Cynthia H. McCollough PhD, Presenter: Research grant, Siemens AG Research grant, RTI Electronics AB Research grant, Bayer AG

Lifeng Yu PhD, Abstract Co-Author: Research grant, Xoran Technologies, Inc

To demonstrate the increased noise levels for small patients and increased dose levels for obese patients that occur with automatic exposure control (AEC) systems designed to maintain constant image noise.

Relative tube current (mA) vs. relative patient attenuation was determined using the model: mA = reference mA x (attenuation/reference attenuation)^b, where the exponent b controls the strength of the mA modulation. For b=1, image noise remains constant as patient attenuation varies. For b < 1, the strength of the mA modulation decreases with decreasing values of b. mA was assumed to be directly proportional to dose and inversely proportional to the square root of noise. Dose and noise were calculated as a function of patient width for an AEC requiring constant noise (b=1) and an AEC providing weak, average and strong mA modulation (b = 0.3, 0.5, and 0.7, respectively). All results were normalized to our manual technique chart and 100% at 36 cm patient width.

The requirement of constant noise resulted in relative doses of 1, 5, 22, 466, and 1476% and relative noise levels of 435, 267, 163, 61 and 42% for patient widths of 12, 20, 28, 36, 44, and 50 cm, respectively. Use of an AEC system that adapts the strength of the mA modulation as patient attenuation varies provided almost an exact match to our manual technique charts for b=0.36, while still decreasing dose in small patients to 19, 33, and 58% for 12, 20 and 28 cm widths, respectively, compared to a 36 cm width. For obese patients, b=0.36 resulted in relative doses of 174 and 264% for 44 and 50 cm widths.

AEC systems that prescribe constant image noise as patient attenuation varies deliver excessive noise levels in small patients and excessive doses in obese patients. Reduction in the strength of the mA modulation mimicked our previously determined manual technique charts for b=0.36. The requirement of decreased noise in small patients and allowance of increased noise in obese patients is consistent with other reports in the literature. Use of noise-level technique charts and upper and lower mA boundaries are required to avoid unintentional imaging errors using constant noise AEC paradigms.

AEC systems must limit the decrease in mA for small patients and the increase in mA in large patients in order to avoid excessive noise or dose, respectively.

McCollough, C, Yu, L, Unintentional Errors in CT Imaging with Use of a Constant Noise Automatic Exposure Control (AEC) Paradigm.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6017195.html