31P MR Imaging and Concentration Measurements
Arthur Coste1, Alexandre Vignaud1, Philippe Ciuciu1,2, Fawzi Boumezbeur1, Franck Mauconduit3, Alexis Amadon1, Sandro Romanzetti4, Denis Le Bihan1, and C├ęcile Lerman1

1MR Imaging and Spectroscopy Unit, NeuroSpin, Gif sur Yvette, France, 2Parietal, INRIA Saclay, Saclay, France, 3Siemens Healthcare, Saint Denis, France, 4University Clinic RWTH Aachen, Aachen, Germany

Synopsis

This work describes a novel method accounting for MR acquisition properties (B0, B1+,B1-,T1,T2*) to perform concentration measurements in the framework of in vitro Phosphorus MRI. Our pipeline uses a non-Cartesian 3D sequence for efficient signal sampling and a wavelet regularized least square method for reconstruction. We demonstrated within an acceptable time for human experiment, for 5mm isotropic resolution, that we are able to calibrate and measure absolute 31P concentrations.

Introduction

Phosphorus MR Spectroscopy provides quantitative information about energy metabolism[1, 2]. At ultra-high magnetic fields (UHF), 31P-MR imaging of specific metabolites like phosphocreatine (PCr) is a possible alternative to lengthy MRSI approaches. In this study, we adapted an existing pipeline used for 1H MRI[3] to X-nuclei. At UHF we need to account for B0 inhomogeneity, coil excitation field (B1+) and sensitivity (B1-) and finally the intrinsic properties of studied nucleus (longitudinal T1 and transversal T2* relaxation rates).

Purpose

This work describes a novel method accounting for previously mentioned properties and leading to accurate absolute Phosphorus concentration calibration, manageable within acceptable time for Human investigations.

Material and Methods

Our complete quantification pipeline is depicted in Figure 1.

Measurements were performed on a 7T Magnetom MR scanner (Siemens Healthcare, Erlangen, Germany) with a double-tuned (1H/31P) mono-channel transmission coil, 8-channels phased array head coil (Resonance Research Inc. Billerica, USA)[4].

Prior to all acquisitions B0 shimming was performed using the proton channel.

The field for the 31P coil was computed using the XFL sequence[5]. Multiple runs of Twisted Projection Imaging[6] (TPI) sequences were acquired with 5 different TE ranging from 4.5 to 40ms. Estimation of a B0 map is done with phase difference of the two first images with TE=4.5 and 10ms[7]. T2* was computed by fitting a mono-exponential decay based on all magnitude images.

Unless otherwise stated, TPI parameters were TR/TE = 100/0.5ms, FA=10°, isotropic FOV of 320mm and a 5mm isotropic resolution, linear portion p=0.75 and 7780 projections. TPI Images have been reconstructed by minimizing a wavelet-based least square criterion using FISTA algorithm[8,9].T1 was determined using the Variable Flip Angle Method[10] with 2 TPI acquisitions of FA of 5° and 20°. Combining VFA acquisitions and previously determined parameters, we computed T1 and M0 by linear regression and obtained the spin density. The coil sensitivity profile has been derived from previously acquired TPI image with the lower FA (5°) by applying the low pass filter method[11] to model its slow spatial variation. Our goal is to measure the concentration of a sphere filled with Phosphate Buffered Saline (PBS) solution at 40mmol/L. Tubes containing either pure water or diluted Phosphorus at known concentration surrounded the sphere. Their spatial arrangement is illustrated in Figure 2.

Results

Intermediate and final results for our 31P Phosphorus quantification protocol are reported in Figure 3.

Panels (a) and (b) show a transversal slice of the 3D TPI images with flip angles of 5° and 20°, respectively. Panel (c) shows the map acquired with the XFL sequence and then interpolated. We can clearly notice that the targeted FA is not homogeneously reached everywhere which creates some inaccuracies. Panel (d) presents the B1- map computed from low order polynomial interpolation inside the spherical compartment of image (a) and assuming constant sensitivity elsewhere. Panel (e) illustrates the T1 map with estimated mean value of 5.5s. T2* was estimated at about 10ms in average. Panel (f) represents the magnetization map and panel (g) depicts the corrected spin density of our phantom.

Absolute concentration was obtained by means of a calibration step performed by measuring the spin density in each compartment (panel (g) presents the used ROI). Such setup allows estimating a 36.6mmol/L concentration in the sphere to be compared with a theoretical concentration of 40mmol/L. Figure (h) illustrates the estimated concentration distribution in our phantom based on calibration equation of Figure 4.

Discussion and Conclusion

In this study we demonstrate the ability to perform accurate concentrations measurements of in vitro 31P by taking all properties of UHF acquisition in account.

Concentrations used in this study are in agreement with in vivo concentration of nuclei such as Sodium for instance. We achieved a satisfying isotropic resolution of 5mm with a total acquisition protocol lasting about 1.5 hour. More information about the reconstruction pipeline for TPI images is given in other submitted work[9]. Moreover, using Multiple Echo Sequence might allow acquiring all echoes at the same time providing a 2-fold acceleration. In this study, perfect spoiling was assumed leading to a slightly overestimated T1 value[12]. Comparison with reference methods for both T1[13] and T2*[14] estimation will be performed. The computed B0 map has not yet been taken into account as the result is not satisfying enough due to high TE values. Nevertheless, the initial B0 shimming step ensures relatively good homogeneity. In the future, we plan to apply this pipeline for in vivo quantification of Phosphorylated metabolites and Sodium in the human brain.

Acknowledgements

No acknowledgement found.

References

[1] XH. Zhu, NeuroImage 60:2107-2117, 2012

[2] A. Lu et al., MRM 69:538-544, 2013

[3] A. Lecocq et al., Magn Reson Mater Phy 28:87-100, 2014

[4] N. Avidievich, Appl Magn Reson. 41(2-4): 483-506, 2011

[5] A. Amadon et al., Proc. Intl Soc Mag Res in Med 20, 2012.

[6] F. E. Boada et al., MRM 37, 706-715, 1997

[7] E. A. Schneider et al., MRM 18:335-347, 1991

[8] A. Beck et al., FISTA J Imaging Sciences 2(1):183-202, 2009

[9] A. Coste et al., submitted ISMRM 2016

[10] H. M. Cheng et al., MRM 55: 566–574, 2006

[11] J. Wang et al., MRM 53:666 – 674, 2005

[12] C. Preibisch et al., MRM 61 (1): 125-135, 2009

[13] S.A. Hurley et al., MRM 68:54-64, 2012

[14] D. Hernando et al., MRM 68:830-840, 2012

Figures

Figure 1 : Illustration of the proposed Quantification pipeline with associated acquisition time. The computed B0 map has not yet been taken into account.

Figure 2: Phantom used for our measurements with 31P concentration indicated in mmol/L. White tubes only contain water.

Figure 3: Results: (a) TPI 5°, (b) TPI 20°, (c) relative B1+ map, (d) relative B1- map, (e) T1 map, (f) Magnetization map, (g) Spin Density map, (h) 31P Concentration Distribution

Figure 4: Calibration results showing spin density according to concentration (blue diamonds). Red square illustrates concentration measurement in spherical compartment. Uncertainty on spin density is the standard deviation of each measure. For PBS concentration, we considered a maximum uncertainty of 10% due to the dilution of the 100mmol/L mother solution.




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