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Background Photon-counting cardiac CT (PCCT) overcomes conventional CT limitations by combining improved spectral characterisation with enhanced geometric fidelity. Myocardial injury characterisation by late iodine enhancement (LIE) PCCT (Flash, dual-scan analysis) compares favourably to cardiovascular magnetic resonance (CMR) characterisation in ST-segment elevation acute coronary syndrome (STEACS). This study provides an intra-individual comparison of PCCT acquisition and analysis workflows for quantifying microvascular obstruction (MVO) and infarct size (IS).Methods Between 11/2024 and 04/2025, 19 consecutive patients (OxAMI) underwent PCCT LIE myocardial evaluation with a first-generation NAEOTOM Alpha system (Siemens Healthineers). All participants underwent PCCTA(0.6 gI/kg), followed by an 8-minute delay and two consecutive LIE acquisitions: a high-pitch spiral technique (Flash) and a sequential technique (Flex) with a 250-350ms window. Same-day late gadolinium enhancement (LGE) CMR (1.5T, Siemens Healthineers) was performed in 12 patients. PCCT LIE myocardial analysis was based on iodine-derived extracellular volume, using dual-scan, Flash-based and single-scan, Flex-based workflows (figures 1 and 2).Results There is excellent agreement between a single-scan, Flex-based and a dual-scan, Flash-based PCCT workflow for MVO and IS; intraclass correlation coefficient (ICC) 0.98 95%CI:0.95-0.99 and 0.96 95%CI:0.89-0.98, respectively. Bland-Altman analysis shows no systematic bias and acceptable limits of agreement for MVO and IS evaluation; mean bias/limits of agreement: 0.03%/3.14% and 0.18%/6.01%, respectively. There is good agreement between a single-scan, Flex-based PCCT and CMR LGE for MVO and IS evaluation; ICC 0.79 95%CI:0.28-0.94 and 0.90 95%CI:0.63-0.96), respectively.Conclusion Single-scan Flex LIE PCCT provides accurate quantification of MVO and IS, with good level of agreement to CMR LGE and dual-scan Flash LIE PCCT. Question:In patients with ST-segment elevation acute coronary syndrome (STEACS), how does the single-scan Flex-based photon-counting CT (PCCT) workflow compare to the dual-scan Flash-based workflow and cardiovascular magnetic resonance (CMR) in quantifying microvascular obstruction (MVO) and infarct size (IS)?A. It shows poor agreement with both dual-scan Flash PCCT and CMR, limiting its clinical utility.B. It demonstrates excellent agreement with dual-scan Flash PCCT but no agreement with CMR.C. It demonstrates excellent agreement with dual-scan Flash PCCT and good agreement with CMR.D. It overestimates MVO and IS compared to both dual-scan Flash PCCT and CMR.Correct answer:Abstract 13 Figure 2Abstract 13 Figure 1C. It demonstrates excellent agreement with dual-scan Flash PCCT and good agreement with CMR.