|Year : 2019 | Volume
| Issue : 3 | Page : 228-229
The physics of CT dosimetry: CTDI and beyond
Department of Health Physics, Gartnavel Royal Hospital, Glasgow G12 OXH, Scotland
|Date of Web Publication||13-Sep-2019|
Dr. Gurpreet Singh
Department of Health Physics, Gartnavel Royal Hospital, 1055 Great Western Road, Glasgow G12 OXH
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Singh G. The physics of CT dosimetry: CTDI and beyond. J Med Phys 2019;44:228-9
Author: Robert L. Dixon
Publisher: CRC Press
Edition: 1st Edition
Number of Pages: 232 Pages
ISBN No: 9780367077594
This book is written by Professor Robert L Dixon who is very famous in the diagnostic radiology, especially for computed tomography (CT) dosimetry work. This book addresses the needs of all medical physicists who are working on the optimization of CT dose. I am confident that who so ever will refer to this book will appreciate its content. Diagnostic radiology is an integral part of any hospital. CT imparts the largest portion of radiation dose among all radiation diagnostic imaging equipment. As the number of examinations is going up every year, 'As Low As Reasonably Achievable' (ALARA) principle has become a central issue for any radiation installation with the intention to bring the doses down. Understanding the concept of CT dose index (CTDI) is a very important topic to optimize the patient doses. This book has been carefully arranged into ten sections.
| Chapter 1: Introduction and History|| |
This chapter outlines the historical evaluation of CTDI100. CTDI100 has been widely used as an indicator of clinical patient dose assessment. It was 1977, when the pencil chamber was introduced. Measuring dose with thermoluminescent dosimeter (TLD) was a tedious task. The author also points out that the use of CTDIw in wide-beam CT has its own disadvantages. The author also described the use of 0.6cc Farmer ion chamber in CT dosimetry.
| Chapter 2: Derivation of Dose Equations for Shift-Invariant Techniques and the Physical Interpretation of the CTDI-Paradigm|| |
This chapter imparts knowledge on derivation of the dose equations and the CTDI-paradigm on the phantom central axis for a shift-invariant helical technique. The equations in this section describe why there is a limitation of the CTDI-paradigm, and there is a need for shift invariance. The CTDI is not an actual dose. The author explained three-dimensional calculation of the total energy deposited in the phantom in convolution format. This chapter also tells the differences between longitudinal average versus angular average for helical scans.
| Chapter 3: Experimental Validation of a Versatile System of CT Dosimetry Using a Conventional Small Ion Chamber|| |
Chapter 3 focuses on the efficacy of small ion chamber. The author has explicitly mentioned that equilibrium dose is not easy to measure and it is a tedious task to build a phantom which can be used for such measurement. The major point of this chapter is a new suggested CT dose measurement protocol with a small ion chamber. To achieve CTDI∞, one only needs to use single scan length. Furthermore, CTDI∞ more closely represents accumulated doses for clinically relevant body scan than CTDI100. The author also explained in detail Farmer versus pencil chamber comparison. He also pointed out that CTDI100 is not a representative of clinical body scan lengths which may exceed 250 mm or more. More focus should be on Deq which can be obtained from CTDI∞. As a clinical physicist, I found this chapter valuable.
| Chapter 4: An Improved Analytical Primary Beam Model for CT Dose Simulation|| |
The author coherently introduced the simple geometric model on the axis of rotation. Later on, it diverged into a detailed primary beam model. He considered small-angle approximation and other parameters such as heel effect, convolution approximation for the tilted anode, and a broader view in which energy deposited in phantom is proportional to aperture and entered dose on the axis of rotation (AOR). Soon after this, the primary beam model was conceptually transformed to the peripheral phantom axis. The author also explained the differences between experimental versus simulated accumulated dose distributions. In one of the sections of this chapter, the results of cumulative dose measurements using a small ion chamber in the form of graphs are shown.
| Chapter 5: Cone Beam CT Dosimetry: A Unified and Self-Consistent Approach Including All Scan Modalities – With or Without Phantom Motion|| |
The author described in this chapter, a constant approach of acquiring dose under CT with table movement and stationary table. The author has also explicitly mentioned that a pencil chamber cannot measure the dose at the center as it can only measure the integral of f(z). Under numerical analysis of experimental stationary phantom cone-beam CT (SCBCT), both Aeq equilibrium dose constant and CTDI∞ are irrelevant. The author has also pointed out that there is the inapplicability of the CTDI-paradigm and pencil chamber for SCBCT. The author has also explained how to approach scatter equilibrium for SCBCT. While modeling the cone beam, a simple beam model predicting the observed dose data has been considered. For this to progress, a line-spread function (LSF) is assumed, with a mathematical 32 cm PMMA phantom of infinite length at 120 kVp. This chapter is one among the best in the book as it deals with cone-beam CT, and every clinical physicist would find it handy because it addresses questions and challenges that clinical physicists would face in the cone-beam dosimetry.
| Chapter 6: Analytical Equations for CT Dose Profiles Derived Using a Scatter Kernel of Monte Carlo Parentage Having Broad Applicability to CT Dosimetry Problems|| |
The author started this chapter stating the advantages of dose profile dosimetry over CTDI. He explained in detail the derivation of the scatter component of the axial profile from the scatter LSF. After this, in another section, it was shown a great match between experiment and theory for the widest cone beam. The author elaborately gave considerations to limit the importance of heel effect in quantitative CT dosimetry.
The CTDI equation looked behind its integral facade. The new scatter LSF kernel was mathematically incorporated. The power of this method was that now each profile could be separated with respect to aperture, mA. An analytical CT dose simulator-SIMDOSE, a mathematical function, which runs under MATLAB, faster than Monte Carlo simulation, is noise free. It gives the level of detail far better than Monte Carlo.
| Chapter 7: Dose Equations for Tube Current Modulation in CT Scanning and the Interpretation of the Associated CTDIVol|| |
In this chapter, the convolution equations describing the accumulated dose distribution for helical and axial scan trajectories have been derived. The author first revised the meaning of CTDIvol based on constant tube current and later derived the theoretical equations for Automatic Tube Current Modulation (ATCM). The author has mentioned in this section published values of CTDI100. The TCM CTDIvol has little information to offer which is generated by shift variant current. Furthermore, he described that a pencil chamber measurement has neither any utility for auto-TCM nor for shift-variant techniques. There are also explanations on the mA (z) profiles used in the simulations.
| Chapter 8: Dose Equations for Shift-Variant CT Acquisition Modes Using Variable Pitch, Tube Current, and Aperture, and the Meaning of Their Associated CTDIVol|| |
This chapter covers the derivation for additional shift-variant techniques including variable pitch, simultaneous tube current modulation, and changing collimator aperture. These equations may be helpful in optimization of patient doses. Furthermore, focus was on the troubles with scanner-reported values of CTDIvol with shift-variant techniques.
| Chapter 9: Stationary Table CT Dosimetry and Anomalous Scanner-Reported Values of CTDIVol|| |
The author's focus was on anomalous values of CTDIvol in the ACR dose registry. The author also addressed that if there is a fault in CTDIvols, it will reflect in SDDE (size-specific dose estimation).
| Chapter 10: Future Directions of CT Dosimetry and a Book Summary|| |
This chapter questions the usefulness of calculating exact organ doses in CT if we keep using CTDIvol in SSDE. This chapter also sheds light on risks from CT exams.
Overall, this book is valuable. I certainly recommend this book to all physicists, teachers, and students who want to get involved in CT dose optimization.