ABSTRACT
Cellular apoptosis, a common pathway towards tumor regression, is induced by many radiotherapy and chemotherapy regimens. Imaging methods that can detect apoptosis may be able to assess treatment response earlier than typical tumor volume measurements. In this paper, a wide range of diffusion experiments and a simple model of diffusion in tissues were used to probe the microstructural effects of apoptosis. Experiments were conducted on acute myeloid leukemia cell pellets, where apoptosis was induced by treatment with the chemotherapeutic agent cisplatin. Seventy-two hours following cisplatin treatment, pulsed and oscillating gradient diffusion measurements were utilized to assess effects across a broad range of structural scales. The presence of apoptosis, which was histologically confirmed by TUNEL (terminal deoxynucleotidyl transferase UTP nick end labelling) staining, significantly changed diffusion properties. To describe these changes, the data were fit to the parallel plane model, which characterizes the effects of restricted diffusion in terms of three parameters: d, the restricted size, Dfree , the intrinsic, free diffusion coefficient of the solvent, and Drest , the long time or "restricted" diffusion coefficient. Apoptotic samples exhibited significant decreases in parameters d and Dfree and a significant increase in Drest . These changes appear consistent with the established morphological effects of apoptosis. In particular, the decrease in d may be a result of the combined effects of cell shrinkage, nuclear fragmentation and membrane blebbing, the decrease in Dfree may relate to cytosolic condensation, while the increase in Drest can be attributed to increased membrane permeability and extracellular volume fraction. By non-invasively detecting apoptosis, the methods reported in this study have the potential to improve upon current MRI methods for monitoring therapeutic response. Furthermore, these methods may offer sufficient specificity to differentiate between apoptosis and other modes of cell death, such as oncosis or necrosis.