Comparison of fluorine-18 and bromine-76 imaging in positron emission tomography

State of the art positron emission tomography (PET) systems allow for scatt er and attenuation correction. However, the size of the structure being stu died and the region of interest (ROI) chosen also influence the accuracy of measurements of radioactive concentration. Furthermore, the limited spatia l resolution of PET tomographs, which depends, among other factors, on the range of positrons in matter, can also contribute to a loss in quantitation accuracy. In this paper we address the influence of positron range, struct ure size and ROI size on the quantitation of radioactive concentration usin g PET. ECAT EXACT HR+ (HR+) and ECAT 953B/31 (ECAT 953B) PET systems were u sed in phantom acquisitions performed with two radioisotopes with different positron ranges The 3D Hoffman phantom was scanned nn both ranges. The 3D Hoffman phantom was scanned on both scanners with both radioisotopes, to vi sually analyse the image quality. A resolution phantom having six spheres o f different diameters in a Plexiglas cylinder was used to calculate the val ues of the contrast recovery coefficient or hot spot recovery coefficient a nd of the spill-over or cold spot recovery coefficient under different imag ing conditions used in clinical routine at our institution. Activity ratios were varied between 2 and 30 or between 0.4 and 200 by filling the spheres with fluorine-18 or bromine-76 respectively and the cylinder with C-11. Dy namic scans were performed on each scanner. Data were reconstructed using t he same parameters as are used in the same parameters as are used in clinic al protocols. The variations in sphere and cylinder activities with time we re fitted using the function M(t)=k(1).A(t)+k(2).B(t), where M(t) is the ra dioactivity concentration measured in an ROI placed on each sphere and A(t) and B(t) represent the true radioactivity concentrations present at time t in the spheres and in the cylinder respectively. k(1) and k(2) are factors representing the contrast recovery coefficient and the spill-over from sur rounding activity on measurements respectively. The visual analysis of imag es obtained using a 3D Hoffman phantom showed that image resolution and ima ge contrast between different regions are radioisotope dependent and clearl y better when using F-18. Linear profiles taken on these images confirmed t he visual assessment. For a given scanner, the k(1) values obtained with F- 18 were systematically higher than those measured using Br-76 in the same m achine (especially for the smaller spheres) when using the same ROI. For a sphere of a particular diameter, the use of a wider ROI resulted in lower q uantitative accuracy when using the same isotope and the same camera. Lower quantitative accuracy was found for smaller spheres for all ROI sizes used in image analysis. For the same scanner and for a similar imaging situatio n (same sphere and same ROI), it was found that k(1) and k(2) values depend on the radioisotope used. For the same isotope and tomograph, the k(1) val ues obtained decreased with the size of the structures imaged, as well as w ith the increase in ROI size. The use of a tomograph with better spatial re solution (HR+, rather than ECAT 953B) greatly increased the k(1) values for F-18 while only a mild improvement in these values was observed for 76Br. The use of Br-76 led to k(2) values that were slightly higher than those me asured using F-18 These differences may have been due to the difference in the range of the positrons emitted by the radioisotopes used in this study. The measurements performed in this study show that the comparison of studie s obtained on the same camera depends on the radioisotope used and may requ ire the adaptation of ROI size between examinations. Marked differences are visible if the positron ranges of such radioisotopes are very different. T herefore, when employing commercially available tomographs and imaging prot ocols used in clinical routine, the effects of differences in positron rang e on image quality and quantitation are noticeable and correction for these effects may be of importance. With the arrival of PET imaging systems with better spatial resolution (close to 2 mm full-width at half-maximum for an imal PET systems), positron range will have an increasing influence on the image quality and on the choice of radioisotope for a given application.