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Note 2 - Stock-based Compensation (Tables)
 6 Months Ended
Jun. 30, 2019
Notes Tables  
Share-based Payment Arrangement, Activity [Table Text Block]
   
Awards
Available
  
Options
Outstanding
  
Weighted
Average
Exercise
Price
Balance December 31, 2018    
805
      
17,491
     $
6.49
  
Plan amendment    
4,000
      
—
      
—
  
Stock option awards granted    
(826
 )    
826
      
6.90
  
Stock option awards exercised    
—
      
(251
 )    
3.75
  
Stock option awards cancelled    
123
      
(123
 )    
7.28
  
Balance June 30, 2019    
4,102
      
17,943
     $
6.54
  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
    2019   2018
Expected Life in Years    
5.5
      
5.5
  
Expected Volatility    
81.0
 %    
82.0
 %
Expected Dividend Yield    
0.0
 %    
0.0
 %
Risk-Free Interest Rate    
2.3
 %    
2.7
 %