<PAGE>

                                                                   EXHIBIT 10.18

                                                               EXECUTION VERSION

                      FIRST AMENDMENT TO CREDIT AGREEMENT
                      -----------------------------------

     THIS FIRST AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), dated as of
                                                     ---------
December 16, 1999 is entered into by and among:

          (1) ACUSON CORPORATION, a Delaware corporation ("Borrower");
                                                           --------

          (2) Each of the financial institutions from time to time listed in

     Schedule I to the Credit Agreement referred to in Recital A below
     ----------------------------------                ---------
     (collectively, the "Lenders"); and
                         -------

          (3) ABN AMRO BANK N.V., acting through its San Francisco
     Representative Office, as agent for the Lenders (in such capacity,

     "Agent").
      -----

                                    RECITALS
                                    --------

     A.  Borrower, the Lenders and Agent are parties to that certain Credit
Agreement dated as of April 9, 1999 (the "Credit Agreement").
                                          ----------------

     B.  Borrower has requested that the Lenders and Agent amend the Credit
Agreement in certain respects.

     C.  The Lenders and Agent are so willing to amend the Credit Agreement upon
the terms and subject to the conditions set forth below.

                                   AGREEMENT
                                   ---------

     NOW, THEREFORE, in consideration of the above recitals and for other good
and valuable consideration, the receipt and adequacy of which are hereby
acknowledged, Borrower, the Lenders and Agent hereby agree as follows:

     1.  Definitions, Interpretation.  All capitalized terms defined above and
         ---------------------------
elsewhere in this Amendment shall be used herein as so defined.  Unless
otherwise defined herein, all other capitalized terms used herein shall have the
respective meanings given to those terms in the Credit Agreement, as amended by
this Amendment.  The rules of construction set forth in Section I of the Credit
                                                        -----------------------
Agreement shall, to the extent not inconsistent with the terms of this
---------
Amendment, apply to this Amendment and are hereby incorporated by reference.

     2.  Amendments to Credit Agreement.  Subject to the satisfaction of the
         ------------------------------
conditions set forth in Paragraph 4 below, the Credit Agreement is hereby
                        -----------
amended as follows:

          (a) Paragraph 1.01 is hereby amended by adding thereto, in the
              --------------
     appropriate alphabetical order, the definitions of the terms "Ecton
     Acquisition" and "Ecton Acquisition Charge" to read in their entirety as
     follows:

               "Ecton Acquisition" shall mean the proposed acquisition by
                -----------------
          Borrower of Ecton, Inc., a Pennsylvania corporation, in accordance
          with that certain Plan of
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          Merger and Reorganization, dated as of September 15, 1999, pursuant to
          which the parties thereto will effect a tax-free reorganization within
          the meaning of Section 368(a) of the IRC.

               "Ecton Acquisition Charge" shall mean the non-recurring charge,
                ------------------------
          not to exceed $20,000,000 (pre-tax) in the aggregate, taken by
          Borrower through Borrower's fiscal year 2002 as a result of write-offs
          of in process research and development expenses and other non-cash
          charges incurred in connection with the consummation of the Ecton
          Acquisition.

          (b) Paragraph 1.01 is hereby further amended by changing the
              --------------
     definition of the term "EBIRT" set forth therein to read in its entirety as
     follows:

               "EBIRT" shall mean, with respect to Borrower for any period, the
                -----
          sum, determined on a consolidated basis in accordance with GAAP, of
          the following:

                    (a)  The net income or net loss of Borrower and its
               Subsidiaries for such period before provision for income taxes;

                                      plus
                                      ----

                    (b)  The sum (to the extent deducted in calculating net
               income or loss in clause (a) above and without duplication) of
                                 ----------
               (i) all Interest Expenses of Borrower and its Subsidiaries
               accruing during such period and (ii) all operating lease and
               "synthetic" lease payments of Borrower and its Subsidiaries
               accruing during such period;

                                      plus
                                      ----

                    (c)  To the extent deducted in calculating such net income
               or net loss for such period under clause (a) above, an amount
                                                 ----------
               equal to the pre-tax sum of any Ecton Acquisition Charge taken by
               Borrower during such period.

          (c) Paragraph 1.01 is hereby further amended by changing the
              --------------
     definition of the term "EBITDA" set forth therein to read in its entirety
     as follows:

               "EBITDA" shall mean, with respect to Borrower for any period, the
          sum, determined on a consolidated basis in accordance with GAAP, of
          the following:

                    (a)  The net income or net loss of Borrower and its
               Subsidiaries for such period before provision for income taxes;

                                      plus
                                      ----

                    (b)  The sum (to the extent deducted in calculating net
               income or loss in clause (a) above) of (i) all Interest Expenses
                                 ----------
               of Borrower and its Subsidiaries accruing during such period and
               (ii) all depreciation and

                                       2
<PAGE>

               amortization expenses of Borrower and its Subsidiaries accruing
               during such period.

                                      plus
                                      ----

                    (c)  To the extent deducted in calculating such net income
               or net loss for such period under clause (a) above, an amount
                                                 ----------
               equal to the pre-tax sum of any Ecton Acquisition Charge taken by
               Borrower during such period.

     3.  Representations and Warranties.  Borrower hereby represents and
         ------------------------------
warrants to Agent and the Lenders that the following are true and correct on the
date of this Amendment and that, after giving effect to the amendments set forth
in Paragraph 2 above, the following will be true and correct on the Effective
   -----------
Date (as defined below):

          (a) The representations and warranties of Borrower and its
     Subsidiaries set forth in Paragraph 4.01 of the Credit Agreement and in the
                               --------------------------------------
     other Credit Documents are true and correct in all material respects
     (except with respect to representations and warranties that speak as of a
     particular date, which shall be true and correct as of such date);

          (b) No Default or Event of Default has occurred and is continuing; and

          (c) Each of the Credit Documents is in full force and effect.

(Without limiting the scope of the term "Credit Documents," Borrower expressly
acknowledges in making the representations and warranties set forth in this
Paragraph 3 that, on and after the date hereof, such term includes this
-----------
Amendment.)

     4.  Effective Date.  The amendments set forth in Paragraph 2 above, shall
         --------------                               -----------
become effective on December 16, 1999 (the "Effective Date"), subject to receipt
                                            --------------
by Agent and the Lenders on or prior to the Effective Date of the following,
each in form and substance satisfactory to Agent, the Lenders and their
respective counsel:

          (a) This Amendment duly executed by Borrower, the Lenders and Agent;

          (b) A Certificate of the Secretary of Borrower in the form attached
     hereto as Attachment A, dated the Effective Date, certifying (i) that the
               ------------
     Certificate of Incorporation, Bylaws and resolutions of Borrower, in the
     form delivered to Agent on the Closing Date, are in full force and effect
     and have not been amended, supplemented, revoked or repealed since such
     date, and (ii) the incumbency, signatures and authority of the officers of
     Borrower authorized to execute, deliver and perform this Amendment, and all
     other documents, instruments or agreements related thereto executed or to
     be executed by Borrower; and

          (c) Such other evidence as Agent or any Lender may reasonably request
     to establish the accuracy and completeness of the representations and
     warranties and the

                                       3
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     compliance with the terms and conditions contained in this Amendment and
     the other Credit Documents.

     5.  Effect of this Amendment.  On and after the Effective Date, each
         ------------------------
reference in the Credit Agreement and the other Credit Documents to the Credit
Agreement shall mean the Credit Agreement as amended hereby.  Except as
specifically amended above, (a) the Credit Agreement and the other Credit
Documents shall remain in full force and effect and are hereby ratified and
confirmed and (b) the execution, delivery and effectiveness of this Amendment
shall not, except as expressly provided herein, operate as a waiver of any
right, power, or remedy of the Lenders or Agent, nor constitute a waiver of any
provision of the Credit Agreement or any other Credit Document.

     6.  Miscellaneous.
         -------------

         (a) Counterparts.  This Amendment may be executed in any number of
             ------------
     identical counterparts, any set of which signed by all the parties hereto
     shall be deemed to constitute a complete, executed original for all
     purposes.

         (b) Headings.  Headings in this Amendment are for convenience of
             --------
     reference only and are not part of the substance hereof.

         (c) Governing Law.  This Amendment shall be governed by and construed
             -------------
     in accordance with the laws of the State of California without reference to
     conflicts of law rules.

                            [signature page follows]

                                       4
<PAGE>

     IN WITNESS WHEREOF, Borrower, Agent and the Lenders have caused this
Amendment to be executed as of the day and year first above written.

BORROWER:                    ACUSON CORPORATION, a Delaware
                             corporation

                             By:
                                -----------------------------------------
                                  Name:
                                          -------------------------------
                                  Title:
                                          -------------------------------


AGENT:                       ABN AMRO BANK N.V., as Agent

                             By:
                                -----------------------------------------
                                  Name:
                                          -------------------------------
                                  Title:
                                          -------------------------------

LENDERS:                      ABN AMRO BANK N.V., as a Lender

                              By:
                                -----------------------------------------
                                  Name:
                                          -------------------------------
                                  Title:
                                          -------------------------------

                              BANQUE NATIONALE DE PARIS, as a Lender

                              By:
                                -----------------------------------------
                                  Name:
                                          -------------------------------
                                  Title:
                                          -------------------------------

                                       5
<PAGE>

                                  ATTACHMENT A
                                  ------------
                        FORM OF SECRETARY'S CERTIFICATE]
                        -------------------------------

                               ACUSON CORPORATION
                            SECRETARY'S CERTIFICATE

     Reference is made to that certain Credit Agreement, dated as of April 9,
1999 (the "Credit Agreement"), by and among Acuson Corporation, a Delaware
           ----------------
corporation (the "Company"), each of the financial institutions listed in
                  -------
Schedule I to the Credit Agreement (collectively, the "Lenders"), and ABN AMRO
----------------------------------                     -------
Bank N.V., acting through its San Francisco Representative Office, as agent for
the Lenders (in such capacity, "Agent").  This Certificate is delivered pursuant
                                -----
to Subparagraph 4(b) of the First Amendment with the intention that the Lenders
   -----------------
and Agent shall rely hereon in connection with the execution and delivery of the
First Amendment.  Capitalized terms used herein without definition shall have
the meanings given to such terms in the First Amendment.

     I, Charles Dearborn, do hereby certify that I am the duly elected,
qualified and acting Secretary of the Company, and that, as such, I am
authorized to execute this Certificate on behalf of the Company, and I further
certify that:

          (a) The following persons have been duly elected or appointed to, are
duly qualified for, and on the date hereof do hold, the offices of the Company
set forth opposite their respective names below, and as such, are authorized to
execute and deliver on behalf of the Company the First Amendment, and all other
documents, instruments, or agreements related thereto executed or to be executed
by the Company, and that the signatures appearing below, opposite the name of
such persons are the authentic signature or facsimile thereof, respectively, of
such persons:

<TABLE>
<CAPTION>
Name                         Office                         Signature
----                         ------                         ---------
<S>                           <C>                            <C>

____________________          ____________________           ______________________________

____________________          ____________________           ______________________________

____________________          ____________________           ______________________________

____________________          ____________________           ______________________________
</TABLE>

                                       6
<PAGE>

          (b)    Each of the Company's (i) Certificate of Incorporation, (ii)
Bylaws and (iii) resolutions authorizing the transactions contemplated pursuant
to the Credit Agreement and the other Credit Documents, each in the form
delivered to Agent on the Closing Date, are in full force and effect and have
not been amended, supplemented, revoked or repealed since such date.

     IN WITNESS WHEREOF, I have executed this Certificate on and as of this 16th
day of December, 1999.


                              -----------------------------------
                              Name:  Charles Dearborn
                              Title:  Secretary

                                       7