Magnetic resonance ultrashort echo time spin-echo imaging of the deepest layers of articular cartilage

Abstract

Magnetic resonance imaging (MRI) has emerged as an invasive radiologic technique to assess and characterize cartilage lesions in the setting of injury and degenerative joint disease. However, most of the currently available clinical and research MRI techniques, including proton-density weighted fast spin echo (FSE) [1], T2-weighted FSE [2], T2 mapping [3], and steady state free precession imaging [4] have focused on the superficial layers of cartilage. There is a growing interest in the deepest layers of articular cartilage, including the calcified and deep radial layers located just superficial to the subchondral bone [5,6]. There has been an emphasis on the role of lesions in the deep radial and calcified layers of cartilage in the pathogenesis of osteoarthritis [6,7]. The lack of imaging evaluation of the deep layers of cartilage stems largely from the fact that it is technically difficult to image. Conventional MRI pulse sequences with echo times (TE) of 1 ms or greater provide little or no detectable signal from many tissues and tissue components that have very short T2 relaxation times, such as calcified cartilage, menisci, tendons, ligaments and some forms of soft tissue calcification [8]. By using half-sinc radiofrequency (RF) pulses, radial mapping of k-space, rapid transmit/receive switching, and variable rate selective excitation, nominal TEs as short as 8 μs have been achieved with ultrashort TE (UTE) imaging [9,10]. Therefore, the UTE pulse sequences make it possible to directly image tissues with very short T2 and their adjacent tissues [11]. Although short T2 tissues are detectable with UTE sequences, positive visualization of deep layers of cartilage is limited by the presence of high signals from long T2 water and fat. The dual-echo gradient-echo UTE acquisition has been used previously to suppress long T2 signals in knee cartilage two-dimensional (2D) imaging [8]. With this approach, the second echo is subtracted from the first one, which is equivalent to band pass filtering. This selectively suppresses the signal from the longer T2 components, and typically provides high contrast in the short T2 range. However, the 2D UTE free induction decay (FID) acquisition is sensitive to eddy currents, gradient anisotropy and timing errors [12]. The latter echoes in this acquisition are sensitive to off-resonance effects. Therefore, we developed a dual-echo spin echo UTE acquisition approach. This letter describes the development and implementation of dual-echo spin-echo UTE sequences, and testing its relative efficacy in terms of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in imaging deep radial and calcified layers of articular cartilage.