Note 2 - Summary of Significant Accounting Policies - 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	$ 725	$ 540	$ 1,345	$ 1,045
Research and Development Expense [Member]
Stock-based compensation expense	187	245	420	463
General and Administrative Expense [Member]
Stock-based compensation expense	$ 538	$ 295	$ 925	$ 582