Virtual Angiography (CT Angiography)

By: Prof. Dr. Şükrü Akyüz

Virtual angiography (virtual angiography) is the visualisation of blood vessels using a computed tomography (CT) scanner.

What is CT angiography?

Virtual angiography is also called CT angiography because it is performed with a CT scanner. Virtual angiography can be performed on any vessel in the body, but in this article, virtual angiography refers to the visualisation of the heart vessels (coronary arteries). Therefore, by CT angiography we also mean coronary CT angiography. In CT angiography, X-rays, a type of radiation, provide the image. A dye (contrast medium) is injected into the blood vessel, which makes the normally invisible vessels visible through X-rays.

 

What is the difference between CT angiography and conventional angiography?

Virtual angiography and conventional angiography are both procedures to visualise the vessels of the heart. In conventional angiography, a long, flexible tube (catheter) is inserted into the heart through an artery in the groin (leg) or wrist. Dye is then injected through the catheter directly into the heart vessels. In virtual angiography, however, no catheter is sent to the heart and the dye is not injected directly into the heart vessels, but into a vein in the arm or the back of the hand, from where it reaches the heart.

What is the difference between CT angiography and cardiac tomography?

Computed tomography is one of the devices used in the diagnosis and treatment planning of many diseases such as heart vessels, valves, membrane and congenital abnormalities. The general name of all of them is “cardiac tomography”. Coronary CT angiography is the specific name of cardiac tomography for the heart vessels.

Where is virtual angiography performed?

Virtual angiography is performed in imaging centres or radiology departments of hospitals. A radiology technician takes the images under the supervision of a radiology specialist (radiologist). The images are then processed with special software and interpreted by radiologists. Prepared images are also interpreted by cardiologists.

How to prepare for a virtual angioplasty?

A 4-hour fast is usually required before filming. However, pure water can be drunk until the last 2 hours. Caffeinated beverages (tea, coffee, etc.) should not be drunk on the day of the scan, because they can increase the heart rate. At a high heart rate, the images will not be clear and the reliability of the test will be reduced. To prevent this, it may be necessary to use one of the drugs called beta-blockers (Beloc ZOK, Saneloc, Concor, Vasoxen, etc.) to slow down your heart rate (more precisely, to bring it down to a normal rate of 50-70/min). These drugs can also be given directly intravenously if necessary. In addition, some patients are very anxious and their heart rate increases a lot because they are very stressed during the filming. In this case, temporary use of anti-anxiety medication is very useful in controlling the heart rate.

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How is virtual angiography performed?

A vein (intravenous line) is opened in the back of the hand or arm. The dye is introduced into your body through this vein. Then you lie on a stretcher-like table in front of the device. During imaging, you are asked to remain still and hold your breath from time to time to minimise heart activity. When the dye reaches the heart, the device sends X-rays to the heart and obtains vascular images.

How long does a virtual angioplasty take?

Although the whole process takes up to 15 minutes, the active shooting time is actually 10-15 seconds. In case of problems with rapid heartbeat, this period can reach 1-2 hours.

What are the benefits of virtual angiography?

  • Although the image quality is not as good as that of conventional angiography, it is quite high. For this reason, it has now taken precedence over the effort test (treadmill) for cardiovascular occlusion screening (check-up) in most hospitals.
  • It shows anatomy and abnormal vascular connections in congenital heart diseases very well.
  • It is an examination that is better tolerated by patients as it takes a very short time and does not have the problems of long duration, noise and a more closed area as in MRI.
  • The likelihood of complications is much lower than with conventional angiography, as there is no catheter insertion into the heart vessels and no arterial access to perform the procedure.

Is virtual angiography risky?

Virtual angiography is a safe procedure for most people. However, it still exposes the patient to some radiation. Pregnant women should not have a virtual angiogram because of the possibility of harm to the baby in the womb (foetus). Also, some people may have an allergic reaction to the dye used. For those with known allergies, a steroid (cortisol) medication may be used hours before the procedure to reduce the risk of a reaction. Apart from allergy, the dye may rarely temporarily impair kidney function. Therefore, drinking plenty of water before and after the procedure is important to protect the kidneys. The kidneys of patients with chronic renal failure are more sensitive to the dye. It is recommended that these patients are evaluated by a nephrologist before the procedure and that they receive continuous intravenous fluids hours before the procedure.

Is virtual angiography accurate?

Is a virtual angioplasty accurate? A conventional angiogram is not required to confirm this. However, if a problem is detected, the accuracy rate of the test is partially reduced (80-90% accuracy rate). In particular, the margin of error increases when the heart rate is not sufficiently reduced. In these cases, it may not be possible to distinguish whether the stenosis seen in the vessel appears to be narrow because it is actually narrow, or whether it appears to be narrow by mistake due to a blurred image due to a shooting error. Classical angiography, which is the most accurate test, may be necessary to clarify this. However, if your cardiologist has personally looked at the images rather than relying solely on what is written in the report and is confident that the images are very clear, the likelihood of the test being wrong is very low. Usually, CT angiography is inaccurate by “overestimating” the degree of stenosis. For example, it may mistakenly show a stenosis of 70-80% when it is actually 30-40%. The reverse is rare. For this reason; when <50% stenosis is detected, further examination is usually not required. With the use of an additional feature called CT-FFR in some imaging centres, not only anatomical but also physiological evaluation is performed. This means that if there is a visual stenosis of between 40% and 90% (moderate), the CT-FFR method can determine whether sufficient blood is passing through the stenosis. This greatly increases the accuracy of the test.

In patients with stents or bypass surgery, the margin of error is higher than in other patient groups. In most of these patients, it is actually still accurate. However, there is often interference in the stents under X-rays, and if the stent diameter is <3 mm, the areas inside the stent may not be clearly visible for this reason; in other words, even if the stent is open, it may look as if it is blocked. Similarly, sometimes the image quality may not be good at the sutured connection points of the bypass vessels. In these cases, another imaging method (cardiac scintigraphy or conventional angiography) may be required.

In conclusion, it is important that the imaging centre performing the CT angiography is an experienced centre that frequently performs coronary CT angiography, that the interpreting radiologist is particularly experienced in cardiac radiology and that the scan is performed with a good device with at least 128 slices. CT angiography performed and interpreted in accordance with such standards has a much higher accuracy rate compared to effort testing, cardiac scintigraphy (SPECT or PET) and stress echocardiography. Therefore, the European Society of Cardiology (ESC) 2018 scientific guideline recommends that CT angiography is now the screening test of choice in most patients with suspected cardiovascular occlusion.