International Hospitals – direct sales of products and preservation services.

International Distributors – generally these contracts specify a geographic area that the distributor will service, terms and conditions of the relationship, and purchase targets for the next calendar year.

CardioGenesis Cardiac Laser Console Trials and Sales – CardioGenesis cardiac trialed laser consoles are delivered under separate agreements.

For the years ended December 31, 2019, 2018, and 2017 the sources of revenue were as follows (in thousands):


		2019		2018		2017
Domestic hospitals	$	144,538	$	138,432	$	128,240
International hospitals		85,241		81,203		23,791
International distributors		40,427		36,989		30,805
CardioGenesis cardiac laser therapy		6,016		6,217		6,866
Total sources of revenue	$	276,222	$	262,841	$	189,702

We may generate contract assets during the pre-delivery design and manufacturing stage of E-xtra DESIGN ENGINEERING product order fulfillment. We assess the balance related to any arrangements in process and determine if the enforceable right to payment creates a material contract asset requiring disclosure. No material arrangements in process existed as of December 31, 2019 and 2018.

We also incur contract obligations on general customer purchase orders that have been accepted but unfulfilled. Due to the short duration of time between order acceptance and delivery of the related product or service, we have determined that the balance related to these contract obligations is generally immaterial at any point in time. We monitor the value of orders accepted but unfulfilled at the close of each reporting period to determine if disclosure is appropriate. The value of orders accepted but unfulfilled as of December 31, 2019 and 2018 were not material.