Specific Absorption Rate and Specific Energy Dose: Comparison of 1.5-T Versus 3.0-T Fetal MRI
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ORIGINAL RESEARCH • OBSTETRIC IMAGING Specific Absorption Rate and Specific Energy Dose: Comparison of 1.5-T versus 3.0-T Fetal MRI Christian A. Barrera, MD • Michael L. Francavilla, MD • Suraj D. Serai, PhD • J. Christopher Edgar, PhD • Camilo Jaimes, MD • Michael S. Gee, MD, PhD • Timothy P. L. Roberts, PhD • Hansel J. Otero, MD • N. Scott Adzick, MD • Teresa Victoria, MD, PhD From the Departments of Radiology (C.A.B., M.L.F., S.D.S., J.C.E., T.P.L.R., H.J.O., T.V.) and Surgery (N.S.A.), The Children’s Hospital of Philadelphia, 3401 Civic Center Blvd, Philadelphia, PA 19104; Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pa (J.C.E., T.P.L.R.); Department of Radiology, Boston Children’s Hospital, Boston, Mass (C.J.); and Department of Radiology, Massachusetts General Hospital, Boston, Mass (M.S.G.). Received July 10, 2019; revision requested August 21; revision received February 4, 2020; accepted February 11. Address correspondence to C.A.B. (e-mail: [email protected]). Conflicts of interest are listed at the end of this article. :Radiology 2020; 00:1–11 • https://doi.org/10.1148/radiol.2020191550 • Content codes Background: MRI performed at 3.0 T offers greater signal-to-noise ratio and better spatial resolution than does MRI performed at 1.5 T; however, for fetal MRI, there are concerns about the potential for greater radiofrequency energy administered to the fetus at 3.0-T MRI. Purpose: To compare the specific absorption rate (SAR) and specific energy dose (SED) of fetal MRI at 1.5 and 3.0 T. Materials and Methods: In this retrospective study, all fetal MRI examinations performed with 1.5- and 3.0-T scanners at one institu- tion between July 2012 and October 2016 were evaluated. Two-dimensional (2D) and three-dimensional (3D) steady-state free precession (SSFP), single-shot fast spin-echo, 2D and 3D T1-weighted spoiled gradient-echo (SPGR), and echo-planar imag- ing sequences were performed. SAR, SED, accumulated SED, and acquisition time were retrieved from the Digital Imaging and Communications in Medicine header. Data are presented as mean 6 standard deviation. Two one-sided tests with equivalence bounds of 0.5 (Cohen d effect size) were performed, with statistical equivalence considered at P , .05. Results: A total of 2952 pregnant women were evaluated. Mean maternal age was 30 years 6 6 (age range, 12–49 years), mean gestational age was 24 weeks 6 6 (range, 17–40 weeks). A total of 3247 fetal MRI scans were included, with 2784 (86%) obtained at 1.5 T and 463 (14%) obtained at 3.0 T. In total, 93 764 sequences were performed, with 81 535 (87%) performed at 1.5 T and 12 229 (13%) performed at 3.0 T. When comparing 1.5- with 3.0-T MRI sequences, mean SAR (1.09 W/kg 6 0.69 vs 1.14 W/kg 6 0.61), mean SED (33 J/kg 6 27 vs 38 J/kg 6 26), and mean accumulated SED (965 J/kg 6 408 vs 996 J/kg 6 366, P , .001) were equivalent. Conclusion: Fetal 1.5- and 3.0-T MRI examinations were found to have equivalent energy metrics in most cases. The 3.0-T sequenc- es, such as two-dimensional T1-weighted spoiled gradient-echo and three-dimensional steady-state free precession, may require modification to keep the energy delivered to the patient as low as possible. © RSNA, 2020 Online supplemental material is available for this article. RI performed at 3.0 T has advantages when com- (SAR). This energy metric is defined as the radiofrequency Mpared with MRI performed at 1.5 T (1). The higher power absorbed over time per unit of mass of an object field strength results in higher signal-to-noise ratio and (measured in Watts per kilogram of body weight) (5–8). It spatial resolution. However, the transition from 1.5 to is based on the approximate modeling of the individual’s 3.0 T in fetal MRI brings safety concerns associated with body–in this case, the gravid patient–and depends mainly the higher magnetic field strength and radiofrequency on the field strength and radiofrequency power. power (2). Each excitation and refocusing radiofrequency The Food and Drug Administration sets strict limits pulse of an MRI sequence deposits energy into the indi- on individual exposure to a certain power deposition, with vidual being scanned, which is converted into heat. The an upward SAR limit of 4 W/kg averaged over 15 min- rate of energy deposition depends on the amplitude of utes for the maternal whole body (9,10). There is no set the radiofrequency pulse. When a transition from 1.5 limit for the fetus (9,11). According to the International to 3.0 T is made and all other factors are kept constant, Electrotechnical Commission (IEC), the upper SAR limit energy deposition may quadruple, leading to undesired under the normal operating mode is 2 W/kg averaged over heating (3). 6 minutes (8). Heating is a concern in fetal MRI; temperature in- The specific energy dose (SED) is the cumulative ab- creases experienced by the gravid patient for relatively pro- sorbed dose (measured in Joules per kilogram of body longed periods may be teratogenic (4). There is no direct weight). It represents the total accumulated energy depos- way to measure fetal tissue heating, so tissue power deposi- ited in the individual being scanned rather than an average tion is quantified by measuring the specific absorption rate over time (12). This value can be calculated per sequence, This copy is for personal use only. To order printed copies, contact [email protected] Specific Absorption Rate 28], and Prisma [n = 28]; Siemens Healthcare). The sequences Abbreviations performed included two-dimensional (2D), three-dimensional CNS = central nervous system, EPI = echo-planar imaging, IEC = Inter- (3D), and cine steady-state free precession (SSFP); single-shot national Electrotechnical Commission, SAR = specific absorption rate, SED = specific energy dose, SSFP = steady-state free precession, SSTSE turbo spin-echo (SSTSE); 2D and 3D T1-weighted spoiled = single-shot turbo spin echo, SPGR = spoiled gradient echo, 3D = gradient-echo (SPGR); and echo-planar imaging (EPI). For the three-dimensional, 2D = two-dimensional, TOST = two one-sided test 1.5-T MRI examinations, the imaging parameters for the key Summary sequences were as follows: SSFP (repetition time msec/echo time msec, 4.05/1.65; flip angle, 70°), SSTSE (1100/76; flip Fetal MRI can be performed and optimized to deliver an equivalent amount of energy at 3.0 and 1.5 T. angle, 180°), T1-weighted SPGR (202/4.76; flip angle, 60°), and EPI (5200/75; flip angle, 90°). Key Results For the 3.0-T MRI examinations, the imaging parameters for n A total of 3247 fetal MRI examinations (mean gestational age, 24 the key sequences were as follows: SSFP (4.66/1.93; flip angle, weeks) were compared for radiofrequency energy administration at 1.5 and 3.0 T. 90°), SSTSE (1100/76; flip angle, 180°), T1-weighted SPGR (180/4.76; flip angle, 60°), and EPI (4800/46; flip angle, 90°). n When comparing optimized fetal MRI at 1.5 T with that at 3.0 T, the mean specific absorption rate (1.09 vs 1.14 W/kg), mean specific en- The field of view (range, 280–300 mm) and section thickness ergy dose (33 vs 38 J/kg,), and mean accumulated specific energy dose (range, 3–10 mm) remained constant for these sequences for (965 vs 996 J/kg), respectively, were statistically equivalent. both 1.5- and 3.0-T scanners. Scanner parameters for each se- quence according to field strength and protocol are summarized in Table E1 (online). The number of acquisitions was partly de- termined by the radiologist during each MRI examination. but it is more commonly used to estimate the total energy deliv- At our institution, every fetal MRI examination follows a ered throughout the entire MRI examination. The IEC has set protocol according to abnormality and is then categorized as a an SED limit of less than 14 000 J/kg per MRI examination on fetal central nervous system (CNS) (brain, head, neck, or spine) a first-level controlled MRI mode (8). or body (all structures not included in the brain, head, neck, or There may be clinical concerns when using 3.0-T MRI scan- spine) examination. As an example, a fetal CNS protocol is in- ners as compared with 1.5-T MRI scanners because of the po- dicated to evaluate patients with myelomeningocele or ventricu- tential for increased SAR and SED at 3.0 T (3,13). Previous lomegaly, whereas a fetal body protocol is indicated to evaluate studies showed that radiofrequency energy deposition could be patients with congenital diaphragmatic hernia. CNS protocols reduced by modifying MRI parameters in well-known sequences are heavily based on SSFP sequences, whereas body protocols (14,15). The purpose of this study was to compare and evaluate usually employ a combination of the sequences delineated previ- the equivalence of SAR and SED of fetal MRI obtained at 1.5 ously, with an emphasis on SSFP and SSTSE imaging. and 3.0 T. SAR and SED Materials and Methods The SAR and acquisition time values were retrieved from the This was a retrospective study approved by the institutional Digital Imaging and Communications in Medicine header. review board at the Children's Hospital of Philadelphia and The SED, which reflects the sum of energy absorbed by the pa- performed in compliance with the Health Insurance Portabil- tient over the course of MRI, was calculated as SAR multiplied ity and Accountability Act.