The concerns came to light after an audit in February
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About 900 patients who were referred to a private healthcare company for tests to help cut NHS waiting lists, have been recalled.
The Department of Health said those affected were sent to health firm Atos for ultra sound scans from hospitals in Greater Manchester and Merseyside.
The recall is due to "administrative and technical problems", it said.
Concerns were raised about Atos's patient records and the quality of the scans during an audit in February.
All referrals were immediately halted and a review of patients undertaken. Those recalled were out-patients between December 2006 and March 2007.
They are now being contacted with new appointments.
Patients contacted
The primary care trusts affected are Bolton, Manchester, Salford, Stockport, Oldham, Knowsley, Liverpool and Ashton, Leigh and Wigan.
Mike Farrar, chief executive of NHS North West, said: "We are being vigilant and cautious, but we want to assure ourselves and the public that the additional services we are commissioning on their behalf are safe and of the highest quality."
Health bosses said most ultrasound scans were used to diagnose routine conditions and there was no evidence that patient safety or clinical care had been seriously compromised.
University Hospital Aintree, where 258 patients underwent Atos scans, said it was getting in touch with those affected.
A spokeswoman said: "It has been agreed that all patients scanned by Atos should receive a further scan.
"These will be carried out in the radiology department at University Hospital Aintree and will be reported on by the trust's own radiologists over the next few weeks.
"Our main concern is to ensure that the patients are re-scanned as quickly and safely as possible."