Simple modification of arm position improves B1+ and signal homogeneity in the thoracolumbar spine at 3T

Abstract

Purpose: To evaluate the homogeneity of the radiofrequency magnetic field (B₁⁺ ) and signal intensity using different arm positions during 3T thoracolumbar spinal imaging.

Materials and methods: Twenty volunteers were scanned with a four-channel radiofrequency (RF) transmit coil at 3T, with arms on the bed (conventional), arms elevated by 100 mm (arm lift), or with the arms-up position (elevated arm). Axial B₁⁺ maps and sagittal T₁ -weighted image (T₁ WI)-performed RF shimming were obtained for each arm position. The mean and standard deviation (SD) of the flip angle (FA) at the center of the vertebra on each B₁⁺ map, and contrast noise ratios (CNRs) between the spinal cord and cerebrospinal fluid of sagittal T₁ WI, were calculated and compared among the different arm positions.

Results: Mean FA values (degrees) for the arm lift and elevated arm positions were significantly larger than for the conventional position (P < 0.001 for both) at the twelfth thoracic vertebra (Th12). FA SD values for the arm lift and elevated arm position were significantly smaller than for the conventional position (P < 0.001 for both) at Th12. CNR for the arm lift and elevated arm position were significantly higher than for the conventional position (P = 0.007 and 0.002, respectively). The mean and SD of the FA and the CNR did not differ significantly for the arm lift and elevated arm positions (P = 0.591, 0.958, and 0.927, respectively).

Conclusion: Inhomogeneities of B₁⁺ and signal intensities were improved by simply changing the arm position in 3T thoracolumbar spinal imaging.

Level of evidence: 3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:123-130.

Keywords: B1+ homogeneity; arms position; thoracolumbar spine MRI.