Debt - Additional Information (Detail) - USD ($)				3 Months Ended	12 Months Ended
	May. 20, 2016	May. 09, 2016	Jun. 26, 2015	Mar. 31, 2016	Dec. 31, 2014	Apr. 25, 2016	Jul. 23, 2015
Hercules Term Loan
Debt Instrument [Line Items]
Amount outstanding under the agreement					$ 15,000,000
Stated Percentage Rate Range, Maximum					10.25%
Maturity Date Range, End				Jun. 01, 2017
Beginning period for interest payments				Jan. 01, 2015
Frequency of Periodic Payment				Monthly principal and interest payments
Beginning period for interest payments				Jan. 31, 2014
Unamortized debt discount				$ 466,000
Exercisable from Issuance				10 years
Exercisable after an IPO				5 years
Convertible preferred stock warrant liability				$ 158,000
Fair value of warrant assumption, expected term				2 years
Fair value of warrant assumption, expected volatility				30.00%
Fair value of warrant assumption, risk-free interest rate				0.40%
Fair value of warrant assumption, expected dividend yield				0.00%
Hercules Term Loan | Series C Convertible Preferred Stock
Debt Instrument [Line Items]
Vested and exercisable warrant to purchase				128,231			128,231
Exercise price				$ 5.84
Hercules Term Loan | First 12 months
Debt Instrument [Line Items]
Prepayment penalty				5.00%
Hercules Term Loan | Thereafter Until Maturity
Debt Instrument [Line Items]
Prepayment penalty				1.00%
Hercules Term Loan | Prime Rate
Debt Instrument [Line Items]
Spread on interest rate				7.00%
CRG Term Loan
Debt Instrument [Line Items]
Frequency of Periodic Payment				Quarterly
Maximum borrowing capacity			$ 50,000,000
Current borrowing capacity			30,000,000
Remaining borrowing capacity			20,000,000
Proceeds from term loan			40,000,000
Post Money Valuation Minimum Amount			120,000,000
Debt drawn			$ 30,000,000
Debt maturity date				Jun. 26, 2020
Line of credit facility, covenant terms				In March 2016, the Company and CRG executed an amendment to the original terms of the CRG Term Loan such that, with regard to the conditions for borrowing the remaining $20.0 million available under the CRG Term Loan, the Company may, at its election, draw down (i) an amount of either $10.0 million or $15.0 million in up to two advances upon achievement of a minimum of $15.0 million or aggregate product and service revenue during any consecutive 12 month period ending on or before March 31, 2016 and (ii) an additional $5.0 million (or $10.0 million, if the previous draw made was only in an amount of $10.0 million) upon achievement of a minimum of $25.0 million of aggregate product and service revenue during any consecutive 12 month period ending on or before December 31, 2016 and upon execution of the first sales contract of the Company’s second generation product.
Facility fee			7.00%
CRG Term Loan | Scenario Forecast
Debt Instrument [Line Items]
Proceeds from draw down of debt	$ 10,000,000
CRG Term Loan | Subsequent Event
Debt Instrument [Line Items]
Remaining borrowing capacity		$ 10,000,000
Debt drawn						$ 15,000,000
Proceeds from draw down of debt		$ 5,000,000
CRG Term Loan | Milestone One
Debt Instrument [Line Items]
Remaining borrowing capacity				$ 10,000,000
Required minimum gross revenue				15,000,000
Debt drawn				10,000,000
CRG Term Loan | Milestone Two
Debt Instrument [Line Items]
Remaining borrowing capacity				5,000,000
Required minimum gross revenue				25,000,000
Debt drawn				$ 15,000,000
CRG Term Loan | MRIdian System
Debt Instrument [Line Items]
Required minimum gross revenue			$ 25,000,000
CRG Term Loan | First 12 months
Debt Instrument [Line Items]
Debt instrument, interest rate percentage			12.50%
Prepayment penalty			3.00%
Debt instrument cash interest rate percentage			8.00%
Deferred payment in-kind interest rate			4.50%
CRG Term Loan | After Year 1 but on or Before Year 2
Debt Instrument [Line Items]
Debt instrument, interest rate percentage			12.50%
Prepayment penalty			2.00%
Debt instrument cash interest rate percentage			8.00%
Deferred payment in-kind interest rate			4.50%
CRG Term Loan | After Year 2 but on or Before Year 3
Debt Instrument [Line Items]
Debt instrument, interest rate percentage			12.50%
Prepayment penalty			1.00%
CRG Term Loan | After Year 3 Thereafter Until Maturity
Debt Instrument [Line Items]
Prepayment penalty			0.00%
Deferred Payment In-Kind Interest | Hercules Term Loan
Debt Instrument [Line Items]
Debt instrument, interest rate percentage				1.50%