Double quantum filtered ²³Na MRI with magic angle excitation of human skeletal muscle in the presence of B₀ and B₁ inhomogeneities

Abstract

Double quantum filtered 23Na MRI with magic angle excitation (DQF‐MA) can be used to selectively detect sodium ions located within anisotropic structures such as muscle fibers. It might therefore be a promising tool to analyze the microscopic environment of sodium ions, for example in the context of osmotically neutral sodium retention. However, DQF‐MA imaging is challenging due to various signal dependences, on both measurement parameters and external influences. The aim of this work was to examine how B0 in combination with B1 inhomogeneities alter the DQF‐MA signal intensity. We showed that, in the presence of B0 inhomogeneities, flip angle schemes with only one 54.7° pulse can be favorable compared with the classical 90°‐54.7°‐54.7° scheme. DQF‐MA images of the human lower leg were acquired at B0 = 3 T with a nominal spatial resolution of 12 × 12 × 36 mm3 within an acquisition time of TAcq < 10 min, and compared with spin density weighted (DW), as well as triple quantum filtration (TQF) 23Na images. We found mean normalized signal‐to‐noise ratios of TQF/DW = 13.7 ± 2.3% (tibialis anterior), 11.9 ± 2.3% (soleus) and 11.4 ± 2.2% (gastrocnemius medialis), as well as DQF‐MA/DW = 4.7 ± 1.1% (tibialis anterior), 3.3 ± 0.73% (soleus) and 3.4 ± 0.6% (gastrocnemius medialis). These ratios might serve as additional measures in future clinical studies of sodium retention within human skeletal muscle. However, the influence of B0 and B1 inhomogeneities should be considered when interpreting DQF‐MA images.

X‐nuclei imaging: Current state, technical challenges, and future directions

“…The DQ magic angle sequence nulls signals from biexponential relaxation such that only signal from nuclei experiencing quadrupolar splitting is detected . Deducted indication on tissue anisotropy is valid if B 0 and B 1 + inhomogeneities can be neglected . While B 0 and B 1 + inhomogeneities reduce DQ signal specificity, they reduce TQ signal strength .…”

“…DQ imaging has been applied to study skeletal muscle due to its macroscopical anisotropy (Fig. ). The feasibility has been demonstrated by .…”

“…The feasibility has been demonstrated by . Furthermore, the ratio of 23 Na DQ to SQ signal in muscle was proposed to be a measure for Na + retention . The presence of magic angle DQ signal in the human brain was shown to influence TSC quantification …”

“…From left to right: 23 Na density weighted ( 23 Na DW), triple‐quantum filtered (TQF), double‐quantum filtered (DQF), off‐resonances (ΔB 0 ) and relative B 1 images. Adapted and reprinted from Gast et al 2018, with permission. (b) Measurements in the head at 9.3T with single density weighted (SDW) and TQF 23 Na images.…”

“…TQ imaging was conducted in the knee and has else been predominantly applied in the brain (Fig. ). Increased TQ and constant SQ signals were measured shortly after ischemia .…”