Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Balance at Apr. 30, 2017	$ 31,327	$ 10	$ 89,815	$ (55,855)	$ (2,643)
Balance, shares at Apr. 30, 2017		9,713,042
Issuance of ordinary shares/ common stock, net of issuance costs	9,101	$ 1	9,100
Issuance of ordinary shares/ common stock, net of issuance costs, shares		1,070,589
Issuance of common stock from exercise of stock options	36		36
Issuance of common stock from exercise of stock options, shares		16,264
Stock-based compensation expense	1,060		1,060
Net loss	(15,805)			(15,805)
Foreign currency translation adjustments	1,534				1,534
Balance at Apr. 30, 2018	27,253	$ 11	100,011	(71,660)	(1,109)
Balance, shares at Apr. 30, 2018		10,799,895
Issuance of ordinary shares/ common stock, net of issuance costs	87,910	$ 6	87,904
Issuance of ordinary shares/ common stock, net of issuance costs, shares		6,382,320
Issuance of common stock from exercise of stock options	242		242
Issuance of common stock from exercise of stock options, shares		95,535
Stock-based compensation expense	2,966		2,966
Net loss	(20,816)			(20,816)
Foreign currency translation adjustments	(1,257)				(1,257)
Unrealized holding gains from available-for-sale securities, net of reclassification for realized gains	440				440
Unrealized holding gains on available-for-sale securities	440
Balance at Apr. 30, 2019	96,738	$ 17	191,123	(92,476)	(1,926)
Balance, shares at Apr. 30, 2019		17,277,750
Issuance of ordinary shares/ common stock, net of issuance costs	11,422	$ 1	11,421
Issuance of ordinary shares/ common stock, net of issuance costs, shares		527,221
Issuance of common stock from exercise of stock options	$ 216		216
Issuance of common stock from exercise of stock options, shares	37,494	40,628
Stock-based compensation expense	$ 4,448		4,448
Net loss	(29,116)			(29,116)
Foreign currency translation adjustments	59				59
Unrealized holding gains on available-for-sale securities	285				285
Reclassification adjustment for realized (gain) on available for sale securities included in net loss	(300)				(300)
Balance at Apr. 30, 2020	$ 83,752	$ 18	$ 207,208	$ (121,592)	$ (1,882)
Balance, shares at Apr. 30, 2020		17,845,599