Ardenkjær-Larsen, Jan Henrik

Carvajal, Lucas

Gordon, Jeremy W.

Hansen, Rie Beck

Hanson, Lars G.

Shin, Peter J.

van Criekinge, Mark

Vigneron, Daniel B.

For hyperpolarized 13C MRI acquisitions aimed at metabolic rate constant estimation, the Bloch-Siegert shift enables encoding of the transmit field (B1+-field) amplitude within a single hyperpolarized substrate injection. This ability is needed since most clinical hyperpolarized MRI studies use inhomogeneous transmit coils, and because kinetic modeling based on incorrect flip angles can lead to incorrect rate constant estimations. This study demonstrates the feasibility of integrated B1+ mapping for large volume thermal and hyperpolarized phantoms in a clinical setup using a clamshell transmit coil and a 16-channel receive array, and a 3D stack-of-spirals sequence. Phase-sensitive coil-combination was achieved using ESPIRiT

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Downloaded from orbit.dtu.dk on: Jun 17, 2018Integrated B1+ Mapping for Hyperpolarized 13C MRI in a Clinical Setup using Multi-Channel Receive ArraysHansen, Rie Beck; Shin, Peter J.; Gordon, Jeremy W.; van Criekinge, Mark; Carvajal, Lucas; Hanson,Lars G. ; Ardenkjær-Larsen, Jan Henrik; Vigneron, Daniel B.Publication date:2018Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Hansen, R. B., Shin, P. J., Gordon, J. W., van Criekinge, M., Carvajal, L., Hanson, L. G., ... Vigneron, D. B.(2018). Integrated B1+ Mapping for Hyperpolarized 13C MRI in a Clinical Setup using Multi-Channel ReceiveArrays. Abstract from Joint Annual Meeting ISMRM-ESMRMB 2018, Paris, France.6/11/2018 indexsmart.mirasmart.com/ISMRM2018/PDFﬁles/3698.htmlhttp://indexsmart.mirasmart.com/ISMRM2018/PDFﬁles/3698.html 1/33698Integrated B1+ Mapping for Hyperpolarized 13C MRI in a Clinical Setup using Multi-ChannelReceive ArraysRie B Hansen , Peter J Shin , Jeremy W Gordon , Mark Van Criekinge , Lucas Carvajal , Lars G Hanson , Jan H Ardenkjær-Larsen , and Daniel B Vigneron Department of Electrical Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark, Department of Radiology and Biomedical Imaging, UCSF,San Francisco, CA, United StatesSynopsisFor hyperpolarized C MRI acquisitions aimed at metabolic rate constant estimation, the Bloch-Siegert shift enables encoding of the transmit  eld(B - eld) amplitude within a single hyperpolarized substrate injection. This ability is needed since most clinical hyperpolarized MRI studies useinhomogeneous transmit coils, and because kinetic modeling based on incorrect  ip angles can lead to incorrect rate constant estimations. Thisstudy demonstrates the feasibility of integrated B  mapping for large volume thermal and hyperpolarized phantoms in a clinical setup using aclamshell transmit coil and a 16-channel receive array, and a 3D stack-of-spirals sequence. Phase-sensitive coil-combination was achieved usingESPIRiT.IntroductionBy means of the Bloch-Siegert shift, the transmit  eld (B ) amplitude can be encoded into the signal phase during MRI acquisition.  Forhyperpolarized carbon-13 MRI, this can provide data that enable B - eld mapping together with spatial mapping of metabolic rate constantswithin one hyperpolarized substrate injection.  Integrated B  mapping is motivated by an extensive use of inhomogeneous clamshell-type transmitcoils  in clinical hyperpolarized MRI applications,  together with reduced quantitation accuracy of rate constants when modeling with incorrect  ipangles.  While proof-of-principle integration of Bloch-Siegert B  mapping in a hyperpolarized study has been shown,  this study focuses on aclinical setup using multi-channel arrays that enable parallel imaging for acquisition acceleration. However, for phase-based B  mapping methods,phase-sensitive coil-combination is a challenge. By using ESPIRiT  for coil-combination, SNR-levels comparable to using sum-of-squares can beachieved, while preserving phase information.MethodsBloch-Siegert B  mapping was integrated in a 3D stack-of-spirals sequence with singleband spectral-spatial (SPSP) excitation. By alternatingbetween positive and negative o -resonance Bloch-Siegert pulses through the full dynamic acquisition, B  mapping is possible across multipletime points. This makes the method robust against spatially varying signal support over time; expected in vivo.All data were acquired on an MR750 3T scanner (GE Healthcare, Waukesha, WI, USA) with a C clamshell coil for excitation and a 16-channel coil forreception (Rapid Biomedical, Rimpar, Germany). See coil setup in Figure 1.Multi-compartment thermal phantom: Image data were acquired for a 32x32x12 cm  FOV with a 30-ms single-shot-spiral readout in the axial planeand 10 phase-encodes (superior/inferior). Two measurements were conducted with 4-ms Bloch-Siegert pulses applied +/-2 kHz o -resonance.Other scan parameters: TR 1 s;  ip angle 70°; NEX 32; SPSP frequencies four (Ethylene Glycol, Urea, Alanine, Acetate (∆f = [0,3177,3598,3803] Hz)).The spirals were gridded to a 54x54 matrix and  ltered to result in an isotropic resolution of 1.2 cm. The data were coil-combined using ESPIRiT foreach 3D volume, across frequencies and time points, before estimating B  maps as previously described.Hyperpolarized [1- C]pyruvate phantom: 1.47 g of [1- C]pyruvic acid doped with 15 mM electron paramagnetic agent was polarized for 3 hours ina 5T Spinlab at 0.78 K. The sample was rapidly dissolved and neutralized yielding 42 mL pyruvate (259 mM, 44 % polarization). This was added to asphere containing ~2.5 L water. Imaging started 43 s after injection. The sequence was run three times with 16 time points for each run and a 20-sand 10-s delay between runs. Other scan parameters: TR 200 ms;  ip angle 5°. FOV, spiral, Bloch-Siegert pulse, and reconstruction method wereidentical to those for the thermal phantom experiment. Due to long relaxation times and the short TR, the measured signal depends critically on T ,T , B  and B , so  tting of the measured signal to a signal equation was not attempted. This could have provided validation for the B  maps.Results and DiscussionFigure 2(b-c) illustrate how coil-combination by ESPIRiT results in similar or higher SNR levels compared to sum-of-squares reconstruction. Figure 3shows the B  mapping results together with a simulated B - eld for the clamshell transmitter. All maps show the same pattern. The slightlysmaller inhomogeneity of the simulated map compared to the Bloch-Siegert maps is likely explained by a geometry symmetry assumption for thesimulation, while the B   eld of the clamshell coil has experimentally been shown to be asymmetric as visible in Figure 3(a).Figure 4 shows 24 B  maps extracted from the 24 pairs of consecutive time points of the hyperpolarized experiment. Ideally these would beidentical, but as SNR decreases so does reliability of B  mapping. In the higher SNR region,  ip angles were estimated consistently up to 100 s afteracquisition start (70 s active scan time), while in low SNR regions estimates became less consistent after 40 s active scan time. A minimum SNR of~40 is needed for accurate B  estimation. Figure 5 shows scatter plots for the voxel-wise relative  ip angle values across experiments. A higherlinear correlation was observed for high relative  ip angle voxels compared to voxels with low relative  ip angle; as these voxels also had low SNR,this is not unexpected.ConclusionThis study demonstrated feasibility of integrated B  mapping for a clinical hyperpolarized C coil setup, by means of the Bloch-Siegert shift andESPIRiT for coil-combination. Future studies should optimize the Bloch-Siegert pulse design to avoid SAR issues, test integration of B  mapping inother sequences, validate the results in vivo, and test integrated B  mapping with parallel imaging to fully utilize multi-channel receive coils.Acknowledgements1 2 2 2 2 11 21 2131+1+1+ 11+21+3 4–671+ 21+81+1+1331+ 113 1312 1 0 1+1+1+1+1+1+1+1+ 131+1+6/11/2018 indexsmart.mirasmart.com/ISMRM2018/PDFﬁles/3698.htmlhttp://indexsmart.mirasmart.com/ISMRM2018/PDFﬁles/3698.html 2/3This work has been partly funded by the Independent Research Fund Denmark (DFF – 4005-00531), the Danish National Research Foundation(DNRF124), and the Elite Research travel grant (6161-00043B).References1. Sacolick, L. I., Wiesinger, F., Hancu, I. & Vogel, M. W. B1 mapping by Bloch-Siegert shift. Magn. Reson. Med. 63, 1315–1322 (2010).2. Lau, A. Z., Chen, A. P. & Cunningham, C. H. Integrated Bloch-Siegert B1 mapping and multislice imaging of hyperpolarized 13C pyruvate andbicarbonate in the heart. Magn. Reson. Med. 67, 62–71 (2012).3. Tropp, J. et al. Multi-channel metabolic imaging, with SENSE reconstruction, of hyperpolarized [1-13C] pyruvate in a live rat at 3.0 tesla on a clinicalMR scanner. J. Magn. Reson. 208, 171–177 (2011).4. Cunningham, C. H. et al. Hyperpolarized 13C Metabolic MRI of the Human Heart: Initial Experience. Circ. Res. 119, 1177–1182 (2016).5. Park, I. et al. Dynamic Hyperpolarized 13C Metabolic Imaging of Patients with Brain Tumors. in Proc. Intl. Soc. Mag. Reson. Med. 555 (2017).6. Gordon, J. W. et al. Human Hyperpolarized C-13 MRI Using a Novel Echo-Planar Imaging (EPI) Approach. in Proc. Intl. Soc. Mag. Reson. Med. 728(2017).7. Chen, H.-Y. et al. Phase II Clinical Hyperpolarized 13C 3D-Dynamic Metabolic Imaging of Prostate Cancer using a B1-insensitive Variable Flip AngleDesign. in Proc. Intl. Soc. Mag. Reson. Med. 725 (2017).8. Uecker, M. et al. ESPIRiT - An eigenvalue approach to autocalibrating parallel MRI: Where SENSE meets GRAPPA. Magn. Reson. Med. 71, 990–1001(2014).FiguresFigure 1: Coil setup for (a) the thermally polarized phantom experiment, and (b) the hyperpolarized phantom experiment. The pictures show theclamshell coil and the top paddle of the 16-channel receive array. The bottom paddle is under the linen. The phantom in (a) is a 20-cm long cylinderwith 24 cm inner diameter and eight cylindrical compartments. Six of these are  lled with 1.5 M [1- C]-L-Alanine, 2.0 M C-Urea, and 2.0 M Sodium[1- C]Acetate, respectively, the last two are empty. The large compartment contains natural abundance Ethylene Glycol. The phantom in (b) is asphere with 18 cm diameter. Figure 2: (a) Proton reference and magnitude image results from the thermal phantom experiment (summed over slices and over time). The colorson the reference image indicate the compartment contents, i.e. red for [1- C]-L-Alanine, blue for C-Urea, and yellow for Sodium [1- C]Acetate.(b-c) Individually scaled magnitude images from the hyperpolarized phantom experiment for the center slice (8x6 time points shown left to right,row-by-row). The images result from coil-combination using (b) sum-of-squares reconstruction and (c) ESPIRiT. Figure 3: B  mapping results for the thermal phantom experiment (a-b), and for the hyperpolarized phantom experiment (d). (a) shows the mapestimated from the Ethylene Glycol images, while (b) shows the combined maps from the Alanine, Urea and Acetate images. (c) shows thesimulated B - eld variation. All maps shown are based on the center-slice. Notice that because of normal spatial point spread, the estimated Bmaps for the thermal phantom in (a-b) exceed the compartment borders in high SNR regions. This explains why the outer signal voids for theEthylene Glycol compartment in (a) are hardly noticeable. 13 131313 13 131+1+1+6/11/2018 indexsmart.mirasmart.com/ISMRM2018/PDFﬁles/3698.htmlhttp://indexsmart.mirasmart.com/ISMRM2018/PDFﬁles/3698.html 3/3Figure 4: B  mapping results for the center-slice across the full dynamic acquisition of the hyperpolarized phantom. Peak SNR values wereestimated based on the maximum absolute signal divided by the standard deviation of the real part of the noise. The  rst map is based on twoimages acquired after 4 s, while the last map is based on two images acquired after 126 s. The bottom graph shows estimated relative  ip anglevalues for two di erent voxels, a high and a low SNR voxel, across time points. Figure 5: Scatter plots of voxel-wise estimations of relative  ip angle across experiments. The dashed lines indicate a linear correlation of 1. Ala =Alanine, Ace = Acetate. Coil loading across experiments is assumed to be similar. 1+Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)3698

Integrated B1+ Mapping for Hyperpolarized 13C MRI in a Clinical Setup using Multi-Channel Receive Arrays

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Integrated B1+ Mapping for Hyperpolarized 13C MRI in a Clinical Setup using Multi-Channel Receive Arrays

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