Conventional material decomposition
techniques for dual-energy computed tomography (CT) assume mass or volume conservation, where the CT number of each voxel is fully assigned to predefined materials. We present an image-domain contrast material
extraction process (CMEP) method that preferentially extracts contrast-producing materials while leaving the remaining image intact.
Materials and Methods
Image processing freeware (Fiji) is used to perform consecutive arithmetic operations on a dual-energy ratio map to generate masks, which are then applied to the original images to generate material-specific images. First, a low-energy image is divided by a high-energy image to generate a ratio map. The ratio map is then split into material-specific masks. Ratio intervals known to correspond to particular materials (eg, iodine
) are assigned a multiplier of 1, whereas ratio values in between these intervals are assigned linear gradients from 0 to 1. The masks are then multiplied by an original CT image to produce material-specific images. The method was tested quantitatively at dual-source CT
and rapid kVp-switching CT
(RSCT) with phantoms using pure and mixed formulations of tungsten
, and iodine
. Errors were evaluated by comparing the known material concentrations with those derived from the CMEP material-specific images. Further qualitative evaluation was performed in vivo at RSCT with a rabbit model using identical CMEP parameters to the phantom. Orally administered tungsten
, vascularly administered iodine
, and skeletal calcium
were used as the 3 contrast materials.
All 5 material combinations—tungsten
, and calcium
, and mixtures of tungsten
—showed distinct dual-energy ratios, largely independent of material concentration at both dual-source CT
and RSCT. The CMEP was successful in both phantoms and in vivo. For pure contrast materials in the phantom, the maximum error between the known and CMEP-derived material concentrations was 0.9 mg/mL, 24.9 mg/mL, and 0.4 mg/mL for iodine
, and tungsten
respectively. Mixtures of iodine
showed the highest discrepancies, which reflected the sensitivity of iodine
to the image-type chosen for the extraction of the final material-specific image. The rabbit model was able to clearly show the 3 extracted material phases, vascular iodine
, oral tungsten
, and skeletal calcium
. Some skeletal calcium
was misassigned to the extracted iodine
image; however, this did not impede the depiction of the vasculature.
The CMEP is a straightforward, image-domain approach to extract material signal at dual-energy CT
. It has particular value for separation of experimental high-Z contrast elements from conventional iodine
contrast or calcium
, even when the exact attenuation coefficient profiles of desired contrast materials may be unknown. The CMEP is readily implemented in the image-domain within freeware, and can be adapted for use with images from multiple vendors.