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, 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 equity incentive plans	216		216
Issuance of common stock from equity incentive plans, 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 gains (losses) from available for sale securities	285				285
Reclassification adjustment for realized (gain) loss 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 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 of $132, 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 gains (losses) from available for sale securities	(473)				(473)
Reclassification adjustment for realized (gain) loss 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
Issuance of common stock from equity incentive plans	1,582	$ 1	1,581
Issuance of common stock from equity incentive plans, shares		128,217
Stock-based compensation expense	11,086		11,086
Net loss	(82,339)			(82,339)
Foreign currency translation adjustments	(1,499)				(1,499)
Unrealized holding gains (losses) from available for sale securities	(1,511)				(1,511)
Reclassification adjustment for realized (gain) loss on available for sale securities included in net loss	581				581
Balance at Apr. 30, 2022	$ 185,093	$ 25	$ 439,104	$ (250,175)	$ (3,861)
Balance, shares at Apr. 30, 2022		24,550,748