Document:

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                                                                     EXHIBIT 4.1

      FIRST AMENDMENT TO STOCKHOLDER PROTECTION RIGHTS AGREEMENT

      THIS FIRST AMENDMENT TO STOCKHOLDER PROTECTION RIGHTS AGREEMENT (this
"Amendment") dated as of the 17th day of March, 2005, by and between CTI
Molecular Imaging, Inc., a Delaware corporation (the "Company"), and SunTrust
Bank, as Rights Agent (the "Rights Agent"), constitutes the first amendment to
the Shareholder Protection Rights Agreement dated May 21, 2002 by and between
the Company and the Rights Agent (the "Rights Agreement").

                              W I T N E S S E T H:

      WHEREAS, the parties hereto desire to amend the Rights Agreement in
certain respects on the terms and conditions hereinafter set forth;

      NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto do hereby agree as follows:

      1. The Rights Agreement is hereby amended by:

      (a) Adding the following sentence at the end of the definition of
"Acquiring Person" in Section 1.1 of the Rights Agreement:

      "Notwithstanding any of the terms of the foregoing definition, no Person
      shall become an "Acquiring Person" solely as the result of the execution
      and delivery of, or the consummation of any of the transactions
      contemplated by, the Agreement and Plan of Merger dated as of March 18,
      2005, by and among the Company, Siemens Medical Solutions USA, Inc.
      ("Parent"), and MI Acquisition Co. ("Purchaser") (the "Merger Agreement"),
      and any agreements, documents or instruments executed or entered into in
      connection with the Merger Agreement (collectively with the Merger
      Agreement, the "Parent Transaction Documents")."

      (b) Deleting the definition of "Expiration Time" in Section 1.1 of the
Rights Agreement in its entirety, and inserting the following:

      ""Expiration Time" shall mean the earliest of (i) the Exchange Time, (ii)
      the Termination Time, (iii) June 1, 2012, (iv) other than the merger of
      CTI, Inc. and the Company in connection with the reorganization of CTI,
      Inc. as a Delaware corporation, the time of a merger of the Company into
      another corporation pursuant to an agreement entered into prior to a
      Flip-In Date, and (v) at the effective time of the merger of Purchaser
      with and into the Company pursuant to the terms of the Parent Transaction
      Documents."

      (c) Adding the following sentence at the end of the definition of "Flip-In
Date" in Section 1.1 of the Rights Agreement:

      "Notwithstanding any of the terms of the foregoing definition, no "Flip-In
      Date " will occur solely as the result of the execution and delivery of,
      or the consummation of any of the transactions contemplated by, the Parent
      Transaction Documents."

      (d) Adding the following sentence at the end of the definition of
"Flip-Over Transaction or Event" in Section 1.1 of the Rights Agreement:

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      "Notwithstanding any of the terms of the foregoing definition, no
      "Flip-Over Transaction or Event" will occur solely as the result of the
      execution and delivery of, or the consummation of any of the transactions
      contemplated by, the Parent Transaction Documents."

      (e) Adding the following sentence at the end of the definition of
"Separation Time" in Section 1.1 of the Rights Agreement:

      "Notwithstanding any of the terms of the foregoing definition, no
      "Separation Time" will occur solely as the result of the execution and
      delivery of, or the consummation of any of the transactions contemplated
      by, the Parent Transaction Documents."

      (f) Adding the following sentence at the end of the definition of "Stock
Acquisition Date" in Section 1.1 of the Rights Agreement:

      "Notwithstanding any of the terms of the foregoing definition, no "Stock
      Acquisition Date" will occur solely as the result of the execution and
      delivery of, or the consummation of any of the transactions contemplated
      by, the Parent Transaction Documents, or any public announcement of any of
      the foregoing."

      2. All terms defined in the Rights Agreement which are used herein shall
have the meanings defined in the Rights Agreement, unless specifically defined
otherwise herein.

      3. The term "Agreement" as used in the Rights Agreement shall mean the
Rights Agreement, as amended by this Amendment, or as it may from time to time
be amended in the future by one or more other written amendment or modification
agreements entered into pursuant to the applicable provisions of the Rights
Agreement.

      4. This Amendment shall be binding upon and shall inure to the benefit of
the parties hereto and their respective successors and assigns.

      5. This Amendment shall be governed by and construed and enforced in
accordance with the laws of the State of Delaware.

      6. Except as expressly herein amended, the terms and conditions of the
Rights Agreement shall remain in full force and effect.

      7. This Amendment is not intended to be, nor shall it be construed to be,
a novation.

                                     - 2 -

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IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly
executed as of the date first above written.

                           CTI MOLECULAR IMAGING, INC.

                           By: /s/ Ronald Nutt
                              ----------------------------------------
                           Name: Ronald Nutt
                           Title: President & Chief Executive Officer

                           SUNTRUST BANK

                           By: /s/ Sue Hampton
                              ----------------------------------------
                           Name: Sue Hampton
                           Title: Vice President

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                                                                    EXHIBIT 10.1

                                                               EXECUTION VERSION

                         STOCKHOLDERS SUPPORT AGREEMENT

            STOCKHOLDERS SUPPORT AGREEMENT, dated as of March 18, 2005 (this
"Stockholders Agreement"), among SIEMENS MEDICAL SOLUTIONS USA, INC., a Delaware
corporation ("Parent"), MI MERGER CO., a Delaware corporation and a wholly owned
subsidiary of Parent ("Purchaser"), and each of the stockholders whose names
appear on the signature pages of this Stockholders Agreement (each, a
"Stockholder" and, collectively, the "Stockholders").

            WHEREAS, as of the date hereof, except as set forth on Exhibit A,
each Stockholder owns of record and beneficially and has good, valid and
marketable title to, free and clear of any Lien, proxy, voting restriction,
limitation on disposition, adverse claim of ownership or use or encumbrance of
any kind, other than pursuant to this Stockholders Agreement, and has the sole
power to vote and full right, power and authority to sell, transfer and deliver,
the number of shares of common stock, par value $0.01 per share ("Company Common
Stock"), of CTI Molecular Imaging, Inc., a Delaware corporation (the "Company"),
as set forth opposite such Stockholder's name on Exhibit A hereto (all such
shares of Company Common Stock and any shares of Company Common Stock of which
ownership of record or the power to vote is hereafter acquired by any of the
Stockholders prior to the termination of this Stockholders Agreement being
referred to herein as the "Shares"); and

            WHEREAS, Parent, Purchaser and the Company propose to enter into,
simultaneously herewith, an Agreement and Plan of Merger (the "Merger
Agreement"; except as otherwise noted herein, terms used but not defined in this
Stockholders Agreement shall have the meanings ascribed to them in the Merger
Agreement), a draft of which has been made available to each Stockholder, which
provides, upon the terms and subject to the conditions thereof, for the merger
of Purchaser with and into the Company (the "Merger") following the consummation
of a cash tender offer by Purchaser to acquire all the issued and outstanding
shares of Company Common Stock (the "Offer").

            NOW, THEREFORE, in consideration of the foregoing and the mutual
covenants and agreements contained herein and in the Merger Agreement, and
intending to be legally bound hereby, the Stockholders hereby agree as follows:

            1. Tender of Shares. Each Stockholder hereby agrees that such
Stockholder (a) shall tender, or cause to be tendered, in the Offer, as promptly
as practicable, but in any event no later than five business days after the date
of commencement of the Offer, all of his or its Shares pursuant to the terms of
the Offer and (b) shall neither withdraw, nor cause to be withdrawn, such
Shares. Terry Douglass shall use reasonable best efforts to cause all Shares
pledged as collateral by Revocable Charitable Trust of Terry and Rosann Douglass
under any security arrangement to be tendered in the Offer.

            2. Grant of Proxy. Each Stockholder, by this Stockholders Agreement,
with respect to his or its Shares, hereby grants an irrevocable proxy (to the
extent that the Law of the

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                                       2

state of Delaware may permit) until the termination of this Agreement to Parent
(and agrees to execute such documents or certificates evidencing such proxy as
Parent may reasonably request) to vote, at any meeting of the stockholders of
the Company, and in any action by written consent of the stockholders of the
Company, all of such Stockholder's Shares (a) in favor of the approval and
adoption of the Merger Agreement and approval of the Merger and all other
transactions contemplated by the Merger Agreement and this Stockholders
Agreement, (b) against any action, agreement or transaction (other than the
Merger Agreement or the transactions contemplated thereby) or proposal
(including any Competing Transaction) that would to his or its knowledge result
in a material breach of any covenant, representation or warranty or any other
obligation or agreement of the Company under the Merger Agreement or that could
result in any of the conditions to the Company's obligations under the Merger
Agreement not being fulfilled, and (c) in favor of any other matter necessary to
the consummation of the transactions contemplated by the Merger Agreement and
considered and voted upon by the stockholders of the Company. Each Stockholder
further agrees to vote such Stockholder's Shares in accordance with the
foregoing. THIS PROXY IS IRREVOCABLE AND COUPLED WITH AN INTEREST. Each
Stockholder hereby acknowledges receipt and review of a copy of the Merger
Agreement.

            3. Transfer of Shares. Each Stockholder agrees that he or it shall
not, directly or indirectly, (a) sell, assign, transfer (including by operation
of law), lien, pledge, dispose of or otherwise encumber any of the Shares or
otherwise agree to do any of the foregoing, (b) deposit any Shares into a voting
trust or enter into a voting agreement or arrangement or grant any proxy or
power of attorney with respect thereto that is inconsistent with this
Stockholders Agreement, (c) enter into any contract, option or other arrangement
or undertaking with respect to the direct or indirect acquisition or sale,
assignment, transfer (including by operation of law) or other disposition of any
Shares or (d) except as may be required by applicable Law or permitted pursuant
to Section 10 hereof, take any action that would make any representation or
warranty of such Stockholder herein untrue or incorrect in any material respect
or have the effect of preventing or disabling the Stockholder from performing
his or its obligations hereunder. Notwithstanding the foregoing, a Stockholder
that is an individual may, at any time prior to the Effective Time, transfer any
or all of his Shares (or any interest therein) to another Stockholder or to one
or more members of the Stockholder's family, any trust for the benefit of the
Stockholder's family, any entity controlled by the Stockholder or his family or
a charitable or other not-for-profit entity or organization (including, without
limitation, the ProVision Foundation and Terry and Rosann Douglass Foundation),
so long as the transferee of such Shares agrees in writing to be bound by the
applicable provisions of this Stockholders Agreement.

            4. No Solicitation of Transactions. Each Stockholder agrees that he
or it shall not, directly or indirectly, through any agent or otherwise, (a)
solicit, initiate or encourage (including by way of furnishing nonpublic
information), or take any other action to facilitate, any inquiries or the
making of any proposal or offer (including, without limitation, any proposal or
offer to any other stockholders of the Company) that constitutes, or may
reasonably be expected to lead to, any Competing Transaction, or (b) enter into
or maintain or continue discussions or negotiations with any person in
furtherance of such inquiries or to obtain a proposal or offer for a Competing
Transaction, or (c) agree to, approve, endorse or recommend any Competing
Transaction or enter into any letter of intent or other contract, agreement or
commitment contemplating or otherwise relating to any Competing Transaction;
provided, however, that

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nothing in this Section 4 shall prevent any Stockholder, in his capacity as a
director or executive officer of the Company from engaging in any activity
permitted pursuant to Section 6.05(b) or Section 6.05(c) of the Merger
Agreement. Each Stockholder shall, and shall direct his or its representatives
and agents to, immediately cease and terminate all existing discussions or
negotiations with any parties conducted heretofore with respect to a Competing
Transaction.

            5. Information for Offer Documents and Proxy Statement; Disclosure.
Each Stockholder represents and warrants to Parent and Purchaser that none of
the information relating to such Stockholder and his or its affiliates provided
in writing by, or, with the express prior authorization of such Stockholder (or
his or its affiliates, as applicable), on behalf of, such Stockholder or his or
its affiliates for inclusion in the Schedule TO, Schedule 14D-9, Offer
Documents, or Proxy Statement will, at the respective times the Schedule TO,
Schedule 14D-9, Offer Documents, or Proxy Statement are filed with the SEC or
are first published, sent or given to stockholders of the Company, contain any
untrue statement of material fact or omit to state any material fact required to
be stated therein or necessary in order to make the statements therein, in light
of the circumstances under which they were made, not misleading. Each
Stockholder authorizes and agrees to permit Parent and Purchaser to publish and
disclose in the Offer Documents and the Proxy Statement and related filings
under the securities laws such Stockholder's identity and ownership of Shares
and the nature of his or its commitments, arrangements and understandings under
this Agreement and any other information required by applicable Law.

            6. Termination. This Stockholders Agreement shall terminate upon the
earliest of (i) the Effective Time or (ii) the termination of the Merger
Agreement pursuant to Section 8.01 of the Merger Agreement; provided, however,
that Section 7 of this Stockholders Agreement shall survive the Effective Time
pursuant to its terms. Nothing in this Section 6 shall relieve any party of
liability for any breach of this Stockholders Agreement prior to the termination
of this Stockholders Agreement.

            7. Non-Competition/Non-Solicitation Agreement. (a) During the period
beginning at the Effective Time and ending on the third anniversary thereof,
except with the consent of Parent, no Stockholder shall, and shall not permit
any of his or its agents on his or its behalf (collectively, the "Stockholder
Parties") to, directly or indirectly, as principal for their own account or
jointly with others, or as an equity holder in any person, engage in, distribute
products for, or own, manage, operate, or control, or participate in the
ownership, management or control of, or have (or acquire) an interest in or
assets of, any person, enterprise or business that directly engages in:

            (i) the design, development, manufacture, assembly, distribution or
      sale of any (A) positron emission tomographs, including non-human animal
      positron emission tomographs, (either standalone or in combination with
      any other modalities, such as computed tomography, magnetic resonance, or
      near-infrared imaging or designed for dual modality imaging
      simultaneously) or components (including software, tools, procedures and
      protocols) for the assembly of, incorporation into or use with any
      positron emission tomographs (either standalone or in combination with any
      other modalities, such as computed tomography, magnetic resonance, or
      near-infrared imaging or designed for dual modality imaging
      simultaneously), including, without limitation, probes for the

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      detection of positron emitters, ring tomographs (emission computerized
      axial tomography image devices) and other positron imaging devices; (B)
      positron emission tomography biomarkers, tracers, compounds, precursor
      agents or other positron emission tomography radiopharmaceuticals or (C)
      cyclotrons, cyclotron targets, chemistry boxes, microfluidic chips or
      related tools and equipment used for the production of positron emission
      tomography radiopharmaceuticals and radionuclides that can be used for
      positron emission tomography;

            (ii) the design, development, manufacture, assembly, distribution or
      sale of detector materials for use in the items described in clause (i)(A)
      above;

            (iii) the design, development, manufacture, assembly, distribution
      or sale of radioactive sources used for calibration and quality control
      and/or attenuation correction of positron emission tomographs;

            (iv) the design, development, manufacture, assembly, distribution or
      sale of computer workstations and software for the creation, display and
      analysis of molecular images;

            (v) internet hosting services specifically designed to connect
      physicians, radiopharmacies, patients and positron emission tomography
      providers; or

            (vi) the provision of technical services, training services,
      maintenance and repair services, site planning and installation services,
      radioactive materials licensing, and application development services with
      respect to any of the foregoing (all of the businesses referred to in
      clauses (i) through (vi), inclusive, the "Restricted Business");

            provided, however, that the foregoing shall not prohibit any
Stockholder Party (i) from being the passive owner of not more than 5% of any
class of capital stock of any person which is publicly traded on a national
securities exchange or in the over-the-counter market in the United States or on
a foreign securities exchange, (ii) from engaging in any activities set forth on
Exhibit B hereto under the heading "Permitted Activities" (which, for avoidance
of doubt, shall not be deemed a Restricted Business hereunder) or (iii) from
being an employee, director, advisor or consultant for, any business division or
unit of an entity which engages in the Restricted Business provided that the
business division or unit does not engage in the Restricted Business and does
not require Stockholder to do so.

            (b) Each of the Stockholders agrees that for a period of three years
from the Effective Time, he or it shall not, and shall use his or its reasonable
best efforts to cause his or its respective Stockholder Parties on his or its
behalf not to, in any manner, directly or indirectly, knowingly (i) induce or
attempt to induce any individual that has been an employee of the Company at any
time between October 1, 2004 and the Effective Time, to leave the employ of the
Company or to violate the terms of his or her contract, or any employment
arrangements, with the Company, (ii) except in response to a good faith request
by a person that has not been an employee with the Company during the
twelve-month period preceding such request for a recommendation regarding the
employment qualifications of such employee, recommend to any other person that
they employ any such employee (provided, for avoidance of doubt, that there

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shall be no restriction on the ability to make recommendations for employment to
a Company not engaging in Restricted Business), (iii) interfere with or attempt
to interfere with any officers, employees, representatives or agents of the
Company in a manner adverse to the Company, or (iv) hire any such employee,
unless such employee has not been an employee of the Company at least for a
period of twelve consecutive months. Notwithstanding the foregoing, nothing in
this Section 7(b) shall prohibit any Stockholder or Stockholder Party from
employing or engaging as a consultant an individual (i) with the consent of
Parent or (ii) who responded to general solicitations in publications or on
websites, or through the use of search firms, so long as such general
solicitations or search firm activities are not targeted specifically at an
employee of the Company, Parent or Purchaser or any of their respective
subsidiaries.

            (c) Each of the Stockholders agrees that for a period of three years
from the Effective Time, such Stockholder shall not, and shall use his or its
reasonable best efforts to cause his or its respective Stockholder Parties on
his or its behalf not to, in any manner, directly or indirectly, knowingly (i)
solicit the business (pertaining to the Restricted Business) of any person known
by such Stockholder to be a customer of the Company as of the Effective Time or
seek to induce any such person to reduce or refrain from doing business with any
of Parent, Purchaser, the Company or any of their respective affiliates, or (ii)
disparage Parent, Purchaser, the Company or any of their respective affiliates
or any of their respective products or activities in the Restricted Business.
Parent and Purchaser shall not (and shall use reasonable best efforts to cause
the executive officers and directors of Parent, Purchaser, the Company and their
respective subsidiaries not to) make any derogatory or disparaging statement
regarding any Stockholder or any Stockholder Party. Nothing in this Section 7(c)
will prohibit the making of any truthful statements made by any person in
response to a lawful subpoena or legal proceeding or to enforce such person's
rights under this Stockholders' Agreement and the other documents relating to
the transactions contemplated by the Merger Agreement.

            (d) Each Stockholder hereby acknowledges that this Section 7
constitutes an independent covenant and shall not be affected by performance or
nonperformance of any other provision of this Stockholders Agreement by any
party hereto. Each Stockholder hereby acknowledges that the covenants of each
Stockholder set forth in this Section 7 are an essential element of this
Stockholders Agreement and an inducement for Parent and Purchaser to enter the
Merger Agreement and that, but for the agreement of each Stockholder to comply
with these covenants, neither Parent nor Purchaser would have entered into the
Merger Agreement. Each party hereto agrees that money damages would be an
inadequate remedy for any breach of this Section 7. Therefore, in the event of a
breach or threatened breach of this Section 7, Parent, Purchaser and/or any
Surviving Corporation may, in addition to other rights and remedies existing in
its favor, apply to any court of competent jurisdiction for specific performance
and/or injunctive or other relief in order to enforce, or prevent any violations
of, the provisions thereof. Each Stockholder hereby acknowledges that such
Stockholder has independently consulted with counsel and after such consultation
agrees that the covenants set forth in this Section 7 are intended to be
reasonable and proper in scope, duration and geographical area and in all other
respects. If any such covenant is found to be invalid, void or unenforceable in
any situation in any jurisdiction by a final determination of a court or any
other Governmental Authority of competent jurisdiction, each Stockholder, Parent
and Purchaser agree that: (i) such determination shall not affect the validity
or enforceability of (A) the offending term or provision in any other situation
or in any other jurisdiction or (B) the remaining terms and provisions of this
Section 7

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in any situation in any jurisdiction; (ii) the offending term or provision shall
be reformed rather than voided and the court or Governmental Authority making
such determination shall have the power to reduce the scope, duration or
geographical area of any invalid or unenforceable term or provision, to delete
specific words or phrases, or to replace any invalid or enforceable term or
provision with a term or provision that is valid and enforceable and that comes
closest to expressing the intention of the invalid or unenforceable provision,
in order to render the restrictive covenants set forth in this Section 7
enforceable to the fullest extent permitted by applicable Law; and (iii) the
restrictive covenants set forth in this Section 7 shall be enforceable as so
modified.

            (e) The Stockholders' shall have the right to review and consent to
the Parent, Purchaser and/or Company's allocation to the above covenants of the
consideration to be received by each Stockholder pursuant to the Offer and/or
Merger Agreement for tax reporting purposes.

            8. Representations and Warranties of Stockholder. Each Stockholder
hereby severally represents and warrants to Parent and Purchaser, except as set
forth on Exhibit A, as follows:

            (a) (i) Each Stockholder that is an individual has full legal right
and capacity to execute and deliver this Stockholder Agreement, to perform his
obligations hereunder and to consummate the transactions contemplated hereby,
and (ii) each Stockholder that is not an individual is duly organized, or
formed, as applicable, validly existing and in good standing under the Laws of
its jurisdiction of organization or formation, as applicable, and has all
necessary power and authority to execute and deliver this Stockholders
Agreement, to perform its obligations hereunder and to consummate the
transactions contemplated hereby. This Agreement has been duly executed and
delivered by each Stockholder and, assuming the due authorization, execution and
delivery by Parent and Purchaser, constitutes the legal, valid and binding
obligation of such Stockholder, enforceable against such Stockholder in
accordance with its terms, subject to the effect of any applicable bankruptcy,
insolvency (including, without limitation, all Laws relating to fraudulent
transfers), reorganization, moratorium or similar Laws affecting creditors'
rights generally and subject to the effect of general principles of equity
(regardless of whether considered in a proceeding at law or in equity) and
applicable public policy.

            (b) The execution and delivery of this Stockholder Agreement by each
Stockholder do not, and the performance of this Stockholder Agreement by such
Stockholder will not (i) conflict with or violate any organizational or
formation document of any Stockholder, (ii) assuming that all consents,
approvals, authorizations and other actions described in Section 8(d) have been
obtained and all filings and obligations described in Section 8(d) have been
made, conflict with or violate any order, judgment or decree applicable to any
Stockholder or by which any property or asset of any Stockholder is bound or
affected, (iii) result in any breach of or constitute a default (or an event
which, with notice or lapse of time or both, would become a default) under, or
give to others any right of termination, amendment, acceleration or cancellation
of, any contract or other agreement to which Stockholder is a party.

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            (c) Each Stockholder owns of record and beneficially and has good,
valid and marketable title to, free and clear of any Lien, proxy, voting
restriction, limitation on disposition, adverse claim of ownership or use or
encumbrance of any kind, other than pursuant to this Stockholder Agreement, and
has the sole power to vote and full right, power and authority to sell, transfer
and deliver, the Shares (as set forth on Exhibit A).

            (d) Each Stockholder has had access, prior to the execution of this
Stockholder Agreement, to the information he or it felt he or it needed and
desired in connection with his or its evaluation of this Stockholder Agreement
and the Merger Agreement and the transactions contemplated hereby and thereby
and has had, prior to the execution of this Stockholder Agreement, the
opportunity to ask questions of, and receive answers from, the Company and legal
counsel concerning the terms and conditions of the transactions contemplated by
this Stockholder Agreement and the Merger Agreement and to obtain additional
information necessary to assess the transactions contemplated hereby and
thereby.

            9. Representations and Warranties of Stockholder's Spouse. The
spouse of Terry D. Douglass, Ph.D. (the "Spouse") hereby represents and warrants
to Parent and Purchaser as follows: (a) the Spouse shall not assert or enforce,
and does hereby waive, to the extent permitted by applicable Law, any rights
granted under any community property statute with respect to the Shares held by
Terry D. Douglass, Ph.D. that would adversely affect the covenants made by Terry
D. Douglass, Ph.D. pursuant to this Stockholders Agreement or the sale and
transfer of such Shares to Parent or Purchaser pursuant to the terms of this
Stockholders Agreement; provided, however, that the Spouse shall not be
prohibited from asserting any rights the Spouse may have against the
consideration received by Terry D. Douglass, Ph.D. in exchange for such Shares;
(b) the Spouse hereby acknowledges receipt and review of a copy of the Merger
Agreement and this Stockholders Agreement.

            10. Rights as Director or Officer. Nothing herein will be deemed to
prohibit or restrict any Stockholder that is an individual from taking or
omitting to take any action that the Stockholder determines in good faith he is
required to take or prohibited from taking as an officer or director of the
Company or under applicable Law, the intention of the parties herein being only
to impose obligations on the Stockholders in their capacities as stockholders of
the Company.

            11. Miscellaneous. Except as otherwise provided herein, all costs
and expenses incurred in connection with this Stockholders Agreement and the
transactions contemplated hereby shall be paid by the party incurring such costs
and expenses, whether or not the transactions contemplated hereby are
consummated; all notices, requests, claims, demands and other communications
hereunder shall be in writing and shall be given (and shall be deemed to have
been duly given upon receipt) by delivery in person, by telecopy or e-mail or by
registered or certified mail (postage prepaid, return receipt requested) to the
respective parties at their addresses as specified on the signature page(s) of
this Stockholders Agreement; if any term or other provision of this Stockholders
Agreement is invalid, illegal or incapable of being enforced by any rule of law
or public policy, all other conditions and provisions of this Stockholders
Agreement shall nevertheless remain in full force and effect so long as the
economic or legal substance of the transactions contemplated hereby is not
affected in any manner materially adverse to any party; this Stockholders
Agreement constitutes the entire agreement among the

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parties with respect to the subject matter hereof and supersedes all prior
agreements and undertakings, both written and oral, among the parties, or any of
them, with respect to the subject matter hereof; this Stockholders Agreement
shall not be assigned (whether pursuant to a merger, by operation of law or
otherwise), except that Parent may assign all or any of its rights and
obligations hereunder to any affiliate of Parent and Parent may assign all or
any of its rights under Section 7 of this Stockholders Agreement to any person
who purchases any business unit or all or substantially all of the assets of the
Company or the Subsidiaries; provided that no such assignment shall (i) be
permitted (without the applicable Stockholder's consent) to the extent the
effect of the assignment would be to increase the scope of, or otherwise
adversely effect, any Stockholder's obligations under Section 7 or (ii) relieve
Parent of its obligations hereunder if such assignee does not perform such
obligations; this Stockholders Agreement shall be binding upon and inure solely
to the benefit of each party hereto, and nothing in this Stockholders Agreement,
express or implied, is intended to or shall confer upon any other person any
right, benefit or remedy of any nature whatsoever under or by reason of this
Stockholders Agreement; the parties hereto agree that irreparable damage would
occur in the event any provision of this Stockholders Agreement was not
performed in accordance with the terms hereof and that the parties shall be
entitled to specific performance of the terms hereof (except where performance
is excused pursuant to the terms hereof), in addition to any other remedy at law
or in equity; this Stockholders Agreement shall be governed by, and construed in
accordance with, the Laws of the State of Delaware applicable to contracts
executed in and to be performed in that State; this Stockholders Agreement may
be executed and delivered (including by facsimile transmission) in one or more
counterparts, and by the different parties hereto in separate counterparts, each
of which when executed shall be deemed to be an original but all of which taken
together shall constitute one and the same agreement; from time to time, at the
request of Parent, in the case of any Stockholder, or at the request of the
Stockholders, in the case of Parent and Purchaser, and without further
consideration, each party shall execute and deliver or cause to be executed and
delivered such additional documents and instruments and take all such further
action as may be reasonably necessary or desirable to consummate the
transactions contemplated by this Stockholders Agreement; this Stockholders
Agreement may be amended only if such amendment is in writing and duly executed
or delivered by each of the parties hereto; EACH OF THE PARTIES HERETO HEREBY
WAIVES TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW ANY RIGHT IT MAY HAVE
TO A TRIAL BY JURY WITH RESPECT TO ANY LITIGATION DIRECTLY OR INDIRECTLY ARISING
OUT OF, UNDER OR IN CONNECTION WITH THIS STOCKHOLDERS AGREEMENT OR THE
TRANSACTIONS CONTEMPLATED HEREBY.

Douglass Stockholder Agreement

<PAGE>

            IN WITNESS WHEREOF, the parties have executed, or have caused to be
executed, this Stockholders Agreement as of the date first written above.

                                 SIEMENS MEDICAL SOLUTIONS USA, INC.

                                 /s/ Dr. Hermann Requardt
                                 ------------------------------------------
                                 Name: Dr. Hermann Requardt
                                 Title: Executive Vice President
                                 Address: Heukestr. 127
                                         D-91050 Erlangen

                                 /s/ James R. Ruger
                                 ------------------------------------------
                                 Name: James R. Ruger
                                 Title: Corporate Secretary
                                 Address: 51 Valley Stream Pky.
                                         Malvern, PA 19355

                                 MI MERGER CO.

                                 /s/ Dr. Hermann Requardt
                                 ------------------------------------------
                                 Name: Dr. Hermann Requardt
                                 Title: Executive Vice President
                                 Address: Heukestr. 127
                                         D-91050 Erlangen

                                 /s/ James R. Ruger
                                 ------------------------------------------
                                 Name: James R. Ruger
                                 Title: Corporate Secretary
                                 Address: 51 Valley Stream Pky.
                                         Malvern, PA 19355

<PAGE>

                                 TERRY D. DOUGLASS, PH.D.

                                 /s/ Terry D. Douglass
                                 ------------------------------------------
                                 Address:

                                 REVOCABLE CHARITABLE TRUST OF
                                 TERRY AND ROSANN DOUGLASS
                                 By: Terry D. Douglass, Trustee

                                 /s/ Terry D. Douglass
                                 ------------------------------------------
                                 Address:

                                 VISION INVESTMENTS, LLC
                                 By: Terry D. Douglass, Ph.D., its managing
                                 member

                                 /s/ Terry D. Douglass
                                 ------------------------------------------
                                 Name: Terry D. Douglass, Ph.D.
                                 Address:

                                 PROVISION FOUNDATION
                                 By: Anne Sale, its Chief Financial Officer

                                 /s/ Anne Sale
                                 ------------------------------------------
                                 Address:

                                 ROSANN B. DOUGLASS

                                 /s/ Rosann B. Douglass
                                 ------------------------------------------
                                 Address:

<PAGE>

                                    EXHIBIT A

                              LIST OF STOCKHOLDERS

                                               Number of Shares of Company
                                                   Common Stock Owned
           Name of Stockholder                         of Record**

          Terry D. Douglass, Ph.D.                      883,082

Revocable Charitable Trust of Terry and
          Rosann Douglass                               932,010*

         Vision Investments, LLC                      4,950,000
          Pro Vision Foundation                         157,328

* Currently pledged as collateral to secure a personal loan of Terry Douglass.
Mr. Douglass intends to seek the release of such security interest promptly.

** Does not include 59,846 options held by Mr. Douglass or 5,000 deferred stock
units for which Mr. Douglass does not currently hold direct ownership of or the
right to vote or control the disposition

<PAGE>

                                                                       EXHIBIT B

PERMITTED ACTIVITIES:

  -   Owning, managing, operating, or controlling, or participating in the
      ownership, management or control of, or having (or acquiring) an interest
      in or assets of, any person, enterprise or business that (i) provides
      imaging services (including, without limitation PET/CT imaging services
      and services ancillary to imaging services (including, without limitation,
      start-up, networking and software services, and educational and training
      services)), and the lease, purchase or other utilization by any such
      provider of any related equipment, products (including
      radiopharmaceuticals), software or other services (collectively, the
      "Related Equipment") whether or not the supplier, lessor or other provider
      of such Related Equipment is engaged in the Restricted Business (the
      "Provider Business"), or (ii) engages in the Restricted Business (a) if
      the activities of such enterprise which constitute the Restricted Business
      collectively account for less than 10% of the revenues of the entire
      enterprise, and (b) the Stockholder's relationship to such enterprise does
      not involve participation in such activities.

  -   Except in connection with the sale of any business unit or all or
      substantially all of the assets of the Company, any portion of the
      Restricted Business with respect to which the Provider Business contracts
      services to the extent that the Company actually and intentionally ceases
      to engage in such portion of the Restricted Business at any time after the
      Effective Date with the intent to exit the related business.

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