[MRI techniques applied to the diagnosis of hepatic tumors]

We know that the contrast of MR images can be better than that of CT scans. In the spin echo mode, sequences with long TR and long TE provide great contrast, while, conversely, sequences with short TR and TE produce images with a greater anatomical fidelity. This dual performance, a notion that had never been expressed as explicitly in imaging, has led to distinguishing between the roles of MRI for detection and tissue characterization (diagnosis and nature). While the choice of an appropriate sequence for characterization has apparently been made immediately with a modulation of T2 weighting, the selection of a detection sequence is less easy. The number of parameters involved (signal-to-noise and contrast-to-noise ratios, spatial resolution, artifacts, imaging time) and the compromise arrangements needed to take them into account explain the lack of consensus as to the optimal sequence for detection. Instead of becoming simpler with time, the issue is increasingly complex, as now fast imaging has been developed in addition to conventional spin echo imaging, and may even supersede it.