Ardenkjær-Larsen, Jan Henrik

Eldirdiri, A.

Hansen, A.E.

Hansen, Rie Beck

Hennig, J.

Hövener, J. - B.

Leupold, J.

Müller, C. A.

Skinner, J. G.

MRI with hyperpolarized tracers has enabled new diagnostic applications, e.g. metabolic imaging in cancer research. However, the acquisition of the transient, hyperpolarized signal with spatial and frequency resolution requires dedicated imaging methods. Here, we compare three promising candidates for 2D MR spectroscopic imaging (MRSI): (i) multi-echo balanced steady-state free precession (me-bSSFP), 1,2 (ii) echo planar spectroscopic imaging (EPSI) sequence and (iii) phase-encoded, pulseacquisition chemical-shift imaging (CSI

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G. ; Eldirdiri, A. ; Leupold, J. ; Hennig, J. ; Ardenkjær-Larsen,Jan Henrik; Hansen, A.E.; Hövener, J. - B.Publication date:2018Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Müller, C. A., Hansen, R. B., Skinner, J. G., Eldirdiri, A., Leupold, J., Hennig, J., ... Hövener, J. . B. (2018).Signal to noise comparison of metabolic imaging methods on a clinical 3T MRI. Abstract from 13th EuropeanMolecular Imaging Meeting, San Sebastián, Spain.Title Signal to noise comparison of metabolic imaging methods on a clinical 3T MRI Authors C. A. Müller1, 2, R. B. Hansen3, J. G. Skinner1, A. Eldirdiri1, J. Leupold1, J. Hennig1, J. H. Ardenkjaer-Larsen3, A. E. Hansen4, J. - B. Hövener5 1 Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Dept.of Radiology, Medical Physics, Freiburg, Baden-Württemberg, Germany 2 German Cancer Research Center (DKFZ), German Cancer Consortium (DKTK), partner site Freiburg, Heidelberg, Baden-Württemberg, Germany 3 Technical University of Denmark, Department of Electrical Engineering, Copenhagen, Denmark 4 Reigshospitalet, University of Copenhagen, Department of Clinical Physiology, Nuclear Medicine & PET, Copenhagen, Denmark 5 Section for Biomedical Imaging, Molecular Imaging North Competence Center (MOINCC), Department of Radiology and Neuroradiology, University medical Center Schleswig-Holstein, University of Kiel, Kiel, Germany Introduction (max 600 character) MRI with hyperpolarized tracers has enabled new diagnostic applications, e.g. metabolic imaging in cancer research. However, the acquisition of the transient, hyperpolarized signal with spatial and frequency resolution requires dedicated imaging methods. Here, we compare three promising candidates for 2D MR spectroscopic imaging (MRSI): (i) multi-echo balanced steady-state free precession (me-bSSFP),1,2 (ii) echo planar spectroscopic imaging (EPSI) sequence and (iii) phase-encoded, pulse-acquisition chemical-shift imaging (CSI). Methods (max 800 character) All sequences were implemented on a clinical 3T PET/MR system (Siemens) equipped with a 1H-13C birdcage coil (Rapid Biomedical). A multi-compartment container filled with Gd-doped (c=0.23%v/v) aqueous model solutions of 1.0 M 13C-bicarbonate (bi), 13C-urea (ur) and [1-13C]-acetate (ac) was used for imaging. For better comparison, field of view (FOV), voxel size and scan time was identical for all methods. Faster methods were repeated until one acquisition of the CSI was completed resulting in 1, 16 or 670 averages for CSI, EPSI or me-bSSFP (Tab. 1). Chemical shift (CS) maps were calculated with dedicated methods for each sequence, signal-to-noise (SNR) ratios were obtained by dividing the mean signal of a region of interest (ROI) by the mean signal of an ROI, where no 13C-signal is expected.     me-bSSFP CSI EPSI Flip angle (°) 30 / 60 90 90 TR (ms) 23.9 1000 1000 Acquired data points in time domain 5 512 128 Dwell time (ms) 1.14 0.2 2 Spectral resolution 175 Hz 10 Hz 8 Hz Spectral bandwidth  709 Hz 5000 Hz 500 Hz Averages (NA) 670 1 16 Single scan duration 380 ms 4 min, 16 s 16 s Total acquisition time  4 min, 16 s FOV (mm3) 100 x 100 x 100 Matrix size 16 x 16 x 1 Voxel size (mm3) 6.3 x 6.3 x 100 Zero-filling (x) 2 Final resolution (mm³) 3.15 x 3.15 x 100 13C-SNR (ac / bi / ur) 146 / 123 / 125 62 / 105 / 35  47 / 66 / 31  Table 1: Acquisition parameters for 13C MRSI. Metabolic maps (for ac: +328 Hz, ur: -252 Hz, bi: -328 Hz) were obtained by using a Dixon based IDEAL3,4 for me-bSSFP and peak amplitude per voxel reconstruction for CSI and EPSI. SNR was calculated for ROIs (ac, bi, ur), manually segmented based on 1H MRI (Figure 1, B.).  Results/Discussion (max 1000 character) Non-localized 13C spectroscopy revealed three peaks for ac (+328 Hz), ur (-252 Hz) and bi (-328 Hz) (Figure 1). In a single 13C MRSI acquisition, CSI provided most SNR. Over the entire scan time, me-bSSFP yielded more SNR (max. 146, NA=670) than CSI (max. 105, NA=1) and EPSI (max. 66, NA=16). The direct comparison of the SNR is possibly inaccurate (due to the different number of averages, signal evolution and the reconstruction algorithms), but a similar trend is expected using HP tracers in vivo.  With respect to the max. signal, me-bSSFP revealed the smallest artifacts (which are not apparent on a linear scale, Figure 1, C.1-3).The logarithmic scale (C.4-6) reveals two kinds of artifacts for all methods: localized signal-leakage (e.g. signal at the position of ur in the bi image), as well as signal banding.  The SNR and image quality provided by me-bSSFP may allow shortening the acquisition time further. Simulation of signal evolution may also help understanding the observed results.  Conclusion (max 600 character) We presented and compared 13C MRSI of thermally polarized model solutions using the methods me-bSSFP, CSI and EPSI. For a given time, imaging volume and resolution, me-bSSFP results in highest SNR at the reconstructed metabolite maps and least artefacts (with lowest flip angle and shortest scan time per average).  These results make me-bSSFP a suitable candidate for 3D MRSI of large FOVs. Metabolite separation and robustness appears to be sufficient in vivo, too, as is indicated by first results of a different study that is ongoing.  References 1. Leupold, J. et al. Fast chemical shift mapping with multiecho balanced SSFP. Magn. Reson. Mater. Phys. Biol. Med. 19, 267–273 (2006). 2. Leupold, J., Månsson, S., Petersson, J. S., Hennig, J. & Wieben, O. Fast multiecho balanced SSFP metabolite mapping of 1H and hyperpolarized 13C compounds. Magn. Reson. Mater. Phys. Biol. Med. 22, 251–256 (2009). 3. Reeder, S. B. et al. Multicoil Dixon chemical species separation with an iterative least-squares estimation method. Magn. Reson. Med. 51, 35–45 (2004). 4. Peterson, P. & Månsson, S. Fat quantification using multiecho sequences with bipolar gradients: Investigation of accuracy and noise performance. Magn. Reson. Med. 71, 219–229 (2014).  Acknowledgement Support by the following programs is acknowledged: 1) the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 642773. 2) DKTK. 3) Emmy Noether Programme of the DFG, award No HO-4604/2-1. Figures (max 2) Table is 1 figure  Figure 1: 13C MR-spectrum (A.), 1H MRI (B.) and 13C MRSI (C., linear 1-3 & logarithmic 4-6 scale) of acetate, bicarbonate and urea using me-bSSFP, CSI and EPSI. Spatial localization, metabolite separation as well as high SNR (white numbers) was achieved with each method, with me-bSSFP giving highest SNR. Note that some signal leaking is apparent in all logarithmic images. Due to small chemical shifts the reconstructed metabolite maps can have artifacts at the corresponding positions. 

Signal to noise comparison of metabolic imaging methods on a clinical 3T MRI

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Signal to noise comparison of metabolic imaging methods on a clinical 3T MRI

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