Note 9 - Share-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Stock options outstanding, beginning of period (in shares)		2,182,708
Balance, weighted average exercise price, beginning of period (in dollars per share)		$ 4.04
Stock options outstanding, weighted average remaining contractual term (Year)	6 years 9 months 18 days	7 years 6 months
Stock options outstanding, aggregate intrinsic value	$ 43		$ 343
Stock options granted (in shares)	708,284
Stock options granted, weighted average exercise price (in dollars per share)	$ 1.39
Stock options exercised (in shares)	(199)
Stock options exercised, weighted average exercise price (in dollars per share)	$ 1.49
Stock options forfeited (in shares)	(397,677)
Stock options forfeited, weighted average exercise price (in dollars per share)	$ 2.42
Stock options expired (in shares)	(6,349)
Stock options expired, weighted average exercise price (in dollars per share)	$ 16.09
Stock options outstanding, end of period (in shares)	2,486,767
Balance, weighted average exercise price, end of period (in dollars per share)	$ 3.51
Exercisable, December 31, 2022 (in shares)	1,552,294
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.56
Options exercisable, weighted average remaining contractual term (Year)	5 years 6 months
Options exercisable, aggregate intrinsic value	$ 0
Options vested and expected to vest (in shares)	2,486,767
Options vested and expected to vest, weighted average exercise price (in dollars per share)	$ 3.51
Options vested and expected to vest, weighted average remaining contractual term (Year)	6 years 9 months 18 days
Options vested and expected to vest, aggregate intrinsic value	$ 43