Radiology services in Wales are unsustainable despite being well-managed and meeting waiting times, a report has found.
The Auditor General said rising demand, staffing difficulties and insufficient equipment would see the service struggle to cope in the future.
Adrian Crompton called for the Welsh Government to take action.
A government spokesman said Wales was the only UK nation to have improved imaging waiting times.
A review, which started in late 2016, examined each health board's arrangements to meet demand for radiology examinations and made recommendations for service improvements.
- An improvement in waiting time performance over the last five years
- Demand is increasing, in some areas by as much as 15% per year
- All but one health board is struggling to recruit and retain radiologists and radiographers
- All health boards have some equipment nearing the end of its lifespan
The report added: "Given the nature of some of the issues facing radiology services in Wales, action taken alone by health boards will not be enough to ensure the future sustainability of radiology services - and national strategic planning is required."
The Welsh Government said it has launched an imaging taskforce which will include area for NHS Wales to address.
The Royal College of Radiologists (RCR) has also claimed more than 4,800 angina patients missed out on life-saving scans last year as Welsh hospitals provided only a fifth of tests needed.
This made Wales the worst performing country in that respect, it said.
Mr Crompton said: "Radiology is a vitally important part of our NHS, helping to diagnose, monitor and treat disease and injuries.
"But it's a service under strain and while it may be coping at the moment, this is unlikely to continue in the longer term.
"The report I am publishing today makes some clear recommendations at national and local level and I call on NHS Wales to take clear and targeted action in response."
A Welsh Government spokesman said: "Wales is the only nation in the UK to have improved its imaging waiting times.
"Through additional investment we have nearly doubled the size of the radiology training programme in Wales.
"We have also created a new National Imaging Academy to provide state-of-the-art facilities for training more radiologists."
Responding to the RCR's claim, he added: "Health boards are increasing capacity to improve waiting times and access for cardiac diagnostic tests including cardiac CT."
Analysis by BBC Wales' health correspondent Owain Clarke
This analysis shows a mixed picture.
The positive is that waiting time targets for scans are, on the whole, being met despite demand going up a lot, especially for complicated and time-consuming scans like CT and MRI.
In fact the Welsh Government insists Wales is the only UK country to see a recent improvement.
Why? Well there has been a big drive in recent years to drive down waiting times which has partly been achieved by health boards paying to use private-sector mobile scanning units.
But the report goes on to warn there is a risk that progress could be undone because of a number of challenges.
For example, each health board has ageing equipment that will need to be replaced and all except one said they are struggling to recruit and keep radiology staff.
Problems with IT systems could be adding to the pressure.
The Welsh Government however said it was trying to tackle some of the issues by setting up a national academy to train more radiologists and recently announcing £11m for new scanners across Wales.