Note 6 - Stock-based Compensation - Stock Option Activity for Employees and Non-employees (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Options Outstanding, Beginning Balance (in shares)		2,681,072
Options Outstanding, Weighted Average Exercise Price, Beginning Balance (in dollars per share)		$ 7.18
Options Outstanding, Weighted-average Remaining Contractual Term (Year)		8 years	6 years 292 days
Options Outstanding, Aggregate Intrinsic Value		$ 678	$ 121
Options Granted (in shares)		1,790,000
Options Granted, Weighted Average Exercise Price (in dollars per share)		$ 2.65
Options Exercised (in shares)
Options Exercised, Weighted Average Exercise Price (in dollars per share)
Options Forfeited (in shares)		(83,433)
Options Forfeited, Weighted Average Exercise Price (in dollars per share)		$ 5.82
Options Expired (in shares)		(6,702)
Options Expired, Weighted Average Exercise Price (in dollars per share)		$ 2.38
Options Outstanding, Ending Balance (in shares)		4,380,937	2,681,072
Options Outstanding, Weighted Average Exercise Price, Ending Balance (in dollars per share)		$ 5.37	$ 7.18
Options Exercisable (in shares)		1,716,700
Options Exercisable, Weighted Average Exercise Price (in dollars per share)		$ 7.76
Options Exercisable, Weighted-average Remaining Contractual Term (Year)		6 years
Options Exercisable, Aggregate Intrinsic Value		$ 232
Options Vested or Expected to Vest (in shares)	[1]	4,380,937
Options Vested or Expected to Vest, Weighted Average Exercise Price (in dollars per share)	[1]	$ 5.37
Options Vested or Expected to Vest, Weighted-average Remaining Contractual Term (Year)	[1]	8 years
Options Vested or Expected to Vest, Aggregate Intrinsic Value	[1]	$ 678

[1] Represents the number of vested options at March 31, 2018 plus the number of unvested options expected to vest based on the unvested options outstanding at March 31, 2018.