Note 7 - Stockholders' Equity - Option Activity for Stock Option Plans (Details)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Outstanding, options (in shares) | shares	1,156,027
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.92
Granted, options (in shares) | shares	93,500
Granted, weighted average exercise price, options (in dollars per share) | $ / shares	$ 2.63
Forfeited (in shares) | shares	(21,875)
Forfeited (in dollars per share) | $ / shares	$ 3.26
Expired, options (in shares) | shares	(27,182)
Expired, weighted average exercise price, options (in dollars per share) | $ / shares	$ 4.67
Outstanding, options (in shares) | shares	1,200,470
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.81
Outstanding, weighted average remaining contractual life, options (Year)	8 years 146 days
Outstanding, aggregate intrinsic value, options | $
Vested and Expected to Vest, options (in shares) | shares	960,726
Vested and Expected to Vest, weighted average exercise price, options (in dollars per share) | $ / shares	$ 4.02
Vested and Expected to Vest, weighted average remaining contractual life, options (Year)	8 years 36 days
Vested and Expected to Vest, aggregate intrinsic value, options | $
Exercisable, options (in shares) | shares	303,348
Exercisable, weighted average exercise price, options (in dollars per share) | $ / shares	$ 6.26
Exercisable, weighted average remaining contractual life, options (Year)	5 years 182 days
Exercisable, aggregate intrinsic value, options | $