Significant Accounting Policies - Narrative (Details) - USD ($)	3 Months Ended				9 Months Ended
	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Jan. 31, 2018	Jan. 31, 2019	Jan. 31, 2018	Apr. 30, 2018
Accounting Policies [Abstract]
Restricted cash	$ 0				$ 0		$ 150,000
Net income (loss)	(369,000)	$ 267,000	$ 483,000	$ (76,000)	381,000	$ (844,000)
Net cash used in operating activities					(1,610,000)	1,235,000
Accumulated deficit	(70,445,000)				(70,445,000)		(70,826,000)
Positive working capital	(177,000)				(177,000)
Cash and cash equivalents	3,310,000				3,310,000
Unrecognized tax benefits	151,000				151,000		$ 151,000
Provision for income taxes	$ 0			$ 2,000	$ 0	$ 18,000