Magnets, Spins, And Resonances
Magnets, Spins, and ResonancesAn introduction to the basics ofMagnetic Resonance
Magnets, Spins, and Resonances
Magnets, Spins, and ResonancesAn introduction to the basics ofMagnetic Resonance
Siemens AG 2003All rights reservedSiemens Medical SolutionsMagnetic ResonanceErlangen
A short excursion through MR Physics19About spin relaxation and echoes63From the signal to the image99The wide range of contrasts129Fast image generation159MR systems and their components181Environmental as well as biological effects209MR HighlightsIndex1
Join us in our journey through the fascinatingworld of modern MR imaging.This brochure and its scope of applicationaddress a wide audience, covering topics of interestto radiologists, radiological technicians,medical specialists, as well as researchers.Additionally, the brochure is also an excellententry-level source of information to thosewith an abiding interest in Magnetic Resonance.In this vein, we hope you are enjoyingMagnets, Spins,and Resonancesthe brochure as highly informative,easy-to-understand reading material.Siemens Medical Solutions
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Morphology—details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeMR HighlightsGastroenterologyand MROrthopedics in MRNeurology with MRDiffusion andperfusion imagingProton spectroscopy
MR HighlightsMorphology—details from head to toeMR is a non-invasive imaging technique. Its primary field of application includesthe display of morphology, that is, tissue structures in a series of slice imagesthrough the body.The advantages of MR imagingThe three main advantages ofMR imaging are: excellent soft tissue contrast with highresolution display of several images andoblique cuts no ionizing radiationModern MR systems allow for fast imagesof the body from head to toe.For example, examinations of the entirespine can be completed in as little astwo steps.3As pacemaker of the MR industry,Siemens has incorporated a unique coilconcept known as Integrated PanoramicArray (IPA ).Combined with automatic tablepositioning (Integrated PanoramicPositioning—IPP ), MAGNETOM systemsby Siemens allow for the quick(high-speed) display of large volumes.
Morphology —details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeGastroenterologyand MRMR-imaging provides image contraststhat result from the combination ofseveral parameters. These include the density of the nuclear spinsstimulated, especially that of hydrogenprotons the relaxation time for magnetizationof the tissue under examination, as wellas various different contrast mechanismsOrthopedics in MRNeurology with MRThe different MR contrasts enable precisediagnosis while supporting tissuecharacteristics.High-resolution MR images with a smallfield of view show excellent anatomicdetails.Diffusion andperfusion imagingProton spectroscopy
MR HighlightsComprehensive imaging of the heartCardio-vascular MR imaging (CMR) profits greatly from the ability of magneticresonance to display slice images in any orientation with high spatial as well astemporal resolution. The prerequisites for a diagnostically useful image areperformance-oriented gradients, superb pulse sequences as well as a robust,high-speed hardware.MR imaging of the heart providesexcellent morphological displays.5It also provides a multitude of informationregarding the function of the myocardium,such as vitality, ejection fraction, wallmovement or valve functions.
Comprehensive imaging of the heartMorphology—details from head to toeContrast-enhancedangiographyfrom head to toeGastroenterologyand MROrthopedics in MRNeurology with MRMR-imaging offers methods using contrast agent to display coronaryvessels. To visualize coronary arteries, however, MR provides formethods that do not use contrast agents. These are the so-calledTrueFISP and Dark Blood techniques.Diffusion andperfusion imagingProton spectroscopy
MR HighlightsContrast-enhanced angiography from head to toeConsiderable advances have been made in the area ofcontrast-enhanced MR angiography.The interaction of strong gradients, high-speed MR systemsand Care Bolus results in excellent contrast at optimalcontrast agent consumption.7
Contrast-enhanced angiography from head to toeMorphology—details from head to toeComprehensiveimaging of the heartGastroenterologyand MROrthopedics in MRNeurology with MRDiffusion andperfusion imagingProton spectroscopyContrast-enhancedMR angiographyusing stronggradients, iPAT(integrated ParallelAcquisitionTechniques) andarray coils.Excellent detailrecognition of bloodvessels in a matter ofseconds.
MR HighlightsGastroenterology and MRMR imaging is a highly suitable technique for gastroenterology.New as well as unique pulse sequences bySiemens, such as 3-D VIBE (Volume InterpolatedBreathhold Exam), enable the display ofanatomical details as well as dynamic angioinformation.3-D VIBE with fecal tagging finds extensive use inMR colonography.9
Gastroenterology and MRMorphology—details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeOrthopedics in MRNew techniques such as iPAT (integratedParallel Acquisition Techniques) or PACE(Prospective Acquisition CorrEction) facilitateexaminations and help reduce motionartifacts.New views in virtual endoscopy are obtainedby post-processing 3-D data sets.Neurology with MRDiffusion andperfusion imagingProton spectroscopy
MR HighlightsOrthopedics in MRHigh-resolution imaging of joints and interarticular spacesHigh-resolution images with good contrast are the basisfor satisfactory diagnosis. The images are generatedwith unique pulse techniques, such as 3-D DESS(Double Echo Steady State) and MEDIC (Multi Echo DataImage Combination).11
Orthopedics in MRMorphology—details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeGastroenterologyand MRThe interfering fat signal is suppressedthrough specific water excitation.Neurology with MRDiffusion andperfusion imagingProton spectroscopy
MR HighlightsNeurology and comprehensive, high-speed diagnosis with MRNeuro-imaging presents oneof the most revolutionary applicationsof magnetic resonance.Inline technology enables automaticcomputation and superimposing t-test(z-score) images on anatomicalEPI images.ART (fully automatic motion correction)as well as spatial filtration supportaccurate results.13
Neurology with MRMorphology—details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeGastroenterologyand MRModern technology enables the compact displayof mosaic images, very helpful, for example, inOR planning.Orthopedics in MRDiffusion andperfusion imagingProton spectroscopy
MR HighlightsDiffusion and perfusion imagingDiffusion imaging with Single-Shot-EPI sequences provides 16 different b-values with amaximum b-value of 10,000 s/mm2. The ADC cards (Apparent Diffusion Coefficient) as well asthe trace-weighted images are automatically computed via integrated post-processing (inline).15
Diffusion and perfusion imagingMorphology—details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeGastroenterologyand MROrthopedics in MRNeurology with MRPerfusion imaging with inline computation of the Global Bolus Plot (GBP),the time-to-peak map (TTP) as well as the percentage-of-baseline-at-peak (PBP).Inline computation greatly accelerates neurological examinations.Proton spectroscopy
MR HighlightsProton spectroscopyIn addition to image generation, MR spectroscopy provides forbiochemical quantification.Over the years, clinical MR spectroscopy haschanged from rather complex into simpleprocedures.Modern spectroscopy uses new pulse sequenceswith shorter echo times. The new evaluationsoftware provides for color metabolite images aswell as spectral overview cards.17
Proton spectroscopyMorphology—details from head to toeComprehensiveimaging of the heartContrast-enhancedangiographyfrom head to toeGastroenterologyand MROrthopedics in MRNeurology with MRDiffusion andperfusion imaging
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Nuclei and spinsHow magnetization iscreatedSpin precession in themagnetic fieldMoving spins out of theirstate of equilibriumHow do we obtain anMR signal?What does an MR examination involve?Let’s follow a patient examination step-by-step.The first steps involve moving the patient intothe magnet where he is exposed to a strongmagnetic field. During the course of theexamination, the patient’s body generatesmagnetic reactions that produce a measurablesignal.To sufficiently explain these reactions, we wouldlike to invite you to join us on our shortexcursion through MR physics. During this trip,A short excursionthroughMR Physicsit will become quite obvious that theMAGNETIC SPIN is so-to-speak the power behindMAGNETIC RESONANCE or MR TOMOGRAPHY.
1A short excursion through MR PhysicsNuclei and spinsMagnetic resonance or MR tomography. Our focus will be on the magnetic spinand its magnetic effects. That is why we start our trip by looking at the atomicnuclei present in the human body. True, beginnings are never easy. So it’s best tosimplify matters as much as possible.21Hydrogen is the least complexProtons and billiard ballsAtoms of chemical elements consists of anucleus and its electronic shell. Hydrogenis the most prevalent element and possessthe least complex nucleus: in this case asingle, positively charged PROTON.What makes the hydrogen protons usefulfor MR?MR tomography uses the magneticcharacteristics of the hydrogen proton togenerate images.The two advantages that hydrogen bringsto MR tomography are:1. Hydrogen is an elementary part ofwater and fat which makes it the mostprevalent element in the human body.2. Of all the elements, the nuclei ofhydrogen give the strongest magneticresonance signal.The SPIN is a purely quantummechanical characteristic of atomicbuilding blocks, but it is best to imaginethat we could actually “see” a rotatingproton, just like the spin of a billiard ball the rotation of the earth about its axis the spinning of a child’s top.Protons possess a singular characteristic:the spin.
Nuclei and spinsSpin precession in themagnetic fieldMoving spins out of theirstate of equilibriumHow do we obtain anMR signal?FOR DISCUSSIONHow magnetization iscreatedThe special features of a spinClassic physics or quantum physicsWhile ordinary objects can rotate atdifferent speeds, the spin of a nucleusalways remains the same ; it is a uniqueproperty of the nucleus. The onlyvariation you will see is the change in thedirection of the axis. And there is onemore difference with respect to a billiardball: the spin never rests.Our model that likens the spin to the“rotation” of a sphere is nothing morethan an analogy. You cannot apply it to allatomic nuclei or all shapes of spin.Why are we focusing on the spin?The spin is the source of the magneticresonance signal: a nucleus with spin isalways magnetic.But the following does apply,independent of any kind of analogies: thespin is a measure of the quantum state ofan atomic particle. It can be accuratelydefined through complex state vectors.MR tomography can be applied withoutin-depth knowledge of quantum physics.Note thatMR imaging is notdescribed byindividual spins,but rather by theircollectivecharacteristics.Fortunately, thisleads toeasy-to-understandmodels that can beused with unduedistortion of reality.
1A short excursion through MR PhysicsBar magnet and spin magnetsAlthough the characteristic of spin ispurely quantum mechanical, this does notkeep us from giving it a simple model—inthis case a bar magnet. This type ofmagnet has a magnetic north pole N anda magnetic south pole S.Let’s assume that the proton acts like atiny bar magnet, although we will seebelow that this is somewhat misleading(and will be discussed later).Because, in classical physics, a spinningcharge generates a magnetic field, it iseasy to conclude that the magnetism ofthe proton is caused by the rotation ofthis charged particle. We call thismagnetic force the SPIN MAGNET.23
Nuclei and spinsSpin precession in themagnetic fieldMoving spins out of theirstate of equilibriumHow do we obtain anMR signal?FOR DISCUSSIONHow magnetization iscreatedSpins are characterized by directionThe rotating chargeAlthough individual spins may point invarious directions, we can view theireffects as a single vector, an alignedmagnitude in space. The randomlyselected direction of the spin magnet runsfrom the magnetic south pole to the northpole (as shown by the blue arrow).Classic physics consider the electricalCHARGE of the proton as the source for itsmagnetic effect: the moving charge isnothing more than an electrical current. Inturn, this current generates the associatedmagnetic field. A rotating charge usuallygenerates its magnetic effect in thedirection of the rotating axis. Thismagnetic force is known as the MAGNETICMOMENT.Of course, it is not the proton itself that isa vector, but rather its spin and/ormagnetic effect.In what follows we are not going to look atthe protons themselves. Instead we aregoing to study their coupledcharacteristics: spin and magnetism. Thisis what we mean when we are talkingabout a “spin magnet”.As compared to a proton, the electricallyneutral NEUTRON does not have a charge.But since it still has a spin, it is considereduseful for magnetic resonance.This means that anelectrical charge isnot a prerequisite forthe magnetism of anucleus. Actually, themodern theory ofquarks postulates thereverse effect,namely thatmagnetism is thecause for anelectrical charge.
1A short excursion through MR PhysicsWhat really matters is the right directionThe essentials about vectors and arrowsDo you feel like going over VECTORS onemore time? There are a large number ofphysical magnitudes, for example,temperature or mass, etc. that are knownto be non-directional. What this means isthat they are sufficiently identified bymagnitude and units (e.g. 21 degreesCelsius, 5 kilograms).Spin magnetism, however, is a directionalmagnitude. But the magnitude ofmagnetism alone does not tell us aboutits effect. To determine that we need toknow its direction.25Again, there are a multitude of physicalmagnitudes that depend on spatialorientation (e.g. force or speed). Vectorsare a suitable means for depicting thesemagnitudes.ARROWS are excellent for depictingvectors. The direction of the arrowcorresponds to the orientation of thevector quantity, the length of the arrowcorresponds to the magnitude of thevector.Vector quantities allow for SPATIALADDITIONS. The direction has to be takeninto account and visualized by linking thearrows.If the arrows point into the samedirection, the magnitude of the vectorsum is simply the sum of the magnitudes:(in this case a a).
Nuclei and spinsHow magnetization iscreatedSpin precession in themagnetic fieldMoving spins out of theirstate of equilibriumVectors of the same magnitude CANCELeach other: a – a 0Just as you can add vectors, you can alsodecompose them. Each vector, forexample, can be divided into separateCOMPONENTS. These are the projections ofthe arrow along predefined spatial axes,typically the COORDINATE SYSTEM.In our example, the sum of vectors a bconsists of a vertical component a, andhorizontal component b.How do we obtain anMR signal?Please do notconfuse vectors witharrows and viceversa. A vector is amathematical modelfor a physicalquantity. An arrow ismerely a tool for thevisual display of avector.
1A short excursion through MR PhysicsWhich nuclei are suitable for use with magnetic resonance?We would now like to continue with theatomic nuclei of other elements.Protons and neutrons are ATOMICPARTICLES. Both have the property of spin.Atomic nuclei with an uneven number ofprotons or neutrons have a net spinknown as the NUCLEAR SPIN.Common examples are: carbon 13C,fluorine 19F, sodium 23Na orphosphorus 31. Two thirds of the isotopesfound in nature have a net nuclear spin,making them suitable for use withmagnetic resonance.27Atomic nuclei with aneven number of protonsand neutrons do nothave a net nuclear spin.They are magneticallyneutral.Examples of these areoxygen 16O (with8 protons and8 neutrons each) orcarbon 12C (with6 protons and6 neutrons each). Theseisotopes are not suitablefor use with magneticresonance.
Nuclei and spinsSpin precession in themagnetic fieldMoving spins out of theirstate of equilibriumHow do we obtain anMR signal?FOR DISCUSSIONHow magnetization iscreatedReviewHow do we get a nuclear spin?The nuclear spin is the source of themagnetic resonance signal: a nucleuswith spin is always magnetic.In the atomic nucleus, two identicalparticles cannot be in the same state.They have to align their spin orientationanti-parallel to each other, and the netspin of this “couple” of particles cancels.This means that the “dancing couple” isinvisible to the outside. This rule of natureis known as the PAULI EXCLUSION PRINCIPLE.It’s the “solo dancers” that create thenuclear spin.The spin is a directional quantity. Spins,just as vectors, allow for spatial addition.Along with hydrogen, two thirds of theisotopes found in nature have a netnuclear spin, making them in principalsuitable for use with magnetic resonance.The nuclear spin, as a quantity resultingfrom individual spins, is not a rotation ofthe atomic nucleus as such. In the strictestsense, this applies to the individual protonas well. Its spin results from its internalstructure (quarks and gluons).
1A short excursion through MR PhysicsHow magnetization is createdProtons and atom nuclei with a nuclear spin can be simplified by visualizing themas spin magnets. How do we benefit from this model? It allows us to explain thealignment of these spin magnets in the magnetic field of the MR system and howthey generate magnetization in the patient’s body.Voxels and spinWe are not going to measure the effect ofan individual spin in the body. Instead weare going to measure the entire collection(“ensemble”) of spins.An ENSEMBLE is the total of all proton spinsin a volume element, also known as aVOXEL. A voxel can be a small cube with anedge length of, for example, not morethan 1 mm.Let’s take a look at a voxel in the bodytissue of a patient as well as at thebehavior of the associated spin ensemble.29
How magnetization is createdSpin precession in themagnetic fieldMoving spins out of theirstate of equilibriumHow do we obtain anMR signal?FOR DISCUSSIONNuclei and spinsThe spin ensemble in field-free spaceCan we actually talk about a field-free space?The effect of the ensemble is created bythe spatial addition of the individual spinvectors.A completely random orientation of spinsapplies only to an absolutely field-freespace. The protons “feel” the magneticfield of the earth at all times.While the magnetic field of the earth isapproximately 20,000 times weaker thanthat of an MR magnet, it is neverthelesseffective. In other words, even though themagnetic field of the earth is very weak,the ensemble is magnetically affected byit outside the MR system.Without an external magnetic field,in field-free space, the spins in theensemble are randomly oriented and theireffects cancel each other. This is thereason why the ensemble appears to benon-magnetic.For this reason, magnetic resonance canbe used in the magnetic field of the earth(e.g. for the discovery of subterranean oilfields). However, for clinical imaging,magnetic fields with ten thousand timesthis strength are a must. The patient istherefore positioned in the strongmagnetic field of the MR magnet.
1A short excursion through MR PhysicsThe spin ensemble in the magnetic fieldWhat happens after we moved the patientinto the magnetic field of the MR system?Let’s continue concentrating on a small voxelinside the patient’s tissue.When we look at the spin orientation alongthe field lines, we can see t
Nuclei and spins How magnetization is created Spin precession in the magnetic field Moving spins out of their state of equilibrium How do we obtain an MR signal? While ordinary objects can rotate at different speeds, the spin of a nucleus always remains the same; it is a unique property o
spins are known as nuclear spins. ! An unpaired component has a spin of ½ and two particles with opposite spins cancel one another. ! In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field. Th
Atlas Auto SPINS Rotisserie unit. Keep this manual near the Atlas Auto SPINS Rotisserie unit at all times. Make sure all operators read this manual. This is an Atlas Auto SPINS Rotisserie unit installation/operation manual and no attempt is made or implied herein to instruct the user in specific methods particular to an individual application.
electron spins are not cancelled by the spins of other electrons. The net result is that the atoms in these materials act as tiny, atomic magnets, each with two poles. Investigation 4: If Iron, Nickel, and Cobalt Contain Tiny “Atomic Magnets,” Why Is
poles. The north pole always points north. The south pole always points south. Magnets attract other magnets. Unlike poles attract each other. The north pole of one magnet attracts the south pole of another magnet. But poles that are alike repel, or push away from, each other. Magnets are strongest at there ends. The north pole of one
The Triple Axis and SPINS Spectrometers Volume 98 Number 1 January-February 1993 S. F. Trevino ARDEC Picatinny Arsenal, NJ 07806 and National Institute of Standards and Technology, Gaithersburg, MD 20899 In this paper are described the triple axis and spin polarized inelastic neutron scattering (SPINS) spectrometers which
100 and 1M Spins of the Wheel. 100 spins of Fair Roulette Expected return betting 2 -100.0% . 100 spins of Fair Roulette Expected return betting 2 44.0%
Vorstellung: „kleine (Elementar-)Magnete“ benachbarte Spins antiparallel ("#oder #"): „Energie gespeichert“ benachbarte Spins parallel (##oder ""): „keine Energie gespeichert“ energieerhaltende Änderung möglichst vieler Spins: mit Schachbre muster i
from phytate is very good to enhance animal nutrition (Simons et al., 1990; Adeola et al., 2006; Augspurger et al., 2006; Garcia et al., 2005). Excretion of phosphate can be decrease by as much as .
