4D Contrast-enhanced MR Angiography with the Keyhole Technique in Children: Technique and Clinical Applications

Unlike in adults, contrast agent-enhanced magnetic resonance (MR) angiography in the pediatric population raises unique challenges such as faster heart rates, more rapid arteriovenous transit, smaller structures, smaller volumes of contrast agent used, and more complex disease processes. A need exists for a rapid contrast-enhanced MR angiographic technique that can separate the arterial and venous phases of contrast enhancement in sedated pediatric patients breathing freely during the course of an examination. In time-resolved contrast-enhanced MR angiography with the keyhole method (four-dimensional [4D] contrast-enhanced MR angiography), various spatial and temporal frequency undersampling schemes are used to substantially reduce the time of acquisition without markedly compromising spatial resolution. The keyhole method can be briefly described as an undersampling approach in which only a small region of the k-space (keyhole) around the center is repeatedly sampled while the periphery is sampled only once during acquisition. This method provides a wide range of options that can be used to overcome conventional limitations of contrast-enhanced MR angiography in children and opens the door for several new pediatric applications, including evaluation of congenital heart disease in neonates and infants, thoracic and extremity vascular pathologic conditions, high-flow vascular malformations, systemic vein thrombosis, and pediatric portal hypertension. This review provides a technical overview of 4D contrast-enhanced MR angiography, outlines its advantages and pitfalls in the pediatric population, and also describes various applications in children, including modifications of the technique needed for each application.