Secondary Capture: Storing Legacy Lateral and PA Film scans in DICOM

[Antonio Magni]

Hi,

at Case Western Reserve University, we are still scanning our legacy longitudinal collection of radiographs, and we want to clean them up and store them properly in DICOM, so they can be more useful.

This entails also understanding and defining how to properly encode all necessary information in the proper DICOM tags. Which we also plan to move forward as an ADA Implementation Guideline/Standard or an IHE Profile or both.

Q1:
Should we use legacy Secondary Capture CIOD, or Multi-frame Grayscale Word Secondary Capture Image CIOD? These are 12 or 16 bit grayscale single frame images. The standard seems to encourage the use of multiframe, even for single frame.

Q2:
Nominal Scanned Pixel Spacing (0018,2010) is easy, as it can be easily calculated from calibrated scanner DPI. However, Pixel Spacing depends on the Radiographic Magnification.

One idea is to set Pixel Spacing Calibration Type (0028,0A02) to 'GEOMETRY', and then explain in Pixel Spacing Calibration Description (0028,0A04) (LO 64 chars max) all the known values and reasoning for the proper Pixel Spacing. The explanation would cover the DistanceSourceToDetector (variable limited range, exact number available in burned in annotation) and DistanceSourceToPatient (fixed, well known 1524mm), but it would not fit it in the 64 char limit of the VR.

Another idea would be to use the DistanceSourceToDetector (0018,1110) and DistanceSourceToPatient (0018,1111) tags, and then in the 64 chars refer to those, but they are not part of the Secondary Capture IOD.

Keep in mind that the overwhelming majority of these cephalograms around the world have been taken at the same distances of 1524mm, in order to allow researchers and clinicians to share, compare and analyse the same cephalometric analysis values. This means that the physician expects to be measuring the distorted film and not the actual patient, therefore it would make "functional" (as opposed to logical) sense for Pixel Spacing = Nominal Scanned Pixel Spacing.

Alternatively, should all this information go in the C.7.6.2 Image Plane Module?

Q3:
Related to Q2, we could make use of the Positioner Type (0018,1508) 1 with value “CEPHALOSTAT”, as it might clarify the above issues. However, this tag too isn't available in the SC IOD. Is it time for a Correction Package?


Thanks,
--

Toni Magni, BME

Clinical Instructor

Department of Orthodontics

Case Western Reserve University