ABSTRACT
Given the levels of radiation involved in CT, it is important to understand how CT doses are measured, communicated, and ultimately optimized during clinical use. This chapter introduces the metrics used in radiation dosimetry applicable to all x-raybased radiologic applications and the methods to estimate these metrics, followed by a discussion of CT-specific dosimetry and its limitations, in addition to a recent proposal for new CT dose
Contents 5.1 Introduction 59 5.2 General radiation dose units 60
5.2.1 Exposure and air kerma 60 5.2.2 Absorbed dose 60 5.2.3 Effective dose 60 5.2.4 Estimating absorbed and effective dose 61
5.2.4.1 Monte Carlo simulations 61 5.2.4.2 Empirical estimations 62
5.3 CT-specific dose units 62 5.3.1 CT dose index 63 5.3.2 Dose length product 64 5.3.3 Computing effective dose in CT 64 5.3.4 Dosimetry in CBCT 64
5.4 Image acquisition parameters affecting dose 65 5.4.1 Tube voltage 65 5.4.2 Tube current and tube current-exposure time product 66 5.4.3 x-Ray filters 68 5.4.4 Organ shields 68 5.4.5 Half-revolution acquisitions 68 5.4.6 Reconstructed image thickness 69 5.4.7 x-Ray beam width, geometric efficiency, and pitch 69
5.5 Specific applications of CBCT 69 5.5.1 Cone beam dedicated breast CT 69 5.5.2 CBCT for radiotherapy positioning 70
5.5.2.1 kV CBCT 71 5.5.2.2 MV CBCT 71
Acknowledgments 71 References 71
metrics aimed to overcome these limitations. Also, CT image acquisition parameters and how these parameters affect patient dose are reviewed. Finally, two cone beam computed tomography (CBCT) applications that involve application-specific dosimetry metrics are discussed.
