Three-dimensional ultrasound has gained increased application over the past several years, improving structural display1 and affording volumetric quantitative analysis. 2 Clinical benefit in diagnosis, treatment planning, and family counseling has been reported for evaluation of facial abnormalities, neural tube defects, skeletal defects, congenital heart disease and the central nervous system1 as well as fetal biometry.3With improved operator experience, clinical workflow is enhanced.4 Magnetic resonance imaging (MRI) has become an important secondary obstetrical modality for both the fetus and the mother. Additional advantages of 3-dimensional fetal MRI (FMRI), include true volumetric quantitative analyses (e.g. total lung and fetal body volumes) and enhanced fetal biometry analyses (e.g. multiplanar manipulations, Table 3).10 Technique of image acquisition Clinical utilization of FMRI sequences have 2 essential requirements: 1 fast temporal resolution to account for fetal movement; 2 high, tissue signal-tonoise ratio (SNR), given the inability to administer gadolinium-based contrast agents.
Fetal MRI in the Third Dimension Jeffrey C. Hellinger, MD, and Monica Epelman, MD D iagnostic fetal imaging is an essential requirement for pre- and post-natal care and out- come. Imaging may be applied for screening, targeted evaluations and sur- veillance of fetal abnormalities. Ultra- sound (US) historically has been and remains the principle obstetrical imag- ing modality. It is readily performed in outpatient and inpatient settings at mini- mal inconvenience to the mother, yield- ing reliable information for immediate clinical counseling and management. Initial sonographic screening of all body systems occurs between 20 and 24 weeks. Standard protocols are per- formed with 2-dimensional grayscale sional ultrasound has gained increased ited ability to perform comprehensive and Doppler static and cine techniques application over the past several years, functional evaluations, however, rele- to generate structural and functional improving structural display1 and gates MRI to an adjunctive role, after information. Inherent limitations affording volumetric quantitative analy- ultrasound has been performed. MRI is include operator and acoustic window sis.2 Clinical benefit in diagnosis, treat- often requested to provide more defini- dependence. In addition, interpretation ment planning, and family counseling tive anatomical understanding and is based upon a summation of the has been reported for evaluation of diagnosis for abnormalities identified images and cine loops from multiple facial abnormalities, neural tube on the screening ultrasound, as well as real-time projections. Three-dimen- defects, skeletal defects, congenital to assess for concomitant abnormali- heart disease and the central nervous ties.5 The literature has also advocated Dr. Hellinger is Associate Professor of system1 as well as fetal biometry.3 With its use for anatomical screening in the Radiology and Pediatrics and Director improved operator experience, clinical setting of congenital heart disease of the Advanced Imaging and Informat- workflow is enhanced.4 (CHD)6 and predisposed families.7 ics Laboratory, Departments of Radiol- Magnetic resonance imaging (MRI) Standard MRI protocols utilize high ogy and Pediatrics, Stony Brook Long has become an important secondary resolution, ultra-fast static and dynamic Island Children’s Hospital, Stony Brook University School of Medicine, Stony obstetrical modality for both the fetus cine 2-dimensional T1- and T2- Brook, NY; and Dr. Epelman is Assis- and the mother. Compared with ultra- weighted (T1W and T2W) sequences.8,9 tant Professor of Radiology, Director of sound, it offers superior spatial resolu- Using 2-dimensional MRI, multiple Neonatal Imaging, Department of Radi- tion and structural detail, affording acquisitions, in multiple planes are often ology, The Children’s Hospital of comprehensive anatomical displays required to ensure complete anatomical Philadelphia, University of Pennsylva- from single acquisitions. The high coverage. The number of acquisitions nia School of Medicine, Philadelphia, cost, potentially long exam time in a and the exam time are directly depen- PA. typical closed bore unit, and the lim- dent upon fetal movement, with the total 8 ■ APPLIED RADIOLOGY © www.appliedradiology.com July–August 2010 FETAL MRI IN THE THIRD DIMENSION FIGURE 1. Normal 3-dimensional SSFP FMRI multiplanar interrogation. Depending upon the image quality, a single 3-dimensional SSFP sequence can be manipulated using MPR (B,D,F) and thin-slab MIP (A,D,E,G-J) to generate anatomical images of the placenta (A, asterisks); brain (B); ventricular system and spinal canal (C); vertebral bodies (D); airway and lungs (E); heart (F); stomach and small bowl (G-H); kidneys, ureters, and bladder (I); and extremities (J). number of sequences often ranging Table 1. 3D Fetal MRI Sequences between 20 to 25, yielding an average 3D (T1) Volume Interpolated GE 3D (T2 / T1) Balanced SSFP exam time of 30 to 55 min. As with ultra- sound, interpretation is often based upon Meconium Amniotic Fluid mental reconstruction of the anatomy Hemorrhage Placenta and Umbilical Cord Placenta Central Nervous System and pathology from the multiple acquisi- Colon Craniofacial and Neck tions; communication, treatment plan- Liver Thorax (including airway) ning and family counseling frequently Spleen Cardiovascular require displaying multiple sequences. Thryoid Gland Gastrointestinal Three-dimensional MRI sequences Pituitary Gland Genitourinary can also be acquired through the gravid Musculoskeletal pelvis (Table 1, Figures 1 and 2) with GE = Gradient Echo; SSFP = Steady State Free Precession
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