Document:

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                                                                    EXHIBIT 4.3

THIS WARRANT AND THE STOCK ISSUABLE UPON THE EXERCISE HEREOF HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), AND CAN BE
TRANSFERRED ONLY IN COMPLIANCE WITH THE ACT AND APPLICABLE STATE SECURITIES
LAWS. THIS WARRANT AND SUCH SECURITIES MAY NOT BE SOLD, TRANSFERRED OR ASSIGNED
IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT, UNLESS, IN THE OPINION OF
COUNSEL FOR THE COMPANY OR COUNSEL FOR THE REGISTERED HOLDER (WHICH SHALL BE IN
FORM AND FROM SUCH COUNSEL AS SHALL BE REASONABLY SATISFACTORY TO THE COMPANY),
SUCH REGISTRATION IS NOT THEN REQUIRED.

                           MOBILE P.E.T. SYSTEMS, INC.
                          COMMON STOCK PURCHASE WARRANT

                  1.       ISSUANCE. In consideration of good and valuable
consideration, the receipt of which is hereby acknowledged by MOBILE P.E.T.
SYSTEMS, INC., a Delaware corporation (the "Company"), York, LLC, a Cayman
Islands limited liability company, or registered assigns (the "Holder") is
hereby granted the right to purchase at any time until 5:00 P.M., Pacific Coast
time, on February 28, 2005 (the "Expiration Date"), One Hundred Twenty Thousand
(120,000) fully paid and nonassessable shares of the Company's Common Stock, no
par value per share (the "Common Stock") at an exercise price of $5.00 per share
(the "Exercise Price") subject to further adjustment as set forth in Section 6
hereof.

                  2.       EXERCISE OF WARRANTS. This Warrant is exercisable in
whole or in part for whole shares of the Company's Common Stock at the Exercise
Price per share of Common Stock payable hereunder, payable in cash or by
certified or official bank check. In lieu of paying cash to exercise this
Warrant, the Holder may, by designating a "cashless" exercise on the Notice of
Exercise Form, acquire a number of whole shares of the Company's Common Stock
equal to (a) the difference between (i) the Market Value of the Company's Common
Stock and (ii) the Exercise Price, multiplied by (b) the number of shares of
Common Stock purchasable under the portion of the Warrant tendered to the
Company, divided by (c) the Market Value of the Company's Common Stock. Upon
surrender of this Warrant Certificate with the annexed Notice of Exercise Form
duly executed, together with payment of the Exercise Price for the shares of
Common Stock purchased, the Holder shall be entitled to receive a certificate or
certificates for the shares of Common Stock so purchased. For the purposes of
this Section 2, "Market Value" shall be an amount equal to the average closing
bid price of a share of Common Stock for the five (5) business days immediately
preceding the Company's receipt of the Notice of Exercise Form duly executed.

                  3.       RESERVATION OF SHARES. The Company hereby agrees that
at all times during the term of this Warrant there shall be reserved for
issuance upon exercise of this Warrant such number of shares of its Common Stock
as shall be required for issuance upon exercise of this Warrant (the "Warrant
Shares").

                  4.       MUTILATION OR LOSS OF WARRANT. Upon receipt by the
Company of evidence satisfactory to it of the loss, theft, destruction or
mutilation of this Warrant, and (in the case of loss,

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theft or destruction) receipt of reasonably satisfactory indemnification, and
(in the case of mutilation) upon surrender and cancellation of this Warrant, the
Company will execute and deliver a new Warrant of like tenor and date and any
such lost, stolen, destroyed or mutilated Warrant shall thereupon become void.

                  5.       RIGHTS OF THE HOLDER. The Holder shall not, by virtue
hereof, be entitled to any rights of a stockholder in the Company, either at law
or equity, and the rights of the Holder are limited to those expressed in this
Warrant and are not enforceable against the Company except to the extent set
forth herein.

                  6.       ADJUSTMENTS TO EXERCISE TERMS.

                           If the Company at any time prior to the full
execution of this Warrant shall, by subdivision, combination, merger, spin-off,
re-classification or like capital adjustment of the securities, change any of
the securities to which purchase rights under this Warrant exist into the same
or different number of securities of any class or classes, this Warrant shall
thereafter entitle the Holder to acquire such number and kind of securities as
would have been issuable as a result of such change with respect to the
securities acquirable immediately prior to such transaction. If shares of the
securities acquirable upon exercise of this Warrant are subdivided into a
greater number of securities, including any stock dividend, or if such
securities are combined into a lesser number of securities, then the purchase
price for the securities acquirable upon exercise of this Warrant and the
securities acquirable pursuant to this Warrant shall be proportionately and
equitably adjusted.

                  7.       TRANSFER TO COMPLY WITH THE SECURITIES ACT;
REGISTRATION RIGHTS.

                  (a)      This Warrant has not been registered under the
Securities Act of 1933, as amended, (the "Act") and has been issued to the
Holder for investment and not with a view to the distribution of either the
Warrant or the Warrant Shares. Neither this Warrant nor any of the Warrant
Shares or any other security issued or issuable upon exercise of this Warrant
may be sold, transferred, pledged or hypothecated in the absence of an effective
registration statement under the Act and applicable state securities laws
relating to such security, unless in the opinion of counsel satisfactory to the
Company, such registrations are not required under the Act. Each certificate for
the Warrant, the Warrant Shares and any other security issued or issuable upon
exercise of this Warrant shall contain a legend on the face thereof, in form and
substance satisfactory to counsel for the Company, setting forth the
restrictions on transfer contained in this Section.

                  (b)      The Company agrees to file a registration statement,
which shall include the Warrant Shares, on Form SB-2 or another available form
(the "Registration Statement"), pursuant to the Act, pursuant to a Registration
Rights Agreement between the Company and Holder dated as of the date hereof (the
"Registration Rights Agreement").

                  8.       NOTICES. Any notice or other communication required
or permitted hereunder shall be in writing and shall be delivered personally,
telegraphed, telexed, sent by facsimile transmission or sent by certified,
registered or express mail, postage pre-paid. Any such notice shall

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be deemed given when so delivered personally, telegraphed, telexed or sent by
facsimile transmission, or, if mailed, two days after the date of deposit in the
United States mails, as follows:

                           (i)      if the to Company, to:

                                    Mobile P.E.T. Systems, Inc.
                                    2240 Shelter Island Drive
                                    San Diego, CA  92106
                                    ATTN: CEO
                                    Telephone No.: (619) 226-6738
                                    Telecopier No.: (619) 226-6889

                           (ii)     if to the Holder, to:

                                    c/o Thomson Kernaghan & Co.
                                    365 Bay Street, Suite 1000, 10th Fl.
                                    Toronto, Ontario M5H 2V2
                                    Telephone No.: (416) 860-4160
                                    Telecopier No.: (416) 860-8313

Any party may be notice given in accordance with this Section to the other
parties designate another address or person for receipt of notices hereunder.

                  9.       SUPPLEMENTS AND AMENDMENTS; WHOLE AGREEMENT. This
Warrant may be amended or supplemented only by an instrument in writing signed
by the parties hereto. This Warrant contains the full understanding of the
parties hereto with respect to the subject matter hereof and thereof and there
are no representations, warranties, agreements or understandings other than
expressly contained herein and therein.

                  10.      GOVERNING LAW. This Warrant shall be deemed to be a
contract made under the laws of the State of California and for all purposes
shall be governed by and construed in accordance with the laws of such State
applicable to contracts to be made and performed entirely within such State.

                  11.      DESCRIPTIVE HEADINGS. Descriptive headings of the
several Sections of this Warrant are inserted for convenience only and shall not
control or affect the meaning or construction of any of the provisions hereof.

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         IN WITNESS WHEREOF, the parties hereto have executed this Warrant as of
the 3rd day of March, 2000.

                            MOBILE P.E.T. SYSTEMS, INC., a Delaware corporation

                            By: /s/ PAUL CROWE
                                --------------------------------
                                Name:  Paul Crowe
                                Title:  CEO

Attest:

/s/ THOMAS BROWN
---------------------
Name:  Thomas Brown
Title:  CFO

                                       4<PAGE>

                                                                  EXHIBIT 10.28

                                 [LETTERHEAD]

                            ADIRONDACK CAPITAL, LLC

                                                                29 November 1999

Mobile P.E.T. Systems, Inc.
2240 Shelter Island Drive
San Diego, CA 92106

Attn:  Paul J. Crowe, President and Chief Executive Officer
       Thomas G. Brown, Chief Financial Officer

Gentlemen:

This will confirm the understanding and agreement (the "Agreement") between
Adirondack Capital, LLC and its affiliates ("Adirondack") and Mobile
P.E.T. Systems, Inc. and its affiliates (the "Company") as follows:

1.     The Company hereby engages Adirondack as its financial advisor in
connection with the exploration of certain financial and strategic
alternatives available to the Company.

2.     Adirondack hereby accepts the engagement and in that connection agrees
to:

          (a) become familiar to the extent we deem appropriate with the
       business, operations, properties, financial condition, prospects and
       management philosophy of the Company; and

          (b) advise and assist management of the Company in evaluating
       certain financial and strategic alternatives available to the Company.

3.     The Company hereby agrees that in the event the Company proposes,
during the term of Adirondack engagement hereunder or within six (6) months
thereafter, to engage a third party to act as (i) placement agent in
connection with the issuance and sale of any of the Company's debt or equity
securities, or (ii) financial advisor in connection with a Transaction (as
hereinafter defined), the Company shall engage Adirondack as sole placement
agent, or financial advisor, as the case may be, unless Adirondack shall
notify the Company in writing that it declines to be so engaged. In
connection therewith, the Company shall pay Adirondack its customary fees (as
summarized in Exhibit A) for similar services and shall enter into
documentation in Adirondack's customary form including without limitation any
necessary or appropriate engagement letters, placement, or agreements.

A "Transaction" means any transaction or a series of transactions which
results, directly or indirectly in (i) the transfer of control to the Company
(whether by merger, business combination, acquisition of assets or securities,
tender or exchange offer, lease of assets, partnership, joint venture or
otherwise) of all or a material portion of the assets or securities of
another company or any material businesses; (ii) the sale of the Company
(whether or not the proposal therefor is solicited or unsolicited) of all or a
material

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portion of its assets or securities or any of its material businesses (whether
by merger, business combination, sale of assets or securities, tender or
exchange offer, lease of assets, partnership, joint venture or otherwise);
(iii) the redemption or repurchase by the Company of any of its outstanding
securities; (iv) a recapitalization or restructuring of the Company or its
businesses (including divestitures, spin-offs, split-offs and similar
transactions); (v) a liquidation of the Company; the acquisition by the
Company of an other company or its assets; or (vi) such other form of
transaction that Adirondack, after completing the process provided for in
paragraph 2(a), believes may be of possible interest to the Company.

4.     The Company shall furnish, or cause to be furnished, to Adirondack all
information requested by Adirondack for the purpose of rendering services
hereunder (all such information being "Information"). In addition, the
Company agrees to make available to Adirondack upon request from time to time
the Company's officers, directors, accountants counsel and other advisors.
The Company recognizes and confirms that Adirondack: (a) will use and rely on
the Information and on information available from generally recognized public
sources in performing the services contemplated by this Letter Agreement
without having independently verified the same; (b) does not assume
responsibility for the accuracy or completeness of the Information and such
other information; and (c) will not make an appraisal of any of the assets or
liabilities (contingent or otherwise) of the Company.

5.     The term of Adirondack's engagement hereunder as the Company's
financial advisor shall extend from the date hereof through March 31, 2001.
It is understood that this engagement is an initial engagement and is
renewable with the consent of both the Company and Adirondack. Subject to the
provisions of paragraphs 3 and 4 and 6 through 13, which shall survive any
termination of this Agreement, either party may terminate Adirondack's
engagement hereunder at any time, with or without cause, by giving the other
party at least ten (10) days' prior written notice.

6.     As compensation for the services to be rendered by Adirondack
hereunder, the Company agrees to pay Adirondack a monthly cash retainer of
$3,000 (Three Thousand Dollars) commencing upon the completion by the Company
of an equity, or equity linked transaction in an amount equal to or greater
than $3,000,000 (Three Million Dollars).

7.     In partial consideration for its services hereunder, the Company
shall, on the date hereof, issue to Adirondack or its designee an option (the
"Option") to purchase 50,000 (Fifty Thousand) shares of the Company's common
stock. The Option shall expire on December 31, 2006 and shall have an
exercise price of $2.00 (Two Dollars). In addition, the Company shall grant
to Adirondack or its designee "piggy back" registration rights for the shares
of common stock underlying the Option.

8.     In addition to any fees payable hereunder, the Company shall reimburse
Adirondack and its affiliates promptly upon request for their respective
out-of-pocket and incidental expenses, incurred during the term of its
engagement hereunder, including the fees and expenses of legal counsel and
those of any advisor retained by Adirondack. It is expressly understood and
agreed that Adirondack will not engaged any outside advisor in connection
with this engagement without the express prior written consent of the
Company. Adirondack will bill the Company monthly for such expenses.

9.     Since Adirondack will be acting on behalf of the Company in connection
with this engagement, the Company agrees to indemnify Adirondack as set
forth in the separate letter agreement, dated the date hereof, between
Adirondack and the Company.

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10.     The Company agrees that Adirondack has the right to place advertisements
in financial and other newspapers and journals at its own expense describing its
services to the Company hereunder.

11.    Any advice provided by Adirondack under this Agreement shall be treated
as confidential by the Company and shall not be disclosed or made available to
third parties without Adirondack's prior written consent.

12.    The Company represents and warrants to Adirondack that there are no
brokers, agents, representatives or other persons which have an interest in
compensation paid or payable to Adirondack hereunder.

13.    The benefits of this Agreement shall, together with the separate
indemnity letter, inure to the benefit of respective successors and assigns of
the parties hereto and of the indemnified parties hereunder and their successors
and assigns and representatives, and the obligations and liabilities assumed in
this Agreement by the parties hereto shall be binding upon their respective
successors and assigns.

14.    This Agreement may not be amended or modified except in writing executed
by the Company and Adirondack and shall be governed by and construed in
accordance with the laws of the State of New York, without regard to principles
of conflicts of laws.

Adirondack is delighted to accept this engagement and looks forward to working
with you on this assignment. Please confirm that the foregoing correctly sets
forth our agreement by signing the enclosed duplicate of this letter in the
space provided and returning it, whereupon this letter shall constitute a
binding agreement as of the date first above written.

                                  ADIRONDACK CAPITAL, LLC

                                  By: /s/ K. Ivan F. Gothner
                                     ------------------------
                                     K. Ivan F. Gothner
                                     Managing Director

AGREED:

MOBILE P.E.T. SYSTEMS, INC.

By:   /s/ Paul J. Crowe
      ---------------------

Title:     CEO
      ---------------------

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                                    EXHIBIT A

CUSTOMARY FEES OF ADIRONDACK CAPITAL, LLC

Private Placement of Equity or "Equity-like" Securities:

                     Cash:        8% - 10% of amount raised.
                     Equity:      Options to be negotiated in the context of
                                  the proposed transaction.
                     Expenses:    All fees and expenses incurred in connection
                                  with the proposed transaction.

Private Placement of Debt Securities:

                     Cash:        3% - 6% of amount raised.
                     Equity:      Options to be negotiated in the context of
                                  the proposed transaction.
                     Expenses:    All fees and expenses incurred in connection
                                  with the proposed transaction.

M&A Advisory Services:

                     Cash:        1% - 4% of total transaction value.
                     Equity:      Generally, not applicable.
                     Expenses:    All fees and expenses incurred in connection
                                  with the proposed transaction.

Other Advisory Services:

                     Compensation for: Valuations, Fairness Opinions and other
                     general advisory services are negotiated on a case by case
                     basis to reflect the facts and circumstances of the
                     specific assignment. These advisory services may include
                     advisory services in connection with equity or debt
                     financings where Adirondack has not been engaged as the
                     placement agent.

<PAGE>

                                  [LETTERHEAD]

                            ADIRONDACK CAPITAL, LLC

                                                                10 February 2000

Mobile P.E.T. Systems, Inc.
2240 Shelter Island Drive
San Diego, CA 92106

Attn:    Paul J. Crowe, President and Chief Executive Officer
         Thomas G. Brown, Chief Financial Officer

Gentlemen:

Pursuant to paragraph 3 our letter agreement dated 29 November 1999, the
following sets forth our agreement and understanding regarding the compensation
due to Adirondack Capital, LLC ("Adirondack") in connection with its role as a
financial advisor to Mobile P.E.T. Systems, Inc. (the "Company") in securing the
financing contemplated by the term sheet issued by Southridge Capital Management
and accepted by the Company on 3 February 2000.

Adirondack shall be paid a cash fee equal to ten (10) percent of the gross
proceeds from the sale of Series A Convertible Preferred Stock on the closing
date. Adirondack shall also be paid a cash fee equal to ten (10) percent of the
gross proceeds from each sale of common stock under the proposed "Equity Line of
Credit" on the date each such sale is completed.

In addition to the cash fees described above, the Company shall issue to
Adirondack, or its designee, immediately upon the completion of the sale of the
Series A Convertible Preferred Stock an option (the "Option") to purchase three
hundred thousand (300,000) shares of the Company's common stock at a price of
$3.00/share. The Option will expire on 31 December 2006. The Company shall grant
to Adirondack or its designee "piggy-back" registration rights for the shares of
common stock underlying the Option. Provided, however, that Adirondack's
"piggy-back" registration right hereunder shall not apply to the registration
statements which the Company will be filing in connection with the sale of the
Series A Preferred Stock or the "Equity Line of Credit".

Please confirm that the foregoing correctly sets forth our agreement by signing
the enclosed duplicate of this letter in the space provided and returning it,
whereupon this letter shall constitute a binding agreement as of the date first
above written.

                                    ADIRONDACK CAPITAL, LLC

                                    By: /s/ K. Ivan F. Gothner
                                       --------------------------
                                       K. Ivan F. Gothner
                                       Managing Director

AGREED:

MOBILE P.E.T. SYSTEMS, INC.

By:    /s/ Paul J. Crowe
       --------------------

Title:     CEO
       --------------------

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