CT dose reduction using dynamic collimation

The purpose of this work is to define and calculate the motion requirements of dynamic collimators that can significantly reduce dose in CT scans. Both axial and transverse collimators are modeled and compared in terms of their velocity requirements. Today, helical scans already use the axial collimator. A separate transverse dynamic collimator is proposed that would be used for both helical and axial scans. The leaves of both the helical and transverse collimators move such that only those x-rays that intersect a predefined VOI are exposed to the patient. To assess the motion control requirements for the transverse collimator, the velocity of each leaf is determined for a circular region of interest (ROI) at a given distance from scan center. These velocity profiles are then compared to those of a helical collimator for a typical helical scan with an 8 cm detector and .3 second rotation time. The maximum velocity incurred by the helical collimator for a scan with a pitch factor of 1.4 is about 23 cm/sec. For a 12 cm cardiac scan located 18 cm from scan center, the maximum velocity of a transverse collimator positioned 27 cm from the source is 198 cm/sec. Three options (or combination thereof) to address the higher velocities are: 1) Position the patient such that the VOI is closer to scan center 2) Move the dynamic transverse collimator closer to the source 3) Upgrade the collimator motor controller.