MRI imaging is more adept at detecting fetal abnormalities than conventional ultrasound imaging


MRI scanning can more precisely define and detect head, neck, thoracic, abdominal and spinal malformations in unborn babies, finds a large multidisciplinary study led by King’s College London with Evelina London Children’s Hospital, Great Ormond Street Hospital and UCL.

In the study, published today in Lancet Child and Adolescent Health, the team of researchers and clinicians demonstrate the ways that MRI scanning can show malformations in great detail, including their effect on surrounding structures. Importantly, they note that MRI is a very safe procedure for pregnant women and their babies.

They say the work is invaluable both to clinicians caring for babies before they are born and for teams planning care of the baby after delivery.

Recent research has concentrated on correcting for fetal movement in fetal brain MRI and, more recently, for imaging the fetal heart. However, there is an increasing demand to assess the entire fetus with MRI and research from King’s College London School of Biomedical Engineering & Imaging Sciences at Evelina London Children’s Hospital, have recently been able to develop a post-acquisition pipeline to motion correct and volume reconstruct images of the whole fetal body.

Lead researcher, Professor Mary Rutherford, from the School of Biomedical Engineering & Imaging Sciences said ultrasound remains the gold standard for fetal screening and indeed is complementary to these optimised MRI approaches for evaluating abnormalities of the fetal body.

“Until now, ultrasound has been the modality of choice to diagnose those anomalies. However, sometimes the ability of ultrasound to define the most detailed anatomy is limited. MRI scanning offers the potential to more precisely define malformations that could help support clinicians in their planning of care and counselling of parents”.

MRI is commonly used in the classification of fetal brain anomalies. Although its use in fetal body anomalies is less widely adopted, advances have led to the validation of its role in the antenatal investigation of several conditions including the investigation of fetuses with spina bifida: imaging of the fetal brain along with the spinal cord is an important factor in evaluating which patients could benefit from fetal surgery.

For fetal neck masses, MRI provides a clear advantage over conventional ultrasound for assessing tumour extension and giving a 3D visualisation of the tumour’s relation to the airway.

MRI may also be better than ultrasound for distinguishing between normal and abnormal lung tissue, and in making other diagnoses such as diaphragmatic hernia–particularly in late gestation, when doing so with ultrasound is challenging.

New approaches to imaging the fetal body with MRI allows both motion correction of the fetal images and volume reconstructions of body organs and defects. Researchers say this improves visualisation and therefore detection and characterisation of abnormalities.

The project brought together surgeons, fetal medicine specialists, radiologists and physicists to review the use of magnetic resonance imaging to investigate conditions in the unborn baby; this approach has already been integrated into clinical practice at Evelina London, which is part of Guy’s and St Thomas’ NHS Foundation Trust. Videos and images are also available for viewing.

Ongoing work is focused on a fully automated process suitable for clinical translation and wider dissemination into clinical practice.

Fetal ultrasound has been described as the first-line diagnostic examination to identify prenatal congenital abnormalities [1], and the majority of fetal ultrasound examinations are diagnostic. However, the technique has technical limitations even in experienced hands.

Accordingly, in cases where the pathology is complex, an additional approach is needed to confirm or complement the ultrasound findings. Fetal magnetic resonance (MRI) is a complementary diagnostic technique to fetal ultrasound that does not present the same limitations as the latter in cases of advanced gestational age, oligohydramnios, inadequate fetal position, and maternal obesity.

Moreover, fetal MRI visualization of the brain is not affected by interposition of the cranial vault [2].

Although MRI does not use ionizing radiation and is assumed to be safe, not enough research has been carried out to prove long-term safety [3]. Therefore, MRI should be performed when the benefit outweighs the potential side effects [3]. All fetuses should have a screening ultrasonogram for the detection of fetal anomalies and pregnancy complications before performing a fetal MRI [4].

Limitations of fetal MRI include the lack of availability of equipment and radiology expertise, higher cost, and longer time to perform an examination [4] and lower spatial resolution than ultrasound at early gestational age because of the small size of the fetus [5].

Additionally, fetal movement may cause the appearance of artifacts [5]. To avoid this, measures are taken such as fasting the mother in the hours before the exam or ensuring that the she is comfortable during the procedure, especially at advanced gestational age. In addition, current MRI software and hardware allow MRI exams to be performed with high-quality images obtained in less than 1 second, allowing fetal imaging without maternal or fetal sedation [4].

As for all patients, there are absolute contraindications to MRI in pregnant women (e.g., a ferromagnetic cerebral aneurysm clip, cardiac pacemaker), and some patients are too claustrophobic to undergo the examination. In addition, the use of intravenous contrast is not accepted because gadolinium contrast freely passes through the placenta.

The transferred gadolinium then passes into the fetal circulation where it is excreted into the urinary tract and subsequently into the amniotic fluid. Some of the gadolinium in the amniotic fluid will be reabsorbed by the maternal circulation, while some will be swallowed and reabsorbed through the fetal GI tract [6]. Furthermore, the half-life of gadolinium in the fetus is unknown.

Some authors have recently reported the potential nephrotoxicity risk of gadolinium (nephrogenic systemic fibrosis) [7].

Since the first reports of fetal MRI in 1983 [8], the modality has been increasingly used as an adjunct to ultrasound [9], mainly on cerebral pathology. There are fewer publications describing fetal MRI use in chest and abdominal pathology [10], or other less common anatomical locations such as musculoskeletal pathology [11].

While the diagnostic accuracy of fetal ultrasound (compared with postnatal diagnosis or autopsy) has been widely presented, few studies have assessed the accuracy of fetal MRI in the prenatal diagnosis of different anomalies, and most of these examined central nervous system (CNS) pathology.

In a meta-analysis of 34 studies [12], including 959 cases of cerebral fetal MRI confirmed by postnatal surgery or autopsy, the diagnostic accuracy of fetal MRI was approximately 91%, 16 percentage higher than that of fetal ultrasound. Likewise, Gonçalves et al. [13] reported that the sensitivity of fetal MRI in non-CNS-related pathology was 80%, as compared with 77.8% for fetal ultrasound.

There is also some controversy about the value of fetal MRI in prenatal diagnosis and its ability to provide additional information over fetal ultrasound and the extent to which the additional information is useful for perinatal management and prognosis, with published studies providing conflicting findings.

In a systematic review of prenatal diagnosis of CNS pathology, it was concluded that fetal MRI provided additional information in approximately 22.1% of cases and resulted in a change in clinical management in some 30% [14]. By contrast, Paladini et al. [15] reported that fetal MRI provided clinically relevant additional information in only 7.9% of cases.

Given the disparity in results from different studies, the objective of the present work was to assess the prenatal diagnostic accuracy and usefulness of fetal MRI in CNS and non-CNS pathology in patients with previous fetal ultrasound examinations referred to the Radiology Department.

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Source: King’s College London


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