Waits are too long in many hospitals in England and are hidden from official statistics, a survey suggests.
By 2008 no one should wait longer than 18 weeks
Data gathered from 73 out of 153 NHS Trusts by the Liberal Democrats in 2004 shows some patients are waiting more than a year for diagnostic tests.
Government figures do not include such waits. The clock starts ticking once patients are on surgery waiting lists.
The news comes as the head of the NHS is expected to announce further reductions in waiting times.
Sir Nigel Crisp's end of year report is expected to show fewer patients are waiting for operations, and those who do so wait for much shorter periods than in previous years.
The government has also promised that by 2008, no one will have to wait longer than 18 weeks from being referred by their GP to being treated in hospital.
But Liberal Democrat Shadow Health Secretary Paul Burstow says action is needed now.
His survey found patients were waiting up to six months or more in two out of five NHS Trusts for routine MRI scans.
One in 12 trusts had MRI waiting lists of over a year.
Lack of training
Similarly, about one in 10 reported waiting times for CT scans of six months or more.
Mr Burstow says part of the problem is a lack of trained staff to perform the scans - vacancies for radiographers have risen by a third since 2000.
A third of the trusts said availability of scanners or old equipment was another barrier.
Mr Burstow said: "People are waiting far too long.
"Whenever the government says it has cut waiting times, it may have cut the wait for treatment, but there are still an awful lot of people on these hidden waiting lists just waiting to get a diagnosis.
"For some...we may well find that the diagnosis comes too late to actually save their life."
We are well aware of the problem
A spokesman from the Department of Health
"By 2008 the government might begin to bear down on this problem, but that's far too long and too late."
Shadow Health Secretary Andrew Lansley said government targets were the problem and should be abolished.
Professor Adrian Dixon, of the Royal College of Radiologists, said staff shortage was a big problem, but he did not think there were too few scanners.
He said it was more a question of not being able to use the scanners around the clock to clear waiting lists.
"Some radiographers would work for overtime if that could be funded but most radiology department managers would get their knuckles rapped for going over budget."
He was hopeful that the situation would improve in the future.
"The government has now woken up to this, and they are certainly throwing some money at mobile scanners."
A spokesman from the Department of Health said: "We are well aware of the problem."
He said they were working at increasing staffing and had invested massively to increase scanner capacity.
"There are currently 230 MRI scanners installed in the NHS in England, compared to 110 machines in 1997, representing a 99% increase in total numbers.
"By December 2004, this will increase further to approximately 276 MRI scanners, a 150% increase on 1997. This too will help address waits for imaging tests.
"There are 1,600 (13%) more NHS radiographers than in 1997, and the number of students entering training to become a radiographer has more than doubled since 1996/7."