A Concise Introduction toMRI PhysicsAnthony Wolbarst, Nathan Yanasak, R. Jason Stafford
Outline for Today1.2.3.Introduction to MRI‘Quantum’ NMR and MRI in 0DMagnetization, m(x,t), in a VoxelProton Density MRI in 1DT1 Spin-Relaxation in a VoxelMRI Case Study, and CaveatSketch of the MRI Device‘Classical’ NMR in a VoxelFree Induction Decay in 1DT2 Spin-RelaxationSpin-Echo Reconstruction in 1DTissue Contrast-Weighting in SESpin-Echo / Spin-Warp in 2D
Introduction to MRI
Soft Tissue Contrast: CT vs. MRIposterior reversible encephalopathy syndrome (PRES): edematous changesCTMRI
Magnetic Resonance ImagingYields distinct spatial maps of anatomy, physiology, andpathology of soft tissues.Multiple unique types of contrast ‒ created through,and informing on, subtle aspects of tissue biophysics.No ionizing radiation.Risks from intense magnetic fields, RF power.Expensive.Technology complex, challenging to learn.
Newtonian Contrast among ApplesDifferent biophysical sources, sensorsColorTextureSmellTasteHolesetc.
Medical Imaging ContrastBeam of probes:particles, waves, Differentialtransmission, reflection,emission, etc., of probesImage receptorC
ionizingvλEImagingmodalityPETX-, ldOpticalIRESRIB-fieldMRI(US)8
Operation of Imaging ModalitiesModalityProbe / SignalPlanar R/F X-ray X-rays transitingCTthe bodyDetectorSource of Contrast:Differences in AMFPI, II CCD,GdO, etc., arrayµ( ρ, Z, kVp) dssNuc Med,SPECT,PETGamma-rays,511 keVemittedNaI single crystal;multiple NaI;LSO arrayRadiopharmaceuticaluptake, concentration99mTc, 18FDG, USMHz sound,reflectedPiezoelectrictransducerρ, κ, µUSMRIProtons, photonsprobe mol. mag.environmentsRF radio receivercoilsT1, T2, PD, [O], bloodflow, water diffusion,chemical shift,
Standard Breast ModalitiesMammographyUltrasoundPETMRI10
MRI:Mapping the Spatial Distributionsof Spin-Relaxation Rates, etc,of Hydrogen Nuclei in Strong Bin Soft-Tissue Water and Lipids
Put Another Way Soft-Tissue Water and LipidsHydrogen Nuclei in B0Spin-Relaxation Rates, etc.,NMRSpatial DistributionsMRI
‘Quantum’ NMR and MRI in 0D
Two Approaches to Proton NMR/MRI (incompatible!) ψSimplified QMUp/down spin states , transitions betweenthe two spin statesvLarmor , m0 , T1Full QMClassical Bloch Eqs.for m(t)precession, nutation ofmagnetizationvLarmor , T2, k-space, 2D
MRI Magnetsprincipal magnetic field B0 defines z-axisB0zSuperconducting:ye.g., niobium-titanium wirexzOpen: electromagnetic or permanentB0
Three External Magnetic Field Types in Open-Magnet MRIB0 , Gz , and Gx all point along z. Not BRF!zB0Gz ΔBz(z)/Δ zBRFGx ΔBz(x)/Δ xx
Moving Charge Produces Magnetic Fieldcompass needle; proton ‘spinning’ on axisµNeN SSS
Magnetic Dipole Tends to Align in External FieldµzSµNNz B0SNz-componentof proton µSSz
Energy to Flip Over Needle with Magnetic MomentzBzθμµNºE – μ Bz – μz Bz cos θΔE180º 2 μ z Bzin Bz
Nuclear Zeeman Splitting for Proton: ΔE 2 μz Bzμz points only along or against state μzNorthΔE180º(Bz) 2 μz BzBz
Nuclear Magnetic Resonance: Larmor Frequency–μz63.87 MHzspinfliphvLarmorproton: ΔE180 2 μz Bzphoton E hvvLarmor [MHz] μz60H-10.240P-31200.5vLarmor(Bz) ( γ /2π) Bz10.11.5Energy [(μeV]OrientationEnergyBz [T]
NMR Gedanken-Experiment on Watermonochromatic RF power absorption at (and only at) vLarmor‒H r meterDetectedRF powersweep the RF vResonance at 1.5 T:63.87 MHzTransmitter v [MHz]
3D Sensitive Point ReconstructionThe three gradient fields oscillate wildly,produce a net field stable at only one voxel.NMR occurs there.“Sensitive point” then shifts one voxel over.
Proton Density MRI in 1D
zxBz‒10‒50510 cmGx ΔBz /Δx1.501 T1.500 T
vLarmor(x)Bz(x)xzswept RFvx–10B(x) 10 cmGx1.502 T1.500 T1.498 TSWEEP0 cmBz(x) B0 Gx xxv(x)Bz(x)63.8763.96MHzPD MR Image vLarmor / 42.58
Summary: PD MRI on 1D PatientA(v)63.8763.96 MHzx0vLarmor ofNMR PeakBz (x)Voxel Position, x 510 cmAmplitude ofNMR PeakPD(x)Pixel Brightness, PD
Proton-Density MRIcontrast from differences in PD
Two MRI Motion Artifactsrespirationaortic pulsation
Magnetization, m(t), in a Voxel
Voxel’s MRI Signal Proportional to Its Magnetization, m(x,t)i) What is the magnitude of voxel magnetizationat dynamic thermal equilibrium, m0?ii) How long does it take to get there (T1)?iii) What is the mechanism?
Ensemble of Compass Needles or ProtonsSingle voxel at position xB 0Gentle agitation(noise energy)
Magnetization, m(t), for the Voxel at Position x:magnetic field from the ensemble of protons or needles themselvesSingle voxel at position xBz 0m(t)Gentle agitation(noise energy)
Filling Four Energy Levels of Marbles vs. Noise Levelequilibrium from battle between energy and entropy(all slippery; black balls denser)Shaking (Noise) Energy:too muchtoo littlejust right
Magnetization in Voxel at x, under Dynamic Equilibrium:m0(x,t) [N–(x,t) – N (x,t)] μ , t μBzm000 tesla0.01 T 0ort 0 *»1.5 TN μz1.5 Tt 05 10– 6 N μz(Boltzmann)* after abruptly turning on Bz , or after a 90º pulse.
RF Signal from Voxel, #1Voxel’s MRI signal is proportional to its magnetization,m(x,t)#1:s(x) m(x,t) PD(x)36
T1 Proton-Spin Relaxation, in a Voxel
Voxel’s MRI Signal Proportional to Its Magnetization, m(x,t)i) What is the magnitude of voxel magnetizationat dynamic thermal equilibrium, m0?ii) How long does it take to get there (T1)?iii) What is the mechanism?
Switching Bz On at t 0 Induces Magnetization m(0 )t 0‒B 0m 0
Over Time, m(0 ) Moves from m(0 ) toward m0protons in voxel at xt 0 Bz(0 ) 0mz(0 ) 0Bz(0 )m( ) m0N–( ) N ( )t m0
Polarization after t 0B0mz(1)mz(0)t 0 msmz(t 0) 0mz(2)t 1 msmz(1) 2μt 2 msmz(2) 4μ
Aside: Exponential Decay of Radionucliden(t)n(0)0n(400 ms)250500750 msdn/dt λ n(t)trate parametern(t) /n(0) e λ tphoton atten. (x), tracer conc. (t), cell killing (D), etc.42
Exponential Return of mz(t) to Equilibrium Value, m0[m0 – mz(t)]mz( ) m0mz(t) / m00 250500t750 msrated[m0 – mz (t)] / dt (1/T1) [m0 – mz (t)]z-axis Bloch Equationmz(t)/m0 1 e t / T143
#2#2:T1, Regrowth of mz(t) along z-Axismz(t)/m0 1 e t / T144
Voxel’s MRI Signal Proportional to Its Magnetization, m(x,t)i) What is the magnitude of voxel magnetizationat dynamic thermal equilibrium, m0?ii) How long does it take to get there (T1)?iii) What is the mechanism?
In MRI, the only thing a protonis ever aware of, or reacts to, is thelocal magnetic field, Blocal(t).!!!!!In addition to B0 and G(t),the source of Blocal(t) can be eitherexternal (BRF) orinternal (e.g., dipole-dipolefrom a moving partner-proton).46
T1 Transitions: Magnetic vLarmor ‘Noise’fluctuations in proton magnetic dipole-dipole interactionsHH OO H4 10 – 4 TH1.5 ÅEach water proton produces magnetic field fluctuationsof all frequencies, including local vLarmor(x) , at its partner proton
Factors Affecting vRotation of Water Moleculemore or lessboundhydrationlayerfree
Two Materials in 2 Voxels in 1D Phantom1mz(t) / m0Lipid: T1 250 msx 0CSF: T1 2,000 msx 5 cmtLipidCSFT1 MR image
Approximate Relaxation Times of Various TissuesTissuePDp /mm3, rel.pure H201brainCSF0.95white matter 0.6gray matter 0.7edemagliomaliverhepatomaT1, 1 T(ms)T1, 1.5 T(ms)40002000700800930200080090011001000T1, 3 0muscle0.9700900adipose0.95240260408518004560
T1-w MR ImageCSFT1 2000 mssub-cu. fatT1 260 msGd
MRI Case Study, and Caveatwith T1, FLAIR, MRS, DTI, f MRI,and MR-guided biopsy studies of a glioma
Case Study57 year old medical physicist has had daily headachesfor several months. Responds to ½ Advil.Physical examination unremarkable. Patient appears tobe in good general health, apart from mild hypertension,controlled by medication.Good diet, exercises moderately. Patient reports nomajor stresses, anxieties.CT indicates a lesion in the right posterior temporooccipital region, adjacent to occipital horn of rightlateral ventricle. MRI for better contrast.Principal concern: Vision for reading.
Lesion: Right Posterior Temporo-Occipital Region,adjacent to occipital horn of right lateral ventricleNo enhancementwith Gd contrast.T1-wT1-w FLAIR54
Chemical Shift and Non-invasive MRS ‘Biopsy’acetic acidCH3COOHvLarmor1.56 ppmAstrocytoma
Functional MRI and Diffusion Tensor Imagingvisual stimulusf MRIDTI56
MRI-Guided Stereotactic Fine-Needle BiopsyGrade 1-2 Astrocytomawith scattered cellular pleomorphism and nuclear atypia57
Caveat:strong gradient fields outside borewww.simplyphysics.com
within / with patient, othersin / into imaging suiteaneurysm clip, shrapnelcochlear implant, prosthesesartificial heart valvestent, permanent denturedefibrillator, pacemakerelectrodes, neurostimulatormedical infusion pumpdrug-delivery patch, tattooO2 tank, IV polewheelchair, gurneyhemostat, scalpel, syringescissors, pen, phone, laptoptool, tool chestcleaning bucket, mopfire extinguisher, axgun, handcuffs59
MRI Physics Anthony Wolbarst, Nathan Yanasak, R. Jason Stafford . Introduction to MRI ‘Quantum’ NMR and MRI in 0D Magnetization, m(x,t), in a Voxel Proton Density MRI in 1D T1 Spin-Relaxation in a Voxel MRI Case Study, and Caveat Sketch of the MRI Device ‘Classical’ NMR in a Voxel
magnetic resonance imaging (MRI)-MRI image fusion in assessing the ablative margin (AM) for hepatocellular carcinoma (HCC). METHODS: A newly developed ultrasound workstation for MRI-MRI image fusion was used to evaluate the AM of 62 tumors in 52 HCC patients after radiofrequency ablation (RFA). The lesions were divided into two
The use of magnetic resonance imaging (MRI) is increasing globally, and MRI safety issues regarding medical devices, which are constantly being developed or upgraded, represent an ongoing challenge for MRI personnel. To assist the MRI community, a panel of 10 radiologists with expertise in MRI safety from nine high-volume academic centers .
CASE REPORT Four patients underwent 7.0 T MRI before treatment be-tween April 2009 and July 2009. All of them received 1.5 T MRI at 5 to 9 days before receiving 7.0 T MRI. Three of them under-INTRODUCTION Meningiomas are typically diagnosed by their characteristic appearance on conventional magnetic resonance imaging (MRI).
Select on the patient programmer to say Yes. 2. Press Select on the patient programmer. 3. Press Select. The programmer disables MRI mode. 4. To start stimulation, press the Amplitude Increase key on the top of the programmer. Turning Stimulation Back on After Your MRI Scan Once your MRI scan is complete, disable MRI mode to resume normal .
MRI (Magnetic Resonance Imaging). Somali. MRI (Magnetic Resonance Imaging) MRI (Baaritaanka Ku salaysan Sawirka Magneetiga) An MRI is a safe, painless test. It uses radio waves and a magnetic field to take pictures of soft tissues, bones and blood supplies. The pictures provide information that can help your doctor diagnose the problem that
May 15, 2020 · RCEEM approved MRI-related CE over a period of three years. It is recommended that a minimum of 1 CE hour include MRI safety instruction. Comment: To be relevant to MRI, the course content must address the principles, instrumentation, techniques and/or interpretation of MRI specific to the anatomic area.
clinical practice the role of breast mri in clinical practice 514 reprinted from australian Family physician Vol. 38, No. 7, July 2009 Breast mri Breast MRI is performed using a standard MRI machine with a special attachment (a ‘breast coil’). The patient lies prone for the pro
R&D projects, but there are doubts on how many innovations have effectively gone to the market. The mid-term evaluations show outputs and results coming out of collective actions and support to regional filières and clusters. 2011 is the first year with outputs in the field of