Summary of Significant Accounting Policies - Summary of Revenue Disaggregated by Type and Geography (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,639	$ 6,389	$ 5,820	$ 11,156
Systems
Disaggregation of Revenue [Line Items]
Total revenue	2,737	4,657	4,024	8,111
Instruments and accessories
Disaggregation of Revenue [Line Items]
Total revenue	560	1,510	1,106	2,621
Services
Disaggregation of Revenue [Line Items]
Total revenue	342	222	690	424
U.S.
Disaggregation of Revenue [Line Items]
Total revenue	152	1,312	285	1,333
U.S. | Systems
Disaggregation of Revenue [Line Items]
Total revenue	0	875	0	875
U.S. | Instruments and accessories
Disaggregation of Revenue [Line Items]
Total revenue	25	409	25	409
U.S. | Services
Disaggregation of Revenue [Line Items]
Total revenue	127	28	260	49
Outside of U.S.
Disaggregation of Revenue [Line Items]
Total revenue	3,487	5,077	5,535	9,823
Outside of U.S. | Systems
Disaggregation of Revenue [Line Items]
Total revenue	2,737	3,782	4,024	7,236
Outside of U.S. | Instruments and accessories
Disaggregation of Revenue [Line Items]
Total revenue	535	1,101	1,081	2,212
Outside of U.S. | Services
Disaggregation of Revenue [Line Items]
Total revenue	$ 215	$ 194	$ 430	$ 375