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                                                                   EXHIBIT 10.27

                      STOCK PURCHASE AND EXCHANGE AGREEMENT

         This STOCK PURCHASE AND EXCHANGE AGREEMENT (this "Agreement") is made
and entered into effective as of the date set forth hereinbelow, by and between
the undersigned shareholder (the "Shareholder") of CTI Group, Inc., a Tennessee
corporation ("Group"), SIEMENS GAMMASONICS, INC., a Delaware Corporation
("Siemens"), and CTI, INC., a Tennessee corporation ("CTI").

                                   WITNESSETH:

         WHEREAS, Shareholder is the owner and holder of certain shares of
Common Stock of Group, as set forth on Exhibit A hereto; and

         WHEREAS, Group, CTI and Siemens have entered into a Stock Purchase,
Reorganization and Joint Venture Agreement dated December 10, 1987 (the
"Reorganization Agreement") pursuant to the terms of which Siemens has agreed to
purchase a sufficient number of the shares of Common Stock of Group issued and
outstanding as of the Closing (as defined in the Reorganization Agreement) to
give Siemens, when combined with the shares of Common stock of Group held by
Siemens Affiliate Siemens Capital Corporation, forty-nine and nine-tenths
percent (49.9%) of the then issued Common Stock of Group, and pursuant to which
the remaining outstanding shares of Group (other than those held by Siemens and
its affiliates) will be exchanged for Common Stock of CTI;

         NOW, THEREFORE, in consideration of the premises and the mutual terms,
covenants and conditions set forth herein, Siemens, CTI and Shareholder agree as
follows:

         1. Purchase and Exchange Agreement

             1.1 Purchase and Sale of Group Common Stock. Subject to the terms
and conditions of this Agreement, Shareholder agrees to sell to Siemens, and
Siemens agrees to purchase from Shareholder the number of shares of Common Stock
of Group indicated on Exhibit A hereto for a total aggregate purchase price in
the amount indicated on the said Exhibit A; provided, however, that Siemens
shall not be obligated hereunder until and unless Siemens is able to acquire
from all shareholders of Group a sufficient number of shares of the Common Stock
of Group issued and outstanding as of the Closing to give Siemens, when combined
with the Common Stock of Group held by Siemens Affiliate Siemens Capital
Corporation forty-nine and nine-tenths percent (49.9%) of the then issued and
outstanding Common Stock of Group.

             1.2 Exchange of Group Common Stock. Subject to the terms and
conditions of this Agreement, Shareholder agrees to transfer and assign to CTI
his remaining shares of Common Stock of Group, in the amount indicated on
Exhibit A hereto in exchange for a like number of the shares of Common Stock of
CTI.


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         2. Closing and Required Documents

             2.1 Closing. Subject to the conditions set forth herein; the
Closing of the transactions contemplated by this Agreement shall occur in
conjunction with the closing contemplated by the Reorganization Agreement, which
is expected to occur on or about January 5, 1988 or as soon thereafter as
necessary approvals for the transactions contemplated by the Reorganization
Agreement have been obtained. The Closing of the Purchase and Exchange is herein
referred to as the "Closing" and the date of Closing is herein referred to as
the "Closing Date."

             2.2 Required Documents. Shareholder shall prior to the Closing Date
deliver to Group certificates evidencing all of the shares of Common Stock owned
by Shareholder, along with a duly executed stock power authorizing the transfer
of all shares of Common Stock of Group owned by Shareholder as contemplated
hereby.

         3. Obligations and Delivery

             3.1 Obligations of CTI and Siemens. The obligations of Siemens to
purchase the shares of Common Stock of Group in accordance with the terms hereof
and the obligation of CTI to exchange shares of Common Stock of CTI for shares
of Common Stock of Group in accordance with the terms hereof shall be subject to
and expressly conditioned upon fulfillment of the conditions recited in Section
1.1 above and each of the following conditions on or prior to the Closing Date:

                 a. Approval of and compliance with all terms of the
Reorganization Agreement by all of the parties thereto.

                 b. All representations and warranties of Shareholder contained
herein shall be true and correct in all material respects at and as of the
Closing with the same force and effect as though made at and as of such time.

                 c. Shareholder shall have performed and complied with all of
the obligations, terms, conditions and agreements required to be performed or
complied with by it on or prior to the Closing.

             3.2 Delivery of CTI Certificates and Purchase Price. CTI shall
deliver to Shareholder a certificate or certificates in proper form for the
number of shares of Common Stock of CTI to be obtained by Shareholder hereby.
Siemens shall deliver to Shareholder the purchase price provided for
hereinabove, in cash or cash equivalent.

         4. Representations, Warranties and Acknowledgments of Shareholder. In
connection with the purchase and exchange, Shareholder makes the following
representations and warranties:

             4.1 Due Execution. Shareholder has full power and authority to
execute, deliver and perform this Agreement and the obligations hereunder; and
this Agreement (a) has been duly and validly executed and delivered, (b) is a
valid and legally



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binding agreement of Shareholder, and (c) is enforceable against Shareholder in
accordance with its terms.

             4.2 Ownership of the Stock. Shareholder has good and marketable
title to the Common Stock of Group purchased and exchanged hereunder, free and
clear of all claims, liens, encumbrances, charges, assessments or other rights
or interests of any person. There are no restrictions with respect to the
transferability of the Common Stock of Group to either CTI or Siemens in
accordance with the terms of this Agreement, and upon transfer of the Common
Stock of Group, the holder thereof will receive good and marketable title to the
Group Common Stock, free and clear of all claims, liens, encumbrances, charges,
assessments and restrictions or other rights or interest of other persons
whatsoever.

             4.3 No Obligation to Dispose of Share. Shareholder has no
contemplated agreement, undertaking, arrangement, obligation, indebtedness or
commitment providing for or which is likely to compel a disposition of the
shares of the Common Stock of CTI.

             4.4 Stock Unregistered. Shareholder understands that the shares of
the Common Stock of CTI being offered for exchange are not registered within the
meaning of the Securities Act of 1933 (the "Securities Act") and the Tennessee
Securities Act of 1980 (the "Tennessee Act") and that as a consequence of them
being unregistered, the shares of the Common Stock of CTI will be restricted
securities as that term is defined by various rules and regulations promulgated
pursuant to the Securities Act, and therefore cannot be sold, transferred,
pledged or otherwise disposed of without being registered or by qualifying for
an exemption from the registration requirements of the Securities Act and the
Tennessee Act, and further, that all certificates representing shares of the
Common Stock of Group issued pursuant to this Agreement will be required to bear
a restrictive legend in the form as follows:

                                     NOTICE

                  THE SHARES OF COMMON STOCK REPRESENTED BY THIS CERTIFICATE
                  HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
                  AMENDED (THE "ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS,
                  BUT HAVE BEEN ACQUIRED FOR THE PRIVATE INVESTMENT OF THE
                  HOLDER HEREOF AND MAY NOT BE OFFERED, SOLD OR TRANSFERRED
                  UNTIL (i) A REGISTRATION STATEMENT UNDER THE ACT AND SUCH
                  STATE SECURITIES LAWS AS ARE APPLICABLE SHALL HAVE BECOME
                  EFFECTIVE WITH REGARD THERETO, OR (ii) IN THE OPINION OF
                  COUNSEL ACCEPTABLE TO CTI, INC., REGISTRATION UNDER THE ACT
                  AND SUCH STATE SECURITIES LAWS AS ARE APPLICABLE IS NOT
                  REQUIRED IN CONNECTION WITH SUCH PROPOSED OFFER, SALE OR
                  TRANSFER.



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             4.5 No Registration Rights. Shareholder further understands that
there are no registration rights associated with the shares of the Common Stock
of CTI being acquired pursuant to this Agreement.

             4.6 Information Availability. Shareholder acknowledges that he has
had access to information respecting' the reorganization of Group, including a
summary of the overall transaction, and have been given the opportunity and has
been encouraged to ask any questions and receive additional information that
they consider relevant in making a decision to accept this Agreement.

         5. Survival of Representation and Warranties. The representations of
Shareholder contained herein shall survive the Closing contemplated hereby and
any investigation by CTI or Siemens with respect thereto.

         6. Indemnification of CTI and Siemens. Shareholder shall defend,
indemnify and hold harmless CTI and Siemens from and against any damage,
deficiency, loss or liability resulting from any misrepresentation, omission,
breach of warranty or nonfulfillment of any part of this Agreement by
Shareholder, and any and all actions, suits, proceedings, demands, assessments,
judgments, costs and expenses incident thereto.

         7. Failure of Conditions Precedent. To the extent that any condition
precedent to the obligations of CTI and Siemens hereunder is not met by
Shareholder, CTI and Siemens may, at their option, either waive such condition
and complete the transactions contemplated hereby on the terms and conditions
otherwise contained herein, or decline to purchase or exchange the shares,
whereupon this Agreement will terminate and CTI and Siemens shall be subject to
no liability whatsoever to Shareholder. If the transactions contemplated this
Agreement is not consummated for such reason, or for any other reason, neither
party shall be liable to the other for loss, damage, or expense, and the only
remedy to either party shall be to terminate this Agreement.

         8. Miscellaneous.

             8.1 Binding Effect. The terms, covenants and conditions hereof
shall be binding and shall inure to the benefit of the parties hereto and their
respective heirs, successors, and administrators.

             8.2 No Assignment. This Agreement shall not be assignable.

             8.3 Agreement to Cooperate. The parties hereto agree to execute
such other and additional instruments, documents and resolutions, and to take
such other and additional actions as may be necessary or advisable to carry out
the transactions described herein.

             8.4. Governing Law. This Agreement shall be construed in accordance
with and governed by the laws of the State of Tennessee.

             8.5 Entire Agreement and Amendment. This Agreement constitutes the
entire understanding between the parties with respect to the subject matter


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hereof. Any modification or amendment to this Agreement may be made only by an
instrument in writing executed by the party against whom enforcement thereof is
sought.

             8.6 Captions. The headings contained herein are for reference
purposes only and do not affect in any way the-meaning or interpretation hereof.

             8.7 Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

             8.8 No Default. If any provision of this Agreement is held to be
illegal, invalid or unenforceable for any reason, such provision shall be
severable, and this Agreement shall be construed and enforced as if such
illegal, invalid or unenforceable provision had never comprised a part hereof,
and the remaining provisions hereof shall remain in full force and effect and
shall not be affected by the illegal, invalid or unenforceable provision or by
its severance herefrom.

         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
23rd day of December, 1987.



                                           CTI, INC.

                                           By: /s/ Terry D. Douglass
                                              ----------------------------------
Attest: /s/ Mary Lou Marks                 Title: President
      ----------------------------              --------------------------------
                                           Date: December 23, 1987
                                                 -------------------------------



                                           SIEMENS GAMMASONICS, INC.

                                           By:
                                              ----------------------------------
Attest:                                    Title:
       ---------------------------               -------------------------------
                                           Date:
                                                --------------------------------


                                           SHAREHOLDER


/s/ Julia A. Turner                        /s/ Ronald Nutt
----------------------------------         -------------------------------------
Witness
                                           Date: 12/21/87
                                                 -------------------------------


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               EXHIBIT A TO STOCK PURCHASE AND EXCHANGE AGREEMENT

Shareholder:      Ronald Nutt
            --------------------------------------------------------------------

Total number of shares of Group owned by Shareholder:              726,125
                                                          ----------------
Number of shares of Group to be sold to Siemens:                   326,487
                                                          ----------------
Purchase price for shares to be sold to Siemens:                $3,571,424
                                                          ----------------
Number of shares of Group to be exchanged for CTI
shares:                                                            399,638
                                                          ----------------

If the number of shares of Group owned by Shareholder and to be sold to Siemens
is greater than 1000, the number of shares sold to Siemens, the purchase price
and the number of shares to be exchanged for CTI shares may be adjusted on a pro
rata basis with other shareholders similarly situated to account for any
shareholders declining to participate in the sale of Siemens.