Homogenous Excitation in Whole Brain CEST: Combination of Snapshot CEST and Multiple Interleaved Mode Saturation
Andrzej Liebert1, Moritz Zaiss2, Rene Gumbrecht3, Patrick Liebig3, Benjamin Schmitt3, Frederik Laun1, Arnd Doerfler4, Michael Uder1, and Armin M. Nagel1

1Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, 2Max Planck Institute for Biological Cybernetics, Tuebingen, Germany, 3Siemens Healthcare GmbH, Erlangen, Germany, 4Department of Neuroradiolohy, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany


To perform Chemical Exchange Saturation Transfer MRI of the whole brain a homogeneous saturation and fast readout are required. To achieve a fast and robust 3D acquisition a spiral-centric-reordered GRE readout was used. In addition, a Multiple Interleaved Mode Saturation scheme was applied to mitigate B1+-inhomogeneity effects of the CEST saturation. Combination of these two methods allows acquiring a homogenous CEST contrast in a volume of approximately 220x220x45mm3 within an acquisition time of 7 min 26s.


A major factor limiting the clinical application of chemical exchange saturation transfer (CEST) imaging is the long acquisition time required for whole brain CEST MRI. Thus, CEST studies are often limited to 2D acquisitions with a small number of acquired slices. Recent improvements in sampling schemes[1, 2] enable a fast and robust readout of whole brain data. However, CEST MRI at ultra-high B0 (e.g. 7 Tesla) requires correction[3-5] or mitigation[6, 7] of the transmit (B1+) inhomogeneity. In this work, we combine Multiple Interleaved Mode Saturation (MIMOSA)[7] technique to mitigate B1+-inhomogeneity, with the spiral-centric-reordered GRE readout[1] to perform whole brain CEST measurements.


Pulse sequences were implemented on a 7-Tesla whole-body MR system (MAGNETOM Terra, Siemens Healthineers, Erlangen, Germany). An 8Tx/32Rx head coil (Nova Medical, Wilmington, USA) was used for the measurements. Phantom measurements were performed with an 18 cm diameter sphere filled with egg-white. In vivo measurements were performed on a healthy female volunteer. The images were acquired using a pulsed CEST spiral-centric-reordered -GRE sequence[1, 8].

For the MIMOSA acquisition, two sets of complimentary transmitter phases with constant amplitudes (‘modes’) were used for the saturation pulse train as described previously[7]. The mode A had a 45° phase difference between two adjacent transmitter coil channels (“Circular Polarization - CP”). In mode B a 90° phase increment between the channels was chosen (also called B2+)[9]. Each mode has a distinct effective relative B1+ value distribution. Through interleaving of the two modes during saturation, (c.f. Figure 1) a more homogenous saturation can be achieved. During the GRE multi-slice acquisition the excitation pulses are employed in the CP mode. CEST images were also acquired using the CP mode during both acquisition and saturation. MTRRex(NOE) contrast was calculated for both standard CEST acquisition and MIMOSA acquisition based on the work by Windschuh et al.[4]. B1+ maps of both CP and B2+ mode were acquired using a multi flip angle (FA) sequence. A two-point B1 correction was applied on the CP mode acquisition as presented by Windschuh et al.[4]. The B1+ distribution of MIMOSA was calculated based on the B1 continuous wave power equivalent (CWPE) as described by Zu et al[10]:

$$ B_{1,CWPE} (r,t)=\sqrt{\frac{1}{(\tau_p+\tau_d)}\int_0^{\tau_p} \! B^2_P(r,t) \, \mathrm{d}t} $$

Sequence parameters: CEST Saturation parameters: τp=46.08ms, τD=30 ms, n=50, Trec=1s, B1,Nominal=0.6µT and 1.0µT in CP mode, Gaussian pulse; GRE acquisition parameters: FA=6°, TR=4.5ms, TE=2.3ms, Matrix Size=128x128x16, FoV=220x220x80 mm3, GRAPPA 3, Elongation Factor=0.6,

Results and Discussion

In comparison to the standard saturation using CP mode, MIMOSA reduces the intra slice variation of the B1+-inhomogeneity in all of the acquired slices (c.f. phantom measurement, Figure 2). An influence of the higher rB1 value of the CP mode can be observed in the central slices of the phantoms B1,CWPE maps. Figure 3 shows that homogeneity of the MTRRex(NOE) contrast is comparable to B1 corrected images in most of the slices with MIMOSA saturation. The fitting routine fails in the outermost slices which might be caused by imperfect slab selection as well as the lower B1+ or B0 homogeneity.

In vivo results show similarities to the phantom measurements. MIMOSA reduces the intra slice variation of the B1+-inhomogeneity in all of the acquired slices (c.f. Figure 4). In cranial slices, similarly to the outermost slices of the phantom, a decrease of the rB1 value in the MIMOSA B1,CWPE maps can be observed.

In Figure 5, MTRRex(NOE) values of a healthy human brain derived both without and with the use of MIMOSA are presented. In CP mode without B1 correction, a correlation between the loss of MTRRex(NOE) magnitude and the B1 map can be observed (c.f. Figure 4A). The application of MIMOSA resulted in a more homogeneous image in central slices of the brain (c.f. Figure 5C), which is comparable to B1 corrected images (c.f. Figure 5B). Cranial slices acquired with MIMOSA show reduced MTRRex(NOE) values and slightly reduced homogeneity in comparison to images acquired in CP mode both with and without B1 correction. This is caused by low CWPE B1+ values in these slices (c.f. Figure 4C).


A combination of Snapshot CEST and MIMOSA was implemented and analyzed. Compared to the standard CP mode, the MIMOSA approach is able to provide a more homogeneous saturation over a volume of approximately 220x220x45mm3. A slight drop of homogeneity could be observed in cranial parts of the brain. Thus, in those parts, additional correction methods are still required but are simpler due to lower deviation of the B1+ value. However, in the central part of the brain, B1+ correction methods might be omitted as MIMOSA yields contrast comparable to B1 corrected images.


No acknowledgement found.


1. Zaiss, M., P. Ehses, and K. Scheffler, Snapshot-CEST: Optimizing spiral-centric-reordered gradient echo acquisition for fast and robust 3D CEST MRI at 9.4 T. NMR Biomed, 2018. 31(4): p. e3879.

2. Akbey, S., et al., Single-shot whole-brain CEST imaging using centric-reordered 3D-EPI. Proc. Intl. Soc. Mag. Reson. Med. , 2018. 26.

3. Khlebnikov, V., et al., On the transmit field inhomogeneity correction of relaxation-compensated amide and NOE CEST effects at 7 T. NMR Biomed, 2017. 30(5).

4. Windschuh, J., et al., Correction of B1-inhomogeneities for relaxation-compensated CEST imaging at 7 T. NMR Biomed, 2015. 28(5): p. 529-37.

5. Schuenke, P., et al., Simultaneous mapping of water shift and B1 (WASABI)-Application to field-Inhomogeneity correction of CEST MRI data. Magn Reson Med, 2016.

6. Tse, D.H., et al., B1+ inhomogeneity mitigation in CEST using parallel transmission. Magn Reson Med, 2017.

7. Liebert, A., et al., Multiple Interleaved Mode Saturation (MIMOSA) for B1+ inhomogeneity mitigation in chemical exchange saturation transfer. Proc. Intl. Soc. Mag. Reson. Med. , 2018. 26.

8. Schmitt, B., et al., Optimization of pulse train presaturation for CEST imaging in clinical scanners. Magn Reson Med, 2011. 65(6): p. 1620-9.

9. Orzada, S., et al., Mitigation of B1(+) inhomogeneity on single-channel transmit systems with TIAMO. Magn Reson Med, 2013. 70(1): p. 290-4.

10. Zu, Z., et al., Optimizing pulsed-chemical exchange saturation transfer imaging sequences. Magn Reson Med, 2011. 66(4): p. 1100-8.


Two mode (A-CP mode and B-B2+ mode) interleaving during the saturation train shown for three first transmitter channels. The GRE image acquisition is performed in CP mode.

Relative B1+ (rB1) maps of a spherical phantom (18 cm diameter, filled with egg-white) acquired in CP mode (A), B2+ mode (B) and the calculated value of B1,CWPE (C) for the MIMOSA technique. In the outermost slices the influence of the lower rB1 of the B2+ mode on the MIMOSA B1,CWPE can be depicted.

MTRRex(NOE) maps of the standard CP mode saturation without B1 correction(A), with B1 correction (B) and with MIMOSA saturation (C) in 16 slices of an egg-white phantom. MIMOSA results in the increase of homogeneity and higher average MTRRex(NOE) signal intensity in comparison to B1 uncorrected CP mode. Homogeneity in B1 corrected images is comparable to the MIMOSA acquisition.

Relative B1+ (rB1) maps in vivo acquired with CP mode (A), B2+ mode (B) and B1,CWPE (C) map of the MIMOSA technique. In the cranial slices of the brain, the average CWPE B1+ MIMOSA is reduced.

MTRRex(NOE) maps of the standard CP mode saturation without B1 correction (A) with B1 correction (B) and with MIMOSA saturation (C) in 16 slices. MIMOSA results in increased intra tissue homogeneity in comparison to no B1 correction. B1 corrected images show comparable homogeneity of the MTRRex contrast to the MIMOSA saturation in central slices.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)