Note 1 - Organization (Details Textual) - USD ($)							1 Months Ended	2 Months Ended	12 Months Ended
	Jan. 31, 2020	Aug. 26, 2019	Nov. 01, 2018	Jul. 16, 2018	Feb. 16, 2018	Feb. 14, 2018	Jan. 31, 2020	May 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Jul. 13, 2018	Dec. 31, 2017
Retained Earnings (Accumulated Deficit), Ending Balance									$ (348,335,000)	$ (328,885,000)
Cash and Cash Equivalents, at Carrying Value, Ending Balance									10,943,000	16,410,000		$ 5,389,000
Proceeds from Issuance or Sale of Equity, Net					$ 15,500,000
Proceeds from Warrant Exercises								$ 8,000,000	$ 7,993,000	$ 581,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights					1,797,900
Preferred Stock, Par or Stated Value Per Share					$ 0.001				$ 0.001	$ 0.001
Debt Conversion, Original Debt, Amount										$ 42,794,000
Share Price									$ 1.14
CRG [Member] | Conversion of Principal Amount of Senior Secured Loan to Newly Authorized Series A Preferred Stock [Member]
Debt Conversion, Original Debt, Amount					$ 38,000,000	$ 38,000,000
Debt Conversion, Converted Instrument, Amount					$ 41,800,000	$ 41,800,000
Common Stock Warrants, Issued with Series B Preferred Stock, Warrant One [Member]
Class of Warrant or Right, Number of Warrants Issued for Each Share of Preferred Stock					1
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					50
Class of Warrant or Right, Contingent Expiration Period Following Initial Exercise Date					7 years
Class of Warrant or Right, Exercise Price of Warrants or Rights					$ 4
Common Stock Warrants, Issued with Series B Preferred Stock, Warrant Two [Member]
Class of Warrant or Right, Number of Warrants Issued for Each Share of Preferred Stock					1
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					50
Class of Warrant or Right, Contingent Expiration Period Following Initial Exercise Date					7 years
Class of Warrant or Right, Expiration Period Following FDA Clearance					60 days
Series C Preferred Stock [Member]
Proceeds from Issuance or Sale of Equity, Net			$ 10,200,000
Stock Issued During Period, Shares, New Issues			8,586
Preferred Stock, Par or Stated Value Per Share			$ 0.001
Series B Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues					17,979
Preferred Stock, Par or Stated Value Per Share					$ 0.001
Series A Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues					41,800
Series A Preferred Stock [Member] | Conversion of Principal Amount of Senior Secured Loan to Newly Authorized Series A Preferred Stock [Member]
Convertible Preferred Stock, Shares of Common Stock Issuable on Conversion					2,090,000	2,090,000
January 2020 Public Offering [Member] | Subsequent Event [Member]
Proceeds from Issuance or Sale of Equity, Net	$ 3,700,000						$ 3,700,000
Stock Issued During Period, Shares, New Issues	6,428,572
Share Price	$ 0.70						$ 0.70
August Public Offering [Member]
Proceeds from Issuance or Sale of Equity, Net		$ 3,800,000
Stock Issued During Period, Shares, New Issues		3,813,559
Share Price		$ 1.18
November Public Offering [Member]
Stock Issued During Period, Shares, New Issues			728,500
November Public Offering [Member] | Common Stock Warrants [Member]
Proceeds from Warrant Exercises									$ 8,000,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			2,875,000
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right			1
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 4
November Public Offering [Member] | Series C Preferred Stock [Member]
Proceeds from Issuance or Sale of Equity, Net			$ 10,200,000
Stock Issued During Period, Shares, New Issues			8,586
Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member]
Proceeds from Issuance or Sale of Equity, Net				$ 3,000,000
Sale of Stock, Consideration Received on Transaction				$ 3,000,000
Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member] | Common Stock Warrants [Member]
Stock Issued During Period, Shares, New Issues				216,618
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				108,309							108,309
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right				1
Shares Issued, Price Per Share				$ 16.425
Class of Warrant or Right, Exercise Price of Warrants or Rights				$ 15.80							$ 15.80