Outline 4.1 Principles of MRI uclei, Excitation, Relaxation Carolyn Kaut Roth, RT (R)(MR)(CT)(M)(CV) FSMRT CEO Imaging Education Associates www.imaginged.com candi@imaginged.com What nuclei are MR active? - Hydrogen (fat & ) - Other uclei Why are they MR active? - Mass umber How do they behave in the magnet? Excitation - RF excitation Radiofrequency Pulses - Larmor Frequency Relaxation -T1 -T2 Slide # 2 Objectives Upon completion of this course, the attendee should 1. Learn the various nuclei that are MR active 2. Understand why certain nuclei are MR active 3. Realize how nuclei behave in the presence of the magnetic field. 4. Understand Excitation 5. Understand Relaxation 6. Learn T1 & T2 weighted imaging What Do We Image? What do we image with CT? Soft tissues? nes? What do we image with MRI? Soft tissues? nes? Axial CT T1 PD T2 Slide # 3 Slide # 4 What s in an Atom? The ucleus, What Counts? }ucleus e- Electron Electron shell Atoms have a nucleus Protons ( + positive charge) eutrons (neutral) Orbiting the nucleus Electrons ( - negative charge) e- }ucleus e- Electron Electron shell Mass umber umber of Protons Plus (+) umber of eutrons Atomic umber umber of Protons Slide # 5 Slide # 6 1
What Elements are MR Active? Periodic Table Elements Unique Atomic Structure Odd Mass umber Hydrogen Phosphorous Others? Proton Imaging ucleus e- Electron Electron shell Mass umber umber of Protons Plus (+) umber of eutrons = 1 Atomic umber umber of Protons = 1 Slide # 7 Slide # 8 Hydrogen Imaging Why do fat & appear differently? T1 Image is dark is bright PD Image / bright fat / bright The human body is roughly 75% is H20 Hydrogen in H 2 0 Hydrogen in CH 3 T2 Image /bright fat / darker Sagittal Cervical & Thoracic Spine T1 Image T2 Image is bright is bright is dark is dark Hydrogen in O H H Molecule H 2 O Hydrogen in C H H H Molecule CH 3 Slide # 9 Slide # 10 Before there was imaging Spectroscopy Suppression of fat and/or Each chemical has a different Frequency like (fat & ) The location of the peak tells what chemical The area under the peak tells how much of that chemical The difference in frequency is known as Chemical Shift MR Spectrum displays Chemical Shift The study of the spectrum is known as Spectroscopy CH3 chemical shift MR Spectrum H20 Sagittal T2 breast unsuppressed Silicone Implant CH3 chemical shift = 3.5 ppm MR Spectrum H20 3.5 ppm @ = 220 H @ 3.0T = 440 Hz @ 0.3T = 73 Hz Slide # 11 Slide # 12 2
Suppression of fat & Silicone Suppression Silicone Silicone and suppressed Silicone appears bright Silicone Implant MR Spectrum H20 Silicone suppressed Silicone appears dark Silicone Implant MR Spectrum H20 CH3 CH3 Sagittal T2 breast unsuppressed chemical shift 100 hz @ chemical shift 224 hz @ Sagittal T1 breast unsuppressed chemical shift 100 hz @ chemical shift 224 hz @ Slide # 13 Slide # 14 Spectrocopy for Therapy Monitoring Choline / Citrate Imaging 1 week pre T Day 1 Day 42 Day 70 Choline Choline Choline Choline Choline/Citrate Image Superimposed on H image pre Brachytherapy Choline / Citrate Hydrogen Imaging (shown in black & white) Choline / Citrate Imaging (shown in red) Increased in prostate cancer Reduced after treatment with Brachy-therapy Brachy-therapy also known as radium seeds Choline/Citrate Image Superimposed on H image post Brachytherapy Slide # 15 Slide # 16 Hydrogen for MR Imaging Tiny Proton Magnets ucleus e- Moving charged particles, like positively charged protons, make magnetic fields known as the magnetic moment (μ). Magnetic moment behaves like a tiny bar magnet S S Electron Atom Proton Slide # 17 Slide # 18 3
The Magnetic Moment Vector Magnetic Moment Bipolar magnets Magnetic moments Bar magnets Two Poles orth pole South pole Magnetic field lines run from the south pole to the north pole S The magnetic moment is represented by a vector Vector represents magnetic moment (μ). Magnetic field lines Slide # 19 Slide # 20 Review Vectors Vector Addition The vector has two properties Magnitude the length of the vector Direction the direction to which it points The vector can be added to another vector To add vectors Take the tail of Vector #1 Place near the nose of Vector #2 The Vector sum is between Vector #1 Vector #2 Vector sum Vector #1 Vector #2 Vector Vectors Vector addition Slide # 21 Slide # 22 Magnet to Magnet What happens when two magnets are together Opposite Magnets (poles) attract Like magnets (poles) repel S S S S Slide # 23 S S S S Outline What nuclei are MR active? - Hydrogen (fat & ) - Other uclei Why are they MR active? - Mass umber How do they behave in the magnet? Excitation - RF excitation Radiofrequency Pulses - Larmor Frequency Relaxation -T1 -T2 Slide # 24 4
Magnets in a Magnetic Field Direction of the Main Magnetic Field SS Opposite Magnets (poles) attract Like Magnets (poles) Repel Direction of the magnetic field When the patient is in the MR imager Some H protons attract to the magnetic field - Align with - Low energy Some H protons repel the magnetic field direction -Oppose -High energy SS Image courtesy of Hitachi Medical SS Slide # 25 Slide # 26 Protons in a Magnetic Field Classical Method There are roughly 500,000 protons in a drop of When the patient is placed within the magnetic field, protons either align or oppose SS Protons in the magnetic field Low energy -Attract - Align High energy - Repel - Oppose SS Slide # 27 Slide # 28 Precession Cartesian Coordinate System precessional path spin Remember, protons are moving (spinning) charged particles - Known as spins Protons, align at an angle to the magnetic field - The angle is 37 Because protons spin, on an angle, they begin to wobble or precess -Wobble or precess like a spinning top -Precess at a specific rate or frequency known as the Precessional Frequency or Larmor Frequency -Precess along a path known as the precessional path SS precessional path axis axis Low energy Parallel Spin Up axis High energy Anti-parallel Spin Down Slide # 29 Slide # 30 5
Protons to Vectors Thermal Equilibrium axis axis axis Protons are replaced by vectors on the Cartesian coordinates Immediately after the patient is placed within the magnetic field, there are an even number of spins in the high & low energy states After a few seconds, there are more spins in the low energy state. This condition is known as thermal equilibrium Slide # 31 Slide # 32 Vector Sum et Magnetization () In this case, vector sum is zero For example vector #1 + #6 cancel Vector #1 In this case, vector sum is on ero For example vector #1 + #6 cancel and Vectors #3 + #5 Vector #1 cancel Vector #3 Vector #1 Vector #2 Vectors # 3 + #4 cancel Vector #1 Vector #2 Vector #3 Vector #3 Vector #3 Vector #4 Vectors # 2 + #4 Add to form the et Magnetization () Vector #4 Vector #4 Vector #4 Vectors #2 + #5 cancel Vector #2 Vector #2 The net magnetization is responsible for MR images Vector sum et magnetization magnetization along the axis Slide # 33 Slide # 34 and Field Strength Image 3.0T Image Vector #1 Vector #2 Vector #3 Vector #1 Vector #2 Vector #3 Vector #4 Vector #4 As field strength increases, more spins in line, greater net magnetization, higher image signal. Slide # 35 Outline What nuclei are MR active? - Hydrogen (fat & ) - Other uclei Why are they MR active? - Mass umber How do they behave in the magnet? Excitation - RF excitation Radiofrequency Pulses - Larmor Frequency Relaxation -T1 -T2 Slide # 36 6
How is excitation achieved? Precession Alignment Protons in the magnetic field Thermal equilibrium RF Pulse Larmor Frequency Resonance Remember, protons are moving (spinning) charged particles, Known as spins Protons, align at an angle to the magnetic field Because protons spin, on an angle, they begin to wobble or precess -Wobble or precess like a spinning top -Precess at a specific rate, or frequency known as the Precessional Frequency or the Larmor Frequency This frequency describes the energy that keeps the spins in thermal equilibrium It is this energy that can knock the spins from thermal equilibrium (excite the spins) precessional path spin Slide # 37 Slide # 38 Wobbling Top & Precession Precessional frequency wobbling path precessional path We can determine the energy required to excite the spins In order to calculate this energy we need several components The magnetic moment of the proton The spin angular momentum of the proton The field strength of the magnet Precessional Frequency spin spin This frequency describes the energy that keep the spins in thermal equilibrium It is this energy that can knock the spins from thermal equilibrium spin A Top wobbles because of A proton precessses (in the magnet) because of The weight of the top The magnetic moment of the proton The rate of spin (how fast it spins) The rate of spin (spin angular momentum) The gravity of the earth The magnetic field strength Slide # 39 Slide # 40 Larmor Frequency Units of Measure for Frequency In order to calculate the precessional (Larmor) frequency The magnetic moment of the proton The spin angular momentum of the proton Gyro-magnetic ratio = γ Magneto-gyric ratio = γ The field strength of the magnet Magnetic field strength = This is known as the Larmor equation Larmor or Precessional Frequency = ωο ωo = ΒO γ Magnetic field strength = Βο Gyro-magnetic ratio = γ Magneto-gyric ratio = γ Precessional frequency spin Precessional Path One cycle The Larmor Equation calculates the precessional frequency Precessional Frequency or Larmor frequency - The rate at which the spins wobble, or precess Wobble or precess in cycles per second -One cycle is once around the processional path -One cycle is one sine wave -One cycle per second = I Hertz (Hz) - MHz, megahertz = 1,000,000 cycles per second Slide # 41 Slide # 42 7
Larmor Equation Gyromagnetic Ratio A trick to remember the Larmor Equation Whoa y! One can imagine that a proton wobbles pretty rapidly, hence whoa! The gyro-magnetic ratio is constant for each chemical. ωo Frequency = ωο = ΒO γ Magnetic field strength = Βο Gyro-magnetic ratio = γ Magneto-gyric ratio = γ ωo = ΒOγ Here s the actual equation Gyromagnetic ratio (γ) or the Magneto-gyric ratio (γ) for several chemicals 1H (hydrogen) 42.6 MHz/T 19F (fluorine) 40.1 MHz/T 31P (phosphorous) 17.2 MHz/T Slide # 43 Slide # 44 Calculating the Larmor Frequency ωo γ for 1H (hydrogen) = 42.6 MHz/T If the Field strength () is 1.0 Tesla Then ωο = (1.0T) x (42.6 MHz/T) ωο = 42.6 MHz (Megahertz) = ΒO γ γ for 1H (hydrogen) = 42.6 MHz/T If the Field strength () is 1.5 Tesla Then ωο = () x (42.6 MHz/T) ωο = 63.9 MHz (Megahertz) Radiofrequency Energy? Do we use radiation in MR? Electromagnetic spectrum -rays High energy Ionizing radiation MR Radiofrequency Low energy on-ionizing At the frequency is roughly 64 MHz. In most cities, channel 3 broadcasts at roughly 64 Mhz. Ionizing 10 22 Radiation 10 20 Gamma rays 10 18 rays 10 16 Visible light 10 14 10 12 Microwave 10 10 Cell phone 10 8 10 6 Computer Monitor 10 4 10 2 Radiowaves 10 0 Direct Current Hz Slide # 45 Slide # 46 Resonance RF Transmitter Configurations Once the Larmor frequency is calculated Spins can be excited by the radiofrequency pulse at the Larmor frequency If the RF energy matches the precessional frequency of the spins Resonance is achieved RF energy B1 RF Transmitters Slide # 47 Slide # 48 8
Excitation et Magnetization () Slide # 49 RF Mxy As the result of the RF pulse et magnetization moves from to Mxy Spins achieve phase coherence Some low energy spins - absorb energy - enter the high energy state Vector #1 Vector #2 Vector #3 Vector #4 In this case, vector sum is on ero For example vector #1 + #6 cancel and Vectors #3 + #5 cancel Vector #2 The net magnetization is responsible for MR images Vectors # 2 + #4 Add to form the et Magnetization () Vector #1 Slide # 50 Vector #4 Vector #3 Vector sum et magnetization magnetization along the axis Vector Sum Excitation RF Vector #3 Vector #4 RF Mxy Vector #2 Vector #1 Mxy Vector sum et magnetization Mxy magnetization along the plane As the result of the RF pulse et magnetization moves from to Mxy Spins achieve phase coherance Some low energy spins - absorb energy - enter the high energy state Slide # 51 Slide # 52 Image Contrast Parameters T1WI Short TR Short TE Bright fat PDWI Long TR Short TE Bright fat & Slide # 53 T2WI Long TR Long TE Bright Outline What nuclei are MR active? - Hydrogen (fat & ) - Other uclei Why are they MR active? - Mass umber How do they behave in the magnet? Excitation - RF excitation Radiofrequency Pulses - Larmor Frequency Relaxation - Signal Induction -T1 -T2 Slide # 54 9
RF Receiver Configurations Faraday s Law of Induction Drag a magnet across a conductor, a voltage is created (induced) within the conductor RF Receiver coil MR Signal Spine coil, linear array TMJ coils (3 round) 5 round linear coil Chest coil, volume array db / dt = dv Change of magnet divided by time = voltage ΔB / Δt = ΔV Slide # 55 Slide # 56 Fourier Transformation Converting MR Signal prism Ft White light Light spectrum coil Time domain Spectrum Frequency domain Ft Free Induction Decay () MR Spectrum Slide # 57 Slide # 58 Chemical shift Imaging and Spectroscopy Each chemical has a different Frequency like (fat & ) Parts per million (PPM) / 3.5 ppm @ = 224 Hz Varies with field strength CH3 H20 chemical shift MR Spectrum Slide # 59 Slide # 60 10
Excitation Review Relaxation RF RF Mxy Mxy As the result of the RF pulse et magnetization moves from to Mxy Spins achieve phase coherance all get together Some low energy spins - absorb energy - enter the high energy state some get high Slide # 61 As the result of the RF pulse Relaxation et magnetization moves from to Mxy get out of phase- get apart T2 Spins achieve phase coherance all get together return to longitudingl axis- some get low T1 Some low energy spins - absorb energy - enter the high energy state some get high Slide # 62 Relaxation T2* Decay T2* & T2 Decay RF pulse T2* decay T2* T2 decay Equation for T2* T2 + T2 = T2* Mxy coil Axial T2* Brain Image echo In phase Partially dephased Completely dephased Mx,y = transverse magentization Axial T2* Brain Axial T2 Brain Slide # 63 Slide # 64 Is a susceptibility artifact always a bad thing?? Spin Echo Imaging TR 180 0 T2* T2 decay Timing diagram 90 0 echo 90 0 echo TE 180' RF pulse Axial T2* Brain Axial T2 Brain Axial GE abdomen Image Axial SE abdomen Image Slide # 65 Slide # 66 11
Runners on the Race Runners on the Race Spin Echo Start 90 0 180 0 I m on your heels I m the fast guy 90 0 180 0 Start Echo Thought I was winning Gotcha! Inhomogenieties Phase #1 start together and get apart Runners turn 180 Slide # 67 Phase #2 after the 180 Phase #3 cross Turn around apart Phase #4 starting line, together get apart Slide again# 68 Runners on the Race Gradient Echo Relaxation 45 0 RF Start I m on your heels I m the fast guy Start Mxy Inhomogenieties Phase #1 start together and get apart Phase #2 runners change places Phase #3 cross finish line, together Slide # 69 Phase #4 get apart again As the result of the RF pulse et magnetization moves from to Mxy Spins achieve phase coherance all get together Some low energy spins - absorb energy - enter the high energy state some get high Slide # 70 Relaxation get out of phase- get apart T2 return to longitudingl axis- some get low T1 T2 Relaxation T2 Decay T2 decay Transverse Spin spin Exponential decay Decays in ½ lives in 1 T2 time 63% decay 37% remains In 2 T2 times 81% In 3 T2 times 90% In 4 T2 times 95% In 5 T2 times 98% Slide # 71 Slide # 72 12
T2 Decay & Image Contrast 2 for 1 - Dual Echo Imaging (Multi Echo) 180 0 180 0 T2 times = 50 ms = 200 ms 90 0 Proton density-te1 T2WI-TE2 Less T2 weighted more T2 weighted T2 decay T2 decay H20 CH3 echo echo echo TE 1 TE 2 Slide # 73 Slide # 74 T1 Relaxation T1 Recovery CH3 T1 recovery Spin lattice Longitudinal H20 Exponential recovery Recovers in ½ lives in 1 T1 time 63% recovery 37% remains In 2 T1 times 81% In 3 T1 times 90% In 4 T1 times 95% In 5 T1 times 98% Mx,y Slide # 75 Slide # 76 T1 Recovery and Image Contrast Short & Long TR Imaging T1 times = 150 ms = 2000 ms 90 0 1800 Short TR CH3 more T1 weighted H20 less T1 weighted 90 0 1800 Long TR Mx,y Slide # 77 Slide # 78 13
A Few Fun Facts about T1 & T2 A Few Fun Facts about TR & TE We cannot change. T1 recovery T2 decay unless we change Field strength Temperature or Add contrast agents! We can change TR & TE And TR goes with T1 TE goes with T2 Slide # 79 Slide # 80 A Few Fun Facts about T1 A Few Fun Facts about T2 T1 times at 2000 ms for 150 ms for fat T2 times at 200 ms for 50 ms for fat Slide # 81 Slide # 82 A Few Fun Facts about Image Contrast Let s Make a T1 Image We cannot change. T1 recovery T2 decay unless we change Field strength Temperature or Add contrast agents! T1 times at 2000 ms for 150 ms for fat T1WI Short TR (500 ms) Short TE (20 ms) Bright fat T1 times at 2000 ms for 150 ms for fat We can change TR & TE And TR goes with T1 TE goes with T2 T2 times at 200 ms for 50 ms for fat We can change TR & TE And TR goes with T1 TE goes with T2 T2 times at 200 ms for 50 ms for fat Slide # 83 Slide # 84 14
Let s Make a T2 Image Let s Make a PD Image T2WI Long TR (4000 ms) Long TE (100 ms) Bright T1 times at 2000 ms for 150 ms for fat PDWI Long TR (4000 ms) Short TE (20 ms) Bright fat & T1 times at 2000 ms for 150 ms for fat We can change TR & TE And TR goes with T1 TE goes with T2 T2 times at 200 ms for 50 ms for fat We can change TR & TE And TR goes with T1 TE goes with T2 T2 times at 200 ms for 50 ms for fat Slide # 85 Slide # 86 Image Contrast Parameters What is a Pulse Sequence? T1WI Short TR Short TE Bright fat, short T1 time PDWI Long TR Short TE Bright fat & T2WI Long TR Long TE Bright, long T2 time Spin echo family Longer Scan times Better quality Gradient echo family Faster Scan times lower quality T1Weighted Image SE (TSE) FSE IR Fast IR (T1 FFE) GrE spoiled TOF MRA Enhanced MRA PD Weighted Image SE (TSE) FSE FLAIR Fast FLAIR Looks like PD (PD FFE) GrE EPI Flair T2 Weighted Image SE FSE STIR Fast STIR Looks like T2 T2* Weighted Image (T2* FFE) GrE PC MRA EPI Perfusion Diffusion Slide # 87 Slide # 88 Outline What nuclei are MR active? - Hydrogen (fat & ) - Other uclei Why are they MR active? - Mass umber How do they behave in the magnet? Excitation - RF excitation Radiofrequency Pulses - Larmor Frequency Relaxation -T1 -T2 4.1 Principles of MRI uclei, Excitation, Relaxation Thank you for your attention! Click to take your post test and get your credits Carolyn Kaut Roth, RT (R)(MR)(CT)(M)(CV) FSMRT CEO Imaging Education Associates www.imaginged.com candi@imaginged.com Slide # 89 15