Residual fraction in cold inserts of the NEMA NU 2–1994 phantom. Results for the three non-emitting air (square), water (circle), and Teflon (triangle) inserts as a function of the number of iterations with eight subsets for the four SPECT-CT systems. (A) Philips Brightview XCT. (B) General Electric Discovery NM/CT 670. (C) General Electric Hawkeye 4. (D) Siemens Symbia T6. All reconstructions with attenuation and scatter corrections and resolution recovery.