B1 artifact reduction in abdominal DCE-MRI using kT-points: First clinical assessment of dynamic RF shimming at 3T

Background

The excitation inhomogeneity artifact occurring at 3T in the abdomen can lead to dramatic loss of signal and contrast, thereby hampering diagnosis.

Purpose

To assess excitation homogeneity and image quality achieved by nonselective prototypical k_T-points pulses, compared to tailored static RF shimming, in clinical routine on a commercial dual-transmit scanner.

Study Type

Retrospective study with Institutional Review Board approval; informed consent was waived.

Population

Fifty consecutive patients referred for liver MRI at a single hospital.

Field Strength/Sequence

3D breath-hold dynamic contrast-enhanced (DCE) MRI at 3T.

Assessment

Flip angle homogeneity was estimated via numerical simulation based on measured static and RF field maps. In all, 20 of the 50 patients underwent DCE-MRI while a pulse designer was present. The effect of RF shimming and k_T-point pulses could be compared by repeating the acquisition with each transmit scheme before injection and in the late phase. Signal homogeneity, T₁ contrast, enhancement quality, structure details, and global image quality were assessed on a 4-level scale (0 to 3) by two radiologists.

Statistical tests

Means were compared using Wilcoxon signed-rank tests.

Results

Normalized root mean square flip angle error was significantly reduced with k_T-points compared to static RF shimming (8.5% ± 1.5% [mean ± standard deviation, SD] vs. 20.4% ± 9.8%; P < 0.0001). The worst case (heavy ascites) led to 13.0% (k_T-points) vs. 54.9% (RF shimming). Global image quality was significantly higher for k_T-points (2.3 ± 0.5 vs. 1.9 ± 0.6; P = 0.008). One subject's examination was judged unusable with RF shimming by one reader, none with k_T-points. 85% of k_T-points acquisitions were graded at least 2/3, and only 55% for static RF shimming.

Data Conclusion

K_T-points reduce excitation inhomogeneity quantitatively and qualitatively, especially in patients with ascites and prone to B₁ shading.

Level of Evidence: 1

Technical Efficacy: Stage 1

J. Magn. Reson. Imaging 2017.

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