Aims To characterize acute lesions during cardiac magnetic resonance (CMR)-guided radiofrequency (RF) ablation of cavo-tricuspid isthmus (CTI)-dependent atrial flutter by combining T₂-weighted imaging (T₂WI), T₁ mapping, first-pass perfusion, and late gadolinium enhancement (LGE) imaging. CMR-guided catheter ablation offers a unique opportunity to investigate acute ablation lesions. Until present, studies only used T₂WI and LGE CMR to assess acute lesions. . Methods Fifteen patients with CTI-dependent atrial flutter scheduled for CMR-guided RF ablation were prospectively enrolled. and results Directly after achieving bidirectional block of the CTI line, CMR imaging was performed using: T₂WI (n = 15), T₁ mapping (n = 10), first-pass perfusion (n = 12), and LGE (n = 12) imaging. In case of acute reconnection, additional RF ablation was performed. In all patients, T₂WI demonstrated oedema in the ablation region. Right atrial T₁ mapping was feasible and could be analysed with a high inter-observer agreement (r = 0.931, ICC 0.921). The increase in T₁ values post-ablation was significantly lower in regions showing acute reconnection compared with regions without reconnection [37 ± 90 ms vs. 115 ± 69 ms (P = 0.014), and 3.9 ± 9.0% vs. 11.1 ± 6.8% (P = 0.022)]. Perfusion defects were present in 12/12 patients. The LGE images demonstrated hyper-enhancement with a central area of hypo-enhancement in 12/12 patients. . Conclusion Tissue characterization of acute lesions during CMR-guided CTI-dependent atrial flutter ablation demonstrates oedema, perfusion defects, and necrosis with a core of microvascular damage. Right atrial T₁ mapping is feasible, and may identify regions of acute reconnection that require additional RF ablation.