Ryusuke Kujirai, Presenter: Nothing to Disclose

Yutaka Suzuki RT, Abstract Co-Author: Nothing to Disclose

Hiroshi Kageyama RT, Abstract Co-Author: Nothing to Disclose

Cardiac bowtie filters (wedge), are designed to provide a gradient of x-ray beam strength across the patient with the goal of reducing radiation dose outside the clinical region-of-interest (cardiac structures). However, the assumption is that the target is positioned at the scanner iso-center. The purpose of this study was to assess dose and image noise distribution when using a cardiac wedge, as a function of patient positioning (image center offset from scanner iso-center).

A standard 32-cm-diameter polymethyl methacrylate CT dose index phantom was scanned at the iso-center of a 256-slice CT scanner using a body-wedge(large) and cardiac-wedge(medium) with the following acquisition parameters - 120kVp, 200mA, 1sec rotation. Absorbed radiation dose was measured for each acquisition using a standard 10cm pencil ionization chamber positioned in the phantom chamber inserts located at (distance from iso-center) – 0, 38, 76, 114 & 152mm. Dose measurements were averaged for equidistant positions (from iso-center) of ion-chamber in different directions (i.e. anterior-posterior and lateral aspects). Relative differences in dose and theoretical noise between different wedges were computed. Body-wedge acquisition was used as reference for relative changes obtained with cardiac-wedge.

Dose distribution across the phantom from iso-center laterally outwards was - (distance from iso-center, dose with cardiac-wedge, dose with body-wedge) – 0mm, 2.057mGy, 2.16mGy; 38mm, 2.215mGy, 2.33mGy; 76mm, 2.675mGy, 2.88mGy; 114mm, 3.31mGy, 3.68mGy; and 152mm, 3.80mGy, 4.27 mGy. Depending on distance from iso-center the radiation dose reduction ranged between 4.84% and 11.11% and theoretical noise increases between 2% to 6%.

When cardiac structures are centered with scanner iso-center a cardiac-wedge provides approximately 11% dose reduction for non-cardiac structures (outer lung, breast, etc.), relative to a non-cardiac (body) wedge. Clinically, when cardiac structures are not centered with the scanner iso-center the reconstruction field of view is offset. Our study shows that this offset results in higher noise in the images relative to noise obtained when reconstruction is centered at scanner iso-center.

Improper patient positioning could result in reduced image quality due to the increase in image noise for Cardiac CTA acquisitions.

Kujirai, R, Suzuki, Y, Kageyama, H, Influence of Patient Centering on Image Quality for Cardiac CTA.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12036209.html