One of the major limitations of gamma cameras is their relatively poor energy resolution. The main practical consequence of this is that the detection of both scattered and unscattered photons in the photopeak energy window, affecting image contrast and resolution, makes the data inconsistent with the assumption of scatter-free projection data in reconstruction and attenuation correction algorithms. Here, we proposed a method to improve the effective energy resolution of scintigraphic acquisitions. This method is called photon energy recovery (PER).
Methods: Photon energy recovery is based on a spectral deconvolution analysis and uses iterative recurrent linear regressions. In practice, PER only required splitting the photopeak energy window into several subwindows and did not need list mode acquisitions. The method was fully automated. Photon energy recovery was quantitatively validated on 99mTc planar images using a Monte Carlo simulation and a real phantom and was illustrated by a bone study.
Results: The Monte Carlo simulation demonstrated that convergence was reached within relatively few (10-15) iterations. Photon energy recovery led to a considerable quantitative improvement because the mean error between the photopeak energy window image and the true unscattered image was equal to 8.72 s.d. (the mean error between one image and the true image was the mean of the differences between the two images; the difference is expressed as several s.d., where s.d. was the square root of the true value), whereas the mean error between the 140-keV PER image and the true unscattered image was only equal to 2.70. Moreover, the true and PER spectra were highly correlated. The real phantom data pointed out that the counts in the 140-keV PER image calculated from the images acquired "with scatter" were not very different from the true counts given by the "scatter-free" reference image. Planar pelvic bone scintigraphy demonstrated the advantages of PER because contrast increased when only unscattered photons were selected.
Conclusion: Photon energy recovery is a stable and automated method that allows recovery of the correct value of the photon energy after a scintigraphic acquisition. Its ability to separate scattered from unscattered events has been quantitatively validated.