The main advantage of dual energy CT is the selective extraction of materials with high atomic numbers from the images, made possible by acquiring the scan at two different kilovoltages simultaneously. In myeloma imaging, calcium can be removed from scans to generate a "virtual non-calcium map", which contain the fat/soft tissue partition of the bone marrow alone. This enables more accurate assessment of marrow infiltration.
Colour-coded overlay images allow for ready visualization of potential regions of bone marrow plasma cell infiltration. Density values can also be assessed and may increase diagnostic confidence for distinguishing myeloma from other lesions.
Technical Aspects
At our institution, DECT is performed on a Siemens SOMATOM Force Dual Source CT Scanner from skull vertex to below knee. The arms are placed above the head to avoid spinal artefact; if this is not possible, the arms are placed in front on the lower abdomen and a cushion should be used to minimise spinal artefact. Other parameters are as follows: 90-150 kVp, 100-250 mAs, pitch 0.6, rotation time 0.5 s, and a tin filter. The average dose is approximately 8 mSv.
Reconstructed datasets from the single acquisition include an axial mixed dataset, axial 90 kV, axial 150 kV, VNCa dataset, color map overlay MIP images, VRT and lung images. The mixed dataset combines high and low kV images, simulating a standard CT. Slice thickness is 1 mm.
The impact of DECT upon the patient experience is also relevant; this scan is quick to perform, does not involve any contrast media, and is well-tolerated by patients.
At our institution, the colour look-up tables range from green for soft tissue and blue/purple for fat.
A number of cases are presented below:
Case 1
A 65 year old with relapsed myeloma. A concurent WB-MRI shows multifocal disease. The mixed DECT images shows multifocal disease and the VNCa images demonstrate a diffuse pattern of infiltration.
Case 2
A 77 year old with known multifocal disease with both active and inactive lesions. The sternal VNCa density was 16 HU, suggesting active disease. Density at the left femoral neck was -12 HU which may reflect lower grade infiltration with regions of preserved yellow marrow.
Case 3
A 72 year old with known treated myeloma. This case demonstrates the limitation of standard CT - there are numerous lytic lesions however it is challenging to differentiatie between active and inactive disease. The corresponding BM MIP shows that the density values of the lytic lesions are in keeping with fatty/yellow marrow, indicating treated disease.
Pitfalls
There are numerous potential pitfalls, the most common being image artefact leading to segmentation errors. Examples of this include lesions breaching the cortex, in the spinal cord, and metalwork from, for example, prostheses.
A second potential pitfall is the difficulty in differentiating between active disease, inflammation/oedema and red marrow reconversion as all may have similar marrow density.
Finally, the accuracy of regions-of-interest in small volume disease is questionable.