Something like this?
Graphics[Disk[], Frame -> True,
FrameLabel -> {"\!\(\*FormBox[\(ln\\\ x\),
TraditionalForm]\)",
"\!\(\*FormBox[\(ln \[LeftAngleBracket]\*SuperscriptBox[\(R(t)\), \
\(2\)]\[RightAngleBracket]\),
TraditionalForm]\)"}, BaseStyle -> {FontFamily -> "Times"}]
My own procedure is this: I compose the formula in a text cell, math inline cell, copy it and then paste it into the input cell within quotes "". The front end asks me "Do you want to wrap the pasted TraditionalFormn expression in a FormBox?". I answer "OK", whatever that means. As for the angle brackets, I use the TeX aliases \langle, \rangle.