Numerical observer study of MAP-OSEM regularization methods with anatomical priors for lesion detection in 67Ga images

With the increasing number of multimodality systems, registered anatomical data is becoming available for use as patient-specific attenuation maps and anatomical priors in iterative reconstruction. The goal of this work is to investigate whether the introduction of anatomical information in MAP-EM reconstruction can improve lesion detection in Ga-67 images of the chest region. Specifically we investigated three hypotheses. The first hypothesis was that use of information on organ boundaries in MAP-EM will increase the accuracy of tumor detection. The second hypothesis was that use of lesion boundaries in addition to organ boundaries will increase the detection accuracy of lesions when the lesion actually has an elevated concentration of activity compared to background. The third hypothesis was that use of the organ boundary prior in MAP-EM results in improved lesion detection accuracy for lesions with an elevated activity concentration even when the lesion boundary prior was also used for sites where there was not an elevated activity concentration. These hypotheses were investigated using Monte Carlo simulated projections of the mathematical cardiac-torso (MCAT) phantom. The organ and lesion boundaries for use as anatomical priors were obtained from segmentation of the original MCAT activity slices. The priors were used to determine voxel inclusion in the neighborhood of a voxel for a quadratic smoothing prior employed in De Pierro's MAP-OSEM reconstruction algorithm. Two contrasts for lesion/background were investigated, 12/1, and 22.5/1. Three values for beta, the parameter controlling the weighting of the prior, were also investigated. A numerical observer was used to determine the average lesion detection accuracy for multiple sites throughout the mediastinum. The area under the ROC curve (AUC) for the numerical observer was used as the metric for detection accuracy. No evidence was observed in support of the first hypothesis. That is, use of the anatomical prior including just organ boundaries did not tend to increase the AUC over that of not using priors. Evidence was found in support of the second hypothesis however. That is, especially for the lower contrast lesions, the AUC increased, when the lesion boundary was included in the prior. Finally, evidence was also observed in support of the third hypothesis. That is, again especially at the lower. contrast, the use of the anatomical prior increased detection accuracy for lesions with increased uptake even when the prior was also used for matching sites with no elevation in uptake. Thus, use of the anatomical prior for lesion boundary when there was no. elevated lesion uptake did not tend to increase the false-positive rate as one might have guessed would happen.