<PAGE>

                                                                   Exhibit 10.23

                                                           Boston, Massachusetts

                            SECURED PROMISSORY NOTE

$25,000,000.00                                                September 15, 2000

     FOR VALUE RECEIVED, HOLOGIC, INC., a corporation organized under the laws
of the State of Delaware (herein referred to as "Maker"), promises to pay to the
order of TREX MEDICAL SYSTEMS CORPORATION, a Delaware corporation having its
principal address at 81 Wyman Street, Waltham, Massachusetts (the "Payee"), at
                                                                   -----
Payee's principal address or such other place as Holder hereof may designate in
writing (the legal holder from time to time of this Note, including Payee as the
initial holder, being hereinafter referred to as "Holder"), the principal sum of
                                                  ------
Twenty-Five Million and No/100 Dollars ($25,000,000.00), (hereinafter referred
to as "Principal Indebtedness"), together with interest thereon at an annual
       ----------------------
rate of eleven and one-half percent (11.5%) (the "Interest Rate"), in accordance
                                                  -------------
with the provisions hereinafter set forth.  This Note is delivered under the
terms of a certain Asset Purchase and Sale Agreement, dated August 13, 2000 (the
"Asset Purchase Agreement"), among the Maker, the Payer, Trex Medical
Corporation, ThermoTrex Corporation and Thermo Electron Corporation.

     1.  Terms of Payment.  The Maker shall make an initial interest payment on
         ----------------
September 13, 2001 of all interest accrued on the unpaid Principal Indebtedness
from time to time outstanding  through such date at the Interest Rate.
Thereafter, the Maker shall make semi-annual payments of interest accrued at the
Interest Rate on the unpaid Principal Indebtedness from time to time outstanding
under this Note on September 13 and March 13 of each year.  On September 13,
2003 (the "Maturity Date"), Maker shall pay to Holder the entire Principal
           -------------
Indebtedness then remaining unpaid, together with accrued and unpaid interest
thereon at the Interest Rate and any other charges and amounts of money due
under this Note, the Bedford Mortgage (hereinafter defined) and the Danbury
Mortgage (hereinafter defined), and any other documents evidencing, securing or
pertaining to the incurrence of the Principal Indebtedness, (collectively, the
"Loan Documents").  Notwithstanding anything to the contrary contained herein,
---------------
<PAGE>

after the occurrence of an Event of Default (as hereinafter defined) any and all
sums of money held or received by Holder may be applied to such items, in such
order, at such times and in such manner as Holder deems appropriate.

     2.  Prepayment.  Maker may prepay the Principal Indebtedness in whole or in
         ----------
part, together with any and all accrued interest on the Principal Indebtedness
prepaid and other sums due under the Loan Documents, at any time without premium
or penalty.

     3.  Security.  This Note is secured by, among other things, (a) a Mortgage,
         --------
Security Agreement and Assignment of Leases and Rents (hereinafter referred to
as the "Bedford Mortgage"), given by Maker to Payee, of even date herewith, with
        ----------------
respect to real property and improvements located in Bedford, Massachusetts and
(b) an Open-End Mortgage Deed, Security Agreement and Assignment of Leases and
Rent (hereinafter referred to as the "Danbury Mortgage"), given by Maker to
Payee, of even date herewith, with respect to real property and improvements
located in Danbury, Connecticut.  The Bedford Mortgage and the Danbury Mortgage
are hereinafter collectively referred to as the "Mortgage".

     4.  Location and Medium of Payments.  The sums payable under this Note or
         -------------------------------
under the Mortgage shall be paid to Holder at its principal address hereinabove
set forth, or at such other place as Holder may from time to time hereafter
designate to Maker in writing, in legal tender of the United States of America.

     5.  Acceleration of Maturity.  At the option of Holder, which may be
         ------------------------
exercised at any time after one or more of the following events (each being an
"Event of Default") shall have occurred, the whole of the Principal
-----------------
Indebtedness, together with all interest and other charges due under any of the
Loan Documents, shall immediately become due and payable ("Acceleration of
                                                           ---------------
Maturity"):  (i) failure of Maker to pay any amount due under the Loan Documents
--------
within five (5) days of the due date of such payment, (ii) except as otherwise
provided in this Section 5, a default by Maker in the due performance of any of
its covenants or agreements contained in any Loan Document if such default
continues for more than 30 days after notice thereof from Holder, provided that
if such default cannot be cured with the exercise of diligent efforts within
such 30 days but is reasonably susceptible of cure, such 30 day period shall be
extended for a period of time as shall be reasonably required for Maker, in the
exercise of diligent efforts, to cure the default if the Maker commences such
efforts within 30 days after notice thereof from Holder and thereafter
diligently pursues such efforts, (iii) the making of a false or materially
misleading representation, warranty

                                     - 2 -
<PAGE>

or statement by Maker or any guarantor in any Loan Document, certificate,
affidavit or other instrument executed or delivered in connection with the Loan
Documents; (iv) a material inaccuracy in Sections 3.8 or 3.9 of the Asset
Purchase Agreement which materially and adversely affects the ability of the
Maker to perform its obligations under this Note, (v) a default under any other
mortgage or security agreement which covers or affects the Mortgaged Property
(as defined in the Mortgage) if such default has not been cured at least five
(5) days prior to the expiration of any notice or cure period under such
mortgage or security agreement, (vi) the failure to maintain insurance required
under the Mortgage, (vii) the dissolution, termination or liquidation of Maker,
(viii) the creation or placement of any lien on the Mortgaged Property which
lien was not approved by Holder or permitted by the Loan Documents, (ix) a sale
or other Transfer (as defined in the Mortgage) of the Mortgaged Property, (x) if
a petition under Title 11, United States Code, shall be filed by or against
Maker or any guarantor of Maker's obligations under the Loan Documents (which,
in the case of a petition against the Maker or any such guarantor, is not stayed
or dismissed within 60 days) or if Maker or any guarantor of Maker's obligations
under the Loan Documents shall seek or consent to the appointment of a receiver
or trustee for itself or for any of the Mortgaged Property, file a petition
seeking relief under the bankruptcy or other similar laws of the United States,
any state or any jurisdiction, make a general assignment for the benefit of
creditors, or be unable to pay its debts as they become due; if a court shall
enter an order, judgment or decree appointing, with or without the consent of
Maker or any guarantor of Maker's obligations under the Loan Documents, a
receiver or trustee for it or for any of the Mortgaged Property or approving a
petition filed against Maker or any guarantor of Maker's obligations under the
Loan Documents which seeks relief under the bankruptcy or other similar laws of
the United States, any state or any jurisdiction (any one of the foregoing an
"Insolvency Event"), (xi) a default by the Maker in the due performance of the
covenants contained in Article III of each of the Bedford Mortgage and the
Danbury Mortgage, (xii) the Maker or any subsidiary of Maker shall fail to pay,
within any applicable period of grace, any obligation for borrowed money or
credit received which in the aggregate represents indebtedness of $10,000,000 or
more, or fail to observe or perform any material term, covenant or argument
contained in any agreement by which it is bound evidencing or securing borrowed
money or credit received which in the aggregate represents indebtedness of
$10,000,000 or more which results in the acceleration of the maturity thereof,
(xiii) a Change of Control (as defined below) shall have occurred. As used
herein, "Change of Control" shall mean the occurrence of any of the following
events: (a) the acquisition by any individual, corporation, partnership, trust,
unincorporated association, business, or other legal entity (a "Person")
                                     - 3 -
<PAGE>

(including any syndicate or group deemed to be a "person" under Section 13(d)(3)
of the Securities and Exchange Act of 1934, as amended) of beneficial ownership,
directly or indirectly, through a purchase, merger or other acquisition
transaction or series of transactions, of shares of capital stock of the Maker
entitling such Person to exercise 50% or more of the total voting power of all
shares of capital stock of the Maker entitled to vote generally in the elections
of directors (any shares of voting stock of which such person or group is the
beneficial owner that are not then outstanding being deemed outstanding for the
purposes of calculating such percentage); (b) any consolidation of the Maker
with, or merger of the Maker into, any other Person, any merger of another
Person into the Maker, or any sale or transfer of all or substantially all of
the assets of the Maker to another Person (other than a merger (A) which does
not result in any reclassification, conversion, exchange or cancellation of
outstanding shares of capital stock of the Maker or (B) which is effected solely
to change the jurisdiction of incorporation of the Maker or (C) a merger in
which the holders of a majority of the voting stock of the Maker immediately
prior to the merger hold a majority of the voting stock of the survivor
immediately after the merger); or (c) during any consecutive two-year period,
individuals who at the beginning of such period constituted the Board of
Directors of the Maker (together with any new directors whose election by such
Board of Directors or whose nomination for election by the stockholders of the
Maker was approved by a vote of 66-2/3% of the directors then still in office
who were either directors at the beginning of such period or whose election or
nomination for election was previously so approved) cease for any reason to
constitute a majority of the Board of Directors of the Maker then in office.

     6.  Late Charges; Interest Following Event of Default.  If any payment due
         -------------------------------------------------
under this Note, the Mortgage or any other Loan Document is not paid when due,
after giving effect to any cure or grace period, Maker shall pay and Holder
shall be entitled to collect a late payment charge for each month or fraction
thereof during which such payment is not made when due and for each month
thereafter that such sum remains unpaid, equal to the lesser of four percent
(4%) of such late payment or the maximum amount permitted by law, for the
purpose of defraying the expense incurred by Holder in handling and processing
such delinquent payment, and such amount shall be secured by the Loan Documents.
Such charge shall be payable to Holder as additional interest and not as a
penalty.

     In addition to any late payment charge which may be due under this Note,
Maker shall pay interest on all sums due hereunder at a rate (the "Default
                                                                   -------
Rate") equal to the lesser of (i) the Interest Rate plus four percent (4%)
----
                                     - 4 -
<PAGE>

per annum, or (ii) the maximum rate permitted by law, from and after the first
to occur of the following events: if Holder elects to cause the Acceleration of
Maturity; the occurrence of an Insolvency Event, or if all sums due hereunder
are not paid on the Maturity Date.

     7.  Collection and Enforcement Costs.  Maker, upon demand, shall pay Holder
         --------------------------------
for all costs and expenses, including without limitation attorneys' fees, paid
or incurred by Holder in connection with the collection of any sum due
hereunder, or in connection with enforcement of any of Holder's rights or
Maker's obligations under this Note, the Mortgage, or any of the other Loan
Documents, together with interest thereon at the Default Rate.

     8.  Continuing Liability.  The obligation of Maker to pay the Principal
         --------------------
Indebtedness, interest and all other sums due hereunder shall continue in full
force and effect and shall in no way be impaired until the actual payment
thereof to Holder, and in case of a sale or transfer of all or any part of the
Mortgaged Property, or in case of any further agreement given to secure the
payment of this Note, or in case of any agreement or stipulation extending the
time or modifying the terms of payment above recited, Maker shall nevertheless
continue to be liable on this Note, as extended or modified by any such
agreement or stipulation, unless released and discharged in writing by Holder.

     9.  Joint and Several Liability.  If more than one person, corporation,
         ---------------------------
partnership or other entity shall now or at any time in the future execute this
Note as Maker and/or assume the obligations of Maker hereunder, then each person
and entity shall be fully liable for all such assumed obligations of Maker
hereunder and such obligations shall be joint and several.

     10.  No Oral Changes; Waivers; Offsets.  This Note may not be changed
          ---------------------------------
orally, but only by an agreement in writing signed by the party against whom
enforcement of a change is sought.  The provisions of this Note shall extend and
be applicable to all renewals, amendments, extensions, consolidations and
modifications of the other Loan Documents, and any and all references herein to
the Loan Documents shall be deemed to include any such renewals, amendments,
extensions, consolidations, or modifications thereof.

     Maker and any future indorsers, sureties, indemnitors and guarantors
hereof, jointly and severally, waive presentment for payment, demand, notice of
nonpayment, notice of partial payment, notice of dishonor, protest of any
dishonor, notice of protest, and protest of this Note, and all other notices or
formalities to which they may be

                                     - 5 -
<PAGE>

entitled, and all suretyship defenses of every kind and nature and notices in
connection with the delivery, acceptance, performance, default (except notice of
default required hereby, if any), or enforcement of the payment of this Note,
and they agree that the liability of each of them shall be unconditional without
regard to the liability of any other party and shall not be in any manner
affected by an indulgence, extension of time, renewal, waiver or modification
granted or consented to by Holder; and Maker and all future indemnitors,
indorsers, sureties and guarantors hereof consent to any and all extensions of
time, renewals, waivers or modifications that may be granted by Holder hereof
with respect to the payment or other provisions of this Note and to the release
of the collateral, or any part thereof, with or without substitution, and agree
that additional makers, indorsers, indemnitors, guarantors, or sureties may
become parties hereto without notice to them or affecting their liability
hereunder.

     Holder shall not by any act of omission or commission be deemed to waive
any of its rights or remedies hereunder unless such waiver shall be in writing
and signed by Holder, and then only to the extent specifically set forth
therein;  a waiver with respect to one event shall not be construed as
continuing or as a bar to or waiver of such right or remedy with respect to a
subsequent event.  The acceptance by Holder of payment hereunder that is less
than payment in full of all amounts due at the time of such payment shall not
without the express written consent of Holder:  (i) constitute a waiver of the
right to exercise any of Holder's remedies at that time or at any subsequent
time, (ii) constitute an accord and satisfaction or (iii) nullify any prior
exercise of any remedy.

     No failure to cause an Acceleration of Maturity hereof by reason of an
Event of Default hereunder, acceptance of a past due installment or indulgences
granted from time to time shall be construed (i) as a novation of this Note or
as a reinstatement of the indebtedness evidenced hereby or as a waiver of such
right of acceleration or of the right of Holder thereafter to insist upon strict
compliance with the terms of this Note, or (ii) to prevent the exercise of such
right of acceleration or any other right granted hereunder or by applicable
laws; and, to the maximum extent permitted by law, Maker hereby expressly waives
the benefit of any statute or rule of law or equity now provided, or which may
hereafter be provided, which would produce a result contrary to or in conflict
with the foregoing.

     To the maximum extent permitted by law, Maker hereby waives and renounces
for itself, its heirs, successors and assigns, all rights to the benefits of any
statute of limitations and any moratorium, reinstatement, marshalling,
forbearance, valuation, stay, extension, redemption (after a sale by
foreclosure), appraisement and

                                     - 6 -
<PAGE>

exemption now provided, or which may hereafter be provided, by the Constitution
and laws of the United States of America and of any state thereof, both as to
itself and in and to all of its property, real and personal, against the
enforcement and collection of the obligations evidenced by this Note.

     All payments by the Maker hereunder and under the Mortgage shall be made
without setoff, deduction, defense or counterclaim, howsoever arising.  The
Holder may elect, in its sole discretion, to effect the payment of any
indemnification obligation of the Holder to the Maker pursuant to the terms of
the Asset Purchase Agreement by crediting such amounts against the outstanding
Principal Indebtedness and any accrued interest evidenced by this Note.

     12.  Bind and Inure.  This Note shall bind and inure to the benefit of the
          --------------
parties hereto and their respective legal representatives, heirs, successors and
assigns.

     13.  Applicable Law; Service of Process; Severability.  The provisions of
          ------------------------------------------------
this Note shall be construed and enforceable in accordance with the laws of the
Commonwealth of Massachusetts.  Service of process may be made upon the Maker by
mailing a copy of the summons and any complaint to the Borrower, by certified or
registered mail, return receipt requested, at the address to be used for the
giving of notice to the Maker under this Note.

     If any provision of this Note or the application hereof to any person or
circumstance shall, for any reason and to any extent, be invalid or
unenforceable, neither the remainder of this Note nor the application of such
provision to any other person or circumstance shall be affected thereby, but
rather the same shall be enforced to the greatest extent permitted by law,
except that if such provision relates to the payment of a monetary sum and if
Holder in its reasonable judgment concludes that timely payment of all
principal, interest, fees and expenses provided for in the Loan Documents is
jeopardized or threatened, then Holder may, at its option, declare the entire
indebtedness evidenced hereby due and payable upon sixty (60) days prior written
notice to Maker.

     14.  Usury.  It is hereby expressly agreed that if from any circumstances
          -----
whatsoever fulfillment of any provision of this Note, at the time performance of
such provision shall be due, shall involve transcending the limit of validity
then prescribed by any applicable usury statute or any other law, with regard to
obligations of like character and amount, then ipso facto the obligation to be
                                               ---- -----
fulfilled shall be reduced to the limit of such validity, so that in no

                                    - 7 -



<PAGE>

event shall any exaction be possible under this Note that is in excess of the
limit of such validity. In no event shall Maker be bound to pay for the use,
forbearance or detention of the money loaned pursuant hereto interest of more
than the legal limit in effect at the time, the right to demand any such excess
being hereby expressly waived by Holder.

     15.  Notice.  Any notice, request, demand, statement or consent made
          ------
hereunder shall be in writing signed by the party giving such notice, request,
demand, statement or consent, and shall be deemed to have been properly given if
either delivered personally, delivered to a reputable overnight delivery service
providing a receipt or deposited in the United States Mail, postage prepaid and
certified return receipt requested, at the address set forth below, or at such
other address within the continental United States of America as may have
theretofore been designated in writing.  The effective date of any notice given
as aforesaid shall be the date of personal service, one (1) business day after
delivery to such overnight delivery service, or three (3) business days after
being deposited in the United States Mail, whichever is applicable.  The phrase
"business day" or "Business Day", and similar phrases, as used in this Note
shall mean any day other than a Saturday, a Sunday or a Federal holiday on which
the U.S. Postal Service offices are closed for business in Boston,
Massachusetts.  For purposes hereof, the addresses are as follows:

     If to Holder:      Trex Medical Systems Corporation
                        81 Wyman Street
                        P.O. Box 9046
                        Waltham, Massachusetts 02254-9046
                        Attn:  James H. DaCosta

     With a copy to:    Hale and Dorr LLP
                        60 State Street
                        Boston, Massachusetts  02109
                        Attn:  Hal J. Leibowitz, Esq.

     And to:            Thermo Electron Corporation
                        81 Wyman Street
                        P.O. Box 9046
                        Waltham, Massachusetts 02254-9046
                        Attn:  General Counsel

     If to Maker:       Hologic, Inc.
                        35 Crosby Drive
                        Bedford, Massachusetts 01730
                        Attn:  Glenn Muir

     With a copy to:    Brown, Rudnick, Freed & Gesmer
                        One Financial Center

                                     - 8 -
<PAGE>

                        Boston, Massachusetts  02111
                        Attn:  Philip Flink, Esq.

     16.  Attorneys' Fees.  Any reference to "attorney fees", "attorney's fees",
          ---------------
or "attorneys' fees" in this document includes but is not limited to both the
reasonable fees, charges and costs incurred by Holder through its retention of
outside legal counsel and the allocable fees, costs and charges for services
rendered by Holder's in-house counsel.  Any reference to "attorney fees",
"attorney's fees", or "attorneys' fees" shall also mean those attorneys or legal
fees, costs and charges incurred by Holder in the collection of any Indebtedness
(as defined and described in the Mortgage), the enforcement of any obligations
hereunder, the protection of the Mortgaged Property, the foreclosure of the
Mortgage, the sale of the Mortgaged Property, the defense of actions arising
hereunder and the collection, protection or setoff of any claim Holder may have
in a proceeding under Title 11, United States Code.

     17.  Waiver of Trial by Jury.  MAKER AND HOLDER HEREBY IRREVOCABLY AND
          -----------------------
UNCONDITIONALLY WAIVE THEIR RIGHTS TO A TRIAL BY JURY AS TO ANY ACTION, SUIT,
PROCEEDING OR COUNTERCLAIM THAT RELATES TO OR ARISES OUT OF ANY OF THE LOAN
DOCUMENTS OR THE ACTS OR FAILURE TO ACT OF OR BY HOLDER IN THE ENFORCEMENT OF
ANY OF THE TERMS OR PROVISIONS OF THIS NOTE OR ANY OF THE OTHER LOAN DOCUMENTS.

     18.  Entire Agreement; Amendments in Writing.  THIS NOTE, THE OTHER LOAN
          ---------------------------------------
DOCUMENTS AND THE ASSET PURCHASE AGREEMENT EMBODY THE FINAL, ENTIRE AGREEMENT
AMONG THE PARTIES HERETO AND SUPERSEDE ANY AND ALL PRIOR COMMITMENTS,
AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR ORAL,
RELATING TO THE SUBJECT MATTER HEREOF AND THEREOF AND MAY NOT BE CONTRADICTED OR
VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OR
DISCUSSIONS OF THE PARTIES HEREOF. THERE ARE NO ORAL AGREEMENTS AMONG THE
PARTIES HERETO. The provisions of this Note and the Loan Documents may be
amended or waived only by an instrument in writing signed by the Maker and
Holder.

     19.  Headings.  The headings of the Articles, Sections and Subsections
          --------
hereof are inserted for convenience of reference only and shall in no way alter,
modify or define, or be used in construing the text of such Articles, Sections
or Subsections.

                                     - 9 -





<PAGE>

IN WITNESS WHEREOF, Maker has duly executed this Note as a sealed instrument as
of the day and year first above written.


Witness:                        MAKER:
                                HOLOGIC, INC.

/s/ Julia Steen                 By:  /s/ Glenn P. Muir
-----------------------             ----------------------------
Name:                               Name: Glenn P. Muir
                                    Its: Vice President, Finance and Treasurer
                                    (Principal Financial Officer)

/s/ Curt A. Kramer
-----------------------
Name:

                                    - 10 -