Note 5 - Income Taxes - Components of Net Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net deferred tax liability:
Excess of tax over book depreciation and amortization	$ (1,980)	$ (3,321)
Prepaid expenses	(39)	(55)
Allowance for doubtful accounts	14	18
Accrued expenses	105	133
Stock-based compensation	79	135
R&D tax credits	578	397
Net deferred tax liabilities	$ (1,243)	$ (2,693)