Document:

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                                                                   Exhibit 10.18

                                                               Customer No. 1385
                             MASTER LEASE AGREEMENT

Lessor:           TRANSAMERICA BUSINESS CREDIT CORPORATION
                  RIVERWAY II
                  WEST OFFICE TOWER
                  WEST HIGGINS
                  ROSEMONT, ILLINOIS  60018

Co-Lessees:
Collectively and  ACUSPHERE, INC.
individually      UNIVERSITY PARK @ MIT
referred to       38 SIDNEY STREET
herein as         CAMBRIDGE, MASSACHUSETTS 02139
"Lessee"

                  ACUSPHERE SECURITIES CORPORATION
                  UNIVERSITY PARK @ MIT
                  38 SIDNEY STREET
                  CAMBRIDGE, MASSACHUSETTS 02139

The lessor pursuant to this Master Lease Agreement ("Agreement") dated as of
February 21, 2001, is Transamerica Business Credit Corporation ("Lessor"). All
equipment, together with all present and future additions, parts, accessories,
attachments, substitutions, repairs, improvements, and replacements thereof or
thereto, which are the subject of a Lease (as defined in the next sentence)
shall be referred to as "Equipment." Simultaneous with the execution and
delivery of this Agreement, the parties are entering into one or more Lease
Schedules (each, a "Schedule") substantially in the form attached hereto as
Exhibit A which refer to and incorporate by reference this Agreement, each of
which constitutes a lease (each, a "Lease") for the Equipment specified therein.
Additional details pertaining to each Lease are specified in the applicable
Schedule. Each Schedule that the parties hereafter enter into shall constitute a
Lease. Lessor has no obligation to enter into any additional leases with, or
extend any future financing to, Lessee.

          1.   LEASE. Subject to and upon all of the terms and conditions of
this Agreement and each Schedule, Lessor hereby agrees to lease to Lessee and
Lessee hereby agrees to lease from Lessor the Equipment for the Term (as defined
in Paragraph 2 below) thereof. The timing and financial scope of Lessor's
obligation to enter into Leases hereunder are limited as set forth in the
Commitment Letter executed by Lessor and Lessee, dated as of December 5, 2000
and attached hereto as EXHIBIT A (the "Commitment Letter").

          2.   TERM. Each Lease shall be effective and the term of each Lease
("Term") shall commence on the commencement date specified in the applicable
Schedule and, unless sooner terminated (as hereinafter provided), shall expire
at the end of the term specified in such Schedule; PROVIDED, HOWEVER, that
obligations due to be performed by Lessee during the Term shall continue until
they have been performed in full. Schedules will only be executed after the
delivery of the Equipment to Lessee or upon completion of deliveries of items of
such Equipment with aggregate cost of not less than $50,000.

          3.   RENT. Lessee shall pay as rent to Lessor, for use of the
Equipment during the Term or Renewal Term (as defined in Paragraph 8), rental
payments equal to the sum of all rental payments including, without limitation,
security deposits, advance rents, and interim rents payable in the amounts and
on the dates specified in the applicable Schedule ("Rent"). If any Rent or other
amount payable by Lessee is not paid within five business days after the day on
which it becomes payable, Lessee will pay on demand, as a late charge, an amount
equal to 5% of such unpaid Rent or other amount but only to the extent permitted
by applicable law. All payments provided for herein shall be payable to Lessor
at its address specified above, or at any other place designated by Lessor.

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          4.   LEASE NOT CANCELABLE; LESSEE'S OBLIGATIONS ABSOLUTE. No Lease may
be canceled or terminated except as expressly provided herein. Lessee's
obligation to pay all Rent when due hereunder shall be absolute and
unconditional and shall not be subject to any delay, reduction, set-off,
defense, counterclaim, or recoupment for any reason whatsoever, including any
failure of the Equipment or any representations by the manufacturer or the
vendor thereof. If the Equipment is unsatisfactory for any reason, Lessee shall
make any claim solely against the manufacturer or the vendor thereof and shall,
nevertheless, pay Lessor all Rent payable hereunder.

          5.   SELECTION AND USE OF EQUIPMENT. Lessee agrees that it shall be
responsible for the selection and use of, and results obtained from, the
Equipment and any other associated equipment or services.

          6.   WARRANTIES. LESSOR WARRANTS TO LESSEE, SO LONG AS NO EVENT OF
DEFAULT OR EVENT WHICH, WITH THE GIVING OF NOTICE, THE PASSAGE OF TIME, OR BOTH,
WOULD CONSTITUTE AN EVENT OF DEFAULT, HAS OCCURRED, LESSOR WILL NOT DISTURB
LESSEE'S QUIET AND PEACEFUL POSSESSION AND UNRESTRICTED USE OF THE EQUIPMENT.
LESSOR MAKES NO OTHER WARRANTY, EXPRESS OR IMPLIED, AS TO ANY MATTER WHATSOEVER,
INCLUDING, WITHOUT LIMITATION, THE DESIGN OR CONDITION OF THE EQUIPMENT OR ITS
MERCHANTABILITY, SUITABILITY, QUALITY, OR FITNESS FOR A PARTICULAR PURPOSE, AND
HEREBY DISCLAIMS ANY SUCH WARRANTY. OTHER THAN THE ABOVE WARRANTY WITH RESPECT
TO QUIET AND PEACEFUL POSSESSION AND UNRESTRICTED USE OF THE EQUIPMENT, LESSEE
SPECIFICALLY WAIVES ALL RIGHTS TO MAKE A CLAIM AGAINST LESSOR FOR BREACH OF ANY
OTHER WARRANTY. LESSEE LEASES THE EQUIPMENT "AS IS." IN NO EVENT SHALL LESSOR
HAVE ANY LIABILITY, NOR SHALL LESSEE HAVE ANY REMEDY AGAINST LESSOR, FOR ANY
LIABILITY, CLAIM, LOSS, DAMAGE, OR EXPENSE CAUSED DIRECTLY OR INDIRECTLY BY THE
EQUIPMENT OR ANY DEFICIENCY OR DEFECT THEREOF OR THE OPERATION, MAINTENANCE, OR
REPAIR THEREOF OR ANY CONSEQUENTIAL DAMAGES AS THAT TERM IS USED IN SECTION
2-719(3) OF THE MODEL UNIFORM COMMERCIAL CODE, AS AMENDED FROM TIME TO TIME
("UCC"). Lessor grants to Lessee, for the sole purpose of prosecuting a claim,
the benefits of any and all warranties made available by the manufacturer or the
vendor of the Equipment to the extent assignable.

          7.   DELIVERY. Lessor hereby appoints Lessee as Lessor's agent for the
sole and limited purpose of accepting delivery of the Equipment from each vendor
thereof. Lessee shall pay any and all delivery and installation charges. Lessor
shall not be liable to Lessee for any delay in, or failure of, delivery of the
Equipment.

          8.   RENEWAL. So long as no Event of Default or event which, with the
giving of notice, the passage of time, or both, would constitute an Event of
Default, shall have occurred and be continuing, or Lessee shall not have
exercised its purchase option under Paragraph 9 hereof, each Lease will
automatically renew for a term of twelve months (the "Renewal Term") on the
terms and conditions of this Agreement or as set forth in the applicable
Schedule; PROVIDED, HOWEVER, that Obligations due to be performed by the Lessee
during the Renewal Term shall continue until they have been performed in full.

          9.   PURCHASE OPTION. So long as no Event of Default or event which,
with the giving of notice, the passage of time, or both, would constitute an
Event of Default, shall have occurred and be continuing, Lessee may, upon
written notice to Lessor received at least sixty days before the expiration of a
Term, purchase all, but not less than all, the Equipment covered by the
applicable Lease on the date specified therefor in the applicable Schedule
("Purchase Date"). The purchase price for such Equipment shall be its Fair
Market Value to be determined as set forth in the applicable Schedule
substantially in the form attached hereto as Exhibit A. So long as no Event of
Default or event which, with the giving of notice, the passage of time, or both,
would constitute an Event of Default shall have occurred and be continuing,
Lessee may, upon written notice to Lessor received at least sixty days prior to
the expiration of the Renewal Term, purchase all, but not less than all, the
Equipment covered by the applicable Schedule by the last date of the Renewal
Term (the "Alternative Purchase Date") at a purchase price equal to its then
Fair Market Value to be determined as set forth in the applicable Schedule. On
the Purchase Date or the Alternative Purchase Date, as the case may be, for any
Equipment, Lessee shall pay to Lessor the purchase price, together with all
sales and other taxes applicable to the transfer of the Equipment and any other
amount payable and arising hereunder, in immediately available funds, whereupon
Lessor

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shall transfer to Lessee, without recourse or warranty of any kind, express or
implied, all of Lessor's right, title, and interest in and to such Equipment on
an "As Is, Where Is" basis.

          10.  OWNERSHIP; INSPECTION; MARKING; FINANCING STATEMENTS. Lessee
shall affix to the Equipment any labels supplied by Lessor indicating ownership
of such Equipment. The Equipment is and shall be the sole property of Lessor.
Lessee shall have no right, title, or interest therein, except as lessee under a
Lease. The Equipment, other than leasehold improvements financed herein, is and
shall at all times be and remain personal property and shall not become a
fixture without Lessor's prior written consent. Other than leasehold
improvements financed herein, Lessee shall use commercially reasonable efforts
obtain and record such instruments and take such steps as may be necessary to
prevent any person from acquiring any rights in the Equipment by reason of the
Equipment being claimed or deemed to be real property. Lessee shall make the
Equipment and its maintenance records available for inspection by Lessor at
reasonable times and upon reasonable notice. Lessee shall execute and deliver to
Lessor for filing any UCC financing statements or similar documents Lessor may
reasonably request.

          11.  EQUIPMENT USE. Lessee agrees that the Equipment will be operated
by competent, qualified personnel in connection with Lessee's business for the
purpose for which the Equipment was designed and in accordance with applicable
operating instructions, laws, and government regulations, and that Lessee shall
use all reasonable precautions to prevent loss or damage to the Equipment from
fire and other hazards. Lessee shall procure and maintain in effect all orders,
licenses, certificates, permits, approvals, and consents required by federal,
state, or local laws or by any governmental body, agency, or authority in
connection with the delivery, installation, use, and operation of the Equipment,
except where the failure to so procure or maintain in effect has not or could
not reasonably be expected to adversely affect Lessor's rights in such
Equipment.

          12.  MAINTENANCE. Lessee, at its sole cost and expense, shall keep the
Equipment in a suitable environment as specified by the manufacturer's
guidelines or the equivalent, shall meet all recertification requirements, and
shall maintain the Equipment in its original condition and working order,
ordinary wear and tear excepted. At the reasonable request of Lessor, Lessee
shall furnish all documentary proof of maintenance.

          13.  ALTERATION; MODIFICATIONS; PARTS. Lessee may alter or modify the
Equipment only with the prior written consent of Lessor. Any alteration shall be
removed and the Equipment restored to its normal, unaltered condition at
Lessee's expense (without damaging the Equipment's originally intended function
or its value) prior to its return to Lessor. Any part installed in connection
with warranty or maintenance service or which cannot be removed in accordance
with the preceding sentence shall be the property of Lessor.

          14.  RETURN OF EQUIPMENT. Except for Equipment that has suffered a
Casualty Loss (as defined in Paragraph 15 below) and is not required to be
repaired pursuant to Paragraph 15 below or Equipment purchased by Lessee
pursuant to Paragraph 9 above, upon the expiration of the Renewal Term of a
Lease, or upon demand by Lessor pursuant to Paragraph 22 below, Lessee shall
contact Lessor for shipping instructions and, at Lessee's own risk, promptly
return the Equipment, freight prepaid, to a location in the continental United
States specified by Lessor. At the time of such return to Lessor, the Equipment
shall (i) be in the operating order, repair, and condition as required by or
specified in the original specifications and warranties of each manufacturer and
vendor thereof, ordinary wear and tear excepted, and (ii) be capable of being
promptly assembled and operated by a third party purchaser or third party lessee
without further repair, replacement, alterations, or improvements Lessee shall
return Equipment in a condition to allow Lessor to sell or market the Equipment
in a commercial manner. Except as otherwise provided under Paragraph 9 hereof,
at least sixty days before the expiration of the Renewal Term, Lessee shall give
Lessor notice of its intent to return the Equipment at the end of such Renewal
Term. During the sixty-day period prior to the end of a Term or the Renewal
Term, Lessor and its prospective purchasers or lessees shall have, upon not less
than two business days' prior notice to Lessee and during normal business hours,
or at any time and without prior notice upon the occurrence and continuance of
an Event of Default, the right of access to the premises on which the Equipment
is located to inspect the Equipment, and Lessee shall cooperate in all other
respects with Lessor's remarketing of the Equipment. The provisions of this
Paragraph 14 are of the essence of the Lease, and upon application to any court
of equity having jurisdiction in the premises, Lessor shall be entitled to a
decree against Lessee requiring specific performance of the covenants of Lessee
set forth in this Paragraph 14. If Lessee fails to return the Equipment when
required, the terms and conditions of the Lease shall

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continue to be applicable and Lessee shall continue to pay Rent until the
Equipment is received by Lessor.

          15.  CASUALTY INSURANCE; LOSS OR DAMAGE. Lessee will maintain, at its
own expense, liability and property damage insurance relating to the Equipment,
other than leasehold improvements financed herein, insuring against such risks
as are customarily insured against on the type of equipment leased hereunder by
businesses in which Lessee is engaged in such amounts, in such form, and with
insurers satisfactory to Lessor; PROVIDED, HOWEVER, that the amount of insurance
against damage or loss shall not be less than the greater of (a) the replacement
value of the Equipment and (b) the stipulated loss value of the Equipment
specified in the applicable Schedule ("Stipulated Loss Value"). Each liability
insurance policy shall provide coverage (including, without limitation, personal
injury coverage) of not less than $1,000,000 for each occurrence, and shall name
Lessor as an additional insured with respect to such Equipment; and each
property damage policy shall name Lessor as sole loss payee with respect to such
Equipment and all policies shall contain a clause requiring the insurer to give
Lessor at least thirty days' prior written notice of any alteration in the terms
or cancellation of the policy. Lessee shall furnish to Lessor a certificate of
insurance, and upon request, a copy of each insurance policy (with endorsements)
or other evidence satisfactory to Lessor that the required insurance coverage is
in effect; PROVIDED, HOWEVER, Lessor shall have no duty to ascertain the
existence of or to examine the insurance policies to advise Lessee if the
insurance coverage does not comply with the requirements of this Paragraph. If
Lessee fails to insure the Equipment as required, other than leasehold
improvements financed herein ,Lessor shall have the right but not the obligation
to obtain such insurance, and the cost of the insurance shall be for the account
of Lessee due as part of the next due Rent. Lessee consents to Lessor's release,
upon its failure to obtain appropriate insurance coverage, of any and all
information necessary to obtain insurance with respect to the Equipment or
Lessor's interest therein.

          After the Equipment is delivered to Lessee and until the Equipment is
returned to and received by Lessor as provided in Paragraph 14 above, Lessee
shall bear the entire risk of theft or destruction of, or damage to, the
Equipment including, without limitation, any condemnation, seizure, or
requisition of title or use ("Casualty Loss"). No Casualty Loss shall relieve
Lessee from its obligations to pay Rent except as provided in clause (b) below.
When any Casualty Loss occurs, Lessee shall immediately notify Lessor and, at
the option of Lessee, as long as no Event of Default has occurred or is
continuing, shall promptly (a) place such Equipment in good repair and working
order; or (b) pay Lessor an amount equal to the Stipulated Loss Value of such
Equipment and all other amounts (excluding Rent) due hereunder, whereupon Lessor
shall transfer to Lessee, without recourse or warranty (express or implied), all
of Lessor's interest, if any, in and to such Equipment on an "AS IS, WHERE IS"
basis. The proceeds of any insurance payable with respect to the Equipment shall
be applied, at the option of Lessor, either towards (i) repair of the Equipment
or (ii) payment of any of Lessee's obligations hereunder. Lessee hereby appoints
Lessor as Lessee's attorney-in-fact to make claim for, receive payment of, and
execute and endorse all documents, checks or drafts issued with respect to any
Casualty Loss under any insurance policy relating to the Equipment, as long as
no Event of Default has occurred or is continuing.

          16.  TAXES. Lessee shall pay when due, and indemnify and hold Lessor
harmless from, all sales, use, excise, and other taxes, charges, and fees
(including, without limitation, income, franchise, business and occupation,
gross receipts, licensing, registration, titling, personal property, stamp and
interest equalization taxes, levies, imposts, duties, charges, or withholdings
of any nature), and any fines, penalties, or interest thereon, imposed or levied
by any governmental body, agency, or tax authority upon or in connection with
the Equipment, its purchase, ownership, delivery, leasing, possession, use, or
relocation of the Equipment or otherwise in connection with the transactions
contemplated by each Lease or the Rent thereunder, excluding taxes on or
measured by the net income of Lessor. Upon request, Lessee will provide proof of
payment. Unless Lessor elects otherwise, Lessee will pay all property taxes on
the Equipment. Lessee shall timely prepare and file all reports and returns
which are required to be made with respect to any obligation of Lessee under
this Paragraph 16. Lessee shall, to the extent permitted by law, cause all
billings of such fees, taxes, levies, imposts, duties, withholdings, and
governmental charges to be made to Lessor in care of Lessee. Upon request,
Lessee will provide Lessor with copies of all such billings.

          17.  LESSOR'S PAYMENT. If Lessee fails to perform its obligations
under Paragraph 15 or 16 above, or Paragraph 23 below, Lessor shall have the
right to substitute performance, in which case Lessee shall immediately
reimburse Lessor therefor.

          18.  GENERAL INDEMNITY. Each Lease is a net lease. Therefore, Lessee
shall indemnify Lessor and its successors and assigns against, and hold Lessor
and its successors and assigns harmless

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from, any and all claims, actions, damages, obligations, liabilities, and all
costs and expenses, including, without limitation, reasonable legal fees
incurred by Lessor or its successors and assigns arising out of each Lease
including, without limitation, the purchase, lease, possession, maintenance,
condition, use, or return of the Equipment, or arising by operation of law,
except that Lessee shall not be liable for any claims, actions, damages,
obligations, and costs and expenses determined by a non-appealable, final order
of a court of competent jurisdiction to have occurred as a result of the gross
negligence or willful misconduct of Lessor or its successors and assigns. Lessee
agrees that upon written notice by Lessor of the assertion of any claim, action,
damage, obligation, liability, or lien, Lessee shall assume full responsibility
for the defense thereof, provided that Lessor's failure to give such notice
shall not limit or otherwise affect its rights hereunder. The provisions of this
Paragraph with regard to matters arising during a Lease shall survive the
expiration or termination of such Lease.

          19.  ASSIGNMENT BY LESSEE. Lessee shall not, without the prior written
consent of Lessor, which consent will not be unreasonably withheld (a) assign,
transfer, pledge, or otherwise dispose of any Lease or Equipment, or any
interest therein; (b) sublease or lend any Equipment or permit it to be used by
anyone other than Lessee and its employees, consultants and contractors; or (c)
move any Equipment from the location specified for it in the applicable
Schedule, except that Lessee may move Equipment to another location within the
United States provided that Lessee has delivered to Lessor (A) prior written
notice thereof and (B) duly executed financing statements and other agreements
and instruments (all in form and substance satisfactory to Lessor) necessary or,
in the opinion of the Lessor, desirable to protect Lessor's interest in such
Equipment. In the event Lessee takes any such action without the prior written
consent of Lessor, which consent shall not be unreasonably withheld, the parties
agree that Lessor may terminate any Lease and all applicable Schedules thereto,
and Lessee shall promptly pay Lessor all amounts then due thereunder, including
prepayment, if any, without further obligation to Lessor thereto.
Notwithstanding anything to the contrary in the immediately preceding sentence,
Lessee may keep any Equipment consisting of motor vehicles or rolling stock at
any location in the United States.

          20.  ASSIGNMENT BY LESSOR. Lessor may assign its interest or grant a
security interest in any Lease and the Equipment individually or together, in
whole or in part. If Lessee is given written notice of any such assignment, it
shall after thirty days of its receipt of such notice make all payments of Rent
and other amounts hereunder directly to such assignee. Each such assignee shall
have all of the rights of Lessor under each Lease assigned to it. Lessee shall
not assert against any such assignee any set-off, defense, or counterclaim that
Lessee may have against Lessor or any other person.

          21.  DEFAULT; NO WAIVER. Lessee or any guarantor of any or all of the
obligations of Lessee hereunder (together with Lessee, the "Lease Parties")
shall be in default under each Lease upon the occurrence of any of the following
events (each, an "Event of Default"): (a) Lessee fails to pay within five
business days of when due any amount required to be paid by Lessee under or in
connection with any Lease; (b) any of the Lease Parties fails to perform any
other material provision under or in connection with a Lease or violates any of
the material covenants or agreements of such Lease Party under or in connection
with a Lease; (c) any representation made or financial information delivered or
furnished by any of the Lease Parties under or in connection with a Lease shall
prove to have been inaccurate in any material respect when made; (d) any of the
Lease Parties makes an assignment for the benefit of creditors, whether
voluntary or involuntary, or consents to the appointment of a trustee or
receiver, or if either shall be appointed for any of the Lease Parties or for a
substantial part of its property without its consent and, in the case of any
such involuntary proceeding, such proceeding remains undismissed or unstayed for
forty-five days following the commencement thereof; (e) any petition or
proceeding is filed by or against any of the Lease Parties under any Federal or
State bankruptcy or insolvency code or similar law and, in the case of any such
involuntary petition or proceeding, such petition or proceeding remains
undismissed or unstayed for forty-five days following the filing or commencement
thereof, or any of the Lease Parties takes any action authorizing any such
petition or proceeding; (f) any of the Lease Parties fails to pay when due any
indebtedness for borrowed money or under conditional sales or installment sales
contracts or similar agreements, leases, or obligations evidenced by bonds,
debentures, notes, or other similar agreements or instruments to any creditor
(including Lessor under any other agreement) after any and all applicable cure
periods therefor shall have elapsed; (g) any judgment shall be rendered against
any of the Lease Parties which shall remain unpaid or unstayed for a period of
sixty days; (h) any of the Lease Parties shall dissolve, liquidate, wind up or
cease its business, sell or otherwise dispose of all or substantially all of its
assets, or make any material change in its lines of business; (i) any of the
Lease Parties shall amend or modify its name, unless such Lease Party delivers
to Lessor, thirty days prior to any such proposed amendment or modification,
written notice of such amendment or modification and within ten days before such

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amendment or modification delivers executed financing statements (in form and
substance satisfactory to the Lessor); (j) any of the Lease Parties shall merge
or consolidate with any other entity or make any material change in its capital
structure, in each case without Lessor's prior written consent, which shall not
be unreasonably withheld; (k) any of the Lease Parties shall suffer any loss or
suspension of any material license, permit, or other right or asset necessary to
the profitable conduct of its business, fail generally to pay its debts as they
mature, or call a meeting for purposes of compromising its debts; (l) any of the
Lease Parties shall deny or disaffirm its obligations hereunder or under any of
the documents delivered in connection herewith; (m) there is a change, which
change results from a single transaction or series of related transactions, but
not from the sale of newly issued securities to investors, in more than 35% of
the ownership of any equity interests of any of the Lease Parties on the date
hereof or more than 35% of such interests become subject to any contractual,
judicial or statutory lien, charge, security interest, or encumbrance; or (n)
any of the Lease Parties suffers a material adverse change in the business,
operations, results of operations, assets, liabilities, or condition (financial
or otherwise).

          22.  REMEDIES. Upon the occurrence and continuation of an Event of
Default, Lessor shall have the right, in its sole discretion, to exercise any
one or more of the following remedies: (a) terminate each Lease; (b) declare any
and all Rent and other amounts then due and any and all Rent and other amounts
to become due under each Lease (collectively, the "Lease Obligations")
immediately due and payable; (c) take possession of any or all items of
Equipment, wherever located, without demand, notice, court order, or other
process of law, and without liability for entry to Lessee's premises, for damage
to Lessee's property, or otherwise; (d) demand that Lessee immediately return
any or all Equipment to Lessor in accordance with Paragraph 14 above, and, for
each day that Lessee shall fail to return any item of Equipment, Lessor may
demand an amount equal to the Rent payable for such Equipment in accordance with
Paragraph 14 above; (e) lease, sell, or otherwise dispose of the Equipment in a
commercially reasonable manner, with or without notice and on public or private
bid; and (f) recover the following amounts from the Lessee (as damages,
including reimbursement of costs and expenses, liquidated for all purposes and
not as a penalty): (i) all costs and expenses of Lessor reimbursable to it
hereunder, including, without limitation, expenses of disposition of the
Equipment, reasonable legal fees, and all other amounts specified in Paragraph
23 below; (ii) an amount equal to the sum of (A) any accrued and unpaid Rent
through the later of (1) the date of the applicable default, (2) the date that
Lessor has obtained possession of the Equipment, or (3) such other date as
Lessee has made an effective tender of possession of the Equipment to Lessor
(the "Default Date") and (B) if Lessor resells or re-lets the Equipment, Rent at
the periodic rate provided for in each Lease for the additional period that it
takes Lessor to resell or re-let all of the Equipment; (iii) the present value
of all future Rent reserved in the Leases and contracted to be paid over the
unexpired Term of the Leases discounted at the rate of interest implicit in the
applicable Schedule interest; (iv) the reversionary value of the Equipment as of
the expiration of the Term of the applicable Lease as set forth on the
applicable Schedule; and (v) any indebtedness for Lessee's indemnity under
Paragraph 18 above, plus a late charge at the rate specified in Paragraph 3
above, less the amount received by Lessor, if any, upon sale or re-let of the
Equipment; and (g) exercise any other right or remedy to recover damages or
enforce the terms of the Leases. Upon the occurrence and continuance of an Event
of Default or an event which with the giving of notice or the passage of time,
or both, would result in an Event of Default, Lessor shall have the right,
whether or not Lessor has made any demand or the obligations of Lessee hereunder
have matured, to appropriate and apply to the payment of the obligations of
Lessee hereunder all security deposits and other deposits (general or special,
time or demand, provisional or final) now or hereafter held by and other
indebtedness or property now or hereafter owing by Lessor to Lessee. Lessor may
pursue any other rights or remedies available at law or in equity, including,
without limitation, rights or remedies seeking damages, specific performance,
and injunctive relief. Any failure of Lessor to require strict performance by
Lessee, or any waiver by Lessor of any provision hereunder or under any
Schedule, shall not be construed as a consent or waiver of any other breach of
the same or of any other provision. Any amendment or waiver of any provision
hereof or under any Schedule or consent to any departure by Lessee herefrom or
therefrom shall be in writing and signed by Lessor.

          No right or remedy is exclusive of any other provided herein or
permitted by law or equity. All such rights and remedies shall be cumulative and
may be enforced concurrently or individually from time to time.

          Upon the occurrence of an Event of Default, Lessor shall use
commercially reasonable efforts to mitigate its damages described above.

          23.  LESSOR'S EXPENSE. Lessee shall pay Lessor on demand all costs and
expenses (including legal fees and expenses) incurred in connection with the
preparation, execution and delivery of this Agreement and other agreement and
transaction contemplated hereby and all costs and expenses in protecting and

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enforcing Lessor's rights and interests in each Lease and the Equipment,
including, without limitation, reasonable legal and collection, reasonable
remarketing fees and expenses incurred by Lessor in enforcing the terms,
conditions, or provisions of each Lease or upon the occurrence and continuation
of an Event of Default. Legal expenses shall not exceed $10,000 without the
prior written consent of Lessee.

          24.  LESSEE'S WAIVERS. To the extent permitted by applicable law,
Lessee hereby waives any and all rights and remedies conferred upon a lessee by
Sections 2A-508 through 2A-522 of the UCC. To the extent permitted by applicable
law, Lessee also hereby waives any rights now or hereafter conferred by statute
or otherwise which may limit or modify any of Lessor's rights or remedies under
Paragraph 22. Any action by Lessee against Lessor for any default by Lessor
under any Lease shall be commenced within one year after Lessee's notice of any
such cause of action.

          25.  NOTICES; ADMINISTRATION. Except as otherwise provided herein, all
notices, approvals, consents, correspondence, or other communications required
or desired to be given hereunder shall be given in writing and shall be
delivered by overnight courier, hand delivery, or certified or registered mail,
postage prepaid, if to Lessor, then to Transamerica Technology Finance Division,
76 Batterson Park Road, Farmington, Connecticut 06032, Attention: Legal
Department, with a copy to Lessor at Riverway II, West Office Tower, 9399 West
Higgins Road, Rosemont, Illinois 60018, Attention: Legal Department, if to
Lessee, then to Acusphere, Inc., University Park @ MIT, 38 Sidney Street,
Cambridge, Massachusetts 02139, Attention: Vice President of Finance and
Administration or such other address as shall be designated by Lessee or Lessor
to the other party. All such notices and correspondence shall be effective when
received.

          26.  REPRESENTATIONS. Lessee represents and warrants to Lessor that
(a) Lessee is duly organized, validly existing, and in good standing under the
laws of the State of its incorporation; (b) the execution, delivery, and
performance by Lessee of this Agreement are within Lessee's powers, have been
duly authorized by all necessary action, and do not and will not contravene (i)
Lessee's organizational documents or (ii) any law, regulation, rule, or
contractual restriction binding on or affecting Lessee; (c) no authorization or
approval or other action by, and no notice to or filing with, any governmental
authority or regulatory body is required for the due execution, delivery, and
performance by Lessee of this Agreement; (d) each Lease constitutes the legal,
valid, and binding obligations of Lessee enforceable against Lessee in
accordance with its terms; (e) the cost of each item of Equipment reflects all
discounts, rebates, and allowances for the Equipment (including, without
limitation, discounts for advertising, prompt payment, testing, or other
services) given to the Lessee by the manufacturer, supplier, or any other
person; and (f) all information supplied by Lessee to Lessor in connection
herewith is, to its knowledge, correct and does not omit any material statement
necessary to insure that the information supplied is not misleading.

          27.  FURTHER ASSURANCES. Lessee, upon the request of Lessor, will
execute, acknowledge, record, or file, as the case may be, such further
documents and do such further acts as may be reasonably necessary, desirable, or
proper to carry out more effectively the purposes of this Agreement. Lessee
hereby appoints Lessor as its attorney-in-fact to execute on behalf of Lessee
and authorizes Lessor to file without Lessee's signature any UCC financing
statements and amendments Lessor deems advisable.

          28.  FINANCIAL STATEMENTS. Lessee shall deliver to Lessor: (a) as soon
as available, but not later than 120 days after the end of each fiscal year of
Lessee and its consolidated subsidiaries, the consolidated balance sheet, income
statement, and statements of cash flows and shareholders equity for Lessee and
its consolidated subsidiaries (the "Financial Statements") for such year,
reported on by independent certified public accountants without an adverse
qualification; and (b) as soon as available, but not later than 60 days after
the end of each of the first three fiscal quarters in any fiscal year of Lessee
and its consolidated subsidiaries, the Financial Statements for such fiscal
quarter, together with a certification duly executed by a responsible officer of
Lessee that such Financial Statements have been prepared in accordance with
generally accepted accounting principles and are fairly stated in all material
respects (subject to normal year-end audit adjustments).

          29.  CONSENT TO JURISDICTION. Lessee irrevocably submits to the
jurisdiction of any Illinois state or federal court sitting in Illinois for any
action or proceeding arising out of or relating to this Agreement or the
transactions contemplated hereby, and Lessee irrevocably agrees that all claims
in respect of any such action or proceeding may be heard and determined in such
Illinois state or federal court.

<PAGE>

          30.  WAIVER OF JURY TRIAL. LESSEE AND LESSOR IRREVOCABLY WAIVE ALL
RIGHT TO TRIAL BY JURY IN ANY ACTION, PROCEEDING, OR COUNTERCLAIM ARISING OUT OF
OR RELATING TO THIS AGREEMENT OR THE TRANSACTIONS CONTEMPLATED HEREBY.

          31.  FINANCE LEASE. Lessee and Lessor agree that each Lease is a
"Finance Lease" as defined by Section 2A-103(g) of the UCC. Lessee acknowledges
that Lessee has reviewed and approved each written Supply Contract (as defined
by UCC 2A-103(y)) covering Equipment purchased from each "Supplier" (as defined
by UCC 2A-103(x)) thereof.

          32.  NO AGENCY. Lessee acknowledges and agrees that neither the
manufacturer or supplier, nor any salesman, representative, or other agent of
the manufacturer or supplier, is an agent of Lessor. No salesman,
representative, or agent of the manufacturer or supplier is authorized to waive
or alter any term or condition of this Agreement or any Schedule and no
representation as to the Equipment or any other matter by the manufacturer or
supplier shall in any way affect Lessee's duty to pay Rent and perform its other
obligations as set forth in this Agreement or any Schedule.

          33.  SPECIAL TAX INDEMNIFICATION. Lessee acknowledges that Lessor, in
determining the Rent due hereunder, has assumed that certain tax benefits as are
provided to an owner of property under the Internal Revenue Code of 1986, as
amended (the "Code"), and under applicable state tax law, including, without
limitation, depreciation deductions under Section 168(b) of the Code, and
deductions under Section 163 of the Code in an amount at least equal to the
amount of interest paid or accrued by Lessor with respect to any indebtedness
incurred by Lessor in financing its purchase of the Equipment, are available to
Lessor as a result of the lease of the Equipment. In the event Lessor is unable
to obtain such tax benefits as a result of an act or omission of Lessee, is
required to include in income any amount other than the Rent, or is required to
recognize income in respect of the Rent earlier than anticipated pursuant to
this Agreement, Lessee shall pay Lessor additional rent ("Additional Rent") in a
lump sum in an amount needed to provide Lessor with the same after-tax yield and
after-tax cash flow as would have been realized by Lessor had Lessor (i) been
able to obtain such tax benefits, (ii) not been required to include any amount
in income other than the Rent, and (iii) not been required to recognize income
in respect of the Rent earlier than anticipated pursuant to this Agreement. The
Additional Rent shall be computed by Lessor, which computation shall be binding
on Lessee. The Additional Rent shall be due immediately upon written notice by
Lessor to Lessee of Lessor's inability to obtain tax benefits, the inclusion of
any amount in income other than the Rent or the recognition of income in respect
of the Rent earlier than anticipated pursuant to this Agreement. The provisions
of this Paragraph 33 shall survive the termination of this Agreement.

          34.  GOVERNING LAW; SEVERABILITY. EACH LEASE SHALL BE GOVERNED BY THE
LAWS OF THE STATE OF ILLINOIS WITHOUT GIVING EFFECT TO THE CONFLICT OF LAW
PRINCIPLES THEREOF. IF ANY PROVISION SHALL BE HELD TO BE INVALID OR
UNENFORCEABLE, THE VALIDITY AND ENFORCEABILITY OF THE REMAINING PROVISIONS SHALL
NOT IN ANY WAY BE AFFECTED OR IMPAIRED.

LESSEE ACKNOWLEDGES THAT LESSEE HAS READ THIS AGREEMENT AND THE SCHEDULE HERETO,
UNDERSTANDS THEM, AND AGREES TO BE BOUND BY THEIR TERMS AND CONDITIONS. FURTHER,
LESSEE AND LESSOR AGREE THAT THIS AGREEMENT, THE SCHEDULES DELIVERED IN
CONNECTION HEREWITH FROM TIME TO TIME, AND THE COMMITMENT LETTER ARE THE
COMPLETE AND EXCLUSIVE STATEMENT OF THE AGREEMENT BETWEEN THE PARTIES,
SUPERSEDING ALL PROPOSALS OR PRIOR AGREEMENTS, ORAL OR WRITTEN, AND ALL OTHER
COMMUNICATIONS BETWEEN THE PARTIES RELATING TO THE SUBJECT MATTER HEREOF. SHOULD
THERE EXIST ANY INCONSISTENCY BETWEEN THE TERMS OF THE COMMITMENT LETTER AND
THIS AGREEMENT, THE TERMS OF THIS AGREEMENT SHALL PREVAIL.

<PAGE>

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

                                   ACUSPHERE, INC.

                                   By: /s/ Sherri C. Oberg
                                      ------------------------------------------
                                      Name: Sherri C. Oberg
                                      Title: President and CEO
                                   Federal Tax ID No.:

                                   ACUSPHERE SECURITIES CORPORATION.

                                   By: /s/ Sherri C. Oberg
                                      ------------------------------------------
                                      Name: Sherri C. Oberg
                                      Title: President and CEO
                                   Federal Tax ID No.:

                                   TRANSAMERICA BUSINESS CREDIT CORPORATION

                                   By: /s/ Matthew N. McAlpre
                                      ------------------------------------------
                                      Name: Matthew N. McAlpre
                                      Title: SVP<PAGE>
                                                                   Exhibit 10.19

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, OR ANY STATE SECURITIES LAWS. THEY MAY NOT BE SOLD OR OFFERED FOR SALE
IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO THE SECURITIES UNDER
SAID ACT AND ANY APPLICABLE STATE SECURITIES LAWS OR AN OPINION OF COUNSEL
SATISFACTORY TO THE COMPANY AS TO THE AVAILABILITY OF AN EXEMPTION FROM
REGISTRATION UNDER SAID ACT AND ANY APPLICABLE STATE SECURITIES LAWS.

                                       NO.
                           STOCK SUBSCRIPTION WARRANT

         TO PURCHASE SERIES F NON-VOTING CONVERTIBLE PREFERRED STOCK OF

                         ACUSPHERE, INC. (THE "COMPANY")

                   DATE OF INITIAL ISSUANCE: FEBRUARY 21, 2001

     THIS CERTIFIES THAT for value received, TBCC FUNDING TRUST I or its
registered assigns (hereinafter called the "Holder") is entitled to purchase
from the Company, at any time during the Term of this Warrant, Twenty-Five
Thousand Two Hundred Sixty-Four (25,264) shares of Series F Non-Voting
Convertible Preferred Stock, $0.01 par value, of the Company (the "Preferred
Stock"), at the Warrant Price, payable as provided herein. The exercise of this
Warrant shall be subject to the provisions, limitations and restrictions herein
contained, and may be exercised in whole or in part.

SECTION 1. DEFINITIONS.

     For all purposes of this Warrant, the following terms shall have the
meanings indicated:

     COMMON STOCK - shall mean and include the Company's authorized Common
Stock, $0.01 par value, as constituted at the date hereof.

     EXCHANGE ACT - shall mean the Securities Exchange Act of 1934, as amended
from time to time.

     SECURITIES ACT - shall mean the Securities Act of 1933, as amended.

     TERM OF THIS WARRANT - shall mean the period beginning on the date of
initial issuance hereof and ending on February 21, 2008.

     WARRANT PRICE - $4.75 per share, subject to adjustment in accordance with
Section 5 hereof.

     WARRANTS - this Warrant issued in connection with a Commitment Letter dated
December 5, 2000 executed by the Company and Transamerica Business Credit
Corporation (the "Commitment Letter") to the original holder of this Warrant, or
any transferees from such original holder or this Holder.

<PAGE>

     WARRANT SHARES - shares of Preferred Stock purchased or purchasable by the
Holder of this Warrant upon the exercise hereof.

SECTION 2. EXERCISE OF WARRANT.

     2.1. PROCEDURE FOR EXERCISE OF WARRANT. To exercise this Warrant in whole
or in part (but not as to any fractional share of Preferred Stock), the Holder
shall deliver to the Company at its office referred to in Section 11 hereof at
any time and from time to time during the Term of this Warrant: (i) the Notice
of Exercise in the form attached hereto, (ii) cash, certified or official bank
check payable to the order of the Company, wire transfer of funds to the
Company's account, or evidence of any indebtedness of the Company to the Holder
(or any combination of any of the foregoing) in the amount of the Warrant Price
for each share being purchased, and (iii) this Warrant. Notwithstanding any
provisions herein to the contrary, if the Current Market Price (as defined in
Section 5) of one share of Common Stock is greater than the Warrant Price (at
the date of calculation, as set forth below), in lieu of exercising this Warrant
as hereinabove permitted, the Holder may elect to receive shares of Preferred
Stock equal to the value (as determined below) of this Warrant (or the portion
thereof being canceled) by surrender of this Warrant at the office of the
Company referred to in Section 11 hereof, together with the Notice of Exercise,
in which event the Company shall issue to the Holder that number of shares of
Preferred Stock computed using the following formula:

                               PS = WPS x (CMP-WP)
                                    --------------
                                         CMP

Where

     PS        equals the number of shares of Preferred Stock to be issued to
               the Holder

     WPS       equals the number of shares of Preferred Stock purchasable under
               the Warrant or, if only a portion of the Warrant is being
               exercised, the portion of the Warrant being exercised (at the
               date of such calculation)

     CMP       equals the Current Market Price of one share of Common Stock (at
               the date of such calculation)

     WP        equals the Warrant Price (as adjusted to the date of such
               calculation)

In the event of any exercise of the rights represented by this Warrant, a
certificate or certificates for the shares of Preferred Stock so purchased,
registered in the name of the Holder or such other name or names as may be
designated by the Holder, shall be delivered to the Holder hereof within a
reasonable time, not exceeding fifteen (15) days, after the rights represented
by this Warrant shall have been so exercised; and, unless this Warrant has
expired, a new Warrant representing the number of shares (except a remaining
fractional share), if any, with respect to which this Warrant shall not then
have been exercised shall also be issued to the Holder hereof within such time.
The person in whose name any certificate for shares of Preferred Stock is issued
upon exercise of this Warrant shall for all purposes be deemed to have become
the holder of record of such shares on the date on which the Warrant was
surrendered and payment of the Warrant Price and any applicable taxes was made,
irrespective of the date of delivery of such certificate, except that, if the
date of such surrender and payment is a date when the stock transfer

                                     - 2 -
<PAGE>

books of the Company are closed, such person shall be deemed to have become the
holder of such shares at the close of business on the next succeeding date on
which the stock transfer books are open.

     2.2. TRANSFER RESTRICTION LEGEND. Each certificate for Warrant Shares shall
bear the following legend (and any additional legend required by (i) any
applicable state securities laws and (ii) any securities exchange upon which
such Warrant Shares may, at the time of such exercise, be listed) on the face
thereof unless at the time of exercise such Warrant Shares shall be registered
under the Securities Act:

     "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER
     THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD OR TRANSFERRED
     IN THE ABSENCE OF SUCH REGISTRATION OR AN OPINION OF COUNSEL SATISFACTORY
     TO THE COMPANY AS TO AN EXEMPTION THEREFROM UNDER SAID ACT."

Any certificate issued at any time in exchange or substitution for any
certificate bearing such legend (except a new certificate issued upon completion
of a public distribution under a registration statement of the securities
represented thereby) shall also bear such legend unless, in the opinion of
counsel for the holder thereof (which counsel shall be reasonably satisfactory
to counsel for the Company) the securities represented thereby are not, at such
time, required by law to bear such legend.

SECTION 3. COVENANTS AS TO PREFERRED STOCK. The Company covenants and agrees
that all shares of Preferred Stock that may be issued upon the exercise of the
rights represented by this Warrant will, upon issuance, be validly issued, fully
paid and nonassessable, and free from all liens and charges with respect to the
issue thereof. The Company further covenants and agrees that it will pay when
due and payable any and all federal and state taxes which may be payable in
respect of the issue of this Warrant or any Preferred Stock or certificates
therefor issuable upon the exercise of this Warrant. The Company further
covenants and agrees that the Company will at all times have authorized and
reserved, free from preemptive rights, a sufficient number of shares of
Preferred Stock to provide for the exercise of the rights represented by this
Warrant.

SECTION 4. ADJUSTMENT OF NUMBER OF SHARES. Upon each adjustment of the Warrant
Price as provided in Section 5, the Holder shall thereafter be entitled to
purchase, at the Warrant Price resulting from such adjustment, the number of
shares (calculated to the nearest tenth of a share) obtained by multiplying the
Warrant Price in effect immediately prior to such adjustment by the number of
shares purchasable pursuant hereto immediately prior to such adjustment and
dividing the product thereof by the Warrant Price resulting from such
adjustment.

SECTION 5. ADJUSTMENT OF WARRANT PRICE. The Warrant Price shall be subject to
adjustment from time to time as follows:

     (i)  Antidilution rights applicable to the Preferred Stock purchaseable
hereunder are as set forth in the Company's Certificate of Incorporation, as
amended and in effect from time to time (the "Charter"). The Company shall
promptly provide the Holder with any restatement, amendment, modification or
waiver of the Charter.

     (ii) If, at any time during the Term of this Warrant, the number of shares
of Preferred Stock outstanding is increased by a stock dividend payable in
shares of Preferred Stock or by a subdivision or

                                     - 3 -
<PAGE>

split-up of shares of Preferred Stock, then, following the record date fixed for
the determination of holders of Preferred Stock entitled to receive such stock
dividend, subdivision or split-up, the Warrant Price shall be appropriately
decreased so that the number of shares of Preferred Stock issuable upon the
exercise hereof shall be increased in proportion to such increase in outstanding
shares.

     (iii) If, at any time during the Term of this Warrant, the number of shares
of Preferred Stock outstanding is decreased by a combination of the outstanding
shares of Preferred Stock, then, following the record date for such combination,
the Warrant Price shall appropriately increase so that the number of shares of
Preferred Stock issuable upon the exercise hereof shall be decreased in
proportion to such decrease in outstanding shares.

     (iv) For the purpose of any computation pursuant to this Agreement, the
Current Market Price at any date of one share of Common Stock shall be deemed to
be the average of the daily closing prices for the 15 consecutive business days
ending on the last business day before the day in question (as adjusted for any
stock dividend, split, combination or reclassification that took effect during
such 15 business day period). The closing price for each day shall be the last
reported sales price regular way or, in case no such reported sales took place
on such day, the average of the last reported bid and asked prices regular way,
in either case on the principal national securities exchange on which the Common
Stock is listed or admitted to trading or as reported by Nasdaq (or if the
Common Stock is not at the time listed or admitted for trading on any such
exchange or if prices of the Common Stock are not reported by Nasdaq then such
price shall be equal to the average of the last reported bid and asked prices on
such day as reported by The National Quotation Bureau Incorporated or any
similar reputable quotation and reporting service, if such quotation is not
reported by The National Quotation Bureau Incorporated); provided, however, that
if the Common Stock is not traded in such manner that the quotations referred to
in this clause (v) are available for the period required hereunder, the Current
Market Price shall be determined in good faith by the Board of Directors of the
Company or, if such determination cannot be made, by a nationally recognized
independent investment banking firm selected by the Board of Directors of the
Company (or if such selection cannot be made, by a nationally recognized
independent investment banking firm selected by the American Arbitration
Association in accordance with its rules).

     (v) Whenever the Warrant Price shall be adjusted as provided in this
Section 5, the Company shall prepare a statement showing the facts requiring
such adjustment and the Warrant Price that shall be in effect after such
adjustment. The Company shall cause a copy of such statement to be sent by mail,
first class postage prepaid, to each Holder of this Warrant at its, his or her
address appearing on the Company's records. Where appropriate, such copy may be
given in advance and may be included as part of the notice required to be mailed
under the provisions of subsection (vii) of this Section 5.

     (vi) Adjustments made pursuant to clauses (ii) and (iii) above shall be
made on the date such dividend, subdivision, split-up, combination or
distribution, as the case may be, is made, and shall become effective at the
opening of business on the business day next following the record date for the
determination of stockholders entitled to such dividend, subdivision, split-up,
combination or distribution.

     (vii) In the event the Company shall propose to take any action of the
types described in clauses (ii) and (iii) of this Section 5, the Company shall
forward, at the same time and in the same manner, to the Holder of this Warrant
such notice, if any, which the Company shall give to the holders of capital
stock of the Company.

                                     - 4 -
<PAGE>

     (viii) In any case in which the provisions of this Section 5 shall require
that an adjustment shall become effective immediately after a record date for an
event, the Company may defer until the occurrence of such event issuing to the
Holder of all or any part of this Warrant which is exercised after such record
date and before the occurrence of such event the additional shares of capital
stock issuable upon such exercise by reason of the adjustment required by such
event over and above the shares of capital stock issuable upon such exercise
before giving effect to such adjustment exercise; provided, however, that the
Company shall deliver to such Holder a due bill or other appropriate instrument
evidencing such Holder's right to receive such additional shares upon the
occurrence of the event requiring such adjustment.

SECTION 6. OWNERSHIP.

     6.1. OWNERSHIP OF THIS WARRANT. The Company may deem and treat the person
in whose name this Warrant is registered as the holder and owner hereof
(notwithstanding any notations of ownership or writing hereon made by anyone
other than the Company) for all purposes and shall not be affected by any notice
to the contrary until presentation of this Warrant for registration of transfer
as provided in this Section 6.

     6.2. TRANSFER AND REPLACEMENT. This Warrant and all rights hereunder are
transferable in whole or in part upon the books of the Company by the Holder
hereof in person or by duly authorized attorney, and a new Warrant or Warrants,
of the same tenor as this Warrant but registered in the name of the transferee
or transferees (and in the name of the Holder, if a partial transfer is
effected) shall be made and delivered by the Company upon surrender of this
Warrant duly endorsed, at the office of the Company referred to in Section 12
hereof. Upon receipt by the Company of evidence reasonably satisfactory to it of
the loss, theft or destruction, and, in such case, of indemnity or security
reasonably satisfactory to it, and upon surrender of this Warrant if mutilated,
the Company will make and deliver a new Warrant of like tenor, in lieu of this
Warrant; provided that if the Holder hereof is an instrumentality of a state or
local government or an institutional holder or a nominee for such an
instrumentality or institutional holder an irrevocable agreement of indemnity by
such Holder shall be sufficient for all purposes of this Section 6, and no
evidence of loss or theft or destruction shall be necessary. This Warrant shall
be promptly cancelled by the Company upon the surrender hereof in connection
with any transfer or replacement. Except as otherwise provided above, in the
case of the loss, theft or destruction of a Warrant, the Company shall pay all
expenses, taxes and other charges payable in connection with any transfer or
replacement of this Warrant, other than stock transfer taxes (if any) payable in
connection with a transfer of this Warrant, which shall be payable by the
Holder. Holder will not transfer this Warrant and the rights hereunder except in
compliance with federal and state securities laws.

SECTION 7. MERGERS, CONSOLIDATION, SALES. In the case of any proposed
consolidation or merger of the Company with another entity, or the proposed sale
of all or substantially all of its assets to another person or entity, or any
proposed reorganization or reclassification of the capital stock of the Company,
then, as a condition of such consolidation, merger, sale, reorganization or
reclassification, lawful and adequate provision shall be made whereby the Holder
of this Warrant shall thereafter have the right to receive upon the basis and
upon the terms and conditions specified herein, in lieu of the shares of the
Preferred Stock of the Company immediately theretofore purchasable hereunder,
such shares of stock, securities or assets as may (by virtue of such
consolidation, merger, sale, reorganization or reclassification) be issued or
payable with respect to or in exchange for the number of shares of such
Preferred Stock purchasable hereunder immediately before such consolidation,
merger, sale, reorganization or reclassification. In any such case appropriate
provision shall be made with respect to

                                     - 5 -
<PAGE>

the rights and interests of the Holder of this Warrant to the end that the
provisions hereof shall thereafter be applicable as nearly as may be, in
relation to any shares of stock, securities or assets thereafter deliverable
upon the exercise of this Warrant.

SECTION 8. NOTICE OF DISSOLUTION OR LIQUIDATION. In case of any distribution of
the assets of the Company in dissolution or liquidation (except under
circumstances when the foregoing Section 7 shall be applicable), the Company
shall give notice thereof to the Holder hereof and shall make no distribution to
shareholders until the expiration of thirty (30) days from the date of mailing
of the aforesaid notice and, in any case, the Holder hereof may exercise this
Warrant within thirty (30) days from the date of the giving of such notice, and
all rights herein granted not so exercised within such thirty-day period shall
thereafter become null and void.

SECTION 9. FRACTIONAL SHARES. Fractional shares shall not be issued upon the
exercise of this Warrant but in any case where the Holder would, except for the
provisions of this Section 9, be entitled under the terms hereof to receive a
fractional share upon the complete exercise of this Warrant, the Company shall,
upon the exercise of this Warrant for the largest number of whole shares then
called for, pay a sum in cash equal to the excess of the value of such
fractional share (determined in such reasonable manner as may be prescribed in
good faith by the Board of Directors of the Company) over the Warrant Price for
such fractional share, calculated to the nearest cent.

SECTION 10. SPECIAL ARRANGEMENTS OF THE COMPANY. The Company covenants and
agrees that during the Term of this Warrant, unless otherwise approved by the
Holder of this Warrant:

     10.1. WILL RESERVE SHARES. The Company will reserve and set apart and have
available for issuance at all times, free from preemptive or other preferential
rights, the number of shares of authorized but unissued Preferred Stock
deliverable upon the exercise of this Warrant.

     10.2. WILL NOT AMEND CERTIFICATE. The Company will not amend its
Certificate of Incorporation to eliminate as an authorized class of capital
stock that class denominated as "Series F Non-Voting Convertible Preferred
Stock" on the date hereof.

     10.3. WILL BIND SUCCESSORS. This Warrant shall be binding upon any
corporation or other person or entity succeeding to the Company by merger,
consolidation or acquisition of all or substantially all of the Company's
assets.

SECTION 11. NOTICES. Any notice or other document required or permitted to be
given or delivered to the Holder shall be delivered at, or sent by certified or
registered mail to, the Holder at Transamerica Technology Finance Division, 76
Batterson Park Road, Farmington, Connecticut 06032, Attention: Legal Department,
with a copy to the Lender at Riverway II, West Office Tower, 9399 West Higgins
Road, Rosemont, Illinois 60018, Attention: Legal Department or to such other
address as shall have been furnished to the Company in writing by the Holder.
Any notice or other document required or permitted to be given or delivered to
the Company shall be delivered at, or sent by certified or registered mail to,
the Company at Acusphere, Inc., University Park @ MIT, 38 Sidney Street,
Cambridge, Massachusetts 02139, Attention: Vice President of Finance and
Administration or to such other address as shall have been furnished in writing
to the Holder by the Company. Any notice so addressed and mailed by registered
or certified mail shall be deemed to be given when so mailed. Any notice so
addressed and otherwise delivered shall be deemed to be given when actually
received by the addressee.

                                     - 6 -
<PAGE>

SECTION 12. NO RIGHTS AS STOCKHOLDER; LIMITATION OF LIABILITY. This Warrant
shall not entitle the Holder to any of the rights of a shareholder of the
Company except upon exercise in accordance with the terms hereof. No provision
hereof, in the absence of affirmative action by the Holder to purchase shares of
Preferred Stock, and no mere enumeration herein of the rights or privileges of
the Holder, shall give rise to any liability of the Holder for the Warrant Price
hereunder or as a shareholder of the Company, whether such liability is asserted
by the Company or by creditors of the Company.

SECTION 13. REPRESENTATIONS AND COVENANTS OF HOLDER. This Warrant has been
entered into by the Company in reliance upon the following representations and
covenants of the Holder:

     13.1. INVESTMENT PURPOSE. The right to acquire the Warrant Shares
(including any shares of Common Stock issuable upon conversion thereof) will be
acquired for investment and not with a view to the sale or distribution of any
part thereof, and the Holder has no present intention of selling or engaging in
any public distribution of the same except pursuant to a registration or
exemption.

     13.2. PRIVATE ISSUE. The Holder understands (i) that the Warrant Shares
(including any shares of Common Stock issuable upon conversion thereof) are not
registered under the Securities Act or qualified under applicable state
securities laws on the ground that the issuance contemplated by this Warrant
will be exempt from the registration and qualifications requirements thereof,
and (ii) that the Company's reliance on such exemption is predicated on the
representations set forth in this Section 13.

     13.3. DISPOSITION OF HOLDER'S RIGHTS. In no event will the Holder make a
disposition of any of its rights to acquire Warrant Shares (including any shares
of Common Stock issuable upon conversion thereof) unless and until (i) it shall
have notified the Company of the proposed disposition, and (ii) if requested by
the Company, it shall have furnished the Company with an opinion of counsel
(which counsel may either be inside or outside counsel to the Holder)
satisfactory to the Company and its counsel to the effect that (A) appropriate
action necessary for compliance with the Securities Act has been taken, or (B)
an exemption from the registration requirements of the Securities Act is
available. The restrictions imposed upon the transferability of any of its
rights to acquire Warrant Shares (including any shares of Common Stock issuable
upon conversion thereof) shall terminate as to any particular share of Preferred
Stock or Common Stock, as applicable, when (1) such security shall have been
effectively registered under the Securities Act and sold by the holder thereof
in accordance with such registration or (2) such security shall have been sold
without registration in compliance with Rule 144 under the Securities Act, or
(3) a letter shall have been issued to the Holder at its request by the staff of
the Securities and Exchange Commission or a ruling shall have been issued to the
Holder at its request by such Commission stating that no action shall be
recommended by such staff or taken by such Commission, as the case may be, if
such security is transferred without registration under the Securities Act in
accordance with the conditions set forth in such letter or ruling and such
letter or ruling specifies that no subsequent restrictions on transfer are
required. Whenever the restrictions imposed hereunder shall terminate, as
hereinabove provided, the Holder or holder of a share of Preferred Stock or
Common Stock, as applicable, then outstanding as to which such restrictions have
terminated shall be entitled to receive from the Company, without expense to

                                     - 7 -
<PAGE>

such holder, one or more new certificates for the Warrant or for such shares of
Preferred Stock or Common Stock, as applicable, not bearing any restrictive
legend.

     13.4. FINANCIAL RISK. The Holder has such knowledge and experience in
financial and business matters as to be capable of evaluating the merits and
risks of its investment, and has the ability to bear the economic risks of its
investment.

     13.5. RISK OF NO REGISTRATION. The Holder understands that if the Company
does not register with the Securities and Exchange Commission pursuant to
Section 12 of the Exchange Act, or file reports pursuant to Section 15(d), of
the Exchange Act, or if a registration statement covering the securities under
the Securities Act is not in effect when it desires to sell (i) the rights to
purchase Warrant Shares pursuant to this Warrant, or (ii) the Warrant Shares
issuable upon exercise of the right to purchase, or (iii) the shares of Common
Stock issuable upon conversion of such Warrant Shares, it may be required to
hold such securities for an indefinite period. The Holder also understands that
any sale of its rights of the Holder to purchase Warrant Shares, or the Warrant
Shares issuable upon exercise of the right to purchase, or the shares of Common
Stock issuable upon conversion of such Warrant Shares, which might be made by it
in reliance upon Rule 144 under the Securities Act may be made only in
accordance with the terms and conditions of that Rule.

     13.6. ACCREDITED INVESTOR. The Holder is an "accredited investor" within
the meaning of the Securities and Exchange Rule 501 of Regulation D, as
presently in effect.

SECTION 14. LAW GOVERNING. THE VALIDITY, INTERPRETATION, AND ENFORCEMENT OF THIS
WARRANT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE
STATE OF DELAWARE WITHOUT GIVING EFFECT TO THE CONFLICT OF LAW PRINCIPLES
THEREOF.

SECTION 15 MISCELLANEOUS.

          (a)  This Warrant and any provision hereof may be changed, waived,
discharged or terminated only by an instrument in writing signed by both parties
(or any respective predecessor in interest thereof). The headings in this
Warrant are for purposes of reference only and shall not affect the meaning or
construction of any of the provisions hereof

          (b)  All capitalized terms used herein and not otherwise defined
herein shall have the meanings ascribed to them in that certain Master Lease
Agreement dated as of the date hereof by and between the parties hereto.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                     - 8 -
<PAGE>

     IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by its
duly authorized officer this _______ day of ______, 2001.

                                   ACUSPHERE, INC.
[CORPORATE SEAL]
                                   By:  /s/ Sherri C. Oberg
                                      ------------------------------------------

                                   Title:  President and CEO
                                         ---------------------------------------

                                   TBCC FUNDING TRUST I
[CORPORATE SEAL]
                                   By:  /s/ Matthew McAlpre
                                      ------------------------------------------

                                   Title:  Senior Vice President
                                         ---------------------------------------

                                     - 9 -
<PAGE>

                           FORM OF NOTICE OF EXERCISE

                [TO BE SIGNED ONLY UPON EXERCISE OF THE WARRANT]

                     TO BE EXECUTED BY THE REGISTERED HOLDER
                         TO EXERCISE THE WITHIN WARRANT

     The undersigned hereby exercises the right to purchase _________ shares of
Preferred Stock which the undersigned is entitled to purchase by the terms of
the within Warrant according to the conditions thereof, and herewith

[check one]

                    [ ]  makes payment of $__________ therefor; or

                    [ ]  directs the Company to issue ______ shares, and to
                         withhold ____ shares in lieu of payment of the Warrant
                         Price, as described in Section 2.1 of the Warrant.

All shares to be issued pursuant hereto shall be issued in the name of and the
initial address of such person to be entered on the books of the Company shall
be:

     The shares are to be issued in certificates of the following denominations:

                                   ---------------------------------------------

                                   [Type Name of Holder]

                                   By:
                                      ------------------------------------------

                                   Title:
                                         ---------------------------------------

Dated:
        -------------------------------------------

                                     - 10 -
<PAGE>

                               FORM OF ASSIGNMENT
                                    (ENTIRE)

               [TO BE SIGNED ONLY UPON TRANSFER OF ENTIRE WARRANT]

                     TO BE EXECUTED BY THE REGISTERED HOLDER
                         TO TRANSFER THE WITHIN WARRANT

     FOR VALUE RECEIVED ___________________________ hereby sells, assigns and
transfers unto _______________________________ all rights of the undersigned
under and pursuant to the within Warrant, and the undersigned does hereby
irrevocably constitute and appoint _______________________________ Attorney to
transfer the said Warrant on the books of the Company, with full power of
substitution.

                                   ---------------------------------------------

                                   [Type Name of Holder]

                                   By:
                                      ------------------------------------------

                                   Title:
                                         ---------------------------------------

Dated:
        -------------------------------------------

NOTICE

     The signature to the foregoing Assignment must correspond to the name as
written upon the face of the within Warrant in every particular, without
alteration or enlargement or any change whatsoever.

                                     - 11 -
<PAGE>

                               FORM OF ASSIGNMENT
                                    (PARTIAL)

              [TO BE SIGNED ONLY UPON PARTIAL TRANSFER OF WARRANT]

                     TO BE EXECUTED BY THE REGISTERED HOLDER
                         TO TRANSFER THE WITHIN WARRANT

     FOR VALUE RECEIVED _________________________ hereby sells, assigns and
transfers unto _______________________________ (i) the rights of the undersigned
to purchase ___ shares of Preferred Stock under and pursuant to the within
Warrant, and (ii) on a non-exclusive basis, all other rights of the undersigned
under and pursuant to the within Warrant, it being understood that the
undersigned shall retain, severally (and not jointly) with the transferee(s)
named herein, all rights assigned on such non-exclusive basis. The undersigned
does hereby irrevocably constitute and appoint __________________________
Attorney to transfer the said Warrant on the books of the Company, with full
power of substitution.

                                   ---------------------------------------------

                                   [Type Name of Holder]

                                   By:
                                      ------------------------------------------

                                   Title:
                                         ---------------------------------------

Dated:
        -------------------------------------------

NOTICE

     The signature to the foregoing Assignment must correspond to the name as
written upon the face of the within Warrant in every particular, without
alteration or enlargement or any change whatsoever.

                                     - 12 -

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