Digitization and Quality Control
The NHANES II radiographs were digitized by the University of California at San Francisco and the Radix Corporation. All radiographs have been digitized on either a Lumisys 100 or 150 laser spot scanner, with a spot size of 175 microns. The cervical and lumbar spine images have a resolution of 1463x1755x12 bits (5 MBytes) and 2048x2487x12bits (10 MBytes), respectively. After each image has passed a three-tiered quality control procedure, the data is stored on erasable optical disk and is ready for inclusion in the archives optical jukebox.
Quality control (QC) of the NHANES II x-ray images consists of three independent stages. The QC done at each stage is as follows:
Stage 1: This stage is done by a trained computer operator and laser scanner technician. The following operations are performed:
- re-calibrate laser scanner with each scan
- clean optics every 2-3 months
- use step-wedge films to check scanner calibration every 2-3 months
- clean pinch rollers regularly
- visibly check each image for general contrast, image alignment, and for removal of identification tags
Stage 2: This stage is done by a non-medical person trained to filter out images that do not meet the following criteria:
- inspect each image to ensure that identification tags are not visible
- check for sufficient contrast
- check for correct image orientation
Stage 3: This stage is being done at NLM by a trained physician under contract to NCHS. The physician answers the following questions concerning each image:
- is the digitized image acceptable?
- is the digitized image worse, same, or better than radiograph?
- would a reader be able to detect and score the extent of osteophytes, subluxation, sclerosis, or disk space narrowing?
If an image is rejected at stage 1, the radiograph is re-digitized. Rejection at stage two or three eliminates the image from the archive, although such images might prove useful for future work in automating quality control. A separate database to archive these rejected images might be developed.