The following relates to the imaging arts. It finds particular application in single photon emission computed tomography (SPECT) imaging using radioisotopes emitting at multiple energies, and will be described with particular reference thereto. However, it also finds application in SPECT imaging using more than one different radiopharmaceutical, and in other types of nuclear imaging, nuclear spectroscopy, radiation therapy, and so forth.
In SPECT imaging, one or more radiopharmaceuticals are administered to a subject, a gamma camera detects radioactive emissions from the administered one or more radiopharmaceuticals, and the detections are processed to reconstruct an image of the distribution of the one or more radiopharmaceuticals in the subject. For example, the radiopharmaceutical may include a radioisotope attached to a carrier that concentrates in a target organ or other anatomical feature of a human subject, and the reconstructed image is representative of the target organ or other anatomical feature. The administered radiopharmaceutical dosage is typically relatively low, especially in the case of human imaging subjects, and the radioactive emission detections are analyzed using statistical techniques taking into account factors such as scattering, attenuation, and detector efficiency.
If the radiopharmaceutical generates a single emission peak, the scattering, attenuation, detector efficiency, and so forth are readily accounted for using parameters characteristic of the emission energy. However, where the radiopharmaceutical generates different emission peaks (for example, if the radioisotope emits at two or more energies, or if two or more radiopharmaceuticals are used), then accounting for scattering, attenuation, and so forth is difficult. Examples of multiple-peak radiopharmaceuticals include: complexes of In-111, which has main emission peaks at 171 keV and at 245 keV; complexes of Ga-67, which has main emission peaks at 93 keV, 184 keV, and 300 keV; and complexes of Tl-201, which has main emission peaks at 72 keV and 167 keV. Examples of imaging with two or more radiopharmaceuticals at the same time include dual isotope cardiac imaging allowing a stress Tc-99m image to be acquired simultaneously with a rest Tl-201 imaging, or using Tc-99m labeled blood cells to image anatomy while simultaneously using an In-111 complex to image prostate cancer.
Reconstruction of SPECT images from radiopharmaceuticals that produce different energy peaks is complicated by the fact that photons emitted at different energies will be scattered differently, attenuated differently, have different gamma camera detection efficiencies, and so forth. In addition, photons emitted from higher energies can be down-scattered and contaminate the projection data acquired in lower energy windows.
Typically, for multiple-peak isotopes, the gamma camera acquires or bins photons at different energies in a single projection image set, and the reconstruction employs average or approximate parameter values to account for scattering, attenuation, detector efficiency, and so forth. For example, iterative reconstruction methods typically utilize a single set of projections for comparison to estimated activity maps, even if the projections include photons acquired at different energies. Attenuation, scatter, and other compensations are performed by assuming an average or summed value for specific physical properties. Although the reconstructed images are clinically usable, the averaging or approximation degrades the quantitative accuracy of the image. Moreover, if two or more different radiopharmaceuticals are administered, each targeting a functional or molecular compartment and each emitting photons at different peak energies, then reconstruction of a single projection image set does not provide readily distinguishable imaging of the different functional or molecular compartments.