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Safer way to scan the heart

06-Oct-2011 (Thu) Mind Your Body, The Straits Times

New MRI technology can screen for heart diseases without radiation or contrast dyes.
Tan Lay Leng reports


-- ST PHOTO: TED CHEN

In 2005, Mr Charlie Lee had a heart scan which could well have saved his life.

The computed tomography (CT) angiogram found two blockages in his arteries, which might have caused a heart attack some day.

Mr Lee, a HDB officer, had stents inserted into the blood vessels to clear the blockages.

Still, he found the CT angiogram, a kind of X-ray which requires a special dye to be injected into the blood vessels, a little unnerving.

'I had an injection which felt hot during the scan. The scan was fast, about five minutes, but I know there was X-ray radiation,' he said. 'I didn't like my experience because I have a phobia of injections.'

Even though he was concerned about further blockages in his heart, he did not do any more CT scans because he was worried about the possible health risks.

X-ray radiation increases the risk of cancer, while the contrast dye can cause allergic reactions and kidney damage in some patients.

Still, he found the CT angiogram, a kind of X-ray which requires a special dye to be injected into the blood vessels, a little unnerving.

'I had an injection which felt hot during the scan. The scan was fast, about five minutes, but I know there was X-ray radiation,' he said. 'I didn't like my experience because I have a phobia of injections.'

Even though he was concerned about further blockages in his heart, he did not do any more CT scans because he was worried about the possible health risks.

X-ray radiation increases the risk of cancer, while the contrast dye can cause allergic reactions and kidney damage in some patients.

Mr Lee, who is a patient of the centre, opted to have an MRI scan instead of the CT angiogram.

The painless 15-minute procedure revealed no blockages.

'Now, I want to do an MRI scan every year as it's very safe,' he said.

Aside from detecting blockages, the MRI is useful for showing up inherited abnormalities of the heart, said Dr Michael Lim, medical director and senior consultant cardiologist of the Heart, Stroke and Cancer Centre, a sister company of CMI.

But the improved MRI technology is so new, there are no guidelines about how it should be used yet for the heart.

Currently, the 'gold standard' to confirm a blocked artery is through a coronary angiogram. This is an invasive test which requires a tube to be snaked into the heart arteries.

The patient is injected with a contrast dye that shows up under X-ray to pick up images of the arteries.

The procedure carries a small risk of stroke, heart attack and death, on top of the risks from radiation and the dye.

About seven years ago, major technological advancements made the CT angiogram a quick procedure with significantly reduced radiation, and therefore a viable alternative to the coronary angiogram.

Though not everyone is convinced it is so - CT can yield inconclusive results depending on the level of expertise and experience - it adds to the range of diagnostic tests doctors can tap for those suspected of having heart disease.

For people who have symptoms such as chest pain, the first step is an electrocardiogram to check the heart rhythm.

If it is abnormal, treadmill testing, a stress echocardiogram and nuclear scans can follow to determine if there is enough blood flow to the heart muscles when the heart is put under stress.

But all of these tests do not allow a view of the arteries, said Dr Lim.

If these tests indicate that a patient has a problem, the next step is preferably a coronary angiogram, which will show where the blockages are.

If patients are leery of the invasive procedure, some may opt for a CT angiogram, though this is not recommended under the Ministry of Health's clinical practice guidelines on the screening of heart disease.

Researchers at Mie University Hospital in Japan concluded that MRI could be an alternative to the CT angiogram for detecting coronary artery disease after doing a study this year.

They used a new-generation MRI machine to first scan 67 patients suspected of having coronary heart disease.

These were followed by coronary angiograms. The angiograms found blockages in 39 people, which were also detected by the MRI with an accuracy of 99 per cent.

Dr Teo Ngee, medical director of CMI, said a patient is sent for a coronary angiogram when a significantly blocked artery is seen, either on the MRI or CT angiogram.

Since the MRI machine was installed in July, about 400 patients have had their hearts scanned.

Associate Professor Terrance Chua, deputy medical director of the National Heart Centre Singapore, said that although the results of the study in Japan were certainly encouraging, the findings would have to be reproduced in other studies for confirmation.

He added that the MRI has some technical limitations which have not made it ideal for visualising some aspects of coronary arteries, like the thickening of artery walls due to fatty deposits.

Dr Teo admitted that MRI has limitations. Patients with metallic implants such as moving wires are not suitable for an MRI scan.

Other problems are that patients have to be perfectly still during a scan, which can take 15 minutes, so some have to be sedated or anaesthesised. This is because some patients feel uncomfortable in an enclosed space.

Dr Lynette Teo, consultant radiologist at National University Hospital's department of diagnostic imaging, said patients with permanent pacemakers should not undergo cardiac MRI, even if the devices are newer generation MRI-compatible devices, because the safety of the patient and the devices cannot be assured.

Patients with iron-containing clips, implants and foreign objects in delicate tissues such as the brain and the eyeball should also not undergo MRI, she said.

Although recent published studies showed that MRI is able to detect heart arterial narrowing and multi-vessel coronary artery disease with relatively high accuracy, it remains to be seen if this can be achieved in clinical practice, said

Dr Chai Ping, senior consultant cardiologist at the cardiac department of National University Heart Centre, Singapore (NUHCS).

Regular patients are usually different from subjects recruited in research studies as the latter are likely to already have symptoms, he added.

Dr Raymond Lee, consultant cardiologist at Novena Heart Centre and Pacific Healthcare Specialist Centre, felt it would be a while before the technical limitations of MRI, such as the long scanning time and the lack of uniform scanning procedures, are overcome.

'If I want to scan for blockages in the coronary arteries, I would still opt for cardiac CT because of the greater experience with its use,' he said.

At the CMI, an MRI scan of the heart costs $1,800, while a CT scan is about $1,200. A coronary angiogram costs $3,500 to $5,000.

Ischaemic heart disease, in which not enough blood flows to the heart because of blocked arteries, was the third cause of all hospitalisations here in 2009. It accounted for 19.2 per cent of all deaths that year, making it the second biggest killer after cancer.

At NUHCS alone, more than 2,500 patients get a coronary angiogram while 1,200 undergo balloon angioplasty every year.

In balloon angioplasty, a balloon is threaded into a blocked artery at the end of a tube and inflated to unclog the vessel. Sometimes, a stent an expandable wire mesh tube, is left in the vessel to prop it open.


Background story

Why scans are not for those without symptoms

Diagnostic tests, such as the computed tomography (CT) angiogram and magnetic resonance imaging (MRI), are not recommended for people who do not have symptoms of heart disease.

Instead, they should be screened for risk factors first, according to new clinical practice guidelines on the screening of heart disease released in February by the Ministry of Health.

The purpose of screening is to detect heart disease even before there are symptoms of a heart problem, such as chest pain, so that patients can be given treatment to prevent heart attacks.

But the benefits of using expensive tools, such as CT and MRI scans, for this purpose are not matched by the cost of using them and the risk posed by radiation in CT scans.

People who have no signs of heart disease should first complete a global cardiovascular risk score, known as the Framingham Risk Score, in their screening for heart or cardiovascular disease.

This calculates a person's risk for cardiovascular disease using factors such as age and gender.

The score lets a person know if he should go for further tests.

The new guidelines also suggest that people fill up the Framingham Risk Score before embarking on strenuous activities to ensure their heart can take the stress.

Beyond risk factor screening, the value of performing additional tests to screen for heart artery disease is uncertain, said Associate Professor Terrance Chua, deputy medical director of the National

Heart Centre Singapore who was on the committee that drew up the guidelines.

There are concerns about possible risks such as from irradiation or wrong interpretation of results, and lack of data about its benefits in finding and preventing heart attacks, he said.

In 2005, Mr Charlie Lee had a heart scan which could well have saved his life.

The computed tomography (CT) angiogram found two blockages in his arteries, which might have caused a heart attack some day.

Mr Lee, a HDB officer, had stents inserted into the blood vessels to clear the blockages.


Background story

Ways to detect blockages

Coronary angiogram

An invasive test which requires a tube to be snaked into the heart arteries. A contrast dye is injected for the X-ray to pick up images of the arteries. It is the 'gold standard' to confirm a blocked artery.

Computed tomography angiogram

Commonly known as a CT scan, it is less invasive than a coronary angiogram but also requires a special dye to be injected to the blood vessels. However, results can be inconclusive.

Magnetic resonance imaging scan

The MRI machine uses a strong magnetic field and radio waves to create pictures of the body. MRI technology is now advanced enough to capture pictures of moving organs to good resolution.