WE‐E‐144‐01: The Management of Imaging Procedure Dose 3: Computed Tomography
Authors: Boone J , Cody D , Seibert J 
Published: July 2013

John Boone: Measurements and Indices in CT Dose

Computed tomography has experienced rapid growth in utilization over the past 10 years, due in part to the dramatic increase in image quality and decrease in scan time that helical and multi‐slice CT scanners have allowed. This increased utilization has raised legitimate concerns about the radiation dose levels in CT. Traditional dose metrics such as the volume computed tomography index (CTDIvol) and the dose length product (DLP) will be discussed. The limitations of these metrics in the context of individual patient dosimetry will also be explained. In recent years, a number of new CT dose concepts have been introduced in the peer‐reviewed literature, in task group reports, and in other documents. A number of these new dose metrics will be discussed, including the rise‐to‐equilibrium‐dose, H(L), and the size‐specific dose estimate (SSDE). CT dosimetry has historically been performed used integrating ion chambers. In light of the dynamic scanning capabilities of modern CT scanners, the utility of a real‐time radiation meter will be discussed. Real‐time dose meters can substantially reduce the time required by the physicist in the CT scanner suite, while increasing the quantity and quality of the dose information that is measured. Niche applications include the rapid assessment of beam quality (half value layer) and the characterization of the beam shaping filters used in CT. In summary, this presentation will discuss existing CT dose parameters, and will then review a number of proposed new CT dose parameters which will likely be useful for CT dose assessment in the future. The recent growth of CT technology has outgrown the simple dose metrics of the past, and there is a need for the CT community to embrace new and more accurate CT dose metrics.

Learning Objectives:

1. Identify and discuss the standard parameters used for reporting dose in computed tomography, including the volume CTDI, DLP, and effective dose using the k‐coefficients.

2. Identify and discuss parameters which influence the radiation dose to the patient, including patient size, dose modulation protocols, and scan length.

3. Discuss the limitations of using effective dose in describing radiation dose levels to individual patients.

Dianna Cody: Estimating Patient Dose

Although there are several methods in current use for estimating radiation dose delivered to individual patients, all have specific limitations that should be appreciated when they are utilized in the practice of clinical medical physics. Most patient dose estimates are based on fairly crude mathematical models of human anatomy and are unable to incorporate critical characteristics of patients such as their size and shape. Methods that are based on patient images (“voxelized patient models”) are available at few locations and are quite labor intensive. Recent improvements to patient dosimetry in CT, such as the size specific dose estimate (SSDE), may provide a path for reporting more customized patient dose estimates. Potential future options, and foreseeable pitfalls and complications, will be reviewed.

Learning Objectives:

1. Recognize the limitations of current approaches to estimate CT patient dose.

2. Understand several methods available for estimating CT patient dose.

3. Understand potential future options for patient CT dose estimations.

Tony Seibert

Radiation over‐exposure for computed tomography (CT) perfusion studies occurring in the 2008–2009 timeframe resulted in California Senate Bill 1237, legislation that was authored by Senator Padilla in response to these incidents. The legislation was signed by the Governor in September 2010. The law contains three parts: (1) Recording CT dose indices for each patient, placing these values in the radiology report, and verifying accuracy of the volume Computed Tomography Dose Index (CTDIvol); (2) Requiring accreditation for all CT scanners performing diagnostic exams that are under the authority of the California Department of Public Health; (3) Reporting of radiation exposures that exceed specified limits to organs, cause unanticipated erythema or hair loss, or inappropriate irradiation to body parts not ordered by a physician. Part 1 of the law commenced on July 1, 2012, and the other two parts are to commence on July 1, 2013. This presentation describes the steps taken to comply specifically with Part 1 and 3 of the law. To ensure compliance, an automated extraction and delivery of the CTDIvol and DLP indices to the radiology report were implemented. However, the legislation does not provide guidance on how to: (1) adjust CTDIvol for patient size; (2) deal with CT exams having multiple different series, each with individual dose indices; (3) sum CTDIvol and DLP for the same or different body areas scanned (if appropriate). The consequence is variable reporting at the initial implementation of the law, which requires standardized reporting metrics. Recommendations by the University of California Dose Optimization and Standardization Endeavor (UC DOSE) is discussed in this context, with relevant solutions described and specific examples demonstrated. To conclude, an update from the users perspective of compliance, as well as reporting of the status from the State of California Department of Public Health office is provided.

Learning Objectives:

1. Describe the provisions of the California State law on dose reporting for computed tomography (CT) scanners.

2. Demonstrate ways in which the required elements volume Computed Tomography Dose Index (CTDIvol) and Dose Length Product (DLP) can be placed into the radiology report.

3. Discuss discrepancies regarding the relationship between CTDIvol and patient dose, and issues in accumulating dose indices for CT scans in a multi‐series exam and for individual exams over time.

4. Report on the status of compliance with the statutes of the law.