Note 4 - Stockholders' Equity - Summary of Warrant Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2021	Jun. 30, 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)		$ 5.51
Remaining Contractual Term (Year)		5 years 3 months
Aggregate Intrinsic Value		$ 0
June 2017 Warrants [Member]
Warrants outstanding (in shares)		2,457,501	2,457,501
Warrants exercised (in shares)	(168,226)	0
Warrants exercise price (in dollars per share)		$ 7.50		$ 6.41	$ 7.50
Remaining Contractual Term (Year)		4 years 11 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)	(210,000)	0
Warrants exercise price (in dollars per share)		$ 4.31
Remaining Contractual Term (Year)		3 years 4 months 17 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 1 month 9 days
Aggregate Intrinsic Value		$ 0