<PAGE>   1
                                                                   EXHIBIT 10.53


                       THIRD AMENDMENT TO CREDIT AGREEMENT


         THIS THIRD AMENDMENT TO CREDIT AGREEMENT (this "AMENDMENT") is made and
entered into as of August 9, 2000, by and among RADIOLOGIX, INC., formerly known
as American Physician Partners, Inc., a Delaware corporation ("BORROWER"),
GENERAL ELECTRIC CAPITAL CORPORATION, a New York corporation, as Agent for
Lenders pursuant to the Credit Agreement referred to below (the "AGENT"), and
the other Lenders signatory hereto (each a "LENDER").

                               STATEMENT OF FACTS

         A. Borrower, the Lenders and the Agent are parties to that certain
Credit Agreement, dated November 26, 1997, as amended by the First Amendment to
Credit Agreement and Consent dated as of May 19, 1998, and the Second Amendment
to Credit Agreement and Consent dated as of August 3, 1999 (the "CREDIT
AGREEMENT"; capitalized terms used but not defined in this Amendment have the
meanings given in the Credit Agreement, as amended by this Amendment), whereby
the Lenders have made available certain revolving credit facilities to Borrower,
subject to the terms and conditions contained in the Credit Agreement.

         B. Borrower, the Agent, and the Lenders desire to amend the Credit
Agreement as set forth herein, subject to the terms and conditions contained in
this Amendment.

                               STATEMENT OF TERMS

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

         1. AMENDMENTS TO CREDIT AGREEMENT. Subject to the terms and conditions
of this Amendment, including without limitation Section 5 hereof, the Credit
Agreement is hereby amended as follows:

         (a) Section 1.3(b)(iv) is amended to read as follows:

             (iv) Commencing on March 31, 2001 (the "Amortization Commencement
             Date") and on the last business day of each Fiscal Quarter
             thereafter until the Commitment Termination Date, Borrower shall
             repay the Revolving Loan outstanding in quarterly installments
             equal to the lesser of: (x) the percentage of the aggregate
             principal amount of the Revolving Loan outstanding on the
             Amortization Commencement Date set forth opposite the period in
             which such fiscal quarter ends below and (y) the amount of
             Revolving Loan outstanding concurrent on the date of such
             installment:


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<PAGE>   2

<TABLE>
<CAPTION>
                   PERIOD                             PERCENTAGE
                   ------                             ----------
<S>                                                   <C>
               March 31, 2001                           7.50%
                  through
               March 30, 2003

               March 31, 2003                           10.0%
               and thereafter
</TABLE>

         (b) The following new Section 1.3(b)(vi) is added:

             (vi) If at any time the aggregate cash balance of all accounts of
             all of the Credit Parties exceeds $9,000,000, Borrower shall
             immediately repay the outstanding Revolving Credit Advances by the
             amount of such excess.

         (c) Section 1.5(a) is amended by replacing the second chart contained
therein with the following:

<TABLE>
<CAPTION>

                                                       APPLICABLE MARGINS
                                      ---------------------------------------------------
                                      LEVEL I   LEVEL II   LEVEL III   LEVEL IV   LEVEL V
                                      -------   --------   ---------   --------   -------
<S>                                   <C>       <C>        <C>         <C>        <C>
            Applicable Revolver        1.25%      1.50%       1.75%      2.00%      2.25%
            Index Margin

            Applicable Revolver        2.25%      2.50%       2.75%      3.00%      3.25%
            LIBOR Margin

            Applicable L/C Margin      2.25%      2.50%       2.75%      3.00%      3.25%
</TABLE>

         (d) Section 6.1(a) is amended by inserting, in the second sentence
thereof, immediately after the phrase "Notwithstanding the foregoing clause
(y)," the phrase "but subject to Section 6.1(c),"; and

         (e) Section 6.1(b)(iii) is amended by inserting, prior to subclause (A)
thereof, the phrase "Subject to Section 6.1(c)".

         (f) The following new Section 6.1(c) is added:


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<PAGE>   3

             Notwithstanding anything to the contrary in any portion of this
             Section 6.1 or any other provision of this Agreement, from and
             after the Third Amendment Effective Date, neither Borrower nor any
             Subsidiary shall, without the consent of all of the Lenders in
             their sole and absolute discretion, (i) form or acquire any
             Subsidiary, (ii) merge with, consolidate with, acquire all or
             substantially all of the assets or capital stock of, or otherwise
             combine with or acquire, any Person (except that any Restricted
             Subsidiary may merge or consolidate with any other Restricted
             Subsidiary), (iii) create or acquire Stock in any Permitted Joint
             Venture, or (iv) make any Permitted Acquisition.

         (g) The following new Section 6.19 is added:

             6.19 Location of Assets. The aggregate value, as determined by the
             Lenders, of all assets owned by any Credit Party which are located
             on any premises not operated by a Credit Party shall not exceed
             $2,000,000 at any time.

         (h) The following new Section 6.20 is added:

             6.20 Capital Expenditures. Aggregate Capital Expenditures of all
             Credit Parties other than Capital Expenditures which are directly
             financed with the proceeds of Indebtedness permitted under Section
             6.3(ix) shall not exceed $4,000,000 in any Fiscal Quarter.

         (i) Section 11.2(c) is amended by renumbering clause (vii) as clause
(viii), and by inserting the following new clause (vii): "(vii) amend or modify
Section 6.1;".

         (j) Annex A is amended to add the following new definitions in
alphabetical order:

             "Days Sales Outstanding" shall mean, as to any Person, days sales
             outstanding of such Person calculated in accordance with historical
             practices of Borrower as disclosed in writing to the Lenders on the
             Third Amendment Effective Date.

             "Security Agreements" shall mean the Security Agreement, dated as
             of August 9, 2000, executed by each Subsidiary of Borrower, other
             than a Permitted Joint Venture or Permitted Joint Venture Holding
             Company, in favor of the Agent on behalf of itself and the Lenders
             (the "Security Agreement"), and any other security agreement
             entered into from time to time by any Credit Party in favor of the
             Agent on behalf of itself and the Lenders.

             "Third Amendment Effective Date" shall mean August 9, 2000.


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<PAGE>   4

         (k) Annex A is further amended by replacing the definitions of
"Availability Termination Date", "Commitment Termination Date", and "Collateral
Documents" with the following:

             "Availability Termination Date" shall mean the earlier of: (i)
             March 31, 2001, or (ii) the Commitment Termination Date.

             "Commitment Termination Date" shall mean the earliest of (a)
             November 26, 2003, (b) the date of termination of Lenders'
             obligations to make Advances and/or incur Letter of Credit
             Obligations or permit existing Loans to remain outstanding pursuant
             to Section 8.2(b), and (c) the date of indefeasible prepayment in
             full by Borrower of the Loans and the cancellation and return of
             all Letters of Credit or the cash collateralization (or stand-by
             guarantee) of all Letter of Credit Obligations pursuant to Annex B
             and the permanent reduction of the Revolving Loan Commitment and
             the Swing Line Commitment to zero dollars ($0), in accordance with
             the provisions of Section 1.3(a).

             "Collateral Documents" shall mean the Pledge Agreements, the
             Guaranties, the Security Agreements, and all similar agreements
             entered into guaranteeing payment of, or granting a Lien upon
             property as security for payment of, the Obligations.

         (l) Annex E is amended by replacing, in subsection (a) thereof, the
date "September 30, 2000" with the date "March 31, 2001".

         (m) Annex E is further amended by adding the following new subsection
(f):

             (f) Maximum Days Sales Outstanding. Borrower and its Restricted
             Subsidiaries shall have Days Sales Outstanding, determined on an
             aggregate basis as of the last day of each Fiscal Quarter, of no
             greater than 82 days for such Fiscal Quarter.

         2. REPRESENTATIONS AND WARRANTIES. Borrower hereby represents and
warrants to the Agent and the Lenders that (a) this Amendment, the Confirmation
of Guaranty attached hereto, and the Security Agreement have been duly
authorized, executed and delivered by Borrower and each Credit Party signatory
thereto, (b) no Default or Event of Default has occurred and is continuing as of
this date, and (c) all of the representations and warranties made by Borrower or
any Credit Party in the Credit Agreement are true and correct in all material
respects on and as of the date of this Amendment (except to the extent that any
such representations or warranties expressly referred to a specific prior date
or have changed based upon events expressly permitted by the Credit Agreement).

         3. RATIFICATION. Borrower hereby ratifies and reaffirms each and every
term, covenant and condition set forth in the Credit Agreement and all other
documents delivered by


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<PAGE>   5

Borrower in connection therewith (including without limitation the other Loan
Documents to which Borrower is a party), effective as of the date hereof.

         4. ESTOPPEL. To induce the Agent and the Lenders to enter into this
Amendment, Borrower hereby acknowledges and agrees that, as of the date hereof,
there exists no right of offset, defense or counterclaim in favor of Borrower or
any Credit Party as against the Agent or any Lender with respect to the
obligations of Borrower or such Credit Party to any of such parties under the
Credit Agreement or the other Loan Documents, either with or without giving
effect to this Amendment.

         5. CONDITIONS TO EFFECTIVENESS. This Amendment shall be effective as of
August 9, 2000, subject to the following items, which in the case of documents
to be delivered shall be in form and substance satisfactory to the Agent and the
Lenders in their sole discretion:

            (i)   the Agent shall have received this Amendment, duly executed,
                  completed and delivered by the Agent, each of the Lenders, and
                  Borrower, and the attached Confirmation of Guaranty, duly
                  executed by each other Credit Party;

            (ii)  Borrower shall have paid to the Agent, for the account of the
                  Agent and the Lenders, the fees agreed to by Borrower in that
                  certain letter between the Agent and Borrower related thereto
                  and dated as of the date hereof;

            (iii) the Agent shall have received a duly executed certificate of
                  the Chief Executive Officer and Vice President of Finance of
                  Borrower, dated the date hereof, regarding such matters as the
                  Agent may request; and

            (iv)  the Agent shall have received such other certificates,
                  documents and agreements, as the Agent may, in its sole
                  discretion, request.

         6. POST-CLOSING CONDITIONS. As soon as practical after the date hereof,
and in any event on or prior to the dates set forth below, Borrower shall cause
the conditions set forth below to be fulfilled, and the failure of any Credit
Party to fulfill any such condition shall be deemed an Event of Default for all
purposes under the Credit Agreement:

         (a) on or prior to August 18, 2000, the Agent shall have received the
Security Agreement duly executed, completed (except as expressly permitted
therein) and delivered by the Agent and each Credit Party signatory thereto, in
form and substance satisfactory to the Lenders;

         (b) on or before August 31, 2000:

            (i)   each Credit Party shall have duly completed the Schedules to
                  the Security Agreement, and such Schedules shall be
                  satisfactory to the Lenders;

            (ii)  the Agent shall have received evidence satisfactory to it that
                  the Agent (for the benefit of itself and the Lenders) has a
                  valid and perfected first priority


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<PAGE>   6

                  security interest in the Collateral, including (i) such
                  documents duly executed by each Credit Party (including
                  financing statements under the Code and other applicable
                  documents under the laws of any jurisdiction with respect to
                  the perfection of Liens) as the Agent may request in order to
                  perfect its security interests in the Collateral and (ii)
                  copies of Code search reports listing all effective financing
                  statements that name Borrower or any Subsidiary as debtor,
                  together with copies of such financing statements, none of
                  which shall cover the Collateral, except for those relating to
                  Permitted Encumbrances; and

            (iii) the Agent shall receive opinions of counsel of Haynes and
                  Boone, LLP, counsel to Borrower and the other Credit Parties
                  (and, upon request of the Agent, opinions of local counsel as
                  the Lenders may reasonably require) in form and substance
                  satisfactory to the Lenders; and

         7. REIMBURSEMENT OF EXPENSES. Borrower hereby agrees that it shall
reimburse the Agent and the Lenders on demand for all costs and expenses
(including without limitation attorney's fees) incurred by such parties in
connection with the negotiation, documentation and consummation of this
Amendment and the other documents executed in connection herewith and therewith
and the transactions contemplated hereby and thereby.

         8. GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK FOR CONTRACTS TO BE PERFORMED
ENTIRELY WITHIN SAID STATE.

         9. SEVERABILITY OF PROVISIONS. Any provision of this Amendment which is
prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction,
be ineffective to the extent of such prohibition or unenforceability without
invalidating the remaining provisions hereof or affecting the validity or
enforceability of such provision in any other jurisdiction. To the extent
permitted by applicable law, Borrower hereby waives any provision of law that
renders any provision hereof prohibited or unenforceable in any respect.

         10. COUNTERPARTS. This Amendment may be executed in any number of
counterparts, all of which shall be deemed to constitute but one original and
shall be binding upon all parties, their successors and permitted assigns.

         11. ENTIRE AGREEMENT. The Credit Agreement as amended by this Agreement
embodies the entire agreement between the parties hereto relating to the subject
matter hereof and supersedes all prior agreements, representations and
understandings, if any, relating to the subject matter hereof.



                  [Remainder of page intentionally left blank]


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<PAGE>   7

         IN WITNESS WHEREOF, the parties have caused this Amendment to be duly
executed by their respective officers thereunto duly authorized, as of the date
first above written.



                                       RADIOLOGIX, INC.


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



                                       GENERAL ELECTRIC CAPITAL
                                       CORPORATION, as Agent and Lender

Revolving Loan Commitment
(including a Swing Line
Commitment of $5,000,000):
$46,000,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------



                                       BANK ONE, N.A., as Lender

Revolving Loan Commitment:
$25,000,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------



                                       PARIBAS, as Lender

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

Revolving Loan Commitment:
$14,500,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------


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<PAGE>   8

                                       CREDIT LYONNAIS NEW YORK BRANCH,
                                       as Lender

Revolving Loan Commitment:
$20,000,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------



                                       BANK OF AMERICA, N.A., as Lender

Revolving Loan Commitment:
$19,500,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------



                                       COOPERATIEVE CENTRALE RAIFFEISEN -
                                       BOERENLEENBANK B.A.
                                       "RABOBANK NEDERLAND," NEW YORK
                                       BRANCH, as Lender

Revolving Loan Commitment:
$15,000,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------

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



                                       FLEET CAPITAL  CORPORATION, as Lender

Revolving Loan Commitment:
$20,000,000                            By:
                                           -------------------------------------
                                       Name:
                                             -----------------------------------
                                       Title:
                                              ----------------------------------


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<PAGE>   9

                            CONFIRMATION OF GUARANTY


         Each of the undersigned Guarantors hereby acknowledges, consents and
agrees to the terms of the foregoing Third Amendment to Credit Agreement and
Consent and agrees and confirms that its obligations under the Guaranty to which
it is a party will continue in full force and effect and extend to all
Obligations now outstanding or hereafter incurred pursuant to the terms of the
Credit Agreement as amended and modified hereby.

         This 9th day of August, 2000.


                                       ADVANCED IMAGING PARTNERS, INC.
                                       ADVANCED RADIOLOGY, LLC
                                       COMMUNITY IMAGING PARTNERS, INC.
                                       IDE IMAGING PARTNERS, INC.
                                       M&S IMAGING PARTNERS, INC.
                                       M&S IMAGING PARTNERS, L.P.
                                       By: M&S IMAGING PARTNERS, INC.,
                                           as General Partner
                                       MID ROCKLAND IMAGING
                                         PARTNERS, INC.
                                       PACIFIC IMAGING PARTNERS, INC.
                                       QUESTAR IMAGING, INC.
                                       QUESTAR ATLANTA, INC.
                                       QUESTAR CLEVELAND, INC.
                                       QUESTAR COLUMBUS, INC.
                                       QUESTAR DES PLAINES, INC.
                                       QUESTAR DULUTH, INC.
                                       QUESTAR HENDERSON, INC.
                                       QUESTAR H F M C, INC.
                                       QUESTAR KANSAS, INC.
                                       QUESTAR IMAGING MB, INC.
                                       QUESTAR LINCOLN, INC.
                                       QUESTAR LOS ALAMITOS, INC.
                                       QUESTAR LOWER BUCKS, INC.
                                       QUESTAR MT. LAUREL, INC.
                                       QUESTAR NAPERVILLE, INC.
                                       QUESTAR NORTH GEORGIA, INC.
                                       QUESTAR ORLANDO, INC.
                                       QUESTAR PALM SPRINGS, INC.
                                       QUESTAR PHILADELPHIA, INC.
                                       QUESTAR PVH, INC.
                                       QUESTAR QUAKERTOWN, INC.
                                       QUESTAR SAN FRANCISCO, INC.


<PAGE>   10

                                       QUESTAR SOUTH CHICAGO, INC.
                                       QUESTAR TAMPA, INC.
                                       QUESTAR TOLEDO, INC.
                                       QUESTAR TREASURE COAST, INC.
                                       QUESTAR TRISTATES, INC.
                                       QUESTAR TUCSON, INC.
                                       QUESTAR VICTORVILLE, INC.
                                       QUESTAR WEST PALM, INC.
                                       RADIOLOGY AND NUCLEAR MEDICINE
                                         IMAGING PARTNERS, INC.
                                       TOTAL IMAGING PARTNERS, INC.
                                       TREASURE COAST IMAGING PARTNERS, INC.
                                       VALLEY IMAGING PARTNERS, INC.


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



                                       M&S IMAGING INVESTMENTS, INC.

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

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