Share-Based Payments - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 152,000	$ 237,000	$ 863,000	$ 1,901,000
Option modification charge			56,529
Shares issued in relation to consulting services, value			15,000
Issuance of common stock for services			15,000	$ 20,000
Unrecognized compensation cost	$ 203,420		$ 203,420
Period for recognition			4 years 3 months 19 days
Weighted-average grant date fair value (in usd per share)	$ 2.32	$ 1.35	$ 1.95	$ 1.71
Shares granted (in shares)			267,310
Grants in period, weighted average exercise price (in usd per share)			$ 2.68
Warrants outstanding (in shares)	2,004,284		2,004,284