FEBURARY 15, 2006
To scan or not to scan? The 64-slice cardiac CT scan can show detailed pictures of the heart in just seconds.Shefali Srinivas finds out if the test is really the best thing since sliced bread.
Every year since she had a heart attack four years ago, Ms Chen Qin Rong takes an exercise stress test to check how her heart is doing.
The test is used to detect if there are any problems with blood supply to the heart. It also helps doctors determine the kind and level of exercise that is best for their patients. Ms Chen, a 47-year-old designer, had no reason to worry when she took her test last year as she had
been taking her medication and working out regularly. Hooked up to a machine to monitor her heart, she began a slow walk on the treadmill, gradually working up to a run up a small incline. About 13 minutes into the test, her cardiologist noticed that her heartbeat was becoming irregular, which could mean that her heart was not receiving adequate blood. 'The scary thing is that I felt absolutely fine. I could have gone on running,' Ms Chen said. But something clearly was wrong.
		She was given the option of repeating the stress test, going for a coronary angiography or trying the new 64-slice cardiac CT scan that had just become available in Singapore. Ms Chen was anxious about what it would take to investigate the blip on her stress test result. She was also afraid of the pain and hassle of having another angiography - a procedure in which a thin tube is snaked up the heart arteries through a blood vessel in the groin. The test carries a small risk of a wayward tube rupturing an artery wall or excessive bleeding at the entry point. It also takes four to six hours to complete. Though the coronary angiography is the gold standard for detecting blocked arteries, Ms Chen had an alternative to the procedure. A 64-slice cardiac CT scan can show detailed images of the heart in about five to 10 seconds. She would need just the injection of a contrast dye and a tablet to slow her heartbeat down. The scan is used to study the arteries that supply blood to the heart muscle and to determine if fatty or calcium deposits have built up in the arteries.

The risks here are the potential for kidney damage from a reaction to the contrast dye and being subjected to a radiation dose equal to that of 40 X-rays.

Though well over a thousand dollars, it was still cheaper than an angiography. She decided to have it done.

Most cardiologists say that a cardiac CT scan is useful when a stress test is inconclusive. They also recommend it if a patient complains of unexplained shoulder, chest or abdominal pain.

For Ms Chen, the scan was a means of looking into her heart without surgery. 'It took about 10 minutes and showed that my heart was normal,' she said.

Though the scan showed no blockage in her arteries, the question raised by the stress test remained. 'I still did not know why my heartbeat was erratic, but at least I knew that there was no blockage.'

She now knew her heart could not handle running on an incline and she should stay off vigorous exercise.
'Basically, the scan saved me the expense and risk of an invasive coronary angiography, which might have shown the same thing,' she said.

Dr Kwek Boon Han, a radiologist in private practice and a board member of the College of Radiologists, Singapore, estimated that about 20 to 40 cardiac CT scans are done here every day.

Over the last year, at least six such machines have appeared here: two at Mount Elizabeth Hospital, and one each at Raffles, KK Women's and Children's, National University and Tan Tock Seng hospitals.

There is also one 40-slice and three 16-slice scanners in private and public institutions, bringing the total number of multi-slice scanners here to 10.

The increasing use of cardiac CT scans prompted the Ministry of Health to ask the Academy of Medicine to look into its regulation last year. The 10-member committee of cardiologists and radiologists, headed by Dr Kwek and Dr Ho Kheng Thye, the cardiology department chief at Tan Tock Seng Hospital, spent six months drafting the cardiac CT guidelines.

Dr Ho said the CT scan technology had been around for years and was used to scan the brain. It was only in the last two years that the technology was harnessed for the heart. This is because the heart is constantly in motion and this causes images to be blurred.

The latest CT scanners use technology that divides the heart into 64 image slices instead of 16 slices like the older scanners. This improves resolution, creating sharp images that help cardiologists determine the extent of disease. 'The detail of the 3-D images, the fact that you can zoom in and show people the problem areas - these highly visual elements have captured the imagination of the public,' Dr Ho said.

Indeed, the 64-slice scanner made it to the cover of Time magazine last year and was featured on Oprah Winfrey's hugely popular talk show. To hear the hype surrounding this technology, you would think it can not only detect heart disease but also cure it.

However, the fact remains that there is no single test that can show you everything you need to know about the heart.

In the United States, there has been considerable debate about whether doctors would recommend the test to patients who did not need it, or if patients would ask for one when they did not require it.

With a scanner costing nearly $2.5 million here, such overuse may be possible.

Each scan would cost between $880 at a public hospital and upwards of $1,000 at private institutions.
But doctors in Singapore say there are enough checks here to prevent unnecessary use. Only a qualified physician can request the test and must justify it. Patients also need to go through health screenings beforehand.

For heart patients and others thinking about taking the test, the most important questions are whether they really need it and if there are cheaper options they can rely on.

The new guidelines recommend a cardiac CT scan only when the test result is likely to change the patient's treatment. It is also helpful when cheaper and less complicated diagnostic tests show some problems or give conflicting results.

There are two aspects of a cardiac CT scan: calcium scoring and CT angiography.
The calcium scoring is the simpler part. The patient lies on an examination table, which moves up or down and into and out of a large, square machine with a circular opening.

Within the machine, a rotating gantry with an X-ray tube and multiple detectors moves around the patient's body to produce the images. The unit is attached to a computer and the calcium score is obtained in about five to 10 seconds.

For CT angiography, the patient is first injected with a contrast dye. In the time span of eight heartbeats, the scan takes multiple, finely layered pictures of the heart. The images, which are about as thin as credit cards, are then plugged into a computer program to create a 3-D image of the heart and surrounding blood vessels.

Most patients who undergo a cardiac CT scan will have their calcium score taken first, Dr Kwek said, as this can help them decide whether to go ahead with the CT angiography.

Studies have shown that the greater the amount of calcium deposited in the coronary arteries, the greater the likelihood of a blockage and vice versa.

So a low or negative calcium score may mean there is no need to proceed with CT angiography.

On the other hand, someone with a very high calcium score of, say, more than 1,000 may be better off going for an invasive angiography, which would reveal the blockages straightaway.

And yet, there are patients with a low calcium score who may still have heart blockages, so doctors take into account other risk factors when deciding on whether to proceed with the CT angiography, said Dr Kwek.
The strength of the CT angiography, according to Dr Ho, is that it can confirm that a person is free of heart disease 98 per cent of the time. This is called negative predictive value. 'So if your scan is fine, we can tell you for sure that there is no problem with your heart,' he said.

This is hugely important because no other test except the invasive angiography is so accurate in ruling out disease.

The exercise stress test has a negative predictive value of around 60 to 70 per cent. This means three out 10 people who pass the test may actually have heart disease.

Former US President Bill Clinton passed an exercise stress test just months before he was hospitalised for chest pain. He needed quadruple bypass surgery as all his arteries were blocked.

If he had gone for a cardiac CT scan instead, the blockages might have been detected well before his breathlessness and chest pain.

Despite its ability to rule out disease, said Dr Ho, the cardiac CT scan should not be used as a common screening tool.

This is because it does not make sense to subject large numbers of people to radiation that they may not need, he said. 'If you look at diagnostic tests as a pyramid then, at the bottom layer, we have the simple and inexpensive tests such as EKG, Echo and exercise stress test.'

If these give abnormal results, doctors proceed to second-tier tests such as nuclear perfusion.
Cardiac CT is also on the second tier, helping doctors decide if patients need the invasive angiography, said Dr Ho.

Cardiac CT has its shortcomings. Unlike invasive angiography, it cannot create very clear pictures of the smaller arteries of the heart. It also cannot show if the blockage is partial or total.

This means that the test can positively predict disease only 85 to 95 per cent of the time. And when there are calcium deposits in the arteries, the blockages appear as huge white masses - a little bigger than they really are.
A 60-year-old patient, who did not want to be named, said he had experienced some mild chest discomfort and decided to have the scan. It showed a blockage in his right artery, so the doctor recommended an invasive angiography to explore the nature of the blockage.

But the angiography showed clear arteries, with no sign of the blockage that had showed up on the scan.

The man said he felt unnecessarily subjected to the stress of going through two expensive procedures.
Dr Ho pointed out that, ultimately, every new form of technology raised as many questions as it answered. But there is no denying that the questions raised by the cardiac CT also often saves lives.

Dr Kwek cited the example of a 42-year-old man who was referred by his doctor for a cardiac CT because he was an occasional smoker with a family history of high blood pressure, diabetes and high cholesterol. 'If you were to look at this guy, he appeared young and fit but the CT scan showed a severe blockage in a coronary artery.'

The man was immediately referred for an invasive angiography which revealed a 90 per cent blockage in a coronary artery. Three devices called stents were required to keep the blood vessel open.
'If not for the early diagnosis and treatment of the coronary artery disease, he would have run the risk of having a heart attack,' said Dr Kwek.

Dr Melvin Tan, a senior consultant cardiologist at Mount Elizabeth Hospital, said the decision to scan should ultimately rest on the patient's risk factors, not just his symptoms. 'If a patient has no symptoms but has a lot of risk factors such as smoking, family history of heart disease, high cholesterol or hypertension, we don't want to wait for something to happen. We would like to prevent a heart attack even before it happens.'

E-mail: sshefali@sph.com.sg

 

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