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Ultrafast Diffusion-Sensitive MR Imaging of Brain on an Open Scanner at 0.2 TGo

Fritz SchickGo,a, Thomas Nägelea, Dirk Wildgrubera, Jochen Trübenbacha and Claus D. Claussena

a From the Departments of Diagnostic Radiology (F.S., J.T., C.D.C.), Neuroradiology (T.N.), and Neurology (D.W.), University of Tübingen, Germany.



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  		FIG 1. Single-shot diffusion-weighted imaging with the SPLICE acquisition technique.

A, Sequence of RF pulses and magnetic field gradients. Diffusion weighting is achieved by a spin-echo preparation with strong gradients applied before and after the 180° refocusing pulse. The modified fast spin-echo train with two families of signals (E1 and E2) is generated by repetition of the elements inside the box. Two complete magnitude images from echo trains E1 and E2, respectively, should be added to obtain the final image with higher SNR.

B, Phase-encoding steps for half-Fourier reconstruction from the signals in echo train E1 and E2, respectively. Sixty-four lines in the final image are considered in the given example.

C, Phase encoding with centric reordering of the steps for an acquisition of a complete raw data matrix with 64 steps. Data recording starts after six cycles in the fast spin-echo train. This delay allows the spin system to reach an equilibrium state with similar signal intensities in both echo families E1 and E2.



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  		FIG 2. Examples of different types of SPLICE images from the head of a 36-year-old healthy volunteer. Each image, with a 12-mm thickness, was recorded in less than 1 s. The FOV was 210 x 420 mm with a matrix of 96 x 256. A and C were recorded without diffusion gradients, whereas diffusion sensitizing with b = 1000 s/mm2 (maximum amplitude 13.8 mT/m, from 8.0 mT/m in all three orientations simultaneously) was applied for the images in B and D. The timing scheme in the spin-echo preparation remained unchanged for all images. The images recorded with half-Fourier acquisition (A, B) show less blurring artifacts, whereas the SNR is superior in images recorded with centric reordering (C, D). The displayed images are magnified and do not show the entire matrix in read direction.

A, Half-Fourier recording as shown in figure 1B; 55 phase-encoding steps; TE = 155; no diffusion preparation. SNR of brain is 5.5.

B, Half-Fourier technique, with diffusion preparation. SNR of brain is 2.0.

C, Centric reordering as shown in figure 1C; 96 phase-encoding steps; TE = 148; no diffusion preparation. SNR of brain is 7.3.

D, Centric reordering, with diffusion preparation. SNR of brain is 2.3.



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  		FIG 3. Averaging of several single-shot images allows improved SNR and/or smaller FOV. Averaged transverse sections with a thickness of 10 mm and an FOV of 175 x 350 mm (matrix 96 x 256) from eight single-shot scans in a 28-year-old volunteer. The entire measuring time for one set of images was about 45 s (using a TR of 6 s). Centric reordering and recording of the entire matrix as in figure 1C were applied.

A, TE = 112; no diffusion sensitivity. SNR of brain is 22.

B, TE = 112; diffusion preparation with a b value of 250 s/mm2. SNR of brain is 15.

C, TE = 128; no diffusion sensitivity.

D, TE = 128; diffusion preparation with a b value of 500 s/mm2.

E, TE = 148; no diffusion sensitivity. SNR of brain is 15.

F, TE = 148; diffusion preparation with a b value of 1000 s/mm2. SNR of brain is 3.3.



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  		FIG 4. Effect of the addition of two separate magnitude images from echoes E1 and E2 in the modified fast spin-echo train in figure 1A. Coronal single-shot images from a 74-year-old patient with a large ischemic lesion (arrow, F) were recorded using the half-Fourier approach. Section thickness, 12 mm; FOV, 175 x 350 mm; matrix, 96 x 256.

A–C, TE = 155; no diffusion sensitivity.

D–F, TE = 155; diffusion preparation with a b value of 1000 s/mm2.

A and D are images from echoes E1; B and E are images from echoes E2; C and F are the addition of images from E1 and E2. Signal intensity is doubled, whereas noise increases by a factor of . CNR of the lesion versus normal brain is 11 in F.



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  		FIG 5. Transverse images from one section of the brain of a 74-year-old patient with occlusion of the middle cerebral artery, 10 days after onset of stroke symptoms.

A, Standard T1-weighted fast spin-echo image (400/15/2 [TR/TE/excitations]).

B, Standard T2-weighted fast spin-echo image (4000/117/1).

C, Single-scan half-Fourier image. Section thickness, 12 mm; FOV, 250 x 500 mm; matrix, 96 x 256; TE = 155; no diffusion sensitivity.

D, Single-scan half-Fourier image. Section thickness, 12 mm; FOV, 250 x 500 mm; matrix, 96 x 256; TE = 155; diffusion preparation with a b value of 1000 s/mm2. CNR of the lesion (arrows) versus normal brain is 12.

E, Ten single-scan images from a set of 20 images were averaged. Half-Fourier approach, section thickness, 10 mm; FOV, 175 x 350 mm; matrix, 96 x 256; TE = 155; no diffusion sensitivity.

F, Ten single-scan images from a set of 20 images were averaged. Half-Fourier approach, section thickness, 12 mm; FOV, 250 x 500 mm; matrix, 96 x 256; TE = 155; diffusion preparation with a b value of 1000 s/mm2. CNR of the lesion versus normal brain is 13.



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  		FIG 6. Clinical diffusion imaging in a 58-year-old patient with ischemic brain insults of different ages.

A, C, E, Anatomic details are well depicted on T2-weighted standard fast spin-echo images (4000/106/1).

B, D, F, Images show averaged signals from six single-shot scans with half-Fourier reconstruction. Diffusion-weighting (b = 1000 s/mm2, TE = 155) allows distinction between the relatively recent lesion (6 days old, with markedly restricted diffusion (CNR = 10; arrow, B), the older lesion (4 weeks old) with slight signal enhancement (CNR = 4; arrow, D), and the 1-year-old lesion without any visible diffusion abnormalities. FOV, 175 x 350 mm; matrix, 96 x 256; section thickness, 10 mm.





Copyright © 2008 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X