ISUOG Practice Guidelines (updated): performance of fetal magnetic resonance imaging

Ultrasound in Obstetrics & Gynecology(2023)

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Abstract
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) is a scientific organization that encourages sound clinical practice and high-quality teaching and research related to diagnostic imaging in women's healthcare. The ISUOG Clinical Standards Committee (CSC) has a remit to develop Practice Guidelines and Consensus Statements as educational recommendations that provide healthcare practitioners with a consensus-based approach, from experts, for diagnostic imaging. They are intended to reflect what is considered by ISUOG to be the best practice at the time at which they are issued. Although ISUOG has made every effort to ensure that Guidelines are accurate when issued, neither the Society nor any of its employees or members accepts liability for the consequences of any inaccurate or misleading data, opinions or statements issued by the CSC. The ISUOG CSC documents are not intended to establish a legal standard of care, because interpretation of the evidence that underpins the Guidelines may be influenced by individual circumstances, local protocol and available resources. Approved Guidelines can be distributed freely with the permission of ISUOG (info@isuog.org). Fetal magnetic resonance imaging (MRI) is an important diagnostic imaging adjunct to ultrasonography1, especially for evaluation of the fetal brain, lungs and bowel2 and the placenta3. The aim herein is to provide information and guidelines on fetal MRI procedures for those performing the examination, as well as for clinicians interpreting the results of the examination. The purpose of fetal MRI is to complement an expert ultrasound examination, either by confirmation of the ultrasound findings or through the acquisition of additional information. Currently, MRI is not used as a primary screening tool in prenatal care, although a standardized and almost complete assessment of the fetal anatomy is feasible4. However, in selected high-risk cases (e.g. those at risk for brain abnormalities because of affected first-degree relatives), fetal MRI may be considered as a standard method, when previous targeted ultrasound examinations were considered normal4. MRI is not associated with known adverse fetal effects at any point in pregnancy when performed without administration of contrast media. 1.5 Tesla (1.5 T) or 3 T may be used; when using 3 T, machine-specific parameters are available that regulate the level of energy deposition in order to maintain safe levels5. Applied radiofrequency fields may lead to heating of the fetus, which may be harmful. Thus, whole-body radiofrequency-field transmission is limited by the International Electrotechnical Commission to an operating mode not exceeding a specific absorption rate of 2 W/kg6. Postnatal studies have not demonstrated any impact on hearing or growth following prenatal MRI at 3 T7. There is a general consensus that fetal MRI is indicated following an expert ultrasound examination in which the diagnostic information about an abnormality is incomplete or if there is a suspicion of an abnormality that cannot be confirmed by ultrasound alone. Under these circumstances, MRI may provide important information that may confirm or complement the ultrasound findings and modify patient management. Factors influencing the decision to perform fetal MRI include, but are not limited to: experience/equipment of the ultrasound and MRI facilities, accessibility to MRI, maternal conditions such as obesity, abdominal scarring and oligohydramnios, gestational age, safety concerns, legal considerations regarding termination of pregnancy (TOP) and parental wishes after appropriate counseling8. In general, following ISUOG's minimum recommendations for second-trimester ultrasound with basic brain examination9 provides insufficient information to justify requesting MRI. Additional views, such as orthogonal views, higher-frequency probes and/or transvaginal imaging are required to detail specific findings10. In some cases, dedicated ultrasound imaging carried out by a specialist, after routine ultrasound examination, may make the performance of MRI unnecessary11-13. The practice of TOP and associated medicolegal implications may influence the use of fetal MRI at local institutions. In countries in which the decision about TOP has to be made before 24 weeks, the performance of MRI prior to this time may help the parents to decide on the future of their pregnancy12; however, in general, MRI is better reserved for later in the second trimester or in the third trimester14, 15. Although available data are still inconclusive and are heterogeneous due to differences in local expertise and experience with ultrasound and MRI, performing MRI for parental reassurance regarding the absence of associated pathologies in fetuses with apparently isolated conditions may be recommended in fetuses with sonographically isolated findings such as ventriculomegaly (Table 1)16, 17, agenesis of the corpus callosum18, 19, absent septum pellucidum20 and cerebellar or vermian anomalies21. MCD, CC abnormalities Fetal MRI has been found to be complementary and clinically informative in monochorionic twin pregnancy after iatrogenic or natural demise of a cotwin, to assess the surviving twin for pathological changes and other risks for brain injury22, such as severe fetal growth restriction, maternal hypoxia, thrombocytopenia and infection. In this situation, MRI may be indicated both in the absence of ultrasound abnormalities23 and on visualization of ultrasound abnormalities, such as ventriculomegaly (Table 1), that may be associated with conditions which have an impact on further prognosis. In addition to brain and spinal abnormalities, common reasons for referral for fetal MRI include face and neck abnormalities, as well as thoracic and abdominal abnormalities. In one series, fetal MRI was employed in 15% of cases of major fetal structural abnormalities, which represented < 0.3% of pregnancies24. MRI may also help with further characterization of placental adhesion disorders, which usually cannot be detected or excluded without prior suspicion on ultrasound examination3. Fetal MRI performed before 18 weeks does not usually provide information additional to that obtained on (transvaginal) ultrasound examination. In some cases, additional information can be obtained before 22 weeks25, but MRI becomes helpful increasingly thereafter. Specific examples of pathologies that can be evaluated in the third trimester include, but are not limited to, those of cortical development and neck masses that may cause airway compromise26, 27. Generally, organs can be visualized in detail between 26 and 32 weeks of pregnancy, when pathologies related to abnormal development are more fully evolved, but each pregnancy and each fetus will differ. It may become more difficult for the woman to remain comfortable in the scanner with advancing gestation and consideration of left-lateral offset is recommended28, 29. In the setting of single- or multicenter investigational studies including normal pregnancies without clinical indication, following ethical standards as defined elsewhere (declaration of Helsinki), fetal MRI should always be performed and assessed clinically by at least one individual with appropriate (see below) medical expertise in performing and interpreting the examination. When indicated, performed properly and interpreted correctly, MRI not only contributes to diagnosis but may be an important component of treatment choice, delivery planning and counseling. Technical setup of the scanner, onsite patient communication, including prescan safety checks and provision of information, as well as choice of appropriate protocols and techniques require extensive training, which lies beyond the scope of standard educational residency programs in radiology/pediatric radiology/neuroradiology and obstetrics/maternal–fetal medicine and can be offered only by centers with extensive practical clinical experience in fetal MRI. Thus, the performance of fetal MRI should be limited to individuals with specific training and expertise. Obstetrician, radiologist Performs sonographic/neurosonographic examination; provides information to parent(s) regarding findings and possible diagnosis; provides counseling; indicates need for fetal MRI Radiologist, obstetrician Available during MRI examination for acquisition of appropriate planes and advises on changes of protocol as needed; interpretation and reporting of findings; provides counseling Provides counseling and recommendations based on neurosonography, MRI, genetic findings, laboratory findings and/or family history At present, 1.5 T (Figure 2a–c) is the most commonly used field strength, providing acceptable resolution even as early as 18 gestational weeks and not being associated with maternal discomfort related to overheating or to lengthy examination times resulting from long duration of sequences, field inhomogeneities or artifacts, as might be associated with lower or higher field strengths32. Yet, 3 T has the potential to achieve higher-resolution images with a better signal-to-noise ratio than does 1.5 T at a comparable rate of energy deposition on tissue33. In addition, in some centers, only 3-T machines are available. However, with conditions such as polyhydramnios, the use of 1.5 T is preferable to 3 T, as the former is less sensitive to fluid-wave-related artifacts34. Currently, in Europe, about 30% of MRI examinations are performed at 3T (Figure 2d–f)35. Although, usually, measurements will already have been made with ultrasound, measuring certain structures at the MRI examination may be of benefit in particular cases12. Normal values for several cerebral structures have been defined by MRI44. Super-resolution images45 and machine-learning-based automatic measurement methods have been developed46. When measuring fluid-containing structures, it is important to remember that MRI measurements are usually around 10% greater than the corresponding ultrasound measurements47. In lung volumetry, normal gestational-age-related MRI measurements correlate with fetal body volume and are considered predictive of outcome in the case of lung pathology48. As MRI is usually not a first-line examination, but a complementary one following an ultrasound examination performed in the second trimester, the emphasis of the examination and report should be on structures that are more difficult to assess with ultrasound. A detailed anatomical assessment may be performed on request. These Guidelines should be cited as: ‘Prayer D, Malinger G, De Catte L, De Keersmaecker B, Goncalves LF, Kasprian G, Laifer-Narin S, Lee W, Millischer A-E, Platt L, Prayer F, Pugash D, Salomon LJ, Sanz Cortes M, Stuhr F, Timor-Tritsch IE, Tutschek B, Twickler D, Raine-Fenning N, on behalf of the ISUOG Clinical Standards Committee. ISUOG Practice Guidelines (updated): performance of fetal magnetic resonance imaging. Ultrasound Obstet Gynecol 2023; 61: 278–287. At least one meta-analysis, systematic review or randomized controlled trial rated as 1 ++ and applicable directly to the target population; or a systematic review of randomized controlled trials or a body of evidence consisting principally of studies rated as 1 + applicable directly to the target population and demonstrating overall consistency of results Body of evidence including studies rated as 2 ++ applicable directly to the target population and demonstrating overall consistency of results; or extrapolated evidence from studies rated as 1 ++ or 1 + Data sharing not applicable to this article as no datasets were generated or analysed during the current study.
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magnetic resonance imaging
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