The HSE has claimed that the technology company it deals with on its national imaging system was aware of a flaw in the scan software in January 2016.
In a letter issued to top members of the HSE, seen by RTÉ News, HSE Director Tony O'Brien said that during a teleconference yesterday the organisation was told by Change Healthcare that the issue was discovered on 14 January 2016.
They were told it was part of a problem with a number of characters including the hashtag and 'less than' symbols.
Mr O'Brien said it was of grave concern that the company did not inform the HSE about the issue at the time in 2016.
He said that the National Integrated Medical Imaging System team receives patches two or three times a year from Change Healthcare to fix a range of issues.
A patch repair was issued by the company to the HSE in August 2016 but the HSE said it was not made aware it was for the character fix.
Mr O'Brien said that significant questions have still to be answered to the HSE's satisfaction by Change Healthcare.
When scans were archived, the "less than" symbol was omitted. So instead of the system saying, for example, a narrowing of an artery was less than 50%, it said wrongly the narrowing was 50%.
The HSE is also unhappy with the time it took and the completeness of the field safety notice issued by the company to customers around the world.
The contract for the system was originally awarded to McKesson.
Since that time, McKesson and Change Healthcare combined into a new €3bn organisation which became known as Change Healthcare.
The HSE said its dealings are now with Change Healthcare which is a Vancouver-based organisation with a presence in Ireland.
Mr O'Brien said the recall of so many diagnostic procedures here is likely to be expensive and will have a significant impact on waiting lists and capacity across the system.
Once the HSE understands the size of the problem, it will "commence discussions with Change Healthcare regarding liability for the cost of the re-call process".
The Office of the Chief Information Officer at the HSE is leading an investigation into the technology element of the issue and the situation is being treated as a major incident.
Meanwhile the company that installed the system has issued a "field safety notice" to all customers around the world who use its system.
It is believed that a large number of US hospitals may also be affected.
The field safety notice was sent out by Canadian firm McKesson Medical Imaging in the last 24 hours.
Some patients in Ireland who had scans over the past six years may have received unnecessary treatment as a result of incorrect information. Others may need to have their medical tests redone.
A HSE investigation is under way to establish the extent of the issue.