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139 CT and MR features of postmortem change in deaths 8 INTRODUCTION Hospital autopsy rates today are as low as 0-5%, having decreased from a rate of 30% or higher in the 1990s. 22,32,205 This low rate is alarming, since one in five autopsies show majordiscrepanciesbetweenantemortemandpostmortemdiagnosesdespiteimproved diagnostic testing. 13 A possible cause for this decline may be the invasiveness of the conventional autopsy procedure. 102 To provide a less invasive alternative to conventional autopsy,imagingbasedautopsymethodsweredeveloped,primarilyinforensicmedicine. These modern autopsies include total-body postmortem CT (PMCT) and MR (PMMR), sometimes combined with CT angiography (PMCTA) and image-guided biopsies. 206-208 More recently the imaging autopsy is steadily emerging in clinical radiology and there is a growing number of diagnostic studies analyzing the performance of the noninvasive (imaging only) and minimally invasive autopsy (imaging with angiography and / or biopsies). 56,57,209 Combined PMCT, PMCTA and image-guided biopsies appear most sensitive in diagnosing cause of death, however more clinical studies are needed to accurately determine the diagnostic value of the imaging autopsy. 56,209 In forensic centers access to MR scanners is often limited, so PMCT is most commonly performed. In hospitals, MRI is more widely available, and its high performance to visualize organ parenchyma and soft tissues make PMMR a valuable addition to PMCT. Postmortem imaging is not the same as imaging the living. Directly after death various chemical and physical processes affect the body in ways that can change PMCT and PMMR features of organs and soft-tissues. These processes can generally be divided into gravity dependent changes (including sedimentation of blood and livor mortis; also known as lividity or hypostasis), decomposition (including putrefaction), rigor mortis (muscle stiffness) and algor mortis (cooling of the body). Livormortis is caused by blood settling in the dependent parts of the body due to gravity. Livores can be observed both internally, on imaging and autopsy, and externally upon visual inspection. External livores manifest as dark bluish (or livid) areas of the skin within several hours after death. Internal livores are noted as increased attenuation or signal changes of the dependent areas of organs. The combination of postmortem leakage of cell membranes and subsequent increased osmolality of the interstitial fluid, together with the effect of gravity leads to accumulation of fluids in dependent areas, such as the subcutaneous fat, thoracic cavity and abdominal cavity. 116,210,211 Decompositionconsistsofmanyprocessesthatcauseorganicmaterialtobreakdowninto simplerformsofmatter.Itincludesputrefaction,autolysisandinsectandanimalpredation. Putrefaction leads to gas formation, it is found intravascular in an early decomposition stage and in more advanced stages also in soft tissues and organ parenchyma. Rigor mortis leads to muscle contraction after death that results in muscle stiffness.