Evaluation of Simultaneous Dual-radioisotope SPECT Imaging Using (18)F-fluorodeoxyglucose and (99m)Tc-tetrofosmin

Abstract

Objectives: Use of a positron emission tomography (PET)/single-photon emission computed tomography (SPECT) system facilitates the simultaneous acquisition of images with fluorine-18 fluorodeoxyglucose ((18)F-FDG) and technetium ((99m)Tc)-tetrofosmin. However, (18)F has a short half-life, and 511 keV Compton-scattered photons are detected in the (99m)Tc energy window. Therefore, in this study, we aimed to investigate the consequences of these facts.

Methods: The crosstalk correction for images in the (99m)Tc energy window involved the dual energy window (DEW) subtraction method. In phantom studies, changes in the count of uniform parts in a phantom (due to attenuation from decay), signal detectability in the hot-rod part of the phantom, and the defect contrast ratio in a cardiac phantom were examined.

Results: For (18)F-FDG in the step-and-shoot mode, nearly a 9% difference was observed in the count of projection data between the start and end positions of acquisition in the uniform part of the phantom. Based on the findings, the detectability of 12 mm hot rods was relatively poor. In the continuous acquisition mode, the count difference was corrected, and detectability of the hot rods was improved. The crosstalk from (18)F to the (99m)Tc energy window was approximately 13%. In the cardiac phantom, the defect contrast in (99m)Tc images from simultaneous dual-radionuclide acquisition was improved by approximately 9% after DEW correction; the contrast after correction was similar to acquisition with (99m)Tc alone.

Conclusion: Based on the findings, the continuous mode is useful for (18)F-FDG acquisition, and DEW crosstalk correction is necessary for (99m)Tc-tetrofosmin imaging.

Keywords: 18F-FDG; 99mTc-tetrofosmin; DEW method; PET/SPECT system.

Figure 1

Count distribution of projection data. The continuous and step-and-shoot modes were applied in SPECT imaging

Figure 2

SPECT image of the hot-rod phantom (top: ¹⁸F, bottom: ^99mTc, left: step-and-shoot mode, right: continuous mode)

Figure 3

The energy spectra of ¹⁸F and ^99mTc by Infinia 8. The intensity was similar to that of the clinical study

Figure 4

The count ratio of each single radionuclide and the combination of two radionuclides (the concentrations were changed). Single radionuclides were assessed with and without DEW crosstalk method. X-axis: concentration of ^99mTc as a single radionuclide (100%) and combination of two radionuclides (20%, 40%, 60%, and 80%). Y-axis: The count of the cardiac phantom per pixel

Figure 5

Bull’s eye map for the normal cardiac phantom without a defect for single and dual-radionuclide acquisitions

Figure 6

Bull’s eye map for the cardiac phantom, placed on the anterior defect for single and dual-radionuclide acquisitions

Figure 7

Bull’s eye map; SA: short axis, VLA: vertical long axis, and HLA: horizontal long axis in the evaluated patient. ^99mTc-tetrofosmin SPECT images were obtained before and after DEW crosstalk correction. An 81-year-old male was admitted to our center with chest pain. His risk factors for coronary artery disease included a prior history of hypertension and a family history of ischemic heart disease. The electrocardiogram showed ST-segment elevation in II, III, and AVF. The patient underwent selective left and right coronary artery angiography. The left coronary angiography showed moderate stenosis in segment VI. The right coronary angiography revealed 90% stenosis in segment I. He underwent stent placement in segment I. Seven days after stenting, simultaneous ¹⁸F-FDG and ^99mTc-tetrofosmin SPECT was performed