CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Beginning balance, shares at Apr. 30, 2015	1,574,764	27,620,947
Beginning balance, value at Apr. 30, 2015	$ 2,000	$ 28,000	$ 512,629,000	$ (453,624,000)	$ 59,035,000
Series E preferred stock issued for cash, net of issuance costs, shares	2,676
Series E preferred stock issued for cash, net of issuance costs, value			59,000		59,000
Series E preferred stock dividends			(4,139,000)		(4,139,000)
Common stock issued for cash, net of issuance costs, shares		1,232,821
Common stock issued for cash, net of issuance costs, value		$ 1,000	11,144,000		11,145,000
Common stock issued for cash, net of issuance costs, shares		964,523
Common stock issued for cash, net of issuance costs, value		$ 1,000	7,256,000		7,257,000
Common stock issued for cash, net of issuance costs, shares		1,210,328
Common stock issued for cash, net of issuance costs, value		$ 1,000	6,793,000		6,794,000
Common stock issued for cash, net of issuance costs, shares		2,645,503
Common stock issued for cash, net of issuance costs, value		$ 3,000	19,996,000		19,999,000
Common stock issued under Employee Stock Purchase Plan, shares		147,769
Common stock issued under Employee Stock Purchase Plan, value			540,000		540,000
Common stock issued upon exercise of options, shares		25,322
Common stock issued upon exercise of options, value			138,000		138,000
Share-based compensation			4,898,000		4,898,000
Net loss				(55,652,000)	(55,652,000)
Ending balance, shares at Apr. 30, 2016	1,577,440	33,847,213
Ending balance, value at Apr. 30, 2016	$ 2,000	$ 34,000	559,314,000	(509,276,000)	50,074,000
Series E preferred stock issued for cash, net of issuance costs, shares	70,320
Series E preferred stock issued for cash, net of issuance costs, value			1,576,000		1,576,000
Series E preferred stock dividends			(4,279,000)		(4,279,000)
Common stock issued for cash, net of issuance costs, shares		6,137,403
Common stock issued for cash, net of issuance costs, value		$ 6,000	17,753,000		17,759,000
Common stock issued for cash, net of issuance costs, shares		3,750,323
Common stock issued for cash, net of issuance costs, value		$ 4,000	12,687,000		12,691,000
Common stock issued under Employee Stock Purchase Plan, shares		270,075
Common stock issued under Employee Stock Purchase Plan, value			526,000		526,000
Common stock issued upon exercise of options, shares		9,026
Common stock issued upon exercise of options, value			31,000		31,000
Share-based compensation			3,363,000		3,363,000
Net loss				(28,159,000)	(28,159,000)
Ending balance, shares at Apr. 30, 2017	1,647,760	44,014,040
Ending balance, value at Apr. 30, 2017	$ 2,000	$ 44,000	590,971,000	(537,435,000)	53,582,000
Series E preferred stock dividends			(4,325,000)		(4,325,000)
Cumulative-effect adjustment to accumulated deficit			(119,000)	119,000
Common stock issued for cash, net of issuance costs, shares		1,051,259
Common stock issued for cash, net of issuance costs, value		$ 1,000	4,192,000		4,193,000
Common stock issued for cash under Public Offering, net of issuance costs, Shares		10,294,445
Common stock issued for cash under Public Offering, net of issuance costs, Value		$ 10,000	21,484,000		21,494,000
Common stock issued under Employee Stock Purchase Plan, shares		88,327
Common stock issued under Employee Stock Purchase Plan, value			317,000		317,000
Common stock issued upon exercise of options, shares		222,255
Common stock issued upon exercise of options, value			752,000		752,000
Fractional shares issued pursuant to reverse stock split		18,896
Share-based compensation			1,538,000		1,538,000
Net loss				(21,813,000)	(21,813,000)
Ending balance, shares at Apr. 30, 2018	1,647,760	55,689,222
Ending balance, value at Apr. 30, 2018	$ 2,000	$ 55,000	$ 614,810,000	$ (559,129,000)	$ 55,738,000