Analysis of imaging parameters, relaxation time (T1,T2), and relative proton density as they influence the signal intensity of spin echo images.

It is difficult to recognize signal intensity changes on spin echo (SE) images. Clinically both T1-weighted and T2-weighted images are now used in most institutions to evaluate signal intensity changes due to different T1 and T2 of the lesions in MRI. However, these are not sufficient to understand signal intensity changes completely. The authors attempted to summarize on a two-dimensional graph the effects of T1, T2, proton density, and imaging parameters that influence signal intensities on SE images. Both end point (E) and start point (S), as defined later, depended mainly on proton density, but the former also depended on T2 and the later on T1 value. The image obtained in the former situation (near point E) was then called the T2-weighted image. Conditions to accelerate recovery and attenuation velocities (VR and VA), also mentioned later, were the same as those that increased points E and S, respectively. However, when TR was relatively short, the effect of a shorter T1 on VR was greatly emphasized. Such an image was then called the T1-weighted image.