expansion of the spectral bandwidth by spatial and chemical shift selective saturation in high-speed...

Expansion of the Spectral Bandwidth by Spatial and Chemical Shift Selective Saturation in High-speed Magnetic Resonance Spectroscopic Imaging Satoshi Hirata, Yoshitaka Bito, Etsuji Yamamoto

A new spectral bandwidth expansion technique for high- speed magnetic resonance spectroscopic imaging (MRSI) based on an echo-planar technique Is presented. This expansion can be achieved by spatial and chemical shift selective saturation without increasing the total measurement time. In addition, displacement along the slice-select direction due to chemical-shift differences between the measured compounds 1s also suppressed. Experimental results are shown using a phantom consisting of benzene and acetone. High spatial resolution (1 x 1 mm2) and wide spectral bandwidth (1.5-1.8 kHz; the effective spectral bandwidth has been doubled) are obtained without the displacement along the slice-select direction. Key words: magnetic resonance spectroscopic Imaging; high- speed MRSI; echo-planar; expansion of spectral bandwidth.

INTRODUCTION

In conventional magnetic resonance spectroscopic imaging (MRSI) where spectroscopic free induction decays (FIDs) are acquired directly, total measurement time is extremely long, because a large number of excitations and measurements must be repeated in proportion to the spatial resolution (number of gradient phase encodes) (1-6). To decrease the total measurement time, various high-speed MRSI techniques have been proposed (7-15). Echo-planar-based techniques have been proposed by Mansfield (9, lo) , Goilfoyle and Mansfield (11). Doyle and Mansfield (12). and Matsui et al. (13-15). The technique proposed by Matsui et al. reduces measurement dimensions by one compared with multidimensional phase-encoded MRSI by using a periodically inverting readout gradient. Recently, three-dimensional measurement using Matsui's method with short echo times has been performed on the human brain by Posse et a]. (16).

However, in high-speed MRSI based on the echo-pla- technique, the measurement spectral bandwidth is

limited by the reciprocal of the inversion period of the readout gradient, whose strength must increase to obtain high spatial resolution. By inverting the readout gradient, the accumulated phase shift is periodically canceled at

MRM 35:011-616 (1996) From the Medical Electronics Research Center, Central Research Labora- toy. Hitachi, Ltd., Tokyo, Japan. Address Correspondence to: Satoshi Hirata, Medical Systems Research Department, Central Research Laboratory. Hitachi, Ltd., 1-280 Higashi- koigakubo. Kokubunji-shi, Tokyo 185, Japan. Received April 6, 1995; revised November 10, 1995; accepted November 11. 1995. 0740-3194/96 $3.00 Copyright 0 1996 by Williams 8 Wilkins All rights of reproduction in any form reserved.

each echo maximum. Spatial resolution is also propor- tional to the integral amplitude during a half period of the readout gradient. Therefore, if the measurement spectral bandwidth is expanded (i.e., the strong readout gradient must be inverted faster than before), spatial resolution will be lowered because the integral amplitude during a half period of the readout gradient is limited by the slew rate of the gradient (e.g., when the shape of the readout gradient is trapezoidal, the period of constant strength will be shortened, and when the shape of the readout gradient is sinusoidal, the maximum strength of the readout gradient will become low). In other words, improvement of the spatial resolution (increasing the integral amplitude during a half period of the readout gradient) narrows the measurement spectral bandwidth. In this case, the resonances located outside the measurement spectral bandwidth will overlap the spectrum located inside due to aliasing. This problem becomes seri- ous when a strong static magnetic field is used or the chemical shift difference of compounds is intrinsically wide (e.g., '3C-compounds). The conventional spectral bandwidth expansion technique proposed by Matsui et al. (14,15) involving delayed echo trains has a disadvan- tage in that the total measurement time increases in proportion to the expanded bandwidth. In our technique, limited spectral bandwidths in which a resonance exists are excited individually in a single measurement with suppression of aliasing resonances by spatial and chemical shift selective saturation. Each limited spectral bandwidth in which a resonance exists is excited by an RF pulse with a corresponding resonance frequency one after another during the repetition time. Thus the spectral bandwidth can be expanded without increasing total measurement time.

The excitation bandwidth is set to be about five to six times as large as the chemical-shift difference between the farthest left and farthest right resonances in general in vivo 'H MRSI. As a result, displacement along the slice- select direction due to the chemical-shift difference between the farthest left and farthest right resonances is equal to approximately 20-17% of the slice thickness, depending on the ratio of the excitation bandwidth to spectral separation of the excited spins. As a secondary effect, the spectral bandwidth expansion technique can suppress this displacement along the slice-select direction. In this report, by applying this technique to a phantom consisting of benzene and acetone (the chemical- shift difference Acr = 1.0 kHz at static magnetic field B, = 4.7 T), we will demonstrate the feasibility of this expansion technique.

611

612 Hirata et al.

METHODS High-speed MRSI Based on An Echo-Planar Technique

Figure 1 shows a schematic diagram of a sequence from a high-speed MRSI using a spin-echo method reported by Matsui et al. (13-15); this type of MRSI is based on the echo-planar technique. A periodically inverting readout gradient in the x-axis direction is added during data acquisition. The inverting readout gradient encodes, in a gradient-echo train, spatial information in the x-axis direction and the passage of time encodes chemical shift. This is in contrast with the traditional echo-planar technique of encoding spatial information in two directions (17). Other spatial information in the y-axis direction is encoded to the echo train by a phase-encode gradient in the same direction. Therefore, measurement time becomes shorter than that of three-dimensional chemical shift imaging (3D CSI), because three-dimensional information (spectroscopic information and spatial information in the x- and y-axis directions) is obtained by a two-dimensional measurement.

Three-dimensional information is extracted from phase-encoded echo trains obtained by two-dimensional measurement by first separating each echo train into odd- and even-numbered echoes. Thus, two sets of three-dimensional data arrays are obtained. These two sets are reconstructed by three-dimensional Fourier transform. Linear phase corrections of these odd- and even-numbered echo spectra are performed, because the even-numbered echoes and odd-numbered echoes are approximately considered to be shifted a half period of the readout gradient in the time-axis. In this case, the phase difference between the even-numbered and the odd- numbered echo spectra can be represented by linear equation of chemical-shift. After the phase correction, these spectra are added.

I TR I

1 ' I TW2 , TW2

90' 180" 90'

n RF -@ s .__._

Gz \ Y r,

Gx ............

.......-.-.. GY

Sig. ............

............ AD FIG. 1. A schematic diagram of a high-speed MRSl sequence based on an echo-planar technique; the MRSI uses a spin-echo excitation method.

Spectral Bandwidth Expansion Technique

In this technique, each limited spectral bandwidth where a resonance exists is excited in a single measurement. This makes it possible to avoid measuring regions where a useful resonance does not exist, and to expand the relevant spectral bandwidth by lining up the obtained resonances in the direction of the chemical shift. How- ever, the resonances located outside the measurement spectral bandwidth will overlap the spectrum located inside due to aliasing, when the measurement spectral bandwidth is narrowed. To measure only a resonance, it is necessary to suppress the aliasing resonances by applying spatial and chemical shift selective saturation.

An example of measuring an object consisting of two compounds, having two different resonance frequencies, is shown in Fig. 2. Figure 2a shows a sequence that includes spatial and chemical shift selective saturation at the beginning of the high-speed MRSI sequence shown in Fig. 1. Figure 2b shows the region in an excited state and

Selective pseudo-saturation I I t.

RF

Gz

Gx

GY

a

Hoz

compound A

cnmpound n

b

I/. +YO' V

.....

.....

n I I.....

..... 4 llmc

Selective pseudo-saturation r i h I 1st Stage I 2nd Stage I 3rd Stage ]High-speed MRSI

-c ./' n : Thcrmnl cqullllirlum SIOIC Z r measuremen;'?

slice : Excitd stole .. : Saturntd state

FIG. 2. (a) Spatial and chemical shift selective saturation sequence. The central frequency of the RF pulses is set at the frequency of compound A (m), and the excitation bandwidth is set to equal the difference between the resonance frequencies of compounds A and 6. (b) The region in an excited state and in a saturated state. During)he first stage, both the spins corresponding to the compound A resonance in the measurement slice, and those of the compound 6 resonance in the region outside the measurement slice are excited. During the second stage, the spins of compound A in the measurement slice are recovered and those of compound 6 in the region outside the measurement slice are excited, because the signs of RF and Gz are reversed. During the third stage, the spins of compound B in an excited state are saturated by spoiler gradients. Thus, only the spins of compound A in the measurement slice are excited in high-speed MRSI.

Expansion of the Spectral Bandwidth in High-speed MRSI 613

in a saturated state. In this example of measuring a resonance from compound A, the central frequency of the RF pulses shown in Fig. 2a is set at the resonance frequency of compound A (va), and the excitation bandwidth is set to equal the difference between the resonance frequencies of compounds A and B. During the

resonance from each compound exists can be excited by an RF pulse whose central frequency is set at each resonance frequency.

EXPERIMENTS first stage of the sequence,both the spins corresponding to the compound A resonance in the measurement slice, and those of the compound B resonance in the region outside the measurement slice are excited. The difference between the slice positions of compounds A and B to be excited is just one slice thickness because of the excitation bandwidth. During the second stage, the spins of compound A in the measurement slice are recovered and those of compound B in the region outside the measurement slice are excited, because the signs of RF and Gz are reversed. In the third stage, the spins of compound B in an excited state are saturated by spoiler gradients. As a result, the spins of compound A in the measurement slice are excited, and those of compound B outside the measurement slice are selectively saturated. Therefore, when the resonance from compound A is measured, the resonance from compound B is located outside the measurement spectral bandwidth and does not overlap the spectrum located inside the bandwidth, even by aliasing. The spins of compound B in the measurement slice are neither excited nor saturated. It is possible to continuously excite the spins of compound B in the measurement slice after the measurement of the compound A resonance during the original recovery time of the longi- tudinal magnetization, shown in Fig. 3. Then the central frequency of the RF pulses is set at the resonance frequency of compound B (ub), and the spins of compound A located outside the measurement slice are selectively saturated. Therefore, the two regions where the resonances from compounds A and B exist can be excited and measured in a single pass, and the sequence time is the same as that of the conventional method. That is, the spectral bandwidth can be substantially expanded without increasing the total measurement time.

Furthermore, displacement along tho slice-select direction can be suppressed as a secondary effect of applying this expansion technique, because the region where the

TR i' . recovery time j

d

! ?

4 0 : ThrrmnI quilihrlum stntc Z .... ...

"measuremen;'? : Ei r i td btntr

slice : Snturntnl stntc

FIG. 3. The measurement steps of the procedure to shorten the measurement time. m and tb are the resonance frequencies of compounds A and B respectively.

Experiments were performed using a 4.7-T magnet (40-cm horizontal bore, Oxford Instruments, Oxford, UK) equipped with 11-cm bore actively shielded gradient coils. The maximum strength of the gradients is 75 mT/m and ramp time from zero to the maximum amplitude is 200 ps. An 8-cm diameter multiple-element resonator is used for RF transmission and signal reception. We performed two types of experiments to demonstrate the feasibility of this expansion technique.

The first experiment was to evaluate the ability of the spectral bandwidth expansion technique using a phantom consisting of two concentric glass tubes. The small tube is 11 mm in diameter and 155 mm in length and is filled with acetone. It is immersed within a larger tube which is 16 mm in diameter and 180 mm in length and filled with benzene as shown in Fig. 4a. The chemical shift difference between benzene and acetone is 1.0 W z at 4.7 T. We compared the images and spectra of benzene and acetone with and without the expansion technique. We also estimated the effect of spatial and chemical shift selective saturation pulses when using the expansion technique.

The second experiment was to determine whether displacement along the slice-select direction due to the chemical-shift difference could be suppressed. We used a phantom consisting of a bottle 22 mm in diameter and 50

180 mm c: 155 mm

+ benzene acetone i

measurement slice 2

a

mixture of benzene and acetone 50 mm s

38 mm

f t- measurement slice 4 x b Z

b

FIG. 4. (a) A phantom consisting of two concentric glass tubes. The small tube filled with acetone is immersed within the larger tube filled with benzene. (b) A phantom consisting of a bottle filled with a mixture of benzene and acetone, in which a wooden rod is placed at an angle of 30".

614 Hirata et al.

mm in length filled with a mixture of benzene and acetone, inside of which a wooden rod 2 mm in diameter is placed at an angle of 30° as shown in Fig. 4b. We compared the difference in the rod positions in the benzene and acetone images with and without the expansion technique.

Standard parameters in these experiments were as fol- lows. For excitation and focusing, six-lobed sinc pulses with a duration of 7.5 ms and a bandwidth of 0.80 kHz were used in the first experiment and six-lobed sinc pulses with a duration of 6.0 ms and a bandwidth of 1.0 kHz were used in the second experiment, to select a 3-mm thick axial slice. The readout gradient was de- signed to have a 64-cycle trapezoidal shape with an amplitude of 34 mT/m and inversion time of 1.4 ms (twice full scale ramp duration). A 90-ms echo train was acquired symmetrically at an effective echo time (TE) of 136 ms, and was sampled with an analog-to-digital rate of 11 ps at 128 points per inversion time (i.e,, 64 points per echo) and a repetition time (TR) of 2 s. The acquisition parameters were set at a spectral bandwidth of 0.70 kHz, a spectral resolution of 11 Hz, a spatial resolution of 1 X 1 mm', and a 32-mm field of view (double sampling).

Y 4

Y 4

RESULTS AND DISCUSSION

The first experimental results of high-speed MRSI using the phantom consisting of two concentric glass tubes are shown in Fig. 5. Figures 5a-5c show the images and spectrum of benzene and acetone obtained using a 1.4- kHz measurement spectral bandwidth. The spatial resolution of these images is 4 X 4 mm'. Figures 5d-5g show the images and spectra of benzene and acetone obtained using the proposed expansion technique with a measurement spectral bandwidth of 0.70 kHz and without spatial and chemical shift selective saturation pulses. The spatial resolution of these images is 1 x 1 mm'. Higher spatial resolution could be obtained by making the measurement spectral bandwidth narrower in a single measurement. The measurement spectral bandwidth is expanded almost twofold by performing two measurements; one of which excited the benzene and the other the acetone. However, the resonances indicated by the arrows show aliasing due to resonances located outside the measurement spectral bandwidth (Figs. 5f and 5g). Figures 5h-5k show the images and the spectra obtained using the proposed expansion technique with measurement spectral bandwidth of 0.70 kHz and spatial and chemical shift selective saturation pulses.

e 4.0 ppm

L X a b 4.0 ppm

.ihJI1 -0 PPm .!L[E] PPm 3.5 PPm

Y .3 -.

frequency f g

frequency C

L x h i

FIG. 5. (a)-@) Experimental results of high-speed MRSl with a 1.4-kHz measurement spectral bandwidth. (d)-(g) Experimental results of high- speed MRSI obtained using the expansion technique with a measurement spectral bandwidth of 0.70 kHz but without the spatial and chemical shift selective saturation pulses. (h)-(k) Experimental results of high-speed MRSl obtained using the expansion technique with measurement spectral bandwidth of 0.70 kHz and including spatial and chemical shift selective saturation pulses: (a) is an image of benzene, which corresponds to the left resonance in spectrum (c); (b) is an image of acetone, which cormponds to the right resonance in spectrum (c); (d) and (h) are images of benzene, which correspond to spectra (9 and (i), respectively; and (e) and (i) are images of acetone, which correspond to spectra (g) and (k). respectively. In the images of (a) and (b), 8 x 8 pixel areas cut from the central regions of the original 32 x 32 pixel areas are linearly interpolated to 48 x 48 pixel areas in the spatial domain. In the images of (d), (e), (h) and (i), original 32 x 32 pixels are linearly interpolated to 192 x 192 in spatial domain.

frequency j k

Expansion of the Spectral Bandwidth in High-speed MRSI 615

In this measurement, the measurement spectral bandwidth is clearly expanded almost twofold without aliasing resonances. The intensities of the spectra shown in Figs. 5c, 5f, 5g, 5j, and 5k are all normalized by the same standard. It took the same total measurement time to perform each measurement. Therefore, it is clear that only the aliasing resonances are suppressed by adding spatial and chemical shift selective saturation pulses and the measured resonances are almost free from affection of the saturation pulses except signal decrease due to the B, inhomogeneity.

In these experiments, the chemical shift difference between benzene and acetone appeared to be 0.80 kHz, although it is actually 1.0 kHz. This difference is mainly caused by the difference in the magnetic susceptibilities of benzene and acetone in the two concentric glass tubes, which is estimated to be 0.13 kHz according to Eq. [7] of ref. 18, which assumes cylindrical samples of infinite length. The remainder may be caused by a concentric B, inhomogeneity or the sample of finite length.

The results of the second experiment using the phantom consisting of a bottle with a rod inside as shown in Fig. 4b, are shown in Fig. 6. These figures show another advantage of this expansion technique, which is the suppression of displacement along the slice-select direction due to the chemical-shift difference. Figures 6a-6c show

the images and spectrum of benzene and acetone from 3D CSI with a measurement spectral bandwidth of 1.4 kHz but without using the expansion technique. We can see the shift of the stick in images of Figs. 6a and 6b. In this case, the excitation bandwidth was set to equal the chemical shift difference of 1.0 kHz between benzene and acetone so that the distance between the slices in the slice selection would equal the slice thickness. For this reason, the positions of the stick in the benzene and acetone images in the y-axis direction did not coincide. Figures 6d-6k show the images and the spectra of benzene and acetone when utilizing the expansion technique with a 0.70-kHz measurement spectral bandwidth. Figures 6d-6g and 6h-6k were obtained with 3D CSI and high-speed MRSI, respectively. The signal was repeated 32 times for the high-speed MRSI measurement shown in Figs. 6h-6k to make the measurement comparable with the 3D CSI measurement. The stick positions in the benzene and acetone images coincide as shown in these figures. Therefore, it is clear that the proposed expansion technique can suppress displacement along the slice- select direction due to a chemical-shift difference in both 3D CSI and high-speed MRSI.

This method can also be applied to an object consisting of N(>2) kinds of compounds. It will be possible to

FIG. 6. (a)+) Experimental results from 3D-CSI with a measurement spectral bandwidth of 1.4 kHz but without using the expansion technique. (dHg) Experimental results of 3D-CSI utilizing the expansion

~ ~ ~ ! . . . - - - - - - - - ~ technique with a measurement spectral bandwidth of 0.70 kHz. (hHk) Experimental results of high- speed MRSI utilizing the expansion

__.-.-..--. technique with a measurement 7.0 ppm .5 PPm 3.5 PPm spectral bandwidth of 0.70 kHz and

32 repetitions: (a) is an image of f g benzene, which corresponds to the

left resonance in spectrum (c); (b) is

e 5.3 ppm 5.3 ppm

3 .I

frequency frequency C

s; L x h i

S.3 ppm ..

an image of acetone, which corresponds to the right resonance in spectrum (c); (d) and (h) are images of benzene, which correspond to spectra (0 and (j), respectively; and (e) and (i) are images of acetone, which correspond to spectra (9) and (k), respectively. In these images, the original 32 x 32 pixel areas are linearly interpolated to 192 X 192 pixel areas in the spatial domain. Grid lines are superimposed on the images to make the rod positions clear.

k-3.5 p p m 4 k - 3 . 5 ppm -1 frequency k j

616 Hirata et al.

substantially expand the spectral bandwidth through in- dividual excitation and measurement of each compound having unique, single-valued resonance frequency without increasing total measurement time. Then the excitation bandwidth is set to equal the difference between the resonance frequencies of the closest two chemical shifts. The spins of each compound in the measurement slice are continuously measured with each resonance frequency during the repetition time, and the spins of other compounds located outside the measurement slice are selectively saturated. The only case where this is not possible is when the time from the beginning of excitation to the end of the measurement for one compound is more than 1/N of the repetition time.

This expansion technique is sensitive to the B,, field homogeneity. When the B,, field homogeneity decreases, the measured resonance width will be wider. The spins whose resonance frequency differs slightly from the central frequency will have phase deviation and will not be recovered perfectly. Therefore, the recoveries of spins depend on the B, field homogeneity, and likewise, measurements of the intensities of the resonances depend on that. In addition, when T,* relaxation occurs very fast, the signal intensity will decrease a little during the period of spatial and chemical shift selective saturation. The phase deviation and signal decrease become bigger as the interval of RF pulses in the spatial and chemical shift selective saturation is wider. To minimize the sen- sitivity to B, field inhomogeneity and T,* relaxation, the time between saturation pulses should be minimized.

In addition, high-speed MRSI has another disadvan- tage because the signal receive bandwidth is wider than that of 3D CSI, which leads to a decrease in the signal- to-noise ratio (SNR). We are now improving the SNR of high-speed MRSI using multiple echo trains.

CONCLUSION

We have reported a new spectral bandwidth expansion technique for high-speed MRSI based on an echo-planar technique. Both high spatial resolution and wide spectral bandwidth can be obtained independently by using the proposed expansion technique, which will increase the range of possible application for conventional high- speed MRSI; potential applications include measurement in a strong static magnetic field or of compounds whose chemical shift difference is intrinsically wide l e g , “C-compounds). As a secondary effect, this technique also suppresses displacement along the slice-select direction caused by the chemical-shift difference.

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expansion of the spectral bandwidth by spatial and chemical shift selective saturation in high-speed...

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