As of December 31, 2015 and 2014, our deferred tax assets were approximately $74.2 million and $63.2 million, respectively.  The components of our deferred tax assets are summarized as follows:

Current accounting standards require a valuation allowance against deferred tax assets if, based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets may not be realized.  Due to the uncertainty surrounding the realization of these deferred tax assets in future tax returns, all of the deferred tax assets have been fully offset by a valuation allowance at December 31, 2015 and 2014.

In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized.  The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible.  Management considers the scheduled reversal of deferred tax liabilities (including the impact of available carryback and carryforward periods), projected future taxable income, and tax-planning strategies in making this assessment.  Based upon the level of historical taxable income and projections for future taxable income over the periods in which the deferred tax assets are deductible, management believes it is more likely than not that the Company will not realize the benefits of these deductible differences or tax carryforwards as of December 31, 2015.

As of December 31, 2015 and 2014, we had U.S. net operating loss carryforwards of approximately $177.6 million and $153.1 million, respectively.  Of those amounts, $15.3 million and $15.3 million relates to stock-based compensation tax deductions in excess of book compensation expense (APIC NOLs) as of December 31, 2015 and 2014, respectively, that will be credited to additional paid-in capital when such deductions reduce taxes payable as determined on a "with-and-without" basis.  Accordingly, these APIC NOLs will reduce federal taxes payable if realized in future periods, but NOLs related to such benefits are not included in the table above.

At December 31, 2015 and 2014, we had U.S. R&D credit carryforwards of approximately $9.1 million and $8.0 million, respectively.

There were no expirations of U.S. net operating loss carryforwards or R&D credit carryforwards during 2015 or 2014.  The details of our U.S. net operating loss and federal R&D credit carryforward amounts and expiration dates are summarized as follows:

As of December 31, 2015 and 2014, we also had state net operating loss carryforwards of approximately $24.7 million and $16.9 million, respectively.  The state net operating loss carryforwards will begin expiring in 2032.

As of December 31, 2015 and 2014, Cardiosonix had tax loss carryforwards in Israel of approximately $7.6 million.  Under current Israeli tax law, net operating loss carryforwards do not expire.  Due to the uncertainty surrounding the realization of the related deferred tax assets in future tax returns, all of the deferred tax assets have been fully offset by a valuation allowance at December 31, 2015 and 2014.

Under Sections 382 and 383 of the IRC of 1986, as amended, the utilization of U.S. net operating loss and R&D tax credit carryforwards may be limited under the change in stock ownership rules of the IRC.  The Company previously completed a Section 382 analysis in 2013 and does not believe a Section 382 ownership change has occurred since then that would impact utilization of the Company’s net operating loss and R&D tax credit carryforwards.

Reconciliations between the statutory federal income tax rate and our effective tax rate for continuing operations are as follows: