MRI

MRI IMAGING SYSTEM

1. TOPIC 3 1 X-RAY IMAGING SYSTEM Magnetic resonance imaging ( ZME 336 MEDICAL INSTRUMENTATION Dr. Nik Noor Ashikin Nik Ab Razak School of Physics Universiti Sains Malaysia [email protected]

2. Course outcome 2 ZME 336 MEDICAL INSTRUMENTATION

3. Objective 3 The students will be able to: Explain the phenomenon of precession of nuclei in a static magnetic field . Trace the steps involved in reception of the MR signal beginning with insertion of the patient into the magnet . Describe the behavior of bulk magnetization of a sample in the presence of static and radio frequency magnetic fields Course outcome

4. 3.0 Introduction 3.1 MRI Principles 3.2 MRI System 4

5. 3.0 INTRODUCTION 5

6. MRI IS A TUNNEL, OF COURSE! 3.0 What is the similarity? 6 INTRODUCTION

7. 7 3.0 INTRODUCTION

8. 8 3.0 INTRODUCTION

9. MAGNETIC RESONANCE IMAGING (WHAT DO YOU SEE?) 9 3.0 INTRODUCTION

10. MAGNETIC RESONANCE IMAGING (WHAT DO YOU SEE?) 3.0 INTRODUCTION

11. 1920 1930 1940 1950 1960 1970 1980 1990 2000 1924 - Pauli suggests that nuclear particles may have angular momentum (spin). 1937 – Rabi measures magnetic moment of nucleus. Coins “magnetic resonance”. 1946 – Purcell shows that matter absorbs energy at a resonant frequency . 1946 – Bloch demonstrates that nuclear precession can be measured in detector coils . 1972 – Damadian patents idea for large NMR scanner to detect malignant tissue . 1959 – Singer measures blood flow using NMR (in mice). 1973 – Lauterbur publishes method for generating images using NMR gradients. 1973 – Mansfield independently publishes gradient approach to MR . 1975 – Ernst develops 2D-Fourier transform for MR . NMR renamed MRI MRI scanners become clinically prevalent. 1990 – Ogawa and colleagues create functional images using endogenous, blood-oxygenation contrast . 1985 – Insurance reimbursements for MRI exams begin. 4.1 History 3.0 11 INTRODUCTION : HISTORY

12. Damadian - 1977 First ever MRI image of human body Created using the “Indomitable” scanner Field strength was 0.05T Homogeneous part of field very limited so patient table was moved to collect each voxel! Took 4hrs to collect single slice 12 4.1 History 4.1 History 3.0 INTRODUCTION : HISTORY

13. 13 4.1 History 4.1 History 3.0 INTRODUCTION Diagnostic technique which does not involve the use of ionizing radiations It is based on the principles of nuclear magnetic resonance (NMR) ( a spectroscopic technique to obtain microscopic chemical and physical information about molecules ) The NMR a property of nuclei in a magnetic field where they are able to absorb applied radiofrequency ( Rf ) energy and subsequently release it at a specific frequency Produced high quality images of the soft tissues of the human body offers superior soft tissue contrast that is not possible with other imaging modalities

14. 14 MRI uses a combination of Magnetic and Electromagnetic Fields NMR measures magnetization of atomic nuclei in the presence of magnetic fields Magnetization can be manipulated by manipulating the magnetic fields (this is how we get images) Static magnetic fields don’t change (< 0.1 ppm / hr ): The main field is static and (nearly) homogeneous RF (radio frequency) fields are electromagnetic fields that oscillate at radio frequencies (tens of millions of times per second) Gradient magnetic fields change gradually over space and can change quickly over time (thousands of times per second) 3.0 INTRODUCTION

15. SYN OPS IS OF MRI 1) Put subject in big magnetic field 2) Transmit radio waves into subject [2~10 ms ] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject 5) Convert measured RF data to image 3.0 INTRODUCTION

16. 16 4.1 History 4.1 History 3.0 INTRODUCTION

17. 17 4.1 History 4.1 History 3.0 OPEN MRI CLOSED MRI UPRIGHT MRI INTRODUCTION

18. 18 WHAT CAN BE DIAGNOSED BY AN MRI SCAN? • Most ailments of the brain, including tumours • Sport injuries • Musculoskeletal problems • Most spinal conditions/injuries • Vascular abnormalities • Female pelvic problems • Prostate problems • Some gastrointestinal tract conditions • Certain ear, nose and throat (ENT) conditions • Soft tissue and bone pathology/conditions 3.0 INTRODUCTION

19. 19 WHO CAN’T HAVE AN MRI SCAN? • A cardiac pacemaker • Certain clips in your head from brain operations • A cochlear implant • A metallic foreign body in your eye • Had surgery in the last 8 weeks • If you are pregnant 3.0 INTRODUCTION

20. 3.1 MRI PRINCIPLES 20

21. What kinds of nuclei can be used for NMR? Nucleus needs to have 2 properties: Spin charge Nuclei are made of protons and neutrons Both have spin ½ Protons have charge Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin Good MR nuclei are 1 H, 13 C, 19 F, 23 Na, 31 P 3.0 MRI PRINCIPLES: MICROSPIC PRINCIPLES

22. 22 4.1 History 4.1 History 3.0 MRI involves imaging the nucleus of the hydrogen atom , i.e. the proton but WHY HYDROGEN? 63% of human body is hydrogen atoms Hydrogen nuclei have an NMR signal When in ionic state (H+), it is nothing but a proton Has only one proton and it aligns easily with the MRI magnet Proton is not only positively charged, but also has magnetic spin (wobble) MRI utilizes this magnetic spin property of protons of hydrogen to obtain images MRI PRINCIPLES: MICROSPIC PRINCIPLES

23. A Single Proton + + + There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment M . The proton also has mass which generates an angular momentum J when it is spinning. J m Thus proton “magnet” differs from the magnetic bar in that it also possesses angular momentum caused by spinning. 3.0 MRI PRINCIPLES: MAGNETIC PRINCIPLES

24. How do protons interact with a magnetic field? Moving (spinning) charged particle generates its own little magnetic field - Such particles will tend to line up with external magnetic field lines (think of iron filings around a magnet) Spinning particles with mass have angular momentum - Angular momentum resists attempts to change the spin orientation (think of a gyroscope) 3.0 MRI PRINCIPLES: MAGNETIC PRINCIPLES

25. 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

26. 26 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak Absence of external magnetic field , the spins will be aligned in random orientations in space, resulting in zero net magnetization If an external magnetic field, B, is applied, each spin will align either parallel or antiparallel to the direction of the applied field ; they become polarized Randomly arranged hydrogen atom After the strong magnetic field applied 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

27. Proton pointing in opposite direction cancels each others magnetic effect in respective direction. 9 proton align up and 5 down, resulting in 4 proton up force 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

28. 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 28 The spins precess at the resonance frequency ( ωo , Larmor frequency) about the direction of B The frequency f of precession of a proton in units of megahertz (10 cycles or rotations per second) depends upon its gyromagnetic ratio γ (in megahertz per tesla) and the strength B (in tesla, T) of the static magnetic field. 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

29. 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

30. 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 30 3.1 Note that since the net magnetization is aligned along the z axis , it remains stationary and does not precess about any axis. Thus, in the presence of a constant external magnetic field, B , the net magnetization remains constant and oriented along the direction of the field the magnetization is said to be at its equilibrium magnetization, M o MRI PRINCIPLES: MAGNETIC PRINCIPLES

31. 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 31 3.1 To measure magnetization we must perturb it by apply additional magnetic fields to tip protons out of alignment M agnetization will deviate from its equilibrium position Such perturbation of the magnetization is needed for signal detection Amount of energy needed depends on nucleus and applied field strength ( Larmor frequency ) 8. RF frequency must be resonance with H nuclei at 90 degree MRI PRINCIPLES: MAGNETIC PRINCIPLES

32. After protons aligned along the external magnetic force, RF pulse is send for energy exchange. Which is only possible if RF pulse has same frequency as protons precession frequency RF frequency = Protons precession frequency RF frequency = Protons precession frequency 3.1 RESONANCE MRI PRINCIPLES: MAGNETIC PRINCIPLES

33. The RF pulse exchange energy with the protons (a), and some of them are lifted to a higher level of energy, pointing down (b), in effect the magnetization along the z-axis decreases, as the protons which point down neutralize the same number of proton pointing up What happened when RF applied at the L armor frequency Disturb the nuclei from their orderly precession Magnetization ( Mz ) tilted away from the equilibrium Protons can absorb that energy and the energy lost to its surroundings through the so-called spin–lattice relaxation, and the time constant for this phenomenon is termed T1 Protons jump to a higher energy state 3. 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

34. Spin System After Irradiation Basic Quantum Mechanics Theory of MR Send in RF pulse Lower Higher RF pulse Signal 3.1 MRI PRINCIPLES: MAGNETIC PRINCIPLES

35. What happened when RF is OFF 3. 3.1 1. The absorbed RF energy is retransmitted 2. The excited spins begin to return to the original Mz orientation ( B0 at the Larmor frequency ) ( T1 recovery to equilibrium state) 3. Initially in phase, the excited protons begin to dephase (T2 and T2* relaxation) MRI PRINCIPLES: MAGNETIC PRINCIPLES

36. 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 36 3.1 NMR signal decays in time T1 relaxation – Flipped nuclei realign with the magnetic field T2 relaxation – Flipped nuclei start off all spinning together, but quickly become incoherent (out of phase) T2* relaxation – Disturbances in magnetic field (magnetic susceptibility) increase rate of spin coherence T2 relaxation NMR signal is a combination of the total number of nuclei (proton density), minus the T1 relaxation and T2 relaxation components MRI PRINCIPLES: MAGNETIC PRINCIPLES

37. 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 37 3.1 Different tissues have different relaxation times MRI PRINCIPLES: MAGNETIC PRINCIPLES

38. 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 38 3.1 Relaxation times are important for generating image contrast T1 - Gray/White matter T2 - Tissue CSF T2* - Susceptibility (functional MRI) MRI PRINCIPLES: MAGNETIC PRINCIPLES

39. HOW MRI WORKS 39 5/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

40. 3.1 MRI PRINCIPLES: HOW MRI WORKS?

41. 3.1 MRI PRINCIPLES: HOW MRI WORKS?

42. 3.1 MRI PRINCIPLES: HOW MRI WORKS?

43. 3.1 MRI PRINCIPLES: HOW MRI WORKS?

44. 3.1 MRI PRINCIPLES: HOW MRI WORKS?

45. 3.1 MRI PRINCIPLES: HOW MRI WORKS?