Images of coronary calcium imaging by optical coherence tomography (OCT) and high-definition intravascular ultrasound (IVUS). A) OCT shows superficial calcium extending to the medial layer. The calcium, where thickness can be measured due to the optical properties of light penetration through the calcium crystal. B) Co-registered image on high-definition IVUS does not allow measurement of calcium thickness. C) OCT shows superficial calcium extending to the medial layer, where thickness cannot be measured due to the limited penetration depth of light. D) Co-registered image on high-definition IVUS does not allow measurement of calcium thickness. Images and caption courtesy of EuroIntervention and Ziad A. Ali, MD, DPhil, et al. DOI: 10.4244/EIJ-D-21-00089