Nathan Tibbitts Roberts^{1,2}, Diego Hernando^{1,3}, Timothy J Colgan^{1}, Louis A Hinshaw^{1}, Dylan M Kernan^{1}, and Scott B Reeder^{1,3,4,5,6}

Inhomogeneities in the static (B0) and transmitted (B1) magnetic fields can lead to artifacts and image degradation for a large variety of imaging applications. Quantitative MRI applications that fail to account for B0 and B1 inhomogeneities may suffer from substantial errors. Understanding the range of expected B0 and B1 inhomogeneities experienced in vivo is essential to engineer solutions aimed at avoiding or correcting for these effects. In this work, we measure the B0 and B1 inhomogeneities in the liver of 60 and 312 patients, respectively, at both 1.5T and 3.0T.

Rapid 3D spoiled gradient recalled echo (SGRE) acquisitions are commonly used for imaging in the liver (1,2). However, SGRE acquisitions are sensitive to B0 and B1 inhomogeneities (3-5).

B0 inhomogeneities in the static magnetic field and are known to diminish the accuracy of quantitative parameter mapping. B0 inhomogeneities can lead to substantial image distortion in diffusion weighted echo planar imaging (6), as well as fat-water swaps with chemical-shift encoded MRI (CSE-MRI).

B1 inhomogeneities in the local transmitted magnetic field are known to degrade image quality and cause spatially varying errors in the transmitted flip angle (7-9). Flip angle errors can lead to significant inaccuracies for quantitative T1 mapping applications, including inversion recovery and variable flip angle acquisitions (4,10-17).

In order to engineer robust solutions that work
broadly in a large number of patients, it is essential to know the scope of B0
and B1 inhomogeneities experienced in vivo. Therefore, **the purpose of this work** is to determine the magnitude
and variability of B0 and B1 inhomogeneities in the liver at both
1.5T and 3.0T, in large cohorts of patients.

Acquisition

In two separate cohorts of patients, B0 and B1 maps were acquired for quality assurance purposes as part of routine clinical MRI exams. Data were analyzed retrospectively in a HIPAA compliant manner after approval from the local IRB.

3D B0 maps were acquired using a commercially available
version of a quantitative CSE-MRI method (IDEAL IQ, GE Healthcare, Waukesha, WI)
(18)
on a variety of 1.5T and 3.0T clinical MRI systems (Table 1) with the following
parameters: 42x42cm^{2} FOV, 8mm slice thickness, 32 slices, 3^{o}
prescribed flip angle, 128x128 matrix size, ±83.33kHz receiver bandwidth, with 6 echoes. At 1.5T, all
echoes (TE_{1}=0.9ms, ΔTE=1.48ms)
were acquired in a single TR. At 3.0T, echoes (TE_{1}=0.8ms, ΔTE=1.65ms) were acquired in 2 shots.
Images were acquired in a single 17s breath-hold.

2D interleaved B1 maps were acquired using a commercially
available version of the Bloch-Siegert method (GE Healthcare, Waukesha, WI) (10)
on a variety of 1.5T and 3.0T clinical MRI systems (Table 1) with the following
parameters: 44x44cm^{2} FOV, 10/10mm slice thickness/gap, 15^{o}
prescribed flip angle, 64x64 matrix size, ±15.63kHz receiver bandwidth, using an 8ms Fermi excitation pulse.
Images were acquired in two 13s breath-holds.

All RF transmission was performed using standard transmit/receive system body coil and standard B0 and B1 shimming routines were used in the auto-prescan functionality.

Analysis

B0 fieldmap values (denoted ψ [Hz]) are related to relative changes
in the static magnetic field (denoted ΔB0 [T]) by the Larmor equation ψ= γΔB0/2π,
where γ/2π is the gyromagnetic ratio of ^{1}H (42.58MHz/T).

For B1
inhomogeneities, the transmitted flip angle (α_{T}) is related to
prescribed flip angle (α_{P}) by the equation α_{T}=βα_{P},
where β is defined as the B1 calibration
coefficient. The acquired B1 maps were normalized by α_{P} to provide estimates
of β.

Fieldmap (B0) values and β (B1) values were measured in regions-of-interest (ROIs) in all nine Couinaud segments of the liver (19) in acquired fieldmaps and β maps, respectively.

**Results**

Acquisition

Collectively, 372 B0 and B1 maps were acquired on 15 MR systems (See Table 1). Examples of acquired images and maps are shown in Figure 1 (B0 maps) and Figure 2 (B1 maps).

Analysis

Figure 3 plots the median and quartile statistics of fieldmap (B0) values per segment across all patients.

Figure 4 plots the median and quartile statistics of β (B1) per segment across all patients.

B0 and B1 inhomogeneity can impact many MR applications in the liver including quantitative MRI methods. In this work we successfully characterized the magnitude and variability of both B0 and B1 inhomogeneity in the liver at 1.5T and 3.0T, in 372 patients.

As expected, B0 and B1 inhomogeneities were shown to exhibit higher magnitude and variability in the liver at 3.0T compared with 1.5T. Further, we observed that more severe B1 inhomogeneities were experienced in the lateral segment of the left lobe of the liver (segments II and III), where shading artifacts related to B1 inhomogeneities are most commonly observed (20,21).

Understanding the range of expected B0 and B1 inhomogeneities experienced in vivo is essential prior to developing engineering solutions aimed at avoiding or correcting for these confounders. Although MR systems were limited to a single vendor, this work provides data from a large cohort of patients, which can be reliably used to guide future MR application development for liver imaging.

1. Pandharipande PV, Krinsky GA, Rusinek H, Lee VS. Perfusion imaging of the liver: current challenges and future goals. Radiology 2005;234(3):661-673.

2. Liu CY, McKenzie CA, Yu H, Brittain JH, Reeder SB. Fat quantification with IDEAL gradient echo imaging: correction of bias from T(1) and noise. Magn Reson Med 2007;58(2):354-364.

3. Hernando D, Vigen KK, Shimakawa A, Reeder SB. R*(2) mapping in the presence of macroscopic B(0) field variations. Magn Reson Med 2012;68(3):830-840.

4. Deoni SC. Correction of main and transmit magnetic field (B0 and B1) inhomogeneity effects in multicomponent-driven equilibrium single-pulse observation of T1 and T2. Magn Reson Med 2011;65(4):1021-1035.

5. Cheng H-LM, Wright GA. Rapid high-resolution T1 mapping by variable flip angles: Accurate and precise measurements in the presence of radiofrequency field inhomogeneity. Magnetic Resonance in Medicine 2006;55(3):566-574.

6. Reese TG, Heid O, Weisskoff RM, Wedeen VJ. Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo. Magn Reson Med 2003;49(1):177-182.

7. Ibrahim TS, Lee R, Abduljalil AM, Baertlein BA, Robitaille PM. Dielectric resonances and B(1) field inhomogeneity in UHFMRI: computational analysis and experimental findings. Magn Reson Imaging 2001;19(2):219-226.

8. Yang QX, Wang J, Zhang X, Collins CM, Smith MB, Liu H, Zhu XH, Vaughan JT, Ugurbil K, Chen W. Analysis of wave behavior in lossy dielectric samples at high field. Magn Reson Med 2002;47(5):982-989.

9. Collins CM, Liu W, Schreiber W, Yang QX, Smith MB. Central brightening due to constructive interference with, without, and despite dielectric resonance. J Magn Reson Imaging 2005;21(2):192-196.

10. Sacolick LI, Wiesinger F, Hancu I, Vogel MW. B1 mapping by Bloch-Siegert shift. Magnetic Resonance in Medicine 2010;63(5):1315-1322.

11. Warntjes JB, Dahlqvist O, Lundberg P. Novel method for rapid, simultaneous T1, T2*, and proton density quantification. Magn Reson Med 2007;57(3):528-537.

12. Warntjes JB, Leinhard OD, West J, Lundberg P. Rapid magnetic resonance quantification on the brain: Optimization for clinical usage. Magn Reson Med 2008;60(2):320-329.

13. Cohen MS, DuBois RM, Zeineh MM. Rapid and effective correction of RF inhomogeneity for high field magnetic resonance imaging. Human brain mapping 2000;10(4):204-211.

14. Bottomley PA, Andrew ER. RF magnetic field penetration, phase shift and power dissipation in biological tissue: implications for NMR imaging. Physics in medicine and biology 1978;23(4):630-643.

15. Glover GH, Hayes CE, Pelc NJ, Edelstein WA, Mueller OM, Hart HR, Hardy CJ, O'Donnell M, Barber WD. Comparison of linear and circular polarization for magnetic resonance imaging. Journal of Magnetic Resonance (1969) 1985;64(2):255-270.

16. Saekho S, Yip C-y, Noll DC, Boada FE, Stenger VA. Fast-kz three-dimensional tailored radiofrequency pulse for reduced B1 inhomogeneity. Magnetic Resonance in Medicine 2006;55(4):719-724.

17. Soher BJ, Dale BM, Merkle EM. A Review of MR Physics: 3T versus 1.5T. Magnetic Resonance Imaging Clinics of North America 2007;15(3):277-290.

18. Reeder SB, Wen Z, Yu H, Pineda AR, Gold GE, Markl M, Pelc NJ. Multicoil Dixon chemical species separation with an iterative least-squares estimation method. Magn Reson Med 2004;51(1):35-45.

19. Campo CA, Hernando D, Schubert T, Bookwalter CA, Pay AJV, Reeder SB. Standardized Approach for ROI-Based Measurements of Proton Density Fat Fraction and R2* in the Liver. American Journal of Roentgenology 2017;209(3):592-603.

20. Bernstein MA, Huston J, Ward HA. Imaging artifacts at 3.0T. Journal of Magnetic Resonance Imaging 2006;24(4):735-746. 21. Andrews T, Ghostine J, Gonyea J, Ebert G, Braff S, Filippi C. Reduction in dielectric shading in liver on clinical 3T parallel transmission MR system. 2010.

Table 1. B0 and B1
maps were acquired and analyzed in a total of 372 patients presenting for
routine clinical MRI exams on a variety of GEHC Scanners (GE Healthcare,
Waukesha, WI).

Figure 1. Fieldmap inhomogeneities vary spatially across the
liver and tend to increase with field strength. Note the pronounced difference
between the 1.5T and 3.0T fieldmap color scales and the variability within a
single liver at 3.0T (D, dashed oval). Examples from both field strengths of IDEAL IQ B0 fieldmaps
(B,D) are shown above with their accompanying water maps (A,C). Note that
fieldmap values (denoted ψ [Hz]) are related to changes in the static magnetic
field (denoted ΔB0 [T]) by the Larmor equation ψ= γΔB0/2π, where γ is the
gyromagnetic ratio of ^{1}H.

Figure 2. Flip angle (B1) errors
vary spatially across the liver. Note the large B1 inhomogeneities in segments II/III of
the 3.0T example [dashed oval]. Examples from both field strengths of Bloch-Seigert
B1 calibration
coefficient maps (B,D) are shown above with their accompanying magnitude
gradient echo images (A,C). Note that transmitted flip angle (α_{T}) is related
to prescribed flip angle (α_{P}) by the equation α_{T}=βα_{P}.

Figure 3. Fieldmap errors exhibit greater
magnitude and variability in the liver at 3.0T than at 1.5T, however, they are
present in all segments of the human liver. Quartile and range statistics of fieldmap
measurements (Hz) for each segment across all 56 patients are plotted
(statistical outliers excluded). Note that fieldmap values
(denoted ψ [Hz]) are related to changes in the static magnetic field (denoted
ΔB0 [T]) by the Larmor equation ψ= γΔB0, where γ is the gyromagnetic ratio of ^{1}H.

Figure 4. B1 inhomogeneities are
present in the liver at both 1.5T and 3.0T, with flip angle errors in 3.0T
acquisitions exhibiting larger magnitude and variability. Of particular note
are the average β values in the lateral
segment of the left lobe of the liver (segments II and III) at 3.0T which
manifest the largest average flip angle errors. Quartile and range statistics of
β measurements for each segment across all 312
patients are plotted (statistical outliers excluded). Note that transmitted
flip angle (α_{T})
is related to prescribed flip angle (α_{P}) by the equation α_{T}=βα_{P}.