Changes in Properties of Polyvinyl Alcohol when Prepared under a Deoxygenated Environment

Introduction There are several materials currently used as tissue mimicking phantoms depending on the imaging modality in question. Poly(vinyl alcohol) cryogel (PVA-C) not only has the potential to be used as a multi-modality phantom, but it also has a variety of biomedical advantages, including its nontoxic, non-carcinogenic and biodegradable characteristics (1). This polymer is easily produced and readily modified with electric, elastic and MRI properties shown to be comparable to that of human tissue (2-4). In an effort to control various factors during the process of PVA-C manufacture, our group implemented a procedure where the PVA formulation process was carried out in an inert gas (nitrogen) environment. The amorphous regions of PVA-C act as a reservoir for water, in which oxygen gas readily dissolves. Since oxygen is paramagnetic, the magnetic resonance imaging (MRI) relaxation times, T1 and T2, of PVA-C manufactured in the presence of oxygen may hypothetically differ from those prepared in deoxygenated (nitrogen) conditions. The following study investigates the novelty of PVA-C as a tissue mimicking phantom by exploring the MR relaxation times of the cryogel prepared in a deoxygenated environment. The variation in the MRI properties will serve as an indicator of how dependent the MRI signal is on paramagnetic oxygen. Methods A 15% (w/w) PVA solution was prepared by adding 45g of PVA (MW 124,000 – 164,000; Aldrich 36316-2) to a flask containing 255-mL of distilled water. A nitrogen supply attached to the flask was initiated for 2 minutes in order to purge the air from inside the flask, thereby creating an oxygenfree environment. The remaining oxygen in the PVA solution was removed by heating and stirring the solution for 15 minutes, followed by an additional 2 minute nitrogen purge. The solution was then heated for 2 hours at 90° C to form a PVA hydrogel solution, which was injected into cylindrical cavity moulds and subjected to either 3or 6-freeze/thaw cycle(s) (FTC) in a temperature controlled (–20 oC to +20 oC; 0.1 oC per minute) ethylene glycol bath. The samples were then removed and stored in distilled water and thereafter referred to as PVA-C(N). The aforementioned procedure was then repeated, except the nitrogen purge was not performed and these samples were referred to as PVA-C. Image based measurements of T1 & T2 were performed using custom built 1T, 1.89T, and 3T MR systems. T1 was measured using a 2D inversion recovery imaging sequence repeated with 8 different inversion times (TI), spaced from 20 to 1000ms. T1 maps were created using pixel by pixel nonlinear least square fit to the equation S = K[1-2exp(-TI/T1)], then an ROI was drawn and the average T1 was calculated using the variation across the ROI. T2 was measured using a spin echo imaging sequence repeated with 6 different echo times (TE), which were spaced from 25 to 500ms. T2 maps were created using pixel by pixel log-linear least square fit, an ROI was drawn and the average T2 was calculated using the variation across the ROI. Results The effects on the MR relaxation times due to the introduction of nitrogen are shown in Figures 1 and 2. T1 significantly increased (p<0.05) with increasing field strength for the PVA-C samples, whereas there was no significant increase for the PVA-C(N) samples. T2 showed no significant increase for either PVA-C or PVA-C(N) with increasing field strength. The 6 FTC samples showed similar results to PVA-C(N) for T1 and T2. Discussion PVA-C showed a field dependence in T1 for both the 3 and 6 FTC samples. However, the introduction of nitrogen caused a loss of T1 field dependence for both the 3 and 6 FTC samples. This loss was the result of the removal of paramagnetic oxygen within the sample. No field dependence in T2 was shown for either PVA-C(N) or PVA-C. It has been shown previously that the mechanical properties of PVA-C depend greatly on the number of FTCs. More work on mapping the mechanical, electrical, and MRI properties is needed before using PVA-C as human tissue phantoms.