Download Magnetic susceptibility (χ)

MRI, The BASICS
Microwaves
Infrared
Visible Light
Ultraviolet
102 : 106
108
1012
1014
1015
1017
Wavelength
1m
Energy
1μ
1 nm
1 eV
1 keV
and X-rays
MRI
Frequency
Gamma rays
Wave
Radiowaves
In MRI, we deal with much lower energies than X-ray or even visible light.
1018 :1024
1 MeV
The electromagnetic pulse that used in MRI to get a signal is called RF pulse, because it is in
the radio frequency range.
Spins and electromagnetic fields
 All particles in atoms have spin. Spin is an intrinsic property of atomic particles,
just like charge and mass are intrinsic properties. The spin of the particles can no
more be increased or decreased than can its mass or charge. Electrons, protons
and neutrons all rotate, or spin.
 Each particle spins on its axis, generating a magnetic field. Electrons have a larger
magnetic field than other particles. In fact, the magnetic fields for protons and
neutrons are so weak, they have little effect. But the sum of the spins of all the
particles in the atoms (known as the Net Nuclear Spin) contributes to strength of the
magnetic field used in MRI.
 Any spinning charged particle creates an electromagnetic field which emerging
from the south pole to the north one.
 Each atomic nuclei have a specific energy levels related to “spin quantum number
S” where the number of energy states = 2S + 1.
 If the number of protons in the nucleus is even, the net magnetic field will be zero.
While with odd number of protons, there is a net magnetic field to the nucleus that
is called magnetic dipole moment (MDM).
 Spin is the property most responsible for creating an MRI signal. The human body
is comprised mostly of water (H20) and fat (-CH2-).
This results in common
hydrogen (1H) being the most abundant element in the body (about 60%).
Hydrogen has the strongest magnetic moment of all the elements (emits the
strongest signal)
Magnetic susceptibility (χ)
 All substances get magnetized to a degree when placed in a magnetic field.
 χ is the ratio between the induced magnetic field and the applied one.
 According to χ, there are three types of substances; diamagnetic, paramagnetic and
ferromagnetic.
 Diamagnetic substances have no unpaired electrons (χ < 0). They are basically
nonmagnetic. The majority of body tissues have this property.
 Paramagnetic substances have unpaired electrons, so they become magnetized
while B0 is on and become demagnetized once the field has been turned off.
 Hemosiderin, the end-stage of hemorrhage, contains more than 10,000 unpaired
electrons, so it is referred as superparamagnetic substance.
 Ferromagnetic substances become permanently magnetized even after the
magnetic field has been turned off (like; iron, cobalt and nickel).
NMR depends on nuclei whereas bulk magnetism depends on electrons, although
electrons demonstrate a magnetic field much higher than that of protons due to their
much larger mass.
In MRI, low frequency radiowaves penetrate the tissue and reflect back off
magnetized spins within the object.
RF and MR Signal
 If spinning, unpaired protons are placed in an external magnetic field, they will
line up with that field.
 If a radio frequency wave (RF) of a very specific frequency is then sent to the
patient. Spins will change their alignment as a result of this new magnetic field.
 After the RF pulse, spins generate a signal as they return to their original
alignment.
B0 Field
 The external magnetic field that is used in MRI, is denoted B0 and it is on order of
one Tesla (1T = 104 Gauss) while the earth magnetic field is 0.5 Gauss. This field
isn’t uniform in reality but the standard is on the order of 6-7 ppm, shim coils are
usually used for this purpose.
Types of Magnets
 MRI magnets are large cylinders with a 55 and 60 centimeter opening at the center.
 According to field strength, magnets can be divided into five types; Ultrahigh
(spectroscopy), high, midfield, low and ultralow fields.
 According to magnet design, there are three basic types; permanent, resistive and
superconducting.
 Permanent magnets can’t be turned off and have a lower cost.
 Resistive magnets are based on current flows through coil, so they can be turned
off.
 Superconducting magnets; operate near absolute zero, almost wires resistance is
zero, cryogens like liquid helium are used for cooling and used in the majority of
scanners.
Coils
They either generate a magnetic field or detect a changing one (induced electric
current)
Transmit/Receive Coils
 The typical MRI system has at least two RF coils: one for scanning the body, and
one for scanning the head. The RF exciter generates a low power RF pulse, which is
passed to the RF amplifier to drive the body or head coil.
 Closest to the patient in the bore is the RF body coil, which is the smallest of the
fiberglass cylinders, installed in the magnet.
 The body coil and the removable head coil are called volume RF coils because the
objects they scan fit entirely inside them.
 Certain smaller surface coils also fall into the category of volume coils: some knee
coils, some neck coils, and others that completely surround the anatomy of interest.
Surface coils are mainly used to improve SNR in the region of interest.
 Quadrature coils; two receivers are present 900 to one another to distinguish real
and imaginary component and hence increase SNR.
 Solenoid coils; are wrapped around the patient to increase SNR, usually used in
surface and volume coils, used in open scanners (low field magnets), which have a
vertical magnetic field.
 Phased-array coils; small surface coils that are positioned on either side of the
anatomy of interest, allow fast scanning with finer details (Pelvic array coil)
Gradient Coils
They usually add a linear variation to the external magnetic field. Three orthogonal
gradient coils are used for slice selection and spatial encoding.
 The gradient subsystem converts digital gradient pulses into the analog format that
drives the amplifiers.
 Two important properties of the gradient system are the maximum amplitude, or
strength of the gradients, and the minimum time (called rise time) for the
gradients to reach the peak amplitude when energized. The peak gradient amplitude
is the maximum strength. The slew rate of the gradient system is the ratio of the
maximum gradient amplitude to the gradient rise time.
Shim Coils
 Magnet shimming is the process of adjusting the homogeneity of the magnet to
compensate for imperfections in the magnet and the environmental factors that can
alter the system’s magnetic field thereby degrading its homogeneity, and hampering
imaging performance.
 There are two types of shimming: active shimming and passive shimming. Active
shimming is accomplished via eighteen to thirty small shim coils built into the
magnet itself or placed on a cylinder within the bore of the magnet. A computer
program adjusts the shim coil currents for the best homogeneity and, therefore, the
best images. The shim coils have their own power supply.
 Passive shimming is accomplished by placing small pieces of iron, or shims, in
drawers located at key points in the magnet, based on the MR’s shim program.
How to Increase the Signal
 There are two main methods of increasing the signal. One is to reduce the
temperature to absolute zero (-273‫؛‬C) so that all the nuclei align. In this state, all
the spins would occupy the low energy state. But this wouldn’t be a safe alternative
for the patient.
 The other method for increasing the signal is to increase the field strength of the
magnet (see Field Strength Chart). If the field strength of the magnet is doubled, the
MRI signal will be quadrupled. If the field strength is halved, the MRI signal will be
reduced to one-fourth.
Effect of Magnet Field Strength on Humans
1.5T
 Humans are
comfortable with
magnet either on or
off.
 Scan time - 20
minutes or longer
4T
8T
 Flashes appear in eyes
 All symptoms from 4T
field strength appear to
 Nausea may occur
be squared in intensity
 Involuntary muscle motion
rather than doubled
occurs
 Humans become acclimated  Humans become
acclimated and
and comfortable at about 15
comfortable at about 45
minutes.
minutes.
 Scan time - 8-10 minutes
 Scan time - about 4
minutes
Magnetic Dipole Moment (MDM)
 Normally, spins are oriented randomly, so the net magnetic field is zero. After
turning the magnetic field on, approximately half of the spins are lined up with the
external magnetic field and the other half is lined up in the opposite direction. Over
time, more spins line up with the magnet and the induced magnetic field increases
exponentially. The time constant of the curve depends on the tissue and the magnet
strength.
 T1 relaxation time (recovery of magnetization): M = 1- e-t/T1. As the strength of the
applied magnet increases, T1 increases.
 Proton (Spin) Density: magnetization depends also on spin density (N(H)); this is
the number of mobile protons that are able to change their direction.
M = N(H) (1- e-t/T1)
Precession
 Protons not only spin around their own axes, they also spin around B0. Each proton
spins much faster about its own axis than it rotates or precesses around the axis of
the external magnetic field.
 How fast do the nuclei precess? Knowing that the precessional rates of the nuclei
change when the magnetic field strength changes is critical to the creation and
detection of the MRI signal. Precessional rate is given by the Larmor equation:
ω = γ B0 ,
where γ is gyromagnetic ratio that is fixed for each nucleus. γ (H) = 42.6 MHz/Tesla.
1H Frequencies in Common Field Strengths
Magnetic Field
Precessional Frequency
Strength in Tesla
in MHz
0.2
8.516
0.5
21.29
1.0
42.58
1.5
63.87
4.0
170.3
