright.
CSF and muscle are dark.
Blood vessels are bright.
Grey matter is darker than white matter.
February, 2002BME 1450 Introduction to MRI3
MRI Imagers
GE 1.5 T Signa Imager
GE 0.2T Profile/i imager
February, 2002BME 1450 Introduction to MRI4
MR Imaging: Parts of an Imager
Main Magnet– High, constant,Uniform Field,
B0.
Gradient Coils– Produce pulsed, linear
gradients in this field.– Gx, Gy, & Gz
RF coils– Transmit: B1 Excites NMR
signal ( FID).– Receive: Senses FID.
Basics
B0
B0
B0
B1
February, 2002BME 1450 Introduction to MRI5
MR Imaging: Pulse Sequence
______________________________________________________________________________________________________________________________
A
B
C
D
E
Excitation
Slice Selection
Phase Encode
Readout
RF Detected Signal
K Space
DFT
ExcitationRF pulse
Gz
GX
Gy
Basics
Coherent detector
Complex numbers
Image Space
‘Real numbers’
BME595 Intro to MRIOctober 2000
The BasicsThe Details – PhysicsThe Details – Imaging
February, 2002BME 1450 Introduction to MRI7
Magnetic Resonance (MR)
An object in a magnetic field B0 will become magnetized and develop a net Magnetization, M.
Most of M arises from the orbital electrons but a small part is the Nuclear Magnetization.
The nucleus has a magnetic dipole moment, , and angular momentum, J.
||/|J| = , the gyromagnetic ratio. For Hydrogen = 43 MHz/T.
J and
The Details - Physics
Magnetization is “magnetic dipole moment per unit volume”.
February, 2002BME 1450 Introduction to MRI8
MR: Precession
The 1.5T magnetic, B0 field of the MR Imager makes the Hydrogen Nuclei precess around it.
The precession rate,, is the Larmor frequency.
fL = B0 = 43*1.5 = 64MHz for Hydrogen.
Y
Z
J or
X
B0
|B0|••t
The Details - Physics
February, 2002BME 1450 Introduction to MRI9
MR : Summary
The magnetization,M, is the density of nuclear magnetic dipole moments.
If you tip M away from B0 it will precess, at frequency B0, producing a measurable RF magnetic field.
Y
Z
J or or M
X
B0
|B0|••t
The Details - Physics
February, 2002BME 1450 Introduction to MRI10
MR Excitation
You can tip M by applying a circularly polarized RF magnetic field pulse, B1, to the sample.
If B1 is at the Larmor frequency, B0 you get this.
M is now precessing about two magnetic fields.
B1 is effective because it tracks M.
Y
Z
J or or M
X
B0
|B0|••tB1
|B0|••t
|B1|••t
The Details - Physics
B0
B1
February, 2002BME 1450 Introduction to MRI11
Magnitisation Relaxation
The transverse (M) and longitudinal (M||) components of the magnitization change with time.
Two relaxation times T1 (longitudinal) and T2
(transverse). T1 T2
M(t)||
M0
tT2
Y
X
Z
M 0
M(t)M (t)||
The Details - Physics
BME595 Intro to MRIOctober 2000
The BasicsThe Details – PhysicsThe Details – Imaging
February, 2002BME 1450 Introduction to MRI13
Magnitisation Relaxation
MRI Contrast is created since different tissues have different T1 and T2.
Gray Matter: (ms) T1= 810, T2= 101
White Matter: (ms) T1= 680, T2= 92
The Details - Imaging
February, 2002BME 1450 Introduction to MRI14
MR: The FID
As the magnetization precesses it creates its own RF magnetic field.
This field is much smaller than the Exciting RF field.
It can be detected with a standard radio receiver.
The resulting signal from precession is called the FID.
Y
Z
J or or M
X
B0
|B0|••t
How do you maximize the FID?
The Details - Imaging
Lab Frame
M
+ V(t) -
February, 2002BME 1450 Introduction to MRI15
MR: The MR Signal
The FID can be detected by a ‘read out coil’ and amplified in a standard RF amplifier.
It is then input to a coherent detector with two outputs, I and Q.
The detector is phase locked to the excitation pulse. Thus
– My’ “In Phase” output, I
– Mx’ “Quadrature output, Q = 0
Y’
Z
M
X’
My’
MZ
The Details - Imaging
Rotating Frame
M
+ V(t) -
February, 2002BME 1450 Introduction to MRI16
Gradient Pulses
______________________________________________________________________________________________________________________________
A
B
C
D
E
Excitation
Slice Selection
Phase Encode
Readout
RF Detected Signal
K Space
DFT
ExcitationRF pulse
Gz
GX
Gy
Details - Imaging
Coherent detector
Complex numbers
Image Space
‘Real numbers’
{
February, 2002BME 1450 Introduction to MRI17
MRI: The imaging pulses
The phase gradient pulse will cause more precession.
Precession occurs during the readout gradient pulse as well.
During readout I and Q are digitized into a complex value I+jQ and stored in K space.
Y’
Z
M
X’
MZ
x•GxI My’
Q Mx’ x•Gx t
Details - Imaging
February, 2002BME 1450 Introduction to MRI18
MRI: Kspace
If kx(t) and ky(t) are defined as shown, then they represent the row and column that the value, digitized at time t, should be assigned to in Kspace
dttGtk
dttGtk
t
yy
t
xx
0
0
)()(
)()(
Details - Imaging
February, 2002BME 1450 Introduction to MRI19
MRI: Driving through Kspace
times the integral of the Gx(t) and Gy(t) gives the position in Kspace
Kx
Ky
A
BC
DE
A
B
C
D
E
RF pulse
Gz
Gx
Gy
Details - Imaging
BME595 Intro to MRIOctober 2000
The BasicsThe Details – PhysicsThe Details – ImagingDetails not discussed
February, 2002BME 1450 Introduction to MRI21
MR: The Rotating Frame
It is much easier to visualize all this if you observe it from a frame of reference which is rotating at the Larmor frequency, fL=B0.
B1 appears motionless in this rotating frame and B0 effectively disappears and…
During the excitation pulse, M precesses only about B1 at frequency B1!!
Y’
Z
M
X’
B1
|B1|••t
My’
MZ
The Details - Physics
Rotating Frame
February, 2002BME 1450 Introduction to MRI22
MR: The Rotating Frame
When the excitation pulse is over, M is stationary in the rotating frame.
In the Lab frame, however, it is still precessing.
Y’
Z
M
X’
My’
MZ
The Details - Physics
Rotating Frame
February, 2002BME 1450 Introduction to MRI23
MRI – Meaning of “Z Gradient”
X
Y
Z
•A “Z gradient” introduces a gradient in the magnetic field in the Z direction. The gradient is produced with resistive coils.
•Traditionally the Z gradient is associated with the RF excitation pulse and slice selection.
zGz
Details - Imaging
B0
February, 2002BME 1450 Introduction to MRI24
MRI – Meaning of “X&Y Gradients”
xGx
X
Y
Z
Details - Imaging
B0
•An “X or Y gradient” introduces a gradient in the B0 magnetic field in the X or Y direction.
•These gradients are traditionally associated with readout and phase encode, respectively.