MRI Technical Considerations

Transcription

1 MRI Technicl Considertions Positioning nd Fixtion of the Knee Right knee It is not possile to dignose ACL pthology using sgittl MR imges cquired with fully or overextended knees. It is therefore importnt tht the knee is slightly fl exed within the coil (see Chp. 3 for more detils). Lterl Medil 2.2 Acquisition of Imges in the Sgittl Plne It is essentil tht the rnge of imge cquisition covers the entire femorl condyles nd the tiil plteu from the medil to the lterl edge in the xil imge (Fig. 2.1 ). In typicl dult mle ptient, more thn 25 slices will e required t the slice thickness of 3 mm. In old dys, cquisition of MR imges in digonl slices ws severely limited y foldover rtifcts. In those circumstnces, it ws necessry to externlly rotte the distl lower lim y to visulize ACL, which runs digonlly cross the intercondyloid foss, in sgittl imges. Thnks to improvement in oth hrdwre nd softwre in MR imging, this limittion hs ecome less signi fi cnt. However, one should e creful not to unnecessrily internlly or externlly rotte the knee to prevent distortion of ligmentous structures including crucite nd collterl ligments (Fig. 2.2 ). Fig. 2.1 Setup of sgittl slices (right knee). In this exmple, there re only 17 slices, ut idelly, there should e more thn 25 slices t slice thickness of 3 mm to visulize fi ne chnges in crtilge nd menisci In sgittl cquisition, direction of phse encoding is usully nterior to posterior. In this cse, however, imges will e hindered y rtifcts rising from lood fl ow in the poplitel rtery nd vein (Fig. 2.3 ). This cn e prevented y setting the phse encoding direction to superior to inferior, ut one should ttempt to minimize foldover rtifcts. M. Niitsu et l., Mgnetic Resonnce Imging of the Knee, DOI / _2, Springer-Verlg Berlin Heidelerg

2 12 2 MRI Technicl Considertions Phse encoding direction Medil Lterl Fig. 2.2 Exmple of unsuccessful imge cquisition due to excessive externl rottion of the distl lower lim (right knee). Excessive externl rottion of the distl lower lim leds to the direction of sgittl slice prlleling the lterl wll of intercondyloid foss ( ). In sgittl imges, there will e prtil volume effect from the one cortex, which lso hinders the delinetion of ACL (, rrows ) Fig. 2.3 Blood fl ow rtifcts rising from inpproprite phse encoding direction. In sgittl cquisition, direction of phse encoding is usully nterior to posterior. In this cse, however, imges will e hindered y rtifcts (, rrows ) rising from lood fl ow in the poplitel rtery nd vein (*). This cn e prevented y setting the phse encoding direction to superior-to-inferior ( ) 2.3 T1-Weighted nd Proton Density-Weighted Fst Spin-Echo Sequences Although oth T1- nd T2-weighted imges re utomticlly cquired t mny institutions without much considertion, T1-weighted imges do not hve much vlue in delineting ligmentous nd meniscl lesions. Norml ligments nd menisci show low intensity signls, nd thus proton density- or intermedite-weighted imges will llow etter contrst with the surrounding crtilge nd joint fl uid (Fig. 2.4 ). Fst spin-echo (FSE) sequence requires much shorter cquisition time compre to conventionl spin-echo (SE) sequence nd enles cquisition of imges with higher ntomicl resolution. Thus, FSE is often utilized in knee MRI. However, one needs to e cutious regrding the following issues: 1. Echo trin length ( ETL) should e kept to the minimum to prevent occurrence of lurring of imge.

3 2.3 T1-Weighted nd Proton Density-Weighted Fst Spin-Echo Sequences 13 c Fig. 2.4 Comprison of T1WI nd intermedite-weighted (close to PDW) imges. () nd ( c ) T1WI (SE 350/14). ( ) nd ( d ) intermediteweighted (close to PDW) imges (FSE 1.324/17, ET 5). The ltter d demonstrtes the mrgins of ACL nd crtilge etter thn the T1WI, lso with etter contrst of meniscl ter ( d, rrow )

4 14 2 MRI Technicl Considertions Fig. 2.5 PDWI with DRIVE (), in which joint fl uid is depicted s hyperintensity, nd conventionl PDWI (). In ( ), joint fl uid is depicted s hyperintensity which cretes etter contrst with crtilge nd ACL, llowing these structures to e more clerly delineted (FOV 150 mm, slice thickness 3 mm, slice gp 0.3 mm, 23 slices, 512 scn mtrix, 864 ZIP, scn time 6 m) 2. Ft my e depicted s hyperintensity nd my lower the contrst ginst meniscl lesions. This cn e prevented y ppliction of ft suppression. To cquire proton density-weighted FSE imges, ETL should e kept to the minimum (mx 5 or 6). Using proton density-weighted FSE sequences with wter-highlighted technique (dding 90 pulse t the end of the echo trin to forcefully recover verticl mgnetiztion, such s DRIVE (Philips), FRFSE (GE), RESTORE (Siemens) nd T2 Plus (Toshi)), one cn emphsize the T2-weighted contrst even with reltive short TR. Joint fl uid will e depicted s hyperintensity nd with etter delinetion of crtilge, ligments, nd menisci (Fig. 2.5 ). Cre must e tken not to mistke this s pthologicl fi nding such s ligmentous ter. Mgic ngle effect is prticulrly notle with short TE sequences such s T1-weighted, proton densityweighted, or T2*-weighted (which is sed on grdientreclled echo sequence using low fl ip ngle) sequences (Fig. 2.6 ). Mgic ngle effect cn lso ffect the posterior horn of the lterl meniscus (Fig. 2.7 ). Mgic ngle effect cn e voided y using long TE. Therefore, if mgic ngle effect is seen in T2*-weighted imges, it cn e eliminted y using SE sequences with long TE or T2-weighted FSE sequences (Fig. 2.8 ). 2.4 Mgic Angle Effect The mgic ngle effect is phenomenon tht results in rtifctul hyperintensity in structures with ordered collgen, such s tendons nd ligments. This is ecuse when collgen is oriented t 55 to the min mgnetic fi eld, dipole-dipole interctions ecomes zero, resulting in prolongtion of T2 relxtion time. References Erickson SJ, et l. The mgic ngle effect : ckground nd clinicl relevnce. Rdiology. 1993;188:23 5. Peterfy CT, et l. Mgic-ngle phenomenon: cse of incresed signl in the norml lterl meniscus on short-te MR imges of the knee. AJR. 1994;163:

5 2.4 Mgic Angle Effect Bo Fig. 2.6 Mgic ngle effect ffecting the ptellr tendon. T2*WI (GRE 560/14, fl ip ngle 30 ). Superior spect of the ptellr tendon exhiits loclized hyperintensity ( rrows ). This phenomenon cn e seen when the tendon is oriented t 55 to the min mgnetic fi eld (Bo, superior-inferior direction) Fig. 2.8 TE -dependent nture of the mgic ngle effect. T2*WI (GRE 560/14, fl ip ngle 30 ) ( ) nd T2WI (FSE 3,000/90) ( ). Loclized hyperintensity t the inferior spect of the ptellr tendon seen in (, rrows ) disppers if TE is mde longer ( ) Fig. 2.7 Mgic ngle effect ffecting the posterior horn of the lterl meniscus. Coronl T2*WI. Norml posterior horn of the lterl meniscus exhiits loclized hyperintensity ( rrows ) T2-Weighted nd T2*-Weighted Imges T2- nd T2*-weighted imges re useful sequences for knee imging ecuse it cretes good contrst etween joint fl uid (hyperintensity) nd the lesions of ligments nd menisci. A grdient-reclled echo (T2*-weighted) sequence is prticulrly useful for delineting fi ne lesions. However, ering in mind tht the mgic ngle effect cn cuse n unwnted rtifct, long TE, SE, or FSE sequence should e dded (either sgittl or coronl).

6 16 2 MRI Technicl Considertions Fig. 2.9 TE-dependent vriility of signl strengths in grdientreclled echo sequence (From left, TE = 14, 15, 16, 17, nd 18 ms). TE = 14 nd 18 ms results in in-phse, while out-of-phse imges will e creted t TE = pprox. 16 ms. When out-of-phse, signls from wter nd ft within the sme pixel cncel out nd the signls re lost. For exmple, the oundries etween sucutneous ft nd muscle or lood vessels will pper lck ( oundry effect ( lck nd white rrows )) Tle 2.1 TE (ms) representing in-phse nd out-of-phse t 1.5 T In-phse Out-of-phse Tle 2.2 TE (ms) representing in-phse t 1.0 nd 1.5 T In-phse t 1.0 T In-phse t 0.5 T In-Phse nd Out-of-Phse Imging In grdient-reclled echo sequences, signls from ft nd wter vry depending on TE. Resonnce frequency of wter is higher thn tht of ft y 3.5 ppm. This equtes to out 220 Hz (63.9 MHz 3.5 ppm) in 1.5-T system. Thus, resonnce frequencies of wter nd ft synchronize every 4.5 ms of TE (in-phse 220 Hz = 4.5 ms). When out-of-phse, signls from wter nd ft within the sme pixel cncel out nd the signls re lost. For exmple, the oundries etween sucutneous ft nd muscle or lood vessels will pper lck ( oundry effect ) (Fig. 2.9, Tle 2.1 ). Multiply these vlues y 0.5 t 3.0 T, y 1.5 t 1.0 T, nd y 3 t 0.5 T (Tle 2.2 ) Note: TE = pprox. 14 ms results in in-phse in ny cse 2.6 Usefulness of Axil Imges Axil imges dd useful informtion to sgittl nd coronl imges, which re minly used for ssessment of ligments running in the superior-inferior direction nd menisci. Axil imges re prticulrly suitle for delinetion of the femorl ttchment site of the crucite ligments, for exmple, prtil ter of ACL which cn e dif fi cult to detect in sgittl imges Cross-sectionl res of the hmstrings nd ptellr tendon, which is used for ACL reconstruction, cn e mesured Medil synovil plic nd ptellofemorl crtilge (the thickest crtilge of the knee joint) re clerly visulized in xil imges nd evlution of ptellr suluxtion Fluid collection round menisci, including meniscl cysts, cn lso e clerly visulized Reference Roychowdhury S, et l. Using MR imging to dignose prtil ters of the nterior crucite ligment: vlue of xil imges. AJR. 1997;168: Techniques for Ft Suppression Lesions tht re present within the one mrrow, which comprises mostly ftty mrrow, nd sucutneous ft should e ssessed using ft suppression techniques. Chemicl shift selective (CHESS) method, Chem St method = method which utilizes the difference in resonnce frequency etween ft nd wter (224 Hz t 1.5 T) to dd suppress pulse only to ft signls.

7 2.8 Metllic Artifcts 17 Tle 2.3 Comprison of ft suppression techniques Technique Pros Cons CHESS Prolongtion of scn time is little, nd there is little limittion in the imge cquisition techniques Mgnetic fi eld inhomogeneity my led to filure of ft suppression in n uneven fshion, prticulrly t the periphery of lrge FOV STIR Homogeneous nd lmost perfect ft suppression cn e expected Need for ddition of n IR pulse cuse numer of restrictions in the imge cquisition technique (especilly the need for incresed interslice gp nd prolonged scn time) Selective wter excittion Prolongtion of scn time is miniml Very sensitive to mgnetic fi eld inhomogeneity Short TI (tu) inversion recovery (STIR) = method sed on IR technique which sets the TI (tu) to e the null point for the ft signl. There is new method clled wter selective excittion. This is n ddition of n excittion pulse to the wter, rther thn dding suppression pulse to the ft. Excittion pulses, which re clled inomil pulse (e.g., 1-1, 1-2-1, ), re split, nd the phse difference etween the resonnce frequencies of wter nd ft is utilized (Fig ). Ech technique hs pros nd cons (Tle 2.3 ). 2.8 Metllic Artifcts Inevitly, metllic rtifct will rise if there is ferromgnetic component within the humn ody. Stples used in the ACL reconstruction surgery will distort the imge due to loclized mgnetic fi eld inhomogeneity. In this cse, chrcteristic signl dropout in the direction of frequency encoding nd overlpping of the rtifct in wider rnge in the phse encoding direction (Fig ). Metllic rtifcts re prticulrly notle with the grdient-reclled echo technique, which is sensitive to mgnetic field inhomogeneity. Very smll metllic prticles my e incidentlly discovered t MR imging (Fig ). Metllic rtifcts rising from such smll ojects re loclized to smll re, ut one needs to e creful ecuse it cn cuse urn injury. Fig Metllic rtifct. Sttus post-acl reconstruction. ( ) Surgicl stles re oserved in the femur nd tii in this lterl knee rdiogrph. ( ) On sgittl PDWI, imge distortion due to loclized mgnetic fi eld inhomogeneity ( rrows ) nd signl dropout re seen. The phse encoding direction is superior-inferior, while the frequency encoding direction is nterior-posterior. ( c ) Metllic rtifcts re prticulrly notle with the grdient-reclled echo technique, which is sensitive to mgnetic fi eld inhomogeneity (coronl T2*WI)

8 18 2 MRI Technicl Considertions c phse freq Fig (continued) Fig Metllic rtifct due to very smll metl prticle. () In this T2*WI, there is loclized signl dropout nd imge distortion t the inferior spect of the ptell ( rrows ). () This ws due to very smll metl prticle ( rrow ) which is just visile on the lterl rdiogrph

9 2.10 Imging Techniques for Crtilge 19 Fig Delinetion of crtilge using the MTC method. ( ) T2*WI (GRE, 545/15, fl ip ngle 30 ) nd ( ) T2*WI with dded MTC (sme prmeter s ). In T2*WI, crtilge nd joint fl uid re oth exhiiting hyperintensity. By ddition of MT effect, crtilge signl is specificlly suppressed (rrows ) nd improves the contrst ginst hyperintense joint fl uid ( rrowhed ) 2.9 Mgnetiztion Trnsfer Contrst (MTC) Method, MT Effect MRI mostly visulizes protons of free wter molecules. Other thn free wter, there re wter molecules tht re ound to high-moleculr-weight proteins, nd their resonnce frequency rnges few thousnd Hz. In MTC method, contrst is creted y suppression of signls from free wter y irrditing off-resonnce pulse (i.e., the pulse tht is more thn few thousnd Hz wy from the resonnce frequency of free wter molecules). MTC method improves the contrst on T2-weighted imges etween joint fl uid nd hyline crtilge, which is minly composed of collgen nd proteoglycn, y speci fi clly suppressing signls from crtilge (Fig ). However, y irrditing MT pulses: 1. Het will e generted in the ody s determined y the speci fi c sorption rte (SAR) 2. Scn time will e slightly prolonged FSE techniques tht utilize mny 180 degree pulses lso involve the MT effects 2.10 Imging Techniques for Crtilge The two most commonly used MR sequences for crtilge imging re the following (Tle 2.4 ) (Fig ): Blnced stedy-stte free precession (3D lnced grdient echo) technique offers new method to delinete crtilge nd includes sequences such s TrueFISP (Siemens) nd Blnced FFE (Philips). Use of reltively lrge fl ip ngle leds to depiction of joint fl uid s hyperintensity nd enles cquisition of high-contrst imges. Also, TR cn e shortened nd thus high-qulity crtilge imging within short scn time cn e chieved. Tle 2.4 Comprison of dignostic performnce of T2-weighted FSE nd T1-weighted GRE sequences Sensitivity for crtilge defect detection T2-weighted FSE 94% 99% with MTC Ft-suppressed T1-weighted GRE 75 85% 97% Specificity for crtilge defect detection

10 20 2 MRI Technicl Considertions Fig Delinetion of crtilge using 3D lnced grdient-echo sequence. Blnced FFE (TR/TE = 12/6.0, fl ip ngle 70, selective wter excittion pplied, slice thickness 1.6 mm, FOV 140 mm, mtrix , scn time 4 min 06 s). Joint fl uid ppers hyperintense, creting good contrst ginst super fi cil crtilge dmge ( rrow ) Reference Disler DG, et l. Ft-suppressed three-dimensionl spoiled grdient-echo MR imging of hyline crtilge defects in the knee: comprison with stndrd MR imging nd rthroscopy. AJR. 1996;167: Fig Imging of hyline crtilge. In FSE T2WI with MTC ( ), crtilge ppers hypointense in contrst to hyperintense joint fl uid. In FS GRE T1WI ( ), crtilge ppers hyperintense. ( ) FSE T2WI with MTC (TR/TE = 38/14, fl ip ngle 30, off-resonnce MTC, scn time 4 min 32 s). ( ) FS GRE T1WI (TR/TE = 32/6.8, fl ip ngle 25, ft suppression, scn time 5 min 03 s). Both were 1.5 mm slice thickness, 130 mm FOV, nd 256 x 512 mtrix

11 2.10 Imging Techniques for Crtilge 21 c Fig D FS GRE T2*WI of the knee. () sgittl (TR/ TE = 19/ ), ( ) coronl reconstruction imge, nd ( c ) xil reconstruction imge. ( ) nd (c) were reconstructed from the sgittl imge ( slices, depending on the size of the knee) to enle evlution of crtilge, menisci, synovium, nd intr-rticulr free odies

12 22 2 MRI Technicl Considertions C B III II I c Fig T MRI of the knee. Coronl imge ( ) nd the mgni fi ed imges ( nd c ). () FSE 2,025/20, 3.0/0.3, FOV 160, mtrix ( ) Mgni fi ed imge of the medil comprtment shows three lyers of the medil collterl ligment (I, II, III, rrows ). The qulity of imge is equivlent to tht offered y high-resolution imges cquired using microscopy coil (see Chp. 5, Fig. 5.3 ). (c ) Mgnified imge of the lterl comprtment clerly shows focl crtilge defect of the lterl femorl condyle ( rrow ) nd distortion of lterl meniscl free edge ( rrow hed )

13 2.10 Imging Techniques for Crtilge 23 Advntges nd Disdvntge of 3 T MRI Improved signl-to-noise rtio: High-resolution imge: thin slice thickness, smll FOV, 1024 mtrix size Shorter scn time, incresed cquisition series Prolongtion of T1 vlues nd slight shortening of T2 vlues (ut this is not signi fi cnt disdvntge for knee imging) Chemicl shift ecomes more notle (nd thus djustment of ndwidth nd the use of ft suppression is required) Lower RF in fi ltrtion, signl inhomogeneity (multichnnel, prllel imging my e needed) Incresed mgnetiztion effect (incresed metlrelted rtifct, ppliction to susceptiility imging) Incresed SAR (ewre of excessive het production) Reference Rmnth RR, et l. Accurcy of 3-T MRI using fst spin-echo technique to detect meniscl ters of the knee. AJR. 2006;187: Imge Acquisition Protocol for the Most of Imges Used in This Book 1.5 T Slice thickness mm, slice gp mm Sgittl imges: 23 slices, coronl nd xil imges: 18 slices FOV mm, mtrix or Intermedite-weighted (close to proton densityweighted, nd thus in this ook, it will e clled proton density-weighted ) FSE: 1,300 2,500/13 17, ET 4 6 (+DRIVE if pproprite). Ft-suppressed proton density-weighted imges: ft-suppression (e.g., wter excittion pulse) is dded to the ove sequence. T2*-weighted imge: GRE /14 15, fl ip ngle T2-weighted imges: FSE 2,500 3,500/90 100, ET T1-weighted imges (tumors nd one mrrow pthologies, only with contrst-enhnced imging): SE / T 2D imging Slice thickness mm, slice gp mm Sgittl imges: slices, coronl nd xil imges: 26 slices FOV 150 mm, mtrix or Proton density-weighted FSE: 2,400 2,800/17 30, ET 4 7 (+DRIVE if pproprite). Ft-suppressed proton density-weighted imges: ft suppression (e.g., wter excittion pulse) is dded to the ove sequence. T2-weighted nd T1-weighted imges: lmost identicl to 1.5 T imging. 3D imging Slice thickness 0.6 mm/ 0.3 mm (overlpping) Sgittl imges: 280 slices, coronl nd xil imges: reconstructed from the sgittl imges, FOV 150 mm, mtrix ( mm isovoxel) Ft-suppressed T2*-weighted 3D GRE: 19/ (ddition of fi rst echo nd second echo), ft suppression (e.g., wter excittion pulse) Arevitions: SE spin echo, FSE fst spin echo, GRE grdient echo, ET echo trin length

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