3D-T1ρ prepared zero echo time-based PETRA sequence for in vivo biexponential relaxation mapping of semisolid short-T2 tissues at 3 T

Abstract

Background: In addition to the articular cartilage, osteoarthritis (OA) affects several other tissues such as tendons, ligaments, and subchondral bone. T1_ρ relaxation study of these short T₂ tissues may provide a more comprehensive evaluation of OA.

Purpose: To develop a 3D spin-lattice relaxation in the rotating frame (T_1ρ ) prepared zero echo time (ZTE)-based pointwise encoding time reduction with radial acquisition (3D-T_1ρ -PETRA) sequence for relaxation mapping of semisolid short-T₂ tissues on a clinical 3 T scanner.

Study type: Prospective.

Population: Phantom, two bovine whole knee joint and Achilles tendon specimens, 10 healthy volunteers with no known inflammation, trauma or pain in the knee or ankle.

Field strength/sequence: A customized PETRA sequence to acquire fat-suppressed 3D T_1ρ -weighted images tissues with semisolid short T₂ / $T_2^{*}$ relaxation times in the knee and ankle joints at 3 T.

Assessment: Mono- and biexponential T_1ρ relaxation components were assessed in the patellar tendon (PT), anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and Achilles tendon (AT).

Statistical tests: Kruskal-Wallis with post-hoc Dunn's test for multiple pairwise comparisons.

Results: Phantom and ex vivo studies showed the feasibility of T_1ρ relaxation mapping using the proposed 3D-T_1ρ -PETRA sequence. The in vivo study demonstrated an averaged mono-T_1ρ relaxation of (median [IQR]) 15.9 [14.5] msec, 23.6 [9.4] msec, 17.4 [7.4] msec, and 5.8 [10.2] msec in the PT, ACL, PCL, and AT, respectively. The bicomponent analysis showed the short and long components (with their relative fractions) of 0.65 [1.0] msec (46.9 [15.3]%) and 37.3 [18.4] msec (53.1 [15.3]%) for PT, 1.7 [2.1] msec (42.5 [12.5]%) and 43.7 [17.8] msec (57.5 [12.5]%) for ACL, and 1.2 [1.9] msec (42.6 [14.0]%) and 27.7 [14.7] msec (57.3 [14.0]%) for PCL and 0.4 [0.02] msec (58.8 [13.3]%/) and 31.3 [10.8] msec (41.2 [13.3]%) for AT. Statistically significant (P ≤ 0.05) differences were observed in the mono- and biexponential relaxation between several regions.

Data conclusion: The 3D-T_1ρ -PETRA sequence allows volumetric, isotropic (0.78 × 0.78 × 0.78 mm), biexponential T_1ρ assessment with corresponding fractions of the tissues with semisolid short T₂ / $T_2^{*}$ .

Level of evidence: 2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:1207-1218.

Keywords: PETRA; T1ρ relaxation; ZTE; bi-exponential fitting; short-T2.