Non-invasive Techniques in Angiography

Introduction

Angiography is one of the most common words of medical parlance, used commonly by general public. An investigation used to diagnose coronary heart disease (also called Coronary Artery Disease, CAD), which has taken in its grip every 2nd or 3rd person. CAD is a very common group of diseases occurring as a result of deficient blood supply to heart. They can result in conditions like angina and even fatal heart attack (Myocardial Infraction).

CAD is described to be an "epidemic" in several advanced western countries including US and is steadily increasing incidence in India as well. Almost everyone knows someone who has been diagnosed with CAD, or in unfortunate circumstances has been killed by the disease.

The causes of CAD are multifactorial and complex but a common denominator in the different diseases is the development of Atherosclerosis. Atherosclerosis is best described as a patch of fibro fatty tissue on the inside of arteries. Although some amount of atherosclerosis can be seen even in a normal person, but patients of CAD have much increased atherosclerosis. The patches individually are called Atheroma. An atheroma by its physical presence can decrease the caliber of the artery and hence block the lumen completely or partially, and this blockage is called stenosis. The arteries supplying blood to the heart are called Coronary arteries. When atherosclerosis and resultant stenosis affects these arteries, it results in CAD.

Angiography or better called Conventional Coronary Angiography (CCA) is visualization of arteries by direct injection of a radio-contrast dye into coronary arteries following which the arteries following which the arteries can be seen using fluoroscopy (simply defined as real-time X-ray). Since the dye needs to be injected directly into the coronary artery, a catheter is inserted into an artery in thigh and advanced up into the aorta to reach the coronary artery. Obviously this can carry risk of complications like puncture or rupture of artery and/or allergy to dye besides a long list of other possible complications. With modern techniques, the risk of these complications has been reduced to 2%. Although the risk percentage is low, but is still high enough to look for other non-invasive methods of investigations. To prevent the complications and avoid high costs of CCA, some non-invasive techniques have recently come up as an alternative to CCA in selective cases. These are

• Multirow Detector (Spiral) Computed Tomography (MDCT or MSCT or angiography)
• Electron Beam Tomography (EBT)
• Magnetic Resonance Angiography (MRA)

Before moving further it needs to be clarified that although we do feel the need for alternatives but CCA still remains a commonly used investigative technique. It is also the "Gold Standard" Investigation against which all non-invasive methods are compared to prove their mettle.

Returning to the list above, among the three mentioned, the most widely accepted for CAD is MDCT.

What is or Computerized Tomography (CT) & MDCT?

Tomography is a technique using X rays to see the details of the organs at depth. Computerized Tomography (CT) involves use of a complex machine and computers for the purpose. The first machine for CT was invented by Godfrey Hounsfeld & Alan Cornmack, a feat for which they received Noble Prize in the year 1979. We are aware of what is an X-ray and what an X-ray film of say, chest looks like. CT uses x-rays, but the films look nothing like a regular X-ray. They actually show a cross-section of the body, which can be morbidly described as what the cut surface will look like when body is cut perpendicular to the long axis. When several such "cut" or cross sections are available, a computer creates a 3-dimensional image of the structures inside, something left to the physician's imagination before the advent of this technology. The initial CT machines had a table on which the patient lied down, with the X-ray source above him and a detector below. The basic principle of functioning still remains the same. A pencil thin x-ray beam was passed through him to the detector. The beam was altered while passing though different structures in body and was "attenuated" in the process. This attenuation would be detected and recorded by the detector. After this the X-ray source rotated by a small angle in axis perpendicular to the long axis of the patient and process repeated. The whole process would be repeated till 180o of total images was taken. The data generated were analyzed by the computer to make one 'slice". The patient was rolled further in to the machine and the whole process was repeated for the next slice. The complete process took several minutes.

If this very same machine is used to see the heart then we have to consider few things first. Human heart beat at a rate of average 72 beats per minute and within a single beat the heart undergoes changes in its size, shape etc. several times (constituting a cardiac cycle). Now if the above mentioned CT scan is used to make an image of heart then within the say 5 min taken to complete the scanning, the heart would have beaten 360 times. Thus we will have several slices but all taken at a different time and at different phases of the cardiac cycle. Hence the computer will be trying to analyze and join together several pieces of data very different from each other. Something like painting a portrait of a man's youth with several photographs beginning from his childhood to old age, without having the specific time's photograph. The best we can make is a guessestimate, and that too a blurred one. The image produced is not totally useless, it can tell us a few things about the heart, but not in the detail that we can get through CCA. And this detail is paramount to diagnosing and treating the CADs.

With passage of time, over last 2-3 decades the CT machines have gone through several modifications and the recent generations of the machines have multiple detectors (several hundred) instead of one as in the older versions. In these machines the process of moving the patients through the machine, X ray beam production and detection, rotation of the X-ray beam production and detection, rotation of the X-ray tube and detector all occur automatically, simultaneously and at a predetermined rate. Somewhat like as shown in the figure below.

Hence effectively, the X-ray beam moves in a spiral fashion around the patient lying on the table (like the spirals over a screw) - therefore the name spiral or Helical CT. these machines can complete a CT scan in milliseconds to seconds. The latest technique at our disposal is MDCT; with-out going into the specifics it would be sufficient to state that this technique has the capacity to image very thin slices (up to 1 mm) of body, with multiple parallel slices taken (4,8 or 16) simultaneously. This means that the imaging procedure of whole heart takes few hundred milliseconds to complete. Coming back to our calculation of heart rate, at 72beats/min a single beat would take around 0.83 sec or 830 msec. Certain factors limit the time which the images can be acquired to a minimum 500 msec. Hence the whole heart can be imaged in few beats. The image taken is synchronized with ECG, to ensure that those taken at different times are at least in the similar phases of the cardiac cycles. Using highly complex computerized algorithms, the image of heart, with maximal resolution is generated in a matter of few seconds. Images can also be produced after injection of dye into the blood circulation (but the injection here is not done directly into the coronary arteries, but rather into a peripheral vessel). Using these images the heart and its coronary circulation cab be accurately seen in detail, as in CCA. These images help to look for any stenosis, plaque in the coronary artery. This information is routinely used to diagnose or treat cases of CAD.

Unfortunately, the MDCT too has its own set of weaknesses, e.g. drop in the quality of images not only occur with heart movement (which would increase with say increased heart rate) but also with lung, chest movements, obstruction of X-ray beam by metallic implant in chest, etc. This was the case with earlier machines as well but since the time taken by them was several minutes, the probability of an unwanted movement was very high.

EBCT is also a variant of CT scan, which has found comparisons with MDCT. Though many workers consider MDCT superior to it. MRA is based on a different technology, and presently is not as popular for coronary angiography as MDCT.

To conclude, while conventional coronary angiography still remains the reference method and that too a widely used one, there is an active need and attempt to develop less invasive alternatives. MDCT meets this demand and has proved to be highly promising and has proved to be highly promising and helpful and in selected cases offers an effective alternative.

Source

Invention Intelligence, September - October 2006