Identification of water compartments in spinal cords by ²H double quantum filtered NMR

Abstract

In 2H double quantum filtered (DQF) NMR, the various water compartments are characterized by their different residual quadrupolar interactions. The spectral separation between the different signals enables the measurement of the relaxation of each compartment and the magnetization transfer (MT) between them. In the current study, five water compartments were identified in the 2H DQF spectra of porcine spinal cord. The most prominent signal was the pair of satellites with a quadrupolar splitting of about 550 Hz. 2H DQF MRI optimized for the 550 Hz quadrupolar splitting indicated that this signal originated mainly from the white matter and it was assigned to the myelin water. This splitting does not change upon changing the orientation of the spinal cord relative to the magnetic field, indicating a liquid crystalline nature. Another site exhibiting splitting of about 1500 Hz was assigned to collagenous connective tissue. The narrow central peak was assigned to a combination of intra‐ and inter‐axonal water. The assignment of the other two sites is not certain and requires further study. The rates of MT between the various sites were recorded.

Residual quadrupolar couplings observed in 7 Tesla deuterium MR spectra of skeletal muscle

“…Moreover, residual dipolar couplings tend to average out due to chemical exchange between protons/ deuterons between water molecules. 35 The sizes of the observed splittings (the largest splitting was observed in TA when the lower leg was positioned parallel with B 0 , with a mean of 31.0 ± 0.5 Hz among the 3 subjects) are only a fraction of the deuteron quadrupolar coupling constant of ~250 kHz in liquid water 23,24 and are also smaller than the splittings observed in tendon and spinal cord. 25,35 This indicates only a very small preferential alignment of the HDO molecules in the muscle fibers, which can be described by an alignment tensor that describes the average F I G U R E 3 Muscle fiber tracking results from DTI data in the right lower leg (subject 1), positioned parallel with B 0 (A,C,E) and at an angle of approximately 45° with respect to B 0 (B,D,F).…”

“…However, in an ex vivo sample of macroscopically ordered bovine Achilles tendon, immersed in D 2 O, a well‐resolved splitting of the deuterated water signal could be detected by deuterium double‐quantum–filtered NMR spectroscopy 25 . Recently, similar experiments were performed on ex vivo spinal cord in which 5 different water compartments were identified from the deuterium double‐quantum–filtered spectra 35 . The most prominent deuterated water signal showed a residual quadrupolar splitting of about 550 Hz and was assigned to the myelin water.…”

“… 25 Recently, similar experiments were performed on ex vivo spinal cord in which 5 different water compartments were identified from the deuterium double‐quantum–filtered spectra. 35 The most prominent deuterated water signal showed a residual quadrupolar splitting of about 550 Hz and was assigned to the myelin water. This splitting was invariant with respect to the orientation of the spinal cord relative to the magnetic field, indicating that myelin acts as a liquid crystalline medium, which orients itself with respect to the magnetic field and not with the spinal cord.…”

“…These splittings will originate mainly from the residual deuteron quadrupolar coupling, which is typically an order of magnitude larger than the dipolar couplings. Moreover, residual dipolar couplings tend to average out due to chemical exchange between protons/deuterons between water molecules 35 . The sizes of the observed splittings (the largest splitting was observed in TA when the lower leg was positioned parallel with B 0 , with a mean of 31.0 ± 0.5 Hz among the 3 subjects) are only a fraction of the deuteron quadrupolar coupling constant of ~250 kHz in liquid water 23,24 and are also smaller than the splittings observed in tendon and spinal cord 25,35 .…”

“…Moreover, residual dipolar couplings tend to average out due to chemical exchange between protons/deuterons between water molecules 35 . The sizes of the observed splittings (the largest splitting was observed in TA when the lower leg was positioned parallel with B 0 , with a mean of 31.0 ± 0.5 Hz among the 3 subjects) are only a fraction of the deuteron quadrupolar coupling constant of ~250 kHz in liquid water 23,24 and are also smaller than the splittings observed in tendon and spinal cord 25,35 . This indicates only a very small preferential alignment of the HDO molecules in the muscle fibers, which can be described by an alignment tensor that describes the average orientation of the molecule with respect to the magnetic field.…”