BME1450 Intro to MRI February 2002

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BME1450 Intro to MRIFebruary 2002

• The Basics
• The Details Physics
• The Details Imaging

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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.

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MRI Imagers

• GE 1.5 T Signa Imager

GE 0.2T Profile/i imager
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MR Imaging Parts of an Imager
Basics

• 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.

B0
B0
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MR Imaging Pulse Sequence
Basics
Excitation
Excitation
Slice Selection
A
B
Phase Encode
D
Readout
E
C
RF Detected Signal
K Space
Image Space
Coherent detector Complex numbers
__________________
__________________
? DFT ?
__________________
__________________
__________________
__________________
__________________
Real numbers
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BME595 Intro to MRIOctober 2000

• The Basics
• The Details Physics
• The Details Imaging

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Magnetic Resonance (MR)
The Details - Physics

• 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.

? Magnetization is magnetic dipole moment per
unit volume.
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MR Precession
The Details - Physics

• 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 431.5 64MHz for Hydrogen.

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MR Summary
The Details - Physics

• 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.

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MR Excitation
The Details - Physics

• 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.

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Magnitisation Relaxation
The Details - Physics

• The transverse (M?) and longitudinal (M)
  components of the magnitization change with time.
• Two relaxation times T1 (longitudinal) and T2
  (transverse). T1 ? T2

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BME595 Intro to MRIOctober 2000

• The Basics
• The Details Physics
• The Details Imaging

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Magnitisation Relaxation
The Details - Imaging

• 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
  

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MR The FID
The Details - Imaging

• 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.

Z
B0
J or ? or M
Y
B0?t
Lab Frame
X
How do you maximize the FID?
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MR The MR Signal
The Details - Imaging

• 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

Z
MZ
M
Y
My
Rotating Frame
X
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Gradient Pulses
Details - Imaging
Excitation
Excitation

Slice Selection
A
B
Phase Encode
D
Readout
E
C
RF Detected Signal
K Space
Image Space
Coherent detector Complex numbers
__________________
__________________
? DFT ?
__________________
__________________
__________________
__________________
__________________
Real numbers
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MRI The imaging pulses
Details - Imaging

• 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 IjQ and stored in K space.

Z
MZ
M
xGx
Y
xGx ? t
X
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MRI Kspace
Details - Imaging

• 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

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MRI Driving through Kspace
Details - Imaging

• ? times the integral of the Gx(t) and Gy(t) gives
  the position in Kspace

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BME595 Intro to MRIOctober 2000

• The Basics
• The Details Physics
• The Details Imaging
• Details not discussed

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MR The Rotating Frame
The Details - Physics

• 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!!

Z
B1?t
MZ
M
Y
My
Rotating Frame
B1
X
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MR The Rotating Frame
The Details - Physics

• When the excitation pulse is over, M is
  stationary in the rotating frame.
• In the Lab frame, however, it is still precessing.

Z
MZ
M
Y
My
Rotating Frame
X
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MRI Meaning of Z Gradient
Details - Imaging
X
Z
Y

• 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.

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MRI Meaning of XY Gradients
Details - Imaging
X
x?Gx
Z
Y

• 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.