In vivo measurement of a new source of contrast, the dipolar relaxation time, T_1D, using a modified inhomogeneous magnetization transfer (ihMT) sequence

Abstract

The model and experiments described provide access to a new relaxation characteristic of tissue with potentially unique diagnostic information. Magn Reson Med 78:1362-1372, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Optimization of inhomogeneous magnetization transfer (ihMT) MRI contrast for preclinical studies using dipolar relaxation time (T1D) filtering

“…(A) M 0 image including location of internal capsule (IC), cortical gray matter (cGM) and muscle (Mu) structures (top left). Typical mouse brain T 1D map (bottom left) obtained using the method described in Varma et al and showing long T 1D values in white matter (WM) and gray matter (GM) structures and short T 1D values in muscle. Right: ihMT ratio (ihMTR) images (%) obtained for dual‐frequency‐offset saturation achieved with cosine‐modulated (CM) pulses and frequency‐alternating pulses with increasing values of Δ t .…”

“…For illustration, in the limiting case in which this switching time increased towards infinity, the ‘dual‐frequency’ saturation scenario would become equivalent to the single‐frequency scenario and the ihMT signal would naturally vanish. In a separate work, we reported a theoretical model for this decay which, combined with a modified version of the dual saturation, based on the insertion of several pulses at one frequency before alternating to the opposite one, allowed in vivo estimations of T 1D values . Here, instead, for T 1D ‐filtered ihMT imaging, we used a dual‐saturation scheme with a single pulse at alternating frequency (Figure C) and adapted the switching time Δ t .…”

“…Although no conclusion regarding neuroimaging can be derived from ihMT experiments on hair conditioners, the latter can be advantageously used for technical set up and adjustment purposes . T 1D values of hc and ag4% were estimated to be T 1D = 16.1 ± 6.0 ms and T 1D = 1.6 ± 0.3 ms, respectively, by fitting experimental data to an extended ihMT‐normalized model, as outlined in Varma et al A single Δ t value ( Δ t = 1.3 ms) was used for dual‐frequency‐offset saturation performed with cosine modulation for which no T 1D filtering is expected, and Δ t = 1.3 ms, 2.3 ms, 3.3 ms, 4.3 ms, 5.3 ms and 7.3 ms were used for dual‐frequency‐offset saturation performed with alternating pulses. Other ihMT saturation parameters were as follows: Δ f = ±10 kHz; PW = 1 ms; B 1,RMS = 6.7 μT and τ = 0.9 s, leading to E TR = 40 μT 2 s. Experiments were conducted at 25°C.…”

“…Values of T 1D in mouse tissues were estimated with the technique outlined in Varma et al to be T 1D = 6.1 ± 0.8 ms in IC, T 1D = 5.6 ± 1.2 ms in cGM and T 1D = 2.2 ± 0.6 ms in Mu. The proposed T 1D filtering strategy was thus used to attenuate the ihMT signal of the shorter T 1D components, represented by Mu here, whilst preserving the large amount of signal of long T 1D WM and GM, leading to an overall optimization of the contrast of ihMT for these myelinated structures.…”

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.…”