Quantitative pulmonary single photon emission computed tomography for radiotherapy applications

Pulmonary imaging using single photon emission computed tomography (SPECT) is the focus of current radiotherapy research, including dose-response anal ysis and three-dimensional (3D) radiation treatment planning. Improvement i n the quantitative capability of SPECT may help establish its potential rol e in this application as well as others requiring accurate knowledge of pul monary blood flow. The purposes of this study were to quantitatively evalua te SPECT filtered backprojection (FBP) and ordered subset-expectation maxim ization (OS-EM) reconstruction implementations for measuring absolute activ ity concentration in lung phantom experiments, and to incorporate quantitat ive SPECT techniques in 3D-RTP for lung cancer. Quantitative FBP (nonunifor m iterative Chang attenuation compensation, scatter correction, and 3D post reconstruction Metz filtering) and OS-EM implementations were compared with a "clinical" implementation of FBP (uniform multiplicative Chang attenuati on compensation and post-reconstruction von Hann filtering), for their abil ity to improve quantification of inactive and active spherical defects in t he lungs of an anthropomorphic torso phantom. Activity concentration estima tes were found to depend on many factors, such as region of interest size, scatter subtraction constant (k), postreconstruction deconvolution filterin g and, in the case of OS-EM, total number of iterations. In general, recons truction implementations incorporating compensation for nonuniform attenuat ion and scatter provided reduced bias relative to the clinical implementati on. Potential applications to lung radiotherapy, including dose-functional histograms and treatment planning are also discussed. SPECT has the potenti al to provide accurate estimates of lung activity distributions that, toget her with improved image quality, may be useful for the study and prediction of therapeutic response. (C) 1999 American Association of Physicists in Me dicine. [S0094-2405(99)00708-7].