Virtual Radiologic (vRad) has signed a definitive agreement to acquire all the outstanding common stock of NightHawk Radiology Holdings, a provider of radiology technologies to U.S. radiology groups, for a transaction valued at approximately $170 million.

The combined entity will have 325 radiologists serving nearly 2,700 healthcare facilities across all 50 U.S. states and reading approximately six million studies annually, according to the Eden Prairie, Minn.-based vRad and the Scottsdale, Ariz.-based NightHawk. Also, more than 75 percent of the affiliated radiologists will be fellowship-trained subspecialists.

“Local radiology practices and hospitals are under intense pressure to deliver the highest quality care in the most efficient manner possible,” said vRad’s President and CEO Rob Kill. “This combination … will expand access to much-needed subspecialty expertise.”

Upon the completion of the transaction, Kill will continue to serve as president and CEO of the combined organization. Dave Engert, NightHawk’s president and CEO, will remain as a board advisor following the close of the transaction. The remainder of the leadership team will be drawn from the management teams of both companies.

NightHawk said its board of directors unanimously approved the agreement and recommends that stockholders vote in favor of the transaction. The transaction is expected to be completed in the first quarter of 2011, subject to customary closing conditions, including the approval of NightHawk’s stockholders.