Reviewer: James Anthony Seibert, PhD (UC Davis Health)
Description: This comprehensive review of the issues related to CT dosimetry uses derivations of analytical equations in a rigorous and complete manner for quantitative accuracy and provides physical descriptions and associated illustrations demonstrating these concepts for those interested in a more qualitative approach. From the historical beginnings of CT dose index (CTDI) and the reasons for implementation, through the significant shortcomings with current CT technology, the book suggests better, more accurate tools for CT dosimetry for consideration in the future. Each chapter has references to historical and recent state-of-the-art publications.
Purpose: Going beyond CTDI as a computed tomography dose metric is the overall focus of the book. It clearly demonstrates the limitations of CTDI and describes common misconceptions with its use, particularly in situations of tube current modulation and acquisitions without table movement, such as in brain perfusion studies. The long-range travel and extent of scattered x-rays from the primary beam, particularly with wider collimation, are shown to contribute significantly to the local dose, and create a mismatch between local tube current factors and predicted CTDI. The book proposes solutions to these problems, which are demonstrated to be reasonable, more accurate alternatives. Given the many publications on this topic referenced in each chapter, these are worthy objectives to pursue, and this book succeeds in meeting them.
Audience: This book is directed primarily to stakeholders in the CT radiation dosimetry arena, including qualified medical physicists, regulators, standards-making organizations, and manufacturers. The excellent quantitative derivations and qualitative descriptions can be used as an introduction to the issues to medical physics graduate programs. Medical physics residency programs can use the book as a framework for more advanced and in-depth understanding of CT dosimetry. Dr. Dixon, with many publications in CT dosimetry over the past two decades, is a well-known and respected medical physicist in the national and international community.
Features: Each of the 10 chapters is self-contained, with an introduction, summary, references, appendixes, and glossary (the latter two in most chapters). This structure is a nice attribute of the book, with the glossary assisting in decoding the many acronyms, and the appendixes providing an in-depth derivation of equations. Consistency among chapters necessarily requires some repetition and redundancy, which is welcome and helpful in reiterating and bridging important concepts. Chapter 1 provides a historical view of CT dosimetry, from initial, labor-intensive methods using thermoluminescent dosimetry to the introduction of the 100 mm pencil ion chamber and introduction of the computed tomography dose index (CTDI) concept in 1981. It describes modifications to CTDI with advances in multi detector-row CT technology, helical scanning, and others. Baseline rationale, viewpoints, and misconceptions of the CTDI paradigm are nicely described for subsequent topics. Consideration of shift-invariant measurement techniques and derivations for explaining the quantitative shortcomings of CTDI are covered in chapter 2, and validation of a small ion (Farmer) chamber to provide accurate and robust accumulated dose is presented in chapter 3. In chapter 4, the primary x-ray beam is modeled to consider the anode angle and its impact on asymmetric penumbra and heel effect along the z-axis, particularly important with wider collimation beams that introduce shift-variant aspects of the acquisition on the central and peripheral axes. Extension to cone-beam CT, the widest of beam collimation along the z-axis is reviewed in chapter 5, demonstrating the in-applicability of the 100 mm pencil ion chamber and CTDI paradigm in this situation, and the requisite need for a small Farmer chamber to estimate radiation dose. Chapter 6 covers scattered x-ray models based upon Monte-Carlo methods with analytical equations derived from such, demonstrating applicability to a wide variety of CT dosimetry issues. Chapters 7 and 8 deal with shift-variance, including tube current modulation, variable table velocity (variable pitch) and variable z-collimator aperture, and chapter 9 focuses on stationary table acquisitions and the substantial impact on CTDI (in)accuracy. Chapter 10 provides a summary and a look to the future beyond CTDI.
Assessment: For medical physicists, regulators, manufacturers, and other stakeholders interested in CT radiation dosimetry, this uniquely focused work is a must-have, since there are no other resources available that concatenate, describe, deliver the issues and references as succinctly. I fully recommend this book for its mathematical rigor, quantitative methodology, and associated qualitative descriptions - certainly of assistance to those just beginning to explore, as well as advanced physicists and others who are actively working in the field.
