The ACR small MRI phantom is a short, hollow cylinder of acrylic plastic closed at both ends. The inside length is 100 mm; the inside diameter is 100 mm. It is filled with a solution of nickel chloride and sodium chloride: 10 mM NiCl2 and 0.45% by weight aqueous NaCl. The separate vial is filled with 20 mM NiCl2 but no aqueous NaCl.