vasculature It is crucial to decide carefully the timing of acquisition of the images after injection because arteries or veins will appear with greater contrast at different times In clinical practice, a series of data sets are acquired consecutively to depict the entire thoracic vasculature in the venous and arterial phases.8 When reviewing and analyzing the images, it is important to remember that the reconstruction represents a mean shape throughout the cardiac cycle because the acquisition is not ECG gated Therefore it is not possible to measure the vessel in systole or diastole Moreover, being non-ECG gated, the images will be affected by cardiac motion and therefore intracardiac anatomy cannot be adequately investigated FIG 21.5 Three-dimensional volume-rendered, contrast-enhanced magnetic resonance angiography (A) Double aortic arch viewed from posterior (B) Infracardiac total anomalous pulmonary venous drainage into the portal vein (arrowhead) viewed from posterior Note the stenosis as the descending vein passes through the diaphragm (arrow) (C) Small left ventricle to pulmonary trunk conduit (arrow), viewed from the left side in a patient with congenitally corrected transposition of the great arteries Note very dilated pulmonary trunk Stress Perfusion Imaging (Video 21.6) Stress perfusion MRI has limited application in children However, it can be used to assess inducible myocardial ischemia in conditions involving the coronary arteries, such as Kawasaki disease,21 or when the coronary arteries have been manipulated (e.g., post arterial switch operation)22 and ischemia is suspected It can be performed using either a vasodilator agent (adenosine/regadenoson) or, more rarely, an inotropic agent (dobutamine) Perfusion images are acquired at peak stress during infusion of gadoliniumbased contrast media The perfusion defects are visualized as hypointense dark regions of the myocardium that correspond to the ischemic territories Compared with nuclear imaging techniques, adenosine stress CMR has the advantage of being radiation free and much quicker, allowing the acquisition of both stress and resting perfusion in a single examination Careful evaluation of the patients, including medical history, medications, and ECG, is necessary prior to administration of adenosine, to screen for contraindications (asthma, allergy, conduction defects) Tissue Characterization Sequences Tissue characterization sequences are increasingly used in pediatric CMR It is important to know that CMR can offer not only anatomic and functional information but also reveal insights on the myocardial tissue characteristics noninvasively By choosing the appropriate sequences, myocardial edema, scarring, fibrosis, iron, and lipid overload can be investigated These sequences are particularly useful also in the assessment of cardiac masses and cardiomyopathies T2 Short Tau Inversion Recovery Precontrast T2-weighted images are used to evaluate the presence of myocardial edema These sequences can be useful in the context of acute myocarditis and myocardial infarction and in the assessment of cardiac masses T2* (Star) This is a noncontrast sequence that allows for quantification of myocardial and hepatic iron overload in iron-loading conditions It consists in the acquisition of a single midventricular short-axis slice and a single axial slice of the liver It is recommended to perform the analysis of T2* by drawing a region of interest on the interventricular septum to minimize artifacts.23 Serial T2* examinations are used to assess the progression of the iron overload in the myocardium and liver and to guide therapy in conditions such as thalassemia major.24 T1 Mapping and Extracellular Volume These novel techniques are used to evaluate the presence of diffuse interstitial fibrosis and generally to assess whether there is an increase of the volume of the extracellular space The most common CMR sequences used are MOLLI and ShMOLLI Native T1 time is an intrinsic characteristic of every tissue Increases in the extracellular compartment lead to an increase of native T1 values.23,25 This can typically happen in myocardial edema, diffuse fibrosis, or protein deposition (like in amyloidosis) In contrast, the accumulation of iron and lipids tends to shorten the native T1 values Native T1 mapping is a useful tool in investigating patients with LV hypertrophy when Anderson-Fabry disease is suspected.26 Late Gadolinium Enhancement Imaging Late gadolinium enhancement (LGE) images should be acquired ideally 5 to 20 minutes after contrast administration The gadolinium-based contrast agents tend to accumulate in the extracellular space, whereas they have a rapid washout from the normal myocardium The areas in which the gadolinium accumulates will appear bright (enhanced), compared with the normal myocardium that appears black Pathologic processes that lead to an increase of the extracellular space (myocardial scar or fibrosis) will present LGE The pattern of distribution of the LGE follows the underlying pathophysiologic process, being full thickness or subendocardial when secondary to diseases affecting the coronary arteries and intramyocardial or subendocardial in nonischemic conditions (Fig 21.6) Although the role of LGE in adults with acquired and CHD has been well established, the applications in children remain to be fully established FIG 21.6 Late gadolinium enhancement (A) Vertical long axis in a patient with hypertrophic cardiomyopathy, showing patchy enhancement ... conduction defects) Tissue Characterization Sequences Tissue characterization sequences are increasingly used in pediatric CMR It is important to know that CMR can offer not only anatomic and functional information but also reveal insights on the myocardial tissue characteristics