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Note 7 - Stockholders' Equity - Option Valuation Assumptions (Details)
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Employee Stock Option [Member]    
Risk-free interest rate 2.00% 1.50%
Dividend yield 0.00% 0.00%
Volatility 81.40% 90.60%
Expected life (years) (Year) 6 years 6 years
Employee Stock Purchase Plan [Member]    
Risk-free interest rate 1.00% 0.40%
Dividend yield 0.00% 0.00%
Volatility 56.00% 85.10%
Expected life (years) (Year) 180 days 180 days