The document discusses the principles and techniques of magnetic resonance imaging (MRI). It begins by explaining how MRI works by manipulating the spin of protons in a strong magnetic field and detecting their signals. It then describes how different sequences are used to encode spatial information and form images. Finally, it briefly outlines the history and applications of MRI and possibilities for future developments with higher field strengths and improved techniques.

1. Magnetic Resonance Imaging MRI

2. Magnetic Resonance Imaging MRI

3. Outline Medical Imaging Techniques MRI Principles 2.1 Fundamental information 2.2 Manipulating magnetization 2.3 Relaxation times 2.4 MR signal detection 2.5 Structure of MRI machine About MRI 3.1 History of MRI developments 3.2 Applications 3.3 Future

4. 1. Medical Imaging Techniques X-ray imaging Digital subtraction angiography Computed tomography CT Positron computed tomography Diagnostic ultrasound N uclear M agnetic R esonance I maging NMR or MRI

5. Advantages of MRI over others Non-invasive High resolution Cross-sectional images available Yielding images that depend on physiology or functional properties Allow direct information about metabolic processes in vivo Providing real-time images

6. MRI images

7. 2. MRI Principles 2.1 Fundamental information 2.2 Manipulating magnetization 2.3 Relaxation times 2.4 MR signal detection 2.5 Structure of MRI machine

8. Classically, any circulating charged particle possesses a magnetic moment: In QM, all quantum particles have intrinsic spin property and so have the spin angular momentum : In case of proton of the H atom: γ p = 2.675 × 10 8 rad T -1 s -1 which is termed the gyromagnetic ratio 2.1 Fundamental information Spin angular momentum

9. 2.1 Fundamental information Basic concepts Main character: Protons Hydrogen nuclei in water molecules Hydrogen nuclei in organic substances Proton is a fermion. It’s a spin ½ particle. When placed inside a magnetic field, a proton would experience a torque, as a result, it has an additional magnetic potential energy. Splitting of energy level by magnetic field: Zeeman effect Magnetization = Magnetic dipole moment per unit volume

10. 2.2 Manipulating magnetization Application of magnetic field B 0 In the first stage of MRI: a uniform B 0 is applied along z-axis (~0.07 T to 1.5 T) From classical EM, the spin magnetic moment will experience a torque: Subsequent motion: In 2-dimension: similar to SHM In 3-dimension: precess about z-axis with Larmor frequency: At the same time, splitting of energy levels of protons occurs with energy difference: Similar to Zeeman effect for the electron in a H atom.

11. 2.2 Manipulating magnetization Application of magnetic field B 0 Precessing proton in B 0 -> Splitting of energy states-> ↓

12. 2.2 Manipulating magnetization Arise of Magnetization M z The precession will not last too long as some of the magnetic energy transfers to its surrounding and finally reaches the equilibrium state. The population of protons is described by Boltzmann energy distribution: On average, at room temp and B 0 = 0.1 T, there are 7 protons lined up with the B 0 among 10 7 protons. As a result, a net magnetization M z occurs parallel B 0

13. In the second stage of MRI: An oscillating pulse B 1 is applied in x-direction for a while : Thus the magnetization flips onto xy-plane and produce a rotating net magnetization M xy. B 1 also known as “90° pulse”. 2.2 Manipulating magnetization Arise of Magnetization M xy

14. 2.2 Manipulating magnetization Pictures of magnetization ← Spin distribution Magnetization-> Fluctuating magnetization vector ↓

15. 2.2 Manipulating magnetization Magnetic Resonance B 1 has to give the right amount of energy to the protons: So, the excitation will take place only for a very definite frequency f of B 1 : This is called magnetic resonance .

16. 2.2 Manipulating magnetization How to map? Main idea: To map these spins which depend on the physical and chemical properties of its surrounding Methods: Applying different combinations of magnetic field Monitoring the respective characteristics resulted

17. 2.3 Relaxation times T 1 and T 2 Just after B 1 pulse: M z regrows slightly and finally attains its original eqm state. T 1 Spin-Lattice Relaxation: M xy dephases slightly and finally no more magnetization on x-y plane. T 2 Spin-Spin Relaxation:

18. 2.3 Relaxation times T 1 and T 2 Bloch equations:

19. Due to presence of intrinsic inhomogeneities - arise from magnetic field generator itself - and varied from person to person: chemical shift - these also contributes to the dephase of M xy Actual decay of T 2 is even faster: 2.3 Relaxation times Free Induction Decay

20. 2.4 MR signal detection TR and TE Only the rotating M xy produce emf signal by means of Faraday EM induction. Repetition Time TR: time interval between two successive excitations of the same slice. Echo Time TE: time interval between application of the excitation pulse and collection of the MR signal.

21. 2.4 MR signal detection Contrast of image T 1 weighted image: Short TR, strong T 1 weighting Long TR, low T 1 weighting Tissue with short T 1 appears bright Tissue with long T 1 appears dark T 2 weighted image: Short TE, low T 2 weighting Long TE, strong T 2 weighting Tissue with short T 2 appears dark Tissue with long T 2 appears bright

22. 2.4 MR signal detection Spatial Encoding So what is A? Ans: Fourier transform of Detected MR signal …… …… Result: Originally unknown _______________________ …… ……

23. 2.4 MR signal detection Spatial Encoding 1. Phase Encoding: Switch on a magnetic field gradient of amplitude G y in y-direction just after the spins have been excited and precess on xy-plane: - “Phase Shift” of spins relative to each others - phase angle:

24. 2.4 MR signal detection Spatial Encoding 2. Frequency Encoding: At time t y , the gradient G y is turned off and then an orthogonal gradient G x is applied for a time t x : Thus the spins precess at frequency: 3. What do we know now? The exact values of both phase and frequency of precession at each point ( x , y ) on the tissue In “Spin-Wrap imaging”, the amplitude of gradient is raised incrementally and thus forms N 1 × N 2 picture elements.

25. 2.4 MR signal detection Spatial Encoding 4. Fourier Transform: - Through Fourier Transform of the MR signal: - we can know the amplitudes of different frequencies and phases in the k -space, which in turns proportional to the brightness of the picture elements.

26. 2.4 MR signal detection Spatial Encoding

27. 2.5 Structure of MRI machine

28. Patient Patient table Scanner Magnet Gradient Coils Radio Frequency Coils 2.5 Structure of MRI machine

29. 3. About MRI 3.1 History of MRI developments 3.2 Applications 3.3 Future

30. 3.1 History of MRI developments 1938 Nuclear magnetic resonance by I.I. Rabi Mid-1940s First detection of NMR in bulk matter 1950s Discovery of chemical shift and spin-spin coupling 1960s Development of pulse Fourier-transform NMR 1973 First NMR image by Paul Lauterbur, who shared the Nobel Prize in medicine in 2003 1975 2D NMR by Ernst, which earned him the 1991 Nobel Prize in chemistry 1977 First study performed on human 1980s k -space formalism

31. 3.1 History of MRI developments Paul Lauterbur ’s images ← Oil in peanuts Cross-section of a mouse -> (shadows are lungs)

32. 3.2 Applications Clinical diagnosis Physiological research Petrophysical analysis Ceramic manufacturing Food processing And so on…

33. 3.3 Future More powerful computer “ Electron-nuclear double resonance” Electron-nuclear Overhauser effect Emergence of new superconducting materials Up to 10 Tesla Increase signal-to-noise ratio Improve spatial resolution

34. Summary Nuclear magnetic resonance Flipping of spinning nuclei by B 1 of suitable frequencies Apply combinations of fields & detect the resultant signals How “Fourier”? Distribution funct. from a signal funct. Method: G y -> phase G x -> frequency Signal Phase knows y Freq. knows x Lock a position ( x , y ) Amplitude knows

35. Thanks!

36. Medical imaging techniques P.R. Moran, R.J. Nickles and J.A. Zagzebski, The Physics of Medical Imaging , Physics Today (July 1983) pp.36-42. Electron-nuclear Overhauser effect Overhauser, Albert W. (1953-10-15). " Polarization of Nuclei in Metals " Phys. Rev. 92 (2): 411-5.

37. Is it dangerous? Remain mysterious. Radio frequencies is used. It’s not energetic enough to harm any molecule in human body. Typically, x-ray are 10 11 times more energetic & can break up molecules. RF waves of high intensities may cause brief local heating, but less than 0.5K. For B fields below 2T, no biological hazards appear to present. Cell physiology, such as enzyme reactivity, may not be affected.

38. Other methods? Yes… Irradiate the nuclei with rf energy of const. energy, scan it with a constant B . Given up Pulsed rf excitation followed by detection of resultant free-precession signal. Still in use Etc…

39. Who are banned from using MRI? Patients with metal instruments inside his body E.g. heart pacemakers, ferromagnetic prostheses, clips, etc. Someone who is allergy to magnetic field may feel dizzy. Mobile phones, watches should be banned from MRI.