Principles of MRI

Principle and Physics of MRI

1. Principles of MRI By : Dr. Muhannad M. Hadi Al-Mukhtar Final stage - Anaesthesia and Intensive Care Iraqi Board for Medical Specializations Medical City Baghdad Hospital December 2017 Supervised by : Dr. Ayad Abbas Salman Chairman of scientific council of Anesthesia and intensive care Iraqi Board for Medical Specialities

2. History MRI was initially called Nuclear Magnetic Resonance Imaging after its early use for chemical analysis. The " Nuclear " was dropped off about 35 years ago because of fears that people would think there was something radioactive involved, which there is not.

3. History The magnetic resonance phenomenon has been known since the 1940s . Initially employed to determine the structures of molecules. It was discovered by Felix Bloch and Edward Purcell, two American scientists, who were awarded the Nobel Prize in physics in 1952 for their discovery.

4. History - The use of NMR to produce 2D images was accomplished by Paul Lauterbur , and Sir Peter Mansfield who imaged the fingers of a research student, Dr Andrew Maudsley in 1976. - In 2003, Paul C. Lauterbur and Sir Peter Mansfield were awarded the Nobel Prize in medicine for their contribution to the development of MRI for medical purposes. Paul Lauterbur Sir Peter Mansfield

5. Definition MRI (Magnetic Resonance Imaging) is an imaging modality based on an interaction between transmitted radiofrequency (RF) waves and hydrogen nuclei in human body under the influence of a strong magnetic field.

6. Principles of MRI Simply stated, MRI is based on measurements of energy emitted from hydrogen nuclei following their stimulation by radio-frequency signals. The energy emitted varies according to the tissues from which the signals emanate. This allow MRI to distinguish between different tissues.

7. Almost 99% of the mass of the human body is made up of six elements: oxygen , carbon , hydrogen , nitrogen , calcium , and phosphorus . Only about 0.85% is composed of another five elements: potassium , sulfur , sodium , chlorine , and magnesium . All 11 are necessary for life.

8. How do protons help in MR imaging? Protons are positively charged and have rotatory movement called Spin . Any moving charge generates current. Every current has a small magnetic field around it. So every spinning proton has a small magnetic field around it, also called magnetic dipole moment.

9. Why Proton only? Other substances can also be utilized for MR imaging. The requirements are that their nuclei should have spin and should have odd number of protons within them. Hence theoretically 13C, 19F, 23Na, 31P can be used for MR imaging. Hydrogen atom has only one proton . Hence H + ion is equivalent to a proton. Hydrogen ions are present in abundance in body water. H+ gives best and most intense signal among all nuclei.

10. Why Hydrogen Simplest element with atomic number of 1 and atomic weight of 1. When in ionic state (H+), it is nothing but a proton. Proton is not only positively charged, but also has magnetic spin. MRI utilizes this magnetic spin property of protons of hydrogen to elicit images.

11. Elements of MRI

12. PRIMARY MAGNETIC FIELD PRIMARY MAGNETIC FIELD GRADIENT COILS GRADIENT COILS RF COILS RF COILS COMPUTER RF DETECTOR COMPUTER IMAGE

13. Elements of MRI

14. Magnetic Field measured by Tesla (T). Clinical MRI (1.5 - 3 )Tesla Earth Magnetic field = 0.00003 T 1 Tesla = 20,000 times the strength of the earth’s magnetic field.

15. Gradient Coils Three gradient coils, one for each of the orthogonal planes, are located within the core of the MRI unit. Alter primary magnetic field. Responsible for loud noises of MRI. Allow spatial encoding for MRI images in the X,Y, and Z axis i.e localization. Z gradient runs along the Long axis to produce Axial images. Y gradient runs along the Vertical axis to produce Coronal images. X gradient runs along the Horizontal axis to produce Sagittal images

16. Z X Y

17. Radio-frequency Coils The RF coils serve two purposes 1. Transmitting the RF pulses that alter the alignment of the protons 2. Receiving the signals emitted from the protons

18. Steps to get MR images 1. Placing the patient in the magnet. 2. Sending Radiofrequency (RF) pulse by coil 3. The radiowave is turned off 4. The patient’s body emits a signal 5. Receiving signals from the patient by coil 6. Transformation of signals into image by complex processing in the computers.

19. Protons in Human Body Normally the protons in human body (outside the magnetic field) move randomly in any direction. When external magnetic field is applied, i.e. patient is placed in the magnet, these randomly moving protons align (i.e. their magnetic moment align) and spin in the direction of external magnetic field. Some of them align parallel and others anti-parallel to the external magnetic field.

20. Human protons and Magnetic Field When a proton aligns along external magnetic field, not only it rotates around itself (called spin) but also its axis of rotation moves forming a ‘cone’. This movement of the axis of rotation of a proton is called as precession. The number of precessions of a proton per second is called precession frequency. It is measured in Hertz. Precession frequency is directly proportional to strength of external magnetic field. Precession frequency of the hydrogen proton at 1, 1.5 and 3 Tesla is roughly 42, 64 and 128 MHz respectively.

21. Protons before and After applying Magnetic Field

22. Basic four steps of MR imaging include: 1. Patient is placed in the magnet— All randomly moving protons in patent’s body align and precess along the external magnetic field. Longitudinal magnetization is formed long the Z-axis.

23. 2. RF pulse is sent The precession frequency of protons should be same as RF pulse frequency for the exchange of energy to occur between protons and RF pulse. When RF pulse and protons have the same frequency protons can pick up some energy from the RF pulse. This phenomenon is called as “resonance”- the R of MRI. Precessing protons pick up energy from RF pulse to go to higher energy level and precess in phase with each other. This results in reduction in longitudinal magnetization and formation of transverse magnetization in X-Y plane .

24. RESONANCE Resonance relates to the transfer/exchange of energies between two systems at a specific frequency. It is analogous to talking to someone on your cell phone. In magnetic resonance, only protons with the same frequency as the RF pulse will respond. During RF pulse delivery nuclei become excited and then return to equilibrium. During equilibrium, they emit energy in the form of electromagnetic waves.

25. T1 AND T2 RELAXATION When RF pulse is stopped higher energy gained by proton is retransmitted and hydrogen nuclei relax by two mechanisms T1 or spin lattice relaxation- by which original magnetization (Mz) begins to recover. T2 relaxation or spin spin relaxation - by which magnetization in X-Y plane decays towards zero in an exponential fashion. It is due to incoherence of H nuclei. T2 values of CNS tissues are shorter than T1 values

26. T1 is defined as the time it takes for the hydrogen nucleus to recover 63% of its longitudinal magnetization

27. T2 relaxation time is the time for 63% of the protons to become dephased owing to interactions among nearby protons.

28. 3. MR signal is received The transverse magnetization vector precesses in transverse plane and generates current. This current is received as signal by the RF coil. 4. Image formation— MR signal received by the coil is transformed into image by complex mathematical process such as Fourier Transformation by computers.

29. EXCITATION AND RELAXATION

30. TE is the time at which the signal is captured. TR is the time at which the RF pulse is repeated to again displace the protons.

31. TR ( repetition time) The time between two excitation pulses is called repetition time.

32. TE ( time of echo) TE (echo time) : It is the time between the excitation pulse and the echo.

33. TYPES OF MRI IMAGINES T1WI T2WI FLAIR STIR DWI ADC GRE MRS MT Post-Gd images MRA MRV

34. T1-Weighted Imaging characterized by short TR and TE times. Thus the signal is caught early, at a time when the difference in relaxation characteristics for fat and water is most noticeable and tissues that rapidly recover their longitudinal magnetization, such as fat, give rise to high signal intensity (create a bright image). When short TE is employed, tissues that are slow to regain longitudinal magnetization, such as tissues with high free-water content, render low signal intensity .These tissues appear dark on T1-weighted images.

35. T1 TR: short TE: short fat: bright fluid: dark

36. T2 • TR: long • TE: long • fat: intermediate-bright • fluid: bright

37. Long TR and TE times characterize T2 imaging. Because in T2-weighted imaging the signal is measured late in the decay process, tissues that are most reluctant to give up energy are selectively imaged. Free water is slow to give up its energy and consequently renders high signal intensity on T2 sequences. Fat, which gives up its energy rapidly, gives rise to low intensity on T2. T2-Weighted Imaging

38. The Difference Between T1- and T2-Weighted Imaging T1 imaging measures energy from structures such as fat, which give up energy rapidly, early in the process of longitudinal remagnetization. T1 imaging provides images of good anatomic detail, displaying the tissues in a fairly balanced manner. T2 imaging measures energy late in the decay of transverse relaxation and selectively images structures that do not readily give up energy, such as water. It is particularly valuable for detecting inflammation

39. Proton Density (PD) 1. TR is long (more than 2,000 ms) and TE is short (20 to 30 ms). 2. Images are based on measurements of proton density and are similar in appearance to T1 images but with greater anatomic detail.

40. Advantages of MRI 1. Non-ionising , for they do not use X-ray as medium for imaging. 2. Multiplanar imaging is automatically possible, for images in sagittal, coronal and transverse planes are generated simultaneously. 3. Superior contrast in tissue give exquisite anatomical details. 4. Certain tissue diagnosis is possible , e.g. lipoma, edema, age of hemorrhage, etc. 5. MR myelogram is created without injection of any contrast medium. 6. In tumor imaging , it gives exact anatomical details regarding the tumor limits, edema limits, vascularity, etc.

41. Disadvantages of MRI (compared to CT scan) 1. It has low sensitivity for calcium, therefore cannot diagnose calcification clearly. 2. It has low sensitivity for acute hemorrhage. 3. Scan time is prolonged. 4. Contraindications prevent certain patients from entering the MRI system. Patients with metallic implants like cochlear implant , steel sutures , pacemakers , etc. are not allowed inside the MR scanner. 5. Patients with claustrophobia cannot tolerate the study and some young children may need anesthesia. 6. Intravenous contrast agents may be needed.