The most important disadvantage of CCT (in- cluding CT angiography) as an imaging technique is the associated exposure to ionizing radiation. This is especially problematic in patients with ACHD in whom serial assessments are contem- plated over a lifetime.S3.4.5-1 Gating CCT to the ECG allows image acquisition during multiple phases of the cardiac cycle, thereby providing cine im- aging and the ability to select phases of the cycle of specific interest (usually end-systole and end- diastole), at the cost of increased radiation dose. Electrocardiographic gating is generally unneces- sary when the focus is assessment of extracardiac vascular structures, which can consequently be imaged using substantially lower doses of ionizing radiation. Ongoing development of protocols and equipment that reduce radiation exposure are 3.4.6. Cardiac Catheterization Recommendations for Cardiac Catheterization Referenced studies that support recommendations are summarized in Online Data Supplement 10. COR LOE Recommendations I C-LD Cardiac catheterization (hemodynamic and/or angiographic) in patients with ACHD AP classification II and III, or interventional cardiac catheterization in patients with ACHD AP classification I to III should be performed by, or in collaboration with, cardiologists with expertise in ACHD.S3.4.6-1 S3.4.6-4 IIa B-NR In patients with a low or intermediate pretest probability of coronary artery disease (CAD), use of CT coronary angiography is reasonable to exclude significant obstructive CAD when cardiac catheterization has significant risk or because of patient Recommendation-Specific Supportive Text 1. Cardiac catheterization remains a standard tool when diagnosis, prognosis, or management re- quire a) more precise definition of anatomy than is achievable via advanced noninvasive imaging (eg, structures with low flow or those shielded from other techniques), b) calculation of pressures and resistances, or c) physiological or anatomic simu- lation to allow additional calculation or anatomic visualization. 