Dual-energy and spectral CT

Because tissues absorb low- and high-energy X-rays differently, scanning at two energies lets software separate materials. Vendors reach this in different ways: two tube and detector pairs at different voltages (Siemens dual-source), one tube that switches voltage thousands of times a second (GE), a two-layer detector that sorts photons by energy (Philips), or a split filter or two sequential spins (Canon and others). The outputs are iodine maps that quantify tumour enhancement, virtual non-contrast images that replace a separate unenhanced scan, virtual monoenergetic images that boost contrast or suppress metal artefacts from hip implants and spinal hardware, and material-specific images such as virtual bone removal or calcium subtraction to reveal marrow disease.

In oncology it helps characterise kidney and adrenal masses, distinguish bland from tumour thrombus, detect hypervascular liver and pancreatic tumours at lower contrast doses, and see myeloma in the marrow. Photon-counting CT gives the same information natively; dual-energy remains the way most installed scanners obtain it.