A method of preparing a polyvinyl alcohol hydrogel contains the steps of charging an aqueous solution of a polyvinyl alcohol having a degree of hydrolysis of not less than 95 mol % and an average polymerization degree of not less than 500 into a container, the aqueous solution containing the polyvinyl alcohol in a concentration of not less than 5 wt. %, cooling the aqueous solution of the polyvinyl alcohol at a temperature of from -(minus) 9.degree. C. to - (minus) 3.degree. C., subjecting the aqueous solution of the polyvinyl alcohol to physical stimulation, and then maintaining the aqueous solution of the polyvinyl alcohol at a temperature of from - (minus) 5.degree. C. to + (plus) 5.degree. C. The polyvinyl alcohol hydrogel may be contained in a plastic container. An MRI phanton is composed of the plastic container containing such polyvinyl alcohol hydrogel.
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