Note 14 - Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.S. federal net operating losses	$ 11,410	$ 11,478
State net operating losses (net of federal)	2,234	2,865
U.S. federal research and development credits	948	849
U.K. net operating losses	1,132	496
Share options	1,742	1,724
Accrued liabilities	406	1,052
Operating lease liabilities	101
State credits	536	512
Other	99	136
Total deferred tax assets	18,608	19,112
Valuation allowance	(16,127)	(17,991)	$ (48,098)
Total deferred tax assets	2,481	1,121
Intangibles	(24)
Operating lease right-of-use assets	(113)
Depreciation	(118)
Other assets	(63)	(69)
Total deferred tax liabilities	$ (318)	$ (69)