Post-mortem Tissue Degassing Using Positive Pressure Is Superior to Negative Pressure

Objective: To compare the effectiveness of positive pressure (PP) and negative pressure (NP) for reducing gas inclusions in biological tissues in preparation for acoustic imaging. Methods: Eighteen pieces of porcine liver in degassed saline were included in this study. For the PP group (n = 9 samples), a wristwatch waterproof tester was used to pressurize samples to 0.41 MPa (59 psi) for 10 min. For the NP group (n = 9 samples), a desiccator at -0.08 MPa (-12 psi) was used for 30 min. Backscatter coefficients (BSCs) were calculated over the central frequency range of the backscattered spectra and paired-samples t-tests were performed. Results: Utilization of PP resulted in a decrease in BSC for all samples, indicating less gas post-PP (pre-PP -13.0 +/- 4.3 dB [mean +/- SD], post-PP -18.9 +/- 5.0 dB, p = .001). Utilization of NP resulted in an increase in BSC for the majority of samples (pre-NP -14.6 +/- 6.0 dB, post-NP -13.1 +/- 5.3 dB, p = .177). Conclusion: Utilization of a simple PP chamber consistently resulted in a decrease in tissue gas, at lower pressures than previously reported. The vacuum method is ineffective, may result in a paradoxical increase in tissue gas, and may not be recommended for tissue degassing.