Abstract
Computed Tomography CT refers to an imaging procedure that has been popular over the last decade in the imaging of the human body. In coronary artery imaging, this procedure is called Computed Tomography Coronary Angiography (CTCA). This technique is used specifically in the imaging and assessment of the cardiac and coronary anatomy to determine if one has a coronary artery disease or to determine the extent to which the individual has been affected by the disease. This procedure is also used to evaluate the coronary arteries and their anatomy. There are also other methods or techniques that are used in imaging of coronary arteries to determine the presence or extent to which coronary artery calcification in the cardiac of an individual. Such techniques include; Magnetic Resonance Imaging, Ultrasonography, x-ray angiography as well as nuclear imaging.
These techniques all have their strong points and weaknesses. However, Coronary Tomography Coronary Angiography has been known to be the most efficient of them all. This could be because it involves the use of the latest technology. Currently, it involves the use of a 64 multi slice detector which is very powerful. There are tests being undertaken currently to determine whether the 256 multi slice detector can be used for the clinical procedure of coronary artery imaging. The development of CTCA has come in handy to supplement other techniques of Coronary imaging as well as improve the process of evaluating coronary anatomy.
Introduction
Computed tomography (CT), is the scientific procedure that is concerned with the assessment of the human body through imaging the affected body part. This procedure can be used in imaging the coronary arteries through a process called Computed Tomography Coronary Angiography (CTCA). This was triggered by improvements in technology and invention of better equipment and procedures than before that have led to the treatment of coronary artery diseases to be managed. This first began way back in 1958 when the first coronary artery assessment was done. It was until the previous decade that the latest and most effective of all forms of coronary artery disease assessment were discovered. The use of computed tomography using multi slices (MSCT). These slides are used to image coronary arteries to determine how they have been affected by the disease. The computed tomography angiograpy procedure is noninvasive, meaning the arteries are not interfered with; they are simply assessed as they are using the electronic procedure.
The coronary artery disease (CAD) refers to a health defect leading to minimal or evens no blood flow in at least one or more arteries that supply blood to the human heart. This disease could be caused at birth but that is rear, most cases arise as a degenerative disease, mostly occurring to people over the age of 60. Coronary arteries are the arteries that lie on the heart and to supply the heart with oxygen. Assessment of the coronary artery disease has been taking different approaches since 1958 when it was introduced. The improvements in technology and discoveries in the health science sector have led to better methods of assessing this disease. Initially, there was the coronary angiography. However, this method had several disadvantages. It had a relatively high level of incidence cases like myocardial infarction, death and stroke. Angiography also came along with lots of pain for those who received the therapy, it was also very expensive. This approach is also ineffective since information obtained usually is about the lumen of the coronary artery alone.
There are different ways of carrying out the computed tomography process. The major ones include; the magnetic resonance Imaging (MRI) and computed tomography coronary angiography (CTCA). Imaging using the above mentioned techniques allows for evaluation of coronary arteries. The Magnetic Resonance Angiography (MRA) is a kind of Magnetic Resonance Imaging that was developed back in the 1990s when dedicated cardiac coils and gradient techniques of high speed were introduced. This approach was advantageous to patients because they were not exposed to ionizing radiations or contrast media that is iodinated. This technique can also be used alongside other coronary artery techniques such as magnetic resonance (MR) approaches of imaging to assess the structure, function, viability and blood flow of the cardiac system.
Imaging of coronary arteries requires the use of multi detector row Coronary Tomography (MDCT)or use of MRI can be used to help in the clarification of coronary anatomy as well as determining if a particular vessel is occluded or not. Imaging of stress usually has a complementary duty to show or determine zones that have inducible ischemia-lower supply of blood that what the tissue requires. To produce stress, one can infuse a medication which will help in raising the strength of the contraction of cardiac muscles. The infusion of medications such as dipyridamole and adenosine will help dilate vessels hence reducing the supply of blood to areas that are affected by the disease.
Ten years ago, coronary artery imaging using MRI was known to demonstrate stenoses even without injecting contrast material or catheterization. Currently, MDCT provides a very fast and handy option for the definition of coronary anatomy. MDCT usually takes a shorter period of time than MRI; it also provides more coronary anatomy details. However, MRI does not involve use of ionizing radiations or iodinated contrast agents. The speed and image resolutions have significantly been improved due to the developments in CTCA and MRI. This has made CTCA and MRI very useful modalities in evaluating clinical Coronary Artery Diseases and at the same time enhancing their convenience and safety. Furthermore, CTCA and MRI could be utilized in the identification of areas within the heart that have blood supply anomalies through studying the arrivals of the blood that is labeled contrast agent. Another is the use of MRI is the identification of thickness and location of myocardial scars. In real sense, x-ray angiography (XRA) has been replaced by neither MRI nor CTA as the standard clinical approach for diagnosis of coronary stenosis. The use of MRI and CTA in the determination of whether vessels are open is on the rise. Lately, the 64 slice MCDT angiography has proved to be a reliable alternative to x-ray angiography in identifying coronary blockages.
Treatment
When there are symptoms of a possible heart attack, that is; gasping for breath, sudden sweating, weakness, persistent chest pains or radiating pressure to the jaws, an electrocardiogram needs to be acquired as soon as possible with close observation for any kind of insufficient blood supply or arrhythmia. MRI and CTCA have enhanced the ability to identify zones of insufficient supply of blood and to show the coronary anatomy. These approaches do not need stress; they provide sensitivity as well as specificity identical to that of nuclear imaging. They provide a better resolution than nuclear imaging since they can demonstrate a coronary anatomy in 3 dimensions. This implies that CTCA and MRI is a complement of stress test and catheterization combination. It also shows that in other conditions, CTCA and MRI could possibly substitute the other techniques since they provide normal outcomes.
Electronic Beam Tomography EBT, is a kind if CTCA where electronic beams are used by rotating it around the patient instead of a whole x-ray source. EBT and CTCA can also be used to screen for coronary artery classifications with the aim of marking risks of potential coronary diseases in young patients. In monitoring of angiogenesis, delayed arrival and collateral sensitive MRI usually look far more sensitive compared to any other technique.
Computed Tomography
CTCA imaging in coronary arteries is attained using quick CTCA and EBT systems gated by ECG so as to collect information when the heart is diastole mode. The use of a 64 section multi detector row CT (MDCT) is the latest technology in coronary artery imaging. There have been rumors of tests for the use of a 256 section MDCT for clinical purposes. With just a section thickness ranging between 0.5 and 1mm, a coronary anatomy is laid for viewing in 3 dimensions. The processing of the image can help in the viewing of the branches, course of vessels, as well as the degree and presence of stenoses. The tree of a coronary artery can be observed in solid form, creeping on the heart’s surface although parts of the artery tree may not be viewable due to obstruction.
Given a proper view of the coronary artery tree, one should be able to have a clean observation of the aortic root, the left ventricle blood pool, and other extra cardiac features should be removed. Also to be seen in the clean views of the branch are margins of partial volume effects as well as vascular projections which are restricted to areas including the vessel interested in. in normal circumstances, threshold based renderings should not be reliable since they lead to false stenosis and obstruction; which leads to the missing of the intravascular thrombus. Experts use two volumes before and after administering contrast material for purposes of elastic matching which significantly helps to evaluate coronary arteries since it automatically separates the coronary tree from other cardiac material without thresholding.
CTCA facilitates a comprehensive and clear evaluation of the delivery of blood. Basically, when used alongside with catheterization will allow precise definition of the level of myocardium that is collaterally dependent. Imaging that is done using elastic matching automatically shows the image volume differences. Obtaining a set of chest CTCA images that have been contras enhanced through coronaries together with a set of non enhanced CTCA images will give a 3 dimension coronary artery tree view. The coronary tree which is match elastic is overlaid, while the non enhanced volume information is regarded to be holographic projections so as to give an anatomic context. Apart from automation, this approach does not involve thresholding, where filling defects and small branches are properly represented in case they are present.
Research has shown that the MDCT diagnosis accuracy in detection of left anterior descending of coronary artery stenoses can be enhanced by transthoracic Doppler echocardiography. Out of successive 144 patients, assessment of coronary anatomy using MDCT, coronary flow reserve was calculated using echocardiography. This was achieved by measurement of the hyperemic ratio to the velocity of the peak baseline; this showed that outcomes of both approaches were attained by use of coronary angiography invasively. The prediction of LAD stenosis significantly through a univariate model is more convenient with the coronary flow reserve than with MDCT. This is evident in the percentages of positivity and negativity. Once the MDCT and transthoracic Doppler echocardiography were accepted, the level of accuracy rose to 96%. Out of the 13 patients who were missed by MDCT, the test for transthoracic Doppler echocardiography turned out to be 100% precise in terms of predicting a significant level of LAD stenosis.
Level of confidence
The capability of computed tomography to produce an image of the anatomy and notable obstruction absence has been increasing over the years in a rapid and uniform manner. The value of Computed Tomography should be assessed basing on a double blind approach to achieve a standardize result, where the most reliable techniques are used. CTCA always depicts a culprit lesion and a normal coronary tissue. It then helps in establishing the extent of severity and zones that get blood supply from the suspect vessel. It gives a significant advantage to directly assess the areas that have abnormal delivery of blood supply.
Demerits of Computed Tomography (CT)
CTCA depicts calcifications to be white and identical to blood that has been contrast filled while MRI will depict calcifications to be signal voids or black in color.
Any heavy calcification may lead to the electronic beam hardening the artifact on CTCA which will interfere with the visualization. Stents may lead to a local disturbance, which is usually stronger on MRI than in CTCA. While using a 3 dimension, whether with CTCA or MRI, one should be able to understand how images account for the local curvature outside and inside of an imaging plane. To find the best vessel showing plane, radiologists are prone to mistake local curves which could be out of the plane for stenosis. This problem can be resolved by processing images properly.
There are other techniques of obtaining coronary artery images apart from the use of CTCA. they include; Magnetic Resonance Imaging (MRI), x ray angiography (XRA), Nuclear Imaging and Ultrasonography.
X-ray Angiography
This is a radiographic approach that depicts vessels’ origin, their branching, level and presence of obstructions, thrombi or dissections and the zones where they supply blood. This technique is used to delineate coronary anatomy although it is inferior to Computed Tomography in terms of myocardium identification of areas with abnormal blood delivery, assessment of functional outcomes and depicting micro vascular collaterals development. Despite the fact that XRA is used in coronary anatomy identification and stenoses, it can be selectively used to image. This technique will however give very poor quality imaging. There are also possibilities of false results because it is possible to miss lesion hidden beyond other vessels. X ray angiography cannot also be used in detecting diseases of small vessels that are epitomizing cardiac syndrome X. Furthermore, this technique disrupts obstruction in blood vessels making them to appear as if they are fully recovered only to show minimal cross sectional recovery.
Magnetic Resonance Imaging
This technique has been an enhancement of the earlier approaches and equipment that is used to show normal courses and proximal coronary arteries. It is however, not a technique devised to replace the other clinical techniques of coronary artery assessment such as the X ray angiography. This technique, just like CTCA, is performed in 3 dimension volumes although a trade off in resolution and time favours the imaging on selective planes addressing every branch that is of interest. MRI can be used to view the transverse section of coronary anatomy, distal transverse images, and posterior descending artery views. It can also be used in right ventricle infarction, myocardial scars, stenosis, and assessment of potential bypass grafts.
The technique is very efficient in sensing any changes in the functions of the cardiac walls like radial movements and wall thickening. It is also excellent in identification and rating the abnormal blood delivery. The speed and quality of information obtained from imaging is quite high meaning that this approach is very efficient in coronary artery imaging. It also save a lot of time and is less expensive compared to X ray angiography. This approach is regarded equally as CT, meaning it is tested and proven to be one of the most efficient coronary artery imaging techniques.
Demerits of MRI
First, the estimation of a stenosis analysis maybe blown out of proportion due to differences in local velocities that leads to signal voids. Furthermore, magnetic susceptibility artifacts generate stents, voids, and clips as well as wire producing disturbances locally. Calcification through MRI is viewed as b black spot, also called a signal void unlike in CTCA where it is viewed to be white, identical to contrast agent filled blood. The estimation of stenoses can be affected by these appearances. It is also prone to errors because radiologists could make mistakes by assuming a local curve which is out of the plane for stenosis purposes. The proper use of imaging will resolve this abnormally. When using MRI, the disturbance flow which causes velocity shear leads to a reduction in local signal intensity which could in turn result in exaggerated stenosis.
Ultrasonography
Ultrasonography includes two types of techniques. The echo cardiology technique and the IVUS approach. The main left coronary artery can be identified using the echocardiography technique. This approach involves acquiring recordings of ultrasonic images of the cardiac anatomy then interpreting them. Imaging is done by use of ultra sound waves to form a picture of the cardiac system.
The other Ultrasonography technique is the use of IVUS to perform the examination of coronary arteries which is done from the inside of a vessel to view the plaque. The demerit is that the device diameter limits the passage through tight stenoses. Injecting the sonographic contrast agent in the arteries as well as combining it with esophageal Ultrasonography is useful in the identification of perfusion territories. This technique can be used alongside MDCT to identify left anterior descending (LAD) of coronary artery stenosis. This technique has been quite successful in the past although it is not as effective as the Coronary Tomography.
Nuclear imaging
This approach is meant to demonstrate useful metabolites that detect perfusion anomalies. It is not used depict coronary arteries as many people think. In this technique, the common methods are technetium-99m sestamibi and thallium-201. They are combined together at times to reduce the duration of studying the myocardial radioactive tracer uptake while at rest and when one is stressed.
This technique’s disadvantages are that breast attenuation could result in apparent anomalies on radionuclide images. Such a problem can be solved with the use of attenuation correction. Unusual movements, for example bundle branch block when imaging may result in positively false outcomes. Persistent anomalies are commonly interpreted as defects that are fixed or just a scar, even though it may be a reflection of prolonged and reversible ischemic defect of uptake of tracer. This approach has low resolution when compared to other techniques such as CT and MRI.
Conclusion
There are various techniques that can be used in coronary artery imaging,, the main ones of which have been discussed above. However, of all these techniques, Coronary Tomography Coronary Angiography stands out as the most successful, outstanding and cheap. The CTCA technique is in every word the most suitable technique to use because it has all the necessary attributes; cheap, 3 dimensions views, speedy, high quality, versatile and can be used in analysis of coronary anatomy as well. As much as it is the best of all coronary artery imaging approaches, CTCA will not replace the other techniques. In fact, other techniques such as MRI and Ultrasonography have been used alongside CTCA and found to produce results of greater quality. Therefore, CTCA will continue to be used independently or in combination with other techniques of CT imaging but no chance of it replacing them completely.
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