ipat Applications in Clinical Routine and Beyond: Imaging from Head to Toe

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1 ipat Applictions in Clinicl Routine nd Beyond: Imging from Hed to Toe Olf Dietrich, PhD; Stefn O. Schoenerg, MD Deprtment of Clinicl Rdiology Großhdern (Chirmn: Mximilin F. Reiser, MD), Ludwig-Mximilins-University of Munich, Germny ELECTRONIC PREPRINT VERSION: Not for commercil purposes or for ny externl distriution y third prty. Corresponding uthor: Olf Dietrich, PhD Ludwig-Mximilins-University of Munich Deprtment of Clinicl Rdiology Großhdern Mrchioninistrße 15, Munich, Germny Phone: Fx: E-mil: od@dtrx.net Introduction With the relese of Numris 4 version MR2002B n importnt new feture hs ecome ville for routine exmintion: integrted Prllel Acquisition Techniques (ipat). The generl ide ehind ipat is to cquire imge dt simultneously y two or more receiver coils with different sptil sensitivities. Initilly, this technique ws motivted y the wish to ccelerte imge cquisition without reducing the sptil resolution of the imge. However, it turned out tht ipat provides severl other dvntges depending on the ppliction. Technicl Bckground Acquisition of MR imges works y susequently cquiring phse-encoded lines in k-spce. These lines of dt re finlly trnsformed into the imge slice (or sl in the cse of 3D cquisitions) y mthemticl process clled Fourier trnsform. An importnt property of dt in k-spce is tht the density or distnce of the lines in k-spce corresponds inversely to the field of view (FOV) of the finl imge wheres the dt rnge in k-spce corresponds to the sptil resolution of the imge. I.e. reducing the line smpling density y fctor of 2 (y not cquiring every other line) leds to n imge with hlf the FOV in phse-encoding direction in comprison with the originl imge. In this cse, the cquisition time is lso reduced y fctor of 2 s is well known from using rectngulr FOVs. ipat methods use exctly this effect to ccelerte imge cquisition, ut without decresing the FOV due to specil ipat imge reconstruction. Using the complementry dt from the different receiver coils, the missing lines in k-spce cn e clculted during imge reconstruction. There re two groups of ipat lgorithms: lgorithms tht explicitly clculte missing k-spce lines efore Fourier trnsforming the dt, nd lgorithms tht first reconstruct imges with reduced FOV for ll receiver coil elements nd then merge these different imges into one with full FOV. Numris 4 MR2002B provides oth types of lgorithms: GeneRlized Auto-clirting Prtilly Prllel Acquisition (GRAPPA) is n lgorithm of the first type [1]; it is sed on nother well-known lgorithm of the sme clss clled Simultneous Acquisition of Sptil Hrmonics (SMASH) [2]. The est-known lgorithm of the second type hs een clled SENSitivity-Encoded (SENSE) MRI [3] nd modified SENSE lgorithm clled msense is ville under MR2002B. It depends on the specific ppliction (e.g., on the ntomicl region nd pulse sequence) which of these two ipat lgorithm will yield etter imge qulity. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 1 of 19

2 Both GRAPPA nd msense lgorithm require some dditionl informtion out the sptil coil sensitivities, i.e. which prt of the FOV is covered y ech coil element. This informtion cn e cquired s seprte extr scn with low resolution or, typicl for the GRAPPA nd msense lgorithm, y dditionlly cquiring some of the missing dt lines in the center of k-spce (so-clled reference lines) integrted into the cquisition. Totl imging time will e slightly incresed due to the red-out of the reference lines nd in certin sitution it might e preferle to use pulse sequence with externl reference scn. Generlly, the signl-to-noise rtio (SNR) in ipat imges is decresed compred to cquisitions with the full k- spce dt. This is the sme effect s in conventionl imging with rectngulr FOV: cquiring fewer lines in k- spce decreses the SNR of the imge. Additionlly, ipat imges suffer n SNR loss due to the specil reconstruction scheme: this effect depends on the efficcy of the geometry of coil distriution nd is descried y the so-clled geometry fctor g [3]. Advntges nd Disdvntges of ipat The ovious dvntge of ipat is the ccelertion of imging due to the reduced numer of phse-encoding lines to e cquired. With n ccelertion fctor (or ipat fctor) of 2, i.e. cquisition of only every second line in k- spce, the imging time is reduced y 40 % to 50 % depending on the numer of reference lines. This cn e used to decrese the overll exmintion time nd thus improve the ptient throughput nd exmintion efficcy. Alterntively, the sptil imge resolution cn e improved in n ipat scn compred to conventionl scn of the sme durtion. Both shorter scn times nd higher resolution re especilly importnt in reth-hold imging: either reth-hold times cn e shortened or the sptil resolution cn e improved without prolonging the reth-hold time. Another importnt ipat ppliction is dynmic imging like mesurements of perfusion or crdic function ecuse imge ccelertion llows for higher temporl resolution. However, s mentioned ove the resulting SNR will e decresed compred to non-ipat cquisitions; hence, ipat is especilly useful for high-snr pplictions like contrst-enhnced ngiogrphy. Using ipat to cquire more verges in the sme totl scn time cn improve imge qulity, prticulrly in ntomicl res tht re prone to motion rtifcts. ipat imging is less sensitive to motion, ecuse every single cquisition is shorter thn in conventionl sequence. By verging imge dt, the ipat-relted SNR loss is lmost compensted nd remining motion rtifcts re further reduced. Single-shot pulse sequences like echo-plnr imging (EPI) or hlf-fourier cquired single-shot turo spin echo (HASTE) often suffer from imge rtifcts due to their long echo trins. EPI is especilly sensitive to susceptiility rtifcts, wheres HASTE imges often pper lurred due to the T2-relted signl decy during the redout of the echo trin. Both prolems cn e reduced y pplying ipat to shorten the length of the echo trin without loss of sptil resolution. In contrst to other pulse sequences, single-shot methods cn even gin SNR due to ipat ecuse lte echoes with reltively low signl intensity tht re cquired in conventionl sequences re not contined in the shortened ipat echo trin. In conclusion, ipat cn e dvntgeous in very different pplictions with very different wys of using ipat. This is demonstrted in the following sections with exmples rnging from clinicl routine imging to dvnced study protocols. Imging in ll presented pplictions is performed on 1.5 T MAGNETOM Sont Mestro Clss system. Stndrd Numris sequences re used in most cses; however, some pplictions require sequences from specil work in progress (WIP) pckges y Siemens Medicl Solutions. Diffusion Tensor Imging Diffusion tensor imging (DTI) is n dvnced MR imging technique for mesuring the strength, nisotropy, nd direction of wter diffusion in tissue. The term wter diffusion refers to the property of ll wter molecules to move stochsticlly due to their therml energy (Brownin motion). The extent of this motion is restricted y tissue properties, prticulrly y the cellulr microstructure nd the sptil orienttion of cells. Especilly in fier structures like muscle tissue or the cererl white mtter, moleculr motion is restricted y cell memrnes or myelin sheths, nd molecules move preferly prllel to the fier direction wheres diffusion orthogonl to the fier direction is decresed. Thus, the resulting wter diffusion is nisotropic. Informtion out the diffusion Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 2 of 19

3 strength (pprent diffusion coefficient, ADC), diffusion nisotropy, nd diffusion direction re contined in the so-clled diffusion tensor, mthemticl oject (symmetric 3 3 mtrix) consisting of 6 independent numers. The most common pulse sequences to mesure the diffusion tensor re diffusion-weighted EPI sequences with diffusion grdients pplied in t lest six different directions [4, 5]. Single-shot EPI sequences hve the dvntge tht imging is fst (out 100 ms/imge) nd thus very insensitive to motion. However, EPI sequences re very prone to susceptiility rtifcts mnifesting s distortions in the frontl rin nd the crnil se. This disdvntge cn e overcome y using ipat sequences to shorten the length of the EPI grdient echo trin. Hence, we use spin echo EPI diffusion sequence with GRAPPA reconstruction, n ccelertion fctor of 2, nd 24 reference lines for DTI exmintions. A dedicted ipat hed coil consisting of 8 surfce coil elements (Fig. 1) provides the required numer of receiver chnnels. Figure 1 8-chnnel phsed-rry hed coil for cquisition of ipat dt. 8 surfce coil elements re locted cylindriclly round the AP xis; the inner dimeter of the coil is 24 cm. Acquisition with the 8-chnnel hed coil results in imges with n improved SNR compred to the stndrd qudrture hed coil (Fig. 2). This cn e explined y the smller dimeter of the 8-chnnel hed coil (24 cm vs. 26 cm) nd the reduced coil size in crnio-cudl direction. Imges were cquired with mtrix in 36 slices, mm 2 FOV, phse-encoding in nterior-posterior direction, slice thickness of 3.6 mm, 10 verges, nd n ipat fctor of 2 (24 reference lines); the echo time ws 71 ms nd the repet time 6000 ms. DTI with ipat displys less distortion rtifcts thn DTI with conventionl EPI sequences (Fig. 3). Evluting the diffusionweighted imges, prmeter mps with the men ADC, the diffusion nisotropy, nd the min direction of diffusion cn e clculted (Fig. 4). Although n N/2 rtifct in phse-encoding direction (nterior-posterior) is visile in some of the originl EPI imges, this rtifct seems not to influence the clculted prmeter mps. An dditionl dvntge of ipat imging is the reduced durtion of the redout tht llows for the cquisition of n incresed numer of slices within the given repetition time (TR) compred to conventionl sequences; e.g., 38 slices without ipat vs. 50 slices with ipat given TR of 6000 ms. Figure 2 Comprison of SNR with stndrd hed coil () nd 8-chnnel hed coil (). Both imges re cquired with identicl sequence prmeters (diffusionweighted EPI sequence, = 1000 s/mm 2, no verging) nd without ipat. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 3 of 19

4 c d Figure 3 Comprison of spin echo EPI imges in two slices with (, c) nd without (, d) ipat (GRAPPA lgorithm). Some ovious susceptiility rtifcts re mrked with rrows in () nd (d). c d e f g h i Figure 4 Exmples of DTI evlution from diffusion-weighted ipat imges: ADC mp (, d, g) with diffusion coefficients from 0 (lck) to mm 2 /s (white); frctionl nisotropy (, e, h) from 0 (lck) to 1 (white); color-coded min diffusion direction (c, f, i), left-right: red, nterior-posterior: green, crnio-cudl (lue). Note especilly the high nisotropy nd left-right diffusion direction in the corpus cllosum. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 4 of 19

5 Lrynx Imging Mgnetic resonnce imging of the lrynx is difficult due to tissue motion cused y swllowing nd respirtion [6]. Generlly, MR pulse sequences with long cquisition times re more sensitive to motion thn fst imging sequences or even single-shot sequences. Therefore, MRI of the lrynx t conventionl speed often leds to imges with excessive motion rtifcts. Since moving orgns like the lrynx cn e more clerly visulized y reducing the imge cquisition time, ipat sequences cn reduce the sensitivity to motion rtifcts y ccelerting imge cquisition while mintining the sme imge resolution. Thus, we use T1-weighted nd T2-weighted ipat sequences for routine lrynx imging, e.g. in ptients suffering from suspected lryngel crcinom. A pir of dedicted ipat surfce coil systems with 2 6 coil elements is rrnged round hed nd neck of the ptient. To compenste for the ipat intrinsic SNR loss, we increse the numer of cquisitions to 5 (T1-weighted grdient echo sequence with TR/TE = 176 ms/4.8 ms, 24 ipat reference lines) nd 3 (T2-weighted TSE sequence with TR/TE = 3970 ms/89 ms, echo trin length 15, 45 ipat reference lines). Both sequences hve mtrix, FOV of mm 2 nd slice thickness of 3.5 mm. The GRAPPA lgorithm is used for ipat reconstruction. The cquired imges show etter delinetion of tumor extent nd less motion rtifcts thn MRI using nonipat techniques, thus llowing ccurte dignosis of lryngel crcinom (Fig. 5). In our experience, MRI with ipat using flexile 12-element phsed-rry coil is suitle for relile dignosis when lryngel crcinom is suspected. Generlly, in imging moving tissue it ppers preferle to cquire more verges with reduced imging time using ipat nd thus gin imges with identicl resolution nd comprle SNR ut less motion rtifcts thn in conventionl non-ipat imging. c d Figure 5 Axil MRI scn through the suprglottic lrynx, ll imges show the sme scn position. The imges demonstrte lrge suprglottic tumor infiltrting the preepiglottic spce, the left prglottic spce nd the left ryepiglottic fold. () T1-weighted imge showing muscle-isointense tumor. () T1- weighted imge demonstrting contrst-enhncement of the tumor. (c) T2-weighted xil imge showing slightly hyperintense tumor tissue. (d) T1-weighted ftst imge demonstrting contrst enhncement of the tumor. Spinocellulr crcinom ws found t surgery. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 5 of 19

6 Lung Imging MR Screening of Infiltrtes Rdiologicl lung screening is typiclly performed either y conventionl x-ry (CXR) exmintion or y highresolution computed tomogrphy (HR-CT) of the thorx nd therefore exposes ptients to considerle mount of rdition, prticulrly fter repeted exmintions. This rdition dose could e reduced y using MRI s screening modlity insted of x-ry sed methods. Unfortuntely, MRI of the lung is still technicl chllenge ecuse of the very low proton density of the lung tissue nd the strong vrition of susceptiility leding to very short T2* relxtion times. Both fctors together re the reson for very low MR signl intensities from lung prenchym nd hence for low SNR. A further difficulty in lung MRI is tissue motion ecuse of respirtion nd crdic motion. The introduction of ipat opened new possiilities to lung imging with T2-weighted HASTE sequences. The min disdvntge of conventionl HASTE sequences is the lurring of imges cused y the long echo trin nd the T2-relted signl decy during its redout; this effect severely limited the ctul mximum imge resolution [7]. By using ipat, the echo trin cn e reduced to hlf of its originl length nd thus lurring rtifcts re reduced. Since lte echoes with low signl intensity re not cquired, SNR cn even improve compred to non-ipat sequences. Additionlly, the imge cquisition is ccelerted such tht more slices cn e cquired during one reth-hold period. To evlute the use of ipat HASTE sequences for lung screening, we compred HR-CT nd MR imges in immunosuppressed ptients with symptoms of pneumoni ut norml or unspecific CXR. After compring ipat imges reconstructed with the GRAPPA nd msense lgorithm (Fig. 6), we decided to use the GRAPPA lgorithm ecuse of the occurrence of reconstruction rtifcts in the imge center of the msense imges. Coronl slices of the lung re cquired with FOV of mm 2 nd resolution of pixels; xil slices with FOV of mm 2 nd mtrix. The slice thickness is 8 mm nd the TE is 27 ms in oth sequences. To reduce the echo trin length s fr s possile, Siemens WIP sequence with externl ipat reference scn ws used; the sequence cquires the reference lines immeditely efore the ctul imge cquisition within the sme reth-hold. Exmples of the findings re shown in Fig We found tht lung MRI with ipat HASTE sequences is nerly s good s HR-CT for the detection of pulmonry infiltrtes with only few flse-negtive nd flse-positive cses such tht MRI cn e recommended especilly s follow-up tool fter initil HR-CT dignosis. Figure 6 ipat HASTE imges of helthy volunteer reconstructed with GRAPPA () nd msense () lgorithm. Note the reconstruction rtifcts superposing the spine in (). Figure 7 Ground glss infiltrte in immunosuppressed ptient. Multidetector HR-CT () nd ipat HASTE MRI (). Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 6 of 19

7 Figure 8 Smll irregulr infiltrtes in immunosuppressed ptient. Multi-detector HR-CT () nd ipat HASTE MRI (). Figure 9 Discrete typicl infiltrtes in immunosuppressed ptient. Multi-detector HR-CT () nd ipat HASTE MRI (). Figure 10 Discrete typicl infiltrtes in immunosuppressed ptient. Multi-detector HR-CT () nd ipat HASTE MRI (). MR Angiogrphy nd Perfusion Imging Contrst-enhnced vsculr lung MRI requires good sptil resolution nd especilly in the cse of perfusion imging lso good temporl resolution. Both re limited rther y the reth-hold durtion for the ptient thn y SNR considertions due to the high-contrst sitution in contrst-enhnced MRI. Experiences on MR perfusion imging of the lung re still limited nd vrious pproches like conventionl FLASH nd HASTE sequences or flow-sensitive inversion recovery techniques re eing used [8 10]. However, using ipat techniques, oth temporl nd sptil resolution cn e significntly incresed compred with conventionl imging. Therefore, we dded ipat FLASH sequences to our protocol for 3D contrst-enhnced MR ngiogrphy (MRA) nd MR perfusion imging of ptients with primry nd secondry pulmonry rteril hypertension. Using GRAPPA with the dedicted 12-element ipat coil, temporl resolution of 1.2 seconds per phse is possile for dynmic perfusion imging, cquiring 25 dynmic phses in 30 seconds; the imge resolution is mm 3 cquired with mtrix in 24 slices. High resolution ngiogrms cn e cquired with 512 mtrix ( mm 3 voxel size) in 20 seconds reth-hold time. For oth dynmic perfusion nd high-resolution ngiogrphy, n ipat ccelertion fctor of 2 is used with 24 dditionlly cquired reference lines. Imge exmples of these sequences re shown in Fig. 11 nd 12. Using the prllel cquisition technique, excellent visuliztion of susegmentl vessels is possile in the ngiogrphic imges. Time-resolved perfusion imging llows relile detection of smll segmentl nd susegmentl perfusion defects. Using non-ipat methods, visuliztion of perfusion defects nd intrvsculr thromi is generlly possile s well, lthough with lower temporl nd sptil resolution thn using ipat methods. In conclusion, we could sustntilly improve the temporl resolution s well s the sptil resolution y using ipat. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 7 of 19

8 Figure 11 Dynmic pulmonry perfusion imging using ipat to cquire sl of 24 imges ech 1.2 seconds. Perfusion defects re shown in the left upper loe nd right lower loe. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 8 of 19

9 Figure 12 Exmple of n ipat high-resolution pulmonry MR ngiogrphy of the sme ptient s in Fig. 11. A significnt reduction of rteril enhncement cn e demonstrted in the left upper loe nd right lower loe due to centrl thromoemolic occlusions. Functionl Crdic Imging Glol nd Regionl Crdic Function Crdic mgnetic resonnce imging hs een extensively used in ssessment of glol nd regionl myocrdil function. There is no dout tht MRI represents the current stndrd of reference. Although dtset cquisition cn e performed in virtully ny plne, the clcultion of functionl prmeters is most commonly sed on stck of slices in doule olique short xis orienttion. To llow for high sptil s well s high temporl resolution, the current sequence techniques cquire single-slice cine dt set ech reth-hold. Although innovtions of recent yers llowed for speed up of techniques, completion of stndrdized functionl study still tkes out minutes including ptient recovery periods. Rel-time imging techniques using stedy-stte free precession (SSFP) sequences such s TrueFISP, llow for mjor speed up in dt cquisition due to the completion of short xis dtset within single reth-hold [11, 12]. However, this comes long with restriction in sptil nd temporl resolution. And in terms of volumetric ccurcy, temporl resolution is y fr more crucil thn sptil resolution s recently shown y Miller nd co-workers [13]. The current recommendtion for functionl crdic imging requests temporl resolution of 50 ms or even etter. ipat llows meeting this criterion when implemented in conjunction with rel-time TrueFISP. Compred to previous studies performed y Brkhusen nd Lee [11, 12], the temporl resolution tht cn e chieved is in the order of ms. And s most recently shown, this improvement in temporl resolution now leds to n ccurcy of results comprle to tht of segmented TrueFISP [14] (Fig ); the ipat imges re cquired with n ccelertion fctor of 2 nd 12 reference lines. So ipat llows for drmtic time svings in crdic function nlysis without losing ccurcy of volumetric results. It even llows for multiplnr dt cquisition within single reth-hold. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 9 of 19

10 Figure 13 Short xis view of mle ptient with impired right ventriculr function due to pericrdil disese. Imges cquired with segmented TrueFISP technique showing distole () nd systole (). Figure 14 A single slice of the multi-slice ipat rel-time cine dt set t exctly the sme slice position s in Fig. 13. Comprle time points in () distole nd () systole. Figure 15 Ptient fter myocrdil infrction with ischemic diltting crdiomyopthy. Distolic () nd systolic () imges cquired with segmented TrueFISP show lmost no chnge in ventriculr shpe (ejection frction < 25 %). Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 10 of 19

11 Figure 16 Comprison to Fig. 15 t identicl slice position. ipat rel-time TrueFISP shows the sme mrked thinning of the nterior wll without chnge within distole () nd distole (). Bsed on experiences nd comprisons, the GRAPPA reconstruction shows more roust imge qulity in crdic MRI thn msense [1]. Due to the higher sensitivity of SENSE-relted methods to folding rtifcts, these techniques seem to e less useful in crdic imging ecuse of the necessry lrger FOV tht leds either to n dditionl loss of sptil resolution or to loss of cquisition time when more phse-encoding lines re cquired. Aprt from the use of ipat with rel-time techniques it lso llows for further improvement of sptil or temporl resolution in segmented single-slice cquisitions compred to stndrd techniques (Fig. 17). In generl, when using cine TrueFISP techniques the loss in SNR due to ipat is lmost negligile. Figure 17 Comintion of segmented cine TrueFISP with ipat. In comprison to stndrd techniques (; pixel size mm 2 ), the use of ipat llows for mrked increse of sptil resolution (; pixel size 1 1 mm 2 ). Myocrdil Perfusion Imging Myocrdil perfusion imging is promising nd rpidly incresing field in crdic MRI. The rpid development of scnner hrdwre llows lso for n improvement in sequence technologies which hs een of mjor enefit in techniques tht require n ultr-fst dt cquisition such s myocrdil perfusion imging. MR perfusion imging hs intrinsic enefits compred to routinely used techniques of nucler medicine such s single photon emission computed tomogrphy (SPECT) imging or even positron emission tomogrphy (PET) which represents the current gold stndrd in clinicl perfusion imging. Besides the higher sptil resolution nd lck of rdition exposure, myocrdil MR perfusion imging hs no ttenution prolem relted to ntomicl limittions. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 11 of 19

12 However, with the use of mgnetiztion-prepred TuroFLASH techniques (e.g. sturtion recovery Turo- FLASH) the SNR hs come to limit (Fig. 18). Therefore comintion with ipat techniques seems to e of less enefit, s further loss in SNR comes long. Newly developed techniques for myocrdil perfusion imging sed on SSFP techniques re currently under investigtion [15]. In comprison with TuroFLASH techniques, these sequences (ville s Siemens WIP pckge) show considerle higher intrinsic SNR therefore llowing for miniml to moderte loss of SNR when comined with ipat (Fig. 19). Figure 18 Imges of MR perfusion dt sets using sturtion recovery TuroFLASH technique. Comprison of non-ipat Turo- FLASH technique () with n ipat (GRAPPA) TuroFLASH technique (). There is considerle more noise within the ipat imge which hmpers depiction of perfusion normlities sed on the low SNR. Figure 19 Sturtion recovery TrueFISP perfusion imges comined with ipat (GRAPPA) nd high in-plne resolution of mm 2. In contrst to sturtion recovery TuroFLASH, the sptil resolution is still high enough to follow signl dynmics. Liver Imging MR liver imging is most severely restricted y respirtory movement. Therefore, imge qulity ws considerly improved with the introduction of T2-weighted turo spin echo (TSE) nd single-shot sequences. With these techniques, reth-hold exmintions of the liver ecme possile, which most uthors consider superior to conventionl spin echo sequences [16 18]. Generlly, mximum reth-hold time of out 20 seconds, tolerle even for ptients in d helth condition, is limiting prmeter for ll sequences used for liver imging. Respirtory-triggered T2-weighted sequences s n lterntive to the reth-hold strtegy hve een studied with contrdictory results [16, 19, 20]. An dvntge of respirtory-triggered sequences is the possiility to perform high-resolution exmintions with 5 mm slice thickness which hs not een possile with reth-hold sequences due to the limited reth-hold time. If one ttempts to overcome this limittion of reth-hold imging y exmintions with multiple reth-holds such tht the liver is exmined in severl stcks of slices, then prts of the liver cn e missed if the ptient does not meet the sme position of the diphrgm in ll stcks [19]. The development of ipat llows for sustntil reduction of cquisition time, nd thus reth-hold sequences with improved sptil resolution cn e used. 2D nvigtor-sed techniques, s the Prospective Acquisition Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 12 of 19

13 Correction (PACE) technique, known from crdic imging, cn dpt the stcks of slices ccording to the respirtory position y registering the diphrgm position, so tht the whole liver cn e covered even if the ptient does not hold his reth t the sme position [21]. We compred four high-resolution T2-weighted sequences with 5 mm slice thickness nd mtrix for routine liver imging: reth-hold TSE sequence with nd without ipat nd PACE (echo trin length: 27, TR = 2120 ms, TE = 87 ms, 4 reth-hold cycles, ipat fctor 2, 24 reference lines), nd respirtorytriggered TSE sequence with nd without ipat (echo trin length: 25, min. TR = 2680 ms, TE = 117 ms, ipat fctor 2, 24 reference lines). All imges were cquired with 12-element surfce coil system dedicted to ipat pplictions. A respirtion elt ws used for respirtory triggering. The im ws to demonstrte the fesiility of ipat nd PACE for T2-weighted liver imging nd to evlute imge qulity of the different sequences. Imge exmples of ll sequences re shown in Fig. 20 nd 21. In generl, imging with ipat reduced the cquisition time y out 40 % without visile SNR loss. Compring reth-hold nd respirtory-triggered techniques, the ltter turned out to e more roust in ptients wheres no difference in imge qulity ws oserved in volunteers. An explntion for this result is tht ptients hve more difficulties with the reth-hold period of up to 20 seconds. In conclusion, ipat liver exmintions with respirtory triggering pper to e the most roust pproch for clinicl routine exmintions. c d Figure 20 Exmples of T2- weighted liver imges: Breth-hold sequence without ipat () nd with ipat (), respirtory triggering without ipat (c) nd with ipat (d). Note the mrkedly reduced rtifcts in the reth-hold sequence with ipat () of this suject, who hd prolems holding his reth. Respirtory triggering s well compensted this prolem. c d Figure 21 Exmples of T2- weighted liver imges: Breth-hold sequence without ipat () nd with ipat (), respirtory triggering without ipat (c) nd with ipat (d). Respirtory triggering shows less motion rtifcts nd etter delinetion of the diffuse HCC due to etter T2 contrst. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 13 of 19

14 High-Resolution Renl MR Angiogrphy Three dimensionl gdolinium-enhnced mgnetic resonnce ngiogrphy (3D-Gd-MRA) hs gined high populrity s non-invsive imging lterntive for grding of renl rtery stenosis [22]. High ccurcies of over 90 % hve een reported y numerous reserchers in the pst five yers [23]. Nevertheless, the technique is still notoriously known for overgrding high-grde renl rtery stenoses nd missing low-grde lesions, therey limiting its overll clinicl cceptnce [24]. A recent Dutch multicenter tril presented less encourging results with overll ccurcies of only 85 % compred to DSA. In ddition, no relile dt on grding of stenoses of the more distl min renl rtery or segmentl rteries exists, yet [25]. One mjor limiting fctor is sptil resolution. For stndrd reth-hold cquisitions with olus dministrtion of extrcellulr, non-intrvsculr gdolinium cheltes, the mximum chievle sptil resolution represents compromise etween scn time, ntomic coverge nd SNR. Current imging protocols usully otin imges with mximum of 1.5 mm 3 isotropic resolution which still represents 5 to 7 fold less thn tht of digitl sutrction ngiogrphy (DSA). In renl rtery with dimeter of 7 8 mm, n isotropic voxel size of t lest 1 mm 3 is required for ccurte depiction of 90 % reduction in lumen dimeter. Prllel cquisition techniques llow for improvement of sptil resolution without prolonging dt cquisition nd re well suited for imges with high SNR such s 3D-Gd-MRA. Bsed on previous clcultions, it is expected tht voxel sizes of less thn 1 mm 3 re sustntilly limited y SNR constrints [26]. Therefore, it ws our im to increse sptil resolution to mximum vlues within this rnge. The ipat strtegy ws pplied on n 8- chnnel Mgnetom Sont Mestro Clss System in comintion with fst 3D FLASH sequence (TR = 3.79, TE = 1.3, Bndwidth = 350 Hz/pixel, flip ngle = 25 ). Nerly isotropic dt sets with sptil resolution of mm 3 could e cquired within 23 seconds [27]. For signl reception the dedicted 12-element rry coil system ws used. An ccelertion fctor of 2 ws used with 24 reference lines for uto-clirtion of the coils. For dt cquisition nd reconstruction, the GRAPPA nd SENSE lgorithms were compred in terms of rtifcts. To improve the contrst-to-noise rtio, the one-molr contrst gent gdoutrol (Gdovist, Schering AG, Germny) ws dministered t dose of 1.25 mmol/kg ody weight with n injection rte of 2 ml/s. In the ipat imges, SNR decresed y fctor of out 1.5 compred to the dt without ipat. This decrese in SNR could e visully noticed in the source imges, however the intrvsculr signl ws still cceptle. In the MIP imges, the overll decrese in SNR ws hrdly detected. The high-resolution renl 3D-Gd-MRA dt sets were compred to selective x-ry ngiogrphy in more thn 20 ptients with renl rtery stenosis rnging from 20 % luminl nrrowing to occlusion. Imge nlysis of the isotropic dt sets consisted of multiplnr reformts long the vessel xis to ssess the degree of dimeter reduction. In ddition, reformts perpendiculr to the vessel xis were performed to ssess the degree of reduction of vessel re. Using multiplnr reformts the degree of stenosis ws correctly ssessed in 18 of 20 ptients. In 2 cses, the degree of stenosis ws overestimted. However, when reformts were performed in the isotropic dt sets perpendiculr to the vessel xis ll stenoses could e correctly identified compred to x-ry ngiogrphy (Fig. 22 nd 23). One limittion is the propgtion of lising rtifcts into the center of the imge. These rtifcts could e theoreticlly voided y extending the FOV in the left- right direction so tht no lising occurs t ll. In clinicl prctice however, this would men sustntil increse in scn time, in prticulr in lrge ptients. In ddition, not ll ptients re le to put their rms over the hed. Therefore some degree of lising into the mrgins of the FOV hs to e ccepted. Using the GRAPPA lgorithm, rtifcts propgting from tissue outside the FOV into the center of the imge were kept t minimum. Only slight ring-like rtifcts occurred, which did not ffect the imge interprettion. However, when the msense technique ws lterntively used, these rtifcts were more severe (Fig. 24). In conclusion, high-resolution renl 3D-Gd-MRA using ipat llows for sustntil improvement of sptil resolution, therey incresing dignostic ccurcy compred to digitl sutrction ngiogrphy. Using the GRAPPA sed lgorithm, rtifcts propgting into the center of the FOV cn e kept t minimum. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 14 of 19

15 Figure 22 Multiplnr reformts of highresolution 3D-Gd-MRA. In the cross-sectionl reformts of the vessel (lower imge series) even the re of the stenotic lumen cn e clerly demonstrted due to the isotropic sptil resolution. Figure 23 Coronl MIP imge of high-resolution MRA with ipat () revels excellent greement to DSA (). Both high-grde renl rtery stenoses re seen including the residul vessel lumen. Note the sence of ny mjor lising rtifcts in the center of the imge. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 15 of 19

16 Figure 24 Comprison of propgted lising rtifcts in the sme ptient using the msense () nd GRAPPA () lgorithm. The field of view ws on purpose set to only 32 cm to enforce lising of the rms. In the msense imges severe rtifcts occur in the center of the imge (rrows) while these rtifcts re virtully sent on the GRAPPA imges. Whole-Body Imging/Screening Becuse of the recent improvements in hrdwre nd softwre nd the lck of ionizing rdition, mgnetic resonnce imging hs ecome cndidte for screening imging [28]. We developed n MR exmintion which comines well estlished components including functionl crdic imging together with myocrdil perfusion imging, imging of the lung, rin, n overll view of liver, kidneys, spleen, nd pncres, s well s the rteril system. Figure 25 Single reth-hold evlution of glol crdic function with rel-time ipat TrueFISP. Figure 26 Imging of the rin, lungs, nd domen s prt of the whole-ody screening exmintion. The whole-ody exmintion is performed in two prts. In the first prt, the ptient is in hed first position; the spine rry, two ody rrys, nd hed rry re used s receiver coils. In the second prt, the ptient is in feet first position; the spine rry, the lrge FOV dpter, one or two ody rrys (depending to the height of the ptient), nd the peripherl ngio rry re used s receiver coils. ipat with n ccelertion fctor of two is pplied for most scns of the exmintion including rel-time TrueFISP imging of the hert (Fig. 25), highresolution imging of the lung (Fig. 26) s well s dynmic crdic perfusion MRI with TrueFISP. In ddition, Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 16 of 19

17 ipat of 3D-Gd-MRA in comintion with the lrge FOV dpter is performed for ll studies llowing totl scn time of only 62 seconds to cover the re from the thorcic ort down to the toes t sptil resolution of less thn mm 3 (Fig. 27). By pplying the GRAPPA lgorithm with its integrted uto-clirtion scn, it is possile to use flexile comintions of receiver coils with flexile choice of ipat directions nd to move the ptient tle for different ipat cquisitions. The dvntge of ipat in this kind of exm is the coverge of lrge ntomic region nd gin of time so tht whole-ody scn cn e done within 90 minutes without loss of imge qulity. In the lst two months twenty individuls sent y their referring physicin while prticipting in mnger helthcre progrm underwent the whole-ody scn in our deprtment. All twenty individuls tolerted the MR exmintion well. Compred to the conventionl exmintion techniques like ultrsound nd ECG we hve estlished more comprehensive exm within resonle scn time. First results of pthologic findings (scr in lung, ortic stenosis, renl rtery stenosis) show good correltion with the gold stndrd exmintions. Figure 27 Exmple of gdolinium-enhnced MR ngiogrphy s prt of the whole-ody screening exmintion. Note the excellent visuliztion of vessel segments down to the pedl rch. No stenoses re present. Conclusion This overview on pplictions nd ongoing studies in different res of the ody supports the current trend to use prllel imging in the mjority of clinicl scn protocols. The generl dvntges of prllel imging re now well estlished. This includes the possiility for higher sptil resolution for 3D-Gd-MRA with shorter reth-holds which improves the ccurcy of this technique for grding of renl rtery stenosis. The comintion of timeresolved nd high-resolution 3D-Gd-MRA improves the detection nd differentition of pulmonry hypertension. The use of shorter echo trins for single shot HASTE or echo plnr imging results in less imge distortion nd less signl decy. Initil results show enefits for EPI diffusion tensor imging in the rin s well s detection of erly infiltrtes in the lung with HASTE imging. Imging with multiple verges in shorter cquisition times improves detection of tumors in res with incresed motion such s the lrynx. Higher temporl resolution improves the ccurcy of crdic reltime techniques using SSFP sequences to mesure glol crdic function within single reth-hold. In ddition to the generl enefits of prllel imging, the ipat methods GRAPPA nd msense feture some unique dvntges. Artifcts in the center of coronl imges resulting from lising of tissue outside the FOV re sustntil suppressed using the GRAPPA lgorithm. The ipat lgorithms with uto-clirtion integrted into the individul scn re less sensitive to ptient motion s other prllel imging techniques with single mesurement of the coil sensitivity profiles t the strt of the exmintion. In ddition, the integrted uto-clirtion llows flexile comintion of multiple receiver coil systems. Therefore, lrge ntomic coverge with vrious receiver coils nd flexile choice of the ipat directions is possile. This is prticulrly helpful for whole-ody imging where multiple receiver coil systems re comined to scn the entire ody with prllel imging techniques. Scn time for complete crdiovsculr exm is sustntilly reduced while sptil nd temporl resolution of the individul scns re preserved. Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 17 of 19

18 In conclusion, ipat cn e used to improve most clinicl protocols for comprehensive morphologic nd functionl imging. Depending on the specific ppliction its min dvntges re decrese in imging rtifcts or n increse in speed, sptil, or temporl resolution. Co-Workers on Prllel Imging Roger Eiel (pulmonry imging) Wilhelm Fltz (imging of the lrynx) Peter Herzog (pulmonry imging) Armin Huer (crdic imging) Wolfgng Klinger (MR technicin) Hrld Krmer (whole-ody imging nd screening) Konstntin Nikolou (crdic imging nd pulmonry imging) Crol Schmid (MR technicin) Frnk Stdie (MR technicin) Roert Sthl (diffusion tensor imging) Anj Struwe (MR technicin) Bernd J. Wintersperger (crdic imging) Christoph Zech (dominl imging) Acknowledgements We would like to thnk the Mgnetic Resonnce Development Deprtment of Siemens Medicl Systems nd especilly Mthis Nittk, Berthold Kiefer, nd Rolf Suter for their technicl support. References [1] Griswold MA, Jko PM, Heidemnn RM, Nittk M, Jellus V, Wng J, Kiefer B, Hse A. Generlized utoclirting prtilly prllel cquisitions (GRAPPA). Mgn Reson Med 2002; 47: [2] Sodickson DK, Mnning WJ. Simultneous cquisition of sptil hrmonics (SMASH): fst imging with rdiofrequency coil rrys. Mgn Reson Med 1997; 38: [3] Pruessmnn KP, Weiger M, Scheidegger MB, Boesiger P. SENSE: sensitivity encoding for fst MRI. Mgn Reson Med 1999; 42: [4] Bsser PJ, Pierpoli C. A simplified method to mesure the diffusion tensor from seven MR imges. Mgn Reson Med 1998; 39: [5] Le Bihn D, Mngin JF, Poupon C, Clrk CA, Pppt S, Molko N, Chrit H. Diffusion tensor imging: concepts nd pplictions. J Mgn Reson Imging 2001; 13: [6] Keerl M, Kenn W, Hhn D. Current concepts in imging of lryngel nd hypophryngel cncer. Eur Rdiol. 2002; 12: [7] Biederer J, Busse I, Grimm J, Reuter M, Muhle C, Freitg S, Heller M. Sensitivity of MRI in detecting lveolr Infiltrtes: Experimentl studies. Rofo Fortschr Ge Rontgenstr Neuen Bildge Verfhr. 2002; 174: [8] Amundsen T, Torheim G, Kvistd KA, Wge A, Bjermer L, Nordlid KK, Johnsen H, Aserg A, Hrldseth O. Perfusion normlities in pulmonry emolism studied with perfusion MRI nd ventiltion-perfusion scintigrphy: n intr-modlity nd inter-modlity greement study. J Mgn Reson Imging 2002; 15: [9] Htu H, Tdmur E, Prsd PV, Chen Q, Buxton R, Edelmn RR. Noninvsive pulmonry perfusion imging y STAR- HASTE sequence. Mgn Reson Med 2000; 44: [10] Mi VM, Hgspiel KD, Christopher JM, Do HM, Altes T, Knight-Scott J, Stith AL, Mier T, Berr SS. Perfusion imging of the humn lung using flow-sensitive lternting inversion recovery with n extr rdiofrequency pulse (FAIRER). Mgn Reson Imging 1999; 17: [11] Brkhusen J, Goyen M, Ruhm SG, Eggerecht H, Detin JF, Ldd ME. Assessment of ventriculr function with single reth-hold rel-time stedy-stte free precession cine MR imging. Am J Roentgenol 2002; 178: [12] Lee VS, Resnick D, Bundy JM, Simonetti OP, Lee PWeinre JC. Crdic function: MR evlution in one reth hold with rel-time true fst imging with stedy-stte precession. Rdiology 2002; 222: Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 18 of 19

19 [13] Miller S, Simonetti OP, Crr J, Krmer U, Finn JP. MR imging of the hert with cine true fst imging with stedy-stte precession: influence of sptil nd temporl resolutions on left ventriculr functionl prmeters. Rdiology 2002; 223: [14] Wintersperger BJ, Nikolou K, Dietrich O, Rieer, J, Nittk M, Reiser MF, Schoenerg SO. Single reth-hold rel-time cine MR imging: Improved temporl resolution using generlized utoclirting prtilly prllel cquisition (GRAPPA) lgorithm. Eur Rdiol 2003 (in press) [15] Schreier WG, Schmitt M, Klden P, Mohrs OK, Kreitner KF, Thelen M. Dynmic contrst-enhnced myocrdil perfusion imging using sturtion-prepred TrueFISP. J Mgn Reson Imging 2002; 16: [16] Ktym M, Msui T, Koyshi S, Ito T, Tkhshi M, Skhr H, Nozki A, Ksw H. Ft-suppressed T2-weighted MRI of the liver: comprison of respirtory-triggered fst spin-echo, reth-hold single-shot fst spin-echo, nd reth-hold fst-recovery fst spin-echo sequences. J Mgn Reson Imging 2001; 14: [17] G J, Htu H, Jenkins RL, Finn JP, Edelmn RR. Liver msses: replcement of conventionl T2-weighted spin-echo MR imging with reth-hold MR imging. Rdiology 1996; 200: [18] Hori M, Murkmi T, Kim T, Knemtsu M, Tsud K, Tkhshi S, Tkmur M, Hoshi H, Nkmur H. Single rethhold T2-weighted MR imging of the liver: vlue of single-shot fst spin-echo nd multishot spin-echo echoplnr imging. AJR Am J Roentgenol 2000; 174: [19] Augui J, Vignux O, Argud C, Coste J, Gouy H, Legmnn P. Liver: T2-weighted MR imging with reth-hold fstrecovery optimized fst spin-echo compred with reth-hold hlf-fourier nd non-reth-hold respirtory-triggered fst spinecho pulse sequences. Rdiology 2002; 223: [20] Tng Y, Ymshit Y, Nmimoto T, Ae Y, Tkhshi M. Liver T2-weighted MR imging: comprison of fst nd conventionl hlf-fourier single-shot turo spin-echo, reth-hold turo spin-echo, nd respirtory-triggered turo spin-echo sequences. Rdiology 1997; 203: [21] Liu YL, Riederer SJ, Rossmn PJ, Grimm RC, Deins JP, Ehmn RL. A monitoring, feedck, nd triggering system for reproducile reth-hold MR imging. Mgn Reson Med 1993; 30: [22] Prince MR, Nrsimhm DL, Stnley JC, Chenevert TL, Willims DM, Mrx MV, Cho KJ. Breth-hold gdoliniumenhnced MR ngiogrphy of the dominl ort nd its mjor rnches. Rdiology 1995; 197: [23] Dong Q, Schoenerg SO, Crlos RC, Neimtllh M, Cho KJ, Willims DM, Kznjin SN, Prince MR. Dignosis of renl vsculr disese. Rdiogrphics 1999; 19: [24] Schoenerg SO, Bock M, Knopp, MV, Essig M, Lu G, Hwighorst, H, Zun I, Kllinowski F, vn Kick G. Renl rteries: Optimiztion of 3D Gdolinium MR Angiogrphy with olus-timing independent fst multiphse cquisition in single reth hold. Rdiology 1999; 201: [25] Schoenerg SO, Knopp MV, Londy F, Krishnn S, Zun I, Lng N, Essig M, Hwighorst H, Mki JH, Stfford-Johnson D, Kllinowski F, Chenevert TL, Prince MR. Morphologic nd functionl mgnetic resonnce imging of renl rtery stenosis: multireder tricenter study. J Am Soc Nephrology 2002; 13: [26] Heid O. The outer limits of contrst enhnced MR ngio (grdient performnce, resolution, speed, etc). In: Proceedings of the tenth interntionl workshop on mgnetic resonnce ngiogrphy, Prk City, Uth, The Interntionl MR Angio Clu 1998: 75 [27] Schoenerg SO, Rieger J, Johnnson LO, Dietrich O, Bock M, Prince MR, Reiser MF. Dignosis of renl rtery stenosis with mgnetic resonnce ngiogrphy updte Nephrol Dil Trnsplnt 2003, 18: [28] Goyen M, Herorn CU, Kroger K, Luenstein TC, Detin JF, Ruehm SG. Detection of therosclerosis: systemic imging for systemic disese with whole-ody three-dimensionl MR ngiogrphy initil experience. Rdiology. 2003; 227: Mgnetom Flsh 2003; 27 (2.2003): 6 20 Pge 19 of 19