MRI的基本知识

Weighting
The process of weighting involves emphasizing some aspects of a phenomenon, or of a set of data — giving them 'more weight' in the final effect or result.
magnetization of the sample
The flip angles typically range from 1 to 180 degrees, depending on the amplitude and duration of applied RF.
Relaxation: Return to equilibrium
Main magnet radiofrequency (RF) transmitter and receiver (coil) gradient coil
Data processing, image reconstruction, displaying and storage analog converter Computer with monitor Storage devices (disk…) printer
When placed in an external magnetic field, magnetic moments of protons align either parallel or anti-parallel to the magnetic field.
Net magnetic moment
Raymond Damadian Paul Lauterbur
➢ In 1977, Peter Mansfield developed the echo-planar imaging technique (Nobel Prize in 2003)
➢ In 1977, Raymond Damadian completed the first MR scanner (Indomitable).
Imaging contrast is determined by densities of different tissues in X-ray and CT image, but in MRI it is
determined by signal intensities of different tissues.
Protons in and outside external magnetic field
Net magnetic moment = 0
B0
N
Without external magnetic field, magnetic moments of protons are randomly orientated.
Electromagnetic wave is used in MRI , not ionizing radiation wave (X-ray ,γ-ray) or mechanical wave(ultrasound)
MR describes the phenomenon whereby the nuclei of certain atoms, when placed in a magnetic field, absorb and emit energy of a specific frequency.
Mz and Mxy: Vector component of the magnetization in the
longitudinal (z) direction and
transverse plane (xy) respectively
M0: Equilibrium magnetization, the maximum longitudinal
S
z is "longitudinal" direction x-y is "transverse" plane
Static Field, B0
z
Applied RF y
x
Mo
EXCITATION : causing the magnetic moments flip away from equilibrium position (z- axis)
In any MR image, proton density, T1, and T2 contrast are all present, although one type of contrast usually predominate.
A T1-weighted image is one in which the T1 contrast of the tissues predominate.
ຫໍສະໝຸດ Baidu
z
z
y
x
y
x
z
z
z
y
x
y
x
y
x
Energy released in the form of radio waves
T2 relaxation
Decay of the transverse component of magnetization T2 relaxation constant: the elapsed time between the peak transverse signal and 37% of the peak level (1/e)
T1 weighted imaging (T1WI)
➢ Inversion recovery (IR) sequence
➢ Echo planar imaging (EPI) sequence
3.2 Basic Equipments of MRI
Classification of MR according to the type of main magnet: permanent, resistive, superconductive Generation, detection & coding of MR signal
900 1800
900 1800
TR
TE TE
2
2
SE
TE
TR: the time over which a basic pulse sequence is repeated.
TE: time between RF excitation and the peak of the echo signal.
The proton density, T1, and T2 relaxation constants are fundamental properties of tissues, and therefore these tissue properties can be exploited by MRI to aid in the diagnosis of pathologic conditions such as cancer, multiple sclerosis, or hematoma
Richard Ernst Peter Mansfield
3.1 Basic Principles of MRI
MRI reveals chemical compositions of objects as well as their molecular structure & arrangement , but not density.
Hydrogen nucleus (one proton, no neutron) is the best element for general clinical utility of MRI due to its strongest magnetic moment and greatest abundance in human tissues.
➢ In 1975 Richard Ernst proposed MR imaging using phase and frequency encoding, and the Fourier Transform, which are the basis of current MRI techniques. (Nobel Prize in 1991)
MRI
900 pulse－－ TE/2－－1800pulse－－TE/2－－signal coding， [spin echo pulse sequence]
By altering the pulse sequence, the MR signal produced by various structures could be dramatically changed. ➢ Gradient echo (GRE) sequence
➢ In 1971, Raymond Damadian showed that nuclear magnetic relaxation times of tissues and tumors differed
Felix Bloch Edward Purcell
➢ In 1973, By utilizing gradients in the magnetic field, Paul Lauterbur demonstrated MR imaging on small test tube samples (Nobel Prize in 2003)
Main coil
RF coil
Magnet
tunnel-shaped
Scanning aperture
Gradient coil
Power supply
Data processor
MRI System
Manipulation and display
3.3 Factors Affecting MR Signals
Pulse sequence
Pulse sequences refer to the manner in which the radiofrequencies and gradients are applied with appropriate time intervals between them in order to highlight a certain inherent tissue property.
Magnetic resonance imaging（MRI）
HISTORY OF MRI
➢ In 1946, Felix Bloch and Edward Purcell independently discovered the magnetic resonance (MR) phenomena (Nobel Prize in 1952)
Nuclei suitable for MRI are those which have an odd number of protons or neutrons and therefore possess a net charge and have angular magnetic momentum (tiny magnetic field generated by the spinning motion of atomic nuclei)
T1 relaxation
Increase in the longitudinal magnetization T1 relaxation constant: the time needed to recover 63% of the longitudinal magnetization T1 is significantly longer than T2
Spin echo pulse sequence is the most
important sequence in clinical imaging and forms the basis of MR tissue characterization.
Spin echo (SE) sequence