Multidetector CT in Cardiothoracic Surgery
Volumetric CT, also known as helical or spiral CT, involves continuous data acquisition while the patient is moved at a constant speed through the CT gantry, resulting in a contiguous set of images without interscan delay, minimizing misregistration of artifacts, which degrades image quality. With the first generation of volumetric CT, or helical CT scanners, one single data set per gantry rotation was acquired, which still had a long acquisition time for the lungs, heart, and great vessels.
The introduction of multidetector CT (MDCT) in the early 1990s, was revolutionary for the imaging evaluation of the cardiovascular system by significantly improving temporal and spatial resolution. Initially with a 2-channel (or 2 slices) system (1992), followed by a 4-channel system (1998), this technology has evolved to a 320-channel system today. But it was after the introduction of the 16-slice MDCT in 2002 that coronary artery imaging could be routinely obtained.
Unlike the 1-channel system in which only one data set is acquired per gantry rotation, MDCT allows multiple data sets to be acquired simultaneously by multiple rows of detectors, with a significant reduction in scan time, reduction in contrast volume needed, and reduction in motion-related image degradation. Thin-section scanning (< 1 mm slice thickness) produces very high (submillimeter) spatial resolution images in the axial plane, but because in most modern MDCT scanners the voxel data used to reconstruct the images is isotropic (cuboidal in shape), post-processing allows reconstruction of images in any plane as well as 3D- reconstructions without detriment to image quality.
In 2000, an additional refinement was the introduction of electrocardiographic (ECG) gating, to reduce motion artifact caused by cardiac continuous pulsation, which is a prerequisite for coronary artery and cardiac imaging. The ECG synchronization, which can be performed prospectively or retrospectively, in combination with faster gantry rotation time, allows “freezing” of cardiac motion during diastole and/or systole of the cardiac cycle, thus preventing misregistration of artifacts. For a heart rate of 70 bpm, a temporal resolution of at least 250 milliseconds is needed for good image quality. Modern scanners have a temporal resolution in the range of 75 to 150 milliseconds. Routine use of beta blockers or other negative chronotropic medication and sublingual nitroglycerine before scanning significantly improves image quality by reducing heart rate (< 65 bpm) and dilating the coronary arteries. The use of a power injector capable of delivering intravenous contrast at a high rate (4.5-6.5 ml/s) is also critical for a good quality examination. An important limitation of coronary computed tomography angiography CCTA is the presence of a significant amount of coronary artery calcification, which significantly limits the evaluation of luminal narrowing as well as cardiac arrhythmias such as atrial fibrillation, frequent extra systoles, or premature ventricular contractions, all of which negatively affect the ECG gating.
Radiation dose from CT has always been a significant concern due to the potential cancer risk over cumulative lifetime radiation exposure. During the last 10 years significant reduction in radiation dose has been achieved after the introduction of improved imaging protocols and software and hardware technological refinements, including tube-current modulation, use of lower peak tube voltage, iterative data reconstruction, high pitch/faster imaging technique, etc., bringing radiation dose from around 20 mSv to less than 1 mSv per study, which is lower than the radiation dose from a nuclear medicine perfusion scan or coronary artery catheter angiography.
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