Basic principle of MRI

this presentation aims to deliver the basic knowledge of MRI it's working principle , resonance in detail , relaxation and different weighting in MRI image in easy language and with appropriate examples and images.

1. Basic principle of MRI Presenter:- Sagar Chaulagain Roll no.:-157 B.Sc. MIT 2 nd year MMC,IOM

2. Contents Introduction History MR active nuclei Alignment Precession and precessional frequency Resonance Flip angle MR signal Pulse timing parameters TR TE Relaxation T1 Relaxation T2 Decay Image contrast T1 Contrast T2 Contrast Proton Density Contrast Weighting T1 Weighting T2 Weighting Proton Density Weighting Summary Reference

3. Introduction MRI stands for magnetic resonance imaging. It is a non-invasive imaging technology that produces three dimensional detailed anatomical images of almost every internal structure in the human body, using a large magnetic field and radio waves. No ionizing radiation is produced during an MRI exam, unlike X-rays.

4. Introduction MRI is based on the principle of Nuclear magnetic resonance (NMR). Nuclear magnetic resonance (NMR) is the spectroscopic study of the magnetic properties of the nucleus of the atom. NMR is basically a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation.

5. Introduction There are essentially 2 ways of explaining the fundamentals of MRI: classically and via quantum physics. Using classical theory, MRI is explained by using the concepts of mass, spin and angular momentum on a large or bulk scale. Quantum theory operates at a much smaller subatomic scale and refers to the energy levels of the protons, neutrons and electrons.

6. History 1946, Felix Bloch, working at Stanford University, and Edward Purcell, from Harvard University, discovered NMR. 1972, Paul Lauterber & Peter Mansfield produced the first MR image 1975, Raymond Damadian produced first live animal MR image 1977, Dr. Damadian completed construction of the first whole-body MRI scanner and produced the first MRI scan of the human body

7. NMR VS MRI NMR is not an imaging technique rather provides a spectroscopic data for samples In the early 1970s, it was realized that magnetic field gradients could be used to localize the NMR signal and to generate images that display magnetic properties of the proton. In the mid 1980 With technological advancement and body-size magnets facilitate the increased in clinical imaging applications , the “nuclear” connotation was dropped, and magnetic resonance imaging MRI gained widespread acceptance

8. Atomic structure All things are made of atoms. The most abundant atom in the human body is hydrogen atom, but there are other elements such as oxygen, carbon, and nitrogen. The atom consists of a central nucleus and orbiting electrons Atoms are characterized in two ways. The atomic number:- it is the sum of the protons in the nucleus. This number gives an atom its chemical identity. The mass number or atomic weight:- it is the sum of the protons and neutrons in the nucleus.

9. Motion in the atom Three types of motion are present within the atom : Electrons spinning on their own axis Electrons orbiting the nucleus The nucleus itself spinning about its own axis. The principles of MRI rely on the spinning motion of specific nuclei present in biological tissues. Fig:- The atom

10. Motion in the atom A nucleus has no spin if it has an even atomic and mass number because half of the nucleons spin in one direction and half in the other. The forces of rotation cancel out, and the nucleus itself has no net spin. However, in nuclei with an odd number of protons or neutrons the spin directions are not equal and opposite, so the nucleus itself has a net spin or angular momentum. In general, only nuclei with an odd mass number or atomic weight are used in MRI and are known as MR-active nuclei.

11. Fig :- Nucleus of the atom

12. MR active nuclei MR-active nuclei are characterized by their tendency to align their axis of rotation to an applied magnetic field as they have angular momentum. According to EMI Nuclei that have a net charge and are spinning acquire a magnetic moment and are able to align with an external magnetic field. Important examples of MR-active nuclei, together with their mass numbers are listed below: 1H (hydrogen) 13C (carbon) 15N (nitrogen) 17O (oxygen) 19F (fluorine) 23Na (sodium)

13. The Hydrogen Nucleus The protium isotope of hydrogen is most commonly used MR-active nucleus in MRI. It has a mass and atomic number of 1, so the nucleus consists of a single proton and has no neutrons. It is used because hydrogen is very abundant in the human body and because the solitary proton gives it a relatively large magnetic moment. The protium nucleus contains one positively charged proton that spins Therefore, the nucleus has a magnetic field induced around it and acts as a small bar magnet.

14. Fig:- The magnetic moment of the hydrogen nucleus

15. Alignment In the absence of an applied magnetic field, the magnetic moments of hydrogen nuclei are randomly orientated and produce no overall magnetic effect. However, when placed in a strong static external magnetic field ,the magnetic moments of hydrogen nuclei orientate with this magnetic field. This is called alignment. Alignment is best described using classical and quantum theories as follows

16. Classical theory It uses the direction of the magnetic moments of spins of hydrogen nuclei to illustrate alignment:- Parallel alignment : Alignment of magnetic moments in the same direction as the main B0 field also referred to as spin-up. Antiparallel alignment : Alignment of magnetic moments in the opposite direction to the main B0 field also referred to as spin-down. There are always more spins with their magnetic moments aligned parallel than antiparallel. The net magnetic vector, NMV is alwayse aligned parallel to the main B0 field in the longitudinal plane or z-axis

17. Alignment Fig:- Alignment – classical theory

18. Quantum theory Quantum theory uses the energy level of the spins of hydrogen nuclei to illustrate alignment. For hydrogen nuclei, there are only two possible energy states;- Low-energy nuclei ;-These are nuclei that align their magnetic moments parallel or spin-up to the main B0 field. This nuclei do not have enough energy to oppose the main B0 field . High-energy nuclei :-These are nuclei that align their magnetic moments antiparallel or spin-down to the main B0 field This nuclei do have enough energy to oppose the main B0 field .

19. Fig:- Alignment – quantum theory

20. Net Magnetic Vector (NMV) In thermal equilibrium, at any moment in time, there are a greater proportion of spins with their magnetic moments aligned in the same direction as B0 than against it. As there is a larger number aligned parallel, there is always a small excess in this direction that produces a net magnetic moment called NMV NMV reflects the relative balance between spin-up and spin-down nuclei. It is the sum of all magnetic moments of excess spin-up nuclei and is measurable It aligns in the same direction as the main field in the longitudinal plane or z-axis

21. Fig:- The net magnetic vector

22. Precession And Precessional Frequency Each hydrogen nucleus spins on its axis called the primary spin. The influence of B0 produces an additional spin or wobble of the magnetic moments of hydrogen around B0, this secondary spin is called precession and causes the magnetic moments to circle around B0. The course they take is called the precessional path, and the speed at which they precess around B0 is called the precessional frequency. The precessional frequency is often called the Larmor frequency because it is determined by the Larmor equation The unit of precessional frequency is hertz.

23. P Fig:-Precession Fig:-Precession of the spin-up and spin- down populations

24. Larmor equation ω0 = γB0 /2π ω0  = γB0 ω0 is the precessional or Larmor frequency (MHz) γ is the gyromagnetic ratio (MHz/T) B0 is the strength of the external magnetic field (T) γ is a constant, for a given MR-active nucleus ω0 is proportional to B0

25. Gyromagnetic ratio The gyromagnetic ratio expresses the relationship between angular momentum and the magnetic moment of each MR-active nucleus. It is constant and is expressed as the precessional frequency of the magnetic moment of a specific MR-active nucleus at 1 T. The unit of the gyromagnetic ratio is therefore MHz/T.

26. Gyromagnetic ratio Element Gyromagnetic ratio (MHz/T) Larmor frequency at 1.5T (MHz) 1H (Hydrogen) 42.5774 63.8646 13C (Carbon) 10.7084 16.0621 15N (Nitrogen) 4.3173 6.4759 17O (Oxygen) 5.7743 8.6614

27. Precessional Frequency magnetic moments of MR-active nuclei have different precessional frequencies at different field strengths., for example:For hydrogen At 1.5 T, the precessional frequency is 63.87 MHz (42.58 MHz × 1.5 T). At 1.0 T, the precessional frequency is 42.57 MHz (42.58 MHz × 1.0 T). At 0.5 T, the precessional frequency is 21.29 MHz (42.58 MHz × 0.5 T).

28. Precessional Phase Phase refers to the position of magnetic moments on their precessional path at any moment in time. The unit of phase is a radian. A magnetic moment travels through 360 rad or 360° during one rotation In MRI, the relative phase positions of all magnetic moments of hydrogen are important.. Phase is describe as inphase or coherent phase and out of phase or incoherent phase

29. Precessional Phase Out of phase or incoherent:- means that magnetic moments of hydrogen are at different places on the precessional path at a moment in time. In phase or coherent:- means that magnetic moments of hydrogen are at the same place on the precessional path at a moment in time When there is only influence of B0 ,the magnetic moments of the nuclei are out of phase with each other, and therefore the NMV does not precess .

30. Resonance Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation. Resonance is achieved by transmitting an RF pulse called an RF excitation pulse which is produced by a transmit coil . The RF excitation pulse is derived from the magnetic component only the electric field produces heat. Unlike the B0 field, which is stationary, the RF excitation pulse produces an oscillating magnetic field, termed B1 . B1 field is applied at 90° to B0 and is very weak compared with that of the main external field B0.

31. The results of resonance From the classical theory perspective, application of the B1 field in a plane at 90° to B0 termed the transverse plane or x-y palne , causes magnetic moments of the spins to precess around this axis rather than about the longitudinal plane or z-axis. nutation Another consequence of the RF excitation pulse is that the magnetic moments of the spin-up and spin-down nuclei move into phase with each other

32. The results of resonance From the classical theory perspective following are the results, It produces an oscillating magnetic field (B1) at 90° to B0 in transverse plane or x-y plane. Nutation Cause the magnetic moments of the spin-up and spin-down nuclei move into phase with each other From the quantum theory perspective RF excitation pulse gives energy to hydrogen nuclei and causes a net increase in the number of high-energy, spin-down nuclei.

33. Flip angle The magnitude of the flip angle depends on the amplitude and duration of the RF excitation pulse. Usually, the flip angle is 90°, i.e. the NMV is given enough energy by the RF excitation pulse to move through 90° relative to Bo. With a flip angle of 90°, the nuclei are given sufficient energy so that the longitudinal NMV is completely transferred into a transverse NMV. When flip angles less than or more than 90° are used, only a portion of the NMV is transferred to the transverse plane.

34. θ = ω 1 τ from the Larmor equation θ = γB1τ θ is the flip angle (°) B1 is the magnetic field associated with the RF excitation pulse. ω1 is the precessional frequency of B1. τ is the duration of the RF excitation pulse.

35. MR Signal Because of resonance, in-phase or coherent magnetization precesses in the transverse plane. This changing magnetic field generates an electric current which is explained by faraday’s law of EMI. According to Faraday’s law, a changing magnetic field causes movement of charged particles, i.e. electrons. This flow of electrons is a current. If a receiver coil or any conductive loop is placed in a moving magnetic field, a voltage generated by this current is induced in the receiver coil.

36. MR Signal This voltage is called signal and is produced when coherent (in phase) magnetization cuts across the coil. The frequency of signal depends on the frequency of rotation of the magnetic field and the magnitude of signal depends on the amount of coherent magnetization present in the transverse plane

37. Fig:- Generation of the signal

38. Free induction decay signal When the RF excitation pulse is switched off, the NMV is influenced only by B0 , and it tries to realign with it. The hydrogen nuclei lose energy given to them by the RF excitation pulse and the process is called relaxation. As relaxation occurs, the NMV returns to realign with B0 because some of the high-energy nuclei return to the low-energy population and therefore align their magnetic moments in the spin-up direction.

39. Free induction decay signal At the same time, but independently, the MM of hydrogen lose coherency due to dephasing. This occurs because of inhomogeneities in the B0 field and due to interactions between spins in the patient’s tissue As the magnitude of transverse coherent magnetization decreases, so does the magnitude of the voltage induced in the receiver coil. The induction of decaying voltage is called the free induction decay (FID) signal. This is because spins freely precess influenced only by B0 , signal decays with time, and magnetic moments of the spins induce a current in the receiver coil.

40. Pulse timing parameters A very simplified pulse sequence is a combination of RF pulses, signals, and intervening periods of relaxation A pulse sequence consists of several time periods. The main ones are outlined below TR :- Itis the time from the application of one RF excitation pulse to the application of the next RF excitation pulse for each slice and is measured in millisecond. The TR determines the amount of longitudinal relaxation that occurs between the end of one RF excitation pulse and application of the next. The TR thus determines the amount of T1 relaxation that has occurred when signal is read .

41. Pulse timing parameters TE :- it is the time from the application of the RF excitation pulse to the peak of signal induced in the receiver coil and is also measured in millisecond. The TE determines how much decay of transverse magnetization occurs. TE thus controls the amount of T2 relaxation that has occurred when signal is read

42. Image contrast All clinical diagnostic images must demonstrate contrast between normal anatomical features and between anatomy and pathology. One of the main advantages of MRI compared to other imaging modalities is its excellent soft tissue discrimination. The contrast characteristics of each image depend on many variables, and are usually divided into two categories

43. Intrinsic contrast parameters T1 recovery time T2 decay time Proton density (PD) Flow Apparent diffusion coefficient (ADC) Extrinsic contrast parameters TR TE Flip angle TI Turbo factor/echo train length b value.

44. Relaxation When the RF excitation pulse is switched off ,hydrogen nuclei return to their low-energy state and their magnetic moments diphase, the process by which this occurs is called relaxation During relaxation, hydrogen nuclei give up absorbed RF energy, and the net magnetic vector (NMV) returns to B0 At the same time but independently, magnetic moments of hydrogen nuclei lose phase coherence.

45. Relaxation Relaxation therefore results in recovery of magnetization in the longitudinal plane and decay of coherent magnetization in the transverse plane. The recovery of longitudinal magnetization is caused by a process termed T1 recovery (longitudinal relaxation) The decay of coherent transverse magnetization is caused by a process termed T2 decay (Transverse relaxation)

46. T1 Recovery T1 recovery is caused by hydrogen nuclei giving up their energy to the surrounding environment or molecular lattice, also called spin–lattice energy transfer. Energy released by spins to the surrounding molecular lattice causes magnetic moments of hydrogen nuclei to recover their longitudinal magnetization. the NMV gradually realigns itself in the longitudinal plane.. The rate of T1 recovery is an exponential process and occurs at different rates in different tissues .

47. T1 Recovery There is a time constant associated with this exponential relationship called T1 recovery time and it is the time it takes for 63% of the longitudinal magnetization to recover in a tissue. Mzt= Mz (1 − e−t/T1 Mzt is the amount of longitudinal magnetization at time t ( ms ) after the removal of the excitation pulse Mz is full longitudinal magnetization T1 is the T1 recovery time ( ms ) and is the time taken to increase the longitudinal magnetization by a factor of e.

48. Tissue T1 recovery time ( ms ) Water 2500 Fat 200 CSF 2000 White matter 500 Fig:- The T1 recovery curve

49. T2 Decay T2 decay is caused by the magnetic fields of neighboring hydrogen nuclei interacting with each other. This type of relaxation is termed spin–spin relaxation and causes dephasing of magnetic moments of the spins. The term decay refers to the loss of coherent transverse magnetization. Spin–spin relaxation is caused by one spin transferring energy to another spin rather than into the lattice Dephasing is also caused by inhomogeneities in the B0 field. The rate of T2 decay is also an exponential process and occurs at different rates in different tissues

50. T2 Decay There is a time constant associated with this exponential relationship. It is called the T2 decay time and is the time it takes for 63% of the transverse magnetization to dephase (37% is left in phase) in a tissue. The time during which this occurs is the time between an RF excitation pulse and when signal is collected in the receiver coil The echo time (TE) therefore determines how much T2 decay occurs in a tissue when signal is collected.

51. Tissue T2 decay time ( ms ) Water 2500 Fat 100 CSF 300 White matter 100 Fig:- The T2 decay curve

52. Contrast mechanism An MR image contrast depends upon the signal intensity. hyperintensity – white in the image hypointensity – black in the image A tissue has a high signal if it has a large transverse component of coherent magnetization at time TE. If there is a large component of coherent transverse magnetization, the amplitude of signal received by the coil is large, resulting in a hyperintense area on the image If there is a small or no component of transverse coherent magnetization, the amplitude of signal received by the coil is small, resulting in a hypointense area on the image. Images obtain contrast mainly through the mechanisms of T1 recovery, T2 decay, and proton or spin density

53. T1 contrast The term T1 contrast means that image contrast is derived from differences in the T1 recovery times of the tissues rather than any other mechanism. T1 contrast is likely to occur if vectors do not fully recover their longitudinal magnetization between each RF excitation pulse. T1 contrast is controlled by the TR, For good T1 contrast, the TR must be short If TR is longer than the relaxation times of the tissues, full recovery occurs in all tissues, and, therefore, it is not possible to produce an image that demonstrates contrast based on the differences in their T1 recovery times. The T1 recovery time of fat is much shorter than that of water, so the fat vector realigns with B0 faster than the water vector. Fat therefore has a high signal and is hyperintense and Water has a low signal and appears relatively hypointense.

54. Fig:-T1 contrast generation

55. T2 contrast The term T2 contrast means that image contrast is derived from differences in the T2 decay times of the tissues rather than any other mechanism. T2 contrast is likely to occur if vectors dephase and there is a difference in coherent transverse magnetization in each tissue. T2 contrast is controlled by the TE, For good T2 contrast, the TE must be long. If the TE is short, then little dephasing occurs, and therefore it is not possible to produce images that demonstrate differences in T2 decay times of the tissues. The T2 decay time of fat is shorter than that of water so there is more coherent transverse magnetization in water than in fat. therefore, water has a high signal and is hyperintense, and fat has low signal and is relatively hypointense on a T2 contrast image

56. Fig:-T2 contrast generation

57. Proton density contrast Proton density contrast refers to differences in signal intensity between tissues that are a consequence of their relative number of mobile hydrogen protons per unit volume. Tissues with a high proton density have a large transverse component of magnetization and therefore a high signal and are hyperintense. Tissues with a low proton density have a small transverse component of magnetization and therefore a low signal and are relatively hypointense Proton density contrast is always present and depends on the patient and the area under examination

58. Proton density contrast SI = PD e–TE/T2 (1 – e–TR/T1) SI is the signal intensity in a tissue PD is proton density TE is the echo time ( ms ) T2 is the T2 relaxation time of the tissue ( ms ) TR is the repetition time ( ms ) T1 is the T1 relaxation time in the tissue ( ms )

59. Weighting The intrinsic contrast parameters affect image contrast, and therefore, it is possible to obtain images of mixed appearance but it is very difficult to determine the relative contribution of each intrinsic contrast parameter to the contrast observed. To minimize this, extrinsic contrast parameters are selected to weight image contrast toward one of the intrinsic contrast parameters To demonstrate T1, T2, or proton density weighting, specific values of TR and TE are selected. The appropriate selection of these parameters weights an image so that one contrast mechanism dominates the other two

60. T1 weighting A T1-weighted image is one where contrast depends predominantly on the differences in the T1 recovery times between fat and water To achieve T1 weighting, the TE and TR must be short enough so that neither the vector in fat nor the vector in water has sufficient time to fully return to B0 If the TR is too long, both the vectors in fat and water return to B0 and fully recover their longitudinal magnetization. When this occurs, T1 recovery is complete in both tissues, and the differences in their T1 recovery times are not demonstrated . For T1 weighting TE must also be short.

61. Fig:-The difference in T1 recovery between fat and water

62. T2 weighting A T2-weighted image is one where contrast predominantly depends on the differences in the T2 decay times between fat and water. The TE controls the amount of T2 decay that occurs before signal is received. To achieve T2 weighting, the TR and TE must be long enough to give the vectors in both fat and water time to dephase . If the TE is too short, neither the vector in fat nor the vector in water has had time to dephase , and, therefore, the differences in their T2 decay times are not demonstrated. T2-weighted images are used to image pathology because most pathology has a high water content and is therefore relatively hyperintense on T2-weighted images

63. Fig:-The difference in T2 decay between fat and water

64. Proton density weighting A PD-weighted image is one where differences in the number of mobile hydrogen nuclei per unit volume of tissue are the main determining factor in forming image contrast. PD weighting is always present to some extent. To achieve PD weighting, the effects of T1 and T2 contrast are diminished so that proton density contrast dominates. A long TR allows the vectors in both fat and water to fully recover their longitudinal magnetization and so diminishes T1 contrast. A short TE does not give the vectors in fat or water time to dephase and so diminishes T2 contrast. PD-weighted images are used to image anatomy and pathology

65. (A) (B) (C)

66. Other contrast mechanism Diffusion-weighted imaging (DWI) Functional MRI (fMRI) Magnetization transfer contrast (MTC) Susceptibility-weighted imaging (SWI) Contrast agents.

67. Summary MRI is based on the principle of Nuclear magnetic resonance (NMR). Only nuclei with an odd mass number or atomic weight are used in MRI as MR-active nuclei. (most commonly used MR- active nuclei – protium ) In the absence of magnetic field, hydrogen nuclei are randomly orientated However, In a strong magnetic field, hydrogen nuclei align either parallel (low-energy) or antiparallel (high-energy) to the field, forming a NMV in longitudinal plane. Magnetic moment of hydrogen precess around the external magnetic field at the Larmor frequency (𝜔0=𝛾𝐵). Resonance occurs when the frequency of the RF excitation pulse is equal to the Larmor frequency of magnetic moments of the hydrogen nuclei.

68. Summary Because of resonance, in-phase or coherent magnetization precesses in the transverse plane this coherent magnetization induces a current in the receiver coil and signal is generated. When the RF excitation pulse is removed, the magnetic moments of all spins dephase and produce a FID. TR is the time between successive RF pulses and TE is the time from the RF pulse to the peak of the signal received. MR image contrast depends on intrinsic (T1 recovery time, T2 decay time, proton density) and extrinsic (TR, TE, flip angle) contrast parameters. The recovery of longitudinal magnetization is T1 recovery (spin-lattice energy transfer) and the decay of coherent transverse magnetization is T2 decay ( spin–spin relaxation )

69. Summary T1 contrast is controlled by the TR, For good T1 contrast, the TR must be short and T2 contrast is controlled by the TE, For good T2 contrast, the TE must be long. Proton density contrast is the consequence of relative number of mobile hydrogen protons present per unit volume Fat has a short T1 recovery time than water therefore fat has high signal and is hyperintense than water in T1 weighted image. The T2 decay time of fat is shorter than that of water so there is more coherent transverse magnetization in water than in fat. therefore, water has a high signal and is hyperintense than fat in T2 weighted image.

70. References MRI IN PRACTICE 5TH EDITION, CATHERINE WESTBROOK THE ESSENTIAL OF PHYSICS OF MEDICAL IMAGING 3RD EDITION, JERROLD T. BUSHBERG, J. ANTHONY SIEBERT, JOHN M. BOONE MRI PHYSICS VARIOUS WEBSITES AND PRIVIOUS PPT

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