ESPACIO DE NEURORRADIOLOGÍA FETAL MAGNETIC RESONANCE A CONTRIBUTION TO THE DIAGNOSIS OF CENTRAL NERVOUS SYSTEM MALFORMATIONS Nicolás Sgarbi MD, Verónica Etchegoimberry MD RESUMEN ABSTRACT Las malformaciones del sistema nervioso son relativa- Central Nervous System malformations are relatively mente frecuentes e impactan de forma significativa en frequent and have a negative impact in postnatal la morbi-mortalidad postnatal. morbidity and mortality. Su diagnóstico pre-natal se basa en una técnica vali- Prenatal diagnosis is based on the obstetric ultra- dada desde hace varias décadas como es la ecografía sound, which has been validated throughout the years cuyo rendimiento general es excelente. with an excellent performance. En los últimos años se ha producido un cambio sustan- In the last years, there has been a substantial change cial en el paradigma diagnóstico de las malformaciones in the prenatal diagnosis, not only for deciding the no sólo por las implicancias que esto tiene en decidir termination of pregnancy, but also to assess in the la continuidad del embarazo, sino además por el de- opportunity of intrauterine surgery. sarrollo de técnicas de cirugía intrauterina. In this revision we will analyze the most important En esta revisión analizaremos los principales aspectos technical aspects and indications of Fetal MRI, and técnicos de la resonancia magnética fetal y sus aportes its importance in the prenatal diagnosis of Central al diagnóstico de las malformaciones más frecuentes Nervous System Malformations. del sistema nervioso. Key Words: Fetal MRI, Central Nervous System Mal- Palabras clave: resonancia magnética fetal, malforma- formations, neurosonography. ciones del sistema nervioso, neurosonografía. INTRODUCTION That is why Fetal Magnetic Resonance (fMR) has become a complementary technique to US in the study of the The study of fetal anatomy is done as a routine part of central nervous system (CNS). prenatal care. Ultrasound (US) is the diagnostic method of choice, because it gives excellent global results for the The objectives of this review are: to analyze the contri- complete anatomic assessment of the fetus during its bution of fMR to the study of the central nervous system different developmental stages. of the fetus and its more frequent malformations, and to It has been long been clearly established that US is a highly highlight the general principles of this technique, as well sensitive method for the detection of malformations, even as its scope and its limitations. in their early stages of development. During the last few years new technologies have advanced the study of fetal anatomy, MR among them. The use of TECHNICAL ASPECTS MR in this field is constantly increasing. In this respect some authors point out that anomalies that MR has developed considerably in the last few years, were not diagnosed by US can be detected by MR in 20% due to the changed paradigm for the study of congenital of patients (1). malformations. Corresponding author: At the same time, changes in law and health policy have This change came about essentially on account of ad- [email protected] allowed for voluntary interruption of pregnancy and have vances in treatment for some malformations (corrective Received april 19th in this way impacted the diagnosis and management of intrauterine fetal surgery), the need of a diagnosis in view Accepted may 5th malformations. of prenatal genetic advice and planned parenthood, and 2018 REVISION WORK / N. Sgarbi MD, V. Etchegoimberry MD 92 the legal modification related to interruption of pregnancy. and anatomy of the fetus. The subsequent use of MR along with the different va- Then comes the planning of the scan. It is necessary to rieties of US has modified the diagnostic approach to obtain fast T2-weighted cranium images in the three spa- malformations. tial planes (4). Sagittal spine images are obtained, axial or coronal planes may be used too if considered necessary The first concept to be highlighted is that MR must be or complementary. performed only after an ultrasound scan was performed by an expert in the assessment of fetal anatomy (2). Many Single-shot sequences must be used, such as Single Shot factors must be taken into account when ordering and Fast Spin Echo (SSFSE) or the Half Fourier Acquisition performing fMR. Turbo Spin-Echo (HASTE). Slice thickness must be adequate to fetal size, not exce- As a general recommendation, in the first place the scan eding 3 mm; an adequate field of vision (FOV) must be must focus on the anatomical region or organ that pre- chosen in order to obtain specific images of the region sented some alteration on US, so that the technique can of interest. be shown to best advantage. In special cases, study protocol may include other sequen- Then other factors concerning the maternal-fetus unit must ces such as T1-weighted sequences for fat analysis, or come into consideration. the susceptibility sequences such as T2- or SWI-weighted To achieve good imaging one must acquire images while GRE sequences for the assessment of hemorrhage, as well the fetus is immobile. as diffusion sequences (DWI) for the study of ischemia. Longer acquisition time is a limiting factor in such cases. Sedation is not recommended, it is preferable to ask the It is important to take into account some safety parameters. mother to fast during 4 to 6 hours, so as to restrict fetal The FDA has set clear limits to the specific absorption rate movements as much as possible. (SAR) of radio frequency, but its effects on the maternal-fe- The mother must receive precise and detailed information tal unit remain unclear (1). about the duration of the study, the position to maintain during that time and the respiratory movements that will All patients must receive clear information about the fMR be asked of her. modality in use, its benefits, its scope and its limitations. The patient must be placed on her back or even on her side, whichever is more comfortable for her, so as to ensure NORMAL CHARACTERISTICS OF FETAL her collaboration during the study. BRAIN Usually fMR is performed with 1.5 T equipment, but in the last few years some centers have reviewed the contribution In order to interpret and analyze an fMR scan correctly of 3T magnets (1), although it is known that their routine it is essential to know the usual appearance of the brain use is not recommended yet (3). during its development. A review of fetal brain reveals three basic components Although image resolution may improve, movement arti- that permit a fairly accurate diagnosis of the stage of de- facts increase likewise. That is why the use of high magnetic velopment. These components are: brain parenchyma, fields calls for more rigorous technique if optimal results germinal matrix and sulcation pattern (1). are to be achieved. During development white matter presents with high signal Surface coils must be used on the body, with as many in T2-weighted scans, on account of its high water content receptor channels as possible, so that high-resolution and scant myelinization. images are obtained in the least possible acquisition time. Brain parenchyma (or cerebral mantle) appears as several The timing of the pregnancy scan is a point of the utmost layers that can be accurately observed between the 28th importance. and the 30th week of development. The germinal matrix is the cell layer that will produce Most centers recommend performing the fMR scan after neurons; it is to be found on the walls of the ventricular the 19-20th week of pregnancy. Earlier on, structures system. In T2-weighted scans it has a low signal on account are very small and some of them (corpus callosum, for of its high cellular density. instance) are undeveloped and both factors make inter- During the second trimester the cells in this layer will mi- pretation difficult. grate to the surface, where they will form the brain cortex. Later on, in the third trimester, only small foci of germinal It is basic to have a previous recent ultrasound study as matrix persist in the temporal and occipital horns of the a guide, not only for the direct assessment of the malfor- lateral ventricles. The pattern of gyri and sulci undergoes mation under study, but also to have some orientation modifications during the process of development, going regarding fetal position and most of all, fetal neural axis, from a relatively agyral brain up to the 20th week to a for the third-trimester scan. First of all a localizer scan in all more complex pattern in the third trimester. Through three planes is done, so as to get an overview of position knowledge of the temporal sequence of appearance of the 93 Rev. Imagenol. 2da Ep. Jan./Jun. 2018 XXI (2): 92-101 FETAL MAGNETIC RESONANCE A CONTRIBUTION TO THE DIAGNOSIS OF CENTRAL NERVOUS SYSTEM MALFORMATIONS main sulci it is possible to assess fetal brain development TABLE 1 chronologically. Table 1 Time of development of the main brain sulci Following this course, we will find that the interhemisphe- Main brain sulci Gestational age in Fissures weeks ric fissure is already present before the 20th week, the callosomarginal fissure appears around the 22nd week, the Interhemispheric fissure 10 - 12 calcarine fissure between the 20th and the 22nd week, the central sulcus in the 26th week and the postcentral Callosomarginal fissure 20 - 24 sulcus is to be found shortly before the 30th week, around Calcarine fissure 18 -22 the 28th week. Figure 1 Morphology and size of lateral ventricles are important Sylvian fissure 16 - 20 items in the assessment, because as we shall see later, Intraparietal sulcus 24 - 26 these values are among the most commonly obtained. The presence of septum pellucidum must be assessed; it Insula 32 - 34 should be present by the 18th week, beyond that date its Central sulcus (fissure of Rolando) 22 - 26 absence is considered abnormal.