During the development and characterization of imaging technology or new imaging protocols, it is usually instructive to perform phantom experiments. Often, very simplified forms of the realistic patient anatomy are used that may be acceptable under certain conditions; however, the implications for patient studies can be misleading. This is particularly true in breast and axillary node imaging. The complexities presented by the anatomy, variable object scatter, attenuation and inhomogeneous distribution of activity in this upper thoracic region provide a significant challenge to the imaging task.
Methods: A tissue-equivalent anthropomorphic phantom of the thorax (Radiology Support Devices, Inc., Long Beach, CA) containing fillable cavities and organs was modified for the studies. The phantom was filled with realistic levels of FDG activity and scanned on a Siemens ECAT HR+ whole-body PET scanner. Breast attachments containing 2.0- and 2.55-cc lesions with lesion-to-background ratios of 5:1 and 7:1, respectively, were imaged. Scatter and attenuation effects were analyzed with various experimental setups. A lymph node experiment and a multibed position whole-phantom scan also were performed to illustrate the extent to which the phantom represents the human thorax.
Results: Regions of interest were drawn on the lesions as well as the background breast tissue in all studies. It was found that the signal-to-noise ratio decreased 65% when a more realistic phantom (lesions plus breasts plus thorax, all containing activity) was used, as compared to a simple phantom (lesions plus breasts containing activity; no thorax), due to the effects of increased scatter and attenuation. A 23% decrease in the contrast also was seen from the scan of the more realistic phantom due to surrounding activity from nearby organs such as the heart, as well as an increase in the volume of attenuating media.
Conclusion: This new phantom allows us to more realistically model the conditions for breast and lymph node imaging, leading to preclinical testing that will produce results that better approximate those that will be found in vivo. The phantom will be a valuable tool in comparing different imaging technologies, data collection strategies and image reconstruction algorithms for applications in breast cancer using PET, SPECT or scintimammography systems.