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`PROCEEDINGS- SOC1ETY OF MAGNETIC RESONAN
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`15.715000 EDIT 3
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`AC~
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`VOL 1
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`1/1
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`General Electric Co. 1025 - Page 1
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`

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`635
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`Single Shot 30 RARE: A Fast Method for Volumetric Acquisition
`
`J.Hennig(*), M.Buchert(*), J.Laubenberger (*), H. Rumpel(#), E.Martin(#)
`(*) = Radiologische Klinik, Freiburg (#) = Kinderspital, Zurich
`
`Introduction
`Single shot RARE is an extremely useful method for
`imaging of liquid filled structures as in MR-myelography
`(I), MR-urography (2) or MR-cholangiography (3). Three(cid:173)
`dimensional acquisition using repetitive excitations has
`been demonstrated (4). The purpose of this paper is to
`present a single-shot 3D-RARE experiment, which allows
`the acquisition of a
`three-dimensional dataset by
`appropriate phase encoding of echoes following a single
`excitation pul se.
`
`Materials and methods
`The 'sequence was implemented on a 1.5 T whole body
`system (Magnetom Vision) with 25 mT/m gradients, a 2T
`whole body system (Bruker S 200 F) equipped with a
`standard gradient set (1 0 mT/m, 1 ms risetime) as well as
`with a gradient head coil (max. 30 mT/m, 125 !lS
`risetime ). Single shot RARE-experiments were performed
`using an echo spacing te between 7 and 10 ms. 256 or 512
`echoes were acquired in order to generate a 128(256) x
`three-dimensional Fourier
`16(32) x 16 dataset . for
`transformation. Two approaches have been pursued: In
`one set of experiments we have used no slice selection in
`order to acquire data from larger organ systems (renal
`pelvis, bladder, bile ducts) with low spatial resolution (I x
`8 x 8 mm 3
`), which is, however, sufficient for volumetric
`analysis. In a second set of experiments we have used
`pre-selection of a column using a 90- and 180-pulse with
`ensuing single-shot 3D-acquisition for isotropic resolution
`of 1 mm within a small selected volume.
`The simplest approach for phase encoding used linear
`phase encoding for the 2D- and centric phase encoding for
`the 3D-phase encoding gradient according to Fig. 1.
`
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`2d-phase encoding
`
`This k-space trajectory has the disadvantage, that the
`timing between the sampling of consecutive points in the
`2d- and 3d-phase encoding direction is different: Whereas
`datapoints along the 2d-gradient are sampled every te,
`sampling in the third direction occurs every te*n2d, where
`n2d is the ·number of points in the 2d-direction. This leads
`
`to stronger attenuation of the signal intensity along the 3rd
`dimension and thus to anisotropically reduced
`image
`sharpness. The signal to noise as well as the sharpness can
`be improved with a more isotropic sampling of data
`following a square spiral as shown in Fig.2a,b
`
`I
`
`a
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`0> c -g
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`2d-phase encoding
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`00 00 000 00 0000 000 li
`oo o oo o o oooooo o oo -6
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`2d-phase encoding
`
`Fig.2a
`
`Fig.2b
`
`The trajectory in Fig.2b (only the beginning of which is
`shown) s~mples complex conjugates consecutively
`in
`order to minimize dispersion artifacts.
`
`Results
`Even the basic trajectory (Fig. I) delivers
`images of
`satisfying quality especially for imaging of liquids with
`extremely long T2 (CSF, urine). Preferred areas of clinical
`applications with
`lmm
`isotropic
`resolution
`are
`3D-imaging of the spine, the eyes and the biliary tracts.
`Further areas of applications with lower spatial resolution
`are functional studies of filling and emptying. of the gall
`bladder, urinary system and the stomach.
`For longer echotrains with 256 or 512 echoes the flip
`angle of the refocusing pulse has
`to be decreased
`in order
`considerably below 180 degrees
`to avoid
`SAR-problems. Even with flip angles of 30-60 degrees
`good image quality could be obtained. The complex
`refocusing behavior at low flip angles (5) does, however,
`require extremely stable gradient and RF hardware.
`
`Conclusions
`Single shout 3D-RARE is a stable and robust technique
`for fast 3D-imaging in clinical routine.
`
`J.Hennig, H.Friedburg, B.Stroebel J.Comp.Assist.
`(1)
`Tomogr. 10(3),375 (1986)
`(2) G.Sigmund, B.StOver, L.B.Zimmerhackl eta!. Pediatr.
`Radio!. 21, 416-420 (1991)
`(3) J.Laubenberger, M.Biichert, B.Schneider et a!. Magn.
`Reson. Med.33: 18:-23 (1995)
`( 4) J.Hennig, H.Friedburg, D.Ott. Magn.Reson.Med.
`5:380-383 (1987)
`(5) J.Hennig. J.Mag.Res.; 78: 397-407 (1988)
`
`General Electric Co. 1025 - Page 2