The effects of collimation and reconstruction algorithm on image noise in CT were investigated using several low dose techniques typical of lung cancer screening protocols. Tube current settings were 10mA, 20mA, 50mA, and 100 mA, all at 120kVp and 0.8 sec rotation time. A homogeneous water phantom was scanned with various mA setting, collimation, and reconstruction algorithm combinations. Noise was measured under each condition and radiation doses for each tube current used were extrapolated from CTDIw values measured at each collimation. Noise values for each mA, collimation, and algorithm combination were compared as a function of radiation dose (CTDIw) and were also compared with the noise and radiation dose values of currently employed lung cancer screening techniques (e.g. 120 kVp, 50 mA, .8 sec, 2.5 mm collimation, bone reconstruction algorithm). The data shows that thinner slices (those < 2.5 mm) at the same mA setting and reconstruction algorithm yield higher noise values and higher radiation dose values than current techniques, as high as nearly 3 times the original CTDIw. In lung cancer screening imaging with CT, moving to thinner slices presents some difficult tradeoffs between dose and noise. Reconstruction algorithm can be used to reduce image noise, but at a price of reduced in-plane spatial resolution, offsetting some of the benefit of using thinner slices to detect smaller lesions.