3.1 Ex utero Intrapartum Treatment

The EXIT concept was proposed in 1982 and successfully implemented in fetal surgery in 1997 [7, 8, 38]. An EXIT procedure is a specialized delivery strategy used to manage fetal airway compromise or other life-threatening congenital anomalies. Unlike standard cesarean delivery, an EXIT procedure involves partial delivery of the fetus, with preservation of the placental circulation and stabilization of the airway (e.g., intubation, tracheostomy, or tumor resection). This technique helps ensure the safety of the neonate before full delivery [39]. These procedures have proven particularly beneficial in cases with fetal airway obstruction such as giant cervical teratomas, enabling safe control and management of the airway and improved outcomes [40, 41]. Nowadays, EXIT procedures are widely used when the prenatal diagnosis indicates potential problems with neonatal airway compromise or cardiopulmonary instability [39]. These procedures can maintain the uteroplacental circulation during cesarean section, avoiding crises and allowing a stable transition from placental to neonatal circulation [42].

3.2 Open Fetal Surgery

OFS is a sophisticated and highly advanced procedure that involves a direct surgical intervention on the fetus while still in the womb. Over the years, OFS has undergone significant advances, establishing itself as a viable approach for managing severe fetal diseases and congenital anomalies. The procedure requires a hysterotomy—an incision through the maternal abdominal wall and uterus—to gain direct access to the fetus [43, 44]. This approach provides optimal visualization and ample space for precise surgical repair. Initially, OFS was primarily targeted at conditions such as spina bifida, producing significantly superior outcomes compared with postnatal surgery [45, 46]. However, it has since been extended to other conditions including pulmonary hypoplasia, congenital heart defects, and tumors [47, 48]. OFS is typically performed during the later stages of pregnancy to optimize fetal growth and development. This type of intervention was shown to improve survival rates and long-term quality of life for affected fetuses. Historically, OFS was reserved for life-threatening conditions or severe fetal anomalies. However, emerging evidence from preclinical studies has suggested that it may confer neurodevelopmental benefits, potentially enhancing fetal brain development. Consequently, the medical community is increasingly exploring the application of this procedure to repair spinal meningeal malformations and other congenital defects with the potential to significantly improve the fetal prognosis [49, 50].

3.3 Fetoscopic Surgery

Fetoscopic surgery is a transformative, minimally invasive alternative to conventional open maternal–fetal surgery. By eliminating the need for large incisions, this technique markedly reduces maternal complications and minimizes the risk of preterm birth, making it a highly favorable option for both maternal and fetal outcomes. Fetoscopic techniques have become standard practice in the treatment of conditions such as twin–twin transfusion syndrome (TTTS) and congenital diaphragmatic hernia [51, 52]. For TTTS, fetoscopic laser surgery using the Solomon technique is particularly effective. This method corrects vascular connections between the twins, balancing blood flow and improving perinatal outcomes [53-55]. Fetoscopy is also utilized to treat amniotic band syndrome by releasing constrictive bands and to remove herniated intestines in cases of diaphragmatic hernia [56, 57]. Compared with OFS, fetoscopic repair of myelomeningocele lesions carries a lower risk of complications such as uterine rupture and preterm birth [58].

The versatility of fetoscopic surgery continues to expand, enabling interventions such as tracheal decompression and tracheotomy in cases of vocal cord fusion and laryngeal atresia [59]. More recently, fetoscopic laser photocoagulation is utilized in the management of vasa previa, effectively mitigating the risk of rupture and facilitating successful vaginal delivery [36]. Overall, the minimally invasive nature of fetoscopic surgery offers significant benefits over traditional open procedures for both the mother and fetus.

3.4 Percutaneous Procedures

Advances in imaging and surgical technology significantly propelled the growth of percutaneous fetal interventions, addressing a wide range of congenital anomalies. These minimally invasive procedures are now considered standard practice at specialized centers [48, 60]. Ultrasound-guided techniques, such as the placement of vesico-amniotic shunts for lower urinary tract obstruction, were performed since the 1980s [48, 61]. Similarly, thoracic shunts play a critical role in the management of conditions such as congenital diaphragmatic hernia and fetal hydrothorax by facilitating the drainage of excess fluid, thereby alleviating pressure on the developing lungs [48, 62].

Radiofrequency ablation emerged as an effective technique for the treatment of vascular lesions and tumors, thus reducing the risk of fetal hydrops and other complications [63, 64]. Intracardiac catheter procedures are also gaining traction for correcting or alleviating congenital heart defects, including the use of fetal aortic valvuloplasty, to prevent the progression of aortic valve stenosis to hypoplastic left heart syndrome [65].

Innovations such as transuterine ultrasound-guided fetal embolization of vein of Galen malformations highlight the remarkable progress in the field of fetal interventions. This procedure enables elimination of the postnatal pathophysiology, offering significant benefits [66]. The minimally invasive nature of these percutaneous procedures allows targeted treatment while minimizing the risks to both the mother and fetus.

EXIT procedures involve partial delivery of the fetus while maintaining placental support for critical interventions, such as airway management, whereas OFS requires direct access to the fetus via a larger abdominal incision to correct major congenital defects. In contrast, fetoscopic surgery is minimally invasive, utilizing small instruments for targeted procedures such as laser treatment, and percutaneous procedures employ needle access for interventions such as cyst drainage.

4.1 EXIT

Fetal MRI has been a critical diagnostic tool for the evaluation of complex fetal conditions, particularly those involving airway obstruction, since 1983 [69, 70]. This advanced imaging technique provides essential diagnostic and anatomical information about large fetal neck masses and the surrounding airways which is crucial for planning EXIT procedures. By utilizing multiple MRI pulse sequences, the full extent of a mass can be delineated, offering significant advantages over ultrasound. These sequences enable detailed characterization of the nature of the mass and assessment of airway compression. Specifically, a single-shot fast spin-echo (SSFSE) sequence minimizes motion artifacts from fetal movement, enabling clear visualization of high T2 signals in liquid-filled fetal airways, including the oropharynx and larynx. If the airway is encased or displaced by a neck mass, fetal MRI can precisely localize the tracheal position and contour, aiding airway management planning and identifying the optimal site for tracheostomy during the EXIT procedure (Figure 3) [69, 71, 72].

Advanced MRI techniques, such as super-resolution tracheal reconstruction (SSR), MRI-based 3D modeling, 3D navigation, and virtual bronchoscopy, were applied to evaluate fetal neck masses and adjacent airways [72-74]. SSR, combined with conventional 2D MRI, offers higher image clarity and further delineation of anatomical details. The 3D technologies enhance prenatal airway assessment, patient counseling, and perinatal management. Moreover, because of the strong associations between fetal head and neck lesions and central nervous system abnormalities, fetal MRI facilitates detailed brain structure assessments that detect coexisting pathological changes [75-77].

Fetal MRI has proven effective in the visualization of chest tumors and their surrounding structures, offering a more accurate basis for prenatal counseling and surgical planning [78, 79]. In cases with congenital high airway obstruction syndrome (CHAOS), fetal MRI can confirm ultrasound findings and localize the obstruction site, guiding decision-making for fetal or neonatal interventions via an EXIT procedure [80, 81]. The MRI findings in CHAOS include dilated airways, increased lung signals, flattened hemidiaphragms, ascites, heart compression, and placentomegaly. Prenatal MRI is useful for determining the likelihood of a successful EXIT procedure, because low tracheal obstruction is associated with failed airway access. In CHAOS cases, 2D MRI and sequence-registered reconstruction techniques can clarify the obstruction levels, such as laryngeal communications, that are missed by ultrasound. This information is critical for surgical planning and optimization of an EXIT procedure [74, 82]. 3D virtual bronchoscopy further aids in CHAOS diagnosis and management [83].

When combined with fast T1-and T2-weighted imaging, MRI can qualitatively and quantitatively analyze the thoracic contents as well as the lung and liver volumes in CDH (congenital diaphragmatic hernia) patients, thereby predicting the clinical outcomes [84-86]. Metrics like the fetal pleural effusion-to-lung tissue volume ratio and mediastinal displacement can guide EXIT procedures [87, 88]. Recent advances in 3D printing and pre-procedure simulations using 3D modeling have enhanced team collaboration and improved EXIT procedure management [89].

Overall, MRI provides invaluable information for EXIT procedures, enabling accurate fetal anatomy assessment and tailored surgical planning for complex airway or congenital abnormalities. Its detailed imaging capabilities are essential for visualizing fetal structures and identifying optimal interventions, ensuring safe delivery and effective postnatal management.

4.2 Open Fetal Surgery

In cases with open neural tube defects, fetal MRI can not only depict the extent of the spinal dysraphism and the protruding spinal cord/meninges to guide the open surgical repair but also characterize the composition of the herniated tissues. Importantly, sagittal T2-weighted imaging can demonstrate the presence and degree of cerebellar tonsillar herniation as well as any associated hydrocephalus. Conversely, closed dysraphism exhibits a more continuous sac wall transition, fewer posterior fossa anomalies, and a higher association with the omphalocele-exstrophy complex than open defects [90]. This information is valuable for prenatal neurosurgical consultations, assessing the potential benefits of interventions, and guiding the decision-making regarding surgery [91-93]. MRI enhances the diagnostic accuracy of fetal neuroimaging and improves the management in a high proportion of neural tube defect cases [94, 95].

Fetal MRI can delineate the location of sacrococcygeal teratoma and characterize the tumor type by revealing whether the mass is cystic or solid. It can also evaluate any associated findings such as the mass effect on the displaced vagina and bladder, bilateral hydroureteronephrosis, vaginal distension, ascites, pulmonary compression, and oligohydramnios [17]. The high T1 signals in the meconium can help to differentiate a type IV cystic teratoma from a cloacal malformation. Multiplanar imaging can clarify the relationship between the mass and the spinal canal, aiding in distinguishing sacrococcygeal teratoma from a myelomeningocele [41, 96].

For fetal open brain procedures, such as ventriculo-amniotic shunt placement, fetal MRI can assess the degree of ventriculomegaly, the integrity of the septum pellucidum and ependymal lining, and the development of the cerebral cortex. Postoperative fetal MRI can not only evaluate the shunt position but also monitor the recovery of the underlying brain pathology such as the improvement in ventriculomegaly and progressive cortical thickening [18].

Because fetal MRI provides a detailed visualization of the fetal anatomy, it plays a crucial role in OFS planning and guidance. The integration of high-resolution MRI data enhances the precision and safety of these surgical procedures, enabling clinicians to make more informed decisions and optimize the surgical approaches.

4.3 Fetoscopic Surgery

Fetal MRI has multiple critical roles for TTTS. It can precisely depict the growth disparities between the fetuses (Figure 4), identify any pathological placental changes, such as increased thickness and heterogeneous signal characteristics, and enhance preoperative planning [97, 98]. Advanced deep learning techniques can simulate fetoscopic views and laser ablation, optimizing surgical strategies and training [99]. The deep Q-CapsNet reinforcement learning framework further refines preoperative planning by generating accurate placental and cavity models [100]. Postoperatively, fetal MRI can assess the placental infarction extent and detect central nervous system complications via diffusion-weighted imaging [101, 102].

For CHAOS cases, in situ fetoscopic laser ablation of the obstructing lesion emerged as an alternative to an EXIT procedure, altering the natural history and improving the long-term survival. Fetal MRI can not only highlight the advantages of the EXIT procedure but also delineate the length of the obstructing segment, providing critical guidance to determine the fetoscopic access and ablation distance [103].

Fetal MRI significantly enhances the evaluation of urogenital abnormalities by providing detailed structural and functional insights beyond those achievable with ultrasound alone. Studies indicated that MRI improves the diagnostic accuracy, enabling better presurgical planning and management of conditions such as lower urinary tract obstruction [104]. Utilizing SSFSE techniques, fetal MRI can comprehensively assess the urinary tract, especially in cases with late-gestation anomalies identified on ultrasound [105]. It effectively visualizes findings related to the posterior urethral valves including bilateral hydronephrosis, hydroureteronephrosis, increased bladder volume, and proximal urethra dilatation. 3D virtual cystoscopy offers non-invasive, accurate imaging to guide fetoscopic interventions [106]. Fetal MRI can also evaluate the relationships between the three bladder angles, namely the bladder neck angle, vesico-urethral angle, and detrusor-bladder neck angle, allowing assessment of the anatomical constraints and physiological curvature of the enlarged fetal bladder, indicating the feasibility of fetoscopic procedures [107].

Since 1988, MRI was utilized to detect vasa previa with sagittal T1-weighted imaging offering clearer visualization of the placental location relative to the cervical os compared with prenatal ultrasound [108]. The half-Fourier single-shot turbo spin-echo T2-weighted sagittal sequence provides further enhancement by revealing the relationship of the flow voids representing the previa vessels to the fetus, placenta, and cervical os. This detailed imaging guides the fetoscopic laser ablation approach [109, 110]. The timing of surgery for amniotic band syndrome is crucial and significantly influenced by MRI findings. Amniotic bands are characterized by their linear structures with low T2 signals, distinguishing them from movement artifacts in the amniotic fluid (Figure 5). MRI provides a clear visualization of the limb involvement in amniotic band syndrome and allows assessment of the potential effects on other organs including the head, neck, and abdomen [111, 112]. This comprehensive evaluation aids in determining the feasibility of surgery and informs the optimal timing for intervention.

4.4 Percutaneous Procedures

MRI assumes a preeminent role in contemporary prenatal surgical planning, particularly in the management of thoracic shunts, urinary decompression procedures, radiofrequency ablations, and intracardiac catheter interventions. By providing high-resolution multiplanar imaging capabilities, MRI significantly enhances diagnostic accuracy and operative precision, thereby contributing to improved maternal-fetal outcomes.

Fetal MRI serves as an indispensable diagnostic modality for the evaluation of congenital pulmonary airway malformations (CPAMs) [113, 114]. It enables precise characterization of the lesion topography, cystic architecture, and histological subtype according to the modified Stocker classification (types 0–4). MRI also allows a detailed assessment of the parenchymal involvement, mediastinal displacement, and hydrops fetalis while differentiating CPAMs from pulmonary sequestration and congenital diaphragmatic hernias. Although not directly utilized for in utero therapeutic interventions, MRI-derived anatomical insights are critical for guiding clinical decision-making including prenatal corticosteroid administration for microcystic CPAMs (Type 3) and cyst aspiration for macrocystic variants (Type 1) (Figure 6). Serial post-treatment imaging facilitates monitoring of lesion progression, informing decisions regarding an EXIT procedure, OFS, or postnatal resection [115].

In the evaluation of vein of Galen malformations, T2-weighted MRI sequences offer distinct advantages by visualizing flow voids that represent anomalous vasculatures. This enables accurate localization of arteriovenous shunts, quantification of hemodynamic parameters, and assessment of cerebral parenchymal involvement. These imaging characteristics are instrumental for preoperative embolization planning and post-interventional follow-up, ensuring timely management of these life-threatening cerebrovascular anomalies [66, 116, 117].

Regarding fetal cardiac interventions, MRI provides unparalleled anatomical and functional assessments that are essential for percutaneous procedures such as balloon valvuloplasty and atrial septostomy [74, 118]. Since its introduction in the 1990s, cardiac MRI has complemented echocardiography for the diagnosis of aortic stenosis by visualizing post-stenotic dilation and quantifying the flow dynamics in the aortic isthmus and descending aorta [119]. This capability aids in identifying fetuses with critical left ventricular outflow tract obstruction, characterized by retrograde aortic flow and reduced cardiac output [120]. Postoperatively, MRI serves as the gold standard for evaluating structural remodeling and functional recovery [121].

In the field of radiofrequency ablation, MRI is increasingly recognized for its role in the management of fetal tumors and other lesions. It facilitates precise localization and characterization of tumors which are essential for planning ablation procedures. By providing detailed information on the relationships between the tumor and its surrounding structures, MRI facilitates safer and more effective interventions [121]. Fetal MRI plays a crucial role in enhancing preoperative planning across these surgical approaches. For EXIT procedures, MRI provides vital anatomical details about the fetal airway, whereas in OFS, it aids in assessing complex structural defects. In fetoscopic surgery, MRI helps to identify placental and vascular anomalies, and for percutaneous procedures, it maps anatomical structures to ensure accurate needle placement and intervention, ultimately improving surgical outcomes.