MRI Technology & Basic Semiology PDF

18/11/2024 Basic technology and semiology Magnetic Resonance Imaging Dr LAAH NJOYO 1 Course outline The MRI equipment Basic physic Basic semiology Limitation and contra indication When to MRI...

18/11/2024 Basic technology and semiology Magnetic Resonance Imaging Dr LAAH NJOYO 1 Course outline The MRI equipment Basic physic Basic semiology Limitation and contra indication When to MRI 2 Objectives At the end of the discussion, the student should be able to: List the basic component of an MRI machine Decribe the used of each of the conponent Explain the mechanism of spin relaxation and their contribution to the image formation To list the basic sequence used in mri To describ the relation between spin rlation and image using as example the brain tissue 3 1  18/11/2024 MRI equipment Main Components Static Magnetic Field Coils Gradient Magnetic Field Coils Magnetic shim coils Radiofrequency Coil Subsystem control computer Data transfer and storage computers Physiological monitoring, stimulus display, and behavioral recording hardware 4 5 6 2  18/11/2024 7 Shimmingrf rf gradient coil coil main main magnet magnet Transmit Receive Control Computer 8 The main magnet The magnetic field is measured in uit of Gauss (G) and Tesla (T), 1testa = 10 000G The earth magnetic field is 0.5G The main magnet field stregnht rag between 0.2 and 3T (B0) The aim is to altered the field strenght and affect the lamour frequency at which th proton naturally precess 9 3  18/11/2024 The main magnet Can be classify according to the physic used to create the magnet – permanent magnet : B0 = 0.2-0.4T – Resistive magnet: B0 less than 0.4T – Superconducting electromagnet: B0= 1.5 – 3T Can also be classify according to th shape of the magnet – Cylindrical or closed magnet : high field magnet – Open field magnet : low field magnet The strengnt of the main magnet is used to classify MRI equipment 10 Gradient Coils Create a varying magnetic field on the X, Y and Z axes Enable localization of each proton postion on the area of interrest 11 Radio frequency Coil RF Coils can transmit and receive RF signals (i.e. apply B1 and monitor Mxy) RF (radio frequency) fields are electromagnetic fields that oscillate at radio frequencies (tens of millions of times per second) RF (radio frequency) fields are electromagnetic fields that oscillate at radio frequencies (tens of millions of times per second) 12 4  18/11/2024 Radio frequency coil Radio Frequency Fields RF electromagnetic fields are used to manipulate the magnetization of specific types of atoms This is because some atomic nuclei are sensitive to magnetic fields and their magnetic properties are tuned to particular RF frequencies Externally applied RF waves can be transmitted into a subject to perturb those nuclei Perturbed nuclei will generate RF signals at the same frequency – these can be detected coming out of the subject 13 X-Ray, CT MRI Electromagnetic Radiation Energy 14 RF coils § 7 standard configuration： QD head coil QD Neck Coil QD Body Coil QD Extremity Coil Flat Spine Coil Breast Coil 15 5  18/11/2024 Radio frequency coil Will function as transmitter to stimulate the proton of the region of interest Receiver : to register the signal stimulated by the stimulated proton 16 MRI Scanner Components 17 Basic physic The proton and the it spin: The effect of an external magnetic field Bo on the proton : the precessing The spin excess and net magnetization Component of net magnetization Relaxation Repetition time and Echo time 18 6  18/11/2024 Properties of Electrical Fields SS + N - 19 Properties of Magnetic Fields S N 20 Magnetic Resonance Imaging Hydrogen protons spin producing a magnetic N + field A magnetic field S creates an electrical charge when it rotates spinning past a coil of wire bar proton magnet 21 7  18/11/2024 Randomly oriented Protons aligned with a protons strong magnetic field Bo Mo net magnetic net magnetic moment is zero moment is positive 22 A Single Proton There is electric charge µ The proton also J on the surface of the has mass which proton, thus creating a generates an small current loop and + angular generating magnetic + momentum + moment µ. J when it is spinning. Thus proton “magnet” differs from a magnetic bar in that it also possesses angular momentum caused by spinning. 23 Protons in a Magnetic Field Bo Parallel (low energy) Anti-Parallel (high energy) Spinning protons in a magnetic field will assume two states. If the temperature is 0 o K, all spins will occupy the lower energy. The difference between the parallel and the antiparallel proton is call the spin excess At room temperature , 3T of Bo, only about 10 out of 1 million proton contribute to spin excess This spin excess is used to generate signal in MRI 24 8  18/11/2024 Effect of Static Field on Protons Parallel= low energy Bo Antiparallel = high energy 25 Net magnetization Spin excess 26 Net magnetization Each proton can be considered as a vector Net magnetization of the precession proton can be found using vector summation Two componnents – Longitudinal – Transversal 27 9  18/11/2024 Net magnetization Longitudinal magnetization – Majority of parallel and antiparallel spin cancel each other – Some additionnal parallel spin called “spin excess” remain and can be used to generate MT signals Transversal component – Phases of proton spins are random, so vector summation yields zero Overall – Net magnetization (M) is in the longitudinal z-direction (the same direction with the external field B0) 28 29 Relaxation The functionning of the RF coils include – Period of stimulation with generate magnetization – period of reception to study the partten of relaxation Relaxation is consider as return to equilibrium of net magnetization at the end of stimulation Two components – T1: longitudinal or spin-latice relaxation – T2: transvrsal or spin-spin relaxation 30 10  18/11/2024 Turn off the radio The higher energy gained by the protons is retransmitted (NMR signal) The original magnetization begins to recover (T1) The excessive spin begins to dephase (T2) 31 T1 relaxation T1 is defined as the time it takes for the hydrogen nucleus to recover 63% of its longitudinal magnetization 32 T2 relaxation T2 relaxation time is the time for 63% of the protons to become dephased owing to interactions among nearby protons 33 11  18/11/2024 34 TR and TE TE (echo time) : time interval in which signals are measured after RF excitation TR (repetition time) : the time between two excitations is called repetition time By varying the TR and TE one can obtain T1WI and T2WI In general a short TR (45ms) scan is T2WI Long TR (>2000ms) and short TE (