PEDIATRIC IMAGING 523 4D Contrast-enhanced MR Angiog- raphy with the Keyhole Technique in Children: Technique and Clinical Applications 1 Unlike in adults, contrast agent–enhanced magnetic resonance (MR) angiography in the pediatric population raises unique chal- lenges 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 angiog- raphy), 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 con- genital 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, in- cluding modifications of the technique needed for each application. © RSNA, 2016฀•฀radiographics.rsna.org Rajesh Krishnamurthy, MD Sara M. Bahouth, MD Raja Muthupillai, PhD Abbreviations: FOV = field of view, 4D = four- dimensional, SNR = signal-to-noise ratio, TR = repetition time, 3D = three-dimensional RadioGraphics 2016; 36:523–537 Published online 10.1148/rg.2016150106 Content Codes: 1 From the Edward B. Singleton Department of Pediatric Radiology, Texas Children’s Hos- pital, Baylor College of Medicine, 6701 Fan- nin St, Suite 1280, Houston, TX 77030 (R.K., S.M.B.); and the Department of Radiology, St Luke’s Episcopal Hospital, Texas Heart Insti- tute, and Baylor College of Medicine, Houston, Tex (R.M.). Received April 7, 2015; revision re- quested June 25 and received August 6; accepted August 20. For this journal-based SA-CME ac- tivity, the author R.K. has provided disclosures (see end of article); all other authors, the editor, and the reviewers have disclosed no relevant re- lationships. Address correspondence to R.K. (e-mail: rxkrishn@texaschildrenshospital.org). © RSNA, 2016 After completing this journal-based SA-CME activity, participants will be able to: ■ Describe the technique of time-re- solved MR angiography with the keyhole technique. ■ Explain how to use the keyhole tech- nique of time-resolved MR angiography for pediatric applications, including con- genital heart disease, vascular malforma- tions, and portal hypertension. ■ Discuss the modifications of the tech- nique for each pediatric application and the pitfalls associated with the technique. See www.rsna.org/education/search/RG. SA-CME LEARNING OBJECTIVES Challenges of Pediatric Contrast-enhanced MR Angiography Depicting vascular structures in the pediatric population by using contrast agent–enhanced magnetic resonance (MR) angiography raises several specific challenges: (a) Higher and more variable heart rates inherent in the pediatric patient population result in faster and more variable arterial-to-venous transit times. (b) Pediatric patients weigh less; therefore, the volume of the bolus of contrast agent ad- ministered is proportionately smaller, and the first-pass transit of the contrast bolus within the target vascular anatomic structures is even more transient than it is in adults. (c) Many infants and children are sedated and cannot hold their breath during the acquisition; if con- trolled ventilation is not available in institutions, it becomes neces- sary to acquire data during free breathing. This copy is for personal use only. To order printed copies, contact reprints@rsna.org