Cardiac CT Imaging: Diagnosis of Cardiovascular Disease
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Coronary Artery Disease is caused by artherosclerosis hardening of the arteries with cholesterol build-up, and calcification calcium build up of the coronary arteries. The cardiac CT scan can detect the presence of calcium at a very early stage, even before any significant blockage has developed, and potentially years before a standard health screening or exercise ECG would detect the disease. Stage 1. Stage 2.
Prior to the scan, your heart needs to be beating at a certain rate to ensure a proper diagnostic result. Medication may need to be given at your appointment if your heart rate is not at the required level. This will be in the form of a tablet known as a beta-blocker. A radiographer will carry out pre-scan checks which will determine this.
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If you are given this medication, you are advised not to drive for hours after your scan. Therefore, please make sure suitable arrangements have been made for your journey home, ie, public transport; taxi; lift.
You will be asked to change into a gown, removing only the top half of your clothing. The gown will be open to the front as we need to place ECG monitoring on your chest during the scan. You will also be asked to:. A cannula drip line will be inserted into a vein in your arm. This will be used for the injection of x-ray dye during the scan.
CT Scan for Coronary Artery Disease
The ECG monitoring will now be placed on your chest. This will display your heart rate throughout the scan. Breathing instructions will be given to you during the scan and it is very important that these are followed. These instructions will be given approximately times during the scan. A radiographer will inform you when the injection of x-ray dye begins. You may experience a hot flush, metallic taste in your mouth, and a realistic sensation of passing urine during this time. For some patients, the less invasive Calcium Score alone is deemed to be sufficient for diagnosis.
There is no preparation required for the Calcium Score.
Cardiac Computed Tomography (CT) Angiography
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The volume of acquisition is typically 12—16 cm for coronary angiography and 18—25 cm for angiography of coronary bypass conduits. The radiation energy level required to obtain adequate image quality depends on the width of the patient's chest, the type of study, and the desired spatial resolution.
Current CCT systems with 64 detectors provide a typical dose range on the order of 8—20 mSv for coronary angiography. Corresponding doses are 2—6 mSv for invasive angiography, 10—27 mSv for rest—stress nuclear myocardial perfusion imaging studies, and 3. It is estimated that the risk of cancer may increase by 1 in 2, CCT studies, depending on the age of the patient. Because of these concerns, the routine application of CCT as a screening test is not justified until more outcome data become available.
Another recent strategy developed by some manufacturers involves step-and-shoot acquisition in a nonhelical mode; preliminary data suggest that in selected patients, the dose may by reduced to a total of 2—4 mSv. Once an adequate heart rate is achieved, if there are no contraindications such as severe hypotension or use of phosphodiesterase inhibitors , sublingual nitrates are given to vasodilate the coronary vessels. The acquisition starts with a scout scan planar x-ray mode , which is used to select the region of interest usually from the carina to slightly below the diaphragm, but from the subclavian artery down if an internal thoracic graft is be assessed.
In many centers, a calcium score scan no contrast material is then acquired during a breath hold. A remotely controlled dual-injection system capable of administering iodinated contrast material and saline separately is used for the contrast-enhanced study. Image acquisition may be triggered manually or automatically when the concentration of contrast material reaches a prespecified Hounsfield unit HU attenuation value — HUs in the descending aorta.
Alternatively, before the scan, a timing bolus of 20 mL of contrast medium may be injected at the same rate as that to be used for the scan, followed by a single-slice axial image acquisition every 2 s at the level of the carina. A time—density curve is created by plotting the attenuation values obtained in the descending aorta, and the interval from the onset of injection to the peak of this curve is selected to determine the scan delay.
Next, the entire heart volume is scanned within a single breath hold 15—20 s.
CCT provides complex and detailed 3-dimensional datasets, which are reconstructed from the raw data file according to specific phases of the cardiac cycle. Once the best phase for analysis is determined, examination of each vessel is performed by use of axial images and multiplanar reconstructed images in any orientation coronal, sagittal, or oblique. Evaluation of the images in the axial projection is done first, because it represents the data in the form that is acquired and is less prone to reconstruction artifacts.
Careful adjustment of image window parameters is done to differentiate the iodine-enhanced lumen from calcified and noncalcified plaques. Other postprocessing formats are also used for assessing cardiac structures. Maximum-intensity-projection images allow the evaluation of longer segments of coronary vessels, but they are limited by overlap from adjacent structures.
Three-dimensional volume-rendered images are useful for assessing the relationships among different anatomic structures. Curved multiplanar images are reformatted on a plane to fit a curve usually the path of a coronary artery and allow display of the entire vessel in a single image Fig. A Axial image showing left main coronary ostium and its divisions into left anterior descending, ramus intermedius, and left circumflex arteries.
B Multiplanar reconstructed image. C Anatomic 3-dimensional volume-rendered image showing relationships among left main artery, branches, and adjacent cardiac structures. D Curved multiplanar reconstruction of entire length of left circumflex artery. The most widely used measure of calcium burden is the calcium score often known as the Agatston score , which is based on the radiographic density—weighted volume of plaques with attenuation values of greater than HUs. Although the utility of screening asymptomatic individuals remains controversial, several studies have indicated that the calcium score provides prognostic information independent of conventional risk factors.
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In a recently published study, a calcium score of greater than was associated with a significant increase in cardiac events compared with that determined by a clinical score alone 8 , supporting the notion that a high calcium score can modify predicted risk; this is especially true for patients in the intermediate-risk category, for whom clinical decision making is most difficult.
Patients determined to be at low risk by clinical criteria, however, appear to derive minimal additional prognostic benefit from calcium scoring. These conclusions are represented in a clinical consensus document recently issued by the American College of Cardiology and the American Heart Association 9.go to link
Non-invasive imaging of the coronary arteries | European Heart Journal | Oxford Academic
One of the most unique applications of CCT is coronary angiography. Several single-center studies investigated the accuracy of channel CCT coronary angiography for the detection of coronary artery stenosis in patients with known or suggested coronary artery disease and referred for invasive coronary angiography 10 — An important advantage of the newer , , and channel CCT systems is their greater craniocaudal coverage per rotation, which allows shorter breath holds and, consequently, smaller contrast injection volumes, fewer artifacts related to patient breath-hold compliance, and less heart rate variability 21 — Studies have shown that the superior performance characteristics of slice CCT in terms of spatial and temporal resolution lead to measurable improvements in image quality Oblique coronal image obtained from patient with anginal symptoms and indeterminate stress test results, showing severe stenosis of ostium of left main coronary artery arrow.
Evaluating coronary artery stenosis in patients with extensive coronary artery calcifications may be difficult and represents a major limiting factor.
In addition, the true lumen results in a low-density area because of beam-hardening artifacts. In some situations, it can be difficult to distinguish these artifacts from noncalcified coronary plaque. Because symptomatic patients with very high calcium scores have a very high probability of having obstructive CAD, it is reasonable to avoid CCT coronary angiography and proceed directly to invasive catheterization in these patients Fig. A Axial image obtained at level of origin of left main artery, showing extensive calcification in left anterior descending coronary artery.
Aortic mechanical prosthetic valve is visualized arrow. In this case, it is difficult to evaluate lumen because of metallic artifacts. Vessels distal to stents are widely patent. A special consideration is the use of this technique in emergency departments. Several recent studies examined the role of CCT in the evaluation of acute chest pain in patients at low risk for acute coronary syndrome 29 — In most of these patients, CCT can reliably exclude obstructive CAD and can help to diagnose patients with other potentially life-threatening etiologies of chest pain such as acute aortic dissection or pulmonary embolism.
However, further studies are needed to determine the safety and cost-effectiveness of CCT compared with those of other imaging modalities in this setting. Recent studies evaluated the feasibility of CCT for quantifying atherosclerotic coronary plaques and differentiating calcified from noncalcified lesions on the basis of their x-ray attenuation. Compared with intravascular ultrasound, CCT tends to underestimate the noncalcified plaque volume but to overestimate the calcified plaque volume.
Accurate assessment of coronary vessels that have stents remains an important limitation of CCT coronary angiography 33 , The ability to evaluate the lumen of vessels with stents depends on the type and the diameter of the stent. Practical delineation of in-stent restenosis remains difficult for stents smaller than 3 mm in diameter; the luminal diameter is often underestimated because of partial-volume averaging and blooming artifacts Fig.
Other potential indications of CCT include the evaluation of bypass grafts. Grafted vessels have larger calibers and are less prone to motion artifacts than native coronary arteries. However, accuracy for the evaluation of distal anastomoses is lower. Metallic artifacts caused by surgical clips may limit the assessment of segments of internal thoracic grafts. Analysis of native vessels is often more difficult in patients who have received coronary artery bypass grafts because of poor distal vessel opacification, more extensive calcification, and smaller lumen size Fig.
Three-dimensional volume-rendered oblique sagittal view obtained from patient with previous bypass surgery. Arrow indicates distal anastomosis of aortocoronary bypass graft to left anterior descending artery. CCT coronary angiography is very useful in evaluating the origin and course of anomalous coronary arteries 37 , In addition, CCT can easily determine the 3-dimensional relationship of anomalous coronary arteries with the aorta and pulmonary arterial trunk.
Also, it can detect aneurysms on coronary vessels, arteriovenous fistulae, and myocardial bridges Fig. CCT image obtained for young patient with chest pain. Arrow indicates anomalous origin and course of right coronary artery between aorta and pulmonary arterial trunk. A Axial view showing normal anatomy of 4 pulmonary veins and left atrial appendage clear of thrombus asterisk. B Axial view from another patient undergoing evaluation before radiofrequency ablation of atrial fibrillation.