Note 8 - Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Stock-based compensation expense	$ 1,444	$ 948	$ 3,018	$ 1,856
Research and Development Expense [Member]
Stock-based compensation expense	174	139	548	298
General and Administrative Expense [Member]
Stock-based compensation expense	$ 1,270	$ 809	$ 2,470	$ 1,558