Note 2 - Summary of Significant Accounting Policies - 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	$ (639)	$ (627)	$ (1,984)	$ (1,672)
Stock-based compensation expense	639	627	1,984	1,672
Research and Development Expense [Member]
Stock-based compensation expense	256	229	676	693
Stock-based compensation expense	(256)	(229)	(676)	(693)
General and Administrative Expense [Member]
Stock-based compensation expense	(383)	(398)	(1,308)	(979)
Stock-based compensation expense	$ 383	$ 398	$ 1,308	$ 979