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

Abstract

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 2018;47:1562-1571.

Keywords: 3T; ascites; dynamic contrast enhancement; kT-points; liver; parallel transmission.