Ultrashort Echo MR Imaging Of Anterior Cruciate Ligament: Relationship Between Mono-exponential And Bi-exponential Decay Coefficients

Quantitative ultrashort echo time (UTE) T2* magnetic resonance imaging can evaluate the structural properties of anterior cruciate ligament (ACL) grafts non-invasively and longitudinally. When the UTE sequence is used, ACL grafts show signal intensity decay as echo-time. Traditionally, the UTE-T2* is described using a mono-exponential decay (T2*mono). However, ACL graft demonstrates a bi-exponential decay, with a rapid decay at short echo times (T2*short), followed by a slower long-lasting decay at longer echo times (T2*long), each describing different structural properties of the graft. It remains to be seen if the bi-exponential description can provide unique insights into the graft's structural properties, which would otherwise be unavailable via the mono-exponential analysis. PURPOSE: To assess the relationship between mono and bi-exponential decay coefficients in ACL grafts. HYPOTHESIS: The mono-exponential and bi-exponential’s short and long decay coefficients are uncorrelated. METHODS: Twenty-three ACL-reconstructed subjects (12 females & 11 males; Age: 14.2-32.2 years) were recruited and tested in this IRB-approved study 6 months post-surgery. Only subjects who had unilateral ACL injury for the first time with minimal injuries to other knee structures were included. A 10-echo MR UTE sequence (TE = 0.3-18.3 ms; 2 ms echo steps) were acquired from the ACLR limbs. The mono- (T2*mono) and bi-exponential (T2*short and T2*long) decay coefficients were calculated for voxels containing the ACL graft. The rms of each coefficient were calculated. Pearson’s correlation analysis was performed between T2* outcomes at p < 0.05. RESULTS: T2*mono was significantly correlated with T2*long (r = -0.508, p = 0.007) but not T2*short (p = 0.259). Also, T2*short and T2*long were significantly correlated (r = 0.470, p = 0.012). CONCLUSIONS: The T2*short coefficient was uncorrelated to T2*mono, suggesting that it provides unique information about the structural quality of healing ACL grafts that the mono-exponential analysis cannot provide. The T2*short and T2*long might be related to the collagenous structures (faster decay) and water content (slower decay) within the graft, respectively. But, these coefficients are not entirely independent and might quantify some overlapping graft properties.