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                                                                   EXHIBIT 10.55

                             SECOND MODIFICATION OF
                      AMENDED AND RESTATED CREDIT AGREEMENT
                               AND LOAN DOCUMENTS

         THIS SECOND MODIFICATION OF AMENDED AND RESTATED CREDIT AGREEMENT AND
LOAN DOCUMENTS (the "Second Modification") is made and entered as of the 31st
day of May, 2002, by and among CTI MOLECULAR IMAGING, INC., a Delaware
corporation (the "New Borrower"); all Subsidiaries of CTI, Inc., as the
Borrower, that are parties to the Credit Agreement as defined herein
(collectively the "Guarantors"); and SUNTRUST BANK, a Georgia state bank
("SunTrust"), as administrative agent (in such capacity the "Administrative
Agent") for the benefit of SunTrust; Bank of America, N.A., a national banking
association ("Bank of America"); Fifth Third Bank, an Ohio state bank ("Fifth
Third"); and National City Bank of Kentucky, a national banking association
("National City"), as Lenders (collectively the "Lenders"); and SunTrust as
Issuing Bank.

                                    RECITALS:

         Pursuant to the terms of an Amended and Restated Credit Agreement,
dated as of March 14, 2002, among CTI, Inc. ("CTI") as the Borrower, the
Guarantors, the Lenders and SunTrust as Administrative Agent and Issuing Bank
(the same may be amended, restated, supplemented, extended, renewed, replaced or
otherwise modified from time to time, the "Credit Agreement"; except as
otherwise defined herein, terms used herein and defined in the Credit Agreement
shall be used herein as so defined), the Lenders have extended Loans to CTI and
have agreed to purchase participations in Letters of Credit issued for the
account of CTI, and the Issuing Bank will issue such Letters of Credit, all as
more specifically described in the Credit Agreement. The Loans are evidenced by:
(a) a Revolving Credit Promissory Note in the principal amount of $40,000,000,
executed by CTI and made payable to SunTrust (the "SunTrust Note"), (b) a
Revolving Credit Promissory Note in the principal amount of $20,000,000,
executed by CTI and made payable to Bank of America (the "Bank of America
Note"), (c) a Revolving Credit Promissory Note in the principal amount of
$15,000,000, executed by CTI and made payable to National City (the "National
City Note"), and (d) a Revolving Credit Promissory Note in the principal amount
of $15,000,000, executed by CTI and made payable to Fifth Third (the "Fifth
Third Note").

         SunTrust has also extended to CTI a Swingline Loan in the amount of
$10,000,000, pursuant to the terms of the Credit Agreement, and evidenced by a
Swingline Note in the principal amount of $10,000,000 (the "Swingline Note").

         The SunTrust Note, the Bank of America Note, the National City Note,
the Fifth Third Note, the Swingline Note, and the Letters of Credit are secured
by:

                  (a)      certain assets of CTI and the Subsidiaries, pledged
                           pursuant to the terms of a Security Agreement, dated
                           as of March 14, 2002 by and among CTI,


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                           certain Subsidiaries, and SunTrust as Administrative
                           Agent (the "Security Agreement"), with such assets
                           including but not limited to certain Pledged Notes,
                           Pledged Stock, and Ownership Interests, and with such
                           assets being perfected by UCC-1 financing statement,
                           numbered 102011485, and UCC-3 financing statements,
                           numbered 102011481, 102011482, 102011483 and
                           102011484, filed with the Tennessee Secretary of
                           State (the "Financing Statements");

                  (b)      a Third Modification of Deed of Trust ("ACT Deed of
                           Trust Modification"), by and between Advanced Crystal
                           Technology, Inc. (ACT, Inc.) ("ACT") and SunTrust as
                           Administrative Agent, encumbering certain property in
                           Blount County, Tennessee and with such ACT Deed of
                           Trust Modification being of record in Book 199, Page
                           899, in the Register's Office for Blount County,
                           Tennessee; and

                  (c)      a Patent and License Security Agreement, dated as of
                           March 14, 2002, in which CTI pledges to SunTrust as
                           Administrative Agent a security interest in certain
                           patents and licenses of the Borrower (the "Patent
                           Security Agreement").

         Upon CTI's request, the Lenders, SunTrust as Administrative Agent, the
Issuing Bank and CTI entered into a First Modification of Amended and Restated
Credit Agreement, dated as of April 4, 2002 (the "First Modification"),
modifying the terms of the Credit Agreement to provide for certain liabilities
of CTI associated with a private placement agreement between CTI and First Union
Capital Partners.

         The Credit Agreement, the SunTrust Note, the Bank of America Note, the
National City Note, the Fifth Third Note, the Swingline Note, the Letters of
Credit, the Security Agreement, the ACT Deed of Trust Modification, the Patent
Security Agreement and the First Modification, along with all other documents
related to the extension of the Loans and the issuance of the Letters of Credit,
are hereby collectively referred to as the "Loan Documents."

         On April 5, 2002, CTI filed a Form S-1 Registration Statement with the
Securities and Exchange Commission in connection with a proposed initial public
offering of common stock of CTI (the "IPO"). In connection with the IPO, CTI has
determined that it is in CTI's best interest to enter into a merger with the New
Borrower, with the New Borrower being the surviving Person of such merger.
Therefore, CTI has requested that the Administrative Agent, the Issuing Bank and
the Lenders consent to the merger of CTI with the New Borrower, with the New
Borrower being the surviving Person of such merger; and the Administrative
Agent, the Issuing Bank and the Lenders have agreed to consent to the merger
between CTI and the New Borrower, upon the condition that the New Borrower and
the Guarantors execute this Second Modification.

         NOW, THEREFORE, in consideration of the foregoing, the Loans extended
pursuant to the terms of the Credit Agreement, the agreements contained herein,
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties agree as follows:


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         1.       Merger of CTI and New Borrower; Substitution of the New
Borrower as Borrower. In connection with the IPO, CTI and the New Borrower have
entered into a merger with the New Borrower being the surviving Person of such
merger. The parties agree that, effective as of the date of such merger, the New
Borrower is substituted for CTI as the Borrower under the Loan Documents as
fully as if the New Borrower had executed the Loan Documents as the Borrower.
The New Borrower hereby assumes all of the Obligations of the Borrower under the
Loan Documents; hereby promises to pay to the order of the Lenders all amounts
due under the SunTrust Note, the Bank of America Note, the National City Note,
the Fifth Third Note, the Swingline Note and the Letters of Credit; and hereby
promises and agrees to perform all obligations, covenants, and agreements of the
Borrower under the Loan Documents as fully as if the New Borrower had executed
the same as the Borrower.

         2.       Financing Statement. The New Borrower hereby authorizes the
Administrative Agent to prepare and file a new UCC-1 financing statement in the
name of the New Borrower, to be filed in the State of Delaware to perfect all of
the interests of the Administrative Agent and Lenders in and to the collateral
of the New Borrower as described in the Security Agreement.

         3.       Patent Security Agreement. The New Borrower hereby agrees to
execute a modification of the Patent Security Agreement to reflect the merger of
CTI and the New Borrower, with the New Borrower being the surviving Person of
such merger.

         4.       Organizational Documents. The New Borrower hereby agrees to
deliver to the Administrative Agent, simultaneously with the execution of this
Second Modification: (a) a copy of the corporate charter of the New Borrower,
certified by the Delaware Secretary of State, (b) a copy of the by-laws of the
New Borrower, and (c) a good standing certificate from the Delaware Secretary of
State.

         5.       Costs and Expenses. The New Borrower hereby agrees to
reimburse the Administrative Agent, on demand, for all reasonable fees and
expenses relating to this Second Modification and the terms hereof, including
but not limited to filing or recording fees, reasonable attorneys' fees, and any
other costs or expenses necessary to modify the Loan Documents and to perfect
the security interests thereunder.

         6.       Representations, Warranties and Covenants. The New Borrower
hereby confirms that: (a) each of the representations and warranties of CTI
contained in the Credit Agreement and any of the other Loan Documents are true
and correct in all material respects as of the date hereof; and (b) the New
Borrower is in compliance with each of the covenants of CTI contained in the
Credit Agreement and any of the other Loan Documents as of the date hereof.

         7.       Ratification and Other Terms. The New Borrower hereby ratifies
and confirms all of the terms and conditions of the Credit Agreement and all of
the Loan Documents as fully as if it had executed the same. All terms and
conditions of the Credit Agreement and all Loan Documents shall remain unaltered
and in full force and effect, unless otherwise modified in this Second
Modification.


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         8.       Guarantors Consent. The Guarantors join in this Second
Modification to consent to the merger of the New Borrower with CTI, with the New
Borrower being the surviving Person of such merger, and to consent to the
substitution of the New Borrower for CTI as the Borrower under the Loan
Documents.

         9.       Chief Executive Office. The chief executive office of the New
Borrower shall be located at 810 Innovation Drive, Knoxville, Tennessee 37922.

         10.      Additional Documents. The New Borrower and the Guarantors
agree to execute such additional documents, instruments and agreements as are
reasonably required for the New Borrower to assume all of the Obligations of CTI
under the Loan Documents and to preserve the security interests of the
Administrative Agent, the Lenders and the Issuing Bank in and to the Collateral.

         11.      Binding Effect; Governing Law. This Second Modification shall
be binding upon and inure to the benefit of the New Borrower, the Guarantors,
the Administrative Agent, the Issuing Bank, and the Lenders, and their
respective successors, heirs and assigns. This Second Modification shall be
governed by and construed in accordance with the laws of the State of Tennessee.

         12.      Severability. In the event that any clause or provision of
this Second Modification shall be held to be invalid by any court of competent
jurisdiction, the invalidity of such clause or provision shall not affect any of
the remaining provisions of this Second Modification.

         13.      Counterparts. This Second Modification may be executed in
counterparts and when executed by all of the parties shall constitute one and
the same Second Modification.

         IN WITNESS WHEREOF, the parties have caused this Second Modification to
be executed as of the date first above written.



                                       NEW BORROWER/BORROWER:



                                       CTI MOLECULAR IMAGING, INC.
                                       a Delaware corporation



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


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                                       GUARANTORS:



                                       CTI PET SYSTEMS, INC.,
                                       a Tennessee corporation



                                       By:       /s/ Michael Templin
                                          --------------------------------------
                                       Name:     Michael Templin
                                            ------------------------------------
                                       Title:    CFO
                                             -----------------------------------



                                       CTI SERVICES, INC.,
                                       a Tennessee corporation



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



                                       P.E.T.NET PHARMACEUTICALS, INC.,
                                       a Tennessee corporation



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



                                       ADVANCED CRYSTAL TECHNOLOGY, INC.
                                       (ACT, INC.), a Tennessee corporation



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


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                                       CTI GMBH, a German corporation



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



                                       ADMINISTRATIVE AGENT:


                                       SUNTRUST BANK, as Administrative
                                       Agent



                                       By:         /s/ Scott C. Folsom
                                          --------------------------------------
                                       Name:       Scott C. Folsom
                                            ------------------------------------
                                       Title:      Group Vice President
                                             -----------------------------------


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