Transcript Physics of Magnetic Resonance Imaging Dr. Sunil Kulatunga Head

Physics of Magnetic Resonance
Imaging
Dr. Sunil Kulatunga
Head
Department of Nuclear Science
University of Colombo
Magnetic Resonance Imaging (MRI)
Can do nearly everything CT does & much more
Very little risk to the patient
UV, visible, infrared & microwave radiation do
not penetrate far into tissue
But radiofrequency (RF) radiation can readily
pass through
RF radiation can be made to interact with atomic
nuclei by means of nuclear magnetic resonance
(NMR)
NMR provides information on proton densities,
T1and T2 relaxation times of different tissues
Magnetic Resonance Imaging
Nucleus consists of protons and neutrons
and they are made of quarks.
Protons and neutrons spins around
different axes.
Nuclei posses a nuclear angular
momentum J, when number of protons or
number of neutrons is odd
Associated with nuclear angular
momentum there is a proportional nuclear
magnetic dipole moment μ
The nuclear spin and charge
gives rise to a magnetic moment
μ  J
Many spins give rise to bulk
magnetisation parallel to Bo
Population difference that
gives rise to net, bulk,
magnetisation of material,
that is detected by NMR
Spins precess at 54o to the z
axis with random phase with
frequency f0 = (γ/2π)B0
Larmor Equation
Net bulk magnetisation is
parallel to Bo
z
y
x
Transitions (spin flipping) can occur when
energy of RF radiation (photon) matches
the energy difference ΔE between the two
energy levels
hf = (γhB0)/2π  f = (γ/2π)B0 = f0
Transitions occur when the frequency of
the RF radiation matches the Larmor
frequency. This condition is called the
resonance
Macroscopically this flipping corresponds
to the rotation of the magnetic dipole
moment vector away from the z axis.
By controlling the intensity (B1) and time
duration of the RF pulse, magnetic dipole
moment vector can be rotated by any
angle
Instrumentation
RF coils
A special coil (antenna) is used to produce
the RF field
Coil is tuned to the appropriate resonant
frequency.
Helmholtz coils & Surface coils
Magnetic Field gradients
Localization of nuclei in 3D requires the
application of three distinct and orthogonal
magnetic field gradients during pulse
sequence
 Slice Select Gradient (SSG)
 Phase Encoding Gradient (PEG)
 Frequency Encoding Gradient (FEG)
Pulse Sequences
Timing, Order , Polarity, Repetition frequency of
RF pulses and x, y and z gradient application
Tailoring pulse sequences emphasizes contrast of tissues
dependant on ρ, T1 and T2
Major Pulse Sequences
Spin Echo (SE)
Inversion Recovery (IR)
Fast Spin Echo (FSE)
Gradient Recalled Echo (GRE)
Echo Planer Image (EPI)
Slice Selection
B = B0 + z . Gz  f0 = (γ/2π).(B0 + z . Gz )
 Larmor frequency changes with z position.
Apply the 90° RF pulse containing frequencies from fL to
fH
Dipoles in a slice of tissue with z coordinates zL to zH
are in resonance condition and they can absorb the RF
energy and rotate into x-y plane
fL = (γ/2π).(B0 + zL . Gz )
fH = (γ/2π).(B0 + zH . Gz )
Δf = (γ/2π).(zH – zL) . Gz
Δf = (γ/2π).Gz .Δz
Thank You