PROMISSORY NOTE
                                 ---------------

U.S.$2,000,000.00                                                  June 29, 2001
                                                   So. San Francisco, California


     The undersigned POSITRON  CORPORATION,  a corporation organized and validly
existing  under  the laws of the  State of Texas,  for  value  received,  hereby
unconditionally  promises to pay to the order of IMATRON INC.  (the  "Payee") at
389 Oyster  Point Blvd.,  So. San  Francisco,  CA 94080,  in lawful money of the
United  States of America and in  immediately  available  funds,  the  principal
amount of TWO MILLION U.S. DOLLARS,  or, if less, the aggregate unpaid principal
amount of all Loans (as more fully shown on the grid attached  hereto and made a
part hereof),  with interest on the principal  amount hereof remaining from time
to time unpaid at such  interest  rates and payable at such times as provided in
the Loan Agreement.  Notwithstanding  the foregoing,  Borrower hereby authorizes
Lender to record and adjust,  on the grid attached hereto,  the principal amount
of Loans, principal payments and balances by Borrower under the Loan Agreement.

     This Note evidences Loans by the Payee to the  undersigned  pursuant to the
Loan Agreement  between the  undersigned and the Payee dated as of June 29, 2001
(the "Loan Agreement") as from time to time may be amended, restated,  replaced,
supplemented,  substituted  for or  renewed,  and the  holder  of  this  Note is
entitled to the benefits  thereof.  Each term defined in the Loan  Agreement and
not otherwise defined herein shall have the same definition when used herein.

     The principal  hereof (together with any accrued but unpaid interest) shall
become  forthwith  due and payable as provided in the Loan  Agreement.  Payments
hereunder  not  made  when due  shall  bear  interest  as  provided  in the Loan
Agreement.  Rights of Borrower,  if any, to prepay the Note are set forth in the
Loan Agreement.

     All payments made pursuant to the terms of this Note shall be made free and
clear of, and without deduction for, withholding,  setoff or counterclaim of any
kind.

     Neither  the  failure on the part of the holder of this Note in  exercising
any right or remedy nor any single or partial  exercise  or the  exercise of any
other right or remedy shall operate as any waiver. No amendment  hereunder shall
be effective unless in writing signed by the undersigned and holder of this Note
and no waiver  hereunder  shall be  effective  unless in writing,  signed by the
party  to  be  charged.  The  undersigned  hereby  waives  demand  for  payment,
presentment,  protest and notice of any kind in  connection  with the  delivery,
acceptance, performance, default or enforcement of this Note and hereby consents
to any extensions of time, renewals, releases of any party to this Note, waivers
or modifications  that may be granted or consented to by the holder of this Note
in respect of the time of payment or any other  matter.  Anything  contained  in
this Note to the  contrary  notwithstanding,  in the event  that any  payment of
interest  hereunder shall exceed the legal limit,  such amount in excess of such
limit shall be deemed a payment of principal hereunder.

     The terms and  provisions  hereof  shall  inure to the  benefit  of, and be
binding upon,  the  respective  successors  and assigns of the  undersigned  and
Payee. THIS NOTE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH THE LAWS
OF THE STATE OF CALIFORNIA  WITHOUT  GIVING EFFECT TO THE PRINCIPLES OF CONFLICT
OF LAWS THEREOF.

     IN WITNESS  WHEREOF,  the undersigned has caused this Promissory Note to be
executed  by its  authorized  representative,  who  certifies  that  he has  all
necessary authority on behalf of the undersigned to execute this Promissory Note
and bind the undersigned to the terms hereof.


                                  POSITRON CORPORATION


                                  By: _______________________________

                                  Its: _______________________________

ATTEST:





<PAGE>



                   GRID TO PROMISSORY NOTE DATED JUNE 29, 2001

                 ISSUED BY POSITRON CORPORATION TO IMATRON INC.

                       ADVANCES AND PAYMENTS OF PRINCIPAL

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<CAPTION>

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<S>             <C>                    <C>                        <C>                        <C>
Date             Amount of Advance     Amount of Principal Paid   Unpaid Principal Balance   Notation Made by
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6/29/01          $1,000,000                                       $1,000,000                 RVR
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</TABLE>