Note 4 - Stockholders' Equity - Summary of Warrant Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Aug. 09, 2017	Jun. 30, 2017
Warrants outstanding (in shares)	6,596,096	6,596,096
Warrants exercised (in shares)	0
Warrants exercise price (in dollars per share)	$ 4.47
Remaining Contractual Term (Year)	4 years 2 months 12 days
Aggregate Intrinsic Value	$ 0
June 2017 Warrants [Member]
Warrants outstanding (in shares)	2,457,501	2,457,501
Warrants exercised (in shares)	0
Warrants exercise price (in dollars per share)	$ 7.50		$ 6.41	$ 7.50
Remaining Contractual Term (Year)	5 years 2 months 12 days
Aggregate Intrinsic Value	$ 0
November 2020 Warrants [Member]
Warrants outstanding (in shares)	4,109,344	4,109,344
Warrants exercised (in shares)	0
Warrants exercise price (in dollars per share)	$ 4.31
Remaining Contractual Term (Year)	3 years 7 months 6 days
Aggregate Intrinsic Value	$ 0
SVB Warrants [Member]
Warrants outstanding (in shares)	29,251	29,251
Warrants exercised (in shares)	0
Warrants exercise price (in dollars per share)	$ 6.41
Remaining Contractual Term (Year)	5 years 4 months 24 days
Aggregate Intrinsic Value	$ 0