<PAGE>

                                                                    EXHIBIT 10.1

SIXTH AMENDMENT attached to and made a part of that certain Lease dated
September 10, 1991, between FIRST INDUSTRIAL MORTGAGE PARTNERSHIP, L.P.,
successor in interest to WS Development Company, as Landlord, and SOMANETICS
CORPORATION, as Tenant, covering Premises at 1653 E. Maple Road, Troy, Michigan.

NOTWITHSTANDING anything to the contrary contained in the Lease, Agreement,
Addenda to Lease, Termination of Agreement, and Extension of Lease to which this
Sixth Amendment is attached to and made a part thereof, the Landlord and Tenant
agree as follows:

1.    The Term of the Lease shall be Five (5) years, Five (5) months, commencing
      August 1, 2004 and terminating December 31, 2009.

2.    The minimum net rent for said extended Five (5) year, Five (5) month Term
      shall be Seven Hundred Seventy-Seven Thousand Three Hundred Seven and
      96/100 Dollars ($777,307.96) payable in monthly installments in advance as
      follows:

<TABLE>
<CAPTION>
              Months                   Monthly Base Rent
              ------                   -----------------
<S>                                    <C>
August 1, 2004  - December 31, 2004       $ 11,696.00
January 1, 2005 - December 31, 2005       $ 11,696.00
January 1, 2006 - December 31, 2006       $ 11,696.00
January 1, 2007 - December 31, 2007       $ 11,929.92
January 1, 2008 - December 31, 2008       $ 12,168.52
January 1, 2009 - December 31, 2009       $ 12,411.89
</TABLE>

3.    TERMINATION RIGHT. Notwithstanding anything to the contrary contained
      herein, if (a) Tenant enters into an agreement for the sale of all of the
      assets of Tenant to an unrelated third party (i.e. an individual or entity
      that is neither an individual or entity that is a direct or indirect
      subsidiary of Tenant, an individual or entity of which Tenant is a direct
      or indirect subsidiary, or any other subsidiaries of any of the foregoing
      individuals or entities), or ( b) Tenant's shareholder(s) enters into an
      agreement for the sale of all of Tenant's stock with an unrelated third
      party (i.e. an individual or entity that is neither an individual or
      entity that is a direct or indirect subsidiary of Tenant, an individual or
      entity of which Tenant is a direct or indirect subsidiary, or any other
      subsidiaries of any of the foregoing individuals or entities) and either
      of the sales described in (a) or (b) is fully consummated on or before
      September 1, 2007, Tenant shall have a one-time option to terminate this
      Lease ("TERMINATION OPTION") in accordance with the following terms and
      conditions:

      TENANT GIVES NOTICE. If Tenant desires to exercise the Termination Option,
      Tenant shall give Landlord irrevocable written notice ("TERMINATION
      NOTICE") of Tenant's exercise of this Termination Option, which shall be
      delivered by certified mail, which Termination Notice must be received by
      Landlord no later than the date that is four (4) full months prior to the
      Termination Date (defined below), that is, such Termination Notice must be
      received by Landlord no later than September 1, 2007. Time is of the
      essence with respect to Landlord's receipt of the Termination Notice and
      all other deadlines in this Sixth Amendment, Paragraph 3.

      TERMINATION DATE. If Tenant gives the Termination Notice and complies with
      all the provisions in this Sixth Amendment, Paragraph 3, the Lease shall
      terminate at 11:59 p.m. on December 31, 2007 (the "TERMINATION DATE").

      TERMINATION FEE MUST ACCOMPANY NOTICE. In order for such Termination
      Notice to be effective, it must be accompanied by the termination fee in
      the amount of Thirty Thousand and 00/100 Dollars ($30,000.00), which shall
      be payable only in cash or certified funds.

      TENANT'S OBLIGATION SURVIVES TERMINATION. Tenant's obligations to pay
      Rent, Additional Rent, and any other costs or charges under this Lease,
      and to perform all other Lease obligations for the period up to and
      including the Termination Date, shall survive the termination of this
      Lease.

<PAGE>

      LANDLORD MAY CANCEL AND VOID TERMINATION IF TENANT IS IN DEFAULT.
      Notwithstanding the foregoing, if at any time during the period on or
      after the date on which Tenant shall exercise its Termination Option (in
      accordance with this Sixth Amendment, Paragraph 3) up to and including the
      Termination Date Tenant shall be in Default of this Lease, then Landlord
      may elect, but is not obligated, to cancel and declare null and void
      Tenant's exercise of the Termination Option and this Lease shall continue
      in full force and effect for the full Lease Term hereof unaffected by
      Tenant's exercise of the Termination Option. If Landlord does not cancel
      Tenant's exercise of the Termination Option after Tenant's Default, Tenant
      shall cure any Default within the period of time specified in this Lease
      and this obligation shall survive the Termination Date.

      TENANT SHALL SURRENDER SPACE BY TERMINATION DATE. In the event Tenant
      exercises the Termination Option, Tenant covenants and agrees to surrender
      full and complete possession of the Premises to Landlord on or before the
      Termination Date vacant, broom-clean, in good order and condition, and, in
      accordance with the provisions of this Lease, and thereafter the Premises
      shall be free and clear of all leases, tenancies, and rights of occupancy
      of any entity claiming by or through Tenant.

      FAILURE TO SURRENDER MAKES TENANT A HOLDOVER. If Tenant shall fail to
      deliver possession of the Premises on or before the Termination Date in
      accordance with the terms hereof, Tenant shall be deemed to be a holdover
      tenant from and after the Termination Date, and in such event, Tenant
      shall be liable to Landlord for payments for the use and occupancy of the
      Premises equal to the fair market value thereof, and shall also be liable
      to Landlord for all costs and expenses incurred by Landlord in securing
      possession of the Premises. Landlord may accept any such sums from Tenant
      without prejudice to Landlord's right to evict Tenant from the Premises by
      any lawful means.

      LEASE CEASES AFTER TERMINATION. If Tenant properly and timely exercises
      the Termination Option and properly and timely satisfies all other
      monetary and non-monetary obligations under this Lease, the Lease shall
      cease and expire on the Termination Date with the same force and effect as
      if said Termination Date were the date originally provided in this Lease
      as the expiration date of the Term hereof.

4.    Section 10 - Repairs and any subsequent sections of the Lease referring to
      repairs and maintenance shall remain the same except for the following:

      Landlord shall be responsible during the Term for the repairs and
      replacements, if and when necessary, of the roof and exterior roof
      structure of the building the foundation, footings and exterior structural
      components of the Building. The cost and expenses incurred by Landlord for
      the repairs and replacement that Landlord must perform under this Sixth
      Amendment, Paragraph 4 shall be allocated as follows: (i) all such repairs
      and replacement shall be at Landlord's expense. Notwithstanding the
      previous sentence to the contrary, in the event that any such repair or
      replacement is required to be performed by Landlord under this Sixth
      Amendment, Paragraph 4 and is necessitated by (w) the performance or
      existence of any Alterations, (x) the installation, use or operation of
      Tenant's Property in the Premises, (y) the moving of Tenant's Property in
      or out of the Property, or (z) any negligent act or omission, misconduct,
      or misuse by tenant, any of its subtenants, or others entering into the
      Premises by act or omission of Tenant or any subtenant (collectively,
      "Tenant Necessitated Repairs"), then Tenant shall be required to reimburse
      Landlord for all cost and expense that Landlord incurs in order to perform
      such Tenant Necessitated Repairs, and such reimbursement shall be paid in
      full within 10 days after Landlord's delivery of demand therefore.

      Landlord shall also be responsible during the Term for the replacement, if
      and when necessary of the HVAC and the repair and replacement of the
      parking lot at the Property. Landlord shall be responsible for all costs
      and expenses incurred of all such repairs and replacements.
      Notwithstanding the previous sentence to the contrary, in the event that
      any such repair or replacement is required to be performed by Landlord
      under this Sixth Amendment, Paragraph 4 and is necessitated by (w) the
      performance or existence of any Alterations, (x) the installation, use or
      operation of Tenant's Property in the Premises, (y) the moving of Tenant's
      Property in or out of the Property, or (z) any negligent act or omission,
      misconduct, or misuse by tenant, any of its subtenants, or others entering
      into the Premises by act or omission of Tenant or any subtenant
      (collectively, "Tenant Necessitated Repairs"), then Tenant shall be
      required to reimburse Landlord for all cost and expense that Landlord

<PAGE>

      incurs in order to perform such Tenant Necessitated Repairs, and such
      reimbursement shall be paid in full within 10 days after Landlord's
      delivery of demand therefore

5.    Subject to any changes required by local authorities, Landlord agrees to
      construct at its cost the tenant improvements described in attached
      Exhibit B-1 (the "Tenant Improvements"). Landlord will use commercially
      reasonable efforts to complete the Tenant Improvements as soon as
      reasonably possible after the Date of Commencement, such construction to
      be completed in a good and workmanlike manner. All installations shall be
      in a location acceptable to Landlord, consistent with the specifications
      attached as Exhibit B-1. For purposes of Section 21 of this Lease, the
      Tenant Improvements shall be treated as Alterations.

All other terms and conditions of said Lease, Agreement, Addenda to Lease,
Termination of Agreement and Extension of Lease to remain in full force and
effect unless in conflict with the terms and conditions of this Sixth Amendment
shall prevail and control.

                                  LANDLORD:
                                  FIRST INDUSTRIAL MORTGAGE
                                  PARTNERSHIP, L.P., a Delaware limited
                                  partnership, successor in interest to WS
                                  Development Company

                                  By: First Industrial Mortgage Corporation, a
                                      Maryland corporation
                                  Its: General Partner

                                      /s/ Richard S. Czerwinski
                                      ------------------------------------------
                                  By: Richard S. Czerwinski
                                      National Regional Director

                                  TENANT:
                                  SOMANETICS CORPORATION, a Michigan
                                  corporation

                                  By:  /s/ Bruce J. Barrett
                                       -----------------------------------------
Dated: April 21, 2004             Its: President and CEO


<PAGE>

                                   EXHIBIT B-1

Landlord shall provide the following:

      -     28 oz. nylon carpet, paint and caulk the joints in the office area.

      -     Install VCT tile in the office area.

      -     Replace stained ceiling tiles.

      -     Caulk windows, where leaking.