Summary of Significant Accounting Policies - Summary of Revenue Disaggregated by Type and Geography (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Total revenue	$ 2,181	$ 4,767
Systems [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	1,287	3,454
Instruments and Accessories [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	546	1,111
Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	348	202
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	133	21
U.S. [Member] | Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	133	21
Outside of U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,048	4,746
Outside of U.S. [Member] | Systems [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	1,287	3,454
Outside of U.S. [Member] | Instruments and Accessories [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	546	1,111
Outside of U.S. [Member] | Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 215	$ 181