<PAGE>

                                                                   Exhibit 10.18

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED
(THE "ACT"), OR APPLICABLE STATE LAW, AND NO INTEREST THEREIN MAY BE SOLD,
DISTRIBUTED, ASSIGNED, OFFERED, PLEDGED OR OTHERWISE TRANSFERRED EXCEPT AS
OTHERWISE SET FORTH HEREIN AND UNLESS (A) THERE IS AN EFFECTIVE REGISTRATION
STATEMENT UNDER THE ACT AND APPLICABLE STATE SECURITIES LAWS COVERING ANY SUCH
TRANSACTION, (B) THE COMPANY RECEIVES AN OPINION OF LEGAL COUNSEL FOR THE HOLDER
SATISFACTORY TO THE COMPANY STATING THAT SUCH TRANSACTION IS EXEMPT FROM
REGISTRATION OR (C) THE COMPANY OTHERWISE SATISFIES ITSELF THAT SUCH TRANSACTION
IS EXEMPT FROM REGISTRATION.

APRIL 14, 2006                                                      $15,000,000

                            IMARX THERAPEUTICS, INC.

                             SECURED PROMISSORY NOTE

     ImaRx Therapeutics, Inc., a Delaware corporation (the "COMPANY"), for value
received, promises to pay to Abbott Laboratories, an Illinois corporation
("HOLDER"), the principal sum of $15,000,000, subject to adjustment from time to
time (as adjusted from time to time, the "PRINCIPAL AMOUNT") in accordance with
the terms of this Note and that certain Asset Purchase Agreement dated as of
April 10, 2006, by and among the Company and Holder (the "ASSET PURCHASE
AGREEMENT"). Capitalized terms not defined herein shall have the meanings given
such terms in the Asset Purchase Agreement.

     The following is a statement of the rights of Holder and the conditions to
which this Note is subject, to which Holder, by the acceptance of this Note,
agrees:

     1. Principal Amount and Interest; Payment; Prepayment.

          (a) Principal Amount and Interest. Simple interest shall accrue on the
unpaid Principal Amount at the annual rate of six percent (6.0%) per annum
("INTEREST"), measured from the date set forth above. Interest will be computed
on the basis of a 365-day year. The Company will pay the Principal Amount on or
before December 31, 2007 (the "PAYMENT DATE") plus all accrued but unpaid
Interest through such date. Notwithstanding the foregoing, if an Event of
Default (as defined below) occurs, then, at the option of Holder, upon 30 days
prior written notice of default given to the Company, the unpaid Principal
Amount (as it may have been adjusted pursuant to the provisions hereof) and all
accrued and unpaid Interest thereon shall be immediately due, payable and
collectible by Holder.

          (b) Prepayment. The Company may at any time prepay in whole or in part
the Principal Amount and any accrued and unpaid Interest without penalty.

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          (c) Withholding. Holder acknowledges that the Company may be required
by law to withhold on payments of Interest and hereby authorizes the Company to
withhold as required by law.

          (d) Payments. All payments made hereunder shall be applied first to
accrued Interest, and thereafter to the Principal Amount. Principal and Interest
and all other amounts due hereunder (collectively, the "OBLIGATIONS") are to be
paid in lawful money of the United States of America in federal or other
immediately available funds.

     2. Security Interest. Pursuant to the terms of the Security Agreement
attached as an exhibit to the Asset Purchase Agreement (the "SECURITY
AGREEMENT"), the Company has granted a continuing, first priority security
interest to Holder in all of the Collateral (as defined in the Security
Agreement) to secure the payment of the Obligations.

     3. Transfer of Note; Restrictions on Transfer. Neither this Note nor any of
the rights, interests or obligations hereunder may be assigned in whole or in
part by the Company or Holder without the prior written consent of the other
party, which consent may not be unreasonably withheld. In addition, this Note
may be transferred only in compliance with applicable federal and state
securities laws and only upon surrender of the original Note for registration of
transfer, duly endorsed, or accompanied by a duly executed written instrument of
transfer in form satisfactory to the Company. A new Note for like Principal
Amount and Interest will be issued to, and registered in the name of, the
transferee. Interest and Principal Amount are payable only to the registered
holder of the Note.

     4. Events of Default. The term "EVENT OF DEFAULT" as used in this Note
means any of the following events:

          (a) Any breach by the Company of the obligation to pay within thirty
(30) days after the date when it is due and payable (i) any amount payable
hereunder or (ii) any amount payable under the Escrow Agreement between the
Company and the Holder of even date herewith into the Escrow Account (as defined
in the Escrow Agreement);

          (b) The Company files or is served with a petition for relief under 11
U.S.C. Section 101 et seq. or any similar federal or state statute, or an action
is commenced to appoint a receiver or trustee for all or substantially all of
the Company's property, and the Company fails to cause such proceeding to be
dismissed within ninety (90) days after it is instituted against the Company;;

          (c) The liquidation, dissolution or winding up of the Company prior to
the payment in full of all Obligations hereunder


                                      -2-

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          (d) The breach by the Company of any representation, warranty,
covenant or agreement in the Asset Purchase Agreement or any of the Other
Agreements (as defined in the Asset Purchase Agreement) beyond any applicable
cure period;

          (e) The entry of any judgment or order against the Company that could
reasonably be expected to have a material adverse effect on the Company's
business and which remains unsatisfied or undischarged and in effect for thirty
(30) days after such entry without a stay of enforcement or execution; or

          (f) The placement of any security interest or other lien or
encumbrance upon the Collateral which is not removed within fifteen (15) days
following notice from Holder.

     5. Effect of Event of Default. If an Event of Default described in Section
4 hereof shall occur, in addition to taking any other action permitted by law,
the Holder may, at its sole option, by written notice to the Company, declare
the Obligations hereunder immediately due and payable, without presentment,
demand, protest or further notice of any kind. The Holder may also exercise any
or all of the rights and remedies available to a secured creditor under the
Uniform Commercial Code of the applicable jurisdiction, as the same may be
amended from time to time, including without limiting the foregoing, the right
to take possession of the Collateral.

     6. Nonrecourse. This Note is a nonrecourse promissory note. Upon any
default, except as set forth in the Escrow Agreement and this Section 6,
Holder's sole recourse with respect to the Obligations under this Note shall be
to the Collateral. Holder shall have no right to recover from the Company
personally or from any of the Company's assets other than the Collateral any
deficiency with respect to the Obligations under this Note. The foregoing
limitation shall not be interpreted or construed to limit (i) the ability of
Holder to seek monetary damages from the Company with respect to any breach of
any covenant under the Security Agreement to maintain the condition of the
Collateral or (ii) the right of the Holder to the Proceeds (as defined in the
Escrow Agreement) in accordance with the provisions of the Escrow Agreement.

     7. Miscellaneous.

          (a) Remedies. The Company hereby waives notice, presentment, protest
and notice of dishonor.

          (b) Holder as Owner. The Company may deem and treat the holder of
record of this Note as the absolute owner for all purposes regardless of any
notice to the contrary.


                                      -3-

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          (c) No Shareholder or Ownership Rights. This Note shall not entitle
Holder to any voting rights or any other rights as a shareholder or owner of the
Company or to any other rights except the rights stated herein.

          (d) Shareholders, Affiliates, Officers and Directors Not Liable. In no
event shall any shareholder, affiliate, officer or director of the Company be
liable for any amounts due or payable pursuant to this Note.

          (e) Notices. All communications, notices and consents provided for
herein shall be in writing and be given in person or by means of telex,
facsimile or other means of wire transmission (with request for assurance of
receipt in a manner typical with respect to communications of that type), by
overnight courier or by mail, and shall become effective: (a) on delivery if
given in person; (b) on the date of transmission if sent by telex, facsimile or
other means of wire transmission; (c) one (1) business day after delivery to the
overnight service; or (d) four (4) business days after being mailed, with proper
postage and documentation, for first-class registered or certified mail,
prepaid.

     Notices shall be addressed as follows:

If to the Company, to: ImaRx Therapeutics, Inc.
                       1635 East 18th Street
                       Tucson, Arizona 85719
                       Attn: Greg Cobb
                       Facsimile Number: (520) 791-2437

       with copies to: DLA Piper Rudnick Gray Cary LLP
                       701 Fifth Avenue, Ste 700
                       Seattle, Washington 98104
                       Attn: Jeff Harmes
                       Facsimile Number: (206) 839-4801

     If to Holder, to: Abbott Laboratories
                       100 Abbott Park Road
                       Building AP6D, Department 364
                       Abbott Park, Illinois 60064-6020
                       Attn: General Counsel
                       Facsimile Number: (847) 938-6277

       with copies to: Kirkland & Ellis LLP
                       200 East Randolph Drive
                       Chicago, Illinois 60601
                       Attn: R. Scott Falk, P.C.
                       Facsimile Number: (312) 861-2200


                                      -4-

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provided, however, at the time of mailing or within three business days
thereafter there is or occurs a labor dispute or other event that might
reasonably be expected to disrupt the delivery of documents by mail, any
communication, notice or consent provided for herein shall be given in person or
by means of telex, facsimile or other means of wire transmission or by overnight
courier, and further provided that if any party shall have designated a
different address by notice to the others, then to the last address so
designated.

          (f) Amendments and Waivers. Any term of this Note may be amended and
the observance of any term may be waived (either generally or in a particular
instance and either retroactively or prospectively) only with the written
consent of the Company and Holder. Any amendment or waiver shall be binding on
each future Holder and the Company.

          (g) Governing Law. This Note shall be subject to the provisions of
Section 12.9 of the Asset Purchase Agreement.

          (h) Successors and Assigns. The terms and conditions of this Note
shall inure to the benefit of and be binding on the respective successors and
assigns of the parties.

          (i) Severability. If one or more provisions of this Note are held to
be unenforceable under applicable law, such provision shall be excluded from
this Note, and the balance of this Note shall be interpreted as if such
provision were so excluded and shall be enforceable in accordance with its
terms.

            [The remainder of this page is intentionally left blank.]


                                      -5-

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     IN WITNESS WHEREOF, the Company has caused this Secured Promissory Note to
be issued as of the date first written above.

                                        IMARX THERAPEUTICS, INC.


                                        By: /s/ Evan C. Unger
                                            ------------------------------------
                                        Name: Evan C. Unger
                                        Title: President and Chief Executive
                                               Officer

AGREED AND ACCEPTED:

ABBOTT LABORATORIES ("HOLDER")


By: /s/ Sean E. Murphy
    ----------------------------------
Name: Sean E. Murphy
Its: Vice President, Global Licensing/
     New Business Development