The spinal cord (SC) is responsible for mediating neurologic function between the brain and peripheral nervous system, and is somatotopically organized: sensory information is conveyed through the dorsal columns, while the lateral columns convey a significant fraction of motor function. Therefore, even small SC lesions (e.g. multiple sclerosis [MS]) can result in severe neurological impairment. In particular, the lumbar cord is integral to lower extremity function: its characterization can significantly improve diagnosis and prognosis of neurodegenerative disease. However, lumbar cord quantitative MRI studies have been limited due to its size (~1cm), location, and composition: it is located directly below the lungs, quickly diminishes in size through the conus medularis and cauda equine, and has significantly less white matter per volume than its cervical counterpart.

A single-point quantitative magnetization transfer (qMT) was recently developed in the brain¹ and cervical SC² that provides rapid, high-resolution indices that relate to grey matter (GM) and white matter (WM) myelin content. However, qMT has not been studied in the lumbar cord; therefore we have implemented for the first time an assessment of qMT at the thoracolumbar bulge to accurately characterize the MT effects observed in the WM and GM of the thoracolumbar cord in healthy volunteers.

Two healthy volunteers (one male, ages 25 and 26 years) were imaged using a 3.0T Achieva whole body scanner (Philips, The Netherlands). A two-channel transmit body coil was used for excitation and a six-channel spine array was used for signal reception. For each volunteer, a transverse volume between T11 and L2 was selected from a total spine T₂-weighted survey image. qMT data were acquired over this volume using a 3D MT-prepared spoiled gradient echo sequence³. MT-preparation used a 20 ms single-lobe sinc-Gauss pulse, saturation flip angle (α_MT) of 820 ̊, and offset frequencies (∆ω) of 2.5 and 100 kHz. Additional imaging parameters included: TR/TE=50/2.7 ms, excitation flip angle (α)=6 ̊, FOV=150×150×70 mm³, resolution = 0.65×0.65×5 mm³, and 2 signal averages. B₁⁺ was measured in the same volume using the actual flip angle imaging method (TR1/TR2= 30/130 ms, α= 60°)⁴; ∆B₀ from gradient echo phase images acquired (∆TE=2.3 ms)⁵; and T₁ using a multiple flip angle acquisition (TR/TE=20/4.6 ms, α = 5, 10, 15, 20, 25, 30°). A high-resolution (0.65×0.65×3 mm³, 2 mm gap) multi-echo gradient echo (mFFE) anatomical image was also acquired for registration (TR/TE/∆TE = 700/8.0/9.3 ms, α = 33°). Total scan time to acquire anatomical, T₁, B₁⁺, ∆B₀, and MT data was ≈22 minutes.

Prior to data fitting, all images were co-registered to the mFFE image using the FLIRT package from FSL⁶. Normalized (to ∆ω = 100 kHz data) signal intensities were then fitted¹ to a two-pool model of the MT effect to find the macromolecular to free pool size ratio (PSR), while fixing the qMT parameters, k_mf, T_2fR_1f, and T_2m, to their respective values derived from the cervical cord², and using the T₁, B₁⁺, and ∆B₀ field maps to correct for fitting errors associated with field inhomogeneities/T₁ variations.

Figure 1 displays the mFFE (left panel) and PSR (right panels) for a single slice at T12 in each volunteer. WM displays higher PSR than GM, while GM is a significant fraction of the total cord volume. Of particular note are the dorsal horns protruding from the cord into the cerebrospinal fluid which are the start of the dorsal nerve roots. The lumbar PSR data agrees well with established values², as the mean PSR in each volunteer was found to be (WM = 0.18±0.04,0.21±0.05, GM = 0.13±0.03,0.15±0.03). Table 1 displays the mean PSR for each volunteer for the dorsal column (DC), lateral column (LC), WM (DC+LC), and GM.The PSR for WM and GM in the lumbar cord is suspected to be similar to cervical cord PSR; healthy cervical PSR values in the WM and GM were found to be 0.19±0.01 and 0.15±0.01, respectively². In particular, note the ability to identify the dorsal nerve roots extending into the CSF beginning in Fig. 1, which are difficult to visualize at lower resolutions. The integrity of these nerve roots is paramount for assessing neural function; therefore, the ability to accurately quantify the PSR in the SC, nerve roots and GM may have important implications for demyelinating diseases such as MS.These results demonstrate the first study of high-resolution qMT and derived PSR maps of the healthy thoracolumbar cord. Future work involves collecting additional healthy controls, assessing the similarity of the constraints to cervical cord estimates, and evaluation in patient populations.