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Radiation Exposure in Computed Tomography: Fundamentals, Influencing Parameters, Dose Assessment, Optimisation, Scanner Data, Terminology, 2nd ed.

Wake Forest University School of Medicine Winston-Salem, NC 27157-1088

Edited by Hans Dieter Nagel with contributions from M. Galanski, N. Hidajat, W. Maier, H. D. Nagel, Th. Schmidt. Hamburg, Germany: Offizin Paul Hartung Druck, 72 pp., 2000. [UNK]25

If you would like to review books for AJR, please send a cover letter stating your interest with a current curriculum vitae to Assistant Editor for Book Reviews, AJR, 101 S. Stratford Rd., Ste. 303, Winston-Salem, NC 27104.

Radiation Exposure in Computed Tomography is an excellent resource for anyone dealing with CT dose, and that includes a lot of us because of the recent interest in dose reduction in CT. This monograph was originally published in the German language; however, the present translation into English is both accurate and easy to read.

As a teacher of radiologic physics, I found the chapter on dosimetry fundamentals to be quite useful in that it gives a clear description of the relationship of the CT dose index (CTDI) to the average dose for a scan series in nonmathematical terms. Also included is a complete description of all the CTDI variants that have recently come into use (and which automatically come up on the monitor of the newer CT scanners).

A study quoted from Leipzig University indicates that improvements in scanner technology have often not been used to reduce dose but rather to shorten scanning times and to further improve image quality. That is, milliampere-second (mAs) is increased if it is no longer restricted by technical limitations; the number of slices is increased if there is enough time or if a fast scanner is available; and the scan length increases with the availability of helical CT.

Chapter 4 presents an excellent description of factors influencing patient dose that should be of interest to medical physicists and radiologists alike, and these are conveniently summarized in a table. If one keeps the image quality (noise) constant by the appropriate reduction in mAs, then dose decreases linearly with increasing kilovoltage, slice thickness, and pitch factor. This result is fairly straightforward; however, the author points out other methods for reducing dose that are not as obvious. For examinations in which high spatial resolution is not critical, one can obtain the same image quality at reduced mAs by using a smoother reconstruction algorithm (filter function [e.g., "soft tissue" vs "standard" for General Electric scanners]). Also, for examinations in which sufficient natural contrast exists, lower mAs can be used, and the increased noise may be reduced in appearance to the observer simply by using a wider window width (e.g., if a setting of 350 H is used instead of 300 H, the dose can be reduced by 26% while noise perception remains the same). Some interesting effects with respect to the relative cutoff frequencies of the filter function and the Nyquist frequency (matrix size/field of view) are also discussed.

A chapter on low-dose CT gives clinical examples of dose reduction techniques. Because the half-value layer for a CT beam is approximately 4 cm of tissue, a reduction in patient diameter of 4 cm would allow a dose (mAs) reduction of one half while keeping the image quality (noise) constant. However, it is far too simplistic to assume that a recipe for pediatric dose reduction based on patient diameter is sufficient when many other mechanisms for even further dose reduction (such as those previously mentioned) are available.

This book provides useful data for the calculation of effective doses for CT examinations, and the last chapter includes several examples of such calculations. These calculations are based on CTDI_air; however, the appendix includes CTDI values (including CTDI_air) for a variety of CT scanners, which are useful data. Also included is a helpful table for estimating fetal dose for a pregnant patient.

In an ideal world, Radiation Exposure in Computed Tomography would be read by every practicing diagnostic radiologist and medical physicist, and its dose reduction techniques would be implemented. In reality, the examination techniques for most CT facilities are not customized, and the settings recommended by the manufacturers or applications specialists are used unless someone on-site takes a proactive approach to dose reduction. At the very least, this monograph should provide users with the understanding of those factors under their control that affect CT dose and how to implement a program of CT dose reduction.

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This Article Full Text (PDF) An erratum has been published Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Dixon, R. L. Search for Related Content PubMed Articles by Dixon, R. L. Social Bookmarking                      What's this?