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3D continuous emission and spiral transmission (CEST) scanning provides a high-throughput whole-body PET study by using two dedicated detectors for 3D emission and singles transmission, together with continuous bed movement. To supress the amount of transmission scatter components (TSC) in singles transmission data, the transmission detector was designed to have a short axial extent with a highly collimated 137Cs point source, and to remove the amount of emission contamination (EC) in post-injection scanning, real-time EC correction was implemented. However, transmission images can be still affected by residual EC and TSC, depending on patient size and injected dose. This produces slight variations in attenuation coefficients, depending on the patient's axial and radial positions. In this study, we developed a new soft-tissue segmentation (STS) method based on histogram scaling at each axial position of the spiral transmission. Peaks, corresponding to soft-tissue in a histogram of attenuation coefficients, were found at each axial position and the transmission image was scaled using the ratio of soft-tissue histogram peaks to the theoretical water attenuation coefficient. In scaled transmission images, pixel values near soft-tissue peaks were replaced with their theoretical water attenuation coefficients. Quantitative evaluation of the transmission images obtained was performed under various acquisition conditions, both with and without the proposed STS method. Final imaging performance evaluations included quantification of emission images reconstructed using both STS attenuation correction and hybrid scatter correction. Results showed that the proposed STS method for spiral transmission scanning provided quantitative images that contained activity, independent of object size.