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, 2018	$ 27,253	$ 11	$ 100,011	$ (71,660)	$ (1,109)
Balance, shares at Apr. 30, 2018		10,799,895
Issuance of common stock from public offering, net of issuance costs	87,910	$ 6	87,904
Issuance of common stock from public offering, 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 gain (loss) on available-for-sale securities	440				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 common stock from public offering, net of issuance costs	11,422	$ 1	11,421
Issuance of common stock from public offering, 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		40,628
Stock-based compensation expense	4,448		4,448
Net loss	(29,116)			(29,116)
Foreign currency translation adjustments	59				59
Unrealized holding gain (loss) 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
Issuance of common stock from public offering, net of issuance costs	$ 208,934	$ 6	208,928
Issuance of common stock from public offering, net of issuance costs, shares		6,181,250
Issuance of common stock from exercise of stock options, shares	246,575
Issuance of common shares under an at-the-market sales agreement, net of issuance costs of $132	$ 1,648		1,648
Issuance of common shares under an at-the-market sales agreement, net of issuance costs, shares		97,332
Issuance of common stock from equity incentive plans	1,535		1,535
Issuance of common stock from equity incentive plans, shares		298,350
Stock-based compensation expense	7,118		7,118
Net loss	(46,244)			(46,244)
Foreign currency translation adjustments	1,076				1,076
Unrealized holding gain (loss) on available-for-sale securities	(473)				(473)
Reclassification adjustment for realized (gain) on available for sale securities included in net loss	(153)				(153)
Balance at Apr. 30, 2021	$ 257,193	$ 24	$ 426,437	$ (167,836)	$ (1,432)
Balance, shares at Apr. 30, 2021		24,422,531