- Email: [email protected]
Fast field mapping by NMR interferometry
Abstracts0 Chairman: MARK HAACKE
559
We desired a sequence giving.the stronger signal of a gradient echo (due to preserving a relatively large longitudinal magnetization at short TR) plus the spatial selectivity of a 180' refocussing pulse (to suppress signal from outside the slice). This was accomplished by simply changing the 9Oo pulse in a standard spin echo sequence to produce larger tip angles. This large angle spin echo (LASE) sequence can produce a larger signal than spin echo because the large tip angle reverses the longitudinal magnetization prior to the 180' pulse reversing it again. At the limit of TE=O, a LASE sequence at a tip angle of A0 'gives exactly the same signal as a FLASH or FISP at a tip angle bf 180°- A'. At nonzero TE, LASE gives less signal than FLASH or FISP due to Tl recovery in the TE/2 between the tip and 180' pulses, but still more signal than spin echo. Finally, at longer TR, LASE gives essentially the same signal as FLASH or FISP, with all giving less signal than standard spin echo. Thus at all TR/TE combinations, LASE behaves in between standard spin echo and its corresponding gradient echo sequence. LASE tends toward FLASH behavior if a spoiler pulse is used to zero the signal by the end of each TR, and tends toward FISP behavior if steady state precession is set up instead. The accompanying figures compare FLASH, FISP, LASE, and spin echo. Our clinical use of LASE has been in suppressing artifacts from respiration and blood flow by heavy averaging (16-128 averages per line). This requires short TR to implement clinically, and LASE has performed as expected as an alternative to FLASH and FISP. LASE does have one significant drawback: it produces more RF absorption and heating than standard spin echo. While not a problem at 0.5 T and less, it could be a significant issue at 1.0 T and higher, eliminating LASE as a short TR sequence on these in which gradient echo sequences systems. Otherwise, LASE can be a helpful sequence for those instances fail to perform properly and standard spin echo sequences take too much time or give too little signal or contrast.
FAST
FIELD
Darrasse
MAPPING
L., Mao
BY NMR
INTERFEROMETRY
L., Saint-Jalmes
H.
Institut d'Electronique Fondamentale - LA 22 associ6 Paris XI - Bbt. 220 91405 Orsay Cedex FRANCE
au CNRS - Universitb
Mapping the field Bo permits a systematic approach for the shimming of MRI Sophisticated methods < 1,2 > using the phase of MR images map B. in a magnets. few minutes. However they have to compensate for other inherent phase deviations in the MRI machine. We propose a new method which achieves in a few seconds a precise contour map of the BC field. It uses RF interference of excitation with free precession so that the steady magnetization is directly amplitude-modulated by the Be contours. Our method is insensitive to the phase deviations. Implementation is straightforward using a standard imaging algorithm. Moreover using the
Fiaure The
1
:
Refocusing-Flash
Fiqure -___ Sequence
2
:
Transverse magnetization versus yGBo.TR
Fiaure amplitude
3
:
10 5 interferometry
maps
MagneticResonanceImaging0 Volume 5,Number 6. 1987
560
mode the previous phase of our contour map we provide with a fast acquisition field-mapping methods. a recent fast imaging sequence (Figure 1) we have called Our method uses the Refocusing-Flash < 3,4 >. It compensates for encoding gradient effect over So we get a SSFP magnetization < 5 > (TR << TI, Tz) whole repetition period TR. which is a periodic function of the field offset 6Bo. Dark contours, i.e. nearzero transverse amplitudes (Figure 2) correspond to the values : where k is an integer and y the gyromagnetic ratio. Y 5Bo .Ta = (2k + 1) n grey contours, i.e. slightly under-maximum amplitudes, For small flip angles, correspond to the intermediate values of 6Bo. We have implemented the interferometry method on our 0.1 Tesla machine obtained in 10 s < 6 >. The figure 3 shows 35 x 35 cm maps on 128 x 128 matrix, Dark-contour spacing is 1.2 UT, corresponding to a 20 ms TR. The alternaeach. tion of dark and grey contours is used to assess the B0 monotony. While our method is very convenient for fast quality control of Bo, other chemical shift separation, interesting applications are susceptibility mapping, visualisation of eddy current effects. and 1. A.A. Maudsley, H.E. Simon, S.K. Hilal J. Phys. E17, 216 (1984). SMRM III New-York (1984). 2. P. Margosian, J. Abart Electromedica 1, 15-18 (1986). 3. A. Oppelt, R. Grauman et al. Montreal SMRM 5th annual meeting, 4. L. Darrasse, L. Mao, H. Saint-Jalmes (1986) Phys. rev. 112, 1693 (1958). 5. H.Y. Carr 6. L. Darrasse, H. Saint-Jalmes, M. Sauzade Magn. Res. Imaging u, 178 (1985).
FAST IMAGING SEQUENCE IN THE SPINE Carolyn W. VanDyke, Jeffrey S. Ross, Thomas J. Masaryk and Michael T. Medic University Hospitals/Case
Western Reserve University,
Department of Radiology, Cleveland, Ohio
A prospective comparison of fast gradient echo steady-state sequences (FLASH) with Tl weighted (TlW) SE sequences was made to determine the accuracy and contrast behavior of these sequences in the evaluation of extradural cervical spine disease. Fifty-one consecutive patients were studied with 4 mm sagittal and axial TlW and loo FLASH (200 msec TR/13 msec TE) sequences. Fifty-five consecutive patients were studied with TIW and 60' FLASH sequences. The sequences were independently evaluated for presence, location, and type of disease, and then these results compared to determine which sequence provided the greatest conspicuity of lesions. FLASH 10' provided the best conspicuity of extradural disease and required l/2 of the study time when compared to TlW SE sequences. Although the FLASH 10" images provided the lowest overall signal intensity, excellent gray scale inversion of the CSF-extradural interface and preservation of the CSF-neural interface was found. FLASH 60" sequences were better than TIW SE but worse than FLASH 10" for conspicuity of disease. TlW SE images provided better evaluation of vertebral body marrow changes and were less affected by metal artifact. These results indicate that a reasonable imaging sequence for cervical extradural disease combining speed and sensitivity would be a sagittal TlW SE sequence and sagittal and axial FLASH 10" sequence.
44106
559
We desired a sequence giving.the stronger signal of a gradient echo (due to preserving a relatively large longitudinal magnetization at short TR) plus the spatial selectivity of a 180' refocussing pulse (to suppress signal from outside the slice). This was accomplished by simply changing the 9Oo pulse in a standard spin echo sequence to produce larger tip angles. This large angle spin echo (LASE) sequence can produce a larger signal than spin echo because the large tip angle reverses the longitudinal magnetization prior to the 180' pulse reversing it again. At the limit of TE=O, a LASE sequence at a tip angle of A0 'gives exactly the same signal as a FLASH or FISP at a tip angle bf 180°- A'. At nonzero TE, LASE gives less signal than FLASH or FISP due to Tl recovery in the TE/2 between the tip and 180' pulses, but still more signal than spin echo. Finally, at longer TR, LASE gives essentially the same signal as FLASH or FISP, with all giving less signal than standard spin echo. Thus at all TR/TE combinations, LASE behaves in between standard spin echo and its corresponding gradient echo sequence. LASE tends toward FLASH behavior if a spoiler pulse is used to zero the signal by the end of each TR, and tends toward FISP behavior if steady state precession is set up instead. The accompanying figures compare FLASH, FISP, LASE, and spin echo. Our clinical use of LASE has been in suppressing artifacts from respiration and blood flow by heavy averaging (16-128 averages per line). This requires short TR to implement clinically, and LASE has performed as expected as an alternative to FLASH and FISP. LASE does have one significant drawback: it produces more RF absorption and heating than standard spin echo. While not a problem at 0.5 T and less, it could be a significant issue at 1.0 T and higher, eliminating LASE as a short TR sequence on these in which gradient echo sequences systems. Otherwise, LASE can be a helpful sequence for those instances fail to perform properly and standard spin echo sequences take too much time or give too little signal or contrast.
FAST
FIELD
Darrasse
MAPPING
L., Mao
BY NMR
INTERFEROMETRY
L., Saint-Jalmes
H.
Institut d'Electronique Fondamentale - LA 22 associ6 Paris XI - Bbt. 220 91405 Orsay Cedex FRANCE
au CNRS - Universitb
Mapping the field Bo permits a systematic approach for the shimming of MRI Sophisticated methods < 1,2 > using the phase of MR images map B. in a magnets. few minutes. However they have to compensate for other inherent phase deviations in the MRI machine. We propose a new method which achieves in a few seconds a precise contour map of the BC field. It uses RF interference of excitation with free precession so that the steady magnetization is directly amplitude-modulated by the Be contours. Our method is insensitive to the phase deviations. Implementation is straightforward using a standard imaging algorithm. Moreover using the
Fiaure The
1
:
Refocusing-Flash
Fiqure -___ Sequence
2
:
Transverse magnetization versus yGBo.TR
Fiaure amplitude
3
:
10 5 interferometry
maps
MagneticResonanceImaging0 Volume 5,Number 6. 1987
560
mode the previous phase of our contour map we provide with a fast acquisition field-mapping methods. a recent fast imaging sequence (Figure 1) we have called Our method uses the Refocusing-Flash < 3,4 >. It compensates for encoding gradient effect over So we get a SSFP magnetization < 5 > (TR << TI, Tz) whole repetition period TR. which is a periodic function of the field offset 6Bo. Dark contours, i.e. nearzero transverse amplitudes (Figure 2) correspond to the values : where k is an integer and y the gyromagnetic ratio. Y 5Bo .Ta = (2k + 1) n grey contours, i.e. slightly under-maximum amplitudes, For small flip angles, correspond to the intermediate values of 6Bo. We have implemented the interferometry method on our 0.1 Tesla machine obtained in 10 s < 6 >. The figure 3 shows 35 x 35 cm maps on 128 x 128 matrix, Dark-contour spacing is 1.2 UT, corresponding to a 20 ms TR. The alternaeach. tion of dark and grey contours is used to assess the B0 monotony. While our method is very convenient for fast quality control of Bo, other chemical shift separation, interesting applications are susceptibility mapping, visualisation of eddy current effects. and 1. A.A. Maudsley, H.E. Simon, S.K. Hilal J. Phys. E17, 216 (1984). SMRM III New-York (1984). 2. P. Margosian, J. Abart Electromedica 1, 15-18 (1986). 3. A. Oppelt, R. Grauman et al. Montreal SMRM 5th annual meeting, 4. L. Darrasse, L. Mao, H. Saint-Jalmes (1986) Phys. rev. 112, 1693 (1958). 5. H.Y. Carr 6. L. Darrasse, H. Saint-Jalmes, M. Sauzade Magn. Res. Imaging u, 178 (1985).
FAST IMAGING SEQUENCE IN THE SPINE Carolyn W. VanDyke, Jeffrey S. Ross, Thomas J. Masaryk and Michael T. Medic University Hospitals/Case
Western Reserve University,
Department of Radiology, Cleveland, Ohio
A prospective comparison of fast gradient echo steady-state sequences (FLASH) with Tl weighted (TlW) SE sequences was made to determine the accuracy and contrast behavior of these sequences in the evaluation of extradural cervical spine disease. Fifty-one consecutive patients were studied with 4 mm sagittal and axial TlW and loo FLASH (200 msec TR/13 msec TE) sequences. Fifty-five consecutive patients were studied with TIW and 60' FLASH sequences. The sequences were independently evaluated for presence, location, and type of disease, and then these results compared to determine which sequence provided the greatest conspicuity of lesions. FLASH 10' provided the best conspicuity of extradural disease and required l/2 of the study time when compared to TlW SE sequences. Although the FLASH 10" images provided the lowest overall signal intensity, excellent gray scale inversion of the CSF-extradural interface and preservation of the CSF-neural interface was found. FLASH 60" sequences were better than TIW SE but worse than FLASH 10" for conspicuity of disease. TlW SE images provided better evaluation of vertebral body marrow changes and were less affected by metal artifact. These results indicate that a reasonable imaging sequence for cervical extradural disease combining speed and sensitivity would be a sagittal TlW SE sequence and sagittal and axial FLASH 10" sequence.
44106