OPTIMIZATION OF A DUAL-ECHO IN THE STEADY-STATE (DESS) FREE-PRECESSION SEQUENCE FOR IMAGING CARTILAGE

Three-dimensional (3D) MR imaging of the knee is useful to detect cart ilage abnormalities, although the tissue contrast in 3D gradient-recal led echo (GRE) sequences such as gradient-recalled acquisition in the steady state (GRASS) or fast low-angle shot (FLASH) is poor. T2 contra st can be added to a GRASS sequence by combining the signals from the first and second gradient echoes, which form immediately after and imm ediately before each radiofrequency (RF) pulse in a 3D GRE sequence, W e have optimized a 3D dual echo in the steady state (DESS) sequence, w hich produces one averaged image from the two echoes, for use in the d etection of articular cartilage abnormalities, In the optimization pro cess, we examined the imaging parameters of flip angle (alpha), repeti tion time (TR), echo time (TE), and bandwidth to maximize the contrast between cartilage and joint fluid, A theoretical simulation of the se quence was; confirmed with experiments conducted on phantoms with know n T1 and T2. On the basis of theoretical predictions and experiments u sing healthy volunteers, we determined that an optimized sequence with a bandwidth of 98 Hz per pixel, a TR of 30 msec, a TE of 7.1 msec, an d an alpha of 60 degrees produced the highest contrast between cartila ge and fluid within a defined acquisition time of 6 minutes, Additiona l contrast was obtained by filtering the second-echo image to eliminat e noise before adding it to the first-echo image.