Fast diffusion-weighted steady state free precession imaging of in vivo knee cartilage

Quantification of molecular diffusion with steady state free precession (SSFP) is complicated by the fact that diffusion effects accumulate over several repetition times (TR) leading to complex signal dependencies on transverse and longitudinal magnetization paths. This issue is commonly addressed by setting TR > T2, yielding strong attenuation of all higher modes, except of the shortest ones. As a result, signal attenuation from diffusion becomes T2 independent but signal-to-noise ratio (SNR) and sequence efficiency are remarkably poor. In this work, we present a new approach for fast in vivo steady state free precession diffusion-weighted imaging of cartilage with TR << T2 offering a considerable increase in signal-to-noise ratio and sequence efficiency. At a first glance, prominent coupling between magnetization paths seems to complicate quantification issues in this limit, however, it is observed that diffusion effects become rather T2(ΔD ∼ 1/10 ΔT2) but not T1 independent (ΔD ∼ 1/2 ΔT1) for low flip angles α ∼ 10 − 15°. As a result, fast high-resolution (0.35 × 0.35 − 0.50 × 0.50 mm2 in-plane resolution) quantitative diffusion-weighted imaging of human articular cartilage is demonstrated at 3.0 T in a clinical setup using estimated T1 and T2 or a combination of measured T1 and estimated T2 values