Summary of Significant Accounting Policies - Summary of Revenue Disaggregated by Type and Geography (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,024	$ 5,422	$ 7,844	$ 16,578
Systems
Disaggregation of Revenue [Line Items]
Total revenue	1,277	4,318	5,301	12,428
Instruments and accessories
Disaggregation of Revenue [Line Items]
Total revenue	373	867	1,479	3,488
Services
Disaggregation of Revenue [Line Items]
Total revenue	374	237	1,064	662
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	142	1,381	427	2,714
U.S. | Systems
Disaggregation of Revenue [Line Items]
Total revenue	0	868	0	1,742
U.S. | Instruments and accessories
Disaggregation of Revenue [Line Items]
Total revenue	43	426	68	835
U.S. | Services
Disaggregation of Revenue [Line Items]
Total revenue	99	87	359	137
Outside of U.S.
Disaggregation of Revenue [Line Items]
Total revenue	1,882	4,041	7,417	13,864
Outside of U.S. | Systems
Disaggregation of Revenue [Line Items]
Total revenue	1,277	3,450	5,301	10,686
Outside of U.S. | Instruments and accessories
Disaggregation of Revenue [Line Items]
Total revenue	330	441	1,411	2,653
Outside of U.S. | Services
Disaggregation of Revenue [Line Items]
Total revenue	$ 275	$ 150	$ 705	$ 525