Small spheres for lesion detection phantoms

Small spheres are frequently used in radioactive phantoms to investigate lesion detectability and quantitation accuracy. The surrounding walls of fillable spheres displace background solution, and therefore reduce the apparent radioactivity concentration in the region of the sphere. For comparison purposes, this is not a problem, but to understand scanner capabilities in a more absolute way, it is desirable to have wall-less radioactive spheres of arbitrary size. The wall thickness is particularly important for small spheres, where partial volume effects are important. We have developed a technique of adding ¹⁸F and ^99mTc to wax, from which can be made radioactive spheres of arbitrary size down to a few millimeter diameter. The wax can be diluted to achieve any radioactivity concentration. The molten wax is measured volumetrically to achieve the desired sphere size. The hydrophobic nature of wax prevents the dissolving of the wax, loss of radioactivity in the background, and collection of background radioactivity into the wax. A pipette with preheated tip can be used for repeated measurement of 0.0141 microliter (3 mm diameter) allotements with less than 3% variation in volume (standard error). The technique was demonstrated in a phantom with both wax (10, 8, 7, 6, 5, and 4 mm diameters) and fillable (10, 8, and 6 mm diameters) spheres at an 8:1 sphere/background radioactivity concentration in a 20 cm diameter phantom imaged in a dedicated PET scanner. The images demonstrate the increased apparent activity and detectability in a non-walled sphere, compared to the same sized fillable sphere. This technique provides a relatively simple way to produce wall-less radioactive spheres of any size, using materials and equipment found in any PET facility.