How to calibrate the reconstruction output to Hounsfield Units? How does calibration depend on scanner settings?
In order to get quantitatively accurate reconstructed image we would recommend scanning a water phantom fitting into the field of view. Check the output water density and correct OPTTAG_SLICESCALE accordingly. For example, if the output water level is 500 and the applied OPTTAG_SLICESCALE = 1.25, it then is necessary to set OPTTAG_SLICESCALE = 2.5 to get the density of water as 1000 (mg/cm3). Please note that you do all calibrations w/o PARTAG_MINUS1000 applied. When the calibration is done this tag is just shifting all values to get -1000 for air and 0 for water so the reconstructed image represents Hounsfield Units.
The calibration (and therefore OPTTAG_SLICESCALE value measurement) in general has to be independently performed for all scanner configurations: all X-Ray source kV's and all bowtie filters (if any).
Please note that PARTAG_SCALEFACTOR in general does not make any impact to the image density profiles.
How can I make a geometry calibration phantom myself?
A geometry calibration phantom is a plastic or alluminum cylinder. It must be made accurately and presizely. It means that any crossection perpendicular to the axis of symmetry must be accurately round. A phantom must be solid and stable. A phantom is hollow inside; phantom wall thickness is recommended to be about 1-2 mm.
There are also a photo in our user manual, see Part 3, section 1.
A phantom holds metal balls (beads). Basically a plastic or alluminum cylindrical body is important as far as it supports metal beads in accurate and known positions
Diameter of bead is typically 1 mm or less. Beads must be put inside holes (hollows, depressions) and glued to a cylinder. Typically a cylinder holds 18-32 beads. Beads are forming 3 or 4 layers. Every bead layer contains 5-8 beads. In general a larger amount of beads increases a calibration accuracy. In the same time too many beads may create problems in distinguishing them on x-ray image
Every bead layer is perfectly flat and the corresponding plane is perpendicular to the axis of cylinder symmetry. Distances between beads within the same layer and distances between layers are presize and uniform (stay the same)
A diameter of phantom and it hight is defined by scanner field of view. It must be 25-40% smaller than FOV.
Please also see Part 3, section 1.1 (how x-ray image of a phantom should look like)