Non-Invasive Multislice CT Coronary Angiography - Essay Example

Introduction Even 10 years earlier, coronary computerized tomographic (CT) angiography was considered to be an emerging non-invasive technique for visualization of the coronary arterial tree (Chan et al. 2009). However, the recent advent of multidetector CT coronary angiography is acceptably a new non-invasive tool for routine imaging and evaluation of the coronary artery (Achenbach 2009). This assignment will evaluate the case study of a patient where MDCT had been performed with 64 slices per revolution (Achenbach 2005). In CT angiography, acquisition of multiple images with a single revolution of the X-ray tube and provision of multislice images (Gopal et al. 2009) have allowed enhanced temporal and spatial resolutions, the images from which could be further enhanced with the use of image reconstruction and analysis software (Gopal et al. 2009). The diagnostic value of MDCTA is in the ability to diagnose small-volume plaque of soft consistency since in most cases (Manghat et al. 2005), since the acute coronary syndromes result from the rupture of these plaques (Manghat et al. 2005). Thus it remains to be investigated in the following case study about how these three parameters may lead to a specific protocol for imaging in the following case. Case report This is a case of a 56-year-old man who presented to the hospital with a history of chest pain suggesting crescendo angina pectoris for a duration of 48 hours. Previous ECG demonstrated labile changes with transient ST segment elevation that settled to deep anterior-inferior ST depression. Primary percutaneous intervention was futile since noslective injection of the cusp failed to detect the ostia of the right and left coronary arteries. An echocardiography done on the table suggested features of acute aortic root dissection indicated by a dilated aortic root. It was decided that an emergency 64-slice multidetector row CT (MDCTA) would be performed, which revealed in the gated scan of thoracic aorta and the coronary arteries that there was a single coronary ostium with anomalous origin higher and anterior from the suprasinotubular ridge. From this anomalous ostium, two vessels arose, one a small anomalous left anterior descending artery and a common trunk that eventually trifurcated into a large anomalous circumflex artery, right coronary artery, and a small septal branch. The culprit lesion was located within the anomalous circumflex artery, which proved to be a noncalcified atherosclerotic plaque. A repeat coronary angiography done with a 6 Fr Amplatz left 1 guidewire reconfirmed these findings, in which setting; a direct stenting was performed leading to excellent recovery of the patient. Comment This case exemplifies the clinical utility of MDCTA in that it could detect a rare anomaly of the coronary artery anatomy. In any other investigative modality, it would have been very difficult to detect multiple vessels arising from a location higher than usual, more so from a single coronary artery ostium. MDCTA provided the clinical imaging pathway for mapping these anomalous coronary arteries with localisation of the site of unstable plaque, and obviously, this facilitated rapid diagnosis, treatment; prevented complications; and improved prognosis (Becker 2002). Pathology: Coronary Artery Disease (CAD) The main indication for which CT angiography is done is to detect and diagnose atherosclerotic coronary artery disease (Schindler 2010). This is the main cause of coronary artery disease. Normally myocardium is highly active tissue, and it is in need of continuous and consistent blood supply through which nutrients and oxygen reach the myocardium (Pontone et al. 2007). When atherosclerosis occurs as a pathological lesion in these arteries, the major physiological problem that occurs is deficiency in blood supply to the myocardium due to narrowing in a segment of the artery due to deposition of fatty plaques (Libby 2002). It has been observed that when atherosclerotic coronary lesions occur, primarily there is accumulation and oxidation of low-density lipoprotein particles leading to complex atherosclerotic plaques (Bastarrika et al. 2009). While at early stages of these developments, there is remodelling of the vessel wall thus preventing this plaque to encroach into the arterial lumen, as the lesion advances with the ongoing pathological changes, eventually narrowing of the arterial lumen happens, which may have a chance to be critical enough to lead to symptomatic heart disease, such as angina pectoris or myocardial infarction that construes to be an indication (Virmani et al. 2006). Pathophysiology It is evident from the pathological factors that the built up plaque occupies the lumen of the coronary artery and hence the narrowing of the artery is expressed as percentage of the vessel area (Min, Lin, and Saba 2009). Plaques are known to cause eccentric stenosis and hence there is a need for imaging in multiple projections in order to be able to evaluate all the coronary segments to avoid underestimation of the luminal narrowing (Min, Lin, and Saba 2009). When CT angiography is used as the modality, it is important to recognise the pathophysiological changes occurring in the plaque area (Bastarrika et al. 2009). The plaque in itself has tissues of varied consistencies with a core of lipid deposits with smooth muscle and fibrous tissue surrounding it, ultimately capped with fibrous tissues (Hoffmann et al. 2006). The true value of CT angiography is in its ability to identify the at risk lesion in a non-invasive manner so adequate protective measures may be taken clinically to be able to avoid adverse cardiac events (Hoffmann et al. 2006). Most adverse events at the vascular level occurs when the plaque is immature and the periphery of the plaque gets damaged with fissuring as a result of vascular flow shears and the ongoing inflammatory process related to lipid peroxidation (Pelzel et al. 2007). Such an imbalanced plaque ruptures at an unorganised tissue site and invokes platelets to cause clotting and critical ischemia due to thrombosis (Pelzel et al. 2007). However, when multiple lipid deposits occur along the course of the affected vessel, specifically in this patient, calcified and organised plaques may also be encountered (Virmani et al. 2006). Moreover, in this patient, there is a need for anatomic evaluation as well as evaluation of the severity of the stenosis (Achenbach 2009). This has prognostic implications in that this can detect not only luminal narrowing or assess its grade, also can detect the plaques in the different stages of developmental process (Achenbach 2009). This would help the clinician to detect the at risk lesions which may need to be intervened in order to prevent future adverse events. It had been regularly observed that acute coronary vascular events occur frequently from innocuous lesions thus stressing on the need for detection of early changes out of pathologic processes that may be intervened in order to halt the progression of the disease to an acute event (Bastarrika et al. 2009). Patient Preparation The consent documents were filled, and the patient was verbally explained the minimal but possible risks out of the procedure, which included the type of examination, why it was needed, what were expected out of it, including what would follow and the reasons for it. Metal objects, jewelleries, and eye glasses may compromise the images, and hence the patient was instructed to remove all these thinks. A detailed history of previous radiological examination with the use of radio contrast agents and history of any allergy to iodinated agents should be elicited since radio contrast agents would be used in this investigation. The patient was asked to remain in empty stomach since some food materials including caffeine may adversely interact with the iodinated contrasts. The history must include history of other associated diseases such as diabetes and heart disease and other relevant diseases such as bronchial asthma or kidney diseases since the radio contrast agent is excreted through kidney and allergy to the agent may precipitate asthma to a life-threatening intensity (Becker 2002). The technologist should handle all these issues to minimise the risks associated with the procedure (Kantor et al. 2009). A premedication with a beta-blocker the evening before and in the morning of the investigation was done in order to reduce the heart rate since the patient was demonstrating a heart rate of 78 (Virmani et al. 2006). It was expected that this dose of beta-blocker will reduce the heart rate to below 65 or at least within 70, so the image quality can be best achieved during the breath holding periods (Virmani et al. 2006). It has been reported by multiple studies that higher heart rates produce motion artefacts, and these may contribute substantially to a poor image quality specially when a 64-slice MDCT is used (Bastarrika et al. 2009). While breath holding may reduce the heart beats by 6 beats per minute, aggressive reduction of heart rate by 50 mg or oral metorpolol about one hour before the study sufficed in this patient, which reduced the heart rate by 11 beats (Bastarrika et al. 2009). Moreover, immediately before scanning, a sublingual nitroglycerin was used to cause vasodilatation of the coronary arteries so the coronary artery lumen visibility was enhanced further. Intravenous access through a large 18-gauge intravenous catheter was established, and the patient was positioned supine on the MDCT examination table. Given the nature of the pathophysiology and the goal of the investigation, a good image quality was of utmost importance (Hoffmann et al. 2006). Increased visualization has been made possible with the passage of the contrast agent being temporally synchronized with the time of passage of the bolus of the contrast agent (Hoffmann et al. 2006). These are known to increase the radiographic density and hence visibility of the coronary arteries by at least 50 Hounsfield Units (HU) to even 500 HU (Achenbach 2009). The contrast was injected through the peripheral vein cannula within the intravenous line, and the contrast material was 100 mL of non-ionic iodinated contrast. Given the size of the patient a volume of 100 mL was chosen and the injection rate was set at 4 mL/second. It has been demonstrated that except in patients who are very obese, a flow rate of 4 mL/sec commonly renders images of acceptable quality. The patient was prepared for the sensation of injection, which would be a light burning sensation. Given the patient's age, a test breath hold and practice were done. A cycle of expiration and inspiration preceded the breath holding during inspiration. It has been demonstrated that this practice increases the stability of breath hold and reduces the episodes of unintentional movements during the imaging process (Gopal et al. 2009). Scanning Protocol After the patient was positioned on the table, an adequate ECG tracing was obtained with three ECG leads. This would be useful for getting a noise free ECG so this signal can be synchronised to the raw image data (Gopal et al. 2009). The patient was strapped to the table to minimise movement using the following parameters: X-ray tube potential 140 kV, effective tube current 680 mA, slice collimation 64 x 0.6 mm2, table feed 9.2 mm/rotation, and pitch 0.24. The table moved through the gantry, and the rotating x-ray beam and the detector could acquire images in multiple planes that included the images of the blood vessels in different levels (Kantor et al. 2009). While the image acquisition was performed, the patient was requested to hold the breath which would accomplish further reduction of the heart rate and reduce movement artefacts further (Kantor et al. 2009). The heart rate achieved in this patient was 65, and the most important images were achieved in five heart beats, and the physician present ensured the safety of the patient during the test and maintained the heart rates at this level (Gopal et al. 2009). Following the image acquisition, these images were verified in terms of quality and interpretability, and then the intravenous line was removed (Gopal et al. 2009). The images were then transferred to the computer where image reconstruction software was used to deliver multidimensional views of the coronary arteries. The contrast medium enhanced the images and the plaques were identified readily. Although this procedure can best be done as an outpatient, in this patient, the requisition came from the inpatient cardiology (Pelzel et al. 2007). Contrast Protocol In our unit, a dual injection system with bolus tracking is used for coronary CTA. This follows the protocol of injection of contrast followed by saline. It has been demonstrated that injection of saline prevents dense opacification of the right heart, so the interpretation of the right coronary artery is facilitated (Bastarrika et al. 2009). A saline bolus would push the contrast into the target area and also would minimise dilution of the contrast while it passes through the central veins (Bastarrika et al. 2009). There is indeed a connection between arterial enhancement and contrast injection (Gopal et al. 2009). There are two parameters of contrast enhancement as it relates to the contrast protocol, the rate of injection and the duration of injection (Pontone et al. 2007). The injection was administered in the right arm since if it is given in the left arm, it will opacify the left brachiocephalic vein leading to a source of artefact during the image acquisition. For this patient, it was determined that a 20 mL of the contrast would suffice, which would be administered at a rate of 4 mL per sec for a total duration of 20 sec of scan time. This is appropriate since the contrast material volume is determined by the contrast injection rate and scan time requited to image the native coronary arteries. It has also been highlighted that the contrast should be present in the target area when the image acquisition occurs (Pontone et al. 2007). Out of two different protocols for bolus administration, the bolus tracking method was used in this case. Thus a series of low-dose axial scans every 2 sec at the level of the carina was used to track the bolus in order to ensure contrast enhancement at the level of the ascending aorta (Schindler 2010). With this technique, when the arterial enhancement reached a predefined value of 100 HU, the image sequence was initiated. The saline bolus is injected at the same rate and in large amount of 100 mL ensuring complete wash out of the iodinated contrast (Schindler 2010). The volume of iodine used in this case was 370 mgI/mL. This is a highly concentrated material, and thus to obtain images of high quality it is important to improve the contrast to noise ratio (Hoffmann et al. 2006). Typically with a 64 MDCT the image acquisition time decreases to 10 s, thus a shorter but compact bolus of 80 mL would be ideal (Hoffmann et al. 2006). Strategies to Reduce Radiation Exposure With the goal of isotropic data acquisition and identical spatial resolution at all planes, the radiation risk and dose minimisation remains a challenge specially with machines such as this. Various strategies were used in this patient through adjustment of scanning parameters to reduce the radiation dose (Hoffmann et al. 2006). Some of these are ECG gating, image data oversampling, and spatial resolution (Manghat et al. 2005). In terms of reduction of radiation risk, the two main scanning parameters that determine the exposure to high-energy photons are effective milliampere second (mAs) and kilovolts (kV) (Achenbach 2009). Effective mAs is defined as mAs divided by CT pitch and is proportional to X-ray CT tube current and scan time, which were set for this patient by the technician at the time of data acquisition (Achenbach 2009). Some of such strategies have been enumerated below. a. The ECG gating technique was used to get a low-energy topogram which permitted accurate positioning of the scan volume. This scanner allowed ECG controlled dose modulation (Achenbach 2005). b. There was need to alter or adjust the dose at all intervals. In this case, this was accomplished by lowering the current or increasing the table speed (Manghat et al .2005). c. ECG gating was an effective strategy in this patient to minimise oversampling of the image data. The strategy of leaving out other areas that are unnecessary could successfully reduce unnecessary scanning time and hence radiation exposure (Hoffmann et al. 2006). d. ECG controlled dose modulation that allows for automated dose reduction during systole is known to provide for 30% to 50% dose reduction in the effective radiation exposure. e. With metorpolol, this patient's heart rate became 65 with no appreciable variability in the rate, and thus this strategy worked fine with this patient (Gopal et al. 2009). f. It has been demonstrated that auto mAs which assesses the tube current necessary can be of great help in dose adjustments. In this patient at the time of image acquisition, a tube potential of 140 kVp was used and at the other times this was automatically reduced to 120 kVp, thus reducing the radiation exposure further (Gopal et al.2009). g. Adjustments in the CT pitch can also reduce exposure. A very small part of the RR interval is used to reconstruct the image. A 64 MDCT was thus suitable in this patient that allowed greater coverage with a larger craniocaudal territory per rotation. Increased pitch and speed adjustment can reduce the radiation dose (Manghat et al. 2005). Image Display, Appearance and Analysis The images were reconstructed in overlapping increments (Bastarrika et al. 2009). In this case the images had a slice thickness of 0.75 mm with about 50% overlap between images with a pixel matrix of 512 X 512. Although a thinner slice would have been advisable, it was also necessary to decrease the image noise so the 3-dimensional dataset can be accurately interpreted in order to identify the pathology (Bastarrika et al. 2009). A medium smooth reconstruction kernel was used to reconstruct the images in this patient (Virmani et al. 2006). It has been suggested that for inclusion of incidental extracardiac findings, the raw data with increased slice thickness and a larger field of view would suffice (Virmani et al. 2006). In this patient, a retrospective ECG-gated reconstruction was done. If optimally reconstructed, this would allow images of the coronary artery without virtually any motion artefacts (Hara,et al. 2009). There are three different visualization techniques that may be used which are multiplanar reformation (MPR), volume rendering (VR), and maximum intensity projections (MIP) (Hoffmann et al .2006). Significant coronary artery stenosis was ruled out in this case through both axial images and MPR data (Hoffmann et al .2006). It has been stated that despite a very good visualization, the stenosis is best detected through the original axial images, which can also facilitate detection of virtually all pathologies (Pelzel et al. 2007). The MIP was used in this case of the ostial narrowing when it was detected through two long-axis views of the anomalous circumflex artery with a 3 to 5-mm thin slab MIP images, which is known to project the data in 4 to 7 original slices in a single image (Chan et al. 2009). Thus reconstruction of a true cross-sectional image of this circumflex artery anomaly orthogonal to the long-axis view was possible (Min, Lin, and Saba 2009). Following this, a comparison of luminal cross-section both proximally and distally to the culprit area could enable a qualitative assessment of the extent of the luminal narrowing. Calcification is determined by calcium scoring, and in this patient the calcium scoring was noted to be within normal limits ruling out deterrent and severe calcification. This has been reported to be of acceptable accuracy with software that detects vessel border automatically (Gopal et al. 2009); however, in our unit it was done by manual measurements with the use of callipers. Image Analysis Cardiac CTA is known to produce excellent images with moderate sensitivity and excellent specificity of coronary artery stenosis (Achenbach 2009). In our facility in this case, the use of 64 slice MDCT provided an in-plane resolution of 0.4 mm and a slice thickness of 0,6 mm that resulted in a temporal resolution of 165 ms. This enabled simultaneous acquisition of 64 parallel cross sections leading to the imaging of the entire coronary artery tree (Achenbach 2009). This investigation also has a very high negative predictive value leading to the possibility to exclude presence of hemodynamically significant CAD (Gopal et al. 2009). In this case, almost all the segments could be evaluated which revealed that the culprit lesion is more than 75% stenosed. it has been reported that a 64-slice MDCT can detect stenosis of less than 50% accurately in 79% cases, that more than 50% in 73% cases and greater than 75% in 80% with a 97% specificity (Pontone et al. 2007). Treatment and Aftercare As indicated earlier, this investigation provided the opportunity to diagnose the culprit lesion specifically in this patient, which this case demonstrates a rare coronary artery anomaly with multiple vessels arising from a higher than usual single coronary ostium. It also highlights the utility of MDCT for non-invasively mapping the anatomy of anomalous coronary arteries, locating the site of the unstable plaque, and avoiding potential delays in diagnosis and treatment (Pontone et al. 2007). These findings could lead to the treatment plan of direct coronary angiography with placement of a stent in the anomalous circumflex artery site of stenosis due to hard plaque, which would keep the area dilated and would facilitate the flow through this area leading to remission of symptoms and reduction of risk. In short the prognosis is supposed to be excellent (Chan, et al. 2009) In order to nullify the effect of postural hypotension caused by the beta-blocker metorpolol, the patient was gradually made to assume an erect posture. The patient was given acetaminophen to counteract the headache caused by the nitroglycerin and was placed in observation where he was made to drink plenty of water in order to flush out the contrast material from the system. Conclusion As has been demonstrated in this case study, the coronary artery angiography using a 64-slice MDCT is a very dependable clinical modality of investigation where a high degree of diagnostic accuracy can be achieved leading to a better and more accurate prospective care plan in patients with coronary artery disease. Although there are many technical adjustments necessary in order to facilitate greatest results, it is worthwhile in terms of ensuring the best prognosis of the patient in such cases. 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