Ordered subset expectation maximization
In mathematical optimization, the ordered subset expectation maximization (OSEM) method is an iterative method that is used in computed tomography. In applications in medical imaging, the OSEM method is used for positron emission tomography (PET), for single-photon emission computed tomography (SPECT), and for X-ray computed tomography (X-ray CT) scans.
Source: Wikipedia — Ordered subset expectation maximization (CC BY-SA 4.0)