An arm positioning system for a diagnostic procedure such as an MRI can include a positioning block and a retention device (such as a thumb strap). The retention device and the positioning block can be coupled together (e.g. by hook and loop fasteners) to allow them to position a patients arm in various desired positions. The retention device and positioning block can be composed of materials that do not interfere with the diagnostic device. The retention device and positioning block also can be configured to accommodate left and right arms and variation among arm, hand, and digit sizes of patients.
