Post-mortem evaluation of ischemic heart disease involves an examination of the coronary arteries, for occlusions, and myocardium, for signs of ischemia.¹The technical feasibility of post-mortem MR angiography (PMMRA) was previously demonstrated in a small cohort.² Its quality relies on a complete filling of the vessels of interest, as well as on attainable contrast and image quality in MRI. Perfusate suitability therefore depends on viscosity and relaxation parameters under the additional consideration of cadaver temperature. To develop an approach to PMMRA, this work investigated the viscosity, T1 and T2 of different liquids considered potentially suitable for achieving post-mortem reperfusion of the vascular system over a relevant temperature range. Additionally, simulations to explore contrast with post-mortem myocardium were performed.Relaxation parameters and dynamic viscosity of candidate liquids (n=10) were characterised at different temperatures between 1 and 23°C. For measurements in MRI, samples were placed in a water bath with regulated temperature (1, 8.5, 16 and 23°C) and scanned at 3T (Skyra, Siemens AG) using TIR and multi-echo SE sequences (Table 1). For data analysis (T1, T2) mono-exponential models were fitted. Temperature was monitored using a real-time fibre optic temperature sensor. Dynamic viscosity values were obtained through conversion of density and kinematic viscosity measurements (Ubbelohde viscometer) performed at 8, 10 and 20°C. Temperature dependence of T1, T2 and dynamic viscosity was modelled using quadratic fits. Bloch equation simulations (FLASH) were performed for selected liquids using the characterised relaxation times and literature values for post-mortem myocardium³ at 1, 8.5, 16 and 23°C. At each temperature, contrast between selected liquids and post-mortem myocardium was examined for flip angles between 0-90° (integer values) using three different repetition times (TR=50, 100 and 150ms). For the TR with best overall contrast (150ms), flip angles maximising contrast between a given liquid and post-mortem myocardium were calculated for each temperature. For each liquid, these flip angles were then applied across the entire temperature range to explore the effect of temperature on contrast differences due to selection of an optimal flip angle corresponding to one of the other investigated temperatures.

The temperature dependence of sample relaxation times and viscosity was successfully described by the established quadratic models, with all fits producing excellent adjusted R² values (Table 2, Figure 1). Mineral oil, paraffin oil, Angiofil® and the paraffin oil/Angiofil® mixture showed only a slight to moderate positive correlation between temperature and T1. All samples except silicon showed a similar, positive correlation between temperature and T2. Dynamic viscosity of the liquids was negatively correlated with temperature, with mineral oil, paraffin oil, Angiofil® and the paraffin oil/Angiofil® mixture demonstrating moderate temperature dependence. Due to these results, mineral oil, paraffin oil and a paraffin oil/Angiofil® mixture were selected for the simulations.

Bloch equation simulations demonstrated that maximum contrast with post-mortem myocardium would be achievable using mineral oil (Figure 2). As seen in this figure, paraffin oil and a mixture of paraffin oil and Angiofil® performed similarly, with contrast in the worst case 6.5% less than mineral oil (TR=150 ms at 23°C). The simulated results additionally demonstrated the influence of temperature on the optimal flip angle required to achieve maximum contrast between liquids and post-mortem myocardium (Figure 3). Optimal flip angles were found to be higher at lower temperatures and varied by up to 10° across the investigated temperature range for a given liquid. Differences in contrast due to the selection of a flip angle corresponding to the optimal flip angle at a different temperature were moderate (max. 6-12%, depending on the sample (Figure 4)).

The established models reliably describe the behaviour of dynamic viscosity, T1 and T2 for potentially PMMRA-suitable liquids over a forensically relevant temperature range. The dynamic viscosity of a liquid is a critical determinant for the calibre of the vessels which can be filled in post-mortem angiography.⁴ Dynamic viscosities in the range of 15mPa.s⁵ to 65mPa.s are preferred at 20°C. Adaption of post-mortem MRI protocols depending on cadaver temperature is facilitated by a minimal temperature dependence of perfusate relaxation parameters (Table 2). Additionally, a generally short T1 was preferred. Based on these criteria, mineral oil, paraffin oil and the paraffin/Angiofil® mixture were deemed suitable. Simulations indicated a moderate decrease in contrast when flip angles optimised for other temperatures were applied, meaning that for a given perfusate, consideration of temperature is recommended when selecting the flip angle to avoid loss of contrast.The temperature-dependent characterisation of physical and MRI properties of candidate liquids performed in this work provides essential information for the simulation and development of MRI protocols in post-mortem MR angiography research.