The ornamental design for a portable open probemagneticresonance instrument, as shown and described.
FIG. 1 is a front perspective view of the portable open probe magnetic resonance instrument;
FIG. 2 is a left side perspective view thereof;
FIG. 3 is a right side perspective view thereof;
FIG. 4 is a back perspective view thereof; and,
FIG. 5 is a top perspective view thereof.
The broken lines in FIGS. 4 and 5 are included for the purpose of illustrating portions of the portable open probe magnetic resonance instrument that form no part of the claimed design