MAGNETIC RESONANCE IN MEDICINE 15,497-500 ( 1990)

Multislice Snapshot FLASH Using SIMUSIM

S. MULLER

MR Center of the University, Klingelbergstrusse 50, CH-4031 Basel, Switzerland

Received May 30, 1990; revised July 3, 1990

A combination of the SIMUSIM technique and snapshot ITASH which allows efficient multislice snapshot FLASH imaging is described. Simultaneously acquired images of de- liberate slice position and thickness are obtained with an increased SNR compared to standard time sequential multislice experiments. Experimental results show four coronal cross sections through the human head measured in four acquisitions. o 1990 Academic

Press, Inc.

INTRODUCTION

SIMUSIM (simultaneous multislice imaging) is a recently proposed multislice im- aging method ( I , 2). In contrast to conventional time-interleaved multislice imaging all slices of interest within the object are measured simultaneously. This has two main advantages: (i) The number of slices available for multislice imaging is not limited by the repetition time TR of the imaging sequence. Therefore ultrafast imaging tech- niques like EPI ( 3 ) or snapshot FLASH ( 4 ) obtain efficient multislice capability. (ii) If the acquisition is triggered by a physiologic parameter (ECG, etc.) all slices of interest can be measured at the same point in time. Thus, e.g., in cardiac imaging morphologic cardiac parameters are available from the whole heart at deliberate points in time (2).

SIMUSIM is based on SIMUFREX (simultaneous multifiequency excitation) rf pulses which excite all slices of interest simultaneously. These SIMUFREX pulses replace the single frequency selective pulses in conventional imaging sequences. In order to distinguish the MR signals from each slice, a Hadamard encoding scheme is applied in successive experiments for each spatial frequency of the rf pulse. This gen- erates an individual phase alteration scheme for each slice position and allows a com- plete ordering of the MR signals according to their spatial origin. As a result the whole set of images is obtained with a similar selectivity as single slice imaging but with a higher sensitivity per measurement time ( 1 ) .

Snapshot FLASH is a high-speed imaging technique with the capability of measuring a complete MR image during a single FID ( 4 ) . As in all ultrafast imaging techniques no time gaps in the sequence are available for the time-interleaved acquisition of multiple slices. Therefore snapshot FLASH is basically restricted to single slice imaging (sometimes in combination with 3D phase encoding). Multiple slices, however, can be imaged by combining SIMUSIM with snapshot FLASH: with this modification all slices of interest are acquired simultaneously and full signal collection efficiency is retained.

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Simultaneous excitation of four slices

X a

Evaluated slice profiles

rh

x l

First slice

Second slice .EEEi&z===-

x l Third slice

Fourth slice X I e

FIG. 1. Slice profiles through phantom containing doped water acquired with SIMUSIM snapshot FLASH. In ( a ) the absolute value profiles after simultaneous multifrequency excitation are shown for varying slice distance (slice thickness, 1 cm; one acquisition per slice distance). Such data are acquired in all SIMUSIM experiments. In (b)-(e) the slice profiles after Hadamard decoding are shown. It can be noted that the signal intensity for each slice is increased by about a factor of 4 compared to the data in (a) .

The implementation of SIMUSIM into the snapshot FLASH experiment is straight- forward: the only changes concern the rf pulses which must be replaced by SIMUFREX pulses. The encoding and decoding as described in (1, 2) can easily be integrated into standard MR imaging software and the rf deposition is no point of concern with the applied range of flip angles. Our results were all obtained with a commercial SIEMENS Magnetom 63 / 84 using LP headcoils.

RESULTS

In Fig. 1 a phantom experiment which demonstrates the selectivity and the gain in sensitivity with SIMUSIM snapshot FLASH is shown. In (a) four slices were excited and the profiles recorded with the read-out gradient parallel to the slice section gradient. The slice distance was varied in successive experiments in the stacked plot. Such data are always obtained as a first step in SIMUSIM experiments. In Figs. I b- le the eval- uated single slice profiles are shown after the Hadamard decoding was applied to the raw data. Note the clean delineation of each slice. Note in addition the gain in signal intensity by about a factor of 4 when comparing (a) with (b)-( e). This translates into a gain in SNR of fi for each slice.

In Fig. 2 coronal SIMUSIM snapshot FLASH images through the head of a normal volunteer are shown. For T1 contrast enhancement a nonselective inversion pulse was applied 300 ms before the start of the image acquisition. The whole set of four images

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FIG. 2. Coronal cross sections through the head of a normal volunteer acquired with SIMUSIM snapshot FLASH. Image parameters, TI = 300 ms, TR = 10 ms, TE = 4.9 ms; acquisition matrix, 128 X 256,4 X 1 acquisition; slice thickness, 10 mm; slice gap, 10 mm. Total measurement time, 6 s.

was acquired during four FIDs. Again this corresponds to a fi improvement in SNR for each slice compared to a conventional “slice to slice” multislice experiment.

DISCUSSION

Snapshot FLASH is a fast and simple imaging sequence which can be used, e.g., for scout views or dynamical studies: after a contrast-inducing part like an inversion- recovery period the z magnetization in a slice of the object is spatially resolved using a series of low flip angle rf pulses.

In this communication an extension of snapshot FLASH which allows efficient multislice snapshot FLASH imaging was proposed. It is based on the SIMUSIM method and can be implemented in each MR imager by replacing the selective read-out rf pulses with SIMUFREX pulses. The SIMUSIM phase-alteration and postprocessing algorithm is easily combined with existing imaging software and provides multiple images from well-separated slices after a deliberate (selective or nonselective) contrast- inducing preparation period.

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In general the minimum measurement time for N slices corresponds to N snapshot FLASH periods. Therefore some highly dynamic studies cannot be optimally time resolved. For the same reason the SIMUSIM encoding is sensitive to motion artifacts similar to 3D imaging. Therefore best results are obtained in static or quasistatic objects. Periodically moving objects like the heart however can be imaged using gated snapshot FLASH.

A property of the Hadamard scheme is that the number of slices N should equal 2". In practical applications up to eight slices can easily be acquired with snapshot FLASH. Note that upon increasing the number of simultaneously acquired images a gain of f i i n SNR per measurement time results in comparison to single slice imaging.

The SIMU rf encoding technique has found a number of additional applications in MR experiments. SIMUVOSP ( 5 ) , a recently proposed simultaneous multivolume spectroscopy method, has been successfully applied for clinical 'H brain spectroscopy ( 6 ) . In ( 7) an extensive theoretical analysis of the applied multifrequency irradiation was made in order to verify the linearity of the Hadamard signal encoding. It was shown that, e.g., for 90" sinc pulses the multifrequency approach leads to slice profiles of sensitivity and selectivity similar to those of corresponding single frequency exci- tation. In snapshot FLASH, on the other hand, the small flip angles of 3"-5" render the linearity problem also intuitively treatable and in practical applications it was found that the encoding-decoding mechanism is very stable for the applied range of flip angles.

In conclusion: Snapshot FLASH, an intrinsic single slice imaging technique, has been combined with SIMUSIM for fast and efficient multislice imaging. After a selective or nonselective contrast-inducing period the imaging part is executed using low flip angle multifrequency selective excitation pulses. A simple Hadamard encoding and decoding scheme creates a number of images of deliberate slice position and thickness with an increased SNR compared to a standard time sequential multislice experiment.

ACKNOWLEDGMENTS

The author thanks A. Rudin, H.-P. Hafner, N. Beckmann, and J. Seelig for useful discussions. The work was supported by the Swiss National Science Foundation under Grant 4.889.085.18.

REFERENCES

1. S. MOLLER, Magn. Reson. Med. 6, 364 ( 1988). 2. S. MOLLER, Magn. Reson. Med. 10, 145 (1989). 3. P. MANSFIELD, J. Phys. C 10, L55 ( 1977). 4. A. HAASE, Magn. Reson. Med. 13, 77 ( 1990). 5. S. MOLLER, H.-P. HAFNER, AND N. BECKMANN, NMR Biomed. 2(5/6) , 209 (1989). 6. H.-P. HAFNER, E. RADO, AND J. SEELIG, Mugn. Reson. Med. 15, 135-141 ( 1990). 7. H.-P. HAFNER, S. MOLLER, AND J. SEELIG, Mugn. Reson. Med. 13,279 ( 1990).