Note 2 - Summary of Significant Accounting Policies - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Stock-based compensation expense	$ 292	$ 639	$ 1,741	$ 1,984
Research and Development Expense [Member]
Stock-based compensation expense	(121)	256	480	676
General and Administrative Expense [Member]
Stock-based compensation expense	$ 413	$ 383	$ 1,261	$ 1,308