EXHIBIT 10.4
                           MASTER REPURCHASE AGREEMENT

             AGREEMENT dated as of the 27th day of September, 1995
        between Thermo Electron Corporation, a Delaware corporation
        ("Seller"), and Trex Medical Corporation, a Delaware corporation
        (the "Buyer").

        1.   Applicability

             From time to time Buyer and Seller may enter into
        transactions in which Seller agrees to transfer to Buyer certain
        securities and/or financial instruments ("Securities") against
        the transfer of funds by Buyer, with a simultaneous agreement by
        Buyer to transfer to Seller such Securities on demand, against
        the transfer of funds by Seller.  Each such transaction shall be
        referred to herein as a "Transaction" and shall be governed by
        this Agreement, unless otherwise agreed in writing.

        2.   Definitions

             (a)  "Act of Insolvency", with respect to either party (i)
        the commencement by such party as debtor of any case or
        proceeding under any bankruptcy, insolvency, reorganization,
        liquidation, dissolution or similar law, or such party seeking
        the appointment of a receiver, trustee, custodian or similar
        official for such party or any substantial part of its property;
        or (ii) the commencement of any such case or proceeding against
        such party, or another seeking such an appointment, which (A) is
        consented to or not timely contested by such party, (B) results
        in the entry of an order for relief, such an appointment or the
        entry of an order having a similar effect, or (C) is not
        dismissed within 15 days; or (iii) the making by a party of a
        general assignment for the benefit of creditors; or (iv) the
        admission in writing by a party of such party's inability to pay
        such party's debts as they become due; 

             (b)  "Additional Purchased Securities", Securities provided
        by Seller to Buyer pursuant to Paragraph 4(a) hereof; 

             (c)  "Income", with respect to any Security at any time, any
        principal thereof then payable and all interest, dividends or
        other distributions thereon; 

             (d)  "Market Value", with respect to any Securities as of
        any date, the price for such Securities on such date obtained
        from a generally recognized source agreed to by the parties or
        the most recent closing bid quotation from such a source, plus
        accrued Income to the extent not included therein (other than any
        Income transferred to Seller pursuant to Paragraph 6 hereof) as
        of such date (unless contrary to market practice for such
        Securities);
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             (e)  "Other Buyers", third parties that have entered into an
        agreement with Seller that is  substantially similar to this
        Agreement; 

             (f)  "Pricing Rate", a rate equal to the Commercial Paper
        Composite rate for 90-day maturities provided by Merrill Lynch,
        Pierce, Fenner & Smith Incorporated (or, if such rate is not
        available, a substantially equivalent rate agreed to by Buyer and
        Seller) plus 25 basis points,  which rate shall be adjusted on
        the first business day of each fiscal quarter and shall be in
        effect for the entirety such fiscal quarter;
         
             (g)  "Purchase Price", the price at which Purchased
        Securities are transferred by Seller to Buyer; 

             (h)  "Purchased Securities", the Securities transferred by
        Seller to Buyer in a Transaction hereunder, and any Securities
        substituted therefor in accordance with Paragraph 9 hereof.  The
        term "Purchased Securities" with respect to any Transaction at
        any time also shall include Additional Purchase Securities
        transferred pursuant to Paragraph 4(a) and shall exclude
        Securities returned pursuant to Paragraph 4(b);  

             (i)  "Repurchase Collateral Account", a book account
        maintained by Seller containing, among other Securities, the
        Purchased Securities; and

             (j)  "Repurchase Price", for any Purchased Security, an
        amount equal to the Purchase Price paid by Buyer to Seller for
        such Purchased Security. 

        3.   Transactions

             (a)  A Transaction may be initiated by Buyer upon the
        transfer of the Purchase Price to Seller's account.  Upon such
        transfer, Seller shall transfer to Buyer Purchased Securities
        having a Market Value equal to 103% of the Purchase Price.

             (b)  Purchased Securities shall be held in custody for Buyer
        by Seller in the Repurchase Collateral Account.  Seller shall
        indicate on its books for such account Buyer's ownership of the
        Purchased Securities.  Upon reasonable request from Buyer, Seller
        shall provide Buyer with a complete list of Purchased Securities
        owned by Buyer.  

             (c)  Upon demand by Buyer or Seller, Seller shall repurchase
        from Buyer, and Buyer shall sell to Seller,  for the Repurchase
        Price all or any part of the Purchased Securities then owned by
        Buyer.

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        4.   Margin Maintenance

             (a)  If at any time the aggregate Market Value of all
        Purchased Securities then owned by Buyer is less than 103% of the
        aggregate Repurchase Price for such Purchased Securities, then
        Seller shall transfer to Buyer additional Securities ("Additional
        Purchased Securities"), so that the aggregate Market Value of
        such Purchased Securities, including any such Additional
        Purchased Securities, will thereupon equal or exceed  103% of
        such aggregate Repurchase Price.

             (b)  If at any time the aggregate Market Value of all
        Purchased Securities then owned by Buyer exceeds 103% of the
        aggregate Repurchase Price for such Purchased Securities, then
        Seller may transfer Purchased Securities to Seller, so that the
        aggregate Market Value of such Purchased Securities will
        thereupon not exceed 103% of such aggregate Repurchase Price.

        5.   Interest Payments

             If during any fiscal month Buyer owned Purchased Securities,
        then on the first day of the next following fiscal month Seller
        shall pay to Buyer an amount equal to the sum of the aggregate
        Repurchase Prices of the Purchased Securities owned by Buyer at
        the close of each day during the preceding fiscal month divided
        by the number of days in such month and the product multiplied by
        the Pricing Rate times the number of days in such month divided
        by 360.

        6.   Income Payments and Voting Rights

             Where a particular Transaction's term extends over an Income
        payment date on the Purchased Securities subject to that
        Transaction, Buyer shall, on the date such Income is payable,
        transfer to Seller an amount equal to such Income payment or
        payments with respect to any Purchased Securities subject to such
        Transaction.  Seller shall retain all voting rights with respect
        to Purchased Securities sold to Buyer under this Agreement.

        7.   Security Interest

             Although the parties intend that all Transactions hereunder
        be sales and purchases and not loans, in the event any such
        Transactions are deemed to be loans, Seller shall be deemed to
        have pledged to Buyer as security for the performance by Seller
        of its obligations under each such Transaction and this
        Agreement, and shall be deemed to have granted to Buyer a
        security interest in, all of the Purchased Securities with
        respect to all Transactions hereunder and all proceeds thereof.

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        8.   Payment and Transfer

             Unless otherwise mutually agreed, all transfers of funds
        hereunder shall be in immediately available funds.  As used
        herein with respect to Securities, "transfer" is intended to have
        the same meaning as when used in Section 8-313 of the
        Massachusetts Uniform Commercial Code or, where applicable, in
        any federal regulation governing transfers of the Securities.

        9.   Substitution

             Buyer hereby grants Seller the authority to manage, in
        Seller's sole discretion, the Purchased Securities held in
        custody for Buyer by Seller in the Repurchase Collateral Account.
        Buyer expressly agrees that Seller may (i) substitute other
        Securities for any Purchased Securities and (ii) commingle
        Purchased Securities with other Securities held in the Repurchase
        Collateral Account.  Substitutions shall be made by transfer to
        Buyer of such other Securities and transfer to Seller of the
        Purchased Securities for which substitution is being made.  After
        substitution, the substituted Securities shall be deemed to be
        Purchased Securities.  Securities which are substituted for
        Purchased Securities shall have a Market Value at the time of
        substitution equal to or greater than the Market Value of the
        Purchase Securities for which such Securities were substituted.

        10.  Representations

             Each of Buyer and Seller represents and warrants to the
        other that (i) it is duly authorized to execute and deliver this
        Agreement, to enter into the Transactions contemplated hereunder
        and to perform its obligations hereunder and has taken all
        necessary action to authorize such execution, delivery and
        performance, (ii) the person signing this Agreement on its behalf
        is duly authorized to do so on its behalf, (iii) it has obtained
        all authorizations of any governmental body required in
        connection with this Agreement and the Transactions hereunder and
        such authorizations are in full force and effect and (iv) the
        execution, delivery and performance of this Agreement and the
        Transactions hereunder will not violate any law, ordinance,
        charter, by-law or rule applicable to it or any agreement by
        which it is bound or by which any of its assets are affected.  On
        the date for any Transaction Buyer and Seller shall each be
        deemed to repeat all the foregoing representations made by it.

        11.  Events of Default

             In the event that (i) Seller fails to repurchase or Buyer
        fails to transfer Purchased Securities upon demand for repurchase
        from either Buyer or Seller, (ii)  Seller or Buyer fails, after
        one business day's notice, to comply with Paragraph 4 hereof,
        (iii) Buyer  fails to make payment to Seller pursuant to  
        Paragraph 6 hereof, (iv) Seller fails to comply with Paragraph 5
        hereof,  (v) an Act of Insolvency occurs with respect to Seller

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        or Buyer, (vi) any representation made by Seller or Buyer shall
        have been incorrect or untrue in any material respect when made
        or repeated or deemed to have been made or repeated, or (vii)
        Seller or Buyer shall admit to the other its inability to, or its
        intention not to, perform any of its obligations hereunder (each
        an "Event of Default"):

             (a)  At the option of the nondefaulting party, exercised by
        written notice to the defaulting party (which option shall be
        deemed to have been exercised, even if no notice is given,
        immediately upon the occurrence of any Act of Insolvency), Seller
        shall become obligated to repurchase, and Buyer shall become
        obligated to sell, all Purchased Securities then owned by Buyer
        for the Repurchase Price of such Purchased Securities.

             (b)  If Seller is the defaulting party and Buyer exercises
        or is deemed to have exercised the option referred to in
        subparagraph (a) of this Paragraph, (i) the Seller's obligations
        hereunder to repurchase all Purchased Securities in such
        Transactions shall thereupon become immediately due and payable,
        (ii) all Income paid after such exercise or deemed exercise shall
        be retained by Buyer and applied to the aggregate unpaid
        Repurchase Prices owed by Seller, and (iii) Seller shall
        immediately deliver to Buyer any Purchased Securities subject to
        such Transactions then in Seller's possession.

             (c)  In all Transactions in which Buyer is the defaulting
        party, upon tender by Seller of payment of the aggregate
        Repurchase Prices for all such Transactions, Buyer's right, title
        and interest in all Purchased Securities subject to such
        Transactions shall be deemed transferred to Seller, and Buyer
        shall deliver all such Purchased Securities to Seller.

             (d)  After one business day's notice to the defaulting party
        (which notice need not be given if an Act of Insolvency shall
        have occurred, and which may be the notice given under
        subparagraph (a) of this Paragraph or the notice referred to in
        clause (ii) of the first sentence of this Paragraph), the
        nondefaulting  party may: 

                  (i)  as to Transactions in which Seller is the
        defaulting party, (A) immediately sell, in a recognized market at
        such price or prices as Buyer may reasonably deem satisfactory,
        any or all Purchased Securities subject to such Transactions and
        apply the proceeds thereof to the aggregate unpaid Repurchase
        Prices and any other amounts owing by Seller hereunder or (B) in
        its sole discretion elect, in lieu of selling all or a portion of
        such Purchased Securities, to give Seller credit for such
        Purchased Securities in an amount equal to the price therefor on
        such date, obtained from a generally recognized source or the
        most recent closing bid quotation from such a source, against the
        aggregate unpaid Repurchase Prices and any other amounts owing by
        Seller hereunder; and

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                  (ii)  as to Transactions in which Buyer is the
        defaulting party, (A) purchase securities ("Replacement
        Securities") of the same class and amount as any Purchased
        Securities that are not delivered by Buyer to Seller as required
        hereunder or (B) in its sole discretion elect, in lieu of
        purchasing Replacement Securities, to be deemed to have purchased
        Replacement Securities at the price therefor on such date,
        obtained from a generally recognized source or the most recent
        closing bid quotation from such a source.

             (e)  As to Transactions in which Buyer is the defaulting
        party , Buyer shall be liable to Seller (i) with respect to
        Purchased Securities (other than Additional Purchased
        Securities), for any excess of the price paid (or deemed paid) by
        Seller for Replacement Securities therefor over the Repurchase
        Price for such Purchased Securities and (ii) with respect to
        Additional Purchased Securities, for the price paid (or deemed
        paid) by Seller for the Replacement Securities therefor.  

             (f)  The defaulting party shall be liable to the
        nondefaulting party for the amount of all reasonable legal or
        other expenses incurred by the nondefaulting party in connection
        with or as a consequence of an Event of Default.

             (g)  The nondefaulting party shall have, in addition to its
        rights hereunder, any rights otherwise available to it under any
        other agreement or applicable law.

        12.  Single Agreement

             Buyer and Seller acknowledge that, and have entered hereinto
        and will enter into each Transaction hereunder in consideration
        of and in reliance upon the fact that, all Transactions hereunder
        constitute a single business and contractual relationship and
        have been made in consideration of each other.  Accordingly, each
        of Buyer and Seller agrees (i) to perform all of its obligations
        in respect of each Transaction hereunder, and that a default in
        the performance of any such obligations shall constitute a
        default by it in respect of all Transactions hereunder, (ii) that
        each of them shall be entitled to set off claims and apply
        property held by them in respect of any Transaction against
        obligations owing to them in respect of any other Transactions
        hereunder and (iii) that payments, deliveries and other transfers
         made by either of them in respect of any Transaction shall be
        deemed to have been made in consideration of payments, deliveries
        and other transfers in respect of any other Transactions
        hereunder, and the obligations to make any such payments,
        deliveries and other transfers may be applied against each other
        and netted.

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        13.  Entire Agreement; Severability

             This Agreement shall supersede any existing agreements
        between the parties containing general terms and conditions for
        repurchase transactions.  Each provision and agreement and
        agreement herein shall be treated as separate and independent
        from any other provision or agreement herein and shall be
        enforceable notwithstanding the unenforceability of any such
        other provision or agreement.

        14.  Non-assignability; Termination

             The rights and obligations of the parties under this
        Agreement and under any Transactions shall not be assigned by
        either party without the prior written consent of the other
        party.  Subject to the foregoing, this Agreement and any
        Transactions shall be binding upon and shall inure to the benefit
        of the parties and their respective successors and assigns.  This
        Agreement may be canceled by either party upon giving written
        notice to the other, except that this Agreement shall,
        notwithstanding such notice, remain applicable to any
        Transactions then outstanding.

        15.  Governing Law

             This Agreement shall be governed by the laws of the
        Commonwealth of Massachusetts without giving effect to the
        conflict of law principles thereof.

        16.  No Waivers, Etc.

             No express or implied waiver of any Event of Default by
        either party shall constitute a waiver of any other Event of
        Default and no exercise of any remedy hereunder by any party
        shall constitute a wavier of its right to exercise any other
        remedy hereunder.  No modification or waiver of any provision of
        this Agreement and no consent by any party to a departure
        herefrom shall be effective unless and until such shall be in
        writing and duly executed by both of the parties hereto. 

        19.  Intent

             (a)  The parties recognize that each Transaction is a
        "repurchase agreement" as that term is defined in Section 101 of
        Title 11 of the United States Code, as amended (except insofar as
        the type of Securities subject to such Transaction or the term of
        such Transaction would render such definition inapplicable), and
        a "securities contract" as that term is defined in Section 741 of
        Title 11 of the United States Code, as amended.

             (b)  It is understood that either party's right to liquidate
        Securities delivered to it in connection with Transactions
        hereunder or to exercise any other remedies pursuant to Paragraph
        11 hereof, is a contractual right to liquidate such Transaction

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        as described in Sections 555 and 559 of Title 11 of the United
        States Code, as amended.

             IN WITNESS WHEREOF, the parties have executed this Agreement
        as of the date first above written.

        THERMO ELECTRON CORPORATION        TREX MEDICAL CORPORATION

        By:  /s/ Melissa F. Riordan        By:  /s/ Hal Kirshner
             Melissa F. Riordan                 Hal Kirshner
             Treasurer                          President