A chronic total occlusion (CTO) is present in 15-20% of patients who are referred for invasive coronary angiography. CTO Percutaneous Coronary Intervention (PCI) procedure success rates have increased over the years and can be as high as 90% when performed by high-volume CTO operators.Procedurally, excess calcification in the CTO anatomy is one factor which makes it difficult to complete the procedure or obtain ideal stent expansion. Excess calcification is best identified by Computed Tomography Coronary Angiography (CTCA) rather than invasive angiography. The investigators plan to retrospectively evaluate CTCA in patients who underwent CTO PCI and correlate calcification characteristics with CTO PCI outcomes and tools utilised for calcium modification.