  Collaborative planning for congenital heart diseases typically involves
creating physical heart models through 3D printing, which are then examined by
both surgeons and cardiologists. Recent developments in mobile augmented
reality (AR) technologies have presented a viable alternative, known for their
ease of use and portability. However, there is still a lack of research
examining the utilization of multi-user mobile AR environments to support
collaborative planning for cardiovascular surgeries. We created ARCollab, an
iOS AR app designed for enabling multiple surgeons and cardiologists to
interact with a patient's 3D heart model in a shared environment. ARCollab
enables surgeons and cardiologists to import heart models, manipulate them
through gestures and collaborate with other users, eliminating the need for
fabricating physical heart models. Our evaluation of ARCollab's usability and
usefulness in enhancing collaboration, conducted with three cardiothoracic
surgeons and two cardiologists, marks the first human evaluation of a
multi-user mobile AR tool for surgical planning. ARCollab is open-source,
available at https://github.com/poloclub/arcollab.
