Note 3 - Liquidity Risks and Management's Plans (Details Textual) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Mar. 25, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Sep. 17, 2020
Cash and Cash Equivalents, at Carrying Value, Ending Balance		$ 38,490		$ 16,930
Liabilities, Current, Total		7,484		6,131
Stock Issued During Period, Shares, New Issues (in shares)	9,230,500
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	9,230,500
Stock and Warrants Issued, Price Per Share (in dollars per share)	$ 3.25
Proceeds from Issuance or Sale of Equity, Total	$ 27,400
Net Income (Loss) Attributable to Parent, Total		(8,961)	$ (6,534)
Retained Earnings (Accumulated Deficit), Ending Balance		$ (726,649)		$ (717,688)
September 2020 At The Market Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)		105,083
Proceeds from Issuance or Sale of Equity, Total		$ 600
Shares Authorized to Be Offered and Sold Under Offering Agreement, Value					$ 10,000