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 (B 1 1 ) 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 B 1 1 maps and sagittal T 1 -weighted image (T 1 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 B 1 1 map, and contrast noise ratios (CNRs) between the spinal cord and cerebrospinal fluid of sagittal T 1 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 5 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 5 0.591, 0.958, and 0.927, respectively). 3Tmagnetic resonance imaging (MRI) has several advantages such as higher spatial and temporal resolution. Based on such advantages, many clinical applications of 3T MRI have been reported. 1-3 Spine imaging has been among the more frequent applications. [4][5][6] However, the RF distribution becomes more inhomogeneous in body imaging at 3T than at 1.5T, because the RF wavelength at 3T is only half as large as that at 1.5T. 7,8 In particular, the signal decrease due to local RF field inhomogeneity in abdominal imaging is caused by the dielectric resonance effect or standing-wave effect. 8,9 These B 1 1 inhomogeneities have been also reported at the thoracolumbar junction in spine imaging.10

Impact of arm position on vertebral bone marrow proton density fat fraction in chemical-shift-encoded magnetic resonance imaging: a preliminary study

“…This suggests that there is a degree of flexibility in arm positioning that allows MRI radiographers to prioritize a patient’s condition or image quality, without significantly impacting PDFF measurements. Additionally, in light of the findings of the present study and that by Ishizaka et al ( 12 ), we recommend that slightly lifted arms should be used as the default position in spine examinations using MRI, as it improves morphological images while having a negligible impact on PDFF. However, the position of the patient may be changed depending on their condition or habitus.…”

“…Even if the B 1 + shimming system is adopted, B 1 + variation caused by arm position occurs in the lumbar spine. To the best of our knowledge, lumbar spine MRI is usually performed in the side arm position; however, raising or slightly lifting the arm has been reported to improve B 1 + inhomogeneity, thus improving diagnostic image quality ( 12 ). Therefore, in the absence of specific guidelines regarding a patient’s arm position during lumbar spine MRI, different MRI radiographers may implement different arm positions to improve image quality.…”

“…A change in B 1 + implies a change in flip angle, thus if the flip angle increases, T1 will be emphasized, leading to a higher measured PDFF due to T1 bias. The B 1 + distribution is more inhomogeneous at 3T (12). Although this drawback can be improved via the use of dual-source parallel RF excitation (13), the degree of improvement depends on the body region or body habitus.…”

“…Although this drawback can be improved via the use of dual-source parallel RF excitation ( 13 ), the degree of improvement depends on the body region or body habitus. Furthermore, the spatial B 1 + variability changes the actual flip angles and can affect PDFF measurements ( 12 , 14 ). In fact, in the liver, it has been found that transmit B 1 + field changes and the PDFF on CSE-MRI can vary depending on the adopted arm position ( 14 ).…”

“…The liver is located in an area spanning from the abdominal side to the dorsal side of the body, whereas the lumbar vertebral bodies are located only on the dorsal side where the B 1 + is more likely to be obstructed by the arm when the subject is lying with the arms alongside the body. Simply changing the arm position during lumbar vertebral MRI has been reported to improve B 1 + and signal intensity inhomogeneity ( 12 ). However, without proper guidelines, different MRI radiographers may implement different arm positions to improve the image quality of lumbar MRI.…”