MR Basic

SIEMENS Healthineers MRI Basics Textmasterformat, Disclaimer Unrestricted © Siemens Healthcare GmbH, 2017 Page 1 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers' Getting around………… • Background • Picture to proton • Resonance , excitation and relaxation. • Spatial encoding • MR Hardware Textmasterformat, Disclaimer Page 2 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Story behind……… SIEMENS Healthineers •For a Nobel laureate , it is a long road to Stockholm. •Magnetic resonance travelled a long road from probing water droplets and paraffin blocks to imaging bell peppers and human body, creating seven Nobel laureate s on the way. "Before every big "It's a different breakthrough, it is first a crazy idea." type of image that carries with it much Paul C. Lauterbur, more information Nobel laureate 2003 about the disease process." Sir Peter Mansfield, Nobel laureate 2003 Textmasterformat, Disclaimer Page 3 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Need a search ???????????? SIEMENS Healthineers GO GLE Simple mri physics Q All Videos Images News Maps More Settings Tools MRI About 1,55,00,000 results (0.81 seconds) Fundamental Physics of MR Imaging | RadioGraphics pubs.rsna.org/doi/full/10.1148/rg.254055027 by RA Pooley - 2005 - Cited by 154 - Related articles It is necessary to have a source of hydrogen protons (protons in the nuclei of hydrogen atoms, which are associated with fat and water molecules) in order to form our MR signal. The hydrogen proton is positively charged and spins about its axis (like a child's spinning top). MRI Physics Made Ridiculously Simple - YouTube MRI PHYSICS https://www.youtube.com/watch?v=rumRGO 2HOE MADE RIDICULOUSLY Jan 23, 2016 - Uploaded by Sarel Gaur MD SIMPLE An introduction to MRI physics. By #SARELGAURMD. #SAREL GAUI 4:57 Basic MRI Physics - YouTube https://www.youtube.com/watch?v=zf50X01bRgkv 6666 Nov 23, 2014 - Uploaded by ImagE Native i kinda wonder why anyone pays for classes when almost anything you want to > 1:04:00 learn is on the internet. my MRI Textmasterformat, Disclaimer Page 4 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Are you keen about it ????? O O @ 123RF* + + @123R @ 123RF + O 123RF @123RF @123RF Textmasterformat, Disclaimer Page 5 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Water, Hydrogen Atom and Proton Healthineers The water molecule consists of one oxygen atom and two hydrogen atoms • The hydrogen atom consists of one proton and one electron • The proton is the nucleus and provides the NMR signal Hydrogen Atom Lots of Water Molecules A Water Molecule Proton H O H Textmasterformat, Disclaimer Page 6 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Inside the magnet Healthineers Hydrogen protons posses magnetic Pre cessing e risemble of spins • moments . The body under the looking glass Typically, the magnetic field of an MR scanner runs parallel to the longitudinal axis of a patient positioned in the magnet bore. Let us take a closer look inside a volume element These are responsible for the week (voxel) of the body. • magnetization of the human body in Resulting magnetization side the magnetic field. External magnetic neid The magnitude of this • magnetization is proportional to the proton density. Textmasterformat, Disclaimer Page 7 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 The beauty of spin…. SIEMENS Healthineers •When exposed to magnetic field, the moment of a protons rotates like a spinning top. •This motion is called precession. Textmasterformat, Disclaimer Page 8 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Larmour frequency SIEMENS Healthineers •The precession frequency of the spins is called Larmour frequency. •Depends on the nucleus and the magnetic field strength. m O=yB (The constant factor y is known as the gyromagnetic ratio of the nuclei.) In the earth's magnetic field, spins precess relatively slowly at approximately 2,000 Hz (2 KHz). Textmasterformat, Disclaimer Page 9 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Concept of excess spins SIEMENS Healthineers •The no of spins pointing in the direction of external magnetic field , is slightly higher than against the field. H •MR imaging focus only on the excess spins. 100 000 000 000 000 000 000 •The no of excess spins Protons 600 000 000 000 000 ⋅Proportional to the proton density. - Excess spins ⋅Magnetic field strength. ⋅Temperature. Textmasterformat, Disclaimer Page 10 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Unaligned Protons Protons in B0 (no B0 ) Anti-parallel E+ 0000000 E 1T E- Protons in a magnetic-free In a magnetic field, however, the If B0 is increased to 1.5 Tesla, In this example, B0 is 1 Tesla Parallel the number of protons aligning and the difference in the E environment will have a random E level grows proportional with the magnetic field B0 orientation. E = 0 without a in the parallel direction levels causes one more proton magnetic field. increases! So, more protons are to align parallel than those in available for imaging and the the anti-parallel direction. signal-to-noise ratio increases. E+ E -0-000 1,5T E- Gives more signal ! Textmasterformat, Disclaimer Page 11 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 The spin packet of excess ……. SIEMENS Healthineers •While all the spins has the same precession frequency , they differ in directions. •The components transverse to the magnetic fields, cancel out each other. •So the net magnetization will build up only in the Z direction, and called longitudinal magnetization. Textmasterformat, Disclaimer Page 12 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Creating the resonance SIEMENS Healthineers Excitation Energy In •To generate an MR signal, the magnetization B has to be tilted to X-Y plane. M0 1 •This can be achieved by applying a short Electromagnetic pulse, called RF pulse. RF x y •The frequency of the applied RF pulse has to be matched the Larmour frequency of the B0 spins. ϖ ϖ = φ × B0 2οf = φ × B0 f = φ/2ο × B0 For the 1H nucleus f = 42,577 MHz for 1T Textmasterformat, Disclaimer Page 13 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Pulses and flip angle SIEMENS Healthineers ⋅The magnetization flip is proportional to the duration and energy of the RF pulse. ⋅The final tilt angle is called flip angle. 45° 90° 180° 180° x y x y x y x y Textmasterformat, Disclaimer Page 14 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Dividing the magnetization components SIEMENS Healthineers ⋅Longitudinal magnetization Mz is the portion of the vector in the Z direction , along the B0 z external magnetic field. MZ M ⋅Transverse magnetization Mxy is the component of vector in XY plane rotates about the external magnetic field. MXY x y Textmasterformat, Disclaimer Page 15 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Relaxation SIEMENS Healthineers ⋅After the longitudinal magnetization has been tilted to transverse plane, it immediately rotates in the Larmour frequency and induces an MR signal ( FID) ⋅Two seemliness phenomena, decay and recovery happens. S Transversal M Longitudinal Signal Magnetization T2 T1 Unrestricted © Siemens Healthcare GmbH, 2016 50 150 250 ms 500 1500 2500 ms Textmasterformat, Disclaimer Page 16 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 The Longitudinal recovery SIEMENS Healthineers ⋅The build up and recovery of longitudinal magnetization is called longitudinal relaxation or T1 relaxation ⋅After the time interval T1 , longitudinal magnetization has been recovered up to 63% of its initial value. ⋅The T1 constant depends 90° on the tissue types. Pulse M0 MZ 63% t T1 2×T1 3×T1 4×T1 5×T1 ms Textmasterformat, Disclaimer Page 17 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers ⋅T1 relaxation is energy given off to Fat White matter Gray matter other molecules (spin-lattice) and is 100% lost from the system. Protons are 6.3% returning to the lower energy level. 240 680 809 2500 tims] ⋅T1 relaxation is relatively slow, in CSF the order of 100s of milliseconds The T1 constant under the magnifying glass Different types of tissue show different relaxation times. This is key to the sharp image contrast obtained with MR. As the T1-Relaxation table shows, the T, constant also depends on the field as well. T1 constants (in ms) 0.2 tesla 1.0 tesla 1.5 tesla Fat 240 Muscle 370 730 863 White matter 388 680 783 Gray matter 492 809 917 Fat has a short Ty water has a long T1. CSF 1,400 2,500 3,000 Textmasterformat, Disclaimer Page 18 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Transverse decay SIEMENS Healthineers ⋅The transverse relaxation time T2 is the decay of the MXY magnetization vector as a result of the interaction between nuclei (Spin-Spin-Relaxation). ⋅T2 is the time after which the transverse magnetization is decayed to 37% of its starting amplitude MXY 37% t T2 Textmasterformat, Disclaimer Page 19 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers ⋅Where T2 relaxation only a de-phasing is Fat White matter of the individual proton magnetic 100% CSF moments due to spin-spin coupling, no 63% energy is lost within the system. Gray matter ⋅T2 relaxation is very fast, in the order of 10s of milliseconds 84 92 101 1400 2500 10ms] T2-Relaxation The T2 constant under the magnifying glass T2 is also tissue-specific, but largely field-independent. T2 constants (in ms) Fat 84 Muscle 47 White matter 92 Gray matter 101 Fat has a short T2, water has a long T2. CSF 1,400 Textmasterformat, Disclaimer Page 20 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 T2 * relaxation SIEMENS Healthineers ⋅In practice the T2 decay time is much shorter. ⋅This is mainly due to B0 field in MXY homogeneities which increases the e –t/T2 dephasing of the protons and thus accelerates the decay effect. FID t ⋅This decay effect is called T2* (tee two –t/T2* star). e Textmasterformat, Disclaimer Page 21 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Spin echoe SIEMENS Healthineers ⋅Recovery of T2* effects is possible through the application of an additional 180° RF pulse. This pulse rephases the protons and a SPIN ECHO results. MXY 90° e –t/T2 180° ⋅The time of the echo is called TE. The TE 180° refocusing pulse is exactly between FID T2 the 90° excitation pulse and the spin echo. T2* t FID e –t/T2* t Spin Echo Echo a result of 180° pulse Textmasterformat, Disclaimer Page 22 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Spin echoe SIEMENS Healthineers B1 Textmasterformat, Disclaimer Page 23 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Textmasterformat, Disclaimer Page 24 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Take home SIEMENS Healthineers Larmour frequency ω = γB RF pulse and FLIP FLIP Angle depends upon the energy Angle and duration of the RF pulse T1 relaxation Spin-lattice relaxation T2 relaxation Spin-spin relaxation Textmasterformat, Disclaimer Page 25 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Image contrast SIEMENS Healthineers In MRI the contrast is determined by: This presentation ƒProton density PD ƒT1 Relaxation time ƒT2 Relaxation time ƒFlow, Diffusion, Susceptibility ƒSequence parameters Textmasterformat, Disclaimer Page 26 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers For the first contrast, we will select a CSF relatively short TR of 500ms... and a short TE of 20ms. … Fat This gives us a T1 weighted image. T1 TR Fat TE MZ S 500. 20 1 , , e T1 White matter e T2 , Fat Gray matter white matter gray matter CSF CSF 500 1000 1500 2000 2500 3000 20 60 100 T1 curve T2 curve ms ms Textmasterformat, Disclaimer Page 27 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers The next contrast, a long TR of 3000ms... CSF and a short TE of … 20ms. Fat This gives us a PD T1 weighted image. PD TR Fat TE MZ S 500. 20 1 , , e T1 White matter e T2 , 2500 1 , e T1 , Gray matter CSF 500 1000 1500 2000 2500 3000 20 60 100 Unrestricted © Siemens Healthcare GmbH, 2017 T1 curve T2 curve Textmasterformat, Disclaimer Page 28 ms Magnetom Cognizance ms Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers The last contrast also uses a long TR ... CSF and a long TE of … 100ms. Fat This gives us a T2 T1 PD weighted image. T2 Fat TR TE MZ S 100 White matter , e T2 2500 1 , e T1 , Gray matter CSF 500 1000 1500 2000 2500 3000 20 60 100 T1 curve T2 curve ms ms Textmasterformat, Disclaimer Page 29 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Textmasterformat, Disclaimer Page 30 Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Take home SIEMENS Healthineers Short TR – Short TE Long TR – Short TE Long TR – Long TE Textmasterformat, Disclaimer Page 31 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 K space – the storage cart…….. SIEMENS Healthineers ⋅K space is raw data space which is used to store the digitized MR signals before the transformation. ⋅When it is full the data can be used for --Using a Fourier (inverse) transform we convert K-space into an image. reconstruction. ⋅Reconstruction is carried out by a mathematical algorithm called Fourier transformation. Textmasterformat, Disclaimer Page 32 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Spatial encoding SIEMENS Healthineers ⋅Spatial encoding is the technique used to have the question answered: WHERE are the protons. ⋅Spatial encoding of the MR signal is achieved by applying linear magnetic field variations known as gradients. ⋅Gradients are applied in all three spatial directions (X, Y, Z) and at different times during the measurement process ⋅The amplitude of the gradients help determine the field of view (FoV) Textmasterformat, Disclaimer Page 33 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Gradient coils SIEMENS Healthineers B ⋅The gradient fields are generated by three pairs B0 Z of gradient coils, one pair for each physical axis X, Y and Z. ⋅These coils are wound to produce linear field changes in each of the three spatial directions. ⋅A Gradient Amplifier provides the current to the coils. Gradient Coil Y B0 X Textmasterformat, Disclaimer Page 34 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Slice selection gradient SIEMENS Healthineers ⋅The first step in spatial encoding is to excite protons within the desired slice region. f ⋅A slice is defined by its position and thickness GS Χ f ⋅Slice excitation is achieved by applying a Z magnetic field gradient along the slice axis and a selective RF pulse, i.e. having a K -- limited bandwidth Χf. ΧZ Textmasterformat, Disclaimer Page 35 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Slice Thickness = 10 mm Position Slice 1 = -30mm Position Slice 2 = 0mm slice 1 slice 2 slice 3 Position Slice 3 = +30mm B f 2.4 mT m B0 f0 slice selection gradient GS 1kHz RF spectrum -30 -20 -10 0 +10 +20 +30 X,Y,Z in mm Textmasterformat, Disclaimer Page 36 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Slice Thickness = 4 mm Position Slice 1 = -30mm Position Slice 2 = 0mm slice 1 Slice 2 slice 3 Position Slice 3 = +30mm 6 mT B m f B0 f0 slice selection gradient GS 1kHz RF spectrum -30 -20 -10 0 +10 +20 +30 X,Y,Z in mm Textmasterformat, Disclaimer Page 37 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Frequency encoding SIEMENS Healthineers ⋅The frequency encoding gradient GR is Z applied along the X axis in this example. GR ⋅The resonant frequency changes linearly along the axis of the applied gradient. ⋅The frequency encoding gradient is active during the period where the MR Y signal is read out, and thus this gradient is also called the readout gradient GR < f0 f 0 > f0 . Textmasterformat, Disclaimer Page 38 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Phase encoding gradient SIEMENS Healthineers ⋅Phase encoding gradient super imposes a different phase on spins from different GP Z Phase locations ⋅This gradient switches briefly between slice selection and frequency encoding gradients. ⋅The number of phase encoding steps is dependant on the Matrix in the phase encoding direction (e.g., 128, 256, 512 or in between) ⋅The amplitude of this gradient is changed Y 1 2 3 4 5 6 7 linearly between each scan line (Fourier line). Textmasterformat, Disclaimer Page 39 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Precess: Phase Encode: Phase gradient Off: Frequency Encode: Textmasterformat, Disclaimer Page 40 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Gradient directions and k space SIEMENS Healthineers ⋅Each axis of K space is represented by PE Signal and FE matrix correspondingly. KPE ⋅Reflects the corresponding gradients along KPEN these directions. Pseudo- ⋅For each TR a single line of K spaces is time KEE filled. ⋅So different lines are filled with different values of phase encoding gradient. > KFE TR Time Textmasterformat, Disclaimer Page 41 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers FOV K PE AKPE KFE K-space Image space * (a) AKFE Textmasterformat, Disclaimer Page 42 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Textmasterformat, Disclaimer Page 43 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Take home SIEMENS Healthineers Gradient coils in the system X , Y and Z gradients Different types of Slice selection gradient Frequency encoding Phase encoding Textmasterformat, Disclaimer Page 44 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Lets us talk technical…………… Textmasterformat, Disclaimer Page 45 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Components of MRI SIEMENS Healthineers ⋅Magnet ⋅Gradient system ⋅RF transmitter system ⋅Signal receiver system ⋅Computer system ⋅Electronics Textmasterformat, Disclaimer Page 46 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Basic Components of MRI SIEMENS Healthineers RF shield Patient support Magnetic shielding Magnet x-Gradient T/R RF Transmitter y-Gradient Switch RF Receiver z-Gradient Gradient RF coil coils Computer system Pulse programmer Textmasterformat, Disclaimer Page 47 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Magnet SIEMENS Healthineers' Helium fill and cold head ⋅The magnet is the main component of the MR Casing system. Outer vacuum Outer cold shield shield ⋅Due to design constraints the static magnetic field Helium vessel is inherently non-uniform and its homogeneity is Shim optimized by a process known as shimming whereby coils pieces of steel and/or electrical coils are incorporated into the magnet to improve the Main windings Inner uniformity over a given volume. cold shield Textmasterformat, Disclaimer Page 48 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Types of Magnet SIEMENS Healthineers SEMENS windings MAGNETOM C! B windings 77777777717717771777 Textmasterformat, Disclaimer Page 49 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Properties of main magnetic field SIEMENS Healthineers Field strength Homogeneity ⋅Magnetic field strength (or flux density) is ⋅The homogeneity of a magnetic field is measured in tesla (T). usually expressed in parts per million ⋅Whole-body magnets have been within a given spherical volume. constructed with field strengths of 0.02–7 T. ⋅The size of this volume is given as the ⋅Typical clinical systems Operate in the diameter of a spherical volume (DSV). range of 0.2–3.0 T. Textmasterformat, Disclaimer Page 50 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers 3.0 4 1 mT 2.0 + 5 mT 40 mT 20 mT 1.0 100 mT 200 mT 4.0 3.0 2.0 1.0 1.0 2.0 3.0 4.0 Magnet axis (m) 1.0 2.0 + 3.0 V Figure 9.20 Fringe field plots for a 1.5 T system. Figure courtesy of Magnex Research Systems Ltd. Textmasterformat, Disclaimer Page 51 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Shimming SIEMENS Healthineers Fixed shimming Dynamic shimming ⋅Passive shimming involves the ⋅Dynamic shimming may be performed by positioning of pieces of steel around the the user to optimize the homogeneity over a magnet, with the amount and position of given volume on a per patient basis. the steel usually determined by a computer ⋅Simple dynamic shimming involves the program. use of the gradient coils to produce the ⋅Active shimming involves the use of necessary static magnetic fields to optimize additional coils in which currents of the uniformity. accurately determined magnitude are running. ⋅The required gradient magnetic fields are calculated either manually or automatically. Textmasterformat, Disclaimer Page 52 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Gradient coils system SIEMENS Healthineers' MRI Scanner Gradient Magnets ⋅Gradient coils mounted on a cylindrical Y Coil former just inside the bore of the magnet. Z Coil ⋅Although the gradients are oriented in the three orthogonal directions, the gradient X Coil magnetic fields themselves are parallel to the main magnetic fieldB0. ⋅ The null point at the centre of the gradient coils, and also the centre of the magnet, is called the isocentre. Transceiver Patient Textmasterformat, Disclaimer Page 53 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Bo Bo ............ .... ....... ...... GZ Gy ... Figure 9.7 Golay coil configuration for transverse (x and y) gradient fields. Only the blue arcs contribute usefully to Figure 9.6 Maxwell pair configuration for a longitudinal (z) forming the gradient, which is linear only for a fraction of gradient field. the length of the coil system. Textmasterformat, Disclaimer Page 54 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Radiofrequency system SIEMENS Healthineers The radiofrequency (RF) system comprises a ⋅transmitter ⋅coil ⋅ receiver. Textmasterformat, Disclaimer Page 55 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Transmitter SIEMENS Healthineers ⋅The transmitter has to generate RF pulses with appropriate centre frequencies, bandwidths, amplitudes and phases in order to excite nuclei within the desired slices. ⋅The slice position and the strength of the slice select gradient at that location determine the centre frequency of the pulse. ⋅The bandwidth, or the range of frequencies within the pulse, controls the thickness of the excited slice. ⋅The shape and duration of the RF pulse envelope determines the bandwidth. ⋅The amplitude of the RF pulse controls how much the magnetization is flipped by the pulse Textmasterformat, Disclaimer Page 56 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers TRANSMITTER Amplitude RF Profile Multiplier DAC Mixer Filter power Offset amplifier Frequency Frequency Phase Oscillator DAC Mixer Coil Analogue signals Fixed Digital signals RF Osc. Textmasterformat, Disclaimer Page 57 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Transmit coil SIEMENS Healthineers ⋅The coils used to excite the MR signal must produce a uniform field B1 at right angles to the static magnetic field. Transmit coils are usually large to optimize their uniformity, and encompass a significant volume of tissue. ⋅A transmit coil may also be used to detect or receive MR signals, provided an appropriate transmit/receive ( T/R) switch is used. ⋅The main transmitting coil is usually the body coil, which surrounds the entire patient. This is usually built into the scanner bore and is not generally visible. ⋅Since this coil is large it has a very uniform transmission field. ⋅In some systems other coils, e.g. head or knee, may also be used for transmission Textmasterformat, Disclaimer Page 58 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Receiver SIEMENS Healthineers ⋅The function of a receiver coil (or simply Volume coils ‘coil’) is to maximize signal detection, whilst minimizing the noise. ⋅Completely encompass the anatomy of interest and are often combined ⋅Usually the major source of noise is from transmit/receive coils. the patient’s tissue (from the Brownian motion of electrolytes). Surface coils ⋅ To minimize the noise, and maximize the ⋅Generally receive only, due to their SNR, it is necessary to minimize the coil inhomogeneous reception field. dimensions, i.e. the coil’s volume should be ⋅As their name suggests, surface coils are filled as much as possible by the sample. good for detecting signal near the surface of the patient. Textmasterformat, Disclaimer Page 59 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Receiver SIEMENS Healthineers RECEIVER DSP DSP 500kHz LPF Mixer Gain/ Pre- Filter Quad. ADC Atten. H amplifier DSP DSP Filter Quad. Analogue signals Digital signals Textmasterformat, Disclaimer Page 60 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Components of receiving side SIEMENS Healthineers Preamplifier ⋅Receiver Prescan ⋅The small MR signal detected ⋅The MR signal only contains ⋅Usually each imaging by the coil needs to be boosted a narrow frequency range of sequence is preceded by a by an extremely low-noise pre- interest, but is embedded in or prescan or preparation phase, amplifier before being fed to carried by the Larmor in which the characteristics of the receiver. frequency. the transmitter and receiver ⋅Pre-amplifiers are often ⋅The function of the receiver is are optimized. mounted on the coil assembly to retrieve or demodulate this to avoid degradation of he signal, i.e. eliminate the high- signal by the leads. frequency carrier. ⋅ In all modern MR systems this is performed digitally. Textmasterformat, Disclaimer Page 61 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Computer system SIEMENS Healthineers ⋅The multi-tasking nature of MRI demands a Patient Applications Transfer Edit View Image Tools Scroll Evaluation System Options Help host computer, and its subsystems which >DEMO_MR-PET_Munich MR H -3/25/1937, M, 73Y /1DBD51EDCB2D01/A9AEF8/ Mondrian STUDY 1 MR B18 >DEMO_MR-PET_J Exam 10/26/2010 HFS :11:13:51 AM 1012 IMA 3 / 1 have their own microprocessors whose Viewing Cor commands can be downloaded from the host. ⋅ A typical MRI system will have a host computer on which the operator will prescribe A ME 1:00 the scan in terms of the pulse sequence, its SIEMENS timing and various geometry factors, etc. SIEMENS - est for like. Textmasterformat, Disclaimer Page 62 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers ⋅These parameters will then be converted into commands that are transferred to another microprocessor system, known as the pulse programmer (PP), that controls the hardware. The PP ensures that the RF, gradients and data acquisition are all properly synchronized. TH1755 FO08 24-24 77 22/07:36 11.THE F51 15000 ⋅Once the data have been acquired, a separate 2031-06-24 341244 31 computer system known as the array processor carries out the image reconstruction. The host computer then manages the image display, processing, for example, windowing, hardcopy production, archiving and networking. Textmasterformat, Disclaimer Page 63 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Textmasterformat, Disclaimer Page 64 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 Take home SIEMENS Healthineers Permanent, Resistive and Magnet superconducting. Magnet strength and Homogeneity Slice , frequency and phase encoding Gradient system gradients Controlled by gradient amplifiers RF system Transmission of RF pulses to the body Receiver system Receives the signal and send to reconstruction step Textmasterformat, Disclaimer Page 65 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018 SIEMENS Healthineers Many thanks : South MR Application Team Thank you for your attention Engineering success. Pioneering healthcare. Textmasterformat, Disclaimer Page 66 Unrestricted © Siemens Healthcare GmbH, 2017 Magnetom Cognizance Author / Department HC XX HOOD05162002737284 Effective date: 01/24/2018