Cardiovascular magnetic resonance imaging is becoming a routine diagnostic technique.

Transcription

1 Crdic mgnetic resonnce imging Crdiovsculr mgnetic resonnce imging is ecoming routine dignostic technique. BRUCE S SPOTTISWOODE, PhD MRC/UCT Medicl Imging Reserch Unit, University of Cpe Town, nd Division of Rdiology, Stellenosch University Bruce Spottiswoode hs BSc in Electricl Engineering from the University of the Witwtersrnd nd PhD in Biomedicl Engineering on crdic MRI from the University of Cpe Town. He hs worked on developing electronics for the CSIR, on MRI imge reconstruction for Siemens, nd on X-ry imging for Lodox Systems. He is currently the Director of the Cpe Universities Brin Imging Centre. His current fields of interest re crdiovsculr nd neuro-mgnetic resonnce imging. Correspondence to: Bruce Spottiswoode (spotty@gmil.com) Crdiovsculr mgnetic resonnce imging (CMR) technology hs dvnced significntly over the pst two decdes, nd the modlity is now eing used routinely in rdiologicl investigtions. CMR is frequently referred to one-stop shop for dignosis s it provides more comined informtion out crdic viility, morphology nd function thn ny other imging modlity. This pper descries the sic principles of CMR nd outlines stndrd CMR investigtion. This is followed y n overview of the vriety of estlished CMR cpilities ville. Mgnetic resonnce imging (MRI) is renowned for its ility to distinguish etween soft-tissue types. Principles of CMR Mgnetic resonnce imging (MRI) is renowned for its ility to distinguish etween soft-tissue types. It is lso non-invsive nd it does not involve ionising rdition or rdioctive compounds. An MRI imge is creted y plcing the ptient in lrge mgnetic field nd using rdio wves to pertur free hydrogen protons which hve ligned themselves with the mgnetic field. These protons then give off rdio wves, which re picked up y receiver. An imge is otined using series of rdio wves nd mgnetic field grdients. The signl given off y the protons cn e divided into severl components (T1, T2, nd T2*) which re unique to the tissue eing imged. It is these components tht re used to crete imges with differing imge intensity contrsts etween tissue types, concept known s imge weighting. This is demonstrted in Fig. 1, which shows imges of the sme trnsverse slice of humn rin otined using three different imge weighting techniques. The contrst etween grey mtter, white mtter nd cererospinl fluid is different in ech imge. Although commercil MRI scnners hve een ville for imging sttionry orgns for lmost 30 yers, the technology for crdic imging hs tken longer to develop ecuse of the chllenges involved in imging moving orgn using reltively slow imging modlity. Structurl MRI imges of sttionry orgns re typiclly constructed over tens of seconds or severl minutes y scnning line-y-line, nlogous to n inkjet printer. Although commercil MRI scnners hve een ville for imging sttionry orgns for lmost 30 yers, the technology for crdic imging hs tken longer to develop ecuse of the chllenges involved in imging moving orgn using reltively slow imging modlity. Imging the eting hert is typiclly chieved y synchronising the MRI scn with the QRS complex from n electrocrdiogrm (ECG) trce. This is known s ECG gting, nd it is illustrted in Fig. 2. Here, n imge of the hert t prticulr time point in the crdic cycle is pieced together from dt collected over severl hert ets. By c Fig. 1. Axil rin MRI imge of the sme tissue showing how imge weighting cn e used to vry the contrst etween grey mtter, white mtter nd cererospinl fluid. () T1 weighted, () proton density weighted, nd (c) T2 weighted MRI sequences. 116 CME MARCH 2011 Vol.29 No.3

2 used to cpture lower resolution crdic imges without ECG gting nd during free rething. 1 In ddition to the stndrd trnsverse, sgittl nd coronl imging plnes, numer of stndrd crdic-specific imging plnes re typiclly used. These re rodly divided into short-xis nd long-xis views, s shown in Fig. 3. An MRI sequence is n MRI technique used to highlight prticulr feture or function, nd involves series of pplied mgnetic field grdients nd rdio wves. For exmple, s shown in Fig. 4, these MRI sequences my include fetures for suppressing lood or ft, which cn respectively e used to more clerly distinguish the endocrdil oundries, or to identify ftty deposits. Fig. 2. Electrocrdiogrm gting in crdic MRI nd the multiphse smpling strtegy. Fig. 3. SSFP imges showing the stndrd crdic imging plnes: () sl (top), mid (middle), nd picl (ottom) short-xis slices; () two-chmer long-xis view showing the left ventricle nd trium; (c) four-chmer long-xis view showing oth ventricles nd tri. LV = left ventricle; RV = right ventricle; LA = left trium; RA = right trium. comining the imges otined t multiple time points in the crdic cycle, movie or cine series is creted, portrying the hert eting s if the imges were cquired in single crdic cycle. In prctice, the scn time for this procedure is 10 out seconds, nd the ptient is sked to hold his/her reth during the scn to minimise respirtory motion rtefcts. In cses where the ptient is unle to hold his/her reth, respirtory gting cn lso e incorported, either using elt round the domen or y monitoring the liver-lung interfce during the MRI scn. Blnced stedy-stte free precession (SSFP) is the most common MRI technique for cine imging. Modern MRI scnners lso provide tools for rel-time crdic imging, where ccelerted imging techniques re A CMR rdiologicl investigtion typiclly involves the following MRI sequences: coronl nd xil lck-lood fst spin echo sequence for ssessing extr-crdic normlities short xis stck of cine SSFP imges to ssess myocrdil function; out dozen slices re cquired, spnning the entire hert from the se to the pex two- nd four-chmer cine SSFP imges to ssess myocrdil function nd vlvulr function short- nd long-xis inversion recovery delyed-enhncement sequences to identify infrcted myocrdil tissue short- nd long-xis T1-weighted imges my e dded to investigte the distriution of intrmyocrdil ft short- nd long-xis T2-weighted imges my e dded for distinguishing oedem. Depending on the clinicl query, dditionl CMR sequences cn e dded to ssess other chrcteristics such s myocrdil perfusion, vlvulr function, lood flow, vessel ntomy, nd myocrdil deformtion. The opertion nd interprettion of these sequences will e explined in the following sections. Perfusion nd delyed-enhncement imging A notle strength of CMR is in imging myocrdil viility, where it is close rivl to the current gold stndrd fluorodeoxyglucose positron emission tomogrphy (FDG-PET). A gdoliniumsed contrst gent (gdolinium-dtpa), which increses the signl intensity in MRI MARCH 2011 Vol.29 No.3 CME 117

3 imges, is dministered intrvenously. By imging the hert immeditely fter dministering the contrst, nd monitoring the regionl myocrdil signl intensity over time, mps of myocrdil perfusion cn e otined. 2 These cn e used to ssess the significnce of coronry rtery disese nd microvsculr dysfunction within the myocrdium. The gdolinium remins in infrcted tissue long fter it wshes out of the surrounding norml myocrdium, nd y otining imges out 10 minutes fter the injection, the loction of the firosis ecomes immeditely pprent. This is known s delyed-enhncement imging. 3 A striking wy of portrying delyed enhncement is using inversion recovery sequences tht null myocrdil tissue nd thus enhnce the contrst of the dmged tissue. Fig. 5 shows series of clinicl exmples of inversion recovery delyed-enhncement MRI. c Fig. 4. () SSFP short-xis imge with high lood signl intensity; () T1-weighted lck lood short-xis imge showing cler endocrdil orders; (c) trnsverse T1-weighted lck lood imge without ft sturtion; (d) trnsverse T1-weighted lck lood imge with ft sturtion. LV = left ventricle; RV = right ventricle; RA = right trium. d MRI provides unprlleled tissue contrst, nd is key in dignosing crdiomyopthies, tumours nd infiltrtion. c d e f Fig. 5. Inversion-recovery grdient echo MRI sequences, 10 minutes fter the dministtion of 0.1 mmol/kg gdolinium-dtpa, my help differentite etween: () ischemic crdiomyopthy (septl nd suendocrdil posterolterl infrcts); () hypertrophic crdiomopthy (scrring t the junctions of right nd left ventricles); (c) rrhythmogenic right ventriculr crdiomyopthy (right ventriculr firosis); (d) idiopthic dilted crdiomyopthy (midwll firosis); (e) infiltrtive crdiomyopthies (crdic srcoidosis); nd (f) systemic vsculitides (Churg Struss Syndrome). LV = left ventricle; RV = right ventricle; LA = left trium; RA = right trium. Imges courtesy of Dr Jn-Peter Smedem. 118 CME MARCH 2011 Vol.29 No.3

4 c Fig. 6. CMR provides informtion on tissue chrcteristics: () ftty infiltrtion in ptient with rrhythmogenic right ventriculr dysplsi; () myocrdil oedem of the nterior left ventriculr wll in ptient with virl myocrditis (T2-weighted sequence); (c) intr-myocrdil iron deposits pper drk compred with norml myocrdium on T2* imges. LV = left ventricle; RV = right ventricle; SVC = superior ven cv; IVC = inferior ven cv; RVOT = right ventriculr outflow trct. Imges () nd () courtesy of Dr Jn-Peter Smedem. Fig. 7. Contrst mgnetic resonnce ngiogrms: () mximum intensity projection showing the gret vessels; () nd (c) show two frmes of dynmic MRA during distole nd systole, respectively. c Fig. 8. Short-xis SSFP imges t () end-distole nd () end-systole, with epicrdil nd endocrdil contours mrked in lue nd red, respectively. LV = left ventricle; RV = right ventricle. Morphologicl imging MRI provides unprlleled tissue contrst nd is key in dignosing crdiomyopthies, tumours nd infiltrtion. For exmple, MRI is cple of distinguishing etween constrictive pericrditis nd restrictive crdiomyopthy, s pericrdil thickness cn e mesured nd pericrdil effusions re clerly visile. Fig. 6 shows few clinicl exmples of the strengths of CMR for tissue clssifiction. Contrst-enhnced MRI ngiogrphy (CE- MRA) cn e performed to give n indiction of the morphology of the gret vessels. Here rpid series of imges re cquired during the trnsit time of the contrst gent. Fig. 7 shows typicl CE-MRA, which is cquired without ECG gting nd portryed s mximum-intensity imge, where only the high signl from the contrst gent is shown. Recent rpid imging techniques llow for time-resolved MRA (TR-MRA), 5 which is done using ECG gting. This cn, for exmple, show the lood within the crdic chmers, s illustrted during distole nd systole in Figs 7 nd 7c, respectively. Functionl imging The stck of short-xis cine SSFP imges covering the entire hert cn e used to quntify numer of useful clinicl indictors of crdic function. Modern post processing softwre cn demrcte the orders of the endocrdium nd epicrdium, s shown in Fig. 8. These cn then e used to extrct prmeters such s ejection frction, stroke volume, myocrdil mss, nd regionl wll thickening. Controlled doses of doutmine MARCH 2011 Vol.29 No.3 CME 119

5 cn lso e dministered to investigte wll motion normlities during stress. 6 CMR cn lso e used to evlute spects of vlvulr hert disese. 7 Vlvulr function cn, for exmple, e oserved in cine grdient echo imges. Fig. 9 shows the mitrl vlve leflets during ventriculr distole (lck rrows). Turulent flow reduces the synchronicity of the protons eing imged nd results in signl loss. A turulent jet from n ortic vlve regurgittion is evident in Fig. 9 s chrcteristic drk strek in the otherwise right lood (white rrow). This is commonly known s flow void. Fig. 9. Imging the crdic vlves during ventriculr distole. The lck rrows show the mitrl vlve leflets nd the white rrow shows flow void indicting ortic vlve incompetency. LV = left ventricle; LA = left trium; AA = scending ort. Fig. 10. Imging volume with velocity mesurements in three directions llows for retrospective prticle trcking, where simulted lood prticle re projected long the velocity fields. The imge shows lood eing ejected from the left ventricle nd trvelling long the ort. The colour represents the lood velocity. Imge courtesy of Dr Michel Mrkl. nd cn e seen to deform s the hert moves. Figs 11 nd 11 show n exmple of MRI tgging t end distole nd end systole, respectively. Tgged imges provide insight into contrctility nd myocrdil mechnics, nd cn e processed to yield meningful mesures of myocrdil strin. 11 Displcement encoding with stimulted echoes (DENSE) MRI is relted technique which provides more ccurte mesures of myocrdil displcement nd strin. 12 Fig. 11c shows n infrcted region (rrow) from delyed enhncement sequence, nd Fig. 11d, derived from DENSE MRI dt, shows the Phse contrst velocity encoded MRI llows one to otin mesures of lood velocity plotted over the crdic cycle through the vlves nd the gret vessels. 8 This ffords quntittive mesures of lood flow which cn, for exmple, e used to estimte the ejected lood volume during systole, or cpture flow profiles proximl nd distl to stenosis. Newer scn techniques mesure lood flow in multiple slices nd in three directions. 9 Fig. 10 shows n exmple of how these dt cn e used for ssessing lood flow in the ort. Severl techniques exist to provide quntittive mesures of intrmyocrdil function. The phse contrst velocity encoding technique cn lso e used for myocrdil tissue velocity mpping. 10 MRI myocrdil tgging is nother technique where the signl in the myocrdium is destroyed in series of drk nds or tgs, which re mteril property of the tissue c Fig. 11. Exmples of regionl myocrdil tissue trcking nd deformtion mpping using myocrdil tgging nd DENSE. () Myocrdil tgging t end-distole, nd () the deformed tgs t end-systole. The deformtion of the tg lines gives n indiction of the intrmyocrdil mechnics. (c) Delyed enhncement showing n infrct, nd (d) DENSE tissue trcking (purple lines) nd the corresponding strin mp showing norml tissue with negtive strin (lue) nd the firotic region with zero or slightly positive strin (yellow). LV = left ventricle; RV = right ventricle. d 120 CME MARCH 2011 Vol.29 No.3

6 corresponding motion trjectories from enddistole to end-systole (purple lines) nd regionl deformtion/strin t end-systole (lue nd yellow). The helthy tissue exhiits strong contrction (lue) nd the region of poor contrctility (yellow) corresponds to the loction of the firosis. Becuse thickening of the vessel wll precedes luminl nrrowing, MRI hs the ility to detect erly coronry therosclerosis. Although MRI coronry imging hs improved considerly over the pst yers, resulting in techniques such s volume imging with respirtory nvigtor tgging, multidetector computed tomogrphy (CT) is still the gold stndrd for coronry ngiogrphy. However, ecuse thickening of the vessel wll precedes luminl nrrowing, MRI hs the ility to detect erly coronry therosclerosis. 13 Conclusion CMR is n estlished technique, which cn e considered one-stop shop for ssessing crdic pthology. However, it lso hs severl limittions, including the high cost of the MRI scns, the fct tht mny ptients with implntle crdic devices cnnot e scnned, nd the limited use in ptients with rrhythmis. In ddition, there hve een few rections to the gdolinium contrst In nutshell gent nd there is evidence to suggest link to nephrogenic systemic firosis. Despite these shortflls, CMR is vlule prolemsolving tool nd currently the est single imging modlity for ssessing myocrdil viility nd function. References ville t Crdiovsculr MRI, or CMR, is verstile one-stop shop for the dignosis of crdic pthology. CMR provides oth morphologicl nd functionl informtion out the hert. Imges re cquired over severl hert ets using ECG gting. CMR is non-invsive nd it does not involve ionising rdition or rdioctive sustnces. A gdolinium contrst gent cn e used to ssess myocrdil viility, to mesure myocrdil perfusion, nd to crete ngiogrms. CMR provides unprlleled tissue contrst informtion, nd is key in dignosing crdiomyopthies, tumours, nd infiltrtion. CMR cn e used to evlute spects of vlvulr hert disese, such s the structure of vlve nd whether or not the vlve is incompetent. A collection of cine imges cn e used to extrct cliniclly useful prmeters such s ejection frction, stroke volume, myocrdil mss, nd regionl wll thickening. Additionl CMR techniques exist to mesure lood flow nd intrmyocrdil deformtion. The inility to depict detiled coronry rtery ntomy is currently mjor shortcoming of CMR. SINGLE SUTURE Smoking puts DNA t risk in 15 minutes Here s nother reson to kick the hit: within minutes of inhling, regulr smokers produce chemicls tht cuse genetic dmge linked with cncer. Polycyclic romtic hydrocrons (PAHs), present in tocco smoke, re one of the min culprits ehind lung cncer. They form metolites tht rect redily with DNA to produce muttions tht in turn cn cuse tumours. Stephen Hecht nd collegues t the University of Minnesot in Minnepolis sked 12 volunteers with history of smoking to smoke cigrette lced with phennthrene, type of PAH tht inds with DNA ut is non-crcinogenic. By collecting lood smples efore, during nd fter smoking the tem were le to trck the concentrtions of phennthrene metolites nd determine the speed t which they formed in the ody. The concentrtion of metolites reched pek round minutes fter smoke inhltion efore flling off, suggesting tht cigrette smoke could potentilly egin to ffect genes within minutes of strting to smoke. New Scientist, 22 Jnury 2011, p. 16. MARCH 2011 Vol.29 No.3 CME 121