Yoshinori Funama PhD, Presenter: Nothing to Disclose

Katsuyuki Taguchi PhD, Abstract Co-Author: Research grant, Siemens AG Research grant, Koninklijke Philips Electronics NV

Kazuo Awai MD, Abstract Co-Author: Nothing to Disclose

Masamichi Shimamura PhD, Abstract Co-Author: Nothing to Disclose

Yasuyuki Yamashita MD, Abstract Co-Author: Nothing to Disclose

To evaluate the effect of the off-centering position, the type of bowtie filters, and the direction of CT projection radiograph on the radiation dose and image noise of 64-detector CT scanner in automatic tube current modulation (ATCM) technique.

We scanned a chest CT phantom using ATCM technique at a 64-detector CT scanner. The phantom was located at on-center or off-centered position along y-axis by 2.0, 4.0, and 6.0 cm toward 12 o’clock direction. Three bowtie filters (small, medium, and large) and two directions of CT projection radiograph (anteroposterior and lateral) were used. The tube current value for each projection was determined by the ATCM technique. The tube current ratio was calculated from the cumulative tube current value at off- and on-center positions with each SPR direction and bowtie filter type. The image noise ratio was directly measured from the image pixel values. The measured image noise ratio was calculated as the ratio of the measured image noise with a setting to the corresponding value with the on-centered phantom position.

In anteroposterior scanogram, the tube current ratio increased with increasing the amount of off-centering positioning of the phantom in each type of bowtie filter (from 1.000 to 1.439 with small, 1.000 to 1.442 with medium, and 1.000 to 1.420 with large bowtie filter, respectively). With lateral scanogram, tube current ratio remained constant for each bowtie filter type regardless of the offset phantom positioning. The measured image noise ratio with large bowtie filter remained nearly constant for four offset phantom positions, while that with small bowtie filter increased to 1.258.

Tube current ratio and measured image noise ratio are influenced by off-centered position, the type of bowtie filters, and the direction of CT projection radiograph though we use the ATCM technique.  

Operators should pay attention to position the patient at the center, because the off-centering could increase the radiation dose and image noise even when the ATCM technique is used.

Funama, Y, Taguchi, K, Awai, K, Shimamura, M, Yamashita, Y, Image Noise and Radiation Dose Using Automatic Tube Current Modulation Technique at 64-detector CT: Effect of Off-centered Position, Bowtie Filter, and CT Projection Radiograph.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8001583.html