Molecular, dynamic, and structural origin of inhomogeneous magnetization transfer in lipid membranes

Abstract

Concerted motions of phospholipids in white matter decrease proton spin diffusion leading to increased proton T times and increased ihMT amplitudes, consistent with decoupling of Zeeman and dipolar spin reservoirs. Molecular specificity and dynamic sensitivity of ihMT contrast make it a suitable candidate for probing myelin membrane disorders. Magn Reson Med 77:1318-1328, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

The physical mechanism of “inhomogeneous” magnetization transfer MRI

“…1C shows the dependence on the offset frequency of the prepulse. The general shape of these curves is similar to plots of aqueous ihMTR vs. jDj in previous studies [4][5][6][7]. At short prepulse lengths, the maximum ihMTR occurs near the resonance condition in the local field (X ¼ 1).…”

“…From this, we see that WT 1D is a key parameter in controlling ihMT. If W is held constant, then, as others have pointed out, ihMT generates T 1D -dependent contrast in MRI [5,7,37]. However, since W is, within limits, under control of the experimenter, it may afford detection of ihMT in systems with short T 1D .…”

“…The ''Continuous-Wave" (CW) type applies a single rectangular prepulse, which is sine-modulated in the case of the S dual experiment [4]. The ''pulse-train" type uses prepulses consisting of a train of shaped pulses (typically Hann or Gaussian) [5][6][7]. We analyze the behavior of a coupled spin system under both varieties.…”

“…Experiments have shown that many non-aqueous non-lipids (such as heatdenatured ovalbumin, agarose, and gelatin [4,7]) result in an equal attenuation of the aqueous signals in S þ ; S À , and S dual . However, materials containing a substantial proportion of lipid bilayers (such as brain and spinal cord WM, hair conditioner, Prolipid-161, and DPPC:Cholesterol phospholipids [4][5][6][7][8][9]) show larger attenuation in S dual . A quantitative measure of this difference, the ihMT ratio, has been defined as [4] ihMTR ¼ S þ þ S À À 2S dual 2S 0 : ð1Þ…”

Evaluation of the Sensitivity of Inhomogeneous Magnetization Transfer (ihMT) MRI for Multiple Sclerosis

“…One promising example is the recently described inhomogeneous magnetization transfer (ihMT) technique, 8 which can be envisioned as a method to image, in vivo, the dipolar order (characterized by a dipolar relaxation time constant) underlying broad tissue macromolecular lines. [9][10][11] The long dipolar relaxation time values associated with myelinated structures compared with others 12,13 allow the ihMT technique to more selectively isolate the contribution of the myelin dipolar order within the broad MT signal, thereby providing higher specificity for myelinated structures.…”