Note 3 - Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Impairment of Long-Lived Assets Held-for-use	$ 0
Revenue Recognition, Sales Returns, Reserve for Sales Returns	200	$ 0
Allowance for Doubtful Accounts Receivable, Current, Ending Balance	$ 0		$ 0
Number of Reportable Segments	1
UNITED STATES
Cash, Uninsured Amount	$ 700		3,700
Cash, Cash Equivalents, and Short-term Investments, Total	$ 13,400		22,400
UNITED STATES \| Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member]
Concentration Risk, Percentage	89.00%	40.00%
AUSTRALIA
Cash, Uninsured Amount	$ 0		$ 0
Other Nonoperating Income (Expense) [Member]
Foreign Currency Transaction Gain (Loss), before Tax, Total	$ (100)	$ 100

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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