MR imaging of joints: analytic optimization of GRE techniques at 1.5 T.

To clarify the choice of imaging parameters for optimal gradient-recalled echo MR scanning of joints, we analyzed the behavior of contrast-to-noise and signal-to-noise ratios for spoiled (i.e., fast low-angle shot [FLASH] or spoiled GRASS) and steady-state (i.e., gradient-recalled acquisition in the steady state [GRASS] or fast imaging with steady precession) techniques at 1.5 T. The analysis is based on tissue characteristics derived from spin-echo measurements of hyaline cartilage and synovial fluid signal in the patellofemoral joints of 11 volunteers. Separate analysis of contrast-to-noise and signal-to-noise ratios for multiplanar (long TR) acquisitions shows that these parameters are each improved compared with single-slice methods. At TRs greater than 250 msec, there is no significant difference in the contrast behavior of FLASH and GRASS. For optimal contrast-to-noise ratio (synovial fluid-cartilage), the best multiplanar sequence (for TE less than 23 msec) is with a short TE and a large flip angle (e.g., 400/9/73 degrees [TR/TE/flip angle]). If a single-scan or three-dimensional technique is desired, than a GRASS sequence at minimal TR and TE and intermediate flip angle (18/9/32 degrees) is best. For optimal signal-to-noise ratio (for both synovial fluid and hyaline cartilage), the best multiplanar sequence uses a short TE and an intermediate flip angle (e.g., 400/9/30 degrees). If a short TR, high signal-to-noise technique is desired, then GRASS (18/9/13 degrees) is superior to FLASH.