Jodie A. Christner PhD, Presenter: Grant, Siemens AG

Andrew Primak PhD, Abstract Co-Author: Research grant, Siemens AG

Christian Eusemann PhD, Abstract Co-Author: Employee, Siemens AG

Cynthia H. McCollough PhD, Abstract Co-Author: Research grant, Siemens AG Research grant, RTI Electronics AB Research grant, Bayer AG

To determine the constancy of z-axis spatial resolution and image noise for non-constant velocity spiral CT data reconstructed using a weighted 3D filtered back projection (FBP) reconstruction algorithm.

A flexible-weighting Feldkamp-based 3D FBP reconstruction algorithm was used to reconstruct stationary (axial), constant velocity spiral (p=0.5,1.0 and 1.5) and non-constant velocity spiral CT data acquired using a Siemens Definition AS+ CT scanner using a 2x64x0.6 mm collimation, z-flying focal spot techniques and a gantry rotation time of 0.3 s. Non-constant velocity acquisitions were acquired using the system’s “adaptive spiral" mode, where the table moves in and out of the gantry in a cyclical manner characterized by the scan length (amplitude of cycle in one direction) and cycle time. Adaptive spiral modes studied had amplitudes/cycle times of 69 mm/1.0s; 79 mm/1.25s and 96 mm/1.5. For all scan modes, the dose was held constant at 10.2 mGy. Measurements of image noise and the FWHM of the slice sensitivity profile (SSP) were performed as a function of pitch and slice width, both at isocenter and 120 mm off isocenter.

At isocenter, the SSP FWHM values for nominal slice widths of 0.6, 0.75, 1.0, 1.5 and 2.0 mm were be 0.68, 0.86, 1.14, 1.59 and 2.31mm, respectively, at pitch=0.5, and increased by 0.12mm on average at 120 mm off isocenter. SSP FWHM values for a nominal slice width of 0.6 mm were 0.68, 0.75, and 0.78mm as spiral pitch increased from 0.5 to 1.5, respectively, and was 0.49mm in axial mode (pitch=0). Axial, constant velocity spiral (pitch = 0.5), and the evaluated adaptive spiral scans, at the center of their scan range, yielded SSP FHWM values of 0.74mm +/- 0.03mm for a nominal 0.6 mm image width. SSP FHWM values were essentially constant up to 30 mm away from the center of the adaptive spiral scan range. Noise values were similar for all scan modes assessed, except for a slight increase in noise near the end of the adaptive spiral scan.

Non-constant velocity adaptive spiral scans can achieve CT image quality comparable to stationary (axial) and constant velocity spiral acquisitions.

A non-constant velocity cyclical “adaptive spiral” scan mode allows 4D CT scan lengths greater than the detector width, without a decrease in z axis spatial resolution or an increase in image noise.

Christner, J, Primak, A, Eusemann, C, McCollough, C, Evaluation of Z Axis Resolution and Image Noise for Nonconstant Velocity Spiral CT Data Reconstructed Using a Weighted 3D Filtered Back Projection (FBP) Reconstruction Algorithm.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6017068.html