Introduction

Electric properties of tissue including conductivity and permittivity can provide auxiliary information for disease diagnosis and monitoring. Magnetic resonance electric properties tomography (MREPT) is emerging as a noninvasive imaging method for mapping tissue electric properties.^1,2 Phase-MREPT using the phase map may be particularly useful for clinical applications.^3,4 Implementation of MREPT in the clinic requires a robust and reliable method and a short scan time. This could be accomplished using Compressed SENSE but the effect of this on the precision and reliability of the calculated variables is unknown. Here, we investigated the impact of increased Compressed SENSE (CS) factors on the reliability of conductivity values. We investigated the effect of field mapping on measurement precision as well as the impact of brain activity, such as video watching on tissue conductivity values.⁵

Methods

Data acquisition

Reliability was tested by scanning five healthy subjects (Age: 35.8 ± 8.5, two females, three males) at 3T (Ingenia CX, Philips Healthcare, Best, The Netherlands) on two separate sessions (different days, similar time of day). Parameters: BFFE, TR/TE = 3.2/1.6 ms, nonselective RF pulses, flip angle 25° , resolution 1.3×1.3×1 mm³, sagittal slices.

Utility of compressed SENSE was tested by scanning additional five healthy subjects (Age: 32.2 ± 12.3, three females, two males) using the BFFE approach above, with increasing CS factors. The default CS factor used for reference was 1.3. Conductivity maps were calculated using CS factors from 2 to 12, and error maps were obtained by subtracting these conductivity maps from the reference. The reference scan duration was 4 min 33 s.

A fast acquisition (dynamic scan time 1.89 s) was tested by scanning one healthy subject (Age: 30, male) using BFFE with CS factor 4. Parameters: BFFE, TR/TE = 1.58/0.79 ms, nonselective RF pulses, flip angle 25° , resolution 3×3×3 mm³, sagittal slices, 520 dynamics.

Comparisons of B₁ mapping methods were tested by scanning four healthy subjects (Age: 40.3 ± 11.1, one female, three males) at 3T. Two bFFE scans were acquired for each B₁ mapping method, including single-slice 2D DREAM, full coverage DREAM and full coverage dual TR. Parameters: BFFE, TR/TE = 2.5/1.25 ms, nonselective RF pulses, flip angle 25° , sagittal slices, acquired resolution 1.3×1.3×1 mm³ reconstructed to 1×1×1 mm³.

Effect of video watching was tested by scanning additional four healthy subjects (Age: 42.0 ± 15.1, two females, two males) using BFFE with or without video watching during scanning. Parameters: BFFE, TR/TE = 2.5/1.25 ms, nonselective RF pulses, flip angle 25° , sagittal slices, acquired resolution 1.3×1.3×1 mm³ reconstructed to 1×1×1 mm³.

3D T1 TFE scans were acquired from each subject for anatomical reference. Parameters: 190 slices (R-L) in the sagittal acquisition plane, 240 mm FOV, 1x1x1 mm³ acquired voxel size, non-selective excitation, TE/TR/TI = 7.25/3.42/850 ms, flip angle 8° with a TFE factor = 117 and TFE shot interval = 2000 ms.

Data analysis

T1W images were co-registered to bFFE images for segmenting brain into white matter, gray matter and CSF by FSL to alleviate boundary artifacts in the calculation. An average parabolic phase fitting method was used to reduce artifacts amplified in the Laplacian.⁶ For each dimension of a voxel, parabolic fitting was conducted on both sides of the voxel. The average of second derivatives of the two quadratic expressions was assigned to the second derivative of the target phase in this dimension.

Results

Figure 1 shows conductivity values of white matter, gray matter and CSF of five healthy subjects in two separate sessions. The measured conductivity values of white matter, gray matter and CSF at 3T Larmor frequency (127.78 MHz) were 0.42 ± 0.02, 0.63 ± 0.03, 2.24 ± 0.10 S/m. Figure 2 shows the variation conductivity of white matter, gray matter and CSF with increasing CS factors. White matter conductivity showed minimal deviation with CS factors up to 5 with deviations increasing at CS factors > 6. The conductivity of gray matter showed minimal deviation up to CS factor 6 with increasing deviation at CS factor >8. The ultra fast BFFE with dynamic 1.89 s also showed good precision with coefficient of variances over the 520-point time course of 6.67% for GM and 7.83% for WM.

Figure 3 shows phase and conductivity maps of the same sagittal slice of one subject. Both full coverage DREAM and full coverage dual TR approach enlarged the regions of good phase coherence, making the conductivity values much closer to the reference as shown in Table 1. Figure 4 shows Bland-Altman plots of conductivity measurements between the three B₁ mapping methods, and the corresponding p-values. Compared with single-slice 2D DREAM, the mean differences were -0.043 S/m for full coverage DREAM and -0.038 S/m for full coverage dual TR approach. There was no significant difference between full coverage DREAM and full coverage dual TR. Interestingly, video watching significantly increased the conductivity values. (Fig. 4)

Discussion and conclusion

In summary, BFFE can be safely accelerated with compressed SENSE without sacrifice of precision. Better field mapping further improves the precision of the conductivity measures. Video watching, or activity during scanning should be avoided.

Acknowledgements

The authors are grateful to Mr. Ryan Castillo for expert radiography. The authors acknowledge the facilities and scientific and technical assistance of the National Imaging Facility, a National Collaborative Research Infrastructure Strategy (NCRIS) capability, at Neuroscience Research Australia and UNSW.