MAP.10 MRI Principles of Nuclear Magnetic Resonance PDF

MAP. 10 Principles of nuclear magnetic resonance PRESENTER: Dr. Andy Ma 1 Learning outcomes Recall the principles of electrical induction and electromagnets. Describe the components and standard working procedure of standard NMR. Describe precession of proton spins in...

MAP. 10 Principles of nuclear magnetic resonance PRESENTER: Dr. Andy Ma 1 Learning outcomes Recall the principles of electrical induction and electromagnets. Describe the components and standard working procedure of standard NMR. Describe precession of proton spins in external magnetic fields and calculate their Larmor frequency. Differentiate between spin flips by resonant absorption of radio frequency (RF) photons or by spontaneous decay. Explain the different origins of the longitudinal and transverse magnetic fields. Explain spin-lattice relaxation T1 and spin-spin relaxation T2.  Electricity + Magnetism Remember that in MAP.5, we mentioned the interaction between electric and magnetic fields create EM wave? Indeed, electricity and magnetism are deeply entangled with one another. Before we embark on the journey in exploring Magnetic Resonance Imaging (MRI), we first need to know how electricity creates magnetism. Ampere’s law An electric current flowing through a wire generates a magnetic field I around it. Ampere's Law says that the magnetic field B created by an electric current I is proportional to the size of that electric current (with a constant of B proportionality equal to the permeability of free space μ0). B ~ μ0 I B- Pol · I and B are directly proportional Coil + Current → Magnetic field Passing electric current N through a long straight coil of wire can be used to generate a nearly uniform magnetic field within the coil. The magnetic outside of the coil is similar to that of a bar magnet. wVe S S e.g. electromagnetic · electromagnetic coil recycling crane turns to when magnet current an electric passes  2 waves w the game frequency combined together 11 Magnetic Resonance Imaging (MRI)  MRI = NMR + Imaging MRI is an imaging technique based on Nuclear Magnetic Resonance (NMR).  NMR instrumentation Magnet is usually very strong. Its field strength is varied/tuned to the absorption by certain chemicals („resonance“). Radiofrequency (RF) coils act like antennae: they are used for sending in pulses (Transmitter) and for detecting signals from sample (Detector). · each chemical field will absorb magnetic ked at diff values  NMR in a nutshell 1. Put sample into a strong magnetic field. 2. Send in a short electromagnetic (RF) pulse which is in part absorbed. 3. Then detect decaying electromagnetic signals from sample. Two different types of decays can be measured: For classical NMR, we are interested in how strong one of these signals is at different magnetic field strengths to detect the presence of specific chemical groups. Strength of each wave from the Chemicals during the decay - For MRI, we are interested in how * fast the two signals that belong to hydrogen atoms decay (called ‚T1‘ and ‚T2‘) to detect local tissue properties. fat-fast day The tissue vanriss ecay of diff water-slow day &  Application of NMR Greater signal Themical absorb RF and release it again magnet the atoms rearrang Nuclear Magnetic Resonance (NMR) has been in use for decades as = a method of chemical characterisation. Signal peaks at different magnetic field strengths inform about the presence of chemical groups in the sample.  Why do atoms/molecules interact with magnetic field?  dipole moment & magnetic Proton magnetization (Bp) ↑ Field Bp of proton Each proton within the atomic nucleus (B) has a positive charge and a spin. (motion ↳ The interaction between these the charge produce a small magnetic field. Each proton is thus a tiny magnet, with a proton magnetic field (Bp). Bp is a vector and so its direction is important. Magnitar + direction ↑ motion magnet Charge = +  magnetic - Field The action of an external magnetic field (Bz) > - z direction (X , Y,2) In the presence of an external magnetic field (Bz), this proton magnetic field Bp of a nucleus can either be at the spin up state (aligned/parallel with Bz, lower energy state) or the spin down state (against/anti-parallel with Bz, higher energy state). ① protons are randomly oriented, low state external energy e magnetic axis if theyre in the same direction Field along 2 our body would be magnetic. coming from MRI Our bodies are neutral b Protons ; and their MF is random. ② When B2 is introduced high energy against MF  w angle of e direction Due to the angular momentum of the spin and the action of the external magnetic field, the proton spins actually precess around the Bz. The spins are tilted at an angle with respect to the direction of Bz. ① now protors not spiel only about itself, but also pression around B2 due to external Fed magnetic Definition of Precession: [Slow movement of the axis of a spinning body around another axis due to a torque (such as gravitational influence) acting to change the direction of the first axis.] It is seen in the circle slowly traced out by the pole of a spinning gyroscope. external field (e.g. gravity) Example: spinning top With the direction of the external magnetic field Bz lying on z-axis, the proton’s spin precess around the Bz axis. Precession occurs on the xy-plane. - Proton X,y plane  The number of times per second that BP precesses around Bz is called the Larmor Frequency FL and is given by the Larmor equation: FL = γ’Bz ↓ O-5 tesla from γ’ is the gyromagnetic ratio which varies * with material, for H-1 proton, γ’ = 42.58 * MHz/T. Form the Larmor eq., it is obvious that the stronger the Bz, the higher the precession frequency FL (i.e. the faster the precession). ex : (42. MHz(x)(2x) 58 -F = 85 46 MH2. Besides the proton, a neutron also has its own magnetic field and the overall magnetization of a nucleus (BN) depends on both the protons and neutrons. This makes it complicated to predict the magnetization of a nuclei of different elements. For nuclei with even number of protons and neutrons, every spin up state proton/neutron will be coupled with another spin down state proton/neutron and nuclear magnetization is thus cancelled (parallel Bp + anti-parallel Bp = 0). each These nuclei do not response to a MRI scan. No magnetic field be even cancelOther · , Luckily, we only need to deal with Hydrogen-1 (H-1) nuclei that present in abundance in various type of human tissues. H-1 has only one proton and no neutron and thus its BN = its Bp. For a H-1 nucleus, it has a net nuclear magnetization, BN due to that of its single proton spin state. This BN of each nucleus can be in the spin up or spin down state.  why H ? ① I has I proton in the nucleus only -> So the It P of gives good signal is senstive to external Magetre Find ② found the abundantly body in If sample which contains H-1 spins is placed under external magnetic field Bz, there is a slightly higher tendency for them to align parallel to Bz. # of Spinup] Spindam)  BP · BT Bp is a vector and thus can be resolved into components along mutually Spin up.... BL = 4 perpendicular directions (the z-axis and the xy-plane in this case). For a spin up proton, the z-component of its Bp is parallel to Bz. For a spin down proton, the z- BL down = 1 component of its Bp is anti-parallel to y - 1 = 3 BL upward Bz. so this is the net magnetization The sum of z-components of all the Bp give rise to a net magnetization in z- axis of the sample. By for all protons of the body is or B2 So we can't measure it Spin dan some direction  B up ! Be down do not cancel z Since the no. of spin up protons > spin down protons, the sum of magnetic fields of the nuclei do not cancel out Bz along the z-axis. A tiny but non-zero net ‘longitudinal’ magnetic field BL is created in the sample, in the same direction as the external magnetic field Bz. Bz The stronger the external BL magnetic field, the bigger the longitudinal total C But still B is very small as compared to magnetisation of the sample. BL > BL - of al Protons but we L , y Bz and thus overshadowed by the latter, can't measure it making BL undetectable. small be its the same very x direction of B2 and is small value compared to it  By cancel diff be are in they directions of other protons On the other hand, the phase of precession of each Bp is random. Thus for a large number of protons in the sample, their xy-components point equally random in all possible directions on the xy-plane. That means every xy-component of a Bp is probably cancelled by another one this is anti-parallel. 3 Thus, there is no net magnetization on the xy-plane which is transverse to the external magnetic field Bz, i.e. no transverse magnetization.  So how do we measure the magnetization of a sample then? The action of an external magnetic field (Bz) + RF pulse When the frequency of the RF pulse, f is equal to the Larmor frequency FL of protons in the sample, a proton can absorb an RF photon and be excited from the low energy ‘spin up’ state to the high energy ‘spin down’ state. Op + introduce igup dan RF Spin > - low energy ΔE = h·f = h·FL high energy Gain highe every This process is called Resonant Absorption. It is very sensitive to the frequency of the RF pulse and only efficiently happens when the RF frequency matches the Larmor frequency. cexcitation of proton Lets look at a sample with H-1 net spin up protons. sample → Bp * 26  Spin up Bl = 4 , as we have just discussed.  all S the net become Spin dam Since they absorb Radioware from radior frequely (Oil From MRI As discussed in slide-20, since more protons are usually in the spin “up” state BL is normally pointing up. Absorption of RF photons will change the relative populations in the spin “up” & spin “down” states and thus effect the magnitude and direction of BL. up Spin dan) Spin Shown here is a 90° RF pulse which leads to the elimination of BL.  900 b BT is +o z & Be is on Xy plane - -t so mow By wor Cancel  #) in phase out of Phase BC is still lost" , as we have discussed. But with RF pulse, around This precessing transverse magnetic field, BT of the tissue is what we can measure from the tissue. The direction of this precessing BT is constantly changing. So how do we measure and quantify a changing magnetic field?  Coil + changing magnetic field = voltage Faraday-Lenz law of induction says that a voltage is induced in a circuit whenever relative Induced motion exists between a coil Voltage and a magnetic field. The magnitude of this voltage is B proportional to the rate of change of the magnetic field. + o magnet close, Induced faster notion-higher roltage e.g. Voltage Time – Far bicycle Alternating voltage, dynamo alternating current. We can make use of the principle of induction to directly measure the precessing transverse magnetic field BT. The frequency of the induced voltage is the same as the precession frequency of the BT.  ma (paitent) e S I Small # nation exects the body field Change of magnetic of tip Listen at 50 min z B+ Thus the RF pulse sor dan - Xyplme flip from Skin up to BL 1) changes (or even Bi - By plane suppresses) magnitude and direction of the longitudinal magnetization BL Reduction of BL x 2) induces a detectable y transverse magnetization BT z that precess about the z- axis at Larmour freq. production B OF BT Phase The RF pulse is only applied for a short amount of time. x Bc =- 0 , Brax y What happens when the RF pulse is switched off? I Relaxation to the proton After RF pulse is switched off: BL decays BT recovers Both processes are independent of each other. BT decays faster than BL recovers. ⑬2 direein X , yplane By and By are magnetic field of protons When the RF pulse is switched off, the spin-down protons de-excite back to the spin-up state and the Bp’s begin to lose phase (de-phase) Spin down > - Spinup loose + pruse Recovery of longitudinal magnetisation BL 63 % = T1 - O By was zero when RF is on  What does T1 tell us? When RE is off they loose Thermal to it · protons of fat loose thermal energy faster (faster +1) (Slower T) energy water surroundings · = RE- makes heat > protes gain thermal Energy  Decay of transverse magnetization BT curras a whe ecays  Spin-Spin Relaxation Decay of transverse magnetisation BT 37 % D takes for The time it its to 66 % of to recover in - time it takes By value pre-pulse & issues cuery neb o What does T2 tell us? Trentices - u. ] sit s - al of its > - make Decay of transverse magnetization BT  Complication by extrinsic contributions to T2 cat na Stage X Extrinsic effects make BT decay much faster. & interstic y t (inter The reason is that the gadimage B magnetic field is not exactly the same at the position of the protons. &Be image Slow jecat Learning outcomes Recall the principles of electrical induction and electromagnets. Describe the components and standard working procedure of standard NMR. Describe precession of proton spins in external magnetic fields and calculate their Larmor frequency. Differentiate between spin flips by resonant absorption of radio frequency (RF) photons or by spontaneous decay. Explain the different origins of the longitudinal and transverse magnetic fields. Explain spin-lattice relaxation T1 and spin-spin relaxation T2. Thank you F O R M O R E I N F O R M AT I O N P L E A S E C O N TA C T Dr Andy Ma EMAIL: [email protected]