16
Over the past two weeks, doctors at the Toronto General Hospital have been quietly running tests on the most sophisticated CT scanner in the world.
The $2.5-million machine - called Aquilion One - represents the next big leap in medical imaging of the heart. The Toronto General's Peter Munk Cardiac Centre is one of the first medical institutions to purchase the device from Toshiba, the Japanese-based manufacturer.
Hospitals in only four other cities - Baltimore, Boston, Berlin and Nagoya, Japan, - can boast of such advanced CT equipment.
The machine looks like a giant white glazed doughnut that stands more than two metres high. A patient is placed in the "hole" of this high-tech doughnut, and within a matter of seconds, the machine can collect enough data to produce three-dimensional images of the heart on a computer screen.
That means doctors can immediately see if the patient has any narrowed or obstructed coronary arteries that could be an early warning sign of an impending heart attack.
So far, the Toronto doctors have been impressed by the scans they've done on volunteers willing to test the new technology.
"The potential for this machine is huge," said Narinder Paul, section chief of cardiothoracic imaging at the University Health Network, which includes the Toronto General.
For one thing, the scanner is a lot less invasive than the current "gold standard" test for detecting clogged coronary arteries - cardiac catheterization. This procedure involves inserting a flexible tube, or catheter, in a blood vessel in the groin and threading it all the way to the heart.
Once in position, the tube releases a special dye that allow X-rays to "see" the vessels supplying blood to the heart muscle. The images are known as angiograms.
Cardiac catheterization is highly effective at pinpointing obstructed arteries, but it also carries risks, said Catherine Zahn, executive vice-president, clinical programs and professional practice at the University Health Network.
The tube can dislodge unstable plaque and cholesterol along blood vessel walls. Those little bits of debris could form a clot that triggers a heart attack or stroke.
The chances of something going wrong are relatively small - about one in 1,000 - but when it happens "it can be devastating," said Dr. Zahn.
So, doctors have been looking at alternatives and, in recent years, they have turned their attention to advances in CT imaging, or Computed Tomography.
The technology - which used to be called a "CAT" scan - works by placing the patient in between a spinning source of X-rays and rows of special receptors. It produces cross-sectional images, or slices, which can be stacked up to created three-dimensional pictures of the internal organs.
"It's like cutting up a salami ... you can look inside every slice," said Patrice Bret, chief of medical imaging at the University Health Network.
When CT scanning was first introduced in the mid-1970s, the machines were slow and could only produce a few slices at a time. The patient had to hold still so the image didn't blur. Capturing a clear picture of a beating heart was simply beyond the scope of the equipment.
As a result, the first scanners were used for studying body parts that can remain motionless - such as the brain of a stroke patient.
Over the years, CTs have become increasingly faster and the number of X-ray slices has jumped from four to eight to 16 to 32 and then to 64 at one time.
One recent study found that a 64-slice CT scanner can "freeze" parts of the heart in motion and reveal telltale signs of narrowed blood vessels. Patients just have to be able to hold their breath for six to eight seconds so the chest doesn't move. The speedy scanner does the rest.
But the 64-slice scanner still can't capture the entire heart in one go - only 3.2 centimetres of it. So multiple sweeps are needed to create a full image of the vital organ. The additional shots, which require some overlapping sections to "stitch" together a complete 3-D picture, mean the patient is exposed to higher levels of radiation.
According to some estimates, a patient gets between three to four times more radiation from a 64-slice CT scan of the heart, compared with the X-rays involved in a cardiac catheterization procedure.
And that's why Toshiba's Aquilion One, with 320 slices, holds out so much promise. "You can get the whole heart at once," Dr. Paul said, noting the image extends 16 centimetres, which is more than enough to cover an adult heart. "And because it is such a quick scan, the radiation dose will be less." Indeed, patients only have to hold their breath for a second or two.
Dr. Paul said there are lots of people who have cardiac risk factors - such as a family history of heart disease or they suffer from diabetes or elevated cholesterol levels - but they show no outward symptoms of a compromised heart.
