Abstract
Objective: Nuclear medicine is becoming increasingly important in the early detection of malignancy. The advantage of nuclear
medicine over other imaging modalities is the high sensitivity of the gamma camera. Nuclear medicine counting equipment has the
capability of detecting levels of radioactivity which exceed background levels by as little as 2.4 to 1. This translates to only a few
hundred counts per minute on a regular gamma camera or as few as 3 counts per minute when using coincidence detection on a
positron emission tomography (PET) camera.
Material and Methods: We have experimentally measured the limits of detectability using a set of hollow spheres in a Jaszczak
phantom at various tumor-to-background ratios. Imaging modalities for this work were (1) planar, (2) SPECT, (3) PET, and (4)
planar camera with coincidence detection capability (MCD).
Results: When there is no background (infinite contrast) activity present, the detectability of tumors is similar for PET and planar
imaging. With the presence of the background activity , PET can detect objects in an order of magnitude smaller in size than that can
be seen by conventional planar imaging especially in the typical clinical low (3:1) T/B ratios. The detection capability of the MCD camera
lies between a conventional nuclear medicine (planar / SPECT) scans and the detection capability of a dedicated PET scanner
Conclusion: Among nuclear medicine’s armamentarium, PET is the closest modality to CT or MR imaging in terms of limits of
detection. Modern clinical PET scanners have a resolution limit of 4 mm, corresponding to the detection of tumors with a volume of
0.2 ml (7 mm diameter) in 5:1 T/B ratio. It is also possible to obtain better resolution limits with dedicated brain and animal scanners.
The future holds promise in development of new detector materials, improved camera design, and new reconstruction algorithms
which will improve sensitivity, resolution, contrast, and thereby further diminish the limits of tumor detectability