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                                                                   Exhibit 10.31


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 September 30, 2005 (the "Expiration Date"), Fifty Thousand (50,000)
fully paid and nonassessable shares of the Company's Common Stock, no par value
per share (the "Common Stock") at an exercise price of $3.00 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 21st day of September 2000.

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



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

Attest:

/s/ Thomas G. Brown
------------------------
Name:  Thomas G. Brown
Title: Secretary


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