Physics of Magnetic Resonance Imaging

1. Physics of Magnetic Resonance Imaging Dr. Sunil Kulatunga Head Department of Nuclear Science University of Colombo

2. 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

3. 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

4. The nuclear spin and charge gives rise to a magnetic moment

7. z y x 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

8. 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

9. 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

17. Instrumentation

20. RF coils • A special coil (antenna) is used to produce the RF field • Coil is tuned to the appropriate resonant frequency.

21. Helmholtz coils & Surface coils

24. 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)

25. 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)

28. 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