Note 7: Field equipment, net

	December 31,
	2016			2015
Field equipment	$	11,167		$	9,226
Less: accumulated depreciation		(2,359	)		(3,197	)
Field equipment, net	$	8,808		$	6,029

Depreciation expense was $3,248, $1,555 and $1,076 for the years ended December 31, 2016, 2015 and 2014, respectively. Write downs of $6,436, $36, and $12 were identified for the years ended December 31, 2016, 2015 and 2014, respectively.

The Company received U.S. Food and Drug Administration (“FDA”) approval on its Premarket Approval supplement application to market its second generation Optune system in the United States on July 13, 2016. The Company made the second generation Optune system available to all patients in the United States during the quarter ended September 30, 2016. Manufacturing of the first generation Optune system has been terminated. In 2016, the Company recorded an impairment loss with respect to the write-down of first generation Optune system field equipment in the amount of $6,412 (finished goods and production stage goods in the amount of $4,830 and $1,582, respectively) presented in cost of revenues.