In vivo 1H and 31P MR spectroscopy in healthy fibroglandular breast tissue at 7 Tesla.
Wybe JM van der Kemp1, Bertine L Stehouwer1, Vincent O Boer1, Peter R Luijten1, Dennis WJ Klomp1, and Jannie P Wijnen1

Synopsis

Water and fat suppressed 1H total choline MR spectroscopy and 31P MR spectroscopy were performed in healthy fibroglandular breast tissue of a group of 8 volunteers. 31P T2 values were determined, and reproducibility of 1H and 31P MR spectroscopy were investigated. The 1H and 31P data were combined to calculate estimates of absolute concentrations of PC, GPC and PE.

Introduction

In vivo proton and phosphorus MR spectroscopy can both be applied in monitoring breast cancer treatment response. Proton MR spectroscopy can be used to measure total choline, while 31P MR spectroscopy can be used to measure the individual 31P containing metabolites of total choline. Here we applied both methods in a group of eight healthy volunteers and also investigated the reproducibility of the two methods to quantify choline composition.

Methods

A group of eight healthy female volunteers (ranged 21-30yr) underwent two MRI scan sessions of the right breast with a dual tuned 1H-31P quadrature coil1 on a 7 tesla scanner (Philips, Cleveland, USA). Each volunteer was scanned twice on the same day, with the same protocol consisting of basic imaging, 31P AMESING2 MRSI and metabolite cycled3 1H semi-laser spectroscopy. Phosphorus MRSI was done by adiabatic multi-echo spectroscopic imaging, AMESING2, with T­R=6s; ΔTE=45ms; matrix 8x8x8; 4x2x4cm3 voxels; acquiring 1 FID and 5 full echoes; with a bandwidth of 8200 Hz and 256 datapoints for the echoes.

Proton spectroscopy was focused on measuring total-choline signal and was done with a water and fat suppressed semi-laser proton sequence with metabolite cycling and TE averaging (118,119,120,121ms) for the overlapping choline resonances at 3.2ppm and a voxel of 15x15x15mm3 carefully placed in fibroglandular tissue.

Results & Discussion

The group averaged 31P FID and echo spectra based on the spectra of the 8 volunteers are shown in Figure 1. Note that the peak labeled GPtC (glycerophosphatidylcholine) is gone from Echo 2 onwards, comparable to the quick disappearance of γ-ATP. Also note that the peak labeled GPC+GPtE loses a substantial intensity from FID to the first echo (GPtE glycerophosphatidylethanolamine has a short T2), after which the decrease in intensity goes much slower, leaving only the slowly decaying GPC signal. In the FID spectrum there appears to be some signal between the peaks labeled PE and PC that seems to disappear in the echoes, this signal could be AMP4.

Figure 2 shows the T2-fits for the different 31P metabolites. Note that the FID signal amplitudes of PE+AMP, PC+AMP, and GPC+GPtC are all well above the fitted mono-exponential curve. The additional bi-exponential fit for these resonances shows a better agree­ment with the data.

Relative metabolite abundance expressed as a percentage of the total 31P signal, for this group of volunteers measured twice, as obtained from the data in Figures 1 and 2 are: PE=8%; AMP=5%; PC=4%; Pi=29%; GPE~ 0%; GPC=4%; GPtE=10%; GPtC=17%; γ-ATP=13%; α-ATP=8%; NADP=2%. A measure for the reproducibility of the 31P measurements and data analyses of the individual volunteers is given by the mean relative error $\langle\overline{RE}_{A_{m}}\rangle$ in amplitude A of a metabolite m defined as: $$\langle\overline{RE}_{A_{m}}\rangle=100\cdot\frac{\langle\overline{sd}_{A_{m}}\rangle}{\langle{\overline{A}_{m}}\rangle}, [1]$$

which is shown in Figure 3, for 31P and 1H MRS. Here${\langle{\overline{A}_{m}}\rangle}$ is the group average of the mean amplitude of metabolite m of the individual duplicate measurements for the eight volunteers, and ${\langle\overline{sd}_{A_{m}}\rangle}$ is the square root of the group average variance from the duplicate measurements of the eight volunteers.

Errors on the individual monoester signals are largest because these signals have on average (see Figure 1) the lowest SNR, overlap partially, and are also influenced by the presence of a disturbing signal from AMP. The sum of monoesters, however, shows only small variability particularly when compared to 1H tChol. Note the reduction in relative errors and standard deviations of relative errors when making use of T2-weighted spectra that combines FID and echo spectra instead of FID spectra alone. Figure 4 shows an example of 31P MR spectra and 1H MR tChol edited spectra for volunteer 2 measured in duplicate. The measured tChol concentrations are shown in Table I.

The average tChol concentration(based on 9 protons per molecule) for the group of volunteers is 0.5mM with a mean relative error of 35% that is calculated analogous to that of the 31P metabolites. Combined with the 31P data and assuming only phosphorylated choline compounds (no free choline) and no mobile phospholipid (MPL) signal in tChol (T2(MPL)<<120 ms), this would amount on average to the following concentrations of the phosphomonoesters and phosphodiesters in healthy fibroglandular tissue: [PE]=0.4 mM; [PC]=0.2 mM; [GPC]=0.2 mM.

Conclusions

In vivo 31P MR spectroscopy offers a better window on cell membrane metabolism and energy metabolism than 1H MR spectroscopy: 31P MRS reproducibility was similar to 1H tChol MRS and gave insight into the 31P metabolite tChol composition. Estimates of phosphomonoester and phosphodiester concentrations in healthy fibroglandular tissue based on a combination of 31P and 1H MR spectroscopy are PE=0.4mM (2 protons); PC=0.2mM (9 protons); GPC=0.2mM (9 protons).

Acknowledgements

No acknowledgement found.

References

1. Klomp DWJ, Bank BL van de, Raaijmakers A, Korteweg MA, Possanzini C, Boer VO, van de Berg CAT, Bosch MAAJ van de, Luijten PR. 31P MRSI and 1H MRS at 7 T: initial results in human breast cancer. NMR Biomed. 2011; 24: 1337–1342.

2. Kemp WJM van der, Boer VO, Luijten PR, Stehouwer BL, Veldhuis WB, Klomp DWJ. Adiabatic Multi-Echo 31P Spectroscopic ImagiNG (AMESING) at 7 tesla for measuring transverse relaxation times and regaining sensitivity in tissues with short T2* values. NMR Biomed. 2013; 26: 1299-1307.

3. Dreher W, Leibfritz D. New method for the simultaneous detection of metabolites and water in localized in vivo 1H nuclear magnetic resonance spectroscopy. Magn Reson Med. 2005; 54:190-195.

4. Smith TA, Glaholm J, Leach MO, Machin L, Collins DJ, Payne GS, McCready VR. A comparison of in vivo and in vitro 31P NMR spectra from human breast tumours: Variations in phospholipid metabolism. Br. J. Cancer. 1991; 63: 514-516.

5. Haddadin IS, McIntosh A, Meisamy S, Corum C, Styczynski Snyder AL, Powell NJ, Nelson MT, Yee D, Garwood M, Bolan PJ. Metabolite quantification and high-field MRS in breast cancer. NMR Biomed. 2009; 22: 65-76.

Figures

Figure 1. Averaged FID and echo 31P MR spectra (n = 16), echo spacing 45 ms, for a group of 8 healthy volunteers scanned twice.

Figure 2. Transverse relaxation times, T2, fitted for various 31P metabolites in fibroglandular tissue derived from the weighted averaged group spectra (1 FID and 5 echoes; n = 16) of eight healthy volunteers measured twice.

Figure 3. Mean relative error in metabolite peak amplitudes as fitted with JMRUI as derived from FID spectra and from T2-weighted spectra of the individual volunteers for 31P MRS and tChol for 1H MRS.

Figure 4. Example of 31P MR spectra and tChol edited 1H spectra for volunteer 2.

Table I. Total choline (tChol) concentrations measured twice in fibroglandular tissue of a group of 8 healthy volunteers. Data are calculated as choline, i.e. assuming that all signal between 3.1 and 3.3 ppm is choline. T1 and T2 values at 7T for water were taken from Haddadin et al.5

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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