Note 11 - Common Stock (Details Textual) - USD ($)						1 Months Ended					2 Months Ended	3 Months Ended		6 Months Ended
	May 04, 2021	Jan. 19, 2021	Jun. 09, 2020	Jun. 08, 2020	Apr. 16, 2020	Mar. 31, 2021	Feb. 28, 2021	Jun. 30, 2020	Mar. 31, 2020	Feb. 29, 2020	Mar. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Jan. 31, 2021	Jan. 18, 2021	Dec. 31, 2020	[1]	Apr. 20, 2020	May 22, 2017
Proceeds from Issuance of Common Stock														$ 25,122,000	$ 0
Proceeds from Warrant Exercises														$ 179,000	4,800,000
Stock Issued During Period, Share, Restricted Common Shares (in shares)								3,453	2,832					0
Stock Issued During Period, Value, Restricted Common Shares								$ 25,000	$ 24,000				$ 25,000
Amortization of Debt Discount (Premium)												$ 1,000	1,000	$ 2,000	2,000
Debt Instrument, Unamortized Discount, Total												7,000		7,000
Fair Value Adjustment of Warrants												$ 86,000	$ 1,838,000	$ 373,000	$ 1,838,000
Class of Warrant or Right Number of Securities Called by Warrants or Rights Cancelled In Period (in shares)												0	0	0	0
Class Of Warrant Or Right, Number Of Securities Called By Warrants Or Rights Expired In Period (in shares)												6	22	6	22
Series C Preferred Stock [Member]
Preferred Stock, Shares Outstanding, Ending Balance (in shares)						0					0	0		0				0
Warrants Issued in Connection with January 2021 Offering [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)																1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)																$ 3.40
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		8,117,640				8,104,880					8,104,880	8,104,880		8,104,880		8,117,640
Stock Issued During Period, Shares, Warrant Exercises (in shares)											12,760
Proceeds from Warrant Exercises											$ 43,000
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)																					$ 9,500.00
Warrants and Rights Outstanding, Term (Year)																					10 years
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)																					223
Series A Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)												0		0
Stock Issued During Period, Shares, Warrant Exercises (in shares)					482,059					102,626
Proceeds from Warrant Exercises										$ 1,591,000
Series B Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 3.40															$ 6.10
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)												285,632		285,632
Stock Issued During Period, Shares, Warrant Exercises (in shares)					24,279		40,000			4,548
Proceeds from Warrant Exercises							$ 136,000			$ 70,000
Series A-2 and Series B-2 Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 3.40															$ 6.371			$ 6.371
Warrants and Rights Outstanding, Term (Year)																				5 years
Fair Value Adjustment of Warrants		$ 287,000
Series B, A-2 and B-2 Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 2.817
Fair Value Adjustment of Warrants	$ 86,000
January 2021 Offering Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)												8,104,880		8,104,880
Stock Issued During Period, Shares, Warrant Exercises (in shares)						4,000	8,760
Proceeds from Warrant Exercises						$ 13,000	$ 30,000
Series A-2 Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)												392,830		392,830						482,059
Stock Issued During Period, Shares, Warrant Exercises (in shares)														392,830
Series B-2 Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)												20,380		20,380						24,279
Stock Issued During Period, Shares, Warrant Exercises (in shares)														20,380
Class A Units [Member] | Warrants Issued in Connection with Class A Units [Member]
Warrants Per Unit (in shares)		1
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)		1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 3.40
Warrants and Rights Outstanding, Term (Year)		5 years
Class B Units [Member] | Warrants Issued in Connection with Class B Units [Member]
Warrants Per Unit (in shares)		1
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)		1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 3.40
Warrants and Rights Outstanding, Term (Year)		5 years
Purchase Agreement with LPC [Member]
Maximum Amount of Shares Issuable				$ 10,000,000
Stock Purchase Agreement Duration Period (Month)				30 months
Stock Purchase Agreement, Maximum Issuable Shares (in shares)				301,762
Stock Purchase Agreement, Maximum Issuable Shares, Percent of Outstanding Stock				19.99%
Stock Purchase Agreement, Share Price Covenant Trigger (in dollars per share)				$ 6.46
Stock Issued During Period, Shares, New Issues (in shares)	250,000		52,500
Shares Issued, Price Per Share (in dollars per share)	$ 2.817		$ 6.50
Proceeds from Issuance of Common Stock	$ 704,000		$ 341,000
Equity Facility, Remaining Financing Commitment												$ 9,000,000		$ 9,000,000
First Amendment to the LPC Purchase Agreement [Member]
Stock Purchase Agreement, Maximum Issuable Shares (in shares)						2,068,342					2,068,342
Stock Purchase Agreement, Maximum Issuable Shares, Percent of Outstanding Stock						19.99%					19.99%
Stock Purchase Agreement, Share Price Covenant Trigger (in dollars per share)						$ 2.99					$ 2.99
Stock Issued During Period, Shares, New Issues (in shares)	250,000
Shares Issued, Price Per Share (in dollars per share)	$ 2.817
Proceeds from Issuance of Common Stock	$ 704,000
January 2021 Offering [Member]
Shares Issued, Price Per Share (in dollars per share)		$ 3.40
Proceeds from Issuance or Sale of Equity, Total		$ 27,600,000
Proceeds from Issuance or Sale of Equity, Net of Issuance Costs		$ 25,122,000
January 2021 Offering [Member] | Series C Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)		2,450,880
January 2021 Offering [Member] | Class A Units [Member]
Shares Issued, Price Per Share (in dollars per share)		$ 3.40
Units Issued, Shares (in shares)		4,607,940
Common Shares Per Unit (in shares)		1
Warrants Per Unit (in shares)		1
January 2021 Offering [Member] | Class B Units [Member]
Shares Issued, Price Per Share (in dollars per share)		$ 3.40
Units Issued, Shares (in shares)		2,450,880
Common Shares Per Unit (in shares)		1
Warrants Per Unit (in shares)		1
Over-Allotment Option [Member]
Stock Issued During Period, Shares, New Issues (in shares)		1,058,820
Over-Allotment Option [Member] | Warrants Issued in Connection with January 2021 Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		1,058,820

[1] The condensed consolidated balance sheet as of December 31, 2020 has been derived from the audited consolidated financial statements as of that date.