CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Beginning balance, shares at Apr. 30, 2016	1,577,440	33,847,213
Beginning balance, value at Apr. 30, 2016	$ 2	$ 34	$ 559,314	$ (509,276)	$ 50,074
Series E preferred stock issued, net of issuance costs, shares	70,230
Series E preferred stock issued, net of issuance costs, value			1,576		1,576
Series E preferred stock dividends			(4,279)		(4,279)
Common stock issued, net of issuance costs, shares		9,887,726
Common stock issued, net of issuance costs, value		$ 10	30,440		30,450
Common stock issued under Employee Stock Purchase Plan, shares		270,075
Common stock issued under Employee Stock Purchase Plan, value			526		526
Exercise of stock options, shares		9,026
Exercise of stock options, value			31		31
Share-based compensation expense			3,363		3,363
Net loss				(28,159)	(28,159)
Ending balance, shares at Apr. 30, 2017	1,647,760	44,014,040
Ending balance, value at Apr. 30, 2017	$ 2	$ 44	590,971	(537,435)	53,582
Series E preferred stock dividends			(4,325)		(4,325)
Cumulative-effect adjustment to accumulated deficit pursuant to adoption of ASU			(119)	119
Common stock issued, net of issuance costs, shares		11,345,704
Common stock issued, net of issuance costs, value		$ 11	25,676		25,687
Common stock issued under Employee Stock Purchase Plan, shares		88,327
Common stock issued under Employee Stock Purchase Plan, value			317		317
Exercise of stock options, shares		222,255
Exercise of stock options, value			752		752
Fractional shares issued pursuant to reverse stock split		18,896
Share-based compensation expense			1,538		1,538
Net loss				(21,813)	(21,813)
Ending balance, shares at Apr. 30, 2018	1,647,760	55,689,222
Ending balance, value at Apr. 30, 2018	$ 2	$ 55	614,810	(559,129)	55,738
Series E preferred stock dividends			(4,325)		(4,325)
Cumulative-effect adjustment to accumulated deficit pursuant to adoption of ASU				2,739	2,739
Common stock issued under Employee Stock Purchase Plan, shares		75,148
Common stock issued under Employee Stock Purchase Plan, value			258		258
Exercise of stock options, shares		371,327
Exercise of stock options, value		$ 1	1,277		1,278
Share-based compensation expense			1,595		1,595
Net loss				(4,215)	(4,215)
Ending balance, shares at Apr. 30, 2019	1,647,760	56,135,697
Ending balance, value at Apr. 30, 2019	$ 2	$ 56	$ 613,615	$ (560,605)	$ 53,068