Ultrasonic Time Reversal Mirror (TRM) is an original solution to detect small cracks in a solid of any geometrical shape. Indeed, nondestructive testing need high resolution capabilities, high reliability and shortest control time. But the more recent techniques need an a priori knowledge of the liquid-solid interface and present limitations. The first of these uses several large prefocussed arrays, one array for each testing depth. Therefore this control system is sensitive to the arrays alignment, limited by the array precision focusing, and requires a complex mechanical follower system. A greater flexibility can be obtained by a second technique that is based on delay lines. But an accurate knowledge of the interface and a high calculs resolution are indispensable.