Magnetic Resonance Imaging of the Gut: A Primer for the Luminal Gastroenterologist

Transcription

1 CINICA AND SYSTEMATIC S nture pulishing group 497 see relted editoril on pge x Mgnetic Resonnce Imging of the Gut: A Primer for the uminl Gstroenterologist Mir i m R omero, M D 1, Jmes. Buxum, MD 2 nd Suznne. Plmer, MD1 Mgnetic resonnce imging (MRI) is well estlished for imging the solid orgns of the domen nd pelvis. In recent yers it hs een hving n incresingly importnt role in the evlution of the gstrointestinl (GI) trct. Fluoroscopy nd dominl computed tomogrphy, the trditionl minstys of owel imging, remin vlule; however, the contemporry emphsis on decresing ptient rdition exposure is driving prctice towrd non-ionizing modlities such s MRI. The inherent dynmic properties of MRI, its superior tissue contrst, nd cross-sectionl cpilities offer dditionl dvntges. Here we review, from esophgus to nus, techniques nd indictions for MRI of the GI lumen with n emphsis on the norml MRI ppernce of the GI trct nd commonly encountered pthology. Am J Gstroenterol 2014; 109: ; doi: /jg ; pulished online 7 Jnury 2014 INTRODUCTION Fluoroscopy of the limentry trct is not osolete, ut it hs moved from eing the first-line study for evlution of the ptient presenting with gstrointestinl (GI) symptoms to prolemsolving tool. Both computed tomogrphy (CT) nd mgnetic resonnce imging (MRI) offer dvntges over trditionl fluoroscopic techniques, especilly for evlution of extrluminl complictions of GI disese. CT is the most utilized imging modlity for ptients with noniliry symptoms, nd nonspecific nd cute dominl pin. However, MRI evlution of the solid orgns of the domen is well estlished nd it hs n incresing role in the evlution of owel disese. The physics of MRI is complex nd will e ddressed only riefly ( 1 ). Clinicl MRI is sed on the interction of protons nd rdiofrequency wves in the presence of strong externl mgnetic field. Unlike CT imging, MRI does not use potentilly hrmful ionizing rdition. Mgnet strength is mesured in Tesl (T) nd rnges from 0.1 to 3 T for clinicl ody imging. Higher field strength cn result in the higher sptil nd temporl resolution needed for imging the owel; currently, imging the GI trct is optiml t high field strength (1.5 nd 3 T). Ultrhigh field MRI scnning cn go s high s 15 T, ut such high fields re restricted to preclinicl reserch. Imges of the ptient re otined through multistep process of energy trnsfer nd signl trnsmission. When the ptient is plced in the mgnet, moile protons ssocited with ft nd wter lign prllel to the externl mgnetic field. A rdiofrequency pulse is then pplied, cusing the protons to move from their lower, stle energy stte to higher, unstle energy stte (excittion). When the rdiofrequency pulse is removed, the protons relx nd emit rdio signl tht is cptured y receiving coil nd reconstructed into imges through complex mthemticl lgorithm: the Fourier trnsform. Different tissues hve different relxtion rtes tht led to different levels of signl intensity (SI) on the reconstructed imge. Tissue with high SI (e.g., ft) is righter thn tissue with low SI (e.g., corticl one). The difference in SI is clled tissue contrst. T1 nd T2 re prmeters of relxtion tht vry y tissue type nd the cquisition progrm. Multiple pulse sequences re cquired for MRI imging. A pulse sequence is set of defined rdiofrequency pulses nd timing prmeters used to cquire imge dt. The dt re cquired in volumes (voxels), reconstructed s twodimensionl pixels nd displyed reltive to tissue SI vritions (tissue contrst). At different points in the exmintion, cquisition is progrmmed to ccentute T1 nd T2 contrst weighting. In T1-weighted sequences, fluid ppers lck (low SI) nd in T2- weighted sequences fluid ppers white (high SI). With the ddition of functionl imging sequences, such s diffusion-weighted imging (DWI), res of ctive inflmmtion, firosis, nd highly cellulr neoplsm ecome more conspicuous. The sence of ionizing rdition, superior tissue contrst, nd dynmic ilities of MRI re prticulrly well suited for ddressing the clinicl questions commonly encountered in luminl gstroenterology prctice. Compred with CT, the weknesses of MRI include lower sptil resolution, longer cquisition times, incresed susceptiility to motion rtifct nd locl field 1 Deprtment of Rdiology, Keck School of Medicine, University of Southern Cliforni, os Angeles, Cliforni, USA ; 2 Deprtment of Internl Medicine, Gstroenterology, Keck School of Medicine, University of Southern Cliforni, os Angeles, Cliforni, USA. Correspondence: Suznne. Plmer, MD, Deprtment of Rdiology, Keck School of Medicine, University of Southern Cliforni, 1500 Sn Plo Street, 2nd Floor Imging, os Angeles, Cliforni 90033, USA. E-mil: suznne.plmer@med.usc.edu Received 4 July 2013; ccepted 24 Novemer y the Americn College of Gstroenterology The Americn Journl of GASTROENTEROOGY

2 498 Romero et l. inhomogeneity, more limited scnner ccess, nd higher cost. In ddition, the longer required imging time my not e suitle in criticlly ill ptients. Moridly oese ptients my not fit into mny scnners nd clustrophoic ptients my require sedtion or nxiolytic mediction. However, if properly used, MRI cn serve s n excellent prolem-solving tool. IMAGING TECHNIQUES Ptient preprtion for routine dominl pelvic MRI is strightforwrd. Fsting for 4 6 h efore the study is recommended to reduce owel peristlsis, minimize intrluminl residue, nd optimize visuliztion of the iliry system, which is prt of the differentil in most ptients presenting with dominl symptoms. Unlike solid orgns, the owel cn e esily displced nd distended. Collpsed loops cn oth simulte nd oscure pthology; therefore, if enteric imging is desired, ttention to good owel distension is mndtory. There re severl specilized protocols tht require specific preprtion. MR enterogrphy (MRE) techniques require lrge volume of orl contrst. MR colonogrphy (MRC) techniques require owel clensing preprtion, similr to tht used for CT colonogrphy (CTC) nd opticl colonoscopy, s well s colonic contrst. uminl contrst gents re ctegorized sed on their T1 nd T2 SI ( Figure 1 ). Positive gents hve high SI on oth T1- nd Contrst T2 T1 Positive Negtive Biphsic Figure 1. Positive gents produce high signl intensity (SI) on oth T1- weighted (T1W) nd T2-weighted (T2W) sequences. Negtive gents produce low SI on oth T1W nd T2W sequences. Biphsic gents produce high SI on T2W nd low SI on T1W sequences. T2-weighted sequences. Exmples include dilute gdolinium cheltes (Gd-C), mngnese ions (some fruit juices contin high levels), high-ft milk, nd ferrous ions. As positive gents my msk murl enhncement on post-contrst sequences, they re not recommended. Negtive gents hve low SI on oth T1- nd T2-weighted sequences. Perfluorooctyl romide, ferumoxide, nd room ir re mong the ville options ( 2 ). Inner loop scess visuliztion is improved with such gents, ut susceptiility rtifct from these gents my e ccentuted. The most commonly used gents re iphsic, low SI on T1-weighted sequences, nd high SI on T2-weighted sequences. These gents offer high contrst etween the high SI lumen nd low SI owel wll on T2- weighed sequences without msking norml enhncement on T1-weighted sequences. Wter, methycellulose, mnnitol, soritol, polyethylene glycol, pinepple juice, lueerry juice, low-ft milk, nd dilute rium re iphsic gents ( 3,4 ). An optiml orl contrst gent is resonly pltle, optimizes luminl distension, hs miniml side effects, is redily ville, nd is inexpensive. Wter stisfies four of the five criteri, ut is sored cross the intestinl mucos, limiting distension. Polyethylene glycol nd mnnitol hve limited sorption y the owel; however, their cthrtic effects often cuse dirrhe ( 5 ). Dilute rium sulfte (98 99 % wter) provides good owel distension, is well-tolerted, nd reltively inexpensive ( Figure 2 ). ow-ft milk my e used with good results s well. Ptients my drink up to 1,200 1,500 cc of orl contrst, especilly if the distl smll owel is the re of interest. The optiml volume of contrst, the speed of contrst ingestion, nd time to imging cn vry gretly in ptients with different pthologic processes nd disese ctivity. Incomplete luminl distention is common prolem, prticulrly in the jejunum. As collpsed owel loops cn oth oscure nd mimic disese, some tiloring my hve to e done on n individul sis. For foregut imging, up to 1 liter of wter is given within 15 min of imging; for smll owel imging to the level of the cecum, the ptient typiclly egins ingesting dilute rium sulfte 90 min efore imging ( 6 ); if smll nd lrge owel contrst is desired, ingestion of dilute rium sulfte my e strted 3 4 h efore the exmintion. Rectl contrst is not generlly used; however, rectl contrst my e helpful for evluting norectl pthology. uminl c Figure 2. Biphsic contrst gent ppernce: 2 % rium sulfte on coronl T2-weighted (T2W) imge ( ) nd coronl T1-weighted (T1W) post contrst with ntiperistltic gent imge ( ). Coronl T1W imge without nti peristltic gent ( c ). Norml stomch with dequte contrst distension; rrows, norml jejunum;, liver. The Americn Journl of GASTROENTEROOGY VOUME 109 APRI

3 Mgnetic Resonnce Imging of the Gut 499 contrst distends the rectl lumen nd cn increse the conspicuity of the norml wll SI, thickening, nd enhncement. Either queous gel or wter my e used. Approximtely cc of gel re dministered efore the eginning of the exmintion or wter my e instilled vi Foley under grvity during the study. Either surfce coil or n endorectl coil my e used; lthough n endorectl coil my provide etter sptil resolution, its nrrow field of view nd ssocited ptient discomfort re significnt drwcks. Endorectl coils my lso distort djcent structures. Bowel motion cn degrde study; therefore, dministrtion of n ntiperistltic gent is recommended for enteric imging ( Figures 2 nd c ). The ntiperistltic is dministered t the eginning of the exmintion, when the ptient is plced on the MRI tle. The choice of ntiperistltic is sed on fmilirity with nd ccess to the gent. Frequently used gents include glucgon nd utylscopolmine, lthough the ltter is not FDA pproved for use in the United Sttes. Glucgon my e dministered intrvenously (IV) or intrmusculrly. A tuerculin syringe is used for intrmusculr injection nd its smll guge cuses reltively little discomfort during injection. However, IV injection my hve fster onset nd longer durtion of nti-peristltic effects thn intrmusculr injection ( 7 ). Sulingul or IV evsin my lso e dministered ( 8 ). IV injection of glucgon my e ssocited with incresed nuse nd vomiting. At our institution, ptients undergoing routine dominl MRI re positioned supine on the tle, wheres ptients undergoing MRE nd MRC exmintions my e positioned prone to mximize dominl compression, provide mximl owel coverge on coronl imges, nd seprte owel loops. However, for ptient comfort resons, ptients my e imged the supine position ( 9 ). MRI imging should e performed on high field mgnet (1.5 3 T). Evlution of the GI trct relies on rpid cquisition sequences while the ptient holds their reth, lthough free rething sequences my e used. Approximtely min re llotted for ech exmintion. IV contrst is routinely used in enteric imging, nd Gd-C re the contrst gents of choice. Administrtion of Gd- C should e voided in ptients with cute kidney injury, end-stge or severe chronic kidney disese, s these ptients re t higher risk of nephrogenic systemic firosis, potentilly life-thretening condition. There is no solute estimted glomerulr filtrtion rte cutoff elow which Gd-C cnnot e dministered; however, the use of Gd-C should e voided when the estimted glomerulr filtrtion rte is < 30 ml /min per 1.73 m 2 (10,11 ). This is not n solute contrindiction; the use of Gd-C should e t the discretion nd judgment of the ordering physicin. If possile, lterntive imging should e considered. If contrst-enhnced MRI is necessry this should e documented in the ptient s record, nd if the ptient is on hemodilysis the MRI should e scheduled immeditely efore dilysis. Although Gd-C gents re effectively removed with hemodilysis, no pulished report hs proved tht erly dilysis prevents the development of nephrogenic systemic firosis (10 13 ). Specific gents re contrindicted in ptients with estimted glomerulr filtrtion rte < 30 ml /min per 1.73 m 2, including nonionic liner Gdodimide nd Gdoverssetmide, nd Gdopentette dimeglumine; hence, lterntive Gd-C gents should e used ( 10 ). MRI my e performed in ll trimesters. Mny institutions, including ours, require ptients to sign informed consent efore the exmintion. Protocols for pregnnt ptients rely on unenhnced imging. Gdolinium crosses the plcentl rrier, nd fetl risk ssocited with gdolinium exposure remins unknown; thus, these gents should not e routinely used in pregnncy. IMAGING FINDINGS Esophgus nd stomch Fluoroscopic esophgogrphy remins the minsty of nonopticl imging for evlution of the esophgus. Motility studies re redily performed with fluoroscopy, nd post-opertive perfortions re esily demonstrted. However, MRI hs potentil vlue in the ssessment of gstroesophgel junction ntomy nd reflux ( 14,15 ), including elucidting the structures t the gstroesophgel junction nd proximl stomch tht constitute the reflux rrier. MRI hs potentil use in dynmic swllow studies, ut further correltion with mnometry nd video fluoroscopy is needed ( 16 ). For esophgel crcinom stging, CT remins the ccepted imging modlity, ut MRI cn potentilly provide detiled ntomic informtion ( 17 ), nd sequences coned down to the posterior medistinum my e helpful for prolem solving in difficult cses or when iodinted contrst is contrindicted ( Figures 3 nd ). MRI cn id in the evlution of oth congenitl lesions nd post-opertive chnges ( Figures 4 nd ). Hitl herni is common incidentl finding esily recognized on MRI ( Figure 5 ). MR esophgogrphy hs lso een fvorly compred with conventionl MRI in tumor locliztion, tumor length, nd stging ( 18 ). Normlly, the gstric wll should pper s thin, uniform, low SI line on T2-weighted imging, mesuring 2 3 mm in thickness ( Figure 2 ). Gstric ruge my increse the pprent thickness of the gstric wll when prominent, ut my e differentited from pthology, s norml ruge will enhnce homogeneously with the rest of the stomch wll ( Figure 2 ). Murl thickening secondry to gstritis is typiclly diffuse nd SI is righter on T2-weighted imges due to edem ( Figure 6 ). In contrst, gstric cncer results in focl wll thickening tht ppers slightly higher in SI on T2- weighted imges, intermedite SI on T1-weighted imges, nd with heterogeneous enhncement on post contrst imges ( Figures 7, nd 8 ) ( 19,20 ). initis plstic will hve lower T2-weighted signl thn other gstric cncers ecuse of its desmoplstic nture nd does not enhnce s rightly with contrst ( 20 ). ymphdenopthy nd heptic metstses re well demonstrted on MRI; however, dditionl work is needed to estlish its efficcy in stging gstric cncer (21 23 ). Over the course of the lst decde, the pplicility of MRI for the evlution of gstric emptying nd gstric motility hs een exmined nd preliminry results re fvorle ( 24 ). However, fluoroscopic swllowing studies nd nucler medicine gstric studies re less costly nd less time intensive. Smll owel Historiclly, the smll owel hs presented the gretest imging chllenge ecuse of its reltive inccessiility nd long length y the Americn College of Gstroenterology The Americn Journl of GASTROENTEROOGY

4 500 Romero et l. H Figure 4. Sixty-one-yer-old mle sttus post esophgectomy for esophgel cncer. Coronl T2-weighted (T2W) ( ) nd xil post contrst T1- weighted (T1W) ( ) imges demonstrte norml, posterior medistinl gstric pull-up with norml gstric wll enhncement (rrows). Figure 3. Appernce of the norml esophgus on sgittl T1-weighted (T1W) ( ) nd xil T2-weighted (T2W) ( ) imges (rrows). The esophgus is typiclly intermedite in signl intensity. The trditionl smll-owel series does not distend the smll owel relily, nd enteroclysis requires uncomfortle nsojejunl intution ( 25 ). Neither modlity gives direct visuliztion of extrluminl pthology. CT enterogrphy nd MRE hve high resolution for evluting enteric nd nonenteric chnges of the smll owel. Crohn s disese is the leding indiction for cross-sectionl smll owel imging. In ddition to the moridity ssocited with inflmmtion, Crohn s disese is complicted y fistuls, strictures, ostruction, scesses, nd cncer. CT enterogrphy nd MRE hve een found to e eqully ccurte in detecting ctive smll owel inflmmtion nd the extr-enteric complictions of Crohn s disese, despite incresed motion rtifct on MRE ( 26 ). Consensus guidelines endorse oth MRE nd CT enterogrphy ( 27 ). However, CT exposes ptients to ionizing rdition nd should e used judiciously in ptients who my e imged multiple times, such s those with inflmmtory owel disese, s these ptients re typiclly young nd my require multiple studies over the course of their lifetimes ( 28,29 ). Cpsule endoscopy llows direct mucosl evlution of the smll owel while minimizing ptient discomfort ( 30 ). However, the cpsule cnnot e mneuvered, nd luminl nrrowing my result in mechnicl ostruction ( 30 ). Although we do not routinely perform smll owel follow-through efore cpsule endoscopy, given the low likelihood of retention, clinicl history suggestive of ostruction mndtes fluoroscopic study to exclude strictures (31 ). Alert et l. ( 32 ) hve shown MRI to hve comprle sensitivity to cpsule endoscopy in the detection of smll owel Crohn s disese, while simultneously llowing evlution of extr-intestinl disese. The different segments of smll owel hve very chrcteristic norml MRI ppernce. Normlly, the smll owel mesures no more thn 3 cm in dimeter nd the wll mesures no more thn 3 4 mm in thickness. The jejunum should hve no fewer thn threefolds per inch; the ileum my hve up to fivefolds per inch. Findings ssocited with Crohn s disese include wll thickening nd incresed enhncement, oth of which vry depending on disese ctivity. In the setting of cute inflmmtion, the thickened owel wll will hve higher SI on T2-weighted imges, nd intense enhncement on post-contrst sequences ( Figures 9 nd ). Chroniclly ffected owel loops re of lower SI on The Americn Journl of GASTROENTEROOGY VOUME 109 APRI

5 Mgnetic Resonnce Imging of the Gut 501 S K G K P K S K G P Figure 5. Coronl T2-weighted (T2W) imge of incidentlly identifi ed, moderte-sized hitl herni (rrow)., liver; S, spleen; K, kidney; drenl glnd. K K S Figure 7. Foty-fi ve-yer-old femle presenting with vgue dominl pin. Axil T2-weighted (T2W) ( ) nd T1-weighted (T1W) with ft suppression ( ) imges demonstrte gstric wll thickening with intermedite signl intensity (SI). Biopsy reveled gstric crcinom., liver; P, pncres; K, kidney; G, gllldder; S, spleen. S Figure 6. Firty-seven-yer-old mle with dominl pin. Axil T2- weighted (T2W) imge demonstrtes diffuse gstric wll thickening (}) nd incresed signl (rrow). Acute gstritis ws demonstrted on upper endoscopy., liver; S, spleen; scites. T2-weighted imges, with delyed rther thn erly enhncement on post-contrst sequences. Mildly incresed enhncement in non-thickened owel should e pproched cutiously, s it cn led to flse positives. The multiphsic cpilities of MRE llow differentition of strictures from res of non-distension, s strictures will persist over time ( 33 ). At our institution, we otin ultrfst single shot T2-weighted spin echo imging in the coronl plne t the eginning nd t the end of the exmintion to help differentite fixed lesions from res of nrrowing due to peristlsis. Coronl plne single shot T2-weighted spin echo cine imging, efore ntiperistltic mediction onset of ction, cn lso e used to evlute for persistent stricture nd dysmotility ( 34 ). Ares of stenosis my e ssocited with prestenotic dilttion ( Figures 10 nd ) (35 ). MRI does not demonstrte erly nd superficil ulcertion routinely due to limited sptil resolution ( 36 ). In these ptients, cpsule endoscopy should e considered. Deep ulcertions of Crohn s disese my e identified on oth T1- nd T2-weighted sequences Figure 8. Thirty-seven-yer-old femle with gstric cncer. Coronl T1- weighted (T1W) post-contrst imge demonstrtes irregulrly thickened, enhncing gstric wll (lrge rrowhed) consistent with gstric crcinom. A stent (rrow) ws plced to relieve ostructive symptoms. Smll owel (smll rrowheds) is thickened due to peritonel disese., liver; scites. s liner owel wll protrusions. Avidly enhncing lymph nodes, mesenteric edem, nd engorged vs rect hve een ssocited with ctive disese. Mesenteric firo-ftty prolifertion round inflmed owel supports dignosis of Crohn s disese ( 37 ). Acute complictions of Crohn s disese such s smll owel ostruction, ileus, nd owel lek, my e seen on MRI ( Figures 11 nd 12, ), ut the vilility in the cute cre setting llows CT to remin the imging modlity of choice for the ptient presenting with cute dominl symptoms. More chronic complictions, 2014 y the Americn College of Gstroenterology The Americn Journl of GASTROENTEROOGY

6 502 Romero et l. B B Figure 9. Fifty-eight-yer-old mle with Crohn s disese. Coronl T2- weighted (T2W) ( ) nd T1-weighted (T1W) post-contrst ( ) imges demonstrte chnges t the level of the terminl ileum nd ileocecl vlve, including wll thickening nd edem, incresed wll enhncement, nd pseudopolyp (white rrow). The reminder of the ilium (dshed ovl) nd jejunum (ovl) re norml. B, ldder. including fistuls, sinus trcts, nd dhesions re well chrcterized on MRI ( Figure 13 ). Fistuls pper high in signl on T2-weighted imging nd the wlls vidly enhnce following the dministrtion of IV contrst. Bowel wll thickening, fold density, nd incresed enhncement re not specific to Crohn s disese. For exmple, oth incresed nd decresed numers of jejunl folds my e seen in celic disese ( 38 ). Other forms of enteritis cn lso present with similr MRI findings; therefore, the distriution of enteric pthology nd the presence or sence of nonenteric findings should e correlted with the clinicl presenttion nd history in order to increse ccurcy of the dignosis ( Figure 10 ). Smll owel tumors, lthough uncommon, my e encountered ( Figures 14 nd 15 ). Mlignnt lesions include denocrcinoms ( Figure 15 ), lymphoms, GI stroml tumors, nd crcinoids. Both Vn Weyenerg et l. ( 39 ) nd Msselli et l. (40 ) hve demonstrted high ccurcy of MR enteroclysis for dignosis of smll owel neoplsms in ptients suspected of hving smll owel mlignncy, lthough it remins n emerging technique, nd MRI my hve role in the surveillnce of polyposis syndromes. MRI my not e s sensitive in the detection of smller polyps s cpsule endoscopy (41 ). However, for polyps > 15 mm, MRI hs detection rte similr to cpsule endoscopy, nd MRI lso loclizes polyps Figure 10. Seventy-one-yer-old femle sttus post externl em rdition therpy for cervicl cncer. Coronl T1-weighted (T1W) post-contrst imge ( ) revels normlly incresed ilel wll enhncement consistent with rdition ileitis (rrow). This is ssocited with dilttion of the proximl smll owel. Spot imge from n X-ry smll owel follow-through confi rms the fi ndings ( ). Direct compression ccentutes the persistent nrrowing nd norml seprtion of the involved loops of ileum. more ccurtely ( Figure 16 ) ( 42 ). Although smll owel ssessment is criticl for Peutz Jeghers, the role for surveillnce eyond the duodenum in other polyposis syndromes, whether y imging, cpsule, or deep enteroscopy, hs not een vlidted ( 43,44 ). Appendix Well-estlished nd redily ville, CT is idel for the dignosis of ppendicitis nd its complictions in dults. MRI provides n lterntive in children nd pregnnt ptients, for whom ionizing rdition is prticulrly undesirle ( 45,46 ). MRI hs demonstrted high specificity nd sensitivity for ppendicitis in peditric nd pregnnt ptients, with 100 % sensitivities reported for oth popultions. Negtive lprotomy rtes in pregnnt women cn lso e decresed without incresing the perfortion rte ( 47 ). MRI hs lso een shown to e cost effective in pregnnt women with suspected ppendicitis following indeterminte ultrsound (US) ( 48 ). eeuwenurgh et l. (49 ) demonstrted similr ccurcy etween MRI nd CT for the detection of MRI in dult ptients ( 50,51 ). No deleterious effects of MRI on the fetus hve een documented nd MRI cn e performed t ny stge of pregnncy. However, The Americn Journl of GASTROENTEROOGY VOUME 109 APRI

7 Mgnetic Resonnce Imging of the Gut 503 G B Figure 11. Sixty-yer-old mle presenting with renl filure nd dominl distention. Coronl T2-weighted (T2W) imge demonstrtes diffusely dilted, fluid filled loops of smll owel nd diffuse ilel wll edem. No point of ostruction ws found nd distention resolved with dilysis, consistent with ileus., liver; G, gllldder; B, ldder; scites. Figure 13. Fifty-nine-yer-old mle with Crohn s disese nd history of multiple surgeries presenting with dominl pin. Axil T2-weighted (T2W) imge demonstrtes n dhesion (rrow) extending from the sigmoid colon (lrge rrowhed) to loop of smll owel (smll rrowhed). Whether this dhesion resulted from heling of n ileocolonic fi stul to the sigmoid or prior surgery could not determined, ut fi stul ws fvored given the loction. Figure 14. Sixty-fi ve-yer-old womn with dominl pin. Coronl T2-weighted (T2W) imge demonstrtes n exophytic, suserosl mss involving the second portion of the duodenum (rrow), consistent with gstrointestinl stroml tumor (GIST). Figure 12. Thirty-nine-yer-old mle presenting with diffuse dominl pin. Axil T1-weighted (T1W) imge ( ) demonstrtes crescentic region of low signl (extrluminl gs, rrow) with fl uid djcent to short segment of smll owel with mild wll irregulrity (dshed ovl). Computed tomogrphy (CT) confirms the presence of extrluminl gs nd fl uid, consistent with smll owel perfortion ( ). we routinely consent pregnnt ptients efore imging ( 52,53 ). At our institution, we hve developed n imging lgorithm for these ptient popultions in conjunction with the Emergency Medicine, Peditrics, nd Generl Surgery deprtments. The first exmintion performed is trgeted right-lower qudrnt US to identify the ppendix. If the study is equivocl MRI is then performed, nd if positive the ptient is sent for surgery. Conversely, norml ppendix excludes the dignosis of ppendicitis, nd other potentil cuses of dominl pin should e sought ( 51 ). Neither IV nor orl contrst is required for MRI evlution of cute ppendicitis. MR contrst gents pss through the plcent nd enter the fetl circultion; the risk to the fetus remins unknown ( 50 ). A grvid uterus will displce the ppendix from its usul loction, requiring thorough inspection to correctly identify it. As on oth CT nd US, the ppendix cn e recognized s lind-ending tuulr structure originting from the cecum. A thickened ( > 6 mm) nd edemtous ppendicel wll nd djcent inflmmtory chnges support the dignosis of cute ppendicitis. Ascesses tht my develop in cses of perfortion re well demonstrted on MRI. DWI sequences my e useful in mking sutle cse of ppendicitis more conspicuous ( Figures 17 nd ). Colon Most colonic MRI is performed to evlute known inflmmtory or neoplstic disese. CTC hs een more extensively vlidted 2014 y the Americn College of Gstroenterology The Americn Journl of GASTROENTEROOGY

8 504 Romero et l. Figure 15. Eighty-one-yer-old femle with dominl pin. Coronl T2-weighted (T2W) imge demonstrtes norml wll thickening of the second portion of the duodenum (rrow). Although nonspecifi c, the length of involvement (short) nd the intermedite signl intensity fvor neoplsm over inflmmtion / infection. Pthology reveled poorly differentited denocrcinom. nd is currently fvored over MRC outside of reserch protocols, especilly in ptients who cnnot undergo opticl colonoscopy. Kuehle et l. ( 54 ) showed sensitivity nd specificity of 83 % nd 90 %, respectively, for MRC in detecting polyps 5 mm; the ACRIN study showed sensitivity for CTC of 90 % for detecting polyps 10 mm with sensitivity for polyps 5 mm of 65 % ( 55 ). Although MRC compres fvorly with CTC, work in this re remins in erlier stges with overll polyp sensitivities rnging from 28.5 % to 100 % reported ( 56 ). The chllenges of MRC re similr to those of CTC nd most re relted to owel preprtion. Fecl tgging my reduce the mount of owel preprtion needed or eliminte it ltogether, ut this technique is not fully optimized nd sensitivity remins poor ( 57,58 ). Drk lumen nd right lumen MRC re the two mjor techniques. Insufflted room ir, CO 2, nd wrm tp wter re the most frequently used types of drk lumen contrst. However, gs cn cuse rtifcts, lthough there remins dete on the severity of the induced rtifcts ( 59 ). Both rium nd Ferumoxsil cn e used s drk lumen tgging gents. For drk lumen MRC, IV Gd is used; crcinom will enhnce, wheres polyps will not ( 59 ). Dilute Gd-C enem my e used in right lumen techniques or T2 imging my e performed with wter enems. As more Gd is needed for n enem thn for n IV injection, right lumen MRC will e more expensive. Air my lso simulte filling defect on right lumen MRC nd the enefits of contrst enhncement for evluting oth colonic lesions nd extrluminl disese is lost ( 59 ). As in CTC, most imging is performed in oth supine nd prone positions ( 56 ). MRC hs demonstrted high sensitivity for polyps > 1 cm, lthough it does not relily identify polyps < 5 mm (60 ). However, s smll polyps tend to remin stle for 3 5 yers without undergoing mlignnt degenertion, this limittion my not e cliniclly significnt. In ddition, polyps < 6 mm found on CTC re not reported. Grser et l. ( 61 ) recently compred the dignostic ccurcy of opticl colonoscopy, MRC, nd fecl occult lood testing for the detection of colonic neoplsi in symptomtic ptients. MRC detected denoms 6 mm with high sensitivity nd specificity, lthough levels of sensitivity remined lower thn opticl colonoscopy (61,62 ). Figure 16. Sixty-seven-yer-old femle with n ilel polyp found y wireless cpsule video endoscopy, locted 10 min from the ileocecl (IC) vlve. Axil T2-weighted (T2W) ( ) nd coronl T1-weighted (T1W) post-contrst ( ) imges from mgnetic imging enterogrphy (MRE) show low T2 signl intensity (SI) polyp with enhncement locted ~ 15 cm from the IC vlve (rrow). The polyp nd loction were confi rmed with iopsy vi colonoscopic pproch. CT lso remins the recommended modlity of choice for stging colon crcinom; however, ecuse of MRI s superior tissue contrst, stging of known colon crcinom is recommended when contrst-enhnced CT is contrindicted ( 63 ). Tumor extension into the djcent ft, lymph nodes, nd the presence of peritonel disese nd distnt metsttic disese my e evluted using MRI. MRE is the preferred technique for evluting oth smll owel nd colonic involvement y inflmmtory owel disese; however, recent studies hve suggested tht MRC with DWI cn detect inflmmtion relted to inflmmtory owel disese, prticulrly ulcertive colitis (UC), without the need for owel preprtion or orl contrst ( 64 ). UC chnges extend proximlly from the rectum without the skip lesions typiclly seen in Crohn s disese ( Figure 18 ). Mucosl erosions develop in ctive UC with djcent pseudopolyps composed of edemtous, heped mucos. Pseudopolyps, representing res of residul mucos etween res of ulcertion nd denuded mucos, re lso well chrcterized on MRI. Chronic UC will led to shortening of the colon nd loss of norml hustrl folds, creting led pipe ppernce ( Figure 19 ). As it is not trnsmurl process, UC is not ssocited with fistul nd scess formtion. Complictions of UC such s toxic megcolon cn e redily imged. The MRI ppernce of UC is similr to Crohn s disese: thickened owel with norml enhncement. Isolted colonic involvement my e seen in Crohn s disese, nd when limited to the norectl region nd distl colon, my not e The Americn Journl of GASTROENTEROOGY VOUME 109 APRI

9 Mgnetic Resonnce Imging of the Gut 505 U Figure 18. Forty-one-yer-old femle with n 8-yer history of ulcertive colitis. Coronl T1-weighted (T1W) post-contrst imge demonstrtes incresed signl intensity (SI) in the thickened wll of the sigmoid colon nd vid contrst enhncement (ovl). This is consistent with moderte to severe disese. Arrow, engorged vs rect; rrowhed, norml colonic hustr; U, uterus. Figure 17. Sixteen-yer-old mle with right lower qudrnt pin. Axil stedy stte free precession (SSFP) imge ( ) demonstrtes multiple loops of fluid filled owel. Axil diffusion-weighted imging (DWI; = 800) imge ( ) demonstrtes restricted diffusion involving the ppendix. The fl uid in the djcent smll owel drops in signl intensity (SI) mking the ppendix more conspicuous (rrows). distinguishle from UC. MRE my e helpful in clrifying ptient s dignosis y identifying skip lesions tht re chrcteristic for Crohn s disese versus the contiguous disese chrcteristic of UC ( Figure 20 ). Bowel wll thickening is nonspecific finding, nd its distriution, the clinicl setting, nd dditionl rdiologicl findings should e correlted in rriving t dignosis ( Figure 21 ). Rectum The prognosis of rectl cncer depends lrgely on the degree of rectl wll tumor infiltrtion nd regionl lymph node involvement. Both endoscopic US (EUS) nd MRI hve een used for stging the primry cncer, nd lthough some studies hve suggested tht EUS is more ccurte ( 65,66 ), others hve demonstrted comprle results for MRI ( 67 ). Accurcies for T stging rnging from 63 to 97 % hve een reported for EUS nd etween 65 nd 86 % for MRI ( 68,69 ). Unlike EUS, MRI is le to evlute stenosing nd high rectl tumors, nd simultneously imge the entire pelvis to evlute for djcent orgn invsion nd lymphdenopthy ( Figure 22 ). MRI is lso less opertor-dependent. Outside of high volume centers, EUS hs shown decresed ccurcies nd sensitivities, primrily ecuse of opertor vriility ( 68,69 ). EUS is very ccurte for discriminting etween T1 nd T2 tumors, nd Puli et l. (69 ) demonstrted sensitivities nd specificities of 96 % nd 91 %, respectively, for T3 lesions nd of 95 % nd 98 %, respectively, for T4 lesions. Most MRI stging filures occur in differentiting T1 nd T2 lesions ecuse the sumucosl lyer is not visulized on phsed rry MRI nd in differentiting T2 nd T3 tumors ecuse of the sme desmoplstic rection tht is prolemtic for EUS. High resolution (3 mm) T2-weighted sequences cn differentite the lyers of the rectl wll llowing for T stging within the TMO stging system ( Figure 22 ). Tumor ppers slightly hyperintense reltive to muscle on T2 imging nd hypointense reltive to the sumucosl nd mesorectl ft. IV contrst is not necessry s it hs not een shown to increse stging ccurcy ( 70 ), nd lthough the use of endoluminl contrst is controversil queous gel my help define the endoluminl component of the tumor. An endoluminl coil is not necessry for imging rectl cncer; lthough it my improve resolution, incresed ptient discomfort limits its use. As with EUS, high rectl tumors nd lesions cusing stenosis re indequtely evluted y endoluminl MRI. Stge T2 tumors invde ut do not penetrte the musculris propri, wheres T3 tumors invde through the musculris propri nd into the suseros ( Figure 22 ). However, differentiting T2 nd T3 tumors my e difficult. T2 lesions my e overstged ecuse of the difficulty in distinguishing whether n irregulr outer rectl wll order represents inflmmtion from desmoplstic rection or tumor nd firosis together ( 70 ). Extrmurl vsculr invsion, predictor of locl nd distnt recurrence, cn e identified on MRI s intermedite SI mteril expnding the vessel ( 70 ). This represents t lest T3. Stge 4 tumors invde djcent orgns nd structures, or hve perforted the peritoneum. Neodjuvnt chemordition therpy is often used in cses of extrmurl spred in T3 T4 tumors. By downstging nd downsizing loclly dvnced cncers, pre-opertive chemordition idelly improves resectility, decreses locl recurrence, preserves sphincter function in low tumors, nd improves survivl ( 71 ). Restging MRI cn demonstrte reduction in tumor volume, decrese in size nd numer of lymph nodes ( 71 ), s well s help decide surgicl pproch. If the residul tumor is confined to the 2014 y the Americn College of Gstroenterology The Americn Journl of GASTROENTEROOGY

10 506 Romero et l. Figure 19. Twenty-five-yer-old mle with history of colitis. Coronl T2-weighted (T2W) imge demonstrtes smooth ppernce of the trnsverse colon wll due to the loss of hustrl folds. This ptient ws dignosed with ulcertive colitis. Figure 21. Sixty-fi ve-yer-old mle with cirrhosis nd portl hypertension. Coronl T2-weighted (T2W) imges revel diffusely edemtous trnsverse colon (rrows). Although the most common loction for owel edem ssocited with portl hypertension is in the cecum nd scending colon, the stomch (rrowhed) nd smll owel ( ) wlls were lso thickened nd edemtous. Figure 20. Twenty-six-yer-old mle presenting with history of ulcertive colitis. Mgnetic resonnce enterogrphy (MRE) demonstrted symmetric nd noncontiguous res of ctive colonic nd terminl ilium disese with the mgnetic resonnce imging (MRI) ppernce of thum-printing in the trnsverse colon (rrow). Colonoscopy confi rmed discontinuous colon nd terminl ilium findings, nd this ptient s dignosis ws chnged to Crohn s disese. Terminl ilium disese not included in this imge. T rectl wll, trns-nl locl excision cn e performed, with lower moridity nd mortlity when compred with trnsdominl excision ( 72 ). Ptients with more dvnced tumors cn e treted with totl mesorectl excision, n en loc resection of the primry tumor nd mesorectum performed y dissecting long the mesorectl fscil plne. This procedure hs drmticlly decresed the rtes of locl recurrence ( 73 ). The mesorectl fsci ppers s thin hypointense line surrounding the mesorectl ft nd represents the circumferentil resection mrgin when totl mesorectl excision surgicl pproch is used. A distnce 5 mm etween the tumor nd mesorectl fsci predicts n uninvolved circumferentil resection mrgin of t lest 1 mm. The presence of tumor or mlignnt node within 1 mm of the surgicl ed predisposes ptients to locl recurrence ( 70 ). MRI following either surgery or chemordition cn e used to determine tretment response nd evlute for residul or Figure 22. Forty-five-yer-old mle with mss seen on colonoscopy. Wtersolule gel ws inserted into the rectum to ccentute rectl pthology. Sgittl T2-weighted (T2W) imge ( ) shows irregulr wll thickening t the level of the recto-sigmoid junction (rrows). Axil T2W imge ( ) shows tumor nd norml wll. Norml mucos is typiclly low signl intensity (SI) line (lck rrow), sumucos is normlly high SI (white rrow), musclris propri is low SI line (rrowhed). Tumor SI (T) is slightly higher thn musculris propri SI. There is extension of tumor into the mesorectl ft ( ). recurrent disese s prolem-solving modlity when CT findings re equivocl, or when crcinoemryonic ntigen is rising despite negtive CT findings. However, tumor my e difficult to distinguish from ctive firosis following chemordition, leding to oth overstging nd understging ( 73,74 ). Both tumor nd ctive firosis my pper s hypointense tissue invding the The Americn Journl of GASTROENTEROOGY VOUME 109 APRI

11 Mgnetic Resonnce Imging of the Gut 507 mesorectl fsci nd pelvic sidewll. DWI my help in determining tretment response following therpy ( 75 ). Nodl stging my e performed with CT, positron emission tomogrphy-ct, MRI, nd loclly with EUS. Nodl stging is limited, on oth MRI nd CT, s enign nd mlignnt nodes overlp significntly in size, lthough the ddition of signl heterogeneity nd order irregulrity cn increse sensitivity nd specificity ( 76 ). However, it is difficult to differentite n irrdited lymph node from metsttic node following tretment. The use of ultrsmll superprmgnetic iron oxide contrst gents my help distinguish enign nd mlignnt lymph nodes ( 77 ). Mcrophges in norml nodes will tke up ultrsmll superprmgnetic iron oxide gents nd pper low in signl on T2- str imges. Nodes involved with metstses will not tke up the gents nd revel persistent res of high SI on T2 str. MRI restging fter chemordition therpy could identify ptients without nodl disese, who could e cndidtes for locl excision. However, work in this re remins preliminry, nd lthough specificity my e incresed sensitivity is similr to tht when using morphologic fetures ( 78 ). The ddition of DWI sequences to conventionl MRI cn increse the sensitivity of detecting lymph node metstses over tht of CT ( 78 ). DWI my lso help identify complete responders following chemordition nd llow for more conservtive mngement of these ptients ( ). In some cses, iopsy is required to provide the dignosis. ing ll trcts cn help prevent simple fistul from ecoming complex nd void post-opertive fistul recurrence ( 86 ). The mjor nl structures cn e identified on CT, ut MRI hs fr superior contrst resolution nd etter depicts the ntomy ( 87 ). Endonl US hs high resolution ut smller field of view tht my limit evlution of trcts extending into the perinl spce nd eyond the levtor ni complex ( 88 ). Studies compring the ccurcy of endonl US with MRI hve yielded wide vriety of results ( 89,90 ). The modlities re complimentry nd choice of imging modlity should tke into ccount the specific ptient s clinicl picture, surgeon preference, modlity vilility, nd expertise nd cost ( 91 ). At our institution we perform imging with phse rry surfce coil; lthough n endorectl coil my mximize resolution, ptient discomfort is significnt in these ptients nd distension ssocited with the coil itself cn distort mesurements ( 87 ). No specil ptient preprtion is required. We perform T1- nd T2- weighted imging without ft sturtion to delinete the muscles nd ft plnes, nd identify the fistul trct. Firotic fistul trcts will e T1 nd T2 hypointense nd demonstrte delyed enhncement on post-contrst imges. Active trcts contining fluid, pus, or grnultion tissue will e T2 hyperintense nd enhnce on T1 post-contrst sequences. T2-weighted imging with ft suppression helps identify res of edem, s well s trcts nd scesses filled with fluid. Pre- nd post-contrst T1-weighted imging with ft suppression revels res of enhncement nd inflmmtion. Ascesses will pper T2 hyperintense, without internl enhncement, nd typiclly with peripherl enhncement. Gs within n scess my pper s susceptiility rtifct. DWI sequences re routinely performed nd cn e prticulrly helpful in ptients tht cnnot receive IV contrst. Imging should e ligned with respect to the nl cnl to depict the nl sphincter complex in surgiclly relevnt plne; olique xil nd coronl nd xil imges should therefore e otined ( 92 ). CONCUSION The sence of ionizing rdition, superior tissue contrst, nd dynmic ilities of MRI re prticulrly well suited for ddressing the clinicl questions commonly encountered in the prctice of the Anus MRI is vlule tool in evlution of norectl disese. Perinl fistuls ( Figure 23 ) re frequently encountered complictions of Crohn s disese nd MRI cn ccurtely depict the fistul course, its reltionship with the sphincter, nd ssocited scesses. MRI hs een shown to provide importnt informtion tht my lter surgicl pproch, prticulrly in ptients with complex nd recurrent inflmmtory nd infectious disese ( 82,83 ). Deep-tissue fistul heling my lg sustntilly ehind more superficil heling, mking MRI vlule tool for monitoring medicl therpy ( 84 ) nd lso potentilly disese ctivity ( 85 ). Inflmmtory ctivity persists on MRI fter clinicl symptoms nd dringe hve resolved ( 86 ). Recent Europen Crohn s nd Colitis Orgniztion guidelines for the mngement of Crohn s disese recommend MRI or EUS for ll fistul cses where tretment is plnned. Correctly identify c Figure 23. Thirty-five-yer-old femle with history of nl fi stul for evlution of residul trct. Coronl T1-weighted (T1W) post-contrst imge demonstrtes n intersphincteric fi stul extending long the right lterl spect of the externl sphincter from the right deep glutel fold (white rrows) ( ). Fluid within the fistul (white rrow), symmetry of the puorectlis muscle (rrowheds), nd reltionship of the fi stul to the muscles re est seen on T2-weighted (T2W) imge ( ). T1W post-contrst imge sttus post plcement of mushroom ctheter (rrows) nd Seton in trns-sphincteric component of the fistul, not defi nitely identifi ed on the erlier study (lck rrows) ( c ) y the Americn College of Gstroenterology The Americn Journl of GASTROENTEROOGY

12 508 Romero et l. luminl gstroenterologist. With creful enteric contrst selection nd ntiperistltic gents, MRI cn serve s n excellent prolemsolving tool, providing comprehensive picture of inflmmtory owel disese nd its complictions, prticulrly in cses of smll owel Crohn s disese. It chieves these ims without exposing young ptients to unnecessry rdition. The strength of MRI for stging rectl cncer nd evluting the extent of norectl fistul involvement hs een confirmed in the literture nd MRI cn e used to optimize surgicl plnning. Recent work with DWI nd ultrsmll ferromgnetic prticles is lso promising in helping to mke the importnt distinction etween enign nd mlignnt disese. As MRI s cpilities evolve, it will develop into key component of the clinicin s dignostic toolkit. CONFICT OF INTEREST Gurntor of the rticle: Suznne. Plmer, MD. Specific uthor contriutions: Mirim Romero: conceiving, inititing, writing, nd editing the mnuscript; finl review nd pprovl of the sumitted mnuscript. Jmes Buxum: writing nd editing the mnuscript, nd finl review nd pprovl of the sumitted mnuscript. Suznne. Plmer: conceiving, inititing, writing, nd editing the mnuscript, finl review nd pprovl of the sumitted mnuscript, nd sumission of the mnuscript. Finncil support: None. Potentil competing interests: None. Study Highlights WHAT IS CURRENT KNOWEDGE 3 Mgnetic resonnce imging (MRI) is pproprite for evluting heptoiliry pthology. 3 MRI lcks ionizing rdition. 3 MRI hs superior tissue contrst nd dynmic imging ilities. WHAT IS NEW HERE 3 MRI is prolem-solving tool for luminl gstroenterologists. 3 MRI is pproprite for evlution of inflmmtory owel disese. 3 MRI is pproprite for stging rectl cncer. 3 MRI is pproprite for evluting norectl fistuls. REFERENCES 1. Hshe mi R H, Br d l e y WG, is nt i C J. M RI : The Bsics, 2nd edn. ippincott Willims & Wilkins: Phildelphi, Arrivee, Coudry C, Azizi et l. Pinepple juice s negtive orl contrst gent in mgnetic resonnce cholngiopncretogrphy. 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