The intervention
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Straight after randomisation, participants in the immediate intervention arm gained access to the digital imagery-competing task intervention with symptom monitoring of IMs for 4 weeks. The intervention was delivered on a secure web platform (i-spero®) via smartphone, tablet, or computer. Participants had an initial researcher-guided session (~1 h, via Microsoft Teams) and thereafter used i-spero® in a self-directed manner (~25 min; with the option for support). The researcher-guided session consisted of step-by-step instructions, animated videos and multiple-choice questions. Participants were instructed to list their different IMs by typing a brief description. They selected one IM from their list, and very briefly brought the image to mind. After instructions on playing the computer game Tetris® using mental rotation, they played for 20 min. Finally, they were instructed on monitoring IMs in i-spero® and encouraged to use the intervention to target each memory on their list.The brief digital intervention on i-spero® and P1vital® ePRO are owned and manufactured by P1vital Products Ltd. Tetris® has been licensed for use within i-spero® from The Tetris Company. P1vital® ePRO, i-spero® and the brief digital intervention have been developed following a formal computerised system validation methodology which complies with Good Clinical Practice, FDA 21CFR Part 11 and ISO13485 Quality Management System.