Note 2 - Summary of Significant Accounting Policies - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Stock-based compensation expense	$ 761	$ 725	$ 1,449	$ 1,345
Research and Development Expense [Member]
Stock-based compensation expense	337	187	601	420
General and Administrative Expense [Member]
Stock-based compensation expense	$ 424	$ 538	$ 848	$ 925