Summary of Significant Accounting Policies - Summary of Revenue Disaggregated by Type and Geography (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Total revenue	$ 24,102	$ 7,111	$ 1,519
Systems [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	18,749	5,063	1,445
Instruments and Accessories [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	4,308	1,470
Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	1,045	578	74
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,778	1,255
U.S. [Member] | Systems [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,556	865
U.S. [Member] | Instruments and Accessories [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	967	390
U.S. [Member] | Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	255
Outside of U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	20,324	5,856	1,519
Outside of U.S. [Member] | Systems [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	16,193	4,198	1,445
Outside of U.S. [Member] | Instruments and Accessories [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,341	1,080
Outside of U.S. [Member] | Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 790	$ 578	$ 74