CT makers get five FDA recommendations, due to overdose controversy

The FDA has released five recommendations for CT manufacturers, including increased training protocols for facilities performing brain perfusion studies, automatic exposure control, pop-up notifications and additional protocol specifications. The agency outlined these recommendations in a letter issued to the Medical Imaging and Technology Alliance (MITA) Nov. 8.

The agency said its investigation began after the “anomalously high radiation exposures” first reported in September 2009, in connection with CT brain perfusion studies. As a result, the FDA summarized some of the “lessons learned,” which could pertain “to re-organizing or developing additional equipment-operation information that users could readily access, understand and apply to reduce the possibility of inadvertent application of unnecessary amounts of radiation. Some points entail changes to software defaults and hardware features.”

The ideas surfaced during the investigation and reflect issues that appeared to contribute to the possibility of unintended high exposure, according to Jeffrey Shuren, MD, JD, director of the FDA’s Center for Devices and Radiological Health, who wrote the letter to MITA.

1. Brain perfusion CT studies
The FDA recommends providing particular information and training on brain-perfusion protocols to all facilities receiving base CT equipment, whether or not the facilities purchase the related software enabling quantitative analysis of cerebral hemodynamics. Specifically:

• Manufacturer-recommended parameter settings, i.e., tabulated or listed for each scanning parameter set and supplemented with the corresponding values expected for the dose indices CTDI_vol and DLP. “Presumably such dose-index values would be typical of each of the associated scanning sequences comprising a complete brain-perfusion protocol,” the agency noted.
• A concise description for each scanning-parameter set that clearly distinguishes the particular role of its corresponding scanning sequence from the roles of the other scanning sequences comprising a complete brain-perfusion study. “Such descriptions may reduce the potential for inadvertently substituting technique values associated with diagnostic-quality imaging phases for the lower-dose scanning-parameter settings actually needed for contrast-perfusion phases,” Shuren wrote.
• Explanation of why a relatively poorer quality image is appropriate to reduce radiation dose in the contrast-perfusion phases in comparison to the relatively better image quality that might be needed for any diagnostic phases included in a protocol.
• Explanation of how peak skin doses relate to CTDI because brain perfusion studies are associated with relatively high skin doses.

 
2. Automatic Exposure Control (AEC)
The FDA is asking for clarification of parameters affecting dose, along with clear instructions on how to appropriately set those parameters, which includes:

• Description on how to choose AEC versus manual modes, including examples of situations such as brain perfusion studies, in which AEC operation might unnecessarily complicate successful operation without additional dose savings and with no improvement in diagnostic efficacy.
• Emphasis on building or modifying protocols--to check the need to change manufacturer-recommended or default AEC parameters to achieve optimal dose reduction, including the potential for an unintended dose increase (compared with manual-mode operation), if AEC parameters are not checked.
• Directions on how to modify manufacturer-recommended scanning parameter sets, if a user were to elect an AEC mode in lieu of a recommended manual-mode, the values of all dose-associated AEC-configurable parameters could default (until otherwise changed) to those that would yield a value for the sequence-maximum mAs that would match the mAs value recommended in the manual mode.

3. Pop-up Notification at Threshold for Deterministic Injury
The agency is recommending instituting a pop-up notification so that prior to scanning, when the operating conditions associated with any protocol yield an expected value of CTDl_vol greater than or equal to 1 Gy, a notification would alert the operator that a high radiation dose, potentially leading to the development of clinically significant cataracts, skin injury or hair loss, would be incurred by the patient were scanning to proceed.
 
4. User-Accessible Organization of Dose-Related Information
Organization of all dose-related information into one section of each user manual, in a dedicated dose manual, or indexed comprehensively in a concordance covering all manuals, is another request of the agency.

5. Protocol Specifications
Finally, the FDA is asking CT manufacturers to provide facilities with hard copies or pdf files specifying the dose-associated parameter settings recommended for particular clinical applications, including distinct "scanning-parameter sets" for the values of each scanning sequence comprising a protocol and supplemented with the corresponding values (or, perhaps, range of values associated with AEC operation) typical for the dose indices CTDI_vol and DLP (with phantom diameter and length identified), each set tabulated or listed.

In particular, the agency investigators said the following elements could be included in the table or list:

• Name or identifier for each manufacturer-recommended scanning-parameter set as part of the various protocols, the anatomical area covered, and a concise description of the purpose of each particular scanning-sequence within a protocol.
• Specification of operating conditions and settings, including type of scan (e.g., axial or helical); scan-acquisition time; slice thickness; mode of operation (e.g., AEC or manual); and values for all dose-related AEC parameters if an AEC mode is recommended or selected by the user.
• Typical values of CTDIvol and DLP with an explanation of how the typical values were determined if the protocol allows for a dose range (e.g., AEC mode).

 
In the conclusion of his letter, Shuren expressed an eagerness to hear the industry’s reaction to these recommendations, and in discussing them further at the upcoming stakeholders meeting at the annual Radiological Society of North America (RSNA) conference in Chicago, Nov. 28 to Dec. 3. (There are multiple sessions at RSNA dealing with CT and radiation dose, see the various session previews at www.HealthImaging.com)

The letter was addressed to Stephen Vastagh, who is director of international and industry programs and industry director of the x-ray section of MITA.

In response, Dave N. Fisher, executive director of MITA, said that the “recommendations in the FDA’s letter are issues we have been working on with the agency over the last year and include an important component of MITA’s CT Dose Check Initiative … In the end, the optimal solution will require action by all stakeholders including, manufacturers, users and regulators."