Jong Bum Son^{1}, Sooyoung Shin^{1}, Ralph Noeske^{2}, Ersin Bayram^{3}, Jingfei Ma^{1}, and Haesun Choi^{1}

^{1}The University of Texas MD Anderson Cancer Center, Houston, TX, United States, ^{2}GE Healthcare Technologies, Potsdam, Germany, ^{3}GE Healthcare Technologies, Waukesha, WI, United States

### Synopsis

**Prostate MR spectroscopic imaging (MRSI) is
limited with long acquisition time and need for an endorectal coil. Using an
acquisition-weighted spectroscopic imaging sequence with an odd symmetric
sampling scheme and a sorted singular value decomposition method for spectral
processing of data from a multiple receive channels, we demonstrated that in
vivo prostate MRSI can be performed at 3T in a substantially reduced scan time
and without using an endorectal coil. Our proposed technique has the potential
to help substantially expand the clinical use of prostate MRSI.**### Introduction

Prostate MR spectroscopic imaging (MRSI) with reduced acquisition time
and without an endorectal coil could improve patient tolerance and throughput,
and allow imaging of patients without rectum or with their prostate in normal
position. The latter is important for
radiation treatment planning but is difficult or impossible when an endorectal
coil is used. In this work, we aim to demonstrate the following two important
improvements in prostate MRSI: (1) a substantial reduction in the total scan
time using a weighted acquisition scheme without SNR or protocol compromise,
(2) a sorted singular value decomposition (SVD) method for combining spectra
from a phased array surface coil.

### Methods

All
experiments were performed on a 3T whole-body MRI scanner (GE Healthcare,
Waukesha, WI) and using only one of the available phased array surface coils (an
8-channel or a 32-channel torso phased array by GE Healthcare, or a 16-channel
“diaper” phased array coil by ScanMed, Omaha, NE). The scan-parameters for the
prostate MRSI acquisition

^{1} were: TR/TE =
1300/85 ms, FOV = 110x55x55 mm, acquisition matrix = 12x8x8, voxel size =
6.9x6.9x6.9 mm, FID sampling points = 512, spectral bandwidth = 2 kHz. If a standard full k-space and symmetric
sampling scheme were used, the total acquisition for the above protocol would
be 16:44 min for a signal average of only 1.
To improve the scan efficiency, we implemented an acquisition-weighted
spectroscopic imaging technique with an odd symmetric sampling scheme, similar
to that proposed by Pohmann et al, and by Scheenen et al

^{2,
3}. Using the modified sequence for the same scan
protocol, the total acquisition time was 9:42 min for a signal average of 4.
For spectral postprocessing,
we developed a sorted singular value decomposition (SVD) method for multi-channel
multi-voxel spectral combination. Among the various techniques for combining
multichannel MR spectra, SVD does not require a priori information on
coil-sensitivity or estimating explicitly the complex coil weighting function4. In our sorted SVD method, the acquired signal
from each receive channel is first Fourier-transformed along all three spatial
dimensions. The resulting FID signal for
each voxel is then sorted in a descending order according to an SNR index and
assembled in an n x c matrix where n is the
total number of the data points for each FID signal and c is the number of coils.
The SNR used for sorting the signals of the different receive channels
is obtained as the ratio of the water signal over the standard deviation of a pre-defined spectral
region of noise for the spectra of each receive channel. Under the rank-one approximation, the n x c
matrix for each voxel is decomposed using a standard SVD algorithm and the
coil-combined spectrum is estimated as the Fourier transform of the left singular
vector multiplied by the corresponding largest component of the diagonal matrix
after the SVD decomposition

^{4}. To compare the
improvement of our proposed modifications, we performed the SVD analysis of the
same data without first sorting the FID signals.

### Results

Fig. 1 shows a
comparison of the 32-channel coil-combined spectrum using the sorted SVD (red)
and using SVD without sorting (green).
The SNR of the water signal was 13.8 and 6.9, and the (choline+creatine)/citrate
ratio was 0.46 and 0.37 for the two methods, respectively. The improvement shown
was representative of the different spatial locations. The sorted SVD was found
to always produce spectra with better quality and quantitation than SVD without
sorting and therefore was used as our chosen method for processing.

Fig. 2 shows the spectrum
of a patient from a region of biopsy proven cancer with an elevated (choline+creatine)/citrate
ratio of 0.88. Fig. 3 shows the spectrum
of another patient from a region of benign tissue with a (choline+creatine)/citrate
ratio of 0.506. The spectrum in Fig. 2
was acquired using an 8-channel coil and the spectrum in Fig. 3 was acquired
using the 16-channel “diaper” coil. Both
were deemed to be of diagnostic quality according to a radiologist experienced
in reading clinical endorectal coil prostate MRI and MRS.

### Conclusion

Using an
acquisition-weighted spectroscopic imaging sequence with an odd symmetric
sampling scheme and a sorted SVD method for spectral processing of data from a
multiple receive channels, we demonstrated that in vivo prostate MRSI can be
performed at 3T in a substantially reduced scan time and without using an
endorectal coil. Therefore, our proposed technique has the potential to help
substantially expand the clinical use of prostate MRSI.

### Acknowledgements

No acknowledgement found.### References

1. Cunningham CH, et al. Sequence design for magnetic resonance
spectroscopic imaging of prostate cancer at 3 T. Magn Reson Med. 2005; 53:1033-9.

2. Pohmann R
and Kienlin MV. Accurate phosphorus metabolite images of the human heart by 3D
acquisition-weighted CSI. Magn Reson Med. 2001; 45:817-26.

3. Scheenen
TW, et al. Fast acquisition-weighted three-dimensional proton MR spectroscopic
imaging of the human prostate. Magn Reson Med. 2004; 52:80-8.

4. Sandgren
N, et al. Spectral analysis of multichannel MRS data. J Magn Reson. 2005; 175:79-91.