Positron emission mammography (PEM): Effect of activity concentration, object size, and object contrast on phantom lesion detection

Purpose: To characterize the relationship between lesion detection sensitivity and injected activity as a function of lesion size and contrast on the PEM (positron emission mammography) Flex Solo II scanner using phantom experiments. Methods: Phantom lesions (spheres 4, 8, 12, 16, and 20 mm diameter) were randomly located in uniform background. Sphere activity concentrations were 3 to 21 times the background activity concentration (BGc). BGc was a surrogate for injected activity; BGc ranged from 0.44-4.1 kBqmL, corresponding to 46-400 MBq injections. Seven radiologists read 108 images containing zero, one, or two spheres. Readers used a 5-point confidence scale to score the presence of spheres. Results: Sensitivity was 100 for lesions 12 mm under all conditions except for one 12 mm sphere with the lowest contrast and lowest BGc (60 sensitivity). Sensitivity was 100 for 8 mm spheres when either contrast or BGc was high, and 100 for 4 mm spheres only when both contrast and BGc were highest. Sphere contrast recovery coefficients (CRC) were 49, 34, 26, 14, and 2.8 for the largest to smallest spheres. Cumulative specificity was 98. Conclusions: Phantom lesion detection sensitivity depends more on sphere size and contrast than on BGc. Detection sensitivity remained 90 for injected activities as low as 100 MBq, for lesions 8 mm. Low CRC in 4 mm objects results in moderate detection sensitivity even for 400 MBq injected activity, making it impractical to optimize injected activity for such lesions. Low CRC indicates that when lesions 8 mm are observed on PEM images they are highly tracer avid with greater potential of clinical significance. High specificity (98) suggests that image statistical noise does not lead to false positive findings. These results apply to the 85 mm thick object used to obtain them; lesion detectability should be better (worse) for thinner (thicker) objects based on the reduced (increased) influence of photon attenuation.