How much myocardial damage is necessary to enable detection of focal late gadolinium enhancement at cardiac MR imaging?

Purpose: To assess the visibility of small myocardial lesions at magnetic resonance (MR) imaging and to estimate how much myocardial damage is necessary to enable detection of late gadolinium enhancement (LGE) in vivo. Materials and Methods: The study was approved by the local bioethics committee. Coronary microembolization was performed by injecting 300 000 microspheres into the distal portion of the left anterior descending artery in 18 anesthetized minipigs to create multifocal areas of myocardial damage. In vivo MR imaging was performed a mean of 6 hours after microem-bolization by using an inversion-recovery spoiled gradient- echo sequence (repetition time msec/echo time msec, 8/4; inversion time, 240-320 msec; flip angle, 20°; spatial resolution, 1.3 × 1.7 × 5.0 mm³) after injection of 0.2 mmol gadopentetate dimeglumine per kilogram of body weight. High-spatial- resolution imaging of the explanted heart was performed by using the same sequence with a higher spatial resolution (0.5 × 0.5 × 2.0 mm ³). Imaging results were verified with histologic examination. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of in vivo and ex vivo images were calculated, and a t test was used to analyze observed differences. Results: Multifocal myocardial damage was successfully induced in all animals. Areas of LGE with low SNR (mean, 36.3 ± 29.4 [standard deviation]) and CNR (23.7 ± 19.8) were observed in vivo in 12 (67%) of 18 animals, whereas ex vivo imaging revealed spotted to streaky areas of LGE with higher SNR (91.4 ± 27.8, P < .0001) and CNR (72.1 ± 25.4, P 7lt; .0001) among normal-appearing myocardium in all cases (100%). Focal myocardial lesions exceeding 5% of myocardium per slice at histologic examination were detected in vivo with a sensitivity of 83%. Conclusion: Focal myocardial damage exceeding 5% of myocardium within the region of interest seems to be necessary for detection of LGE in vivo in an experimental model of coronary microembolization.