Collecting 48,000 CT Exams for the Lung Screening Study of the National Lung Screening Trial

Abstract

From 2002–2004, the Lung Screening Study (LSS) of the National Lung Screening Trial (NLST) enrolled 34,614 participants, aged 55–74 years, at increased risk for lung cancer due to heavy cigarette smoking. Participants, randomized to standard chest X-ray (CXR) or computed tomography (CT) arms at ten screening centers, received up to three imaging screens for lung cancer at annual intervals. Participant medical histories and radiologist-interpreted screening results were transmitted to the LSS coordinating center, while all images were retained at local screening centers. From 2005–2007, all CT exams were uniformly de-identified and delivered to a central repository, the CT Image Library (CTIL), on external hard drives (94%) or CD/DVD (5.9%), or over a secure Internet connection (0.1%). Of 48,723 CT screens performed, only 176 (0.3%) were unavailable (lost, corrupted, compressed) while 48,547 (99.7%) were delivered to the CTIL. Described here is the experience organizing, implementing, and adapting the clinical-trial workflow surrounding the image retrieval, de-identification, delivery, and archiving of available LSS–NLST CT exams for the CTIL, together with the quality assurance procedures associated with those collection tasks. This collection of CT exams, obtained in a specific, well-defined participant population under a common protocol at evenly spaced intervals, and its attending demographic and clinical information, are now available to lung-disease investigators and developers of computer-aided-diagnosis algorithms. The approach to large scale, multi-center trial CT image collection detailed here may serve as a useful model, while the experience reported should be valuable in the planning and execution of future equivalent endeavors.

Appendices

Appendix A

Table 3 Lung Screening Study Screening Center and Satellites with their National Cancer Institute Contract Numbers

Appendix B: CT Parameters

Researchers using the CTIL will require access to the attributes that are found in the DICOM images. These describe the techniques used for each acquisition as well as the equipment that is used. These attributes, from each image series of each CTIL exam, have been forwarded to Westat in order that Westat might readily respond to a researcher’s formal request for CTIL images by determining which exams will satisfy the researcher’s needs; otherwise, each request involving these attributes would require the extra step of first searching the CTIL database.

Table 4 lists DICOM attributes for values of interest during the screening trial and for secondary analysis. Not all LSS scanners provide values for each of these attributes. In some cases, the attribute stored in the database is actually calculated from other data in the DICOM image. In other cases, the images do not contain sufficient information to calculate or infer those values. The value stored in the CTIL database for each attribute is dependent on the vendor. For example, a manufacturer may provide a value for table rotation but not a value for collimation; or, those values may be entirely absent (Table 4)

Table 4 DICOM Tags Stored in CTIL Database

Note 1: in order to calculate ‘effective mAs’ (=mAs/pitch), one needs to first determine mAs and pitch. Both of those required extensive reading of DICOM conformance statements and explanations from the manufacturers. Values needed to determine mAs and/or pitch are sometimes stored in private attributes. Discussions with the manufacturers’ engineers allowed us to interpret those private attributes and extract the information necessary to determine effective mAs, mAs and pitch.

Note 2: values for pitch were never found directly in a public DICOM attribute. In the Philips equipment, the value for pitch was found in a private attribute. For GE scanners, a text string in a private attribute stored a coded value used to look up the pitch. The Siemens scanners did not record pitch; we performed an inverse calculation based on the effective mAs stored by the equipment and our determination of mAs from exposure and current.

The Toshiba CT devices store several parameters in private elements that must be combined to compute pitch. The Toshiba CT system creates a private attribute with a binary object that includes a wide range of data. Among other things, these data include the required CT pitch information but also PHI captured by the modality. Passing that attribute from the Toshiba system through the laptop to the CTIL would result in PHI disclosure; omitting that data would exclude the important CT pitch information. This problem was addressed by modifying the laptop software to interpret the Toshiba binary object, extract the required pitch information, and encode the pitch information in a separate private attribute. The original Toshiba private attribute could then be safely deleted. This is the only instance where the laptop de-identification software was modified to interpret data specific to a manufacturer or device model. All other manufacturer specific data (non PHI) were interpreted with custom software maintained at the CTIL.

Note 3: pixel spacing (reconstruction interval) was computed from Image Position (0020 0032) rather than just extracted from (0028 0030). This provided a backup mechanism to allow us to check for uniformity of slice location to make sure that slices were not missing. This is best done by examining Image Position (0020 0032) and not making assumptions about Instance Number (0020 0013).

Note 4: scanner ID was a scanner identifier copied from a cross-reference table of identifiers built from the DICOM attributes Institution (0008,0080), Station Name (0008, 1010), Manufacturer (0008,0070), Manufacturer’s Model Name (0008, 1090), and Software Version (0018, 1020). The table had been built from information provided by LSS medical physicists for known scanners for which the physicists had routinely supervised calibrations as part of LSS quality assurance. If the attributes in the DICOM header of any CT exam delivered to the CTIL could not be matched in this table, the originating site was questioned because the exam seemed to have come from a scanner not being monitored for quality assurance. Such exceptions were fewer than ten; and most of these cases were caused by scanner software upgrades of which the CTIL had not been informed.