Note 9 - Summary of Stock Options (Details Textual) - USD ($)			1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 29, 2018	Dec. 06, 2017	Jan. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 05, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance				3,690,902		2,694,224
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Ending Balance				$ 5.34		$ 5.80
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term				8 years 229 days		8 years 211 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross				1,253,559
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate				0.00%	0.00%
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options				$ 6,391,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition				3 years 58 days
Nonemployees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate				0.00%
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate				0.00%
The 2006 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance				38,378
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Ending Balance				$ 10.49
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term				4 years 229 days
The 2013 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance				3,652,524
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Ending Balance				$ 5.28
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term				8 years 244 days
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		914,016
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		4,914,016					4,000,000
Common Stock, Capital Shares Reserved for Future Issuance				1,069,228
The 2013 Plan [Member] | Nonemployees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross				2,388	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value				$ 2.98
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				0	0
The 2013 Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number, Ending Balance				22,500
The 2013 Plan [Member] | Restricted Stock [Member] | Board Members [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period				9,637
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value			$ 5.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period			9,637
The 2013 Plan [Member] | Restricted Stock [Member] | Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period			25,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value			$ 5.19
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period			12,500
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period			1 year
2017 Employee Stock Purchase Plan [Member]
Stock Issued During Period, Shares, Employee Stock Purchase Plans	20,744
Employee Stock Purchase Plan, Purchase Price				$ 3.11
Employee Stock Ownership Plan (ESOP), Number of Suspense Shares	361,362
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate				0.00%
2017 Employee Stock Purchase Plan [Member] | Weighted Average [Member]
Share Price				$ 1.30
2017 Employee Stock Purchase Plan [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross				1,251,171	229,860
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value				$ 2.77	$ 1.96
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				0	0