MRI Lecture Notes PDF

10/1/24 Cluster 5: MRI Basic Components of (I. Physical M...

10/1/24 Cluster 5: MRI Basic Components of (I. Physical MRI Principles of MRI) -Simplified Version- 1. Scanner 2. Computer 3. Recording Hardware Prepared by: Juan Carlo C. Bentinganan, MSRT, RRT 1 2 Main Components of Static Magnetic a Scanner: Field (B) Static Magnetic Field Coils Gradient Coils RF (Radiofrequency Coils) 3 4 1  10/1/24 3 Methods to Generate Magnetic Field 1. Fixed Magnet (permanent magnet) 2. Resistive Magnet (electromagnet) 3. Superconducting Magnet Fixed magnets and resistive magnets are generally restricted to field strength below 0.4 tesla High-resolution imaging systems use super conducting magnets Super conducting magnets are large and complex. They need the coils to be soaked in liquid helium to reduce their temperature. 5 6 Gradient Coils Radiofrequency Coils Gradient coils are used to produce deliberate variations in the main magnetic field RF coils act as transmitter and These variations allow for localization of the tissue slicing receiver as well as for phase encoding and frequency encoding. They are the “antennas” of the MRI system They transmit RF signal and receive the return signals. 7 8 2  10/1/24 MRI Hardware Four Basic Steps are involved in getting an MR image Magnet 1. Placing the patient in the magnet RF Coil 2. Sending RF pulse by coil Gradient Echo 3. Receiving signals from the patient by coil. 4. Transformation of signals into image by complex Shim Coil processing in the computers 9 10 Basic Principles of MRI Spin Angular Momentum (h) MRI is based on the principle of NMR (Nuclear Magnetic Resonance) Spin is an intrinsic angular NMR – certain atomic nuclei demonstrate the ability to absorb momentum of charged and re-emit RF energy when placed in a magnetic field. nucleus. Hydrogen atoms are most commonly used as they are There are 2 types of spins called as “spin up” and “spin present everywhere in the body down” 11 12 3  10/1/24 Why Hydrogen? N ucleus 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 13 14 Magnetic Dipole Moment Gyromagnetic Ratio (nu) It is a unique value for each type of nucleus given by Gamma (y) gyromagnetic ratio = Magnetic Dipole Moment (nu) It is defined as the property of a spin angular momentum nucleus that causes it to behave like a tiny bar magnet i.e it tries to align itself parallel in a magnetic field 15 16 4  10/1/24 Precession It is defined as the change in the rotational axis of a rotating body when an external magnetic field ( torque) is applied. “wobble” spin of a hydrogen atom LARMOR FREQUENCY - Defines that each type of nucleus will precess at a unique frequency in the magnetic field (torque) is applied. (w) Omega = (y) gyromagnetic ratio * (B) magnetic field 1. Precession frequency is never constant 2. It is proportional to Bo. (i.e. the external magnetic field) 3. It becomes higher as the strength of the external magnetic field increases. 17 18 19 20 5  10/1/24 Test Questions: 1.What type of nonionizing electromagnetic radiation does MR use? A. x-ray B. Gamma C. Radiofrequency D. Alpha 21 22 1.What type of nonionizing 2. When protons spin in place within the electromagnetic radiation does MR atom they create use? A. A small magnetic field A. x-ray B. A small hydrogen moment C. An electrical charge B. Gamma D. Pressure on the electron C. Radiofrequency D. Alpha 23 24 6  10/1/24 2. When protons spin in place within the 3. Precessional frequency is atom they create determined by the ___equation A. A small magnetic field A. Faraday B. A small hydrogen moment C. An electrical charge B. Larmor D. Pressure on the electron C. Precessional D. Magnetic frequency 25 26 3. Precessional frequency is Concept of Magnetization determined by the ___equation A. Faraday B. Larmor C. Precessional D. Magnetic frequency 27 28 7  10/1/24 Magnetization (M) Transverse Plane: It is perpendicular to Bo (x, y) Transverse Magnetization: is defined as the net single - When a magnetic field perpendicular to B is applied (i.e. B1) the magnetization is flipped vector obtained by adding all out of alignment with B and this angle is called ”flip angle” the individual MDMs - Needs to take place for protons to precess and this is achieved by giving the radiofrequency signal (RF signal) - If the applied B1 is long enough to flip M by 90 degree we call it “90 degree pulse” 29 31 Introduction to Relaxation Free Induction Decay (FID) Precession of the particles is influenced by the radiation frequency pulse given 32 33 8  10/1/24 Transverse When the RF pulse is switch off, the protons precessing lose T2 Relaxation their phase (dephasing) and Time taken by the their precessing decreases. transverse magnetization to return to its original vector is called as "T2 relaxation time' 34 35 T2 Weighted T2 Weighted Images MRI The images are made T2-wighted by keeping the TE longer. At short TE, tissues with long as well as short T2 have strong signal. At longer TE, only those tissues with long T2 will have sufficiently strong signal and the signal difference between tissues with short and long T2 will be pronounced. 36 37 9  10/1/24 SPIN-SPIN Relaxation Time Longitudinal Relaxation When the radiofrequency (RF) pulse is switched off, the high energy protons tend to transfer As the protons lose their precession and begin to relax, energy to surrounding lattice and align themselves along z- axis they transfer their energy to low energy protons in the process of defacement. Hence T2 is also called as spin-spin relaxation time. 38 39 T1 T1 Relaxation Time Time taken by the longitudinal magnetization to return to it's original value after the RF signal has been switched off is called 'T1’ it is the time taken for 63% of nuclei to return to lower energy state following a 90 degree pulse Also called “Spin Lattice Relaxation” Can be measured in NMR technique 40 41 10  10/1/24 T1 Weighted T1 Weighted Images MRI keeping the TR short. If TR is long the tissues with long T1 will also regain maximum LM giving stronger signal with next RF pulse. This will result in no significant difference between signal intensity of tissues with different T1. With short TR only the tissues with short T1 will show high signal intensity. 42 43 Test Questions: 1. The MR signal generated by a single RF-pulse is called a _______. a. Free induction decay (FID) b. Spin echo (SE) c. Gradient echo (GRE) d. Stimulated echo (STE) 44 45 11  10/1/24 1. The MR signal generated by a single RF-pulse is called a 2. A spin-echo sequence using a short TR and long _______. TE produces a. Free induction decay (FID) a. A T1-weighted image b. Spin echo (SE) b. A T2-weigthed image c. Gradient echo (GRE) c. A PD-weighed image d. Stimulated echo (STE) d. A noisy low contrast image 46 47 2. A spin-echo sequence using a short TR and long TE produces a. A T1-weighted image b. A T2-weigthed image c. A PD-weighed image d. A noisy low contrast image 48 49 12  10/1/24 Various MRI Introduction Sequences 50 51 Magnetic Resonance Imaging MRI Pulse Sequence an imaging modality that uses a programmed non-ionizing radiation to create useful diagnostic images. set of changing magnetic gradients. 52 53 13  10/1/24 Number of Parameters MRI Principle TE TR Flip Angle Diffusion Weighting (Multiple sequences are grouped together into an MRI protocol.) 54 55 Two (2) Basic Principles of NMR Atoms with an odd number of protons or neutrons have MRI is based on the Nuclear Magnetic Resonance spin (NMR) A moving electric charge, be it positive or negative, produces a magnetic field Body has many such atoms that can act as good MR nuclei (1H,13C, 19F, 23Na) 56 57 14  10/1/24 MRI Scanner Main Components: TR & TE Magnet – in which the patient lies Radio wave Antenna - to send signals to the body and then receive signals back. Scanner - These returning signals are converted into images by a computer attached to the scanner. 58 59 TR (Repetition Time) TE (Echo Time) the time between two excitations time interval in which signals are measured after RF excitation 60 61 15  10/1/24 In General Why MRI? No Ionizing Radiation Short TR (45 ms) scan is T2 W Superior soft tissue contrast Long TR (>2000 ms) and Short TE (