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Note 1 - Background, Basis of Presentation and Liquidity (Details Textual)
3 Months Ended 12 Months Ended
Oct. 22, 2015
Jul. 31, 2015
USD ($)
Apr. 30, 2015
USD ($)
Apr. 30, 2014
USD ($)
$ / shares
shares
Jul. 14, 2015
$ / shares
Jan. 14, 2015
$ / shares
Proceeds from S-3 Shelf for Period [Member]            
Proceeds from Issuance of Common Stock   $ 0 $ 0      
Minimum [Member]            
Share Price | $ / shares         $ 1 $ 1
Scenario, Forecast [Member]            
Debt Instrument, Convertible, Conversion Ratio 12          
ATM Facility [Member]            
Proceeds from Issuance of Common Stock       $ 9,698,000    
Share Price | $ / shares       $ 3.02    
Stock Issued During Period, Shares, New Issues | shares       3,306,334    
Underwriting Agreement [Member]            
Proceeds from Issuance of Common Stock       $ 10,828,000    
Share Price | $ / shares       $ 3.10    
Stock Issued During Period, Shares, New Issues | shares       3,800,000    
Shelf Registration Amount Remaining of Offering Amount   3,300,000        
Retained Earnings (Accumulated Deficit)   (168,917,096) $ (164,755,055)      
Cash Cash Equivalents and Marketable Securities   14,200,000        
Restricted Cash and Cash Equivalents   $ 500,000        
Debt Instrument, Convertible, Conversion Ratio   5        
Shelf Registration Authorized Amount   $ 40,000,000        
Shelf Registration Amount Remaining of Offering Amount   18,200,000        
Shelf Registration Market Capitalization   9,900,000        
Shelf Registration Limited Amount to Sell Securities   $ 12,000,000