1
                                                                     EXHIBIT 4.6



                               THIRD AMENDMENT TO
                                RIGHTS AGREEMENT

                  This Third Amendment to Rights Agreement (this "Amendment") is
made and entered into as of the 7th day of March, 1996, by and between MEDICAL
IMAGING CENTERS OF AMERICA, INC., a California corporation (the "Company"), and
HARRIS TRUST COMPANY OF CALIFORNIA (the "Rights Agent").



                                    RECITALS

                  A.       Whereas, the Company and Union Bank entered into a 
Rights Agreement (the "Rights Agreement") dated as of October 2, 1991; and

                  B.  Whereas, Harris Trust Company of California has replaced
Union Bank as Rights Agent under the Rights Agreement; and

                  C.  Whereas, the Company and Harris Trust Company of
California previously entered into a First Amendment to Rights Agreement dated
as of January 23, 1996; and

                  D.  Whereas, the Company and Harris Trust Company of
California previously entered into a Second Amendment to Rights Agreement dated
as of March 1, 1996; and

                  E.       Whereas, Section 27 of the Rights Agreement provides
that, subject to certain conditions not applicable here, the Company may
supplement or amend any provision of the Rights Agreement without the approval
of any holders of Right Certificates representing shares of Common Stock; and

                  F.       Whereas, based on the advice of counsel to the
Company, the Board of Directors of the Company believes that certain amendments
to the Rights Agreement, as provided herein, are desirable and in the best
interests of the Company and its shareholders and has authorized certain
amendments to the Rights Agreement in the manner set forth herein.


                                    AGREEMENT

                  NOW THEREFORE, the Company and the Rights Agent hereby agree
as follows:

                  1.       Amendments.  The Rights Agreement is hereby amended
as set forth below.

                  (a)      The first sentence of Section 3(a) of the Rights 
Agreement is hereby amended to read in its entirety as follows:

                           "(a) Until Tuesday, March 12, 1996 at 11:59 p.m.
                  Pacific Standard Time (the "Distribution Date"), (i) the
                  Rights will be evidenced (subject to the provisions of Section
                  3(b) hereof) by the certificates for Common Shares registered
                  in the name of the holders thereof (which certificates shall
                  also be deemed to be Right Certificates) and not by separate
                  Right Certificates, and (ii) the right to receive Right

                                        1
   2
                  Certificates will be transferrable only in connection with the
                  transfer of Common Shares."

                           (b)      Section 23(b) of the Rights Agreement is
hereby amended to read in its entirety as follows:

                           "(b) The Board of Directors of the Company may, at
                  its option, at any time prior to Tuesday, March 12, 1996 at
                  11:59 p.m. Pacific Standard Time redeem all but not less than
                  all of the then outstanding Rights at a redemption price of
                  $.05 per Right (after giving effect to the Reverse Stock
                  Split), appropriately adjusted to reflect any stock split,
                  stock dividend or similar transaction occurring after the date
                  hereof (such redemption price being hereinafter referred to as
                  the "Redemption Price"). The redemption of the Rights by the
                  Board of Directors may be made effective at such time on such
                  basis and with such conditions as the Board of Directors in
                  its sole discretion may establish. Anything contained in this
                  Rights Agreement to the contrary notwithstanding, the Rights
                  shall not be exercisable following a transaction or event
                  described in Section 11(a)(ii) prior to the expiration of the
                  Company's right of redemption hereunder."

                  2.       No Other Changes. Except as specifically set forth 
herein, no change to the Rights Purchase Agreement is intended by the parties
hereto. Except as modified hereby, the parties to the Rights Agreement hereby
reaffirm in all respects all of the covenants, agreements, terms and conditions
set forth in the Rights Agreement, which are incorporated in full herein by
reference, and all terms, conditions and provisions thereof shall remain in full
force and effect, except as amended hereby.

                  3.       Miscellaneous.  The headings and titles of this
Amendment are for convenience only and do not constitute a part hereof. This
Amendment shall be governed by and construed in accordance with the laws of the
State of California. This may be executed in any number of counterparts, any one
of which shall be deemed to be an original and all of which together shall
constitute one and the same instrument.




                                        2
   3
                  IN WITNESS WHEREOF, the parties have executed this Amendment
as of the day and year first above written.


                           THE COMPANY:


                                   MEDICAL IMAGING CENTERS OF AMERICA, INC.,
                                   a California corporation



                                   By: /s/ Robert S. Muehlberg
                                       -----------------------------------------
                                           Name: Robert S. Muehlberg
                                           Its:  President and Chief Executive
                                                    Officer


                           THE RIGHTS AGENT:

                                   HARRIS TRUST COMPANY OF CALIFORNIA

                                   By: /s/ Michael Goedecke
                                       -----------------------------------------
                                           Name: Michael Goedecke
                                           Its: Assistant Vice President




                                        3