Early ischemia in growing piglet skeleton: MR diffusion and perfusion imaging

PURPOSE: To determine whether diffusion changes with ischemia of increasing duration, whether diffusion magnetic resonance (MR) imaging provides different information than does gadolinium-enhanced imaging, and which structural and/or biochemical changes are potentially responsible for any changes in diffusion. MATERIALS AND METHODS: Ischemia was surgically induced in one hip of each piglet (n=8) after approval from the Subcommittee on Research Animal Care; the other hip served as a control. Piglets were imaged at approximately 48 hours and 1, 2, 4, and 8 weeks after surgery at 1.5 T by using line-scan diffusion and dynamic gadolinium-enhanced MR imaging. Apparent diffusion coefficients (ADCs) and enhancement ratios (ERs) were calculated. Significant differences in ADC and ER values over time were evaluated by using the Student t test (P<.05). At 8 weeks, piglets were sacrificed for histologic evaluation. RESULTS: MR images of ischemic hips showed essentially no flow 48 hours after surgery. Spontaneous partial reperfusion was observed 1-4 weeks after surgery (ischemic ER/control ER=66%+/-35 [standard deviation]), and the ER of the ischemic hips was well above that of the control hips at 8 weeks. The ADC of ischemic hips was elevated above that of control hips before reperfusion 1 week after surgery by 47%+/-12 and remained elevated despite flow restoration. Gross structural abnormalities on MR images appeared to coincide with reperfusion. Histologic findings revealed abnormal epiphyseal cartilage thickening, cartilaginous islands within ossified tissue, and less fatty marrow in ischemic hips than in control hips; all of these factors could explain elevated ADC. CONCLUSION: Diffusion is sensitive to early ischemia and follows a different time course than that of changes observed with gadolinium enhancement. ADC remained elevated in this model of severe, prolonged ischemia despite the spontaneous partial restoration of blood flow seen on gadolinium-enhanced images.