Study for automatic exposure control technique (AEC) in SPECT/CT for reducing exposure dose and influencing image quality

Objectives Auto exposure control (AEC) in SPECT/CT automatically controls the exposure dose (mA) according to patient's shape and size. The aim of this study was to evaluate the effect of AEC in SPECT/CT on exposure dose reduction and image quality.

Methods The model of SPECT/CT used in this study was Discovery 670 of GE Co.; Smart mA for AEC; and 99mTc as a radioisotope. To compare SPECT and CT images by CT exposure dose variation, we used a standard technique set at 80/100/120/140kVp, 10/30/50/100/150/200/250mA, and AEC at 80/100/120/140kVp, 10~250mA. To evaluate resolution and contrast of SPECT images, Triple line phantom and IEC body phantom were used. For CT images, noise and uniformity were checked by IEC body phantom. For dose evaluation to find DLP value, anthropomorphic chest phantom was used and the CT protocol of torso was applied by standard technique (120kVp, 100mA) and AEC (120kVp, 10~250mA).

Results When standard and AEC were applied, the resolutions at SPECT images with Attenuation Correction(A.C) were the same as FWHM by Center 3.65, Left 3.48, Right 3.61. Contrasts of standard and AEC showed no significant difference: standard 35/30/22/14/6, AEC 36/29/22/15/7. In CT images, noise values at standard and AEC were 9.2 and 20.8 respectively. The application of AEC increases noise but the value of coefficient variation were 0.74, 0.26 respectively, obtaining uniform noise image. The values of DLP at standard and AEC were 640.2 and 352.1 each, which shows that the application of AEC decreases exposure dose more than standard by approximately 45%.

Conclusions The results of our study show that there was no difference of A.C in SPECT images based on the CT exposure dose variation at SPECT/CT images. It was found that the increased CT exposure dose leads to the improvement of CT image quality but also increases the exposure dose. Thus, the use of AEC in SPECT/CT contributes to obtaining equal A.C SPECT images, and uniform noise in CT images while reducing exposure dose.