Note 10 - Stock-based Compensation Expense - Summary of Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Balance (in shares)	417,000	450,000
Balance, grant date weighted-average exercise price (in dollars per share)	$ 2.79	$ 2.78
Balance, weighted average remaining contractual life (Year)	3 years 3 months 29 days	1 year 7 months 9 days	2 years 5 months 8 days
Granted (in shares)	6,000	10,000
Granted, grant date weighted-average exercise price (in dollars per share)	$ 2.92	$ 1.88
Granted, weighted average remaining contractual life (Year)	7 years	7 years
Forfeited (in shares)	(334,000)	(43,000)
Forfeited, grant date weighted-average exercise price (in dollars per share)	$ 2.81	$ 2.54
Balance, aggregate intrinsic value	$ 0	$ 2,000
Exercised (in shares)	(22,000)	0
Exercised, grant date weighted-average exercise price (in dollars per share)	$ 2.65
Balance (in shares)	67,000	417,000	450,000
Balance, grant date weighted-average exercise price (in dollars per share)	$ 2.72	$ 2.79	$ 2.78
Exercisable (in shares)	58,000	405,000
Exercisable, grant date weighted-average exercise price (in dollars per share)	$ 2.72	$ 2.80
Exercisable, weighted average remaining contractual life (Year)	2 years 11 months 15 days
Minimum [Member]
Exercisable, weighted average remaining contractual life (Year)		1 year 5 months 23 days