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                                                                   Exhibit 10.38

                             SECURED PROMISSORY NOTE

$2,500,000.00                                         Dated:  September 27, 2001

FOR VALUE RECEIVED, the undersigned, ACUSPHERE, INC., a Delaware corporation
("Borrower"), HEREBY PROMISES TO PAY to the order of VENTURE LENDING & LEASING
III, INC. ("Lender") the principal amount of Two Million Five Hundred Thousand
Dollars ($2,500,000.00) or such lesser amount as shall equal the outstanding
principal balance of the Loan made to Borrower by Lender pursuant to the Venture
Loan and Security Agreement referred to below (the "Loan Agreement"), and to pay
all other amounts due with respect to the Loan on the dates and in the amounts
set forth in the Loan Agreement.

Interest on the principal amount of this Note from the date of this Note shall
accrue at the Loan Rate or, if applicable, the Default Rate. The Loan Rate for
this Note is fifteen percent (15%) per annum based on a year of twelve 30-day
months. If the Funding Date is not the first day of the month, interim interest
accruing from the Funding Date through the last day of that month shall be paid
on the Funding Date. Commencing on November 1, 2001, and continuing on the first
day of each month thereafter (each, a "Payment Date"), Borrower shall pay to
Lender six (6) payments of accrued interest only followed by twenty-four (24)
payments of principal in the amount of $104,166.67, plus accrued interest on the
then outstanding principal amount.

Principal, interest and all other amounts due with respect to the Loan, are
payable in lawful money of the United States of America to Lender as set forth
in the Loan Agreement. The principal amount of this Note and the interest rate
applicable thereto, and all payments made with respect thereto, shall be
recorded by Lender and, prior to any transfer hereof, endorsed on the grid
attached hereto which is part of this Note.

This Note is one of the Notes referred to in, and is entitled to the benefits
of, the Venture Loan and Security Agreement, dated as of September 27, 2001, to
which Borrower and Lender are parties. The Loan Agreement, among other things,
(a) provides for the making of a secured Loan to Borrower, and (b) contains
provisions for acceleration of the maturity hereof upon the happening of certain
stated events.

This Note may not be prepaid except in accordance with the Loan Agreement.

This Note and the obligation of Borrower to repay the unpaid principal amount of
the Loan, interest on the Loan and all other amounts due Lender under the Loan
Agreement is secured under the Loan Agreement.

Presentment for payment, demand, notice of protest and all other demands and
notices of any kind in connection with the execution, delivery, performance and
enforcement of this Note are hereby waived.

Borrower shall pay all reasonable fees and expenses, including, without
limitation, reasonable attorneys' fees and costs, incurred by Lenders in the
enforcement or attempt to enforce any of Borrower's obligations hereunder not
performed when due. This Note shall be governed by, and construed and
interpreted in accordance with, the laws of the State of California.

IN WITNESS WHEREOF, Borrower has caused this Note to be duly executed by one of
its officers thereunto duly authorized on the date hereof.

                                    ACUSPHERE, INC.

                                    By:  /s/ Sherri Oberg
                                       -----------------------------------------

                                    Name: Sherri Oberg
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                                    Title: President and CEO
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