The Lib Dems are to promise an extra £350m in health service funding to help cut "hidden" waiting lists for scans.
The Lib Dems claim many people are waiting for diagnosis scans
New figures released by the Liberal Democrats suggest some 500,000 people are having to wait up to six months for an initial scan before diagnosis.
Leader Charles Kennedy has blamed the situation on Labour's targets, which only measure a patient's wait from the time of diagnosis to treatment.
Labour has made pledges on cancer. The Tories say they would reduce waiting.
The Lib Dems claim patients in two out of five NHS trusts are waiting as long as six months for routine MRI scans, used to detect cancers and heart conditions.
Patients in one in seven trusts wait six months or more for CT scans to diagnose cancers, heart problems and neurological conditions, the party says.
Hospital trusts have reported scanning equipment standing idle because radiology departments are under-funded, it adds.
Mr Kennedy will set out his party's plans - including giving more freedom to frontline health service workers to decide priorities - at a news conference on Tuesday.
He will condemn the wait for diagnosis scans as "totally unacceptable".
"At present the government only measures the time taken from when you are diagnosed with an illness until you are actually treated," Mr Kennedy will say.
"These targeted waiting times are indeed being reduced. But those on the hidden waiting lists - those waiting for tests and scans to find out what is wrong with them - have seen no action."
The Lib Dems have attacked Labour's health plans as being too focused on centrally imposed targets.
Labour, which has promised faster access to specialists for women with breast cancer, is expected to highlight its plans for children's health on Tuesday.
This week, ministers have promised to cut waiting times for scans on breast and bowel cancer.
The Conservatives say they would provide more choice in healthcare. They argue their plans to allow patients to use half the NHS cost of their operation to go private would help clear backlogs.