MAGNETIC RESONANCE IN MEDICINE 9,379-388 (1989)

NOTES Hybrid Methods of Chemical-Shift Imaging

DONALD B. PLEWES* JERZY SZUMOWSKI,* JANE K. EISEN, * SIMON VINITSICI,? PETER w. HAAKE,$ AND

Departments of *Radiology and $Computing Center, University of Rochester Medical Center, Rochester, New York 14642, and ?Department of Radiology, Thomas Jefferson University Hospital,

Philadelphia, Pennsylvania 19107

Received June 27, 1988; revised October 6, 1988

We propose a family of hybrid chemical-shift sequences which combines two physical principles for water/lipid separation to minimize artifacts introduced by Bo and B, inho- mogeneities. Hybrid sequences provide improved species discrimination over earlier methods without resorting to postprocessing while maintaining a multislice/multiecho capability. 0 1989 Academic Press, Inc.

INTRODUCTION

The presence of intense lipid signals in MR images can result in several artifacts ( I , 2) and can degrade soft tissue contrast in selected applications of conventional spin-echo and gradient-recalled sequences. Elimination of lipid signal can suppress these artifacts and improve diagnostic accuracy with appropriate clinical application (3, 4 ) . To date, several methods of lipid suppression that are based on chemical shift, J coupling, or differences in lipid/water relaxation times have been proposed (5- 23) . Each of these approaches is subject to imperfect lipid suppression due to Bo and 23, inhomogeneities or distribution of relaxation times, long acquisition, or the need for extensive postprocessing.

We have proposed a simple phase-sensitive variant of the Dixon method (7) of CSI imaging which does not require any postprocessing by utilizing a simple two- excitation sequence. The first excitation generates a spin echo with the water and lipid magnetizations in-phase. The second excitation alternates the phase of the slice- selective pulse and adjusts the timing of the refocusing pulse to introduce a phase shift of T rad between the lipid and the water magnetizations as described by Dixon ( 5 ) . By complex subtraction of these two data sets during signal averaging, the lipid signal is eliminated for each phase-encoding view during image acquisition. This ap- proach to the Dixon method eliminates any potential for misregistration artifacts or the signal nonlinearities typical of manipulation of magnitude reconstructed images.

This “chopper suppression” (CS ) sequence has been applied clinically and found to be practical and clinically advantageous in lipid-suppressed protocols of the orbits, joints, and abdominal imaging (26-28). However, as with any two-point spectro- scopic chemical-shift imaging approach, Bo inhomogeneities can degrade the unifor- mity of lipid suppression. In practice, these artifacts while clearly evident have not interfered with clinical application if the Bo inhomogeneities are less than the chemi- cal-shift differences between lipid and water. This limitation can be eliminated by

379 0740-3 194/89 $3.00 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form r e ~ e ~ e d .

380 NOTES

storing paired data separately and utilizing a phase correction scheme after data col- lection (Z9). While this approach has been clinically demonstrated, it is not in the spirit of the CS sequences which attempts to achieve lipid signal suppression during data acquisition without resorting to any postprocessing. In this report, we introduce a different category of pulse sequences which improves the extent and uniformity of lipid suppression in the presence of Bo inhomogeneities by combining two or more lipid suppression mechanisms in a single sequence.

METHODS

The general approach of the hybrid method involved the use of a preparation pulse followed by the CS sequence. The preparation pulse could be a frequency-selective excitation in the absence of any gradients to either saturate or excite a specific chemi- cal species. Alternatively, the preparation pulse could be an inverting pulse preceding the CS sequence with a time delay chosen so that the lipid longitudinal magnetization vanishes.

Selective Excitation

The details of the hybrid sequence are shown in Fig. 1. One approach for hybrid lipid suppression uses a frequency-selective sinc pulse with the RF carrier centered on the lipid resonance after which the RF frequency for the remaining CS sequence is centered on the water resonance. During the first excitation, the lipid magnetization is rotated into the transverse plane and dephased by the spoiler gradient. This frequency-selective pulse provides partial elimination of the lipid signal subject to the

spoiler l

freq selective 180 out-of-phase

RF2 -

At 1st echo

FIG. 1. A simplified diagram of the hybrid pulse sequence which combines a frequency-selective pulse followed by chopper suppression. A frequency-selective pulse can be either a sinc or a I33 1 composite pulse centered on lipid resonance. The RF carrier is assumed to be centered on water resonance. In the IR hybrid version, a 2662 pulse has been used for lipid magnetization reversal followed by an appropriate time TI before the 90" pulse to allow for the null effect.

NOTES 38 1

limitations from Bo and B, inhomogeneities. The rest of the sequence operates on the remaining lipid and water magnetizations to produce the in-phase echo. During the second excitation, the frequency-selective pulse and dephasing gradient remain in- tact; however, the phase of the slice-selective pulse is reversed and the position of the refocusing pulse is adjusted to produce the opposite-phase echo. Averaging by com- plex subtraction during the data acquisition as in the original CS sequence results in the elimination of the remnant lipid component in the final image. Since two independent mechanisms of lipid suppression are being invoked in this sequence, its tolerance to Bo and B1 inhomogeneities is significantly improved.

One of the limitations of the use of this form of excitation pulse is that it will slightly reduce the water magnetization prior to the final CS sequence. The pulse was of 8 ms duration with a corresponding bandwidth of 250 Hz centered about the lipid reso- nance. This choice of parameters provides a reasonable compromise between low RF power at the water resonance while keeping the duration of the selective pulse short. Calculation of the RF power spectrum from this truncated sinc pulse showed that the RF power at the water line was 3% of that the lipid resonance which reduced the final water signal by approximately 6%. Accordingly, an alternative approach to the sinc- selective pulse was tested in the form of a 133 1 selective excitation (20) with a 0.25- ms duration for each of the hard pulses.

A water-suppressed sequence can be invoked by making the first frequency-selec- tive pulse centered on the water resonance and the CS sequence centered on the lipid resonance.

Selective In version Recovery

Another approach to hybrid CSI uses a frequency-selective inverting pulse to invert the lipid magnetization with a delay time TI before the remainder of the CS sequence is initiated. The timing of the delay is adjusted to allow the longitudinal lipid magne- tization to vanish as implemented with a STIR sequence (3). This hybrid implemen- tation differs from the STIR sequence in that magnitude of the water signal is not substantially reduced since the inverting pulse excites only the lipid resonance. As in the previous examples, the remainder of the CS sequence operates to eliminate the remnant lipid component from the final image.

These three sequences were tested on a 1.5-T General Electric Signa MR system by modifying a standard multislice/multiecho sequence. The spoiler gradient was sinusoidal in shape and applied along the slice-selective axis only. Studies were con- ducted on a phantom containing CuS04-doped water and machine oil as well as on volunteers. In the phantom studies these sequences utilized a TR/TE of 600/25 ms with 1-cm slice widths with field of views of 20 cm. The sequences could be tuned prior to scanning to adjust the frequency and amplitude of the selective pulses to the lipid resonance and the delay times At for the CS refocusing pulse and TI for the initial selective inverting pulse.

RESULTS

The spectrum of the oil/water phantom is shown in Fig. 2 with and without lipid suppression using the selective sinc-CS hybrid sequence. The RF carrier of the selec-

382 NOTES

FIG. 2. NMR spectra of the water/oil phantom (left) and the result of the hybrid (sinc + CS) sequence (right) are shown. Note the elimination of the oil peak which is accompanied by a small decrease in the water signal due to the frequency respond characteristics of sinc pulse employed in this hybrid sequence.

tive pulse was set at 240 Hz offset from the water resonance as measured from Fig. 2 (left). This illustrates that the hybrid sequence provides excellent suppression of the lipid resonance amounting to 1.4% of its unsuppressed value as summarized in Table 1. A slight reduction in the water magnitude is also evident from excitation of the water line with this sequence.

This is further illustrated in Fig. 3 where the image from conventional spin-echo scan of the waterloil phantom is compared to that resulting from the use of CS sup- pression alone, selective lipid saturation with a sinc pulse alone, and in combination. The relative signal levels for the water and lipid components are summarized in Table 1 which shows that either the CS or the selective excitation mechanism provides a

NOTES 383

TABLE 1

Comparison of Water and Lipid Signals for Various Hybrid Sequences"

Method Lipid signal Water signal

Spin echo 100.0 cs 5.2 Selective sinc 5.7 Hybrid (CS + sinc) 1.3 Selective 133 1 3.9

1 .o Hybrid (CS + 1331)

100.0

99.8 93.8 93.7 98.9 98.8

Selective inversion Recovery (IR) 2.6 98.3 Hybrid (CS + IR) 0.7 98.3

a The water and lipid signals are normalized to the signals obtained from the spin-echo sequence.

significant degree of lipid suppression which is markedly improved when the hybrid sequence is employed.

To test the tolerance of the hybrid sequence to errors in the tuning of either the timing parameter At or the selective excitation of the lipid resonance, another set of images was collected where each of these parameters was deliberately maladjusted. The results of this experiment are summarized in Table 2.

With a 10% error in the value of At, lipid suppression by the CS method was de- graded by a factor of 2.6 over that obtained with the tuned sequence. Similarly, with a 15% error in the amplitude of the selective sinc pulse, the lipid suppression from this mechanism was also significantly degraded. However, the combined effect of these maladjustments in the hybrid sequence was to give only slightly degraded lipid suppression compared to that of the properly tuned sequence. This points out the robust nature of the hybrid sequence in terms of its tolerance to errors in tuning parameters and Bo or B1 inhomogeneities. This results from the compounding effect of the two independent mechanisms (chopper suppression and selective excitation) for lipid suppression.

A clinical example of this hybrid approach is shown in Fig. 4 which compares images through the abdomen with a conventional spin echo and the components of the hybrid sequence. The use of either the CS (Fig. 4, top right) or selective excitation (Fig. 4, bottom left) provides a measure of lipid suppression; however, inhomogene- ities result in a very nonuniform degree of suppression throughout the subcutaneous fat. When the hybrid sequence was used (Fig. 4, bottom right) the uniformity of the lipid suppression was markedly improved as would be predicted from the phantom images. The uniformity of final water image is limited primarily by the uniformity of the B1 field from our body coil as can be seen from inspection of the spin-echo image and is not a result of the lipid suppression sequence.

The results of the implementation of the inversion recovery hybrid approach is

384 NOTES

FIG. 3. Axial images of water/oil phantom. TR/TE 600/25, FOV 20 cm, 1 cm thick; (A) spin echo; (B) CS; ( C ) frequency-selective sinc; (D) hybrid (sinc + CS). Results of ROI intensity measurements are summarized in Table 1 .

summarized in Table 1 and as in the case of the other sequences provided excellent lipid suppression. The delay time TI of 55 ms was found to provide maximum ma- chine oil signal suppression. The comparative features for the various hybrid methods are outlined in Table 1 and show very similar degrees of lipid suppression with the IR hybrid sequence providing maximum lipid suppression with only minimal water signal loss.

Clinical examples of the use of the hybrid lipid suppression over the conventional spin-echo sequence are shown in Figs. 5 and 6. The conventional spin-echo (2000/ 30 TR/TE) sequence and 133 1 hybrid sequence of the upper abdomen is shown in Fig. 5 and illustrates how lipid suppression (bottom) provides for expanded intraab- dominal gray scale contrast producing noticeably brighter images of the spleen and stomach. Marked suppression of the subcutaneous and retroperitoneal fat structures is also evident. The spinal cord assumes a relatively bright signal intensity.

In Fig. 6 we see another comparison of the spin-echo (top) and 133 1 hybrid lipid- suppressed image (bottom) of the upper abdomen incorporating cardiac gating. Note

NOTES 385

TABLE 2

Effects of Detuning the Sinc Hybrid Sequence"

Method Lipid signal Water signal ~

CS tuned 5.2 99.8 CS detuned 13.6 99.3 Sinc tuned 5.1 93.8 Sinc detuned 14.9 93.7 Hybrid tuned 1.3 93.1 Hybrid detuned 1.8 93.1

a The water and lipid signals are normalized to the signals obtained from the spin-echo sequence.

the reversal of the relative signal intensities of the kidney, bowel, and liver. Marked suppression of the subcutaneous, intraabdominal, and retroperitoneal fat is demon- strated. Again we see a relatively bright signal intensity over the spinal cord.

FIG. 4. Axial images of volunteer's abdomen. TR/TE 1500/20, FOV 40 cm, 5 mm thick (top left) spin echo; (top right) CS; (bottom left) selective 133 1; (bottom right) hybrid (133 I + CS) lipid suppression.

386 NOTES

FIG. 5. Two consecutive axial images of a volunteer’s abdomen. TR/TE 2000/30, FOV 40 cm, 5 mm thick. Top, spin echo; bottom, hybrid ( 1 33 1 + CS) lipid suppression.

DISCUSSION

The use of hybrid sequences provides a simple means to uniformly suppress lipid (or water) signals without resorting to postprocessing and the excessive data collec- tion required for phase-corrected imaging. The fact that the sequence utilizes two independent mechanisms for lipid suppression provides a sequence that is tolerant to errors in the system timing or frequency tuning. In clinical practice this indicates that the sequence only needs to be tuned to the water resonance and all other tuning and timing parameters can be fixed for all applications. The sequence is flexible and can be incorporated with narrow bandwidth imaging, cardiac or respiratory gating, and flow compensation. The performance of the hybrid sequence will depend upon the particular scanner utilized. In general, it will favor the high-field high-homogene- ity systems.

NOTES 387

FIG. 6. Axial images of abdomen acquired with cardiac trigger. TR/TE 950/25, FOV 38 cm, 5 mm thick. Top, spin echo; bottom, hybrid ( I 331 + CS) lipid suppression.

In our own experience with these sequences a preference for the 1331 hybrid method has been shown because it is the most flexible and robust. While the sinc-CS sequence performance is similar in terms of lipid suppression the loss of water con- trast is overcome by the 133 1 sequence (Table 1). Similarly the 133 1 hybrid sequence provides greater timing flexibility over the inversion hybrid sequence as there is no requirement for long delay interval TI which decreases the total number of available slices with the TR interval. However, where the total number of slices is not a limiting factor, the inversion hybrid sequence appears to provide the best lipid suppression among all the hybrid approaches tested with only minimal water contrast loss.

Studies are ongoing to assess the clinical efficacy of lipid-suppressed images in vari- ous clinical applications and will be the subject of future publications.

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