musculoskeletal mri pulse sequences: a review for residents … · 2020-04-05 · common...

Musculoskeletal MRI Pulse Sequences: A Review for Residents

and FellowsStephanie Jo, MD, PhD, Steffen Sammet, MD, PhD,

Stephen Thomas, MD, G. Scott Stacy, MD

Author affiliations: From the Department of Radiology, University of Pennsylvania, Philadelphia, Pa (S.J.); and Department of Radiology, University of Chicago, Chicago, Ill (S.S., S.T., G.S.S.).

Address correspondence to S.J: Penn University City, 3737 Market St Mailbox 4,

Philadelphia, PA 19104S (email: [email protected]).

This material was partly presented as an electronic exhibit at the 2018 RSNA Annual Meeting (ID 18004613).

All authors have disclosed no relevant relationships.

2D = two dimensional, 3D = three dimensional, ABER = abduction – external rotation, ADC = apparent diffusion coefficient, CHESS = chemical shift selective fat suppression, DWI = diffusion weighted imaging, EPI = echo planar imaging, ETL = echo train length, FABS = flexion, abduction and supination, FSE = Fast Spin Echo, Gd = Gadolinium, GRE = gradient echo, i.v. = intravenous, MAA = magic angle artifact, MR = magnetic resonance, MRI = magnetic resonance imaging, MSK = musculoskeletal, PD = proton density, RF = radiofrequency, SE = spin echo, SNR = signal to noise ratio, STIR = short-T1 inversion recovery, TE = echo time, TI = inversion time, TR = repetition time, TSE = Turbo Spin Echo

Abbreviations

At the end of this presentation, the viewer should be able to: 1. Understand the basic MR physics principles used for creation of

common musculoskeletal MRI pulse sequences 2. Describe the fundamental differences between spin-echo and

gradient-echo pulse sequences 3. Describe the application and appropriate selection of various pulse

sequences used for musculoskeletal MR imaging 4. Describe various methods of fat suppression 5. Understand the basics of motion, susceptibility, and magic angle

artifacts

LearningObjectives

• A comprehensive musculoskeletal (MSK) MRI protocol generally requires different types of pulse sequences to optimally manipulate contrast of tissues and thereby assess different tissue types, including bone and bone marrow, cartilage, synovium, muscle, ligaments and tendons

• Following a brief review of fundamental MR physics, we will provide an overview of the pulse sequences commonly used in MSK MR imaging

Introduction

MultiplecoronalimagesfromwristMRarthrogram.Fat-suppressedT1-weightedimage(topleft),fat-suppressedproton-density-weightedimage(topright),non-fat-suppressedT1-weightedimage(bottomleft),3DT1-FFE(spoiledgradientecho)withwaterexcitation(bottomright).

• MRI involves absorption and emission of energy by nuclei at a specific resonance (Larmor) frequency

• Signals used to generate images arise from hydrogen nuclei (protons) mainly in water and fat molecules

• Each proton spins about an axis, resulting in a magnetic moment

• The magnetic moments behave like bar magnets, which spin in the body with their axes randomly aligned when no external magnetic field is present

BasicMRIPhysics–Simplified

H + N

S~Thehydrogenproton(top),“spins”arounditsaxis,resultinginamagneticmoment.Protonspinsbehavesimilarto“barmagnets”withanorthandasouthpole.

OutsidethemagneticfieldofanMRIunit,theprotonsinthehumanbodyspinwiththeiraxesrandomlyaligned(bottom).

• When a patient is placed in the MR magnet, the protons align with the external magnetic field (Bo), and generate “longitudinal magnetization”

• Each nucleus precesses around its axis • The frequency of precession is

defined by the Larmor equation: ωo = Bo ・ γ where ωo is the precessional frequency, Bo the external magnetic field, and γ the gyromagnetic ratio, a constant for any given nucleus


Patientplacedinexternalmagneticfield(MRImagnet)

Longitudinalmagnetization

VectorMo

WhenthehumanbodyisplacedintheMRIscanner,theprotonsalignwiththemagneticfieldB0,creatingalongitudinalmagnetizationorientedalongtheaxisofthescanner.Theprotonsprecessaroundtheiraxes,andare“primed”toabsorbenergy.

• A radiofrequency (RF) pulsed is applied, which excites the protons and flips their magnetization vector a certain angle away from the longitudinal axis

• Fundamentally, there are two basic types of pulse sequences which are in part characterized by how the magnetization vector is flipped:

• Spin echo (SE) pulse sequences, and

• Gradient echo (GRE) pulse sequences


RFpulse

transversemagnetization

x-axis

z-axis

longitudinalmagnetization

y-axis

x-axis

y-axis

Aradiofrequencypulsedeflectsthemagnetizationvectorbyapre-determinedflipangleawayfromthelongitudinalaxis.Inthisexample,thevectorisflipped90ointothexy-plane(i.e.transverseplane).

SpinEcho(SE)PulseSequences

transversemagnetization

x-axis

y-axis

x-axis

y-axis

x-axis

y-axis

• Spin echo pulse sequences begin with a 90o excitation pulse that flips the net magnetization vector into the transverse plane resulting in “transverse magnetization,” which is necessary for signal detection by a radiofrequency coil

• When the RF pulse is turned off, two relaxation processes occur which are associated with reduction of transverse magnetization:

• Transverse relaxation T2 • Longitudinal relaxation T1

• How quickly transverse and longitudinal relaxation occur depend on intrinsic tissue properties and magnetic field characteristics

RFpulseon

RFpulseoff

• Transverse relaxation or T2 relaxation refers to the tendency of proton spins to dephase (i.e., become incoherent, pointing in different directions in the transverse plane)

• T2 relaxation decreases the transverse magnetization vector and consequently the MR signal used for image production

• T2 is defined as the time by which transverse magnetization is decreased to 37% of its original value

SpinEcho(SE)PulseSequencesy

x Time

37%

T2tissueA

Signal

TimeT2tissueB

tissueA

tissueB

Afterthe90oexcitationpulseisturnedoff,theprotonspinsinthetransverse(xy)planebegintodephase,whichreducestheinducedsignalintheRF-coil.Theratesofdephasingaredifferentfordifferenttissues,whichcanbedepictedontheT2relaxationcurves(below).Inthisexample,theT2oftissueA(e.g.,fat)isshorterthantheT2oftissueB(e.g.,fluid).

y

x

1800refocusingRFpulse

SpinEcho:signal/emittedenergycanbedetectedbyreceivercoil

900excitationRFpulse


Time

90oRFpulseturnedoffà protonspinsstarttodephase

A180opulsecausestheprotonspinstorephaseandtoproduceaso-calledspinecho

Protonsstarttodephaseagain

• In a spin echo sequence, a 180o refocusing pulse is applied at a timepoint TE/2 to rephase the protons spins. At a timepoint TE (echo time), the protons realign, and a so called spin echo can be recorded for image production. TE is the time between the application of the 90o excitation pulse and the peak of the spin echo.

• Longitudinal relaxation or T1 relaxation refers to the tendency of proton spins to re-align along the longitudinal axis after termination of the RF excitation pulse

• T1 is defined as the time by which longitudinal magnetization reaches 63% of its original value

• The next 90o RF excitation pulse is applied at a time point TR (repetition time) after the initial 90o RF excitation pulse, and the spin echo sequence is repeated

• TR is responsible in large part for the duration of a pulse sequence


z

xy

magnetizationvector

longitudinalmagnetization

63%

Signal

TimeT1tissueA

T1tissueB

tissueA

tissueB

Whenthe90oRFpulseisdiscontinued,theprotonsbegintorealignalongthelongitudinalaxisatdifferentratesdependingonthetissue,whichcanbedepictedonaT1curve(below).Inthisexample,theT1oftissueA(e.g.,fat)isshorterthantheT1oftissueB(e.g.,fluid).

• Choosing a relatively short TR (<800 ms) and short TE will result in a T1-weighted image, on which fluid is dark and fat is bright

• T1-weighted images are useful in MSK imaging for delineating anatomic detail, confirming replacement of normal fatty bone marrow (e.g., by infection or neoplasm), determining the degree of fatty atrophy of muscles, and detecting and characterizing lipomatous lesions

SpinEcho(SE)PulseSequencesT1-weightedcoronalMRimageofpelvis(left)showsdiffuselyabnormallowsignalintensityofbonemarrowinthis30-year-oldpatientwithsicklecelldisease.BonemarrowonT2-weightedSTIRimage(right)showsnoappreciableabnormalitywithinfat-suppressedmarrow.

T1-weightedtransverseMRimageofthighsinpatientwithdermatomyositisshowsfattyatrophyofmusculature,particularlyvastuslateralisbilaterally(*),aswellasfocioflow-signalintensitycalcification(arrows).

T1-weightedtransverseMRimageofthighshowsfattymass(arrow,left)invastuslateralismuscle.Notestrand-likenon-adipocyticelementswithinmass,whichwasdiagnosedasatypicallipomatoustumor/well-differentiatedliposarcomaonhistologicexamination.Massislessconspicuousonfat-suppressedT2-weightedimage(right).

**


T2-weightedcoronalMRimageofmanubrium(top)showsfracture/separationofrightcostomanubrialjunction(arrow)depictedbyhighsignalintensitybetweencartilageandbone(arrow).InjuryisnotasapparentonT1-weightedimage(bottom).

T2-weightedtransverseMRimageofthoracicspine(left)inyoungpatientwithbackpainshowssmallosteoidosteoma(arrow)intransverseprocess,mademoreconspicuousbyadjacentedema.CTimage(right)confirmsdiagnosis.

• Choosing a relatively long TE (>70 ms) and long TR will result in a T2-weighted image, on which fluid is bright

• T2-weighted images are useful for detection of fluid, including edema, and therefore pathologic processes


T1-weightedtransverseMRimageofankle(left)appearsverysimilartoT2-weightedimage(center)withoutfatsuppression.Smallamountsoffluidcanbeseenonbothimages,darkonT1-weightedimageandbrightonT2-weightedimage(arrows).Withfatsuppression(right)smallamountsoffluidbecomemoreconspicuousonT2-weightedimage.

• Although fat has less signal on T2-weighted images than on T1-weighted images, it appears relatively bright; therefore fat-suppression is often necessary with T2-weighted images to better emphasize pathology

• Proton density (PD) weighted images minimize T1 and T2 characteristics of tissues by maximizing longitudinal recovery (long TR) and minimizing transverse decay (short TE)

• Tissues with more protons have higher signal intensity; those with fewer protons have lower signal

• PD-weighted images have lower tissue contrast but a higher signal-to-noise ratio compared to T1- or T2-weighted images; this allows identification of signal in signal-poor structures, and is often the sequence of choice for imaging fibrocartilage

SpinEcho(SE)PulseSequencesPD-weightedsagittalMRimageofknee(top)showstearofposteriorhornofmedialmeniscuscontactingtibialarticularsurface(arrow).Thistearismoreconspicuouscomparedwithfat-saturatedT2-weightedimage(bottom).AlthoughPD-weightedsequencesarepreferredformeniscalevaluationinmostpractices,T2-weightedsequencesplayanimportantrolewhenevaluatingpostoperativemeniscibydifferentiatinghealingfromrecurrenttear.

SpinEcho(SE)PulseSequencesPD-weightedsagittalMRimageofknee(left)showsjointfluid(*)similarinsignalintensitytofat.SamefluidisappreciablybrighterthanfatonT2-weightedimage(right).

Fat-saturatedPD-weightedsagittalMRimageofknee(left)withhigh-signal-intensityfluidnicelydepictstearofposteriorhornofmedialmeniscus(arrow).OncorrespondingT1-weightedimage(right),low-signal-intensityfluidinjointisdifficulttodistinguishfrommeniscus,givingappearanceofintactinnerhalfofposteriorhorn.

* *• Although the TR for PD-weighted images is relatively long (>1000 ms), the TE is short (10-30 ms), and fluid on PD weighted images without fat-suppression is not as bright as fluid on T2-weighted images

• Fat-suppression is often used with PD-weighted images to increase the conspicuity of fluid


• “Intermediate-weighted” pulse sequences have TE values in the range of 30-60 ms (between true PD- and T2-weighted sequences) and TR values between 3000 ms and 4000 ms

• These fluid-sensitive sequences have become popular for assessment of articular cartilage Intermediate-weightedsagittalMRimageofknee(left)showsrelativelyincreasedsignalintensityof

jointfluid(*)comparedwithfluidonproton-density-weightedimage(right).Signalintensitychangesrelatedtocartilagedegeneration(arrow)aremoreevidentonintermediate-weightedimage.

* *


Intermediate-weightedsagittalMRimageofknee(top)showsearlydelamination(arrow)ofcartilagealonglateralfemoralcondyle.Inset(topright)showslayersdepictedasblue(cartilage),red(areaofdelamination)andyellow(subchondralboneplate).Intermediate-weightedimageobtainedtwoweekslater(bottom)showscompletedetachmentofcartilage(arrow).

• Intermediate-weighted pulse sequences provide higher signal intensity of cartilage than T2-weighted sequences (allowing better differentiation between cartilage and bone), and provide higher intrinsic contrast of articular cartilage.

• They are also less prone to magic angle artifacts than true PD-weighted sequences; however, PD-weighted sequences are still preferred by many radiologists for meniscal evaluation

• Conventional (“single-echo”) SE pulse sequences, which use a single 180o refocusing pulse, are lengthy and therefore seldomly used in today’s MSK MRI protocols

• Conventional “dual-echo” SE sequences are occasionally used with modern MSK MRI protocols, applying two 180o refocusing pulses following a single excitation pulse to produce PD- (short TE) and T2- (long TE) weighted images in one acquisition (with a long TR)


TEforPD

RFpulse

TR

90o 180o

Echo

T1 T2 PD

TR Short Long Long

TE Short Long Short

½TEforPD180o

½TEforT2

TEforT2

GeneratesPDimage

GeneratesT2image

PD-weighted(left)andT2-weighted(right)coronalobliqueMRimagesoftheanklewereacquiredduringsameacquisition(TR=5400ms)usingdual-echotechnique(TEsof12msand110ms,respectively).

• Most modern MSK MRI protocols make use of echo-train SE (Fast Spin Echo [FSE] or Turbo Spin Echo [TSE]) sequences, with multiple 180o refocusing pulses per single TR (as well as different phase-encoding gradients applied with each pulse) resulting in acquisition of more data per TR and decreased scan time

• The number of echoes is referred to as the “echo train length” (ETL)


TE

RFpulse

TR

90o 180o 90o

Echo

Conventionalspinecho

TE

RFpulse

TR

90o 180o 90o

Echo

180o 180o

Echo-train(fast,turbo)spinecho

• In addition to reduced acquisition time, fast/turbo spin echo pulse sequences result in reduced magnetic susceptibility artifacts

• The disadvantages of fast/turbo spin echo include:

• Lower tissue contrast • Fat signal ~ fluid

• Edge blurring • Motion sensitivity (although

patients may be less likely to move during the relatively short acquisition)


FastspinechoT2-weightedsagittalMRimage(left)showsfluidalongsemimembranosustendon(*)ofsimilarsignalintensitytoadjacentfat.CorrespondingconventionalspinechoT2-weightedimage(right)showsgreatersignalintensityoffluid(*)relativetoadjacentfat.

**

Fastrecoveryecho-trainSEsequencesareamodificationoffast/turboSEsequences,usinga-90o“flip-back”pulseattheendoftheechotraintoquicklyrefocusmagnetizationbackintothelongitudinalaxis,thusfurtherreducingacquisitiontime.ExamplesincludeFRFSE(GE),DRIVE(Philips)andRESTORE(Siemens).

• MSK MRI is dominated by 2D multislice acquisitions; while these sequences provide excellent signal and contrast between tissues, the anisotropic voxels require that multiple planes of data be acquired separately to minimize partial volume artifacts

• 3D techniques obtain a volume of data in one acquisition, generating isotropic voxels

• Although 3D techniques require longer acquisition times than 2D techniques, overall exam time can be decreased, as thin images can be reformatted in any plane from the single 3D acquisition

• 3D sequences, however, can suffer from limited contrast characteristics, blurring, motion and other artifacts


Reformattedcoronalobliqueimagefrom3Dintermediate-weightedFSEdatasetshowsfabellofibularligament(shortorangearrows)andmeniscofemoralligamentofWrisberg(longorangearrows).

• 3D FSE techniques with parallel imaging to reduce scan time and flip angle modulation to reduce blurring have made isotropic imaging with spin echo contrast possible

• Variants of such sequences include CUBE (GE), VISTA (Philips) and SPACE (Siemens); these PD/intermediate-weighted images may or may not be fat-suppressed

• While image quality currently does not quite match that of 2D FSE sequences, 3D FSE sequences can be used to aid diagnosis of cartilage, meniscal and ligament defects, and have been touted as superior to 3D gradient-echo sequences (described later)


Reformattedtransverse(top)andcoronal(bottom)imagesfrom3Dintermediate-weightedFSEdatasetdemonstratecartilagedegeneration,fromtinysurfacedefects(thinorangearrow,top)tofull-thicknesscartilageloss(thickorangearrow,bottom).While3DFSEtechniqueshavenotyetreplacedstandard2DFSEsequences,theyshowgreatpromiseforwhole-organevaluationofdifferentjoints,andcandepictsubchondralbonemarrowabnormalitiesbetterthangradient-echosequences.

GradientEcho(GRE)PulseSequences

TE

RFpulse

TR

90o 180o 90o

Signal

Conventionalspinechosequence

RFpulse

TR

αo(flipangle<90o)

Gradientechosequence

αo

Gradient

TE

dephasingrephasing

Signal

T2relaxationcurve

T2*relaxationcurve(combinationofT2relaxationandmagneticfieldinhomogeneities)

• Gradient Echo (GRE) pulse sequences differ from SE sequences:

• GRE pulse sequences begin with a excitatory pulse that usually flips the magnetization vector less than 90o

• Gradients, instead of 180o RF pulses, are used to dephase and rephase transverse magnetization

• Transverse relaxation is affected by magnetic field inhomogeneities, yielding to T2*-relaxation (as opposed to T2-relaxation with SE sequences)

• The smaller flip angles used in GRE sequences lead to faster recovery of longitudinal magnetization, shorter TR, and faster acquisition; “new” types of tissue contrast can also be obtained

• Since GRE pulse sequences are not efficient at reducing magnetic inhomogeneity, susceptibility artifacts can occur that can degrade image quality or be used to detect hemorrhage or mineralization


Fat-suppressedproton-density-weightedMRimageofshoulder(left)showsinfiltrativemass(arrow)inglenohumeraljointcontainingregionsoflowsignalintensity.Gradient-echoMRimage(right)showsincreasedprominenceoflow-signalintensityregions(“blooming”)resultingfromhemosiderininthispatientwithpigmentedvillonodularsynovitis.

T1-weighted(left)andT2-STIR(center)coronalMRimagesofwristshowscaphoidwaistfracture(arrow).Gradient-echoimage(right)failstoshowfractureduetosusceptibilityeffectsoftrabeculae.


Gradientechopulsesequence

variants(simplified)

Coherent:transverse

magnetizationpreserved

Spoiled:transverse

magnetizationdisrupted

• In addition to “basic” GRE pulse sequences, a large number of GRE variants have been developed, many of which are used in MSK MR imaging

• While a comprehensive analysis of these various sequences is beyond the scope of this presentation, on a basic level, GRE sequences can be categorized based on whether transverse magnetization is preserved (coherent GRE sequences) or disrupted (spoiled GRE sequences)

• Additional GRE sequences, such as ultra-short TE imaging, are being investigated as novel methods of evaluating tissues with short T2-relaxation times such as tendons, ligaments, menisci and cortical bone


RF2αo

RF3αo

RF4αo

TR(extremelyshort)

FIDofRF2

FIDofRF3

FIDofRF4

SEofRF1

SEofRF2

TE–FID(postexcitation)

TEeffective–SE(preexcitation)

Asteady-stateofbothlongitudinalandtransversemagnetizationisachievedbykeepingtheTRshorterthantheT2relaxationtimesofthetissue.SinceTRisshorterthanT2,thereisnotenoughtimefortransversemagnetizationtodecaycompletelybeforethenextRFpulseexcitation;therefore,therewillberesidualtransversemagnetizationleftover.Withcoherentsteady-statesequences,twotypesofsignalsareproduced:apostexcitationsignalandapreexcitationsignal.Thepostexcitationsignal,consistingoffreeinductiondecay(FID)fromthemostrecentRFpulse,hasmixedT1andT2*weighting.Thepreexcitationsignal,resultingfromrefocusingofresidualechofromthepreviousRFexcitation,isstronglyT2-weighted.

RFpulse

Signal

• With coherent GRE sequences, transverse magnetization is refocused to contribute to a steady state in which longitudinal and transverse magnetization is constant from one TR cycle to the next

• Once equilibrium is reached, two types of signals are produced:

• A postexcitation signal consisting of free induction decay (FID)

• A preexcitation signal consisting of a spin echo (SE)

• Depending on the signals sampled and used for image formation, a variety of coherent, steady-state sequences can be obtained with different types of image weighting and applications, including:

• FID refocusing (postexcitation) • SE refocusing (preexcitation) • FID & SE together (“fully-refocused” or

“balanced”) • FID & SE acquired separately & then

combined (“double echo”)

GradientEcho(GRE)PulseSequencesSequence GE Philips Siemens

Coherentsteady-statesequenceswithFIDsampling(partially-refocused“postexcitation”)

GRASS FFE FISP

Coherentsteady-statesequenceswithSEsampling(partially-refocused“preexcitation”)

SSFP T2-FFE PSIF

Coherentsteady-statesequenceswithFIDandSEsampledtogether(fully-refocused“balanced”)

FIESTA BalancedFFE

TrueFISP

Coherentsteady-statesequenceswithFID&SEsampledseparately,thencombined(“doubleecho”)

MENSA DESS


Examplesof3Dcoherentsteady-stateGREsequences.3Dfully-refocused“balanced”steady-statesagittalMRimageofknee(left)showsfluid-filledcartilagedefect(arrow)ofmedialfemoralcondyle.Doubleechosteady-statesequence(bottom)showsfluid-filledcartilagedefect(shortarrow)ofpatellaanddisplacedcartilagefragment(longarrow).Suchsequenceshaveadvantageofgoodfluid-to-tissuecontrastandhavebeenstudiedfortheirabilitytodepictarticularcartilage,aswellaslabraltearsduringMRarthrography,butarenotidealforassessingadjacentsubchondralbone.

2DT2-weightedcoherentsteady-stateGREcoronalMRimageofinfantwithhipdysplasiashowsleftfemoralheaddirectedintodysplasticacetabulum.Notehyperintensefluidinbladder,aswellascontrastinlefthipfromrecentarthrogram.

• In general, coherent, steady-state sequences are used in MSK imaging in situations when bright fluid is desirable

• Postexcitation steady-state sequences have been largely replaced by fully-refocused sequences which are less sensitive to motion; T2-weighted pre-excitation steady-state sequences have been used for MR myelography and diffusion imaging of the spine

• 3D fully-refocused and double-echo steady-state sequences have shown good results in detecting cartilage lesions within reasonable scan times

• Spoiled GRE sequences are those in which transverse magnetization is disrupted (“spoiled”)

• There are different methods of “spoiling,” and the terminology can be confusing:

• The unqualified term “spoiled” usually refers to “RF-spoiling”; RF-spoiled sequences are often used to create T1-weighted images, and include SPGR (GE), T1-FFE (Philips) and FLASH (Siemens)

• “Long TR spoiling” occurs when TR>>T2*, allowing the transverse magnetization to decay to zero “naturally”; sequences that take advantage of this method include multiecho T2*-weighted GRE

• “Gradient spoiled” is a term occasionally used to refer to the previously described steady-state sequences:

• Gradient-spoiled sequences (FID-refocusing): GRASS, FFE, FISP • Reversed gradient-spoiled echo (echo-refocusing): SSFP, T2-FFE, PSIF



Fat-suppressedT1-weightedspoiledgradientechotransverseMRimagesofthighobtainedbefore(left)andafter(right)intravenousadministrationofgadolinium-chelateshowenhancementofseptae(arrow)withinfattymassinvastuslateralis(atypicallipomatoustumor).

Spoiled3DGRET1-weightedaxialimageofwristshowsenhancementoftissuessurroundingdistalulnafollowingresectionofsynovialsarcomainthisarea.Usingthisrapidly-acquired3Dsequence,time-enhancementcurvescanbegeneratedbyplacingregionsofinterestonenhancingtissueandcomparingcurvetothatofnearbyartery.Curvemorphologyinthiscasewasmoreconsistentwithpost-radiationchangesthanrecurrenttumor,whichwasconfirmedonfollow-upscans.

• RF-spoiled GRE sequences are weighted based on TR, TE, and flip angle:

• As TR ↓, T1-weighting ↑ • As TE ↑, T2*-weighting ↑ • As flip angle ↑, T1-weighting ↑ • As flip angle ↓, T2*-weighting ↑

• RF-spoiled GRE sequences with large flip angles are used to acquire fast pre- and post-contrast T1-weighted images

• Spoiled 3D GRE variants include FAME/LAVA (GE), THRIVE (Philips) and VIBE (Siemens), which can be used for dynamic multiphase imaging

GradientEcho(GRE)PulseSequencesFat-suppressedT1-weightedspoiledgradientechosagittalimageofkneeshowsnormallybrightarticularcartilage.Heterogeneous“blooming”mass(arrow)inHoffa’sfatpadrepresentslocalizedintra-articulartenosynovialgiantcelltumor(PVNS).

Spoiled3DGRET1-weightedsagittalimageofankleshowshyperintensearticularcartilage(longarrow).Cystsinsubchondralbonealongsubtalarjointarenearlyisointensetoskeletalmuscle(shortarrow).

Spoiled3DcoronalMRimageofwristobtainedfollowingintra-articularinjectionofgadolinium-chelateintoradiocarpaljointshowsextensionofcontrastintomidcarpaljointthroughtornscapholunateligament(circle).NotesignalabnormalityindistalradiusrepresentingedemafromfracturenotreadilyvisualizedonthisGREsequence.

• 3D RF-spoiled GRE sequences (typically with fat suppression via fat saturation or selective water excitation) have been the standard for quantitative morphologic imaging of cartilage, although they suffer from long imaging times and suboptimal evaluation of surface/internal cartilage defects, menisci, ligaments and marrow

• These sequences produce high cartilage signal using low flip angles (12o-30o, with TR = 20-30 ms, TE = 7-12 ms)

• Adjacent joint fluid is of low signal, although cartilage can be nicely outlined with intra-articular injection of dilute Gd


SpoiledT2*-weightedGREtransverseMRimageofshoulderusingmultiplecombinedfreeinductiondecaysshowsbrightfluidoutliningfull-thicknesstearofsubscapularistendon(arrow).

SpoiledT2*-weightedGREtransverseMRimageofcervicalspineusingmultiplecombinedfreeinductiondecays(left)showsbettergray/whitemattercontrastincordthanT2-weightedspinechoimage(right).

3DspoiledT2*-weightedGREcoronalMRimageofelbowusingmultiplecombinedfreeinductiondecaysshowsbrightfluid(longarrow)andarticularcartilage(shortarrow).

• Long TR-spoiled sequences in which multiple gradient echoes are generated after each RF pulse and combined to form an image can be used to create T2*-weighting; the early echoes provide increased SNR and the later echoes improve contrast

• Such sequences include MERGE (GE), M-FFE (Philips) and MEDIC (Siemens)

• They are commonly used for c-spine imaging, showing excellent gray/white matter contrast in the cord, but can have other uses in MSK imaging as well; they are prone to susceptibility (metal) artifact

• Protons from fat and protons from water precess at slightly different Larmor frequencies

• A GRE in-and-opposed-phase sequence (usually spoiled) can take advantage of this phenomenon to image fat and water protons when their 1H nuclei are spinning in-phase as well as out-of-phase


01.12.23.34.4ms

Water

Fatopposedphase

inphase

• The Larmor frequency difference between fat and water is 220 Hz, equivalent to a TE difference of 4.4 ms at 1.5 Tesla.

• With an echo time (TE) at which the fat and water signals are in phase, the signals add constructively; when they are out of phase, the signals cancel

GradientEcho(GRE)PulseSequences01.12.23.34.45.56.6ms

Water

Fatopposedphase

inphase “opposedphase"

1.5T

Signal-fat

Signal–non-fat

Opposed-phaseimageneedstobeacquiredbeforethein-phaseimagebecausesignallossesduetoT2*-effectscanconfoundsignallossesduetofat-watercancellation.Anapproximate20%signaldropoffisrecommendedtodistinguishnon-neoplasticfromneoplasticlesions.

TE


T2-weightedtransverseMRIimagethroughproximalfemora(topleft)showsheterogeneousbonemarrowsuggestingpossibilityofmetastaticdiseaseinthispatientwithrenalcellcancer.Betweenin-phase(topright)andout-of-phase(bottomright)images,thereisdecreaseinbonemarrowsignalonout-of-phaseimagecompatiblewithbenignredmarrowreconversion.

• With in-and-out-of-phase imaging, signal is suppressed on out-of-phase images if both fat and water protons are present in the same voxel

• This can be used to detect microscopic fat in bone and thus distinguish red marrow from infiltrated marrow

• Echo-planar imaging (EPI) is the fastest MR technique in which an entire two-dimensional image can be acquired with a single excitation (“single shot”) or a small number of excitations (“multi-shot”)

• EPI sequences are extremely fast, and therefore often used to evaluate physiologic processes (e.g., diffusion)

• They are not frequently used with routine MSK MR imaging outside of diffusion-weighted imaging

Echo-PlanarImaging(EPI)

DiagramofaEchoPlanarImaging(EPI)sequence.AfteranRFexcitationpulse,analternatingfrequency-encodinggradientisswitchedsimultaneouslywithablippedlowmagnitudephase-encodinggradient.Thecollectedgradientechoesaresortedinameander-shapeintok-spaceallowingacquisitionofanentireMRimageafterasingleexcitation.

αoRF

Signal

Frequencyencoding

Phaseencoding

• Diffusion-weighted imaging (DWI) evaluates the random motion of water molecules and allows distinction of unrestricted diffusion from restricted diffusion of water protons

• Areas with higher cellularity (e.g., malignant tumors) restrict the motion of water, resulting in a decrease in the apparent diffusion coefficient (ADC)

• MRI measures water diffusivity by applying diffusion sensitizing gradients to T2-weighted images (either fast GRE or echo-planar imaging sequences)

Diffusion-WeightedImaging

Tissueswithlowercellularity(top)allowforgreatermobilityofwatermolecules(bluearrows)thantissueswithhighercellularity(bottom)

• The DWI MR signal equals the T2 signal intensity minus signal loss determined by:

• Free motion of water molecules • Strength of the applied

diffusion weighting, indicated by its “b-value”

• MSK applications include evaluation of bone malignancy and soft tissue tumors, tumor follow-up after therapy, vertebral fractures, and infection

Diffusion-WeightedImaging

Increasingbvalues(strongerdiffusionw

eighting)

Diffusion-weightedMRimageswithincreasingb-values(toptobottom)reflectingstrongerdiffusionweightinginthispatientwithnecroticmalignantperipheralnervesheathtumor.Asb-valueincreases,fluidwithinbladder(*)aswellascentralnecroticareawithintumor(thinarrow)showdecreasedsignalrelativetoviabletumor(thickarrow).TheviabletumorremainshyperintenseduetorestricteddiffusiononDWI.DWsequencesareoftenappliedinconjunctionwithapparentdiffusioncoefficient(ADC)mappingtechniques(notshown);tissueswithrestricteddiffusionappeardarkonADCmaps,andtissueswithunrestricteddiffusionappearbright.

*

*

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• Fat-suppression is important for improving visibility of lesions on PD-, T2-, and contrast-enhanced T1-weighted images, for evaluating fat in soft-tissue lesions, and for differentiating high-signal-intensity structures seen on both T1- and T2-weighted images (e.g., methemoglobin)

• The main techniques of fat suppression are • Inversion-based, e.g., Short-T1 Inversion

Recovery (STIR) • Chemical-shift based, e.g., Chemical Shift

Selective Fat Suppression (CHESS), Dixon techniques, and water excitation

MethodsofSuppressingFatSignalT2-weightedcoronalobliqueMRimageofshoulder(top)ofpatientwithlong-standinginflammatoryarthritisshowsjointeffusionanddiffuselossofarticularcartilage.Additionoffat-suppression(bottom)revealsbonemarrowedemainhumeralheadnotreadilyseenonnon-fat-suppressedimage.

MethodsofSuppressingFatSignal

ShortT1

LongT1

1800“preparatory”pulse

Longitudinalmagnetization

Timeinterval–settoT1offat

Time

900“excitatory”pulse

Cannotgeneratetransversemagnetization

Cangeneratetransversemagnetization

TI80-150ms

WaterFat

• Short-T1 Inversion Recovery (STIR) is a variation of spin-echo that takes advantage of the different longitudinal relaxation properties of fat and water (T1fat < T1water)

• An initial 180o pulse inverts the longitudinal magnetization of fat and water protons

• As the magnetization recovers, a 90o RF pulse is applied at a time TI (inversion time) when the net vector of fat is close to zero (i.e., with little/no longitudinal magnetization of fat)

• Therefore, the 90o pulse generates no signal from fat, but some signal from water

MethodsofSuppressingFatSignalT2-weightedSTIRcoronalMRimageofknee(left)showshomogeneousfatsuppressiondespitepresenceofscrewsinproximaltibia.Samekneescannedwithchemicalshiftselectivetechniqueshowspoorsuppressionofsubcutaneousandmarrowfat.

ImagesfrompatientwhounderwentrighthipMRarthrogramwithdilutegadoliniuminjectedintojoint.OnSTIRimage(left),signalfromshortT1tissues/fluidissuppressed,includingfatandintra-articulargadolinium(arrow).OnT1-weightedCHESSimage(right),fatissuppressedbyfrequencyspecificRFpulse,andintra-articulargadoliniumremainsbright(arrow).

• STIR is not as susceptible to magnetic field heterogeneities as CHESS, and can therefore be used with off-center imaging and in patients with metal implants; it can also be used on MRI systems with lower field strength

• However, signal from ANY short T1 tissue/fluid, including gadolinium, melanin, proteinaceous material, and blood, can also be suppressed

• This technique also tends to be lengthy and results in a relatively low signal-to-noise ratio

• CHESS (often called “fat saturation”) takes advantage of the fact that protons from fat precess at a slightly lower Larmor frequency than protons in water (“chemical shift”)

• Using an RF pulse specific for the fat resonance frequency and a subsequent dephasing gradient, the fat signal can be suppressed

• A standard imaging sequence is then performed, producing images from water protons with minimal net magnetization from fat protons


z

x x

y

“Spoiler”gradientàfattransversemagnetizationsignalislost

ExcitationRFpulsespecificforfat

Standard(e.g.FSEorGRE)sequenceisperformed,andfatcontributesminimalsignalbecauseitsprotonshavedephased

Water

Fat

z

y

x

y

Water

Fat


T1-weightedtransverseimagethroughaxillashowstwohyperintenselesions(thinandthickarrows).Withchemical-shiftselectivefatsuppression,anteriorlesion(thinarrow)loosessignal,indicatinglipoma.Posteriorlesion(thickarrow)remainshyperintense,indicatinghematoma.

Patientwithsubcutaneousshouldermass(arrow).AtedgeofT2-weightedCHESSimage(left)thereispoorfatsuppressionduetomagneticfieldinhomogeneities,resultinginmasshavingappearanceofcyst.STIRimage(right)islessaffectedbymagneticfieldinhomogeneity;thereisuniformfatsuppression,revealingcorrectdiagnosisoflipoma.

Note:Vendorsalsoofferhybridfat-suppressionsequenceswhichcombineachemical-shiftselectivepulsewithinversiondelay;theseincludeSPECIAL(GE),SPIR(Philips)andSPAIR(PhilipsandSiemens)• The advantages of CHESS include

its ability to be used with any imaging sequence, and with T1-shortening contrast agents; it is relatively fast, and yields relatively high SNR

• However, it is sensitive to magnetic field inhomogeneities (resulting in “incomplete fat saturation”), and is not as effective on low-field MRI scanners; it is not optimal for off-center imaging or patients with metallic implants

• Water excitation is based on the selective excitation of non-fat-bound protons

• This method of fat suppression is most commonly used with gradient echo MSK MR imaging, particularly cartilage imaging because of its fast imaging time, and high signal- and contrast-to-noise ratios

• Sequences include Spectral Spatial RF (GE), ProSet (Philips) and Water Excitation (Siemens)


3DcoronalT1-weightedspoiledgradientechoMRimageofankleobtainedwithwaterexcitationmethodoffatsuppression.Notehyperintensearticularcartilageoftibiotalarjoint.


FoursagittalMRimagesofanklegeneratedduringoneacquisition,includingfat-suppressedimageusingDixontechnique.Topleft:in-phaseimage.Topright:out-of-phaseimage.Bottomleft:fat-suppressed(watersignal)image.Bottomright:fat-signal(water-suppressed)image.

• The Dixon technique of fat suppression is based on the previously-described in-and-out-of-phase imaging technique

• The Dixon technique can generate 4 different images:

1. A: In-phase = water + fat signal 2. B: Out-of-phase = water – fat signal 3. A + B = water signal (fat-suppressed) 4. A - B = fat signal (water-suppressed)

• Fat suppression with the Dixon technique is uniform, and can be combined with a variety of image weighting; sequences include IDEAL and Flex (GE), mDixon (Philips) and Dixon (Siemens)

Contrast-EnhancedMRImaging

Fat-suppressedT1-weightedtransverseMRimageofhandfollowingintravenousadministrationofGd-contrastagent(left)showsextensortenosynovitis(arrows).Tenosynovitisisnotreadilyapparentonfat-suppressedT2-weightedimage(right).

• Injection of gadolinium (Gd) contrast agents causes T1-shortening and therefore hyperintensity on T1-weighted images

• When injected intravenously, Gd concentrates in vascular tissue and therefore can help distinguish between synovitis and effusion, solid and cystic components of a tumor, and inflammation versus necrosis; contrast also helps detect abscesses and sinus tracts

Fat-suppressedPD-weightedtransverseimageofknee(left)showsjointeffusion(*).Fat-suppressedT1-weightedimagefollowingintravenousadministrationofGd-contrast(right)showsmoderateenhancementalongjoint(arrows)indicatingsynovitis.

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Fat-suppressedT2-weightedcoronalMRimageoffoot(left)showsincreasedsignalintensityoffirstproximalphalanx(*)duetoosteomyelitis.Fat-suppressedT1-weightedimage(right)bettershowsoutlineofsinustract(arrows)extendingfromskinsurfacetobone.

*


T1-weightedtransverseMRimageofknee(left)showsheterogeneoussofttissuesarcomawithhigh-andlow-signalintensitycomponents.FollowingintravenousadministrationofGd-contrastagent(centerimage),portionsofmassenhance(*),indicatingvascularity.Enhancingsolidcomponentscanberenderedmoreconspicuousbysubtractingnon-enhancedimagefromenhancedimage(right).

* *

• Sequences following i.v. Gd administration are typically T1-weighted, often with fat saturation

• A T1-weighted image obtained without fat suppression prior to i.v. Gd administration can be subtracted from a similar image obtained following Gd administration to render enhancement more conspicuous; this can be beneficial if fat-suppression is limited (e.g., due to metal susceptibility artifact)

Fat-suppressedT2-weightedtransverseMRimageofelbow(left)showshomogeneoushighsignalintensitymass(arrow)posteriortoolecranonprocess,mimickingcyst.Fat-suppressedT1-weightedimagesobtainedpriorto(centerimage)andfollowing(right)intravenousGdadministrationshowsvividenhancementofmass,indicatingsolidvasculartissue.Glomustumorwasdiagnosedfollowingresection.

• When diluted and injected into joints, Gd can help to delineate tears of the glenoid and acetabular labrum as well as tears of small ligaments (MR-arthrography)

• Fat-suppressed T1-weighted images are typically acquired following intra-articular Gd administration, but other sequences including proton-density and intermediate-weighted sequences with or without fat suppression can be obtained


T2-weightedcoronalobliqueMRimageofshouldershowscyst(*)insuprascapularnotch.Nolabraltearisidentified.Fat-suppressedT1-weightedimageobtainedfollowingintra-articularinjectionofdilutegadoliniumshowssuperiorlabraltear(arrow).

PD-weightedaxial-obliqueMRimageofhipfollowingintra-articularinjectionofdilutegadolinium-chelateshowsanteriorlabraltear(arrow).

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QuantitativeCompositionalCartilageImaging

T2-mappingofarticularcartilageinthekneewithapulsesequencethatacquiresimageswithdifferentechotimesTE(toprightgraph).

• Cartilage consists predominantly of interstitial water (60-80% by weight), as well as collagen (15-20%) and proteoglycans with attached glycosaminoglycans (10%)

• In addition to the various pulse sequences previously described that can assess the morphology of cartilage, special sequences can be used to quantitatively assess the biochemical composition of cartilage before morphologic changes are appreciated

• T2-mapping (briefly described on the next slide) is the most common of these sequences; other methods include dGEMRIC, T1ρ-imaging, sodium imaging and DWI

• The T2 relaxation time of cartilage is a function of its water content and collagen

• T2 values of cartilage can be obtained using a pulse sequence with multiple echoes at different TEs; software can then be used to create color-coded maps for assessment

• Superficial cartilage layers have higher water content and longer T2 relaxation times, whereas deeper layers have lower water content and shorter T2 relaxation times

• Cartilage degeneration generally results in increased water content and increased T2 relaxation times

QuantitativeCompositionalCartilageImaging

T2-mappingofpatellofemoralarticularcartilage.PulsesequenceacquiresimagesatmultipledifferentTEs(toprow).Pixel-by-pixelT2calculationsaredisplayedoncolor-codedmap.NotehigherT2-valuesinsuperficialcartilagelayer(green)comparedtolowerT2-valuesindeepercartilagelayer(orange).

• A variety of image artifacts can be encountered on MSK MRI exams which can degrade image quality or even simulate lesions

• Some artifacts are dependent on the pulse sequence used; a few such artifacts, including motion artifact, susceptibility artifact, and magic angle artifact, will be briefly described

MRIArtifacts

Protondensity-weightedsagittalMRimageofknee(left)showsmotionartifactsimulatingmeniscaltear(arrow).Fat-suppressedprotondensity-weightedimagefromthesamescan(right)showsnotear.

T2-weightedsagittalMRimageofkneewithchemicalshiftselectivefatsaturation(left)showshighsignalintensitysurroundingtibialinterferencescrew(arrow)mimickingpathology.T2-weightedimageusingDixonmethodoffatsuppression(right)showsnolesion.

• Motion artifacts are typically propagated in phase encoding direction, and can be caused by patient motion, cardiac motion, respiratory motion, peristalsis, and pulsatile flow

• When motion is periodic, discrete “ghosts” form • For motion-prone patients, using pulse

sequences with short acquisition times (e.g., GRE or EPI) or that use radial k-space filling techniques (e.g., PROPELLER [GE)], MultiVane [Philips], BLADE [Siemens]) can be effective

• Saturation pulses, cardiac/respiratory gating methods, and swapping frequency- and phase-encoding directions are other options

MotionArtifacts

Motionartifact(arrow)onfat-suppressedtransversePD-weightedimageofshoulder(left)isreducedonPROPELLERsequence(right)

Pulsationartifactfrompoplitealarterymimicsbonelesion(arrow,left)andcartilagelesion(arrow,right)inpatellaonthesefat-suppressedPD-weightedkneeMRimages.Swappingphaseandfrequencyencodingdirections(notshown)wouldresultinmoredesirablepropagationofartifacthorizontallyonimage(ratherthanverticallyasshown).

SusceptibilityArtifacts

• Susceptibility artifacts arise due to heterogeneity of the local magnetic field at the interface of structures with differing magnetic susceptibilities

• While such artifacts may point to specific disease processes (e.g., PVNS), more often they degrade quality (e.g., foreign bodies)

• Artifacts from surgical implants depend on the type of metal, with stainless steel causing more severe artifact than titanium alloy

Lateralkneeradiograph(topleft)showssmallmetallicforeignbody(needle)insubcutaneousfatanteriortopatellartendon.ForeignbodyresultsinsignalvoidanddistortiononPD-weightedsagittalMRimage(topright)andregionalfailureoffatsuppression(arrows)onPD-weightedtransverseimagewithchemicalshiftselectivefatsuppression(bottomright).

SusceptibilityArtifacts

MetalartifactismorepronouncedontransverseGREMRimageofshoulder(arrows,left)thanonFSEimage(right).

STIRMRimage(left)showsmorehomogeneousfatsuppressionthanCHESSimage(right)ofpatientwithtotalkneearthroplasty(arrow,prosthetictibialstem).

CoronalSTIRMRimageofkneeinpatientwitharthroplastyshowssignaldistortion(left),whichismarkedlyreducedonMAVRICmetalartifactreductionsequence(right).

• Strategies to reduce magnetic susceptibility artifacts include:

• Avoid GRE sequences (which are prone to susceptibility artifacts); use FSE sequences with short echo times instead

• Lengthen ETL, increase receiver bandwidth, decrease TE and voxel size

• Use STIR and Dixon sequences; these will reduce artifact relative to CHESS sequences and provide more homogeneous fat suppression

• Consider proprietary metal artifact reduction sequences (SEMAC, MAVRIC)

• When collagen-containing tissues with parallel molecular alignment are oriented ~55o relative to the main magnetic field, they may exhibit increased signal intensity on pulse sequences with TE < 30 ms

• Such tissues include tendons, ligaments, entheses, peripheral nerves, labra, menisci and articular cartilage

MagicAngleArtifact(MAA)

IncreasedsignalintensityinsupraspinatustendononPD-weightedMRimagewithTE=14ms(arrow,left)disappearsonT2-weightedimagewithTE=80ms(arrow,right).

• MAA is most common with T1- and PD-weighted sequences, but can also be seen on T2-weighted sequences; the artifact can be eliminated with TE values >70 ms; the artifact can also be seen on GRE, STIR, and DWI sequences

• The angular range (beyond 55o) over which MAA occurs increases with decreasing TE

MagicAngleArtifact(MAA)

SpoiledGREsagittalMRimagewithTE=4ms(topleft)andPDimagewithTE=14ms(topright)showincreasedsignalintensityduetomagicangleartifactatproximalpatellarenthesisandinposteriorcruciateligament(arrows).SignaldisappearsonT2-weightedimagewithTE=90ms(bottom).

• A basic knowledge of MR pulse sequences is essential for planning diagnostic musculoskeletal magnetic resonance imaging examinations

• Understanding fundamental MR physics allows better appreciation of the properties of various pulse sequences and methods of fat suppression

• Fast/turbo spin echo pulse sequences remain the most common pulse sequences used in typical MSK MR imaging protocols; however, gradient echo pulse sequences have the advantage of rapid acquisition and therefore are frequently used to create 3D datasets

• Chemical shift selective fat saturation is a commonly used method of fat suppression, but is more sensitive to magnetic field heterogeneity than inversion-based techniques

InConclusion

• Some artifacts, such as motion artifacts, susceptibility artifacts, and magic angle artifacts, are dependent upon the pulse sequence used; these artifacts often can be minimized with forethought

• We hope that this presentation has served as a useful introduction and/or review for residents and fellows studying musculoskeletal MRI

InConclusion

SummarytablePulseSequence Usefulfor…

T1-weighted Confirmingreplacementoffattybonemarrow,determiningdegreeoffattyatrophyofmuscles,detecting/characterizingfattyandhemorrhagiclesions

T2-weighted(oftenwithfatsuppression) Detectionoffluidandedema(andthereforeabnormalitiesinavarietyoftissues)

Proton-density-weighted Identifyingabnormalsignalinnormallysignal-poorstructures(e.g.,fibrocartilage);conspicuityoffluidisincreasedwithfatsuppression;typicallysequenceofchoiceformeniscalimaging

“Intermediate-weighted” Assessmentofarticular(hyaline)cartilage

Gradient-echo Detectionofsusceptibilityartifacts(e.g.,hemosiderininpatientwithPVNS)Rapidscanningallows- 3Dvolumetricacquisitionà isotropicvoxelsà thin/multiplanarreconstructions(e.g.,morphologic

imagingofcartilage),oftenatexpenseofcontrast,blurring- Dynamicmultiphaseimaging(e.g.,fortumorvascularity)ImagescanbeT1-,T2-orT2*-weighted

In-and-opposed-phase Detectionofmicroscopicfat(e.g.,todistinguishredmarrowfrommarrowinfiltratedbytumor)

Diffusion-weighted Determiningrestrictionofdiffusionduetocellularity(e.g.,boneandsoft-tissuetumors,follow-upoftumorspost-therapy,vertebralfracturesandinfection)

Fat-suppression(e.g.,CHESS,STIR,waterexcitation,Dixontechniques)

ImprovingvisibilityoflesionsonPD-,T2-andcontrast-enhancedT1-weightedimages,evaluatingfatinsoft-tissuelesions,methemoglobin;STIRandDixontechniquesbestwithhardware

Contrast-enhancedT1-weighted IntravascularGdinjection:detectingvasculartissue(e.g.,inflammation,tumor)Intra-articularGdinjection:delineatingsmallintra-articularstructures(e.g.,labrum,ligaments)andassociatedabnormalities

Quantitativecompositionalcartilageimaging(e.g.,T2-mapping,dGEMRIC,T1r-imaging,sodiumimaging)

Quantitativeassessmentofbiochemicalcompositionofcartilage

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2019; 290:435-445 • Singh DR et al. Artifacts in musculoskeletal MR imaging. Semin Musculoskelet Radiol 2014; 18:12-22 • Tanitame N et al. Clinical utility of optimized three-dimensional T1-, T2-, and T2*-weighted sequences in spinal magnetic resonance imaging. Jpn J Radiol 2017;

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• We also suggest the web site www.mriquestions.com by Allen D. Elster (© 2018) as a comprehensive but easily digestible resource for material pertaining to general MRI physics.

http://www.mriquestions.com/

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