 A state-of-art CT scanner Discovery CT750 HD, General Electric GE Healthcare, has been installed at Derriford hospital for further research on CTCA. It uses novel method of scanning, High Definition Computed Tomographic Coronary AngiographyHD-CTCA, analogous to high definition television and image reconstruction Adaptive Statistical Iterative Reconstruction ASIRas opposed to conventional CT scanners using Filtered Back Projection FBPreconstruction. HD-CTCA enables acquisition of sharper images and ASIR offsets the resultant increase in radiation exposure. This is likely to result in images of higher diagnostic quality with an equivalent or slightly lower radiation exposure compared to present technology. Although initial results are encouraging, this needs further assessment before being applied to routine clinical practice. To assess this we have designed a study to perform HD-CTCA on 300 consecutive patients undergoing diagnostic ICA at Derriford hospital, directly comparing the accuracy of HD-CTCA to ICA presently considered the gold standard. 