<PAGE>
 
                                                                   Exhibit 10.35
                          SUMMIT MEDICAL SYSTEMS INC.
                      NONSTATUTORY STOCK OPTION AGREEMENT

          This Nonstatutory Stock Option Agreement is made this 31st day of
December 1996 (the "Effective Date"), by and between Summit Medical Systems,
Inc. (the "Company") and Robert M. Califf, M.D. (the "Consultant").

          WHEREAS, the Company and Consultant desire to enter into an Consulting
Agreement of even date herewith (the "Consulting Agreement") pursuant to which
Consultant would provide certain consulting services to the Company; and

          WHEREAS, an option to purchase shares of the Company's common stock is
an essential inducement for Consultant to enter into the Consulting Agreement.

          NOW THEREFORE, in consideration of the premises and of the mutual
covenants herein contained, the parties hereto agree as follows:
 
1.   Grant of Option.  The Company hereby grants to Consultant an option (the
     ---------------                                                         
"Option") to purchase up to an aggregate amount of  150,000 shares of the
Company's Common Stock on the terms and conditions set forth herein.

2.   Exercise.  (a) Consultant may exercise this Option for up to 112,500 shares
     --------                                                                   
of the Company's Common Stock in cumulative installments, each exercisable in
whole or in part, from the date first exercisable up to the Expiration Date, as
follows:

<TABLE> 
<CAPTION> 
                                           Number of shares as to
          Date First Exercisable           which Option is exercisable
          ----------------------           ---------------------------
          <S>                              <C>
          Effective Date                               16,875
          December 31, 1997                            31,875
          December 31, 1998                            31,875
          December 31, 1999                            31,875
</TABLE>

(b) Consultant may exercise this Option for up to 37,500 additional shares of
the Company's Common Stock as follows:

     In the event that (i) prior to the fifth anniversary of the Effective Date,
     the Company shall have entered into On-line Outcomes Registry Agreements
     (as hereinafter defined) which have in the aggregate Contract Values (as
     hereinafter defined) of at least $40 million, and (ii) as of the date, if
     any, on which clause (i) is satisfied, the average of the 
<PAGE>
 
     last reported sale prices of the Company's common stock on the Nasdaq
     National Market during the period of the thirty (30) most recent trading
     days, ending on the date on which clause (i) is satisfied, is less than
     $28.00 per share, this Option may be exercised by Consultant, in whole or
     in part, as to 18,750 shares of the Company's Common Stock. Notwithstanding
     the foregoing, in the event that, prior to the fifth anniversary of the
     Effective Date, the Company shall have entered into On-line Outcomes
     Registry Agreements which have in the aggregate Contract Values of at least
     $75 million, this Option may be exercised, in whole or in part, as to
     37,500 shares of the Company's Common Stock. "On-line Outcomes Registry
     Agreements" shall mean any written agreement, understanding or arrangement
     pursuant to which the Company will develop, maintain, manage, operate, or
     otherwise provide services related to, a central registry or database (a
     "registry") that collects clinical medical data from participants in the
     registry or database, primarily using On-line Technologies (as hereinafter
     defined) to receive data into the registry, to process queries to the
     registry, and to deliver reports and responses to the participants in the
     registry. "On-line Technologies" shall mean software products and related
     services that require a participant in the registry to access an off-site
     central processing unit or server maintained by the Company or a third
     party in order to input, aggregate, access and/or process the participant's
     clinical medical data and other data from, or reports based on, the
     clinical medical data in the registry. "Contract Value", as to any On-line
     Outcomes Registry Agreement, shall be determined in accordance with Exhibit
     A hereto. The Company agrees that within twenty (20) business days of the
     execution and delivery of each On-line Outcomes Registry Agreement, the
     Company will determine in good faith the Contract Value of such On-line
     Outcomes Registry Agreement in accordance with the provisions of Exhibit A
     and will give written notice to Consultant, which notice shall state the
     Contract Value for such On-line Outcomes Registry Agreement as determined
     by the Company and the aggregate Contract Values for all On-line Outcomes
     Registry Agreements as of the date of such notice. The calculation of
     Contract Value set forth in this notice shall be certified by Donald
     Fortin, M.D. as Chief Scientific Officer of the Company. If Consultant
     objects to the determination of Contract Value within thirty (30) business
     days following the date of the notice, the Company and Consultant shall
     negotiate in good faith for a period of thirty (30) business days to settle
     any dispute regarding the Contract Value. If such dispute has not been
     settled within such thirty (30) business day  

                                      -2-
<PAGE>
 
     period, the dispute shall be settled in accordance with the arbitration
     provision of Section 11 below.

3.   Purchase Price.  The purchase price of the stock shall be $7.50 per share
     --------------                                                           
(subject to adjustment as noted below).

4.   Nonstatutory Option.  The Option is not intended to be an incentive stock
     -------------------                                                      
option within the meaning of Section 422 of the Internal Revenue Code of 1986,
as amended.

5.   Period of Exercise.  The Option will expire on the date (the "Expiration
     ------------------                                                      
Date") ten (10) years from the Effective Date.  The Option may be exercised only
while the Consulting Agreement is in effect between the Company and Consultant,
or alternatively as provided in Section 7, which deals with termination of the
Consulting Agreement; provided, however, that the Option shall not be
exercisable after the Expiration Date.

6.   Transferability. This Option is not transferable except by will or the laws
     ---------------                                                            
of descent and distribution, or with the prior written consent of the Company.

7.   Termination of Consulting Agreement.  The holder hereof shall have the
     -----------------------------------                                   
right to exercise the Option only to the extent of the full number of shares of
the Company's Common Stock the Consultant was entitled to purchase under the
Option on the effective date of termination of the Consulting Agreement;
provided, however, that if the Consulting Agreement terminates for any reason
other than Consultant's material breach thereof, or Consultant's voluntary
termination thereof other than as a result of Company's uncured material breach,
the Option shall be exercisable as to all 150,000 shares of the Company's Common
Stock immediately upon the effective date of such termination, notwithstanding
Section 2 above. In the event of termination of the Consulting Agreement for any
and all reasons, the Option may be exercised by the holder hereof until the
Expiration Date, subject to the extent provided in this Section 7.

8.   Service.  This Agreement shall in no way restrict the right of the Company
     -------                                                                   
or Consultant to terminate the Consulting Agreement in accordance with its
terms.

9.   Method of Exercise.  The Option may be exercised, subject to the terms and
     ------------------                                                        
conditions of this Agreement, by written notice to the Company.  The notice
shall be in the form attached to this Agreement and will be accompanied by
payment by check (bank check, certified check or personal check) or by delivery
to the Company for cancellation shares of Common Stock having a Market Price
(defined in Section 13(f) below) equal to the full purchase price of the shares
to be issued on the date the 

                                      -3-
<PAGE>
 
notice of exercise is received by the Company, and in the event of an exercise
by an executor or administrator or other person authorized under Section 6
hereof appropriate proof of the right of such person to exercise the Option. The
Company will issue and deliver certificates representing the number of shares
purchased under the Option, registered in the name of the holder hereof as soon
as practicable after receipt of the notice.

10.  Withholding.  In any case where withholding is required under federal,
     -----------                                                           
state or local law in connection with any exercise by the Consultant hereunder,
Consultant may elect to (i) authorize the Company to withhold appropriate
amounts from amounts payable to the Consultant, (ii) remit to the Company an
amount equal to such appropriate withholding, or (iii) have the Company withhold
a portion of the shares of Common Stock otherwise to be delivered upon exercise
of such option having a Market Price equal to the amount of such appropriate
withholding.

11.  Arbitration.  Any dispute, claim or controversy arising out of or in
     -----------                                                         
connection with this Agreement or this Option which has not been settled through
negotiation within a period of thirty (30) days after the date on which either
party shall first have notified the other party in writing of the existence of a
dispute shall be settled by final and binding arbitration under the then
applicable Commercial Arbitration Rules of the American Arbitration Association
("AAA"). Any such arbitration shall be conducted by three (3) arbitrators
appointed by mutual agreement of the parties or, failing such agreement, in
accordance with such AAA Rules. At least one (1) arbitrator shall be an
experienced computer software professional, and at least one (1) arbitrator
shall be an experienced business attorney with background in the licensing and
distribution of computer software. Any such arbitration shall be conducted in
Durham, North Carolina. An arbitral award may be enforced in any court of
competent jurisdiction. Notwithstanding any contrary provision in the AAA Rules,
the following additional procedures and rules shall apply to any such
arbitration:

     (a)  Each party shall have the right to request from the arbitrators, and
the arbitrators shall order upon good cause shown, reasonable and limited pre-
hearing discovery, including  (i) exchange of witness lists, (ii) depositions
under oath of named witnesses at a mutually convenient location, (iii) written
interrogatories, and (iv) document requests.

     (b)  Upon conclusion of the pre-hearing discovery, the arbitrators shall
promptly hold a hearing upon the evidence to be adduced by the parties and shall
promptly render a written opinion and award.

                                      -4-
<PAGE>
 
     (c)  The arbitrators may not award or assess punitive damages against
either party.

     (d)  Each party shall bear its own costs and expenses of the arbitration
and one-half (1/2) of the fees and costs of the arbitrators, subject to the
power of the arbitrators, in their sole discretion, to award all such reasonable
cots, expenses and fees to the prevailing party.

12.  Registration.  Company shall use commercially reasonable efforts to
     ------------                                                       
register the shares of Common Stock that Consultant obtains pursuant to an
exercise of the Option on a registration statement on Form S-8 (or any successor
form) and the Company shall use commercially reasonable efforts to maintain the
effectiveness of such registration statement for so long as any portion of the
Option remains exercisable by Consultant hereunder.

13.  Anti-Dilution Adjustments.  The above provisions are, however, subject to
     -------------------------                                                
the following:

          (a)  The purchase price set forth in Section 3 above shall, from and
     after the date of issuance of this Option, be subject to adjustment from
     time to time as hereinafter provided.  Upon each adjustment of the purchase
     price, the holder hereof shall thereafter be entitled to purchase, at the
     purchase price resulting from such adjustment, the number of shares
     obtained by multiplying the purchase 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 purchase price resulting from such adjustment.

          (b)  In case the Company shall at any time subdivide its outstanding
     shares of Common Stock into a greater number of shares, including any
     dividend declared to effect a subdivision of the outstanding shares of
     Common Stock, the purchase price in effect immediately prior to such
     subdivision shall be proportionately reduced, and conversely, in case the
     outstanding shares of Common Stock of the Company shall be combined into a
     smaller number of shares, the purchase price in effect immediately prior to
     such combination shall be proportionately increased.

          (c)  If any capital reorganization or reclassification of the capital
     stock of the Company, or consolidation or merger of the Company with
     another corporation, or the sale of all or substantially all of its assets
     to another corporation shall be effected in such a way that holders of
     Common Stock shall be entitled to receive stock, securities or assets with
     respect to or in 

                                      -5-
<PAGE>
 
     exchange for Common Stock, then, as a condition of such reorganization,
     reclassification, consolidation, merger or sale, lawful and adequate
     provision shall be made whereby the holder hereof shall thereafter have the
     right to purchase and receive, upon the basis and upon the terms and
     conditions specified in this Option and in lieu of the shares of Common
     Stock of the Company immediately theretofore purchasable and receivable
     upon the exercise of the rights represented hereby, such shares of stock,
     securities or assets as may be issued or payable with respect to or in
     exchange for a number of outstanding shares of such Common Stock equal to
     the number of shares of such stock immediately theretofore purchasable and
     receivable upon the exercise of the rights represented hereby had such
     reorganization, reclassification, consolidation, merger or sale not taken
     place, and in any such case appropriate provision shall be made with
     respect to the rights and interests of the holder hereof to the end that
     the provisions hereof (including without limitation provisions for
     adjustments of the purchase price and of the number of shares purchasable
     upon the exercise of this Option) shall thereafter be applicable, as nearly
     as may be, in relation to any shares of stock, securities or assets
     thereafter deliverable upon the exercise hereof. The Company shall not
     effect any such consolidation, merger or sale, unless prior to the
     consummation thereof the successor corporation (if other than the Company)
     resulting from such consolidation or merger or the corporation purchasing
     such assets shall assume, by written instrument executed and mailed to the
     registered holder hereof at the last address of such holder appearing on
     the books of the Company, the obligation to deliver to such holder such
     shares of stock, securities or assets as, in accordance with the foregoing
     provisions, such holder may be entitled to purchase.

          (d)  Upon any adjustment of the purchase price, then and in each such
     case the Company shall give written notice thereof, by first-class mail,
     postage prepaid, addressed to the registered holder of this Option at the
     address of such holder as shown on the books of the Company, which notice
     shall state the purchase price resulting from such adjustment and the
     increase or decrease, if any, in the number of shares purchasable at such
     price upon the exercise of this Option, setting forth in reasonable detail
     the method of calculation and the facts upon which such calculation is
     based.

          (e)  If any event occurs as to which the other provisions of this
     Section 13 are not strictly applicable or if strictly applicable would not
     fairly protect the purchase rights of the holder of this Option or of
     Common Stock in accordance with the essential intent and principles of such
     provisions, then this Option shall be adjusted in the application of such
     provisions, in 

                                      -6-
<PAGE>
 
     accordance with such essential intent and principles, so as to protect such
     purchase rights as aforesaid.

          (f)  No fractional shares of Common Stock shall be issued upon the
     exercise of this Option, but, instead of any fraction of a share which
     would otherwise be issuable, the Company shall pay a cash adjustment (which
     may be effected as a reduction of the amount to be paid by the holder
     hereof upon such exercise) in respect of such fraction in an amount equal
     to the same fraction of the Market Price per share of Common Stock on the
     date of the written notice of exercise required by Section 9 above. "Market
     Price" for purposes of this Section 13(f), Sections 9 and 10 hereof, and
     for the purpose of determining the option purchase price shall mean, if the
     Common Stock is traded on a securities exchange or on the Nasdaq National
     Market, the closing price of the Common Stock on such exchange or the
     Nasdaq National Market, or, if the Common Stock is otherwise traded in the
     over-the-counter market, the closing bid price, in each case averaged over
     a period of 5 consecutive business days prior to the date as of which
     "Market Price" is being determined. If at any time the Common Stock is not
     traded on an exchange or the Nasdaq National Market, or otherwise traded in
     the over-the-counter market, the "Market Price" shall be deemed to be the
     higher of (i) the book value thereof as determined by any firm of
     independent public accountants of recognized standing selected by the Board
     of Directors of the Company as of the last day of any month ending within
     60 days preceding the date as of which the determination is to be made, or
     (ii) the fair value thereof determined in good faith by the Board of
     Directors of the Company as of a date which is within 15 days of the date
     as of which the determination is to be made.

     14.  Common Stock.  As used herein, the term "Common Stock" shall mean
          ------------                                                     
and include the Company's presently authorized Common Stock and shall also
include any capital stock of any class of the Company hereafter authorized which
shall not be limited to a fixed sum or percentage in respect of the rights of
the holders thereof to participate in dividends or in the distribution of assets
upon the voluntary or involuntary liquidation, dissolution or winding up of the
Company; provided that the shares purchasable pursuant to this Option shall
include shares designated as Common Stock of the Company on the date of original
issue of this Option or, in the case of any reclassification of the outstanding
shares thereof, the stock, securities or assets provided for in Section 13(c)
above.

     15.  No Voting Rights.  This Option shall not entitle the holder hereof to
          ----------------                                                     
any voting rights or other rights as a shareholder of the Company.

                                      -7-
<PAGE>
 
     16.  Optional Conversion.
          ------------------- 

          (a)  In addition to and without limiting the rights of the holder of
     this Option under the terms of this Option, such holder shall have the
     right (the "Conversion Right") to convert this Option or any portion
     thereof into shares of Common Stock as provided in this Section 16 at any
     time from and after the Effective Date and to and including the Expiration
     Date, subject to the restrictions set forth in Section 5. Upon exercise of
     the Conversion Right with respect to a particular number of shares subject
     to this Option (the "Converted Option Shares"), the Company shall deliver
     to the holder of this Option, without payment by the holder of any exercise
     price or any cash or other consideration, that number of shares of Common
     Stock equal to the quotient obtained by dividing the Net Value (as
     hereinafter defined) of the Converted Option Shares by the Market Price of
     a single share of Common Stock, determined in each case as of the close of
     business on the Conversion Date (as hereinafter defined). The "Net Value"
     of the Converted Option Shares shall be determined by subtracting the
     aggregate option purchase price of the Converted Option Shares from the
     Market Price of the Converted Option Shares. Notwithstanding anything in
     this Section 16 to the contrary, the Conversion Right cannot be exercised
     with respect to a number of Converted Option Shares having a Net Value
     below $100. No fractional shares shall be issuable upon exercise of the
     Conversion Right, and if the number of shares to be issued in accordance
     with the foregoing formula is other than a whole number, the Company shall
     pay to the holder of this Option an amount in cash equal to the fair market
     value of the resulting fractional share.

          (b)  The Conversion Right may be exercised by the holder of this
     Option by the surrender of this Option at the principal office of the
     Company together with a written statement specifying that the holder
     thereby intends to exercise the Conversion Right and indicating the number
     of shares subject to this Option which are being surrendered (referred to
     in paragraph (a) above as the Converted Option Shares) in exercise of the
     Conversion Right. Such conversion shall be effective upon receipt by the
     Company of this Option together with the aforesaid written statement, or on
     such later date as is specified therein (the "Conversion Date"), but not
     later than the Expiration Date. Certificates for the shares of Common Stock
     issuable upon exercise of the Conversion Right, together with a check in
     payment of any fractional share and, in the case of a partial exercise, a
     new option evidencing the shares remaining subject to this Option, shall be
     issued as of the Conversion Date and shall be delivered to the holder of
     this Option within 7 days following the Conversion Date.

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<PAGE>
 
     17.  Governing Law.  All questions concerning this Option will be governed
          -------------                                                        
and interpreted and enforced in accordance with the internal law of the State of
Minnesota without regard for principles of conflict of law.


ACCEPTED:                           SUMMIT MEDICAL SYSTEMS, INC.

                                    By_____________________________
______________________________
Robert M. Califf, M.D.              
                                    Its____________________________
                                                            

                                      -9-
<PAGE>
 
                                   EXHIBIT A


                                CONTRACT VALUE

          The Contract Value of each On-line Outcomes Registry Agreement shall
be determined in accordance with the following procedures. The Contract Value of
each On-line Outcomes Registry Agreement shall equal the sum of (a) the Fixed
Fees (as hereinafter defined) and (b) the Variable Fees (as hereinafter
defined). "Fixed Fees" shall mean the sum of all sponsorship fees, periodic site
license fees or other fixed fees to be paid to the Company during the term of
the On-line Outcomes Registry Agreement for which a cash payment of an amount
certain is specified in such agreement. "Variable Fees" shall mean the product
of the Estimated Fees (as hereinafter defined) multiplied by .75. "Estimated
Fees" shall mean the sum of all usage, encounter, inquiry, training, service and
other variable fees, based on usage of the registry and related services by
participants, to be paid to the Company during the term of the On-line Outcomes
Registry Agreement. The Company shall estimate the Estimated Fees in good faith
as of the date the On-line Outcomes Registry Agreement is executed and
delivered. The Company represents, warrants and covenants that it will have a
reasonable basis for its estimate of Estimated Fees and that such estimate will
be prepared on a basis consistent with the internal projections of the Company.
The estimate of Estimated Fees will be prepared using the pricing terms as set
forth in the On-line Outcomes Registry Agreement, including, without limitation,
estimates of any volume discounts, based on the usage estimates made in
accordance with the following sentence. The Company shall prepare in good faith
an estimate of the usage of the registry and related services by participants
over the term of the On-line Outcomes Registry Agreement. By way of example
only, an estimate of the Estimated Fees for a stent registry would include (i)
an estimate of the number of cardiac catheter laboratories ("cath labs") to be
covered by the agreement, (ii) an estimate of the size distribution of such cath
labs, (iii) an estimate of usage of the registry and related services by each
cath lab based on its size, and (iv) an estimate of any volume discounts based
on the foregoing. The foregoing procedure shall be applied in a manner similar
to the illustration attached hereto as Exhibit B, subject to the specific terms
and conditions of the applicable On-Line Outcomes Registry Agreement; provided,
that in Exhibit B the calculation of Contract Value illustrated therein shall
not include the Net Present Value discount shown, but, rather, shall include the
multiplication of such Estimated Fees by the factor of .75. 

                                      -10-
<PAGE>
 
                         SUMMIT MEDICAL SYSTEMS, INC.
                         ----------------------------

                      NOTICE OF EXERCISE OF STOCK OPTION
                      ----------------------------------


To:  Board of Directors
     SUMMIT MEDICAL SYSTEMS, INC.

     I hereby exercise my Option dated __________ to purchase _____ shares of
                                
$.01 par value common stock of the Company at the option exercise price of
                                                                          
$__________ per share.  Enclosed is a certified or cashier's check in the total


amount of $__________, or payment in such other form as the Company has

specified.

     I request that my shares be issued in my name as follows:

     _____________________________________________________________________   
               (Print your name in the form in which you wish to
                          have the shares registered)
     _____________________________________________________________________
                            (Social Security Number)
     _____________________________________________________________________
                              (Street and Number)
     _____________________________________________________________________
          (City)                                          (State)  (Zip Code)

     Dated:  __________, 19__.
             
                                    Signature:____________________________

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