Cardiovascular imaging using current MDCT technology applies fast tube rotation times to decrease cardiovascular motion artifacts and routinely produces images with sub-mm isotropic spatial resolution. Increased emphasis on radiation dose reduction with improved scanner design over the past 10 years has significantly reduced patient doses with MDCT imaging [25]. Of the many factors that can affect radiation dose, the most important for reducing radiation exposure are to use techniques that adjust the tube current according to the weight of the patient [26], automatic tube current modulation to automatically adjust the tube current based on the size, shape and density of the body part being scanned [27], and fast gantry rotation times [28, 29]. In the past few years, iterative reconstruction of CT images has made a substantial impact on lowering patient doses, in addition to improving image quality by reducing image noise due to streak artifacts from calcification and metallic artifacts [30–33].

Higher patient doses are generally associated with ECG-gated acquisitions which are not needed for routine angiography of the thoracic vasculature. ECG gating is necessary for evaluation of the aortic root and ascending aortic dimensions, coronary arteries, and ventricular function, and therefore, cardiac MDCT angiography with ECG-gated acquisitions should be restricted to these applications. Retrospective ECG gating is used to obtain images throughout the entire cardiac cycle for quantification of ventricular function, wall motion, and motion of valves [34], and the measurement of ventricular volumes, mass, and function has shown good correlation with measurements from CMR [35]. Retrospective ECG gating delivers a relatively high radiation dose because the x-ray tube is on throughout the cardiac cycle to acquire imaging data, and reconstructing multiple phases requires oversampling of image data and results in longer scan times. When imaging the aorta and coronary arteries, radiation doses can be substantially reduced by using prospective ECG triggering instead of retrospective ECG gating. With prospective ECG triggering, scanning is initiated at a predefined moment in the cardiac cycle from the QRS complex, usually in diastole. This technique delivers a low radiation dose because the x-ray tube is only turned on during a predefined portion of the cardiac cycle [36]. MDCT scanners with 320 detector rows (volumetric scanners) or second-generation dual-source scanners have become available with faster scanning so that sedation and breath holding are no longer necessary [37], and gated acquisitions for assessment of the heart and coronary arteries have significantly improved temporal resolution and allowed substantial reduction in radiation doses [38–40].

An important consideration when performing MDCT angiography of Fontan patients is the timing of the contrast injection and initiation of scanning [41]. Because the flow through the Fontan circulation is passive from the systemic venous system to the pulmonary arteries, homogeneous enhancement of the Fontan pathway cannot be obtained until the venous phase of contrast administration is reached. A common error is to commence scanning a Fontan patient with a standard protocol for routine pulmonary MDCT angiography when the pulmonary arteries just begin to be opacified. Scanning this early will result in inhomogeneous enhancement of the Fontan pathway (Fig. 21.7). There are several options for timing the contrast administration and scanning depending on the information that is needed from the examination and the intravenous access that can be established. Imaging to assess the Fontan pathway and pulmonary artery patency and possible thrombus requires homogeneous opacification which can be obtained with administering contrast via an upper or lower extremity IV and scanning at 50–70 s. Bolus tracking of the IVC for an upper extremity injection and bolus tracking of the SVC for a lower extremity injection can be used, but care must be taken to delay the initiation of the bolus tracking during the contrast injection in order to keep the radiation dose low. Based on this author’s experience, a 60-s scan delay routinely provides adequate contrast opacification of all of the intrathoracic vasculature with only minor inhomogeneity (Fig. 21.8). Others have found that waiting until 3 min after contrast administration provides the most homogeneous contrast opacification [42], but this is at the expense of overall reduction of contrast density, which can make image interpretation difficult, especially if low radiation dose protocols are utilized. Another option is to administer contrast with simultaneous injections of the upper and lower extremity, which allows denser opacification of the entire Fontan circulation but can be difficult if there is poor IV access. For detection of aortopulmonary collaterals, which can be a source of life-threatening hemoptysis, an earlier arterial phase scan for dense opacification of the aorta is required (Fig. 21.9).