Note 8 - Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Stock-based compensation expense	$ 1,287	$ 987	$ 4,305	$ 2,843
Research and Development Expense [Member]
Stock-based compensation expense	170	178	718	476
General and Administrative Expense [Member]
Stock-based compensation expense	$ 1,117	$ 809	$ 3,587	$ 2,367