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Computed tomography has experienced a number of significant technological advances over the past decade, and these have had pronounced impacts on the accuracy of radiation dosimetry and the assessment of image quality. After reviewing CT technology and clinical applications, this Report describes and discusses existing dosimetry methods and then presents new methods for radiation dosimetry, including the evaluation of beam quality, and measurement of CT-scanner output in air and in phantoms. Many of the proposed dosemetric quantities can be measured quickly using a real-time ionization chamber, which is introduced here. Traditional measurements of image quality for computed tomography rely upon simple and subjective observations. A more rigorous approach is proposed, including routine use of the modulation-transfer function for describing spatial resolution along all axes, and of the noise-power spectrum for describing the noise amplitude and texture properties of CT images. This Report focuses on new but practical methods for the assessment of radiation dose and image quality for CT scanners.

In CT scanning, image quality has many components and is influenced by many technical parameters. While image quality has always been a concern for the physics community, clinically acceptable image quality has become even more of an issue as strategies to reduce radiation dose — to all patients, but especially to pediatric patients— has become a focus in many radiology practices.The purpose of this presentation will be to first describe several of the components of CT image quality — noise, slice thickness (Z‐axis resolution), low contrast resolution and high contrast resolution— as well as radiation dose and to describe how each of these may be affected by technical parameter selection. This presentation will pay particular attention to the tradeoffs that exist between different aspects of image quality, especially when the reduction of radiation dose is one of the objectives.The presentation will then explore several mechanisms that can be used to reduce radiation dose in CT exams and the implications for the diagnostic image quality of the exam. Specifically, the implications of varying the tube current*time product (mAs), pitch or tablespeed (or for axial imaging, the table increment), slice thickness, beam energy (kVp), patient (or phantom) size and dose reduction options (such as tube current modulation) will be described for both radiation dose and diagnostic image quality. Finally, this presentation will emphasize that the tradeoffs between radiation dose and image quality are clinical‐task dependent; that is, the goals of the clinically indicated exam dictate what aspect of image quality may be emphasized for that exam (low contrast resolution or high contrast spatial resolution, etc.) and this will have implications for the amount of radiation dose reduction that is acceptable. This will be illustrated with examples from selected diagnostic imaging exams.Educational Objectives:1. Understand key components of image quality in CT scanning as well as reinforce CT radiation dose concepts.2. Understand the impact that technical parameter selection has on the various aspects of image quality and radiation dose.3. Examine the tradeoffs between various aspects of image quality and radiation dose.4. Examine the impact of these tradeoffs on a few clinical imaging protocols and illustrate the task‐dependence of image quality requirements.

An anthropomorphic pediatric phantom ͑5-yr-old equivalent͒ was used to determine organ doses at specific surface and internal locations resulting from computed tomography ͑CT͒ scans. This phantom contains four different tissue-equivalent materials: Soft tissue, bone, brain, and lung. It was imaged on a 64-channel CT scanner with three head protocols ͑one contiguous axial scan and two helical scans ͓pitch= 0.516 and 0.984͔͒ and four chest protocols ͑one contiguous axial scan and three helical scans ͓pitch= 0.516, 0.984, and 1.375͔͒. Effective mA s ͓=͑tube current ϫ rotation time͒ / pitch͔ was kept nearly constant at 200 effective mA s for head and 290 effective mA s for chest protocols. Dose measurements were acquired using thermoluminescent dosimeter powder in capsules placed at locations internal to the phantom and on the phantom surface. The organs of interest were the brain, both eyes, thyroid, sternum, both breasts, and both lungs. The organ dose measurements from helical scans were lower than for contiguous axial scans by 0% to 25% even after adjusting for equivalent effective mA s. There was no significant difference ͑p Ͼ 0.05͒ in organ dose values between the 0.516 and 0.984 pitch values for both head and chest scans. The chest organ dose measurements obtained at a pitch of 1.375 were significantly higher than the dose values obtained at the other helical pitches used for chest scans ͑p Ͻ 0.05͒. This difference was attributed to the automatic selection of the large focal spot due to a higher tube current value. These findings suggest that there may be a previously unsuspected radiation dose benefit associated with the use of helical scan mode during computed tomography scanning.

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Purpose: Texture features have been investigated as a biomarker of response and malignancy. Because these features reflect local differences in density, they may be influenced by acquisition and reconstruction parameters. The purpose of this study was to investigate the effects of radiation dose level and reconstruction method on features derived from lung lesions. Methods: With IRB approval, 33 lung tumor cases were identified from clinically indicated thoracic CT scans in which the raw projection (sinogram) data were available. Based on a previously‐published technique, noise was added to the raw data to simulate reduced‐dose versions of each case at 25%, 10% and 3% of the original dose. Original and simulated reduced dose projection data were reconstructed with conventional and two iterative‐reconstruction settings, yielding 12 combinations of dose/recon conditions. One lesion from each case was contoured. At the reference condition (full dose, conventional recon), 17 lesions were randomly selected for repeat contouring (repeatability). For each lesion at each dose/recon condition, 151 texture measures were calculated. A paired differences approach was employed to compare feature variation from repeat contours at the reference condition to the variation observed in other dose/recon conditions (reproducibility). The ratio of standard deviation of the reproducibility to repeatability was used as the variation measure for each feature. Results: The mean variation (standard deviation) across dose levels and kernel was significantly different with a ratio of 2.24 (±5.85) across texture features (p=0.01). The mean variation (standard deviation) across dose levels with conventional recon was also significantly different with 2.30 (7.11) (p=0.025). The mean variation across reconstruction settings of original dose has a trend in showing difference with 1.35 (2.60) among all features (p=0.09). Conclusion: Texture features varied considerably with variations in dose and reconstruction condition. Care should be taken to standardize these conditions when using texture as a quantitative feature. This effort supported in part by a grant from the National Cancer Institute's Quantitative Imaging Network (QIN): U01 CA181156; The UCLA Department of Radiology has a Master Research Agreement with Siemens Healthcare; Dr. McNitt‐Gray has previously received research support from Siemens Healthcare.