GOODWILL AND OTHER INTANGIBLE ASSETS

Goodwill is recorded as a result of business combinations. Activity in goodwill for the years ended December 31, 2017 and 2018 is provided below (in thousands):



Balance as of December 31, 2016	$239,553
Goodwill acquired through the acquisition of Resolution Imaging Medical Corp	1,901
Goodwill acquired through the acquisition of MRI Centers Inc.	401
Goodwill disposed through the transfer to Santa Monica Imaging Group JV	(1,901	)
Goodwill acquired through the acquisition of D&D Diagnostics, Inc.	1,519
Goodwill acquired through the acquisition of Stockton MRI, Inc.	3,101
Goodwill disposed through the sale of Hematology Oncology	(110	)
Goodwill acquired through the acquisition of DIA, Inc.	9,185
Goodwill disposed through the sale of Breastlink Medical Group, Inc.	(509	)
Goodwill acquired through the acquisition of RDI, Inc.	1,202
Adjustments to our preliminary allocation of the purchase price of DIA, Inc.	1,058
Goodwill acquired through the acquisition of RadSite, LLC	1,665
Goodwill transferred to other assets	(289	)
Balance as of December 31, 2017	256,776
Goodwill acquired through the acquisition of Imaging Services Company of New York, LLC	2,692
Goodwill acquired through the acquisition of certain assets of MemorialCare Medical Foundation	545
Goodwill transferred to assets held for sale	(1,059	)
Goodwill acquired through the acquisition of Women's Imaging Specialists in Healthcare	4,268
Goodwill acquired through the acquisition of Valley Metabolic Imaging	1,469
Goodwill acquired through the acquisition of Sierra Imaging Associates	1,147
Goodwill disposed through the sale of plastic surgery unit	(80	)
Goodwill acquired through the acquisition of Washington Heights Medical Management	2,303
Goodwill acquired through the acquisition of Medical Arts Radiological Group, P.C.	41,469
Goodwill acquired through the acquisition of Arcadia Radiology Imaging Services, LLC	2,582
Goodwill acquired through the acquisition of Southern California Diagnostic Imaging, Inc.	41
Goodwill acquired through the acquisition of Orange County Diagnostics Imaging Center, Inc.	3,618
Goodwill acquired through assuming operational control of New Jersey Imaging Network, LLC	106,122
Goodwill impaired in the Imaging On Call reporting unit	(3,800	)
Balance as of December 31, 2018	$418,093

The amount of goodwill from these acquisitions that is deductible for tax purposes as of December 31, 2018 is $160.1 million.

Other intangible assets are primarily related to the value of management service agreements obtained through our acquisition of Radiologix, Inc. in 2006 and are recorded at a cost of $57.5 million less accumulated amortization of $28.0 million at December 31, 2018. Also included in other intangible assets is the value of covenant not to compete contracts associated with our facility acquisitions totaling $7.7 million less valuation adjustments and accumulated amortization of $6.4 million, as well as the value of trade names associated with acquired imaging facilities totaling $12.1 million less accumulated amortization of $1.5 million and dispositions and impairments totaling $887,000.

Total amortization expense was $2.7 million for the year ended December 31, 2018 and $2.6 million for each of the years ended December 31, 2017 and 2016. Intangible assets are amortized using the straight-line method over their useful life determined at acquisition. Management service agreements are amortized over 25 years using the straight line method.

The following table shows annual amortization expense, by asset classes that will be recorded over the next five years (in thousands):



	2019	2020	2021	2022	2023	Thereafter	Total	Weighted average amortization period remaining in years
Management Service Contracts	2,287	2,287	2,287	2,287	2,287	18,108	29,543	12.9
Covenant not to compete contracts	376	301	240	210	98	—	1,225	3.9
Trade Names amortized	11	11	11	11	9	—	53	5.0
Trade Names indefinite life	—	—	—	—	—	9,772	9,772	—
Total Annual Amortization	2,674	2,599	2,538	2,508	2,394	27,880	40,593

* These trade name intangibles have an indefinite life