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                                                                    EXHIBIT 10.5
                               FOURTH AMENDMENT OF
                         STOCK PURCHASE, REORGANIZATION
                           AND JOINT VENTURE AGREEMENT


         This FOURTH AMENDMENT OF STOCK PURCHASE, REORGANIZATION AND JOINT
VENTURE AGREEMENT (this "Fourth Amendment") is made and entered into as of the
20th day of March, 2002, by and among CTI, INC., a Tennessee corporation
(hereinafter "CTI"); SIEMENS MEDICAL SOLUTIONS USA, INC., f/k/a Siemens Medical
Systems, Inc., a Delaware corporation (hereinafter "SIEMENS"; successor in
interest to Siemens Gammasonics, Inc.); CTI PET SYSTEMS, INC., a Tennessee
corporation ("CPS"); and DR. TERRY D. DOUGLASS, DR. RONALD NUTT, MICHAEL C.
CRABTREE, and J. KELLY MILAM (hereinafter "MANAGEMENT").

         WHEREAS, CTI, Siemens, CPS and Management heretofore made and entered
into a Stock Purchase, Reorganization and Joint Venture Agreement dated as of
December 10, 1987, as amended by (i) Amendment of Stock Purchase, Reorganization
and Joint Venture Agreement dated as of March 31, 1995, (ii) Second Amendment of
Stock Purchase, Reorganization and Joint Venture Agreement dated as of June 30,
1997, and (iii) Third Amendment to Stock Purchase, Reorganization and Joint
Venture Agreement dated April ___, 2001 (hereinafter, the "JOINT VENTURE
AGREEMENT"); and

         WHEREAS, the parties wish to further amend the Joint Venture Agreement
as set forth below.

         NOW, THEREFORE, in consideration of the terms and conditions contained
herein, the parties hereto mutually agree as follows:

         1.       JOINT VENTURE AGREEMENT. The Joint Venture Agreement is
incorporated herein by reference. Capitalized terms not defined herein shall
have the meanings given them in the Joint Venture Agreement. As amended hereby,
the Joint Venture Agreement shall remain in full force and effect.

         2.       OFFICERS OF CPS. Section 13.4 of the Joint Venture Agreement
is amended by deleting the original language thereof in its entirety and
substituting in its place the following:

                  13.4     Officers of CPS.

                  (a)      Siemens shall be entitled (i) to nominate one
                           individual to serve as either Chairman of the Board
                           of Directors of CPS or President of CPS; (ii) to
                           nominate a successor for such individual; and (iii)
                           to propose the removal from office of any person it
                           nominates under either clause (i) or (ii) above.


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                  (b)      CTI shall be entitled (i) to nominate one individual
                           to serve as either Chairman of the Board of Directors
                           of CPS or President of CPS; (ii) to nominate a
                           successor for such individual; and (iii) to propose
                           the removal from office of any person it nominates
                           under either clause (i) or (ii) above.

                  (c)      The Board of Directors of CPS shall, by majority
                           vote, appoint one of the individuals nominated
                           pursuant to subpart (a) or subpart (b) above to serve
                           as Chairman of the Board and the other nominee to
                           serve as President of CPS. Specifically, if the
                           Chairman of the Board of Directors of CPS is the
                           nominee of CTI, then the President of CPS shall be
                           the nominee of Siemens and, similarly, if the
                           Chairman of the Board of Directors of CPS is the
                           nominee of Siemens, then the President of CPS shall
                           be the nominee of CTI. If either Siemens or CTI
                           proposes the removal of its nominee as Chairman of
                           the Board or President, the Board of Directors of CPS
                           shall promptly effect such removal and shall appoint
                           such party's successor nominee to serve as either
                           Chairman of the Board or President of CPS, as the
                           Board deems appropriate. In the event of an
                           abstention by a director that results in a tie vote
                           in connection with the appointment of either the
                           Chairman or President, (i) the nominee designated by
                           Siemens pursuant to subpart (a) above will serve in
                           the position that his or her Siemens designated
                           predecessor served prior to such tie vote; and (ii)
                           the nominee designated by CTI pursuant to subpart (b)
                           above will serve in the position that his or her CTI
                           designated predecessor served prior to such tie vote.
                           If the tie vote in connection with the appointment of
                           either the Chairman or President continues for a
                           period of three (3) years from the date of the
                           original tie vote, then the nominees designated by
                           Siemens and CTI will rotate offices such that (1) the
                           nominee designated by Siemens pursuant to subpart (a)
                           above will serve in the position that his or her CTI
                           designated predecessor served prior to such tie vote;
                           and (2) the nominee designated by CTI pursuant to
                           subpart (b) above will serve in the position that his
                           or her Siemens designated predecessor served prior to
                           such tie vote. The nominees of the parties will
                           continue to rotate such offices on three year terms
                           for so long as there continues to be a tie vote by
                           the Board of Directors in connection with the
                           appointment of either the Chairman or President.


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                  (d)      Dr. Nutt currently serves as both the President and
                           the Vice President of Research and Development of CPS
                           and will continue to serve in such capacities until
                           the Board of Directors selects a new Vice President
                           of Research and Development. Notwithstanding any
                           other term or provision contained herein, the parties
                           agree that the election or appointment of Dr. Nutt's
                           successor as Vice President of Research and
                           Development and Mr. Fred Stuvek's successor as Vice
                           President of Marketing shall require the unanimous
                           approval of the Board of Directors of CPS.

                  (e)      Other than as set forth in subparts (a) through (d)
                           above, each officer of CPS shall be appointed or
                           removed by the majority vote of the Board of
                           Directors of CPS and unless otherwise agreed to in
                           writing by CTI and Siemens, all future nominees to
                           serve as officers of CPS shall be independent of CTI
                           and Siemens.

                  (f)      As soon as reasonably practicable, but not later than
                           October 1, 2002, the Board of Directors of CTI or the
                           Board of Directors of CPS, as the case may be, shall
                           cause (i) Dr. Nutt to resign as an officer of CTI;
                           and (ii) Michael Templin to resign as an officer of
                           either CTI or CPS. Dr. Nutt and Mr. Templin shall
                           cease performing any duties or responsibilities as an
                           officer of CTI or CPS, as the case may be, upon the
                           effective date of their resignations and, thereafter,
                           may serve as an officer of either CTI or CPS but
                           shall not serve as an officer of both entities
                           simultaneously without the prior written consent of
                           Siemens. Notwithstanding the foregoing, the parties
                           agree that Dr. Nutt will continue to serve as a
                           director of CTI and nothing contained herein shall
                           affect his continued service in such capacity.

         3.       CTI BOARD. Section 13.8 of the Joint Venture Agreement is
amended by deleting the original language thereof in its entirety and
substituting in its place the following:

                           13.8     CTI Board. For so long as CTI and any of its
                           Affiliates, in the aggregate, and Siemens and any of
                           its Affiliates, in the aggregate, each holds more
                           than twenty percent (20%) of the outstanding shares
                           of CPS Common Stock, Siemens shall be entitled to
                           nominate one individual to serve as a member of the
                           Board of Directors of CTI, and Management agrees to
                           vote its shares of CTI Common Stock for the election
                           of such nominee.


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         4.       SIEMENS COINCIDENCE OPTION. Section 4 of the Second Amendment
of Stock Purchase, Reorganization and Joint Venture Agreement dated June 30,
1997 (the "Second Amendment") is amended by deleting the original language
thereof in its entirety and substituting in its place the following:

                           4.       Siemens Coincidence Option. Notwithstanding
                           any other term or provision of the Joint Venture
                           Agreement, Siemens shall have the right to
                           manufacture and distribute, without paying any
                           royalty or other consideration to CTI or CPS, a
                           sodium iodide coincidence option on its dual-headed
                           SPECT (single photon emission computerized
                           tomography) cameras having specifications and
                           features not to exceed those set forth on Appendix A
                           attached hereto. Siemens covenants and agrees that it
                           shall not (i) add any new features to its coincidence
                           option, including, without limitation, attenuation
                           correction provided either by radiation sources or
                           x-ray computerized tomographs; or (ii) modify,
                           enhance or improve any existing features of the
                           coincidence option in a manner that results in the
                           option, as so modified, enhanced or improved,
                           exceeding the specifications set forth on Appendix A.
                           The parties agree that the restrictions set forth in
                           subpart (i) above do not apply to SPECT cameras that
                           do not have a coincidence option. The parties further
                           agree that Siemens shall have the right to make image
                           quality maintenance and reliability improvements to
                           the coincidence option on the SPECT cameras, and make
                           such other modifications as may be necessary to
                           correct software defects or deficiencies, so long as
                           such improvements or modifications do not result in
                           the coincidence option on the SPECT cameras, as so
                           improved or modified, exceeding the specifications or
                           features set forth on Appendix A. Subject to the
                           foregoing limitations, the activities of Siemens
                           contemplated in this Section 4 shall not be deemed to
                           be in violation of the non-competition provisions of
                           the Joint Venture Agreement. Siemens will use, as
                           needed, experience, coincidence-related intellectual
                           property, services and other material or resources of
                           CPS at commercially reasonable rates.



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         5.       APPOINTMENT OF CTI AS A DISTRIBUTOR. The parties acknowledge
that CPS is authorized and permitted to designate CTI as a distributor of its
products and that such designation does not constitute a violation of the
non-competition provisions contained in the Joint Venture Agreement.


                            [Signatures on Next Page]



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         IN WITNESS WHEREOF, the parties hereto have executed this Fourth
Amendment on the day and year first above written.

                              CTI, INC.


                              By:      /s/ Terry D. Douglass
                                 -----------------------------------------------
                                    Terry D. Douglass
                                    President


                              SIEMENS MEDICAL SOLUTIONS USA, INC.


                              By:      /s/ Thomas McCausland
                                 -----------------------------------------------
                                    Thomas McCausland
                                    President and Chief Executive Officer


                              CTI PET SYSTEMS, INC.


                              By:      /s/ Ronald Nutt
                                 -----------------------------------------------
                                    Ronald Nutt
                                    President


                              MANAGEMENT:

                                       /s/ Terry D. Douglass
                              --------------------------------------------------
                              Dr. Terry D. Douglass

                                       /s/ Ronald Nutt
                              --------------------------------------------------
                              Dr. Ronald Nutt

                                       /s/ Michael C. Crabtree
                              --------------------------------------------------
                              Michael C. Crabtree

                                       /s/ J. Kelly Milam
                              --------------------------------------------------
                              J. Kelly Milam


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