Cardiac CT Imaging Procedures PDF

Chapter #3 Chest CT  Thoracic Imaging Procedures -CT imaging of the chest presents unique challenges because of the continuous motion of the heart and vascular structures. -Improvements in temporal and spatial resolution that have been realized as a result...

Chapter #3 Chest CT  Thoracic Imaging Procedures -CT imaging of the chest presents unique challenges because of the continuous motion of the heart and vascular structures. -Improvements in temporal and spatial resolution that have been realized as a result of multidetector-row CT (MDCT) have been particularly valuable in thoracic imaging. -This technology (MDCT) allows the entire thorax to be scanned with thin sections during a single breath- hold, making consistent high-resolution imaging possible. -Electrocardiographic (ECG) synchronization with MDCT is another valuable tool used to reduce cardiovascular motion artifact and improve image quality. - Using the shortest scan time possible helps to reduce artifacts created by respiratory motion. -Whenever possible, scans of the chest should be acquired within a single breath-hold, as this will prevent misregistration. -Most thoracic protocols are performed while the patient lies in a supine position on the scan table with the arms elevated above the head. -In a few instances, primarily high resolution CT protocols of the lungs, additional scans are obtained with the patient in the prone position. Important Note: The thorax has the highest intrinsic natural contrast of any body part? Bcz: 1. The pulmonary vessels and ribs have significantly different attenuation values compared with the adjacent aerated lung. In most adults, 2. The mediastinal vessels and lymph nodes are surrounded by enough fat to be easily identified. For this reason, contrast media administration is not necessary for all thoracic indications. For example: The demarcation of the esophagus and the gastroesophageal junction can be improved by giving the patient an oral contrast agent, most often a barium suspension, shortly before beginning the scan, but is not necessary for most thoracic CT examinations.  CT of the Airways - CT is the best imaging techniques for airway? 1. assessment of disease of the central airways and is 2. mostly commonly used to look for airway narrowing. Technical parameters used for CT imaging of the airways include the use of thin sections (1.25 mm or less), a fast acquisition that allows the entire lungs to be scanned during a single breath-hold (to reduce motion artifact), optimal spatial resolution, and the use of postprocessing techniques.  High-resolution CT (HRCT) High-resolution CT (HRCT) is used to evaluate the lung parenchyma in patients with known or suspected diffuse lung diseases such as fibrosis and emphysema. Like airway imaging, HRCT protocols use thin sections (1.5 mm or less), a fast acquisition to reduce motion artifact, and optimal spatial resolution. In addition to the thin sections, spatial resolution is optimized by the selection of an edge-enhancing algorithm (such as a bone algorithm) and a display field of view (DFOV) that is just large enough to include the lungs More recently, as MDCT scanners have become commonplace, the technique known as volumetric HRCT is replacing the HRCT axial protocols. Why? Because: 1. Volumetric HRCT protocols use a helical mode to acquire images of the entire lung, rather than representative slices. 2. These helical protocols cover the entire lung, they result in a more complete assessment of the lung. 3. They allow postprocessing techniques such as maximum (MIP) and minimum (MinIP) intensity projection reformation.  Thoracic CTA -MDCT angiography has become an imaging mainstay in the diagnosis of pulmonary embolism (PE). -Pulmonary embolism (PE) is a blockage of an artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream (embolism). -Symptoms of a PE may include shortness of breath, chest pain particularly upon breathing in, and coughing up blood. -CT pulmonary angiography is considered by many to be better than traditional catheter or invasive pulmonary angiography, which is limited in the number of projections and suffers from vessel overlap. -Thrombosis can be life threatening when it occurs at other times. The danger is that the thrombus can occlude a vessel and stop the blood supply to an organ. If the thrombus separate from its original site, it is referred to as an embolus. -An embolus can be formed from clotted blood, fat, air and tumour tissue.  Cardiac CT -Cardiac CT has emerged as a less-invasive imaging modality for the diagnosis of coronary artery disease (CAD) and is often used to avoid coronary angiography. -Cardiovascular disease (CVD) is the leading cause of death in the United States, accounting for approximately 37% of deaths in 2003. - Visualization of coronary arteries in CT is difficult due to: tortuous in shape, subject to rapid heart motion. -To reduce motion artifacts on cardiac CTA images A. B-blocker B. ECG gating (prospective or retrospective). -The ECG provides a profile of the heart’s electrical activity with time. Each heartbeat in a normally functioning heart exhibits a similar characteristic pattern consisting of five waves referred to as P, Q, R, S, and T (Fig. 20-18). The distance between two R waves represents one complete cardiac cycle and is sometimes referred to as the R-R interval. Hence, a scan that covers the entire heartbeat (a continuous acquisition) might be referred to as covering 100% of the R-R interval.  Two approaches to cardiac gating are typically used: prospective ECG triggering and retrospective ECG gating. 1. Prospective ECG gating - Prospective ECG gating methods use signal, usually derived from the R wave of the patients ECG. - A delay between the R wave and scan can be also introduced by the technologist. - A scan is acquired during a finite portion of the R-R interval, the table then moves to the next position, and the procedure is repeated until the entire area of interest is covered (referred to as a step and shoot system. – Disadvantage Prospective ECG gating A. Suffers from long scan time (degrades the temporal resolution) B. It is more sensitive to patient motion and image misregistration than the retrospective method –Advantage Prospective ECG gating A. Has an advantage of reduced radiation doses. 2. Retrospective ECG gating  CM Use in Cardiac CT - A low osmolar or isoosmolar, nonionic agen. - with an iodine concentration between 300-400 mg/ml. - is injected at rate of 3-6 ml/s. -The volume varies from 70-150 depending on the scan protocol, scanner type, and the patient weight..Target enhancement is typically in the range of 200 300 HU. -Time to start acquisition can be calculated using timing bolus. -Slice or ROI is placed over the ascending aorta for studies of the coronary arteries -Slice or ROI is placed over the left atrium for pulmonary vein imaging. -The advantages of using saline flush after the injection of the iodinated contrast: A) Pushes the remaining CM in the arm vein which makes use of all CM B) Flattens the enhancement peak (extend the enhancement window) C) Eliminates beam hardening artifacts from dense CM.