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Noninvasive follow-up of tissue encapsulation of foreign materials. Are magnetic resonance imaging and spectroscopy breakthroughs?

The development of sensitive and noninvasive magnetic resonance (MR) techniques for the long- and short-term evaluation of vascular prostheses requires detailed knowledge of the evolutionary trend of the MR properties of the perigraft tissue during the healing process. To characterize changes in water MR properties, the water proton relaxation times, T1 and T2, of the muscle in the vicinity of an implanted polyester material were measured as a function of implantation time. To provide better insight into interpretation of the MR results, we carried out histologic and peripheral blood cell activation studies and tissue water content measurements. The MR results illustrated the sensitivity of the relaxation times to changes in cellular response to the presence of an implant. The evolutionary trend of these MR parameters exhibited two distinct phases. The crossover from phase I to phase II occurred around 10 days postimplantation. This crossover is attributed to the transition in the inflammatory response from the acute phase to the chronic phase. During the acute phase, the very high initial T1 and T2s (the slower relaxing component of the transverse relaxation time) values decreased significantly and steadily. The value of T1 dropped by a factor of 2, whereas T2s went down by a factor of 6. During the same time, the diffusion parameter, beta, remained constant. However, during the chronic phase, the diffusion parameter increased sharply. By 30 days postimplantation, the value of beta had increased by a factor of 10. The relaxation times, on the other hand, increased steadily with implantation time. Because the current MR results provide an in vivo and noninvasive follow-up of the healing process around the polyester implant material, they will be of considerable value in the early detection of vascular graft complications by MR imaging.