Introduction
In the world today, Coronary Artery Disease (CAD) is the primary cause of morbidity, disability, and death in most western countries. The upsurge of athermanous plaques in artery walls will lead to narrowed arteries. This leads to decreased flow of blood, which normally provides the heart muscle with nutrients and oxygen. Severe coronary events, such as extensive myocardial infarction or sudden death are the first indicators of coronary artery disease in most patients (Rich and McLaughlin 2002). Various lifestyle habits such as smoking, obesity, sedentary lifestyle without exercise, and hypertension are major risk factors causing the advancement of CAD.
Additionally, there are other risk factors such as age, sex, genetic factors, and diseases such as diabetes (Hankey and Leslie 2001). However, behavioral changes along with definite pharmacologic interventions can be used to prolong the life of persons under high risk of acute coronary events. Choosing a suitable, safe screening procedure that can correctly recognize people with symptoms of coronary artery disease is a vital aspect. Even though there are various modes of imaging modality applied in the diagnosis of CAD, for instance, CT (CTA) angiography and magnetic resonance angiography (MRA), invasive coronary angiography is well thought-out as the ultimate and most efficient assessment of coronary artery disease (Schlosser et al. 2004).
While coronary angiography is arguably the gold standard for imaging CAD, the question that begs across many people is whether any of the innovative modalities could have the perspective of substituting the invasive procedure used in the diagnosis of CAD. In this assignment, we investigate the diagnostic value of not as many invasive imaging modalities, specifically CT and MR angiography used in diagnosing coronary artery disease to be weighed against the invasive coronary angiography, and evaluated using research findings of various studies conducted in these modalities. Consequently, the diagnostic precision of each one of the modalities is matched up to invasive coronary angiography. This assignment also looks at the principles, diagnostic accurateness, and prospective advancements of each modality for the analysis of CAD.
Invasive coronary angiography
Coronary angiography (CA) is a fundamental x-ray examination of blood vessels or heart chambers. A very small tube known as a catheter is inserted into the blood vessel through the patient’s upper arm or thigh by positioning the head of the catheter either in the heart or at the opening of the arteries supplying nutrients and oxygen to the heart. A different medium or dye is infused to enable the x-ray to raise signs of the fluid coming out of the catheter.
This modality has diverse ways of imaging coronary artery diseases. For example, first, quantitative coronary angiography (QCA) which presents a regular 2-dimensional outlook, and next, rotational coronary angiographic (RCA) which offers a 3-dimensional panoramic view is used. The latter, RCA gives more precise results on coronary arterial segment length about standard QCA (Agostoni et al. 2008).
Coronary angiography is still regarded as the generally conventional technique of diagnosing coronary artery disease following its numerous latent advantages. One of its key advantages is its ability to provide a panoramic illustration of the coronary tree, through the multi-perspective, operator-independent image of any abrasion or laceration. Secondly, it provides equal or even greater diagnostic precision of coronary artery disease, and thirdly, it requires a shorter scan time and uses less contrast medium. Fourthly, it ensures less exposure to ionizing radiation than CTA, and finally, it can be carried out on full-size patients, regardless of morbid obesity (S. Rigattieri 2005).
Fifteen patients were used to carry out statistical research on coronary angiographic evaluation of proximal coronary artery stenosis. An examination using paired perpendicular angiographic views along with digital computation produced statistically dissimilar lesion measurements and hemodynamic forecasts for patients with small stenosis diameters (McMahon et al. 1979). A current study by (Swallow et al. 2006) recorded that coronary angiography assisted in presenting more detailed data about the extent of stenosis useful as a proxy to point out damaged flow in a coronary artery, (high-grade stenoses) seen in (Fig 1).
Despite the popular credence, that coronary angiography is the gold standard for imaging coronary arteries; it is connected to several drawbacks in its diagnosing ability. First, it only allows the view of the lumen of coronary arteries with no further details on the vessel wall. As a result, due to positive remodeling, angiography often fails to identify the initial stages of atherosclerosis. Secondly, the accurate assessment of the extent of luminal stenosis is stringently reliant on the selection of an excellent angiographic projection, for example, at right angles to the outline of maximal narrowing. Thirdly, angiography is an invasive assessment modality, coupled with various threats of complications, using ionizing radiation and nephrotoxic contrast mediums. Fourth, in modern clinical practice, it commonly involves hospitalization with resultant high cost (Gami and Garovic 2004).
Over the years, coronary angiography has been the major modality of diagnosing coronary disease in patients in need of coronary revascularization. Even with the fast growth of non-invasive modalities including computed tomography and magnetic resonance imaging, angiography still is the most accurate technique of identifying the occurrence and severity of coronary artery disease.
CT angiography (CTA)
Computed tomography, (CT) is an imaging modality that uses digital geometry. A bulky collection of 2-dimentional slices can generate a 3-dimensional or 4-dimensional representation, which is the fundamental principle behind the high-end CT. Initially, the procedure was described using the term, Computed axial tomography (CAT) derived from layman’s language “CAT scan” (Achenbach 2008a). The initial CT machine used a single slice using one detector array, then moved to 4, 8 slices, followed by 16 and 32 with added detector arrays along with a high support rotation time (Prokop 2003).
At present, the most up-to-date machines are 64, 256, and 320 slice structures amid novel expertise such as a flying z-focal spot which gives a spatial resolution of 0.4 mm or less (Endo et al. 2008). The latest dual-source CT moreover joins two x-ray tubes in one gantry set at a position of 90° (Achenbach 2008b). CTA is implemental since it is a derived procedure for coronary artery disease diagnosis. By the fact that CTA is non-invasive, as expected, it is safer for examination than coronary angiography. Nevertheless, if any CT technique generates ionizing radiation can be detrimental after a repeated experience. Another explanation that justifies CTA being superior to coronary angiography is the ability to identify several conditions that the invasive angiography cannot detect, for instance, non-flow-limiting atherosclerotic lesions (Schuijf et al. 2008).
Additionally, ECG-gated CT angiography is a recent procedure geared towards improving image quality. ECG-gated CT helps in the assessment of the condition of the coronary arteries, which is instrumental to eliminate major coronary artery stenosis or occlusion in patients with inconclusive clinical and ECG findings.
Amid the introduction of sub-second rotation joint with multi-slice CT (with up to 64-slice), high resolution with high speed can be attained simultaneously, enabling exceptional imaging of the coronary arteries (cardiac CT angiography). This also permits images of the CT to be of high resolution through the help of ECG gating. As such, the technique has been noted to depict images of the heart as multiple segments thus enabling recording of the ECG traces. It is via the ECG that the cardiac contractions of the heart are linked to the CT data. After the linking is complete, the data that had been earlier on recorded as the heart was in motion is ignored while that collected when the heart was at rest is now considered.
This way, each frame in a cardiac CT examination has an enhanced temporal resolution compared to the shortest tube rotation time (Hong 2001). Besides, Sun (2110) advocated that radiation dose connected with retrospective ECG-gating raises steadily, in case of the increased number of detector rows and reducing the detector size. Moreover, in the case of the increased number of slice CT scanners, this will see an increase in radiation dose.
However, several strategies have been put in place to lessen the radiation dose when using MSCT angiography for cardiac imaging, and potential ECG-gating is certainly the most efficient and momentous method to reduce both radiation dose and motion artifacts (Sun 2010). ECG-gated multi-detector row CT is rising as a critical imaging device for assessing the coronary arteries. About 4-slice CT, there is largely sensitivity of 59% and specificity of 89% with various studies considering even higher percentages regarding those qualities, particularly for the finding of coronary artery stenoses (Hoffmann et al. 2007).
A recent study that involved 51 patients was undertaken to gauge the computed tomography coronary angiography by a 16-slice for the assessment of coronary artery bypass grafts. The results were fruitfully concluded without problems in all patients recording enhanced analytical accuracy in stenosis detection (sensitivity 96%, specificity 95%) (Schlosser et al. 2004).
Additionally, CTA containing a 16-detector row system has distinctive advantages compared to the old CT generation leading to improved image superiority such as shorter breath-hold times, dropping respiratory artifacts; lessening cardiac motion artifacts; faster rotation of the gantry, and reduced slice thickness. These have enabled superior spatial resolution (Schlosser et al. 2004). CT scan detects stenosis in a grafted artery just like coronary angiography (Fig 2). The innovative 16-slice spiral CT with its prior advantages can adequately assess diverse coronary artery diseases like significant stenosis, complete occlusion, and coronary artery bypass grafts (Zhang et al. 2005).
The CTA of coronary artery segments faces several limitations with motion artifacts and severe coronary being the major factors according to research. A beta-blocker should be administered before examination even in 64-slice scanners. A beta-blocker is however unnecessary with the advent of the latest dual-source CT since the evaluation of coronary arteries is possible without artifacts motion at all heart rates. The BMI (body mass index) in obese patients can also affect the accuracy of CTA in the diagnosis of coronary artery disease. A CTA with 64-slice technology contains high analytical accuracy in assessing artery segments with distal branches as well as detecting coronary artery disease (Sun et al. 2008).
64 CT angiography was carried out on 52 symptomatic patients following bypass surgery to detect all grafts and coronary arteries for stenosis using conventional quantitative angiography as an indication. The analysis included a sum of 109 grafts, 123 distal coronary runoffs, and 116 non-bypassed coronary branches. “Per-segment analysis of graft disease gave a sensitivity of 99% and specificity of 96%. Sensitivity and specificity to detect run-off disease were 89% and 93% with a positive predictive value of 50%. In non-grafted coronary segments, CT detected significant stenosis with a sensitivity and specificity of 97% and 86%” (Malagutti et al. 2007).
Other recent studies performed on 109 patients to establish the diagnostic accuracy of a particular dual-source CT (DSCT) for chest pain evaluation depicted that the sensitivity and specificity of the diagnostic assessments were l00%, 99% correspondingly. Coronary disease for instance atherosclerosis, relevant stenosis, and occlusion was properly assessed giving a general sensitivity of 98%. Dual-source CT is an extremely valuable tool presenting high-speed and consistent diagnosis of CAD even in incredibly high heart rates (Johnson et al. 2008). The latest CT having more simultaneously acquired slices with a 256-slice system has been recently introduced to enable exposure of the whole heart in one rotation. Furthermore, the best image quality of CTA enables an image of minute coronary atherosclerotic plaques (Fig 3).
This system allows cardiac CT imaging at high-speed, necessitating less contrast, plus latently reducing radiation dose (Achenbach 2008b).
The key advantage of CTA is not just about replacing coronary angiography, but also in identifying the patients who do not need such an invasive modality. Implementation of CTA has to a great extent reduced healthcare costs by the introduction of needless cardiac catheterization (Zemanek et al. 2006). With all these factors considered it is expected that CTA will keep on developing image accuracy and sensitivity in diagnosing and examining the treatment of CAD.
Magnetic resonance angiography (MRA)
Magnetic resonance angiography (MRA) imaging has significantly surfaced over the last decade equally as magnetic fields and radio waves team up to produce either 2-D or 3-D imagery of internal bodily structures (Bosmans et al. 2001) MRA procedures of imaging coronary artery diseases come in three generations. First, it is the standard one slice per breath-hold with two-dimensional (2D). However, it has spatial registration problems. Next, it is the principle of free-breathing with three-dimensional (3D) and high resolution that involves lengthy acquisition times, which could take up to 15 min.
Finally, it is the 3D volume in one breath-hold with 3D and a short acquisition time. The problem, however, is that it offers low spatial resolution (Duerinckx 2001). All three techniques can be used with or without contrast agents, with gadolinium being the commonest (Bosmans et al. 2001). There are various methods for imaging the cardiovascular system using MRA typically through stream-based techniques and ultrafast disparity improved acquirement, which allow the evaluation of coronary flow dimension (Bosmans et al. 2001).
An advancement of MRA procedure paved way for consistent visualization of the mid-portion and proximal coronary artery tree for the elimination of considerable coronary artery disease (Spuentrup and Botnar 2006). As much as conventional angiography is still the present gold standard of coronary artery stenosis despite its inherent disadvantages like being the costly and small risk of mortality, MRA is invaluable as a non-invasive modality for imaging the coronary artery lumen, the vessel wall, coronary plaque, over and above identifying coronary thrombosis (Spuentrup and Botnar 2006).
Medical applications of 2D coronary MRA have numerous key advantages such as coronary lesion detection, the delineation of congenital coronary artery anomalies, and the characterization of earlier known coronary lesions. MRA also can assess coronary bypass graft patency, vessel patency and assessment following coronary stent placement, coronary anatomy after heart transplantation, and coronary flow reserve quantification (Duerinckx 2001).
Coronary MRA was carried out on 109 patients to assess its diagnostic accurateness of diagnosing coronary stenosis using free breathing through the three-dimensional (3D) procedure and the outcome contrasted with conventional coronary angiography. The accuracy, sensitivity, and specificity of patients having a disease of the left major coronary artery or three-vessel disease were 100%, 85%, and 87% in that order. While the negative predictive assessments for whichever coronary artery disease moreover, for left main artery or three-vessel disease was recorded as 81% and 100% respectively (Duerinckx 2001).
A more recent study by (Kunimasa et al. 2008) was performed on 39 patients using free-breathing whole-heart coronary MRA through diaphragm drift correction software. All participants were undertaking conventional coronary angiography. The outcome of the study was encouraging as the sensitivity to detect coronary stenosis ≥ 50% was 80% and the specificity to identify luminal narrowing < 50% was 97%.
The accuracy, positive predictive value, and negative predictive values were 92%, 93%, and 92%, respectively (Kunimasa et al. 2008). From the results, a free-breathing whole-heart coronary MRA with diaphragm drift correction software proved reliable for giving excellent diagnostic accuracy in the detection of significant coronary artery disease. Furthermore, it has great prospects in becoming the regular diagnostic modality for patients with alleged coronary artery disease. Whole-heart coronary MRA can happen to be a valuable diagnostic instrument for diagnosing acute coronary syndrome (Fig 4) (Kunimasa et al. 2008).
Another study was carried out by (Nandalur et al. 2007) to observe the diagnostic results of stress cardiac MRA in detecting CAD. In this case, two techniques were used, that is, stress-induced wall motion abnormalities imaging and perfusion imaging (Nandalur et al. 2007). In stress-induced wall, motion abnormalities imaging the results established a sensitivity of 83% and specificity of 86%, and perfusion imaging showed a sensitivity of 91% and specificity of 81%. In effect, these studies of stress cardiac MRA using both techniques have indicated largely high-quality sensitivity and specificity for CAD diagnosis (Nandalur et al. 2007).
They have also revealed various advantages of MRA including the capacity to significantly decrease needless and outmoded coronary angiographies (Pilz et al. 2008). Imminently, stress cardiac MRA is greatly promising with the innovative method that engrosses stress-induced wall motion abnormalities imaging and perfusion imaging (Nandalur et al. 2007). Although MRA has not replaced the most commonly used conventional coronary angiography, apparently it is only a matter of time before it happens (Sato et al. 2007). However, the most outstanding disadvantage is that certain subgroups of patients cannot benefit from this modality presently. For instance, patients heavier than 300 pounds, patients on continuous life support, claustrophobic patients, and patients with metallic objects within their bodies like pacemakers, valves, clips, joints, pins, etc. (Levine et al. 2007).
Figure 4:
- Volume rendering image showing high-grade stenosis in the proximal portion of the Left anterior descending artery (large arrow) and the first diagonal artery (small arrow)
- Soap bubble maximum intensity projection image showing stenosis in the LAD (black arrow) and D-1 (white arrow)
- Maximum intensity projection image demonstrating the area of low signal intensity in the distal portion of the left main coronary artery and the proximal portion of the LAD.
Conclusion
In the world today, Coronary artery disease (CAD) is the primary cause of morbidity, disability, and death in western countries. Approximately, over 50% of these coronary events happen without any preceding symptoms. Diseases like stenosis, occlusion, and luminal dimensions can be examined by invasive coronary angiography, which is still deemed the gold standard despite the rapid advancement of non-invasive techniques. Studies on CTA and MRA have established the drawbacks of coronary angiography and probable perfection in the diagnostic accuracy of coronary artery diseases. Computed tomography angiography (CTA) with new technology has revealed constant and outstanding improvements by providing 16-slices acquisition concurrently, 64-slices, and lately 256 slices scans.
Prospect developments of CTA hold great promise for CAD diagnosis. In addition, it can detect coronary diseases like atherosclerosis, occlusion, relevant stenosis, and bypass grafts with correct identification of overall sensitivity of 98%.
Alternatively, the non-invasive coronary magnetic resonance angiography (MRA) marked by its new technical improvements, has enabled reliable imaging of the proximal and mid-portion of the coronary vessel lumen, and express visualization of vessel wall disease, counting remodeling and detection of plaque without stenosis. Finally, the new-fangled imaging techniques such as computed tomography, magnetic resonance angiography, show potential. However, despite dynamic research in these fields, conventional coronary angiography remains the gold standard procedure for diagnosing clinically significant coronary artery disease.