Note 1 - Organization (Details Textual) - USD ($)						12 Months Ended
	Nov. 01, 2018	Jul. 13, 2018	Jul. 12, 2018	Feb. 16, 2018	Feb. 14, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 03, 2016
Retained Earnings (Accumulated Deficit), Ending Balance						$ (328,885,000)	$ (301,327,000)
Cash and Cash Equivalents, at Carrying Value, Ending Balance						16,410,000	5,389,000	$ 36,096,000
Proceeds from Issuance or Sale of Equity, Net	$ 10,200,000		$ 3,000,000	$ 15,500,000
Common Stock Offering, Maximum Aggregate Value									$ 150,000,000
Proceeds from Issuance of Common Stock						314,000	246,000
Debt Conversion, Original Debt, Amount						$ 42,794,000
Preferred Stock, Par or Stated Value Per Share						$ 0.001	$ 0.001
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	$ 42,800,000
Common Stock Warrants, Issued with Series B Preferred Stock [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				17,979,000		17,688,500
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				500
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				$ 0.40
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				500
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 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				20,900,000	20,900,000
Series C Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues	8,586
Preferred Stock, Par or Stated Value Per Share	$ 0.001
Common Stock [Member]
Stock Issued During Period, Shares, New Issues	7,285,000						234,518
Sales Agreement with Cowen [Member]
Common Stock Offering, Maximum Aggregate Value									$ 50,000,000
Proceeds from Issuance of Common Stock							$ 3,187,000	5,171,000
Underwriting Discount and Commissions							$ 101,000	$ 160,000
Sales Agreement with Cowen [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues							189,684	27,374
Shares Issued, Price Per Share							$ 17.68	$ 194.74
Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member]
Sale of Stock, Consideration Received on Transaction			$ 3,000,000
Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member] | Common Stock Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		1,083,091	1,083,091			1,083,091
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		$ 1.58	$ 1.58
Placement Agency Sale - Prospectus Supplement of July 12, 2018 [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues		2,166,180	2,166,180			2,166,180
Shares Issued, Price Per Share			$ 1.6425	$ 1.58
November Public Offering [Member] | Common Stock Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights	28,750,000
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	$ 0.40
November Public Offering [Member] | Series C Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues	8,586
November Public Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues	7,285,000					7,285,000