Abbreviated quantitative UTE imaging in anterior cruciate ligament reconstruction

Abstract

Background: Existing ultrashort echo time magnetic resonance imaging (UTE MRI) methods require prohibitively long acquisition times (~ 20-40 min) to quantitatively assess the clinically relevant fast decay T₂* component in ligaments and tendons. The purpose of this study was to evaluate the feasibility and clinical translatability of a novel abbreviated quantitative UTE MRI paradigm for monitoring graft remodeling after anterior cruciate ligament (ACL) reconstruction.

Methods: Eight patients who had Graftlink™ hamstring autograft reconstruction were recruited for this prospective study. A 3D double-echo UTE sequence at 3.0 Tesla was performed at 3- and 6-months post-surgery. An abbreviated UTE MRI paradigm was established based on numerical simulations and in vivo validation from healthy knees. This proposed approach was used to assess the T₂* for fast decay component ([Formula: see text]) and bound water signal fraction (f_bw) of ACL graft in regions of interest drawn by a radiologist.

Results: Compared to the conventional bi-exponential model, the abbreviated UTE MRI paradigm achieved low relative estimation bias for [Formula: see text] and f_bw over a range of clinically relevant values for ACL grafts. A decrease in [Formula: see text] of the intra-articular graft was observed in 7 of the 8 ACL reconstruction patients from 3- to 6-months (- 0.11 ± 0.16 ms, P = 0.10). Increases in [Formula: see text] and f_bw from 3- to 6-months were observed in the tibial intra-bone graft ([Formula: see text]: 0.19 ± 0.18 ms, P < 0.05; Δf_bw: 4% ± 4%, P < 0.05). Lower [Formula: see text] (- 0.09 ± 0.11 ms, P < 0.05) was observed at 3-months when comparing the intra-bone graft to the graft/bone interface in the femoral tunnel. The same comparisons at the 6-months also yielded relatively lower [Formula: see text] (- 0.09 ± 0.12 ms, P < 0.05).

Conclusion: The proposed abbreviated 3D UTE MRI paradigm is capable of assessing the ACL graft remodeling process in a clinically translatable acquisition time. Longitudinal changes in [Formula: see text] and f_bw of the ACL graft were observed.

Keywords: ACL reconstruction; Anterior cruciate ligament (ACL); Graft healing; Ligamentization; Tendon-to-bone healing; Ultrashort TE (UTE).

Fig. 1

$T_{2s}^{\ast }$ and f_bw estimation bias over a range of values clinically relevant for ACL grafts. The proposed baseline-corrected mono-exponential model for short T₂* component evaluation was used for estimation of $$T_{2s}^{\ast }$$ (c) and f_bw (d). The bias compares the estimated values of $T_{2s,\mathit{app}}^{\ast }$ (a) and f_{bw, app} (b) to the simulated values of $T_{2s}^{\ast }$ and f_bw. All simulations assumed a $T_{2l}^{\ast }$ value of 20 ms

Fig. 2

$T_{2s}^{\ast }$ and f_bw estimation bias over a large range of f_bw values. The proposed baseline-corrected mono-exponential model for short T₂* component evaluation was used for estimation of $$T_{2s}^{\ast }$$ (c) and f_bw (d). The bias compares the estimated values of $T_{2s,\mathit{app}}^{\ast }$ (a) and f_{bw, app} (b) to the simulated values of $T_{2s}^{\ast }$ and f_bw. All simulations assumed a $T_{2l}^{\ast }$ value of 20 ms

Fig. 3

UTE images and signal fitting at TE of 0.1 ms (a) and 4 ms (b), corresponding signal decay with bi-exponential model fitting (blue lines) and baseline-corrected mono-exponential model fitting (red lines) for the ACL (c) and patellar tendon (d) for a healthy subject

Fig. 4

UTE images at the 6 imaged TEs (a) and the corresponding signal decay profile with baseline-corrected mono-exponential fitting from the intra-articular graft region (b) for a representative ACL reconstruction subject. Note that the TE2 images are scaled by a factor of 1.2 for display purposes

Fig. 5

Sagittal and axial UTE image with TE of 4 ms for a representative ACL reconstruction subject. The left column shows the images in the sagittal plane and the right column shows images from the axial planes denoted by the yellow lines in the sagittal plane images. The red lines delineate the ACL graft. The first row depicts the femoral intra-bone graft and graft/bone interface, the second row depicts the intra-articular graft, and the third row depicts the tibial intra-bone graft and graft/bone interface. The outlines of the ACL graft on the sagittal plane images have been shrunk by 2 voxels for illustration purposes. The ACL graft outlines on the axial plane images were not shrunk by 2 voxels. The ACL graft outlines on the axial plane images of the femoral and tibial graft tunnels are thicker than the ACL graft outlines on the axial plane image of the intra-articular graft because they include the graft/bone interface (outer 2 voxels)

Fig. 6

Regions of interest for a typical ACL reconstruction subject overlaid on the UTE image with TE of 4 ms. ROIs are color coded as follows: orange, femoral graft/bone interface; red, femoral intra-bone graft; yellow, intra-articular graft; green, tibial intra-bone; blue, tibial graft/bone interface

Fig. 7

Ladder plots of the $T_{2s}^{\ast }$, f_bw, and angle relative to B₀ from 8 ACL reconstruction subjects at 3- and 6-months post-ACL reconstruction surgery