Waits for the results of X-rays and other scans in England are often too long, a report finds.
Waits for tests have come down
The Healthcare Commission found waiting times in hospital X-ray departments had dropped - by 33% for some tests.
But two out of three doctors and nurses said they often did not get results when needed, and a half said the delays affected patient decisions daily.
The Department of Health said trusts were working hard to cut result turnaround times.
Imaging departments carry out 33m patient examinations in England each year, including X-rays, CT scans, MRI scans and ultrasound scans.
The Commission examined data from all 196 imaging departments in England, and surveyed 5,500 doctors and nurses.
It found that despite rising demand, most departments had significantly reduced long waits for scans thanks to new equipment and improved productivity.
CUTS IN WAITING TIMES
Average wait for a non-urgent CT scan:
Seven weeks in 2001
Just over five weeks in 2005
Average wait for an MRI scan:
21 weeks in 2001
14 weeks in 2005
However, in many trusts the subsequent wait for the result to be formally reported to the referring doctor had not improved since 2001.
One in four trusts took more than 10 days to report examinations requested from outpatient clinics.
And almost 10% of imaging examinations were never formally reported to the referring doctor by the X-ray department.
The Commission warned that patients might be put at risk of having injuries or serious conditions go undetected.
It said trusts should have formal agreements covering who should report examination results - and how quickly.
The report also found big differences between trusts in rates of referral for imaging.
One in 10 imaging departments received more than 615 referrals per thousand A&E attendances while in other trusts the figure was less than 310.
More than half of those that examine children did not have a specialist with responsibility for paediatric radiology, and 60% did not have protocols dealing with uncooperative children.
Anna Walker, chief executive of the Healthcare Commission, said: "The results of this review have been reported back to individual trusts so where there is a problem, they can address it.
"We will also be asking the relevant strategic health authorities to follow up this issue."
The Department of Health has stipulated that, from this month, all diagnostic tests should be done within 13 weeks of referral.
By the end of 2008 the maximum time from referral to treatment must be 18 weeks.
A spokesperson welcomed the cut in waiting times, and said investments in technology would speed up the process still further.
"The NHS is working hard to tackle the issues of report turn round times.
"For example, excellent work has been undertaken in Royal Liverpool NHS trust around their long term approach to reducing waiting times and reporting times.
"By regularly monitoring and recording report turn round they are moving towards a target of 24 hours for in-patient cases and 48 hours for all others."
Nigel Edwards, of the NHS Confederation which represents over 90% NHS organisations, said it was important that cuts in waiting times for diagnostic tests were not simply transferred into longer waits for results.
"Trusts will therefore be paying close attention to the Commission's recommendations to improve the reporting of results."