B0 and B1 inhomogeneities in the liver at 1.5 T and 3.0 T

Abstract

Purpose: The purpose of this work is to characterize the magnitude and variability of B₀ and B₁ inhomogeneities in the liver in large cohorts of patients at both 1.5 T and 3.0 T.

Methods: Volumetric B₀ and B₁ maps were acquired over the liver of patients presenting for routine abdominal MRI. Regions of interest were drawn in the nine Couinaud segments of the liver, and the average value was recorded. Magnitude and variation of measured averages in each segment were reported across all patients.

Results: A total of 316 B₀ maps and 314 B₁ maps, acquired at 1.5 T and 3.0 T on a variety of GE Healthcare MRI systems in 630 unique exams, were identified, analyzed, and, in the interest of reproducible research, de-identified and made public. Measured B₀ inhomogeneities ranged (5th-95th percentiles) from -31.7 Hz to 164.0 Hz for 3.0 T (-14.5 Hz to 81.3 Hz at 1.5 T), while measured B₁ inhomogeneities (ratio of actual over prescribed flip angle) ranged from 0.59 to 1.13 for 3.0 T (0.83 to 1.11 at 1.5 T).

Conclusion: This study provides robust characterization of B₀ and B₁ inhomogeneities in the liver to guide the development of imaging applications and protocols. Field strength, bore diameter, and sex were determined to be statistically significant effects for both B₀ and B₁ uniformity. Typical clinical liver imaging at 3.0 T should expect B₀ inhomogeneities ranging from approximately -100 Hz to 250 Hz (-50 Hz to 150 Hz at 1.5 T) and B₁ inhomogeneities ranging from approximately 0.4 to 1.3 (0.7 to 1.2 at 1.5 T).

Keywords: B0; B1; dielectric effect; inhomogeneities; liver; quantitative imaging biomarkers.

Figure 1.

B0 field map inhomogeneities vary spatially across the liver and tend to increase with field strength. Note the pronounced difference between the 1.5T and 3.0T field map color scales and the variability within a single liver at 3.0T (D, dashed oval). Examples from both field strengths of IDEAL IQ B0 field maps (B,D) are shown above with their accompanying water maps (A,C). Note that field map 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.

Field map 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. In 3.0T acquisitions, bore diameter was observed to be a small yet statistically significant factor contributing to B0 inhomogeneity. Quartile and range statistics (with statistical outliers shown in gray) of field map measurements (Hz) are plotted across segments (A), across BMI in females (B), and across BMI in males (C). BMI was defined as: underweight (<18.5kg/m²), normal (<25kg/m²), overweight (<30kg/m²) and obese (≥30kg/m²). Numerical values under each BMI x-axis label give the number of patients included in the statistics for 1.5T − 70cm bore, 3.0T − 60cm bore, and 3.0T − 70cm bore, respectively. Note that field map 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 3.

Flip angle (B1) errors vary spatially across the liver. Note the large B1 inhomogeneities in segments II/III of the 3.0T example (D, dashed oval). Examples from both field strengths of Bloch-Siegert 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 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. In 3.0T acquisitions, bore diameter was observed to be a very large and statistically significant factor contributing to B1 inhomogeneity. Of particular note are the average B1 inhomogeneities (β) 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 (with statistical outliers shown in gray) of B1 inhomogeneity measurements are plotted across segments (A), across BMI in females (B), and across BMI in males (C). BMI was defined as: underweight (<18.5kg/m²), normal (<25kg/m²), overweight (<30kg/m²) and obese (≥30kg/m²). Numerical values under each BMI x-axis label give the number of patients included in the statistics for 1.5T - 70cm bore, 3.0T - 60cm bore, and 3.0T - 70cm bore, respectively. Note that transmitted flip angle (αT) is related to prescribed flip angle (αP) by the equation αT=βαP.