Notes on MRI - Final

8/11/2019 Notes on MRI - Final

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Magnetic Resonance Imaging (MRI)

Also called nuclear magnetic resonance imaging (NMRI), or magnetic resonance

tomography (MRT).

MRI is a medical imagingtechnique or a radiology technique used to visualize

internal structures o the !ody in detail.

MRI ma"es use o the property o nuclear magnetic resonance(NMR) to image

nuclei o atoms inside the !ody.

#nli"e $T scans or traditional %&rays, MRI does not use ionizing radiation.

In many cases MRI gives di'erent inormation a!out structures in the !ody than

can !e seen ith an %&ray, ultrasound, or $T scan.

An MRI scan uses magnetism, radio aves, and a computer to produce images o

!ody structures.

Reecting the undamental importance and applica!ility o MRI in medicine,

Paul Lauterburo the #niversity o Illinois at #r!ana&$hampaignand Sir PeterMansfeldo the #niversity o Nottinghamere aarded the 2003 Nobel Prizein Phsiolog or Medicine or their *discoveries concerning magneticresonance imaging*.

Princi!le"

MRI is a test that uses a magnetic +eld and pulses o radio ave energy to ma"e

pictures o organs and structures inside the !ody.

Proton nuclear magnetic resonance (NMR) detects the presence ohydrogens (protons) !y su!ecting them to a large magnetic +eld to partiallypolarize the nuclear spins, then e-citing the spins ith properly tuned radiorequency (R) radiation, and then detecting ea" radio requency radiationrom them as they *rela-* rom this magnetic interaction. The requency o thisproton *signal* is proportional to the magnetic +eld to hich they are su!ectedduring this rela-ation process.

#he !roton NMR signals are $uite sensiti%e to di&erences in !rotoncontent that are characteristic o' di&erent inds o' tissue.

In the medical application "non as Magnetic Resonance Imaging (MRI), an

image o a cross&section o tissue can !e made !y producing a ell&cali!ratedmagnetic +eld gradient across the tissue so that a certain value o magnetic +eldcan !e associated ith a given location in the tissue. /ince the proton signalrequency is proportional to that magnetic +eld, a given proton signal requencycan !e assigned to a location in the tissue. This provides the inormation to mapthe tissue in terms o the protons present there. /ince the proton density variesith the type o tissue, a certain amount o contrast is achieved to image theorgans and other tissue variations in the su!ect tissue.

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/ince the MRI uses proton NMR, it images the concentration o' !rotons.

Many o those protons are the protons in ater, so MRI is particularly ell&suitedor the imaging o' so't tissue lie the brain ees and other so't tissuestructures in the head. The !one o the s"ull doesn2t have many protons, so itshos up dar". Also the sinus cavities image as a dar" region.

3ushong2s assessment is that a!out 456 o the !ody2s atoms are hydrogenatoms, so most parts o the !ody have an a!undance o sources or thehydrogen NMR signals hich ma"e up the magnetic resonance image.

*ater constitutes about t+o thirds o' the human bod +eight and this

high +ater content e,!lains +h magnetic resonance imaging hasbecome +idel a!!licable to medicine- #here are di&erences in +atercontent among tissues and organs- In man diseases the !athological!rocess results in changes o' the +ater content and this is re.ected inthe MRI- *ater is a molecule com!osed o' hdrogen and o,gen atoms-#he nuclei o' the hdrogen atoms are able to act as microsco!iccom!ass needles- *hen the bod is e,!osed to a strong magnetic feld

the nuclei o' the hdrogen atoms are directed into order / stand atattention- *hen submitted to !ulses o' radio +a%es the energcontent o' the nuclei changes- 1'ter the !ulse a resonance +a%e isemitted +hen the nuclei return to their !re%ious state- #he smalldi&erences in the oscillations o' the nuclei are detected- ad%ancedcom!uter !rocessing it is !ossible to build u! a threedimensionalimage that re.ects the chemical structure o' the tissue includingdi&erences in the +ater content and in mo%ements o' the +atermolecules- #his results in a %er detailed image o' tissues and organs inthe in%estigated area o' the bod- In this manner !athological changescan be documented-

Scanner"

MRI scanning uses magnetism, radio aves, and a computer to produce images

o !ody structures.

It does not use radiation (-&rays).

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The MRI scanner is a large tu!e surrounded !y a giant circular magnet.

The patient is placed on a movea!le !ed that is inserted into the magnet.

The magnet creates a strong magnetic +eld that aligns the protons o hydrogen

atoms, hich are then e-posed to a !eam o radio aves.

This spins the various protons o the !ody, and they produce a aint signal that is

detected !y the receiver portion o the MRI scanner.

The receiver inormation is processed !y a computer, and an image is produced.

or some procedures, contrast agents, such as gadolinium, are used to increase

the accuracy o the images.

/ingle MRI images are called slices.

The images can !e stored on a computer or printed on +lm.

8ne e-am produces dozens or sometimes hundreds o images.

(a) Magnetic feld

MRI scans require a magnetic +eldith to properties9

uniorm +eld density

strength

The magnetic +eld cannot vary more than 0:05,555 o 06 and +eld strength

ranges (depending on the scanner) rom 5.7 to ; teslasin strength in scannerscurrently used clinically, ith research scanners investigating higher +eldstrengths such as < teslas.

The loer +eld strengths can !e achieved ith permanent magnets, hich areoten used in *open* MRI scanners or claustropho!icpatients.

=igher +eld strengths can !e achieved only ith superconducting magnets. An

MRI ith a ;.5 tesla strength magnet may !e reerred to as a *3# MRI* or *3tesla MRI*

/ince the gradient coils are ithin the !ore o the scanner, there are large orces

!eteen them and the main +eld coils, producing most o the hammering noisethat is heard during operation. >ithout e'orts to damp this noise, it canapproach 0;5 deci!els(d3) ith strong +elds.

(b) 4ontrast agents and im!lants

MRI contrast agents may !e in5ected intra%enousl to enhance the

appearance o !lood vessels, tumorsor inammation.

$ontrast agents may also !e directl in5ected into a 5oint in the case o

arthrograms9 MRI images o oints.

The most commonly used intravenous contrast agents are !ased on chelateso

gadolinium. Recently, a ne contrast agent named gado,etate, !rand name6o%ist(#/) or Primo%ist(?#), as approved or diagnostic use.

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(c) Portable instruments

@orta!le magnetic resonance instruments are availa!le or use in education and

+eld research. #sing the principles o ?arth2s +eld NMR, they have no poerulpolarizing magnet, so that such instruments can !e small and ine-pensive. /omecan !e used or !oth ?NMR spectroscopy and MRI imaging. The lo strength o

the ?arth2s +eld results in poor signal to noise ratios (/NR), requiring long scantimes to capture spectroscopic data or !uild up MRI images. =oever, thee-tremely lo noise oor o S78I9based MRI detectorsand the lo densityo thermal noise in the lo&requency operating range (tens o "ilo=ertz) mayresult in usa!le /NR approaching that o mid&+eld conventional instruments.urther, the ultra&lo +eld technologies ena!le electron spin resonancedetection, and potentially imaging, at sae operating requencies.

Research ith atomic magnetometershas addressed the possi!ility o cheap and

porta!le MRI instruments ithout a large magnet.

(d) asic MRI scans"

T0&eighted MRI T7&eighted MRI TB7&eighted MRI T/pin density eightedMRI.

(e) S!ecialized MRI scans"

Ciusion MRI Magnetization transer MRI T0rho or T0D MRI luid attenuated

inversion recovery (EAIR) Magnetic resonance angiography Magneticresonance gated intracranial $/ dynamics (MR&FIEC) Magnetic resonancespectroscopy unctional MRI (MRI) Real&time MRI Interventional MRI Radiationtherapy simulation $urrent density imaging ($CI) Magnetic resonance guidedocused ultrasound Multinuclear imaging /uscepti!ility eighted imaging (/>I)@repolarized MRI or @MRI.

(') :ther s!ecialized MRI techni$ues" Ne methods and variants o e-isting methods are oten pu!lished hen they

are a!le to produce !etter results in speci+c +elds. ?-amples o these recentimprovements are T;2+eighted turbo s!inecho (T2 #S6 MRI) double in%ersion reco%erMRI (9IRMRI) or !hasesensiti%e in%ersion reco%er MRI (PSIRMRI), allo them a!le to improve imaging o !rain lesions. Another e-ample is M@&RAF?(magnetization&prepared rapid acquisition ith gradient echo), hich improvesimages o multiple sclerosis cortical lesions.

Procedure" A strong magnetic +eld is created !y passing an electric current through the ire

loops.

>hile this is happening, other coils in the magnet send and receive radio aves.

This triggers protons in the !ody to align themselves.

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8nce aligned, radio aves are a!sor!ed !y the protons, hich stimulate

spinning.

?nergy is released ater *e-citing* the molecules, hich in turn emits energy

signals that are pic"ed up !y the coil.

This inormation is then sent to a computer hich processes all the signals andgenerates it into an image.

The +nal product is a ;&C image representation o the area !eing e-amined.

*oring o' MRI machine

MRI machines ma"e use o the act that !ody tissue contains lots o ater, and

hence protons(0= nuclei), hich ill !e aligned in a large magnetic +eld. ?achater molecule has to hydrogennucleior protons. >hen a person is inside thepoerul magnetic +eldo the scanner, the average magnetic momento manyprotons !ecomes aligned ith the direction o the +eld. A radio requencycurrent is !riey turned on, producing a varying electromagnetic +eld. Thiselectromagnetic +eld has ust the right requency, "non as the resonancerequency, to !e a!sor!ed and ip the spino the protons in the magnetic +eld.Ater the electromagnetic +eld is turned o', the spins o the protons return tothermodynamic equili!rium and the !ul" magnetization !ecomes realigned iththe static magnetic +eld. Curing this rela-ation, a radio requency signal(electromagnetic radiationin the R range) is generated, hich can !e measuredith receiver coils.

Inormation a!out the origin o the signal in ;Cspace can !e learned !y applying

additional magnetic +elds during the scan. These additional magnetic +elds can!e used to generate detecta!le signals only rom speci+c locations in the !ody

(spatial e-citation) and:or to ma"e magnetization at di'erent spatial locationsprecess at di'erent requencies, hich ena!les "&space encoding o spatialinormation. The ;C images o!tained in MRI can !e rotated along ar!itraryorientations and manipulated !y the doctor to !e !etter a!le to detect tinychanges o structures ithin the !ody. These +elds, generated !y passingelectric currents through gradient coils, ma"e the magnetic +eld strength varydepending on the position ithin the magnet. 3ecause this ma"es the requencyo the released radio signal also dependent on its origin in a predicta!le manner,the distri!ution o protons in the !ody can !e mathematically recovered rom thesignal, typically !y the use o inverse ourier transorm.

@rotons in di'erent tissues return to their equili!rium state at di'erent rela-ation

rates. Ci'erent tissue varia!les, including spin density, T0 and T7 rela-ationtimes, and o and spectral shits, can !e used to construct images. 3ychanging the settings on the scanner, this e'ect is used to create contrast!eteen di'erent types o !ody tissue or !eteen other properties, as in MRIand di'usion MRI.

The MRI e-amination requires specialized equipment that uses a poerul,

constant magnetic +eld, rapidly changing local magnetic +elds, radiorequency

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energy, and dedicated equipment including a poerul computer to create veryclear pictures o internal !ody structures.

1d%antages"

?ven though the spatial resolution o MRI is not as great as a conventional -&ray+lm, its contrast resolution is much !etter or tissue. MRI can create moredetailed images o' the human bod than are !ossible +ith


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@ara&sagittal MRI o the head, ith aliasing artiacts (nose and oreheadappear at the !ac" o the head)

In clinical practice, MRI is used to distinguish pathologic tissue (such as a !rain

tumor) rom normal tissue.

8ne advantage o an MRI scan is that it is harmless to the patient. It uses strong

magnetic +elds and non&ionizing electromagnetic +elds in the radio requencyrange, unli"e $T scansand traditional %&rays, hich !oth use ionizing radiation.

>hile $T provides good spatial resolution(the a!ility to distinguish to separate

structures at a small distance rom each other), MRI provides compara!leresolution ith ar !etter contrast resolution (the a!ility to distinguish thedi'erences !eteen to similar !ut not identical tissues). The !asis o thisa!ility is the comple- li!rary o pulse sequences that the modern medical MRIscanner includes, each o hich is optimized to provide image contrast !ased onthe chemical sensitivity o MRI.

6&ects o' #R and #6 on MR signal

or e-ample, ith particular values o the echo time (T?) and the repetition time

(TR), hich are !asic parameters o image acquisition, a sequence ta"es on theproperty o T7&eighting. 8n a T7&eighted scan, ater& and uid&containingtissues are !right (most modern T7 sequences are actually ast T7 sequences)and at&containing tissues are dar". The reverse is true or T0&eighted images.Camaged tissue tends to develop edema, hich ma"es a T7&eighted sequencesensitive or pathology, and generally a!le to distinguish pathologic tissue romnormal tissue. >ith the addition o an additional radio requency pulse andadditional manipulation o the magnetic gradients, a T7&eighted sequence can!e converted to a EAIR sequence, in hich ree ater is no dar", !utedematous tissues remain !right. This sequence in particular is currently themost sensitive ay to evaluate the !rain or demyelinatingdiseases, such asmultiple sclerosis.

The typical MRI e-amination consists o HJ75 sequences, each o hich is chosen

to provide a particular type o inormation a!out the su!ect tissues. Thisinormation is then synthesized !y the interpreting physician.

Molecular imaging o disease !iomar"ers !y MRI9

MRI has the advantages o having very high spatial resolution and is very

adept at morphological imaging and unctional imaging. MRI does have

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several disadvantages though. irst, MRI has a sensitivity o around 05K;mol:Eto 05KH mol:E hich, compared to other types o imaging, can !e verylimiting. This pro!lem stems rom the act that the di'erence !eteen atomsin the high energy state and the lo energy state is very small. or e-ample,at 0.H teslas, a typical +eld strength or clinical MRI, the di'erence !eteenhigh and lo energy states is appro-imately L molecules per 7 million.

Improvements to increase MR sensitivity include increasing magnetic +eldstrength, and hyperpolarization via optical pumping or dynamic nuclearpolarization. There are also a variety o signal ampli+cation schemes !asedon chemical e-change that increase sensitivity.

To achieve molecular imaging o disease !iomar"ers using MRI, targeted MRI

contrast agents ith high speci+city and high rela-ivity (sensitivity) arerequired. To date, many studies have !een devoted to developing targeted&MRI contrast agents to achieve molecular imaging !y MRI. $ommonly,peptides, anti!odies, or small ligands, and small protein domains, such as=?R&7 a!odies, have !een applied to achieve targeting. To enhance thesensitivity o the contrast agents, these targeting moieties are usually lin"ed

to high payload MRI contrast agents or MRI contrast agents ith highrela-ivities.

It is used to +nd pro!lems such as tumors, !leeding, inury, !lood vessel

diseases, or inection.

MRI also may !e done to provide more inormation a!out a pro!lem seen on an

%&ray, ultrasound scan, or $T scan.

$ontrast material may !e used during MRI to sho a!normal tissue more clearly.

An MRI scan can !e done or the9

=ead. MRI can loo" at the !rain or tumors, an aneurysm, !leeding in the

!rain, nerve inury, and other pro!lems, such as damage caused !y a stro"e.MRI can also +nd pro!lems o the eyesand optic nerves, and the earsandauditory nerves.

$hest. MRI o the chest can loo" at the heart, the valves, and coronary !lood

vessels. It can sho i the heart or lungsare damaged. MRI o the chest mayalso !e used to loo" or !reastor lung cancer.

3lood vessels. #sing MRI to loo" at !lood vessels and the o o !lood

through them is called magnetic resonance angiography (MRA). It can +ndpro!lems o the arteriesand veins, such as an aneurysm, a !loc"ed !loodvessel, or the torn lining o a !lood vessel (dissection). /ometimes contrast

material is used to see the !lood vessels more clearly. A!domenand pelvis. MRI can +nd pro!lems in the organs and structures in

the !elly, such as the liver, gall!ladder, pancreas, "idneys, and !ladder. It isused to +nd tumors, !leeding, inection, and !loc"age. In omen, it can loo"at the uterus and ovaries. In men, it loo"s at the prostate.

3ones and oints. MRI can chec" or pro!lems o the !ones and oints, such as

arthritis, pro!lems ith the temporomandi!ular oint, !one marropro!lems,!one tumors, cartilagepro!lems, torn ligamentsor tendons, or inection. MRI

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http://en.wikipedia.org/wiki/Concentration#Molarityhttp://en.wikipedia.org/wiki/Tesla_(unit)http://en.wikipedia.org/wiki/Hyperpolarization_(physics)http://www.webmd.com/hw-popup/aneurysmhttp://www.webmd.com/brain/picture-of-the-brainhttp://www.webmd.com/hw-popup/stroke-7439http://www.webmd.com/eye-health/picture-of-the-eyeshttp://www.webmd.com/hw-popup/optic-nerve-7742http://www.webmd.com/brain/picture-of-the-earhttp://www.webmd.com/hw-popup/auditory-nervehttp://www.webmd.com/heart/picture-of-the-hearthttp://www.webmd.com/hw-popup/coronary-arterieshttp://www.webmd.com/hw-popup/coronary-arterieshttp://www.webmd.com/lung/picture-of-the-lungshttp://www.webmd.com/hw-popup/breast-cancer-8310http://www.webmd.com/hw-popup/lung-cancerhttp://www.webmd.com/heart/anatomy-picture-of-bloodhttp://www.webmd.com/hw-popup/magnetic-resonance-angiogram-mrahttp://www.webmd.com/heart/picture-of-the-arterieshttp://www.webmd.com/digestive-disorders/picture-of-the-abdomenhttp://www.webmd.com/digestive-disorders/picture-of-the-liverhttp://www.webmd.com/digestive-disorders/picture-of-the-gallbladderhttp://www.webmd.com/digestive-disorders/picture-of-the-pancreashttp://www.webmd.com/urinary-incontinence-oab/picture-of-the-kidneyshttp://www.webmd.com/urinary-incontinence-oab/picture-of-the-bladderhttp://www.webmd.com/urinary-incontinence-oab/picture-of-the-prostatehttp://www.webmd.com/hw-popup/arthritishttp://www.webmd.com/hw-popup/temporomandibular-tm-jointshttp://www.webmd.com/hw-popup/bone-marrowhttp://www.webmd.com/cancer/bone-tumorshttp://www.webmd.com/hw-popup/cartilage-8268http://www.webmd.com/hw-popup/ligamenthttp://www.webmd.com/hw-popup/tendonhttp://en.wikipedia.org/wiki/Concentration#Molarityhttp://en.wikipedia.org/wiki/Tesla_(unit)http://en.wikipedia.org/wiki/Hyperpolarization_(physics)http://www.webmd.com/hw-popup/aneurysmhttp://www.webmd.com/brain/picture-of-the-brainhttp://www.webmd.com/hw-popup/stroke-7439http://www.webmd.com/eye-health/picture-of-the-eyeshttp://www.webmd.com/hw-popup/optic-nerve-7742http://www.webmd.com/brain/picture-of-the-earhttp://www.webmd.com/hw-popup/auditory-nervehttp://www.webmd.com/heart/picture-of-the-hearthttp://www.webmd.com/hw-popup/coronary-arterieshttp://www.webmd.com/hw-popup/coronary-arterieshttp://www.webmd.com/lung/picture-of-the-lungshttp://www.webmd.com/hw-popup/breast-cancer-8310http://www.webmd.com/hw-popup/lung-cancerhttp://www.webmd.com/heart/anatomy-picture-of-bloodhttp://www.webmd.com/hw-popup/magnetic-resonance-angiogram-mrahttp://www.webmd.com/heart/picture-of-the-arterieshttp://www.webmd.com/digestive-disorders/picture-of-the-abdomenhttp://www.webmd.com/digestive-disorders/picture-of-the-liverhttp://www.webmd.com/digestive-disorders/picture-of-the-gallbladderhttp://www.webmd.com/digestive-disorders/picture-of-the-pancreashttp://www.webmd.com/urinary-incontinence-oab/picture-of-the-kidneyshttp://www.webmd.com/urinary-incontinence-oab/picture-of-the-bladderhttp://www.webmd.com/urinary-incontinence-oab/picture-of-the-prostatehttp://www.webmd.com/hw-popup/arthritishttp://www.webmd.com/hw-popup/temporomandibular-tm-jointshttp://www.webmd.com/hw-popup/bone-marrowhttp://www.webmd.com/cancer/bone-tumorshttp://www.webmd.com/hw-popup/cartilage-8268http://www.webmd.com/hw-popup/ligamenthttp://www.webmd.com/hw-popup/tendon


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may also !e used to tell i a !one is !ro"en hen %&ray results are not clear.MRI is done more commonly than other tests to chec" or some !one and

oint pro!lems.

/pine. MRI can chec" the discs and nerves o the spine or conditions such as

spinal stenosis, disc!ulges, and spinal tumors.

It is a procedure used in hospitals to scan patients and determine the severity o

certain inuries.

In general, MRI creates pictures that can sho di'erences !eteen healthy and

unhealthy tissue.

Coctors use MRI to e-amine the !rain, spine, oints (e.g., "nee, shoulder, rist,

and an"le), a!domen, pelvic region, !reast, !lood vessels, heart and other !odyparts.

enefts and Riss

#nli"e $T, MRI uses no ionizing radiation and is generally a %er sa'e

!rocedure.

Nonetheless the strong magnetic felds and radio !ulses can a&ect metal

im!lants including cochlear im!lantsand cardiac !acemaers.

There are many electronically activated devices that have approval rom the #/

CA to permit MRI procedures in patients under highly speci+c MRI conditions.

In the case o cochlear implants, the #/ CAhas approved some implants or

MRI compati!ility.

In the case o cardiac pacema"ers, the results can sometimes !e lethal, so

patients ith such implants are generally not eligi!le or MRI.

Cespite !eing !ainless, MRI scans can !e unpleasant or those ho are

claustro!hobic or other+ise uncom'ortable ith the imaging devicesurrounding them. 8lder closed !ore MRI systems have a airly long tu!e ortunnel. The part o the !ody !eing imaged must lie at the center o the magnet,hich is at the a!solute center o the tunnel. 3ecause scan times on these olderscanners may !e long (occasionally up to G5 minutes or the entire procedure),people ith even mild claustropho!ia are sometimes una!le to tolerate an MRIscan ithout management. /ome modern scanners have larger !ores (up to


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A num!er o eatures o MRI scanning can give rise to ris"s. These include9

@oerul magnetic +elds

Radio aves

$ryogenic liquids

Noise

$laustropho!ia

In addition, in cases here MRI contrast agentsare used, these also typically

have associated ris"s.

:%eruse9 Medical societies issue guidelines or hen physicians should use MRI

on patients and recommend against overuse. MRI can detect health pro!lems orcon+rm a diagnosis, !ut medical societies oten recommend that MRI not !e the+rst procedure or creating a plan to diagnose or manage a patient2s complaint.A common case is to use MRI to see" a cause o lo !ac" pain the American

$ollege o @hysicians, or e-ample, recommends against this procedure asunli"ely to result in a positive outcome or the patient. Nevertheless, MRI has theadvantage o not utilizing ionizing radiation to create medical images, unli"eother imaging modalities such as $T and conventional radiography.

Magnetic feld9 /ome types o medical implants are generally considered

contraindications or MRI e-aminations, hile others may !e accepta!le orpatients under high speci+c MRI conditions. @atients are thereore alays as"edor complete inormation a!out all implants !eore entering the room or an MRIscan. /everal deaths have !een reported in patients ith pacema"ers ho haveundergone MRI scanning ithout appropriate precautions. To reduce such ris"s,implants are increasingly !eing developed to ma"e them a!le to !e saelyscanned, and specialized protocols have !een developed to permit the sae

scanning o selected implants and pacing devices. $ardiovascular stents areconsidered sae, hoever.

MR&/ae sign MR&$onditional sign MRnsaesign

erromagneticoreign !odies such as shellragments, or metallic implants suchas surgical prosthesesand erromagnetic aneurysmclips are also potential ris"s.Interaction o the magnetic and radio requency +elds ith such o!ects can lead

to trauma due to movement o the o!ect in the magnetic +eld or thermal inuryrom radio&requency induction heatingo the o!ect.

Titaniumand its alloys are sae rom attraction and torque orces produced !ythe magnetic +eld, though there may !e some ris"s associated ith Eenz e'ectorces acting on titanium implants in sensitive areas ithin the su!ect, such asstapesimplants in the inner ear.

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http://en.wikipedia.org/wiki/MRI_contrast_agenthttp://en.wikipedia.org/wiki/Low_back_painhttp://en.wikipedia.org/wiki/American_College_of_Physicianshttp://en.wikipedia.org/wiki/American_College_of_Physicianshttp://en.wikipedia.org/wiki/Contraindicationhttp://en.wikipedia.org/wiki/Cardiovascular_stenthttp://en.wikipedia.org/wiki/Ferromagnetichttp://en.wikipedia.org/wiki/Shell_(projectile)http://en.wikipedia.org/wiki/Surgical_prostheticshttp://en.wikipedia.org/wiki/Aneurysmhttp://en.wikipedia.org/wiki/Induction_heatinghttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Lenz's_lawhttp://en.wikipedia.org/wiki/Stapeshttp://en.wikipedia.org/wiki/File:MR_unsafe_sign.svghttp://en.wikipedia.org/wiki/File:MR_conditional_sign.svghttp://en.wikipedia.org/wiki/File:MR_safe_sign.svghttp://en.wikipedia.org/wiki/MRI_contrast_agenthttp://en.wikipedia.org/wiki/Low_back_painhttp://en.wikipedia.org/wiki/American_College_of_Physicianshttp://en.wikipedia.org/wiki/American_College_of_Physicianshttp://en.wikipedia.org/wiki/Contraindicationhttp://en.wikipedia.org/wiki/Cardiovascular_stenthttp://en.wikipedia.org/wiki/Ferromagnetichttp://en.wikipedia.org/wiki/Shell_(projectile)http://en.wikipedia.org/wiki/Surgical_prostheticshttp://en.wikipedia.org/wiki/Aneurysmhttp://en.wikipedia.org/wiki/Induction_heatinghttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Lenz's_lawhttp://en.wikipedia.org/wiki/Stapes


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In the #nited /tatesa classi+cation system or implants and ancillary clinicaldevices has !een developed !y A/TM Internationaland is no the standardsupported !y the #/ ood and Crug Administration9

MRSa'e The device or implant is completely non&magnetic, non&

electrically conductive, and non&R reactive, eliminating all o the primary

potential threats during an MRI procedure. MR4onditional A device or implant that may contain magnetic,

electrically conductive or R&reactive components that is sae or operationsin pro-imity to the MRI, provided the conditions or sae operation are de+nedand o!served (such as 2tested sae to 0.H teslasO or Psae in magnetic +elds!elo H55 gauss in strengthO).

MR8nsa'e Nearly sel&e-planatory, this category is reserved or o!ects

that are signi+cantly erromagnetic and pose a clear and direct threat topersons and equipment ithin the magnet room.

The very high strength o the magnetic +eld can also cause *missile&e'ect*accidents, here erromagnetic o!ects are attracted to the center o themagnet, and there have !een incidents o inury and death. To reduce the ris" oproectile accidents, erromagnetic o!ects and devices are typically prohi!ited inthe pro-imity o the MRI scanner and patients undergoing MRI e-aminations arerequired to remove all metallic o!ects, oten !y changing into a gon or scru!s,and erromagnetic detection devices are used at some sites.

There is no evidence or !iological harm rom even very poerul static magnetic+elds.

Peri!heral ner%e stimulation (PNS)" The rapid sitching on and o' o the

magnetic +eld gradients is capa!le o causing nerve stimulation. 1olunteersreport a titching sensation hen e-posed to rapidly sitched +elds, particularly

in their e-tremities. The reason the peripheral nerves are stimulated is that thechanging +eld increases ith distance rom the center o the gradient coils(hich more or less coincides ith the center o the magnet). Although @N/ asnot a pro!lem or the slo, ea" gradients used in the early days o MRI, thestrong, rapidly sitched gradients used in techniques such as ?@I, MRI, di'usionMRI, etc. are indeed capa!le o inducing @N/. American and ?uropean regulatoryagencies insist that manuacturers stay !elo speci+ed d3:dt limits (d3:dt is thechange in +eld per unit time) or else prove that no @N/ is induced or anyimaging sequence. As a result o d3:dt limitation, commercial MRI systemscannot use the ull rated poer o their gradient ampli+ers.

=eating caused b absor!tion o' radio +a%es" ?very MRI scanner has a

poerul radio transmitter to generate the electromagnetic +eld hich e-cites

the spins. I the !ody a!sor!s the energy, heating occurs. or this reason, thetransmitter rate at hich energy is a!sor!ed !y the !ody has to !e limited.

1coustic noise9 /itching o +eld gradients causes a change in the Eorentz

orce e-perienced !y the gradient coils, producing minute e-pansions andcontractions o the coil itsel. As the sitching is typically in the audi!lerequency range, the resulting vi!ration produces loud noises (clic"ing or!eeping). This is most mar"ed ith high&+eld machines and rapid&imaging

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techniques in hich sound pressure levels can reach 075 d3(A)(equivalent to aet engine at ta"e&o'), and thereore appropriate ear protection is essential oranyone inside the MRI scanner room during the e-amination.

4rogens9 Many MRI scanners rely on cryogenic liquids to ena!le the

superconducting capa!ilities o the electromagnetic coils ithin. Though the

cryogenic liquids used are non&to-ic, their physical properties present speci+chazards.

An unintentional shut&don o a superconducting electromagnet, an event"non as *quench*, involves the rapid !oiling o liquid heliumrom the device. Ithe rapidly e-panding helium cannot !e dissipated through an e-ternal vent,sometimes reerred to as a 2quench pipe2, it may !e released into the scannerroom here it may cause displacement o the o-ygen and present a ris" oasphy-iation.

8-ygen de+ciency monitors are usually used as a saety precaution. Eiquidhelium, the most commonly used cryogen in MRI, undergoes near e-plosivee-pansion as it changes rom a liquid to gaseous state. The use o an o-ygen

monitor is important to ensure that o-ygen levels are sae or patient:physicians.Rooms !uilt or superconducting MRI equipment should !e equipped ithpressure relie mechanisms and an e-haust an, in addition to the requiredquench pipe.

3ecause a quench results in rapid loss o cryogens rom the magnet,recommissioning the magnet is e-pensive and time&consuming. /pontaneousquenches are uncommon, !ut a quench may also !e triggered !y an equipmentmalunction, an improper cryogen +ll technique, contaminants inside thecryostat, or e-treme magnetic or vi!rational distur!ances.

4ontrast agents9 The most commonly used intravenous contrast agents are

!ased on chelateso gadolinium. In general, these agents have proved saer

than the iodinated contrast agents used in %&ray radiography or $T.Anaphylactoid reactionsare rare, occurring in appro-. 5.5;J5.06. 8 particularinterest is the loer incidence o nephroto-icity, compared ith iodinatedagents, hen given at usual dosesthis has made contrast&enhanced MRIscanning an option or patients ith renal impairment, ho ould otherise not!e a!le to undergo contrast&enhanced $T.

Although gadolinium agents have proved useul or patients ith renalimpairment, in patients ith severe renal ailure requiring dialysis there is a ris"o a rare !ut serious illness, nephrogenic systemic +!rosis, hich may !e lin"edto the use o certain gadolinium&containing agents. The most requently lin"ed isgadodiamide, !ut other agents have !een lin"ed too. Although a causal lin" has

not !een de+nitively esta!lished, current guidelines in the #nited /tatesare thatdialysis patients should only receive gadolinium agents here essential, and thatdialysisshould !e perormed as soon as possi!le ater the scan to remove theagent rom the !ody promptly. In ?urope, here more gadolinium&containingagents are availa!le, a classi+cation o agents according to potential ris"s has!een released. Recently, a ne contrast agent named gado,etate, !rand name6o%ist (#/) or Primo%ist(?#), as approved or diagnostic use9 this has thetheoretical !ene+t o a dual e-cretion path.

07 & /R1
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Pregnanc9 No e'ects o MRI on the etus have !een demonstrated. In

particular, MRI avoids the use o ionizing radiation, to hich the etus isparticularly sensitive. =oever, as a precaution, current guidelines recommendthat pregnant omen undergo MRI only hen essential. This is particularly thecase during the +rst trimester o pregnancy, as organogenesista"es place duringthis period. The concerns in pregnancy are the same as or MRI in general, !utthe etus may !e more sensitive to the e'ectsparticularly to heating and tonoise. =oever, one additional concern is the use o contrast agents gadoliniumcompounds are "non to cross the placenta and enter the etal !loodstream,and it is recommended that their use !e avoided.

There are no no+n harm'ul e&ects'rom the strong magnetic feldused

or MRI. 3ut the magnet is very poerul. The magnet ma a&ect!acemaers artifcial limbs and other medical de%ices that containiron. The magnet ill sto! a +atch that is close to the magnet. Any loosemetal ob5ect has the ris o' causing damage or in5ur i' it gets !ulledto+ard the strong magnet.

Metal parts in the eyes can damage the retina.

Iron pigments in tattoos or tattooed eyeliner can cause sinor ee irritation.

An MRI can cause a burn +ith some medication!atches.

There is a slight ris o' an allergic reactioni contrast material is used during

the MRI. 3ut most reactions are mildand can be treated using medicine.

#est is not done +hen9

Pregnanc. An MRI test usually is not done during pregnancy. 3ut MRI may

!e done to get more inormation a!out a possi!le pro!lem that cannot !eseen clearly ith ultrasound.

Medical de%ices that use electronics, such as a pacema"er or medicine

inusion pump. The MRI magnet may cause pro!lems ith these devices, andthat may "eep one rom having an MRI.

Medical de%ices that ha%e metal in them. The metal might ma"e some o

the detailed MRI pictures !lurry. This may prevent the doctor rom seeing theorgan that is !eing loo"ed at. or e-ample, an intrauterine device (I#C)ithmetal may prevent doctor rom seeing the uterus clearly.

Inabilit to remain still during the test.

:besit. A person ho is very overeight may not +t into standard MRI

machines.

MRI has !een shon to !e e,tremel sa'e as long as !ro!er sa'et

!recautions are taen.

In general, the MRI procedure produces no !ainand causes no no+n short

term or longterm tissue damage o' an ind.

The poerul magnetic +eld o the scanner can attract certain metallic

ob5ects no+n as >'erromagnetic? ob5ects, causing them to move suddenly

0; & /R1
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and ith great orce toards the center o the MR system. This may pose a ris"to the patient or anyone in the ay o the o!ect. Thereore, great care istaen to !re%ent 'erromagnetic ob5ects 'rom entering the MR systemroom. It is vital to remove metallic o!ects in advance o an MRI e-am, includingatches, eelry, and items o clothing that have metallic threads or asteners.

MRI acilities have screening !rocedures that, hen careully olloed, illensure that the MRI technologist and radiologist "nos a!out the presence ometallic implants and materials so that special precautions can !e ta"en. Insome unusual cases the e-amination may !e canceled !ecause o concernrelated to a particular implant or device. or e-ample, i an MRI is ordered, itmay !e cancelled i the patient has a erromagnetic aneurysm clip !ecause othe ris" dislodging the clip rom the !lood vessel. Also, the magnetic +eld o thescanner can damage an e,ternal hearing aid or cause a heart !acemaerto mal'unction. I a !ullet or other metallic ragment in present in the !odythere is a potential ris" that it could change position, possi!ly causing inury.

Some conditions ma mae MRI e,amination inad%isable9

=eart pacema"er

$ere!ral aneurysm clip (metal clip on a !lood vessel in the !rain)

@regnancy

Implanted insulin pump (or treatment o dia!etes), narcotics pump (or pain

medication), or implanted nerve stimulators (*T?N/*) or !ac" pain

Metal in the eye or eye soc"et

$ochlear (ear) implant or hearing impairment

Implanted spine sta!ilization rods

/evere lung disease (such as tracheomalacia or !ronchopulmonary dysplasia)

Fastroesophageal reu-

>eight o more than ;55 pounds

Ina!ility to lie on !ac" or ;5 to 5 minutes

$laustropho!ia (ear o closed or narro spaces)

Cespite these concerns, MRI is rapidly groing in importance as a ay odiagnosing and monitoring congenital deectso the etus !ecause it can providemore diagnostic inormation than ultrasoundand it lac"s the ionizing radiation o$T. MRI ithout contrast agents is the imaging mode o choice or pre&surgical,

in&utero diagnosis and evaluation o etal tumors, primarily teratomas,acilitating open etal surgery, other etal interventions, and planning orprocedures (such as the ?%IT procedure) to saely deliver and treat !a!ies hosedeects ould otherise !e atal.

4# %s MRI

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The use o %&rays, a type o ionizing radiation, in computed tomography ($T)

allos or e-amination o tissues composed o elements o a higher atomicnum!er than the surrounding tissues. MRI, in contrast, uses non&ionizing radiorequency (R) signals to acquire images and is !est suited or sot tissue(although MRI can also !e used to visualize !ones, teeth and even ossils).

/ince $T scans use ionizing radiation (%&rays) to produce images, there is a ris"o damage to CNA that can su!sequently cause cancer. In 755hile 4# !ro%ides good s!atial resolution(the abilit to distinguish t+o

se!arate structures at a small distance 'rom each other) , MRI providescom!arable resolution +ith 'ar better contrast resolution(the abilit to

0H & /R1
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distinguish the di&erences bet+een t+o similar but not identicaltissues).

It uses strong magnetic +elds and non&ionizing electromagnetic +elds in the

radio requency range, unli"e $T scans and traditional %&rays, hich !oth useionizing radiation.

>hile $T provides good spatial resolution(the a!ility to distinguish to separate

structures at a small distance rom each other), MRI !ro%ides com!arableresolution +ith 'ar better contrast resolution(the abilit to distinguishthe di&erences bet+een t+o similar but not identical tissues) . The !asiso this a!ility is the comple- li!rary o pulse sequences that the modern medicalMRI scanner includes, each o hich is optimized to provide image contrast!ased on the chemical sensitivity o MRI.

The most commonly used intra%enous contrast agents are based on

chelateso' gadolinium. In general, these agents have proved sa'er than theiodinated contrast agentsused in %&ray radiography or $T.

0 & /R1
http://en.wikipedia.org/wiki/Computed_axial_tomographyhttp://en.wikipedia.org/wiki/Radiographyhttp://en.wikipedia.org/wiki/Ionizing_radiationhttp://en.wikipedia.org/wiki/Spatial_resolutionhttp://en.wikipedia.org/wiki/Contrast_resolutionhttp://en.wikipedia.org/wiki/Chelatehttp://en.wikipedia.org/wiki/Gadoliniumhttp://en.wikipedia.org/wiki/Computed_axial_tomographyhttp://en.wikipedia.org/wiki/Radiographyhttp://en.wikipedia.org/wiki/Ionizing_radiationhttp://en.wikipedia.org/wiki/Spatial_resolutionhttp://en.wikipedia.org/wiki/Contrast_resolutionhttp://en.wikipedia.org/wiki/Chelatehttp://en.wikipedia.org/wiki/Gadolinium

Documents

Notes on MRI - Final