Helical CT is an important recent development in x-ray CT. In helical CT, planar projection sets are synthesized from raw projection data via interpolation. Among various interpolation schemes, linear interpolation is usually preferred due to its efficiency and performance. In this paper, image noise variance is derived for typical helical CT linear interpolation techniques, including the full scan (FS), under-scan (US), full scan with interpolation (FI), half-scan (HS), half-scan with interpolation (HI) and half-scan with extrapolation (HE) methods. Image noise deviation ratios of helical CT to conventional 360 degrees reconstruction (CR) are tabulated. These are consistent with previously reported simulation results. The theoretical results provide further understanding of helical CT noise performance. It is shown that helical CT image noise deviation is independent of transaxial position, proportional to the raw projection noise deviation, and not affected by the fan angle (approximately for the HE method). Also, helical CT image noise variance is proportional to the area under the square of the reconstruction filter.