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Background Erector spinae plane block (ESPB) is a truncal fascial block with a disputed mechanism and anatomical site of effect. This study aimed to perform a one-sided ESPB and use MRI to investigate the spread of the local anesthetic (LA) and the corresponding cutaneous loss of sensation to pinprick and cold. Methods Ten volunteers received a right-sided ESPB at the level of the seventh thoracic vertebra (Th7), consisting of 30 mL 2.5 mg/mL ropivacaine with 0.3 mL gadolinium. The primary outcome was the evaluation of the spread of LA on MRI 1-hour postblock. The secondary outcome was the loss of sensation to cold and pinprick 30–50 min after the block was performed. Results All volunteers had a spread of LA on MRI in the erector spinae muscles and to the intercostal space. 9/10 had spread to the paravertebral space and 8/10 had spread to the neural foramina. 4/10 volunteers had spread to the epidural space. One volunteer had extensive epidural spread as well as contralateral epidural and foraminal spread. Four volunteers had a loss of sensation both posterior and anterior to the midaxillary line, while six volunteers had a loss of sensation only on the posterior side. Conclusion We found that LA consistently spreads to the intercostal space, the paravertebral space, and the neural foramina after an ESPB. Epidural spread was evident in four volunteers. Sensory testing 30–50 min after an ESPB shows highly variable results, and generally under-represents what could be expected from the visualized spread on MRI 60 min after block performance. [ABSTRACT FROM AUTHOR]