Background: Patients with repaired tetralogy of Fallot (rTOF) remain at risk of sustained monomorphic ventricular tachycardia (SMVT) related to slow-conducting anatomical isthmuses (SCAI). Invasive electroanatomical mapping (EAM) is the only available method to identify SCAI (SCAIEAM). We aimed to determine rTOF-specific high signal intensity threshold values (HSIt) to identify abnormal myocardium by 3D late gadolinium enhancement cardiac magnetic resonance (LGE-CMR) and assess the performance of LGE-CMR to non-invasively identify SCAIEAM. Methods: Consecutive rTOF patients who underwent right ventricular EAM (RV-EAM) and 3D LGE-CMR were included (2012-2021). A SCAIEAM was defined as an anatomical isthmus (AI) with conduction velocity (CV) <0.5 m/s. LGE-CMR-derived 3D RV reconstructions were merged with 3D RV-EAM data. The HSIt was determined based on the comparison of local bipolar voltages (BV) and the corresponding local SI using ROC analysis. An abnormal AI on LGE-CMR (Abnormal AICMR) was defined as AI showing continuous high SI (>HSIt) between anatomical boundaries. Results Forty-eight rTOF patients (34{plus minus}16 years, 58% male) were included. Of 107 AIs on EAM (AI1 and 3 in all, AI2 in 11), 78 were normal-conducting AIEAM (NCAIEAM), 22 were SCAIEAM (SCAIEAM2 in 2 and SCAIEAM3 in 20), and 7 were blocked AIEAM3. All 14 induced SMVTs were related to SCAIEAM3. A total of 9240 EAM points were analyzed. HSIt was 42% of the maximal SI (AUC 0.80; sensitivity, 74%; specificity, 78%). On 3D-CMR RV construction, all 29 SCAIEAM or Blocked AIEAM were identified as abnormal AICMR. Among the 78 NCAIEAM, 70 were normal AICMR and 8 were abnormal AICMR. The sensitivity and specificity of 3D LGE-CMR for identifying SCAIEAM or blocked AIEAM were 100% and 90% (29/29 and 70/78), respectively. Among patients with NCAIEAM3 (n=28), those with abnormal AICMR3 (n=6) had significantly lower BV and slower CV compared with those with normal AICMR3 (n=22) (BV, 1.91 [1.62-2.60] vs. 3.45 mV [2.22-5.67]; CV, 0.69 [0.62-0.81] vs, 0.95 m/s [0.82-1.09]; both P<0.01). Conclusion: 3D LGE-CMR can identify SCAI with excellent sensitivity and specificity and may identify diseased AI3 even before critical conduction delay occurs, which may enable non-invasive risk stratification of VT and may refine patient selection for invasive EAM.