Magnetic Resonance Angiography of the Great Vessels. Kawasaki Disease

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1 Mgnetic Resonnce Angiogrphy of the Gret Vessels 127 Fig Unenhnced free-rething MRCA demonstrtes stenosis in the proximl portion of the left coronry rtery (rrow). Finding t the sme loction on conventionl ngiogrphy (rrow) (with kind permission of T. Sommer, Bonn University Hospitl). Fig. 6.40, MRCA of the left coronry system in ptient with inflmmtory signs nd suspected Mediterrnen fever shows no pprent normlities (). Conventionl ngiogrphy () confirms this finding (with kind permission of T. Irhim nd R. M. Botner, Technicl University of Munich). Kwski Disese Kwski disese is generlized vsculitis of unknown etiology tht occurs in smll children nd my eventully give rise to coronry vsculr neurysms. Susequent thromosis of these neurysms nd reduction in lood flow my led to myocrdil infrction. The neurysms often rech considerle size, nd most re esily detected y MRCA. Mgnetic Resonnce Angiogrphy of the Gret Vessels K.-F. Kreitner Although MRI hs een used since its introduction in clinicl prctice for the investigtion of flow effects, the development of cliniclly useful MR ngiogrphy techniques (MRA) did not egin until the mid-1980s. The first techniques to e developed were time-of-flight ngiogrphy nd phse-contrst ngiogrphy. Used without intrvenous contrst gents, they were the first techniques tht permitted the selective visuliztion of flowing lood, mking it possile to imge the interior of vessels. However, they did not rech ny clinicl significnce in the dignosis of intrthorcic vsculr diseses, ecuse the intrluminl signl in oth techniques is dependent on complex flow effects. Furthermore, respirtory movements nd crdic pulstions in the chest led to motion rtifcts, nd the irfilled lung tissue induces susceptiility rtifcts. With dvnces in grdient technology, T1-weighted 3D sequences were developed tht mde it possile to cquire complete dtset in less thn 30 s. These sequences could e cquired during single reth hold. The hevy sturtion of flowing spins requires the use of n effective T1-shortening contrst gent to produce n intrluminl signl tht is very right in reltion to the ckground These enhnced intrluminl signls re virtully independent of flow phenomen, mking contrst-enhnced MRA somewht similr to digitl sutrction ngiogrphy (Fig. 6.41). us: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg

2 128 6 Mgnetic Resonnce Imging nd coverge, enling the imges to e cquired during reth holds. The 3D cquisition provides high sptil resolution cross the imged plne with no reks etween the individul prtitions. 138,141,142 A spoiled sequence is one in which ll residul trnsverse mgnetiztion is destroyed efore the next excittion. This increses the T1 weighting of the sequence nd increses the contrst etween the lood vessels nd surrounding tissue. 140 The repetition time of the MRA sequence should e s short s possile, s this setting will directly ffect the cquisition time (TA) of the sequence: TA = N y N z TR (6.1) where N y is the numer of phse-encoding steps in the y direction, N z is the numer of phse-encoding steps in the z direction (numer of prtitions), nd TR is the repetition time. The TR should e shorter thn 5 ms to mintin resonle durtion of reth holding. High-performnce scnners cn now chieve TR times of less thn 2 ms. This mkes it possile to expnd ntomicl coverge without lengthening the cquisition time or losing sptil resolution, or to improve sptil resolution while mintining the sme degree of coverge. When very short TR is used, 3D dtset cn e cquired multiple times in succession, resulting in time-resolved MRA study. 145,149 Fig Aneurysm of the scending ort in 65-yer-old mn. Optimlly timed MRA siclly limits intrvsculr enhncement to the ort nd its side rnches. Bsic Technicl Principles Scnner Hrdwre nd Softwre Although MRA of the gret thorcic vessels cn e performed successfully t 1.0 T, 1.5 T is considered the stndrd field strength. 138,139,142 Incresingly, 3.0-T mgnets re ecoming ville for routine clinicl imging, nd their use is currently eing optimized for thorcic MRA. 143,144 The speed of n imging sequence minly depends on the performnce of the grdient system. To chieve short cquisition times, it must e possile to produce mgnetic field grdients of very high mplitude tht cn e switched rpidly on nd off. Mnufcturers specifictions for mximum mplitudes rnge from 30 to 45 mt/m with rise times rnging etween 100 nd 200 mt/m/ms nd mximum field of view (FOV) etween 40 nd 53 cm Another importnt prmeter is the numer of receiver chnnels tht re ville to receive the signl from single surfce coil element. While four receiver chnnels re sufficient for conventionl thorcic imging, dditionl chnnels must e ville for prllel imging techniques tht re used in modern protocols. 143,148 More recently, receiver coils with up to 32 elements hve een introduced. 144 Pulse Sequences The stndrd pulse sequence for contrst-enhnced MRA is spoiled three-dimensionl (3D) T1-weighted grdient-echo sequence. It provides rpid imging with cceptle resolution Anlogously to the TR, the echo time TE should e s short s possile in MRA to minimize rtifcts due to flow dephsing (e. g., in high-grde stenoses) nd susceptiility differences (ir tissue interfce in pulmonry ngiogrphy). Usully the TE is less thn 2 ms, nd high-performnce scnners cn chieve vlues less thn 1 ms 145,146 (Fig. 6.42). Vrious other techniques cn e used to expedite dt cquisition. 139,142,148 The use of prtil Fourier techniques is sed on the fct tht informtion contined in the upper hlf of the rw dt mtrix duplictes informtion in the lower hlf. This mkes it possile to crry out complete imge reconstruction with equivlent contrst nd resolution using only slightly more thn 50 % of the rw dt, enling us to modify eqution (6.1) s follows: TA = N y N z TR 0.6 (6.2) The cost of this shortcut is reflected in the signl-to-noise rtio, which is decresed y the squre root of the time sved (Fig. 6.42). As n lterntive, slightly more thn hlf of the rw dt mtrix cn e scnned in the frequency-encoding direction. This registers only portion of the complete signl echo. Also know s frctionl echo or symmetric echo, this technique llows primry reduction of TE, leding secondrily to shortening of TR. Zero filling is n interpoltion lgorithm for improving the pprent resolution of individul imges. With zero filling fctor of 2, the rw dt mtrix is douled, the new points t the periphery of the mtrix re filled with zeroes, nd the imges re then sujected to Fourier trnsformtion. The use of zero filling does not primrily ffect imge cquisition, ut it prolongs the reconstruction time. When zero filling is done in ll three sptil plnes with filling fctor of 2, eight times s us: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg

3 Mgnetic Resonnce Angiogrphy of the Gret Vessels 129 c Fig c Imges showing the evolution of contrst-enhnced MRA in terms of scnner hrdwre nd softwre. Note the progressive shortening of cquisition times nd optimiztion of injection prmeters. A totl of 20 ml Gd-DTPA ws injected t rte of 2 ml/s, with n cquisition time of 28 s. The voxel size is 0.9 mm 1.8 mm 1.5 mm using prtil Fourier techniques. A Dcron grft ws plced in the proximl descending ort for corcttion repir in this 65-yer-old mn. A high-performnce grdient system nd the use of prllel imging resulted in n cquisition time of 11 s with totl contrst dose of only 15 ml injected t rte of 4 ml/s. Voxel size is 1.0 mm 1.0 mm 1.2 mm. The ptient ws 56-yer-old mn with ectsi of the scending ort. c Time-resolved MRA with temporl resolution of 3.5 s nd totl contrst dose of 10 ml injected t rte of 4 ml/s. Voxel size is 1.3 mm 1.3 mm 1.5 mm. This 58-yer-old womn underwent suclvin rtery-descending ort ypss for corcttion of the ort. mny voxels must e clculted s when interpoltion is omitted. But if we ssume tht n imging protocol provides sufficient dignostic informtion without zero filling, we cn use the zero filling strtegy to reduce the numer of necessry phse-encoding steps (N y or N z ), which in turn will shorten the cquisition time. A recent pproch to ccelerted dt cquisition is prllel imging. In this technique the sensitivity profile of the coil elements used for signl detection is utilized for sptil encoding, so tht ultimtely the numer of phse-encoding steps cn e reduced t either the rw-dt or imge-dt level. 148 Prior to ppliction, the sensitivity profile of the coil elements tht cn e integrted into the imging sequence must e determined y clirtion. An ccelertion fctor of 2 leds to n ~ 48 % reduction of cquisition time. The cost of this ccelerted cquisition is reflected in the signl-to-noise rtio, which is decresed y fctor of 1/ ccelertion fctor (Fig. 6.42, c). Essentil Point The current stndrd mgnetic field strength for contrst-enhnced MRA of the thorcic vessels is 1.5 T. The speed of dt cquisition depends criticlly on the performnce of the grdient system. The gol is to shorten the TR nd TE times s much s possile. Prtil Fourier techniques, zero filling, nd prllel imging hve ecome estlished techniques for ccelerting dt cquisition. Prllel imging requires multiple coil elements for signl detection nd corresponding numer of interconnectle receiver chnnels Techniques of Exmintion nd Interprettion Contrst Agents The extrcellulr contrst gents tht hve een pproved for clinicl use re low-moleculr hydrophilic gdolinium cheltes. The gents tht hve een pproved in Germny for MR ngiogrphy of the thorcic vessels re the open-chin complex Gd- DTPA (Mgnevist, Byer Schering Phrm), gdodimide (Omniscn, GE Amershm Buchler), nd the neutrl mcrocyclic gent gdoutrol (Gdovist, Byer Schering Phrm). Gdoutrol is chrcterized y its doule concentrtion of gdolinium compred with other extrcellulr contrst gents (1.0 mmol/l), so tht only hlf s much needs to e injected to chieve n equl dose. 146 The use of extrcellulr contrst gents temporrily shortens the T1 relxtion time of the lood to vlues etween 30 nd 80 ms. 140,141 Originl dose recommendtions for MRA of the thorcic vessels were in the rnge mmol/kg w or totl dose of ml. But with current cquisition times of s it is sufficient to dminister smller doses in the rnge mmol/kg w, or totl dose of 20 ml. 138,150,151 Considerle interest hs focused on the development of lood-pool contrst gents tht remin within the lood vessels for n extended period nd cuse little or no enhncement of extrvsculr tissues. In MRA of the gret vessels, these lood-pool gents could mke it possile to exmine vrious regions without the need for dditionl contrst injection. They us: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg

4 130 6 Mgnetic Resonnce Imging would e prticulrly useful in the quntifiction of crdic perfusion nd imging of the coronry rteries (see p. 111 ff) Bolus Timing For successful contrst-enhnced MRA of the thorcic vessels, cquisition of the 3D dtset should coincide with the rrivl of the contrst olus in the vessels of interest. 140,141,150 Vrious methods hve een descried for the optimum timing of contrst-enhnced MRA. In the test olus method, smll initil olus is dministered to determine the physiologicl trnsit time of the contrst gent from injection to its ppernce in the ROI so tht MR dt cquisition cn e initited precisely when the contrst olus rrives in the trget region (Figs. 6.41, 6.42). 152 There re lso techniques in which continuous SI mesurements re tken to time the rrivl of the contrst olus in predefined test region. When the rise in SI per unit time exceeds designted threshold, MRA is utomticlly initited (e. g., Smrt Prep, SmrtScn). Another pproch is the MR fluoroscopic detection of rrivl of the contrst olus in the trget region with mnul initition of the MRA sequence (e. g., CreBolus). Both methods involve centric rther thn liner k-spce cquisition, ecuse idelly MRA is initited to coincide with the pek concentrtion of gdolinium in the desired vessel. Lrge clinicl studies hve confirmed the reliility of these techniques in ensuring dignostic imge qulity. 146,150,153 A mjor dvntge of the test olus method is tht it cn e used in ny MR system nd does not require specil hrdwre or softwre. The imprecise timing of contrst-enhnced MRA cn ffect imge qulity in vrious wys. 154 If the contrst olus ppers too erly in the ROI reltive to cquisition of the 3D dtset, the contrst gent will lredy hve wshed out y the time imging is initited, resulting in the undesired enhncement of veins nd/or other vsculr nd tissue structures. If the contrst olus rrives too lte, little or no contrst gent will e present in the vsculr region of interest. High-pss filter rtifcts my e oserved; these occur when the T1 time of the lood chnges rpidly ecuse of contrst wsh-in during cquisition of the centrl k-spce lines, which determine imge contrst. This results in poor visuliztion nd enhncement of the trget vessels, with or without ringing rtifcts long the vessel wlls, mking it difficult to detect normlities. hlves of the lung re imged simultneously. Generlly, lrger field of view is needed to prevent wrp-round rtifcts from the shoulders nd pposed rms, nd this dversely ffects the voxel size of the 3D dtset. Another option is to extend the rms ove the hed efore imging. The coronl technique is stisfctory for evluting normlities of the pulmonry veins, ut 3D volume cm thick cnnot cover the entire pulmonry tree, nd the voxel size remins nisotropic even when high-end system is used. These restrictions my e overcome y use of multichnnel phsed-rry coils, prllel imging techniques, nd imging t 3 T: here, whole coverge of the lung is relizle with cquisition of isotropic voxel sizes of 1 mm 1 mm 1 mm in totl of 20 s. 144 Sgittl dt cquisition permits the use of smller FOVs, which hs fvorle effect on voxel size nd mkes it esier to cquire isotropic voxels. Becuse of the smller 3D volume, the cquisition time is shorter thn with coronl dt cquisition. The min disdvntge of sgittl cquisition is the fct tht seprte dtsets must e cquired for ech side, which doules the required contrst dose 151, (Fig. 6.43). An olique sgittl plne is usully recommended for imging the thorcic ort. Generlly the prescription of this plne is sed on the orienttion of the ortic rch in the trnsverse plne. Coronl dt cquisition is recommended in cses where it is necessry to investigte the supr-ortic rnch vessels or detect congenitl nomlies of the ortic rch ECG triggering is not solutely essentil ut does improve visuliztion of the scending ort nd is therefore recommended in exmintions for scending ortic disese. 150 In time-resolved contrst-enhnced MRA, the time needed to cquire 3D dtset cn e shortened to less thn 4 s, which is prticulr dvntge in severely dyspneic ptients. With the dvent of multichnnel coil technology, receiver chnnels nd modified strtegies for k-spce smpling, the reduction of inplne resolution, using thicker prtitions, nd decrese of the numer of prtitions my e minimized. 139,143,145,161 With timeresolved exmintion techniques, higher injection rtes (up to 6 ml/s) should e used with concomitnt reduction in the contrst dose 149 (Fig. 6.44). Time-resolved imging techniques cn e useful in congenitl vsculr mlformtions (e. g., ductus rteriosus) nd ortic dissections (perfusion chrcteristics of the true nd flse lumin), nd to document stel effects in ptients with suclvin rtery stenosis. 145,149,162 The recommended protocol for contrst-enhnced 3D MRA of the thorcic vessels is outlined in Tle 6.8. Essentil Point For successful contrst-enhnced MRA of the thorcic vessels, cquisition of the 3D dtset should coincide with the rrivl of the contrst olus in the vessels of interest. Strtegies for olus timing include the test olus method s well s mnufcturer-specific utomted or semiutomted techniques. The min dvntge of the test olus method is tht it does not require specil hrdwre or softwre. Essentil Point With cquisition times from 10 to 20 s, contrst dose of 20 ml or mmol/kg w is generlly dequte. The shorter the cquisition time, the higher the recommended injection rte (up to 4 ml/s). A rte up to 6 ml/s is recommended for time-resolved MRA. Plnning the Exmintion The pulmonry circultion cn e imged in one coronl or two sgittl slice pckges 151,155. In the coronl cquisition, oth Interprettion The interprettion of thorcic MRA is sed on detiled nlysis of the source imges. Comined with multiplnr reforus: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg

5 Congenitl Hert Disese in Adults Hert Defects nd Endocrditis T. Buck, B. Plicht, T. Schlosser, nd R. Erel Congenitl Hert Disese in Adults Congenitl crdiovsculr nomlies re present in % of ll neworns. The numer of different nomlies is so lrge tht their complete description would e eyond the scope of this ook. This chpter therefore focuses on the cpilities of modern crdic imging techniques in the most common types of congenitl hert disese in dults tril septl defects, ptent formen ovle, nd ventriculr septl defects. Atril Septl Defect Antomy nd Pthophysiology Atril septl defects (ASDs) ccount for ~ 10 % of ll congenitl hert diseses nd for % of congenitl hert disese in dults. They re ssocited with vrying degrees of left-to-right shunting of rterilized lood into the pulmonry circultion, depending on the size of the septl defect nd the reltive pressures. In extreme cses the shunt flow my e severl times the volume flow of tht in the systemic circultion. Atril septl defects re primrily chrcterized y volume overlod on the right hert, which eventully led to crdic filure. With left-to-right shunt ove the level of the tricuspid vlve, the gret diltory cpcity of the pulmonry vessels cn forestll pressure rise in the pulmonry rtery nd right ventricle. Comprle to the ventriculr septl defect however, functionl nd/or orgnic rise in vsculr resistnce will develop over time in the pulmonry circultion, cusing the pressure in the right ventricle to rise. When the pulmonry vsculr resistnce exceeds tht of the systemic circultion in the presence of ventriculr or tril septl defect, shunt reversl occurs, leding to mrked cynosis. This phenomenon is termed the Eisenmenger rection. Three etiologicl types of tril septl defect re distinguished: An ostium secundum tril septl defect (ASD II) is locted in the centrl portion of the tril septum in the region of the foss ovlis. It is the most common type, ccounting for % of tril septl defects. An ostium primum tril septl defect (ASD I) is usully lrge defect tht comprises % of tril septl defects. It results from filure of fusion of the septum primum with the endocrdil cushion etween the trioventriculr vlves. As result, it is commonly ssocited with mitrl vlve defects nd occsionlly with tricuspid vlve defects. A sinus venosus tril septl defect is present in 5 15 % of cses. It occurs in the posterosuperior portion of the tril septum etween the termintion of the superior ven cv nd the foss ovlis. It is frequently ssocited with nomlous termintion of the right pulmonry veins in the right trium. Clinicl Fetures Symptoms Generlly symptomtic until the third decde; 70 % of ptients re symptomtic y the fifth decde Dyspne, rpid ftigility (mnifesttions of hert filure). Plpittions (in ptients with tril rrhythmis) Proneness to respirtory infections Approximtely 15 % of ptients show clinicl symptoms of ssocited mitrl insufficiency Complictions Atril firilltion or flutter Mitrl insufficiency Imging Echocrdiogrphy 2D Echocrdiogrphy Trnsthorcic scnning enles direct visuliztion of the septl defect (excepting sinus venosus defects) (Fig. 7.1). Trnsesophgel scnning permits direct visuliztion of sinus venosus defect (Fig. 7.2). Enlrgement of the right trium nd right ventricle Detection of ssocited congenitl nomlies 3D Echocrdiogrphy Enles direct visuliztion of the ASD in the frontl view to ssess the loction, size, nd shpe of the defect. Cn detect n AV cnl in ptients with ASD I. Allows direct visuliztion of cleft in the nterior mitrl vlve leflet in ASD I. Color Doppler Echocrdiogrphy Detection of shunt flow y demonstrting flow jet through the tril septum into the right trium Frequently primry detection of sinus venosus defect is sed on n norml flow signl ner the termintion of the superior ven cv (Fig. 7.2). Detection of n AV cnl in ptients with ASD I Detection of ssocited mitrl insufficiency in ASD I Contrst Echocrdiogrphy Detection of left-to-right shunt sed on the wshout phenomenon in the contrst-filled right trium (Fig. 7.1). us: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg

6 142 7 Hert Defects nd Endocrditis CW Doppler Echocrdiogrphy Pulmonry hypertension cn e dignosed y ssessing the systolic pulmonry rtery pressure sed on the regurgitnt signl from the tricuspid vlve. Essentil Point In the presence of clinicl suspicion of left-to-right shunt t the tril level, it is possile to overlook high-sited sinus venosus type of ASD if the tril septum ppers norml. Often sinus venosus defect cn e detected only y multiplnr trnsesophgel echocrdiogrphy in color Doppler mode. Mgnetic Resonnce Imging nd Computed Tomogrphy The stndrd plnes for detecting n ASD in MRI re horizontl long-xis slices nd short-xis slices through the hert. Spoiled grdient-echo sequences re preferred. They re of dvntge over SE nd lnced SSFP sequences in tht they cn lso detect smller defects y the ssocited zones of turulent flow, for exmple. Determintion of the mximum size of the ASD is essentil in selecting ptients for interventionl repirs, n unsuitle procedure for lrge defects. Both MRI nd multislice CT (MSCT) cn document the enlrgement of the right trium nd right ventricle (Fig. 7.3). In ddition to demonstrting morphologi- Fig. 7.1, Trnsesophgel echocrdiogrphy of ASD II (rrow). Prior to contrst wsh-in. Typicl ppernce of the wshout phenomenon (rrow) in left-to-right shunt t the tril level. Vrious ules cn e seen entering the left trium fter pssing through the shunt. Fig. 7.2, Sinus venosus tril septl defect documented y trnsesophgel echocrdiogrphy. The size of the defect (rrow) is mesured t the entry of the superior ven cv into the right trium. Anorml flow signl detected y color Doppler echocrdiogrphy. us: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg

7 Congenitl Hert Disese in Adults 143 cl fetures, MRI lso enles n ccurte ssessment of the Q p / Q s rtio sed on flow mesurements in the scending ort nd pulmonry trunk. This rtio is importnt in selecting ptients for opertive or interventionl repir. 1 The Q p /Q s rtio cn lso e determined using cine MRI to identify right nd left ventriculr stroke volumes, provided the presence of n dditionl significnt vlvulr defect cn e excluded. MRI nd CT dditionlly detect ny nomlous pulmonry venous termintion tht my e present. Opertive tretment is the only option ville for correcting this nomly 2 (Fig. 7.3). Ptent Formen Ovle Antomy nd Pthophysiology Ptent formen ovle (PFO) is vlvelike opening tht persists postntlly etween the septum primum nd secundum in the region of the foss ovlis owing to filed fusion of these septl elements. A right-to-left shunt t the tril level my occur spontneously or in response to Vlslv mneuver. Autopsy results indicte tht PFO hs prevlence of ~25 % in the generl popultion. Its clinicl significnce is tht it plces ptients t risk for prdoxicl emoli. This risk ppers to e prticulrly high in ptients with hypermoile tril septl neurysm, which frequently coexists with PFO. Becuse the opening nd volume of the shunt re generlly smll, hemodynmic complictions do not occur. Clinicl Fetures Symptoms The mjority of ptients with PFO re symptomtic. Complictions Centrl or peripherl emoli, stroke, trnsient ischemic ttcks, dizzy spells, migrines Imging Echocrdiogrphy M-Mode Echocrdiogrphy M-mode cn document hypermoile excursions ssocited with n tril septl neurysm (Fig. 7.4). 2D Echocrdiogrphy Demonstrtes the vlvelike seprtion etween the septum primum nd secundum; ut cnnot relily detect ptent formen ovle (2D visuliztion only y trnsesophgel scnning). Cn detect n tril septl neurysm (frequently lso detectle y trnsthorcic scnning. Contrst Echocrdiogrphy Cn relily detect or exclude ptent formen ovle sed on the right-to-left pssge of contrst medium, either spontneously or in response to Vlslv mneuver (Fig. 7.4). Fig. 7.3, Lrge ASD II in 32-yer-old womn. Cine imge in the four-chmer view demonstrtes the lrge ASD (rrow). Contrst-enhnced 3D MRA further demonstrtes prtil nomlous pulmonry venous termintion on the right side. The upper nd middle loe veins (rrows) open into the superior ven cv. us: Thelen, Crdic Imging (ISBN ), 2009 Georg Thieme Verlg