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                                                                   EXHIBIT 10.21

                               AMENDMENT NO. 1 TO

                           CHANGE IN CONTROL AGREEMENT

         THIS AMENDMENT NO. 1 TO CHANGE IN CONTROL AGREEMENT (this "Amendment")
is made and entered into as of December 9, 2003, by and between CTI Molecular
Imaging, Inc., formerly CTI, Inc. (the "Company") and Ronald Nutt, Ph.D.
("Executive") and amends that certain Change in Control Agreement, dated as of
May 15, 2002, by and between the Company and Executive (the "Agreement").

                                    PREAMBLE:

         WHEREAS, pursuant to Section 13(a) thereof, the Agreement may be
amended or modified only by a written agreement executed by the parties thereto
or their respective successors and legal representatives; and

         WHEREAS, the Board of Directors of the Company (the "Board") has
determined that it is in the best interests of the Company and its stockholders
to amend the Agreement in further pursuit of obtaining the assurance that the
Company will have the continued dedication of Executive, notwithstanding the
possibility, threat or occurrence of a Change of Control of the Company;

         NOW, THEREFORE, in consideration of the agreements set forth herein and
for other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties agree as follows:

         1.       Section 6(a)(i)B. of the Agreement is amended to read as
follows:

                  "B. the amount equal to the product of (1) three and (2) the
                  sum of (x) Executive's Annual Base Salary and (y) Executive's
                  target annual bonus for the year in which the Date of
                  Termination occurs; and"

         2.       Section 6(a)(ii) of the Agreement is amended to read as
follows:

                  "(ii) for three years after the Date of Termination, or such
                  longer period as may be provided by the terms of the
                  appropriate plan, program, practice or policy, the Company
                  shall continue benefits to Executive and/or Executive's
                  eligible dependents at least equal to those which would have
                  been provided to them in accordance with the plans, programs,
                  practices and policies described in Section 4(b)(iii) of this
                  Agreement if Executive's employment had not been terminated
                  or, if more favorable to Executive, as in effect generally at
                  any time thereafter with respect to other peer executives of
                  the Company and its affiliated companies and their families,
                  provided, however, that if Executive becomes re-employed with
                  another employer and is eligible to receive medical or other
                  welfare benefits

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                  under another employer provided plan, the Company's
                  obligations under this Section 6(a)(ii) shall cease; and"

         3.       This Amendment may be executed in two or more counterparts,
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument.

         4.       Capitalized terms used but not defined in this Amendment shall
have the meaning assigned to them in the Agreement.

         5.       Regardless of any conflict of law or choice of law principles
that might otherwise apply, the parties agree that this Amendment shall be
governed by and construed in all respects in accordance with the laws of the
State of Delaware.

         6.       Except as specifically set forth in this Amendment, the
Agreement shall remain in full force and effect.

         IN WITNESS WHEREOF, the Company and Executive have caused this
Amendment to be duly executed and delivered, as of the day and year first above
written.

                                     EXECUTIVE:

                                     By:  /s/ Ronald Nutt
                                         ------------------------------------
                                         Ronald Nutt, Ph.D.

                                     THE COMPANY:

                                     CTI MOLECULAR IMAGING, INC.

                                     By:  /s/ David N. Gill
                                         ------------------------------------
                                         Name: David N. Gill
                                         Title: CFO