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B0 and B1 Inhomogeneities in the Liver at 1.5T and 3.0T
Nathan Tibbitts Roberts1,2, Diego Hernando1,3, Timothy J Colgan1, Louis A Hinshaw1, Dylan M Kernan1, and Scott B Reeder1,3,4,5,6

1Radiology, University of Wisconsin - Madison, Madison, WI, United States, 2Electrical and Computer Engineering, University of Wisconsin - Madison, Madison, WI, United States, 3Medical Physics, University of Wisconsin - Madison, Madison, WI, United States, 4Medicine, University of Wisconsin - Madison, Madison, WI, United States, 5Emergency Medicine, University of Wisconsin - Madison, Madison, WI, United States, 6Biomedical Engineering, University of Wisconsin - Madison, Madison, WI, United States

Synopsis

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.

Introduction

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.

Methods

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: 42x42cm2 FOV, 8mm slice thickness, 32 slices, 3o prescribed flip angle, 128x128 matrix size, ±83.33kHz receiver bandwidth, with 6 echoes. At 1.5T, all echoes (TE1=0.9ms, ΔTE=1.48ms) were acquired in a single TR. At 3.0T, echoes (TE1=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: 44x44cm2 FOV, 10/10mm slice thickness/gap, 15o 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 1H (42.58MHz/T).

For B1 inhomogeneities, the transmitted flip angle (αT) is related to prescribed flip angle (αP) by the equation αT=βαP, where β is defined as the B1 calibration coefficient. The acquired B1 maps were normalized by αP 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.

Discussion

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.

Acknowledgements

The authors wish to acknowledge support from the NIH (grants R01 DK088925 and K24 DK102595). The authors also acknowledge GE Healthcare who provides research support to the University of Wisconsin-Madison. The authors would also like to thanks David T Harris, PhD from the University of Wisconsin - Madison for his help.

References

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Figures

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 1H.

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 (αT) is related to prescribed flip angle (αP) by the equation αT=βαP.

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 1H.

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 (αT) is related to prescribed flip angle (αP) by the equation αT=βαP.

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