Basis of Presentation and Significant Accounting Policies - Revenues Disaggregated by Major Good or Service Line, Timing of Revenue Recognition, and Sales Channel, Reconciled to Our Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 5,567	$ 7,329	$ 12,118	$ 14,102
Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,277	4,858	7,723	9,167
Service Contracts [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,788	1,829	3,482	3,667
Supply and Source Usage Agreements [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	490	526	861	1,063
Professional Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		8	11	41
Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12	108	41	164
Goods Transferred at a Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,319	4,952	7,831	9,374
Services Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,248	2,377	4,287	4,728
Direct Sales Force [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,514	4,564	7,155	8,487
OEM Partners [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,408	2,346	3,712	4,403
Channel Partners [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	645	419	1,251	1,212
Detection [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4,117	5,209	8,593	9,377
Detection [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,702	3,808	5,802	6,598
Detection [Member] | Service Contracts [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,403	1,354	2,750	2,676
Detection [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12	47	41	103
Detection [Member] | Goods Transferred at a Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,714	3,808	5,843	6,598
Detection [Member] | Services Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,403	1,401	2,750	2,779
Detection [Member] | Direct Sales Force [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,709	2,863	4,881	4,974
Detection [Member] | OEM Partners [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,408	2,346	3,712	4,403
Therapy [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,450	2,120	3,525	4,725
Therapy [Member] | Products [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	575	1,050	1,921	2,569
Therapy [Member] | Service Contracts [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	385	475	732	991
Therapy [Member] | Supply and Source Usage Agreements [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	490	526	861	1,063
Therapy [Member] | Professional Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		8	11	41
Therapy [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		61		61
Therapy [Member] | Goods Transferred at a Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	605	1,144	1,988	2,776
Therapy [Member] | Services Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	845	976	1,537	1,949
Therapy [Member] | Direct Sales Force [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	805	1,701	2,274	3,513
Therapy [Member] | Channel Partners [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 645	$ 419	$ 1,251	$ 1,212