Multi-modality imaging systems perform diagnostic scans using multiple modalities, such as, for example, magnetic resonance (MR/MRI), computed tomography (CT), positron emission tomography (PET), and/or single photon emission computed tomography (SPECT). Multiple modalities are combined to provide complimentary and/or overlapping data sets. During operation, image quality of one or more imaging modalities, such as a SPECT modality, can be affected by motion during imaging, for example, respiratory motion. When using a SPECT modality, imaging artifacts may be generated during image acquisition due to motion of the patient. In multi-modality systems, the SPECT modality requires a relatively long duration data acquisition period, on the order of several minutes (e.g., about 2 to 30 minutes per image) for a typical clinically sufficient image. Typically, a large number of SPECT data acquisitions (e.g., frames) are acquired at many different time points during this period. Consequently, patient movement is a qualitative and quantitative limiting factor in SPECT scanning.
Common current practice is to reacquire data for a patient who moves during acquisition. In current SPECT environments, list mode data (e.g., data wherein each event, or projection, is listed sequentially, parameter by parameter) is not available. Current systems acquire projections in a step and shoot fashion and cover only a limited number of angles. A projection image is generated by a sum of all detected events (e.g., projections) over a defined dwell time and incorporates gamma detections which have undergone various physical and biological phenomena, including patient movement. Motion is estimated using a global correction which averages the motion present over the whole acquisition duration and neglects any motion that happens between the different acquired views. Severe motion artifacts are generated due to the averaging.
Data sets may be gated in an attempt to reduce motion impact on the data set. Gating results in lower statistics in each gate for nuclear (e.g., SPECT, PET) modalities, producing fewer projections per gate. The separate gates are noisier than summed datasets, as each of the gates includes a set of independent measurements each distorted by a different motion matrix. The reconstruction of the image and the noise in each gate have a higher correlation than summed reconstructions.