Dynamic Parameter in selection screen

Hi all

Following is the selection screen code through which I am trying to assign dynamic parameter name to PARAMETERS, but it is giving sytax error.

SELECTION-SCREEN BEGIN OF BLOCK B WITH FRAME.

Loop at i_text.

CHKIDX = SY-TABIX.

CONCATENATE 'CHKTEXT' chkidx into CHKTXT.

condense chktxt no-gaps.

ASSIGN (chktxt) TO <char>.

PARAMETERS: <char> AS CHECKBOX DEFAULT 'X'.

endloop.

SELECTION-SCREEN END OF BLOCK B.

Please advice whether i can define dynamic parameter in selection screen.

Thanks

Ravindra Suvarna