Michael Hock1, Maxim Terekhov1, Markus Johannes Ankenbrand1, David Lohr1, Theresa Reiter1,2, Christoph Juchem3, and Laura Maria Schreiber1
1Chair of Cellular and Molecular Imaging, Comprehensive Heart Failure Center (CHFC), University Hospital Wuerzburg, Wuerzburg, Germany, 2Department of Internal Medicine I, University Hospital Wuerzburg, Wuerzburg, Germany, 3Departments of Biomedical Engineering and Radiology, Columbia University, New York City, NY, United States
Synopsis
Spatio-temporal inhomogeneities of the static
magnetic (B0) field are a major limiting factor in cardiac magnetic
resonance applications at 7T. A previously developed shim strategy was demonstrated
to correct spatial myocardial B0-field inhomogeneities in a
preliminary in vivo implementation. To correct localized spots of B0-inhomogeneities,
third-order terms were found to be beneficial. Cardiac phase-specific shimming
was evaluated in simulations based on the in vivo field map data, and optimal shim
settings were shown to differ between cardiac phases. Future work will address
the application of a shim averaged over all cardiac phases to each individual
phase.
Introduction
Susceptibility differences of the myocardial
tissue and surrounding lungs induce inhomogeneities of the static magnetic (B0)
field, which lead to signal loss and image distortions in human cardiac magnetic
resonance (CMR) at 7T1. These B0-inhomogeneities also
vary temporally over different cardiac phases2 and breath-hold
positions3. However, a homogeneous B0-field within the
heart is required for a number of applications including T2*-mapping
to analyze microstructure4, steady-state free precision CINE imaging
to assess cardiac function5, or fast imaging techniques such as
spiral readout trajectories6 and echo-planar imaging7. Very
importantly at 7T, the peak amplitudes of radiofrequency pulses are optimized
for a target range of B0-field variations8.
To correct, i.e. shim the cardiac B0-inhomogeneities, a dedicated strategy
was recently developed9. This strategy consists of the triggered
field map acquisition, the anatomy-matched shim-region-of-interest (SROI)
selection, and the calibration-based spherical harmonics (SH) B0-field
modeling in dedicated software10. Pilot results from the in vivo
implementation demonstrated the capability to shim large myocardial B0-inhomogeneities11.
In this work, we further evaluate the role of third-order shim terms for CMR at
7T as well as the potential benefit of extending this static B0-shim
to cardiac phase-specific shimming (CPSS)12,13, since higher-order dynamic
CPSS is not a standard vendor-supplied tool.Methods
All field map data was acquired with a 1TX/16RX
thorax coil (MRI Tools) on a 7T whole-body MR system (MAGNETOMTM
Terra, Siemens Healthineers). The scanner is equipped with full second- and
partial third-order SH shims Z3, Z2X, Z2Y and Z(X2-Y2). Cardiac triggering was
based on acoustic (ACT, MRI Tools) and ECG monitoring of the heartbeat. Three
mid-ventricular transversal slices were acquired for six healthy volunteers (average
age 24 years, average weight 75 kg) after approval by the local ethics
committee (7/17-sc). Parameters for the multi-echo GRE pulse sequence were TE=1.69/2.83/3.96/6.07/9.72
ms, matrix size 128×128, FOV=300×300 mm2, TR=400 ms, slice
thickness=6 mm. B0-maps were reconstructed voxel-wise by the phase
method in B0DETOX software10, which was extended by spatial
phase-unwrapping (FSL library)14. Anatomy-matched SROI selections and
calibration-based B0-field modeling were carried out within the shim
software. Further calculation of standard deviations (SD) and interquartile
ranges (IQR) of the B0 distributions was performed in MATLAB R2015b
(MathWorks Inc, USA).Results
The B0-field within the complete left
ventricle (LV) showed a largely homogeneous pattern (Fig. 1). However, extreme spatial
B0-inhomogeneities were found within the myocardium such as the
anterolateral region (Fig.1C,D). Localized spots of B0-inhomogeneities
showed a temporal variation, which is visible from the same slice acquired at a
different cardiac phase (Fig. 1E,F). For the left-ventricular myocardium, this variation
accounted for a ΔSD of 10±5 Hz and ΔIQR of 19±12 Hz between
systolic and diastolic phases (mean ± standard deviation) for the six
subjects. First in vivo shimming in a healthy subject demonstrated the
capability of the developed shim strategy to correct large myocardial B0-inhomogeneities
at a delay of 400 ms post R wave (Fig. 2). Simulation
results (Fig. 2B) matched
well with the residual B0-field
remaining after the experimental third-order shim (Fig. 2C). B0-field
homogeneity was improved within the anterolateral segment of the LV
and the lateral segment of the right ventricle (Figure 2, arrows). Smaller
localized B0-inhomogeneities remained within the myocardium. The
role of higher-order SH terms was evaluated for the scanner-integrated third-order
terms (Fig. 3). A comparison between second-order only and second- plus
third-order shims demonstrated that, though the reduction of SD for
the overall heart was small, the third-order terms were in particular beneficial
to correct the myocardial B0-inhomogeneities in the anterolateral
segment of the LV. However, residuals of the third-order terms
remained due to limited hardware dynamic ranges of 0.68 Hz/cm3 for Z3, 0.35 Hz/cm3
for Z2X, 0.34 Hz/cm3 for Z2Y, and 0.10 Hz/cm3 for Z(X2-Y2).
The second-order terms did not exceed the hardware limitations for any shim. To
address the temporal B0-field variations, a shim
was computed for eight cardiac phases (Fig. 4). B0-field homogeneity was
improved by the simulated CPSS by 29±6 Hz for all phases. The corresponding currents varied from phase to phase (Fig. 5). Z,
which had a dynamic range of 2129 Hz/cm, was on average employed on 1.82±0.77%
with a peak-to-peak range of 1.91%. The third-order terms were used an average
on -96±48% for Z3, 96±30% for Z2X, and 18±25% for Z(X2-Y2).The corresponding
peak-to-peak ranges were 127%, 72%, and 58%, respectively.Discussion
Extreme spatial B0-inhomogeneities
within the heart were located in localized spots within the
myocardium, and showed a temporal variation over the cardiac cycle. A
calibration-based third-order shim strategy for an anatomy-matched SROI was
developed and initial in vivo implementation demonstrated its feasibilityfor shimming at individual cardiac
phases. The scanner-integrated third-order terms were advantageous to obtain a
homogeneous B0-field. Simulations of CPSS for one volunteer resulted in not negligible
shim current variations. Analysis of other subjects will allow a
more accurate determination of the required shim amplifiers, which are not
integrated in the MR system up-to-date. Future work will address the application of a shim
averaged over all cardiac phases to each individual phase.Conclusion
Detrimental B0-inhomogeneities at
myocardial MRI at 7T can be reduced with a dedicated third-order, ECG-gated
shim strategy using calibration-based SH B0-field modeling. Computed
cardiac phase-specific shims differed over the cardiac cycle.Acknowledgements
Financial support was obtained from the German
Ministry of Education and Research (BMBF) under grant #01EO1504.References
[1] Snyder CJ,
DelaBarre L, Metzger GJ, Van de Moortele PF, Akgun C, Ugurbil K, Vaughan JT. Initial
results of cardiac imaging at 7 Tesla. Magn Reson Med 2009;61:517-524.
[2] Jaffer FA,
Wen H, Balaban RS, Wolff SD. A method to improve the B0 homogeneity
of the heart in vivo. Magn
Reson Med 1996;36:375-383.
[3]
Schmitter S, Wu X, Ugurbil K, Van de Moortele PF. Design of Parallel Transmission Radiofrequency Pulses
Robust Against Respiration in Cardiac MRI at 7 Tesla. Magn Reson Med
2015;74:1291-1305.
[4] Huelnhagen
T, Hezel F, Serradas Duarte T, Pohlmann A, Oezerdem C, Flemming B, Seeliger E,
Prothmann M, Schulz-Menger J, Niendorf T. Myocardial effective transverse
relaxation time T2* correlates with left ventricular wall
thickness: A 7.0 T MRI study. Magn Reson Med 2017;77:2381 – 2389.
[5] Schär M,
Kozerke S, Fischer SE, Boesiger P. Cardiac SSFP imaging at 3 Tesla. Magn Reson
Med 2004;51:799-806.
[6] Nayak KS,
Cunningham CH, Santos JM, Pauly JM. Real-Time Cardiac MRI at 3 Tesla. Magn
Reson Med 2004;51:655-660.
[7] Ding S,
Wolff SD, Epstein FH. Improved Coverage in Dynamic Contrast-Enhanced Cardiac
MRI Using Interleaved Gradient-Echo EPI. Magn Reson Med 1998;39:514-519.
[8] Tao Y,
Hess AT, Keith GA, Rodgers CT, Liu A, Francis JM, Neubauer S, Robson MD.
Optimized saturation pulse train for human first-pass myocardial perfusion
imaging at 7T. Magn
Reson Med 2015;73:1450-1456.
[9] Hock M, Stefanescu MR, Terekhov M, Lohr D,
Herz S, Juchem C, Schreiber LM. Third-Order
Cardiac B0-Shimming at 7 T in Humans. ISMRM Workshop on Ultrahigh
Field Magnetic Resonance: Technological Advances, Translational Research Promises
& Clinical Applications, Dubrovnik, 2019:3.
[10] Juchem C.
B0DETOX – B0 Detoxification Software for Magnetic Field Shimming.
Innovation.columbia.edu/technologies/cu17326_b0detox (2017). Columbia TechVenture
(CTV), license CU17326.
[11] Hock M, Terekhov M, Reiter T, Lohr D, Juchem
C, Schreiber LM. Correction of myocardial B0-inhomogeneities
at 7 T with ECG-gated spherical harmonics shimming. 2019 Minnesota Workshop on
High and Ultra-high Field Imaging, Minnesota, MN, USA, 2019.
[12] Kubach MR,
Bornstedt A, Hombach V, Merkle N, Schär M, Spiess J, Nienhaus GU, Rasche V.
Cardiac phase-specific shimming (CPSS) for SSFP MR cine imaging at 3 T. Phys
Med Biol 2009;54:N467-N478.
[13] Mattar W, Juchem C, Terekhov M, Schreiber
LM. Multi-coil B0 shimming of the human heart:
a theoretical assessment. In: Proceedings of the 24th Annual Meeting of ISMRM,
Singapore, 2016:1151.
[14] Smith SM,
Jenkinson M, Woolrich MW, Beckman CF, Behrens TEJ, Johansen-Berg H, Bannister
PR, De Luca M, Drobnjak I, Flitney DE, Niazy R, Saunders J, Vickers J, Zhang Y,
De Stefano N, Brady JM, Matthews PM. Advances in functional and structural MR
image analysis and implementation as FSL. NeuroImage 2004;23:S208-S219.