Physics of magnetic resonance imaging.pptx

physics of magnetic resonance imaging

1. Physics of Magnetic Resonance Imaging By: Abdelrahman and Abdelrazzak  Kassem LIU Physics Department 22/5/2024

2. Table of contents Chapter 1: Introduction:               Section 1.1: Brief history               Section 1.2: Proton spin Chapter 2: The Physics:                Section 2.1: Magnetic field generation               Section 2.2: Transmission and reception of RF pulses               Section 2.3: Signal detection and image reconstruction Chapter 3: Conclusion                Section 3.1: Main findings                Section 3.2: References

3. Section 1.1: Brief histroy Brief History The timeless physics behind MRIs

4. Section 1.2: Proton spin Protons spin around their axis of rotation Protons are rotating charge Creating a spin angular momentum S=h/2 Moving charge induce magnetic field Protons behave as tiny bar magnets Protons align with an external magnetic field Bo

5. Needed knowledge before Chapter 2: Precession: Protons have their own magnetic dipole mew Placing the dipole in Bo creates a torque The torque causes S to rotate around Bo S will precess around z ( ie . Bo) at a certain frequency The higher Bo strength is the stronger the torque So the precession frequency is directly proportional to B0

6. Needed knowledge before Chapter 2: (cont.) Net magnetization: A human body is placed in Bo Most protons will align with Bo Some will align against Bo Aligning against means higher energy state A net magnetic field M is created M align with Bo along z axis (more protons align with than against) Protons don't  precess  at the same direction and time causing their net magnetization to cancel in the x-y plane

7. Section 2.1:Magnetic field generation The Main Magnet: The main magnet is responsible for producing Bo Bo must be strong enough to have effects Superconductivity is needed to create strong Bo Huge coils are placed in super coolant like liquid helium Conductors have less resistance for electricity at low temperatures Currents will flow continuously for long time A constant Bo is created

8. Section 2.1: (cont.) The Gradient Magnet: Additional magnet placed to alter the strength of Bo Causing different body areas to experience different Bo strength Precession rate directly proportional to Bo strength So different areas emit different signals This help to construct a 3 dimentional  image  This is achieved by altering Bo along the z axis and in the x-y axis

9. Section 2.1: (cont.) Axial Magnets: Changes Bo across the z-axis (along the body) Created by symmetrically positioned coil pairs Each  pair have 2 coils with opposite currents Making Bo strength low the closer it is to the opposite current And Bo is stronger closer to the coil with same current direction Making Bo increase or decrease as function of z

10. Section 2.1: (cont.) Transverse Gradients: The main field is altered along x or y axis 4 circular arcs are symmetrically placed around the center The upper pair of arcs carry the current in same direction The lower pair carry the current opposite to the upper one Causing Bo to change as a function of x or y

11. Section 2.2:Transmation and reception of radio pulses An oscillating magnetic field is needed to create a signal Hence a high frequency electromagnetic radiation is used The RF pulses interact with protons  2 things happen after sending the RF pulse, longitudinal demagnetization and transversal magnetization

12. Section 2.2: (cont.) Longitudinal demagnetization: An RF pulse is send at a similar frequency to that of the proton precession The RF pulse will excite some protons to higher energy state in which they face opposite to Bo Causing more protons to face opposite to each other canceling their net magnetic field along the z axis

13. Section 2.2: (cont.) Transversal magnetization: The RF pulse force the protons to precess at the same direction at the same time Protons facing together will cause a net magnetic field in the x-y plane

14. Section 2.2: (cont.) Relaxation is when protons are de-excited back to their initial state It occurs when the RF pulse is switched off The two relaxation processes occur also longitudinally and transversely 

15. Section 2.2: (cont.) Relaxations times: They are the main piece of information we rely on when forming an image T1 is the time required for the protons to de-excite longitudinally T2 is the time required for the protons to de-excite transversely Each human tissue has its specific relaxation time

16. Section 2.3: Signal detection and image reconstruction Signals detection: Signals are detected by studying new parameters TR: time to repeat TE: time to echo The parameters are adjustable

17. Section 2.3: (cont.) Image reconstruction: Relies on different combinations of using the available magnets and RF pulse Different contrasts of images can be produced Namely: either T1 weighted image or T2 weighted image

18. Conclusion Proton spin is the main focus in MRI An MRI machine consists of 2 types of magnets and an RF pulse The magnets and pulse affect the spin Data is collected from the spin This data is then used to form the image

19. References [1]: Richard Ansorge, Martin Graves – The physics and mathematics of MRI – 2016 [2]: Prof. Hans H. Schild – MRI made easy – 1990 [3]: Alfred L. Horowitz – MRI for Radiologists; A visual approach-1991