Fetal Magnetic Resonance Imaging

Introduction

Over the last ten years, the interest of radiologists and obstetricians in imaging the unborn child by means of magnetic resonance imaging (MRI) has grown significantly, not least facilitated by accelerated technical developments, especially in terms of fast imaging sequences. Thanks to scientific progress, fetal MRI features nowadays more and more in the multidisciplinary approach to fetal medicine.

In routine clinical practice, ultrasound remains the screening modality of choice in prenatal imaging. It has the inherent advantages of being a bedside and real-time technique which is cheap, available and general accepted. Nevertheless, the role of fetal MRI is gaining importance. Where fetal MRI used to be exclusively indicated for central nervous system (CNS) pathologies, its role is gradually expanding as the knowledge and understanding of how pathology is presented on MR images grows.

The aim of this chapter is to give an overview of the current indications, strengths and limitations of fetal MRI.

MRI Safety in the Pregnant Patient

To date, no hazardous effects of MRI on humans have been reported and the potential prevalence of delayed sequelae seems to be very low or even non-existent. Since the fetus is known to be very sensitive during organogenesis (7 to 57 days after conception when the rudiments of all major structures are developing), injuries caused by physical agents such as static magnetic fields, time-varying magnetic gradient and pulsed radio frequency fields have been extensively studied in animal as well as in human research. None of these studies can clearly point to any adverse effects of MRI during pregnancy. A large survey undertaken by Kanal et al. on 1421 pregnant MRI technologists showed no statistically significant changes in fertility, length of gestation, birth weight or pregnancy outcome.

However since it is difficult to adequately evaluate the fetus before 18 weeks of gestation due to the small size of the fetal organs and because of the theoretical risk of interfering with the organogenesis, fetal MRI is not performed in the first trimester in most centres.

Intravenous administration of contrast agents during MRI is not recommended since gadolinium-based contrast material has been shown to cross the placenta and appears within the fetal bladder and amniotic fluid. Although some recent studies suggest back diffusion of gadolinium across the placenta, little is known about the biological half-life of gadolinium in human amniotic fluid. Since the non-chelated form is potentially toxic and there is a relationship with nephrogenic systemic fibrosis (a rare and serious syndrome that involves fibrosis of skin, joints, eyes, and internal organs), we recommend avoiding its use in pregnant women until the safety of gadolinium-based contrast material is proven in larger studies.

Contraindications to MRI in pregnant patients are the same as for non-pregnant patients. Magnetically activated implanted devices such as cardiac pacemakers and neurostimulators may be de-programmed. Ferromagnetic intracranial devices such as cerebral aneurysm clips are known contraindications. More specifically in pregnant women is a hip prosthesis which may interfere with optimal imaging of the fetus due to its typical artefacts.

Fetal MRI Aspects and Techniques

At most institutions, fetal MRI will be performed on routine clinical 1.5 Tesla units which constitute the bulk of the machines used worldwide.

Although there is no specific instrumentation for fetal MRI, a few specific conditions need to be met in order to perform successful imaging.

The unit should be equipped with an advanced gradient system, allowing for fast data acquisition. This reduces the possibility of motion blurring during the acquisition of the images themselves. In addition newer scanning techniques such as steady state imaging require high gradient power.
Another important qualitative factor is the choice of the MR coil. We recommend the use of a phased-array coil with multiple elements lying as close as possible to the fetus. Using a combined eight-channel phase-array body and spine coil positioned over the lower pelvic area can achieve this.
Patients are positioned mostly in a supine or left-lateral position to ensure comfort during the examination. In particular with polyhydramnios or late in the third trimester, the left lateral position can prevent supine hypotension syndrome during the examination.

Three-dimensional scout images are performed which are then used to acquire the three planes of the fetal head and body. For practical reasons we use the performed sequence as a scout image for the next sequences in order to have perfect transverse, coronal and sagittal planes of the fetus. In our centre the fetus is always scanned in total in order to evaluate for potential associated pathologies.

The most important imaging sequences in fetal imaging are briefly outlined below. Similar acquisitions can be obtained in most clinical MR systems, but the specific names and sequences vary by manufacturers.

T2-Weighted MRI (T2-WI)

T2-WI is the most used sequence in fetal MRI since it provides us with excellent contrast resolution of the fetal tissues and it is therefore considered to be the work horse in fetal MRI. T2-WI can be obtained either by using spin echo (SE) sequences such as single shot turbo spin echo (SSTSE) or gradient echo (GE) sequences such as balanced steady-state free precession (SSFP). The main advantage of GE above SE is the ability to visualize fetal cardiac anatomy and vessels which is not possible with the SSTSE. However dynamic SSFP may give information about the global movements of the fetus and the cardiac contractility.

The revolution in fetal MRI scanning arrived with the development of single shot imaging providing us with sequentially acquired images in less than 1 second, avoiding fetal motion-related artefacts. Consequently, fetal movements will only affect the slice that was acquired during the movement and not the whole sequence as is the case in high-resolution spin echo pelvic imaging. Due to this fast imaging no breath-holding is required of the patient. Sequences that are degraded by fetal motion will be repeated with the same parameters. In order to maintain the desired plane, which fetal movements can impair during each sequence, as mentioned above, SSTSE images would serve as the scout image for the subsequent acquisition.

A typical protocol of a SSTSE sequence consists of a TR/TE = 1000 msec/88 ms, field-of-view 380 × 380 mm, 4 mm slice thickness, no intersection gap, matrix of 173 × 256, partial Fourier factor of 5/8 with a resulting pixel resolution of 1.8 × 1.5 × 6.0 mm ³ and a bandwidth of 475. The slice thickness can be reduced to 3 mm when a fetus is examined early in gestation or when the evaluated structure is small, unfortunately with the consequence of decreasing image quality.

T2-WIs are used to delineate the fetal brain, to accurately evaluate biometry and sulcation, and also to image structures with a high water content. These can be either normal tissue such as lungs, stomach, small bowel and bladder or pathological abnormalities such as cysts.

T1-Weighted MRI (T1-WI)

T1-WI, using a fast low-angle shot (FLASH) technique is much more difficult. Not only is the soft tissue contrast low due to large components of water in the fetus but also these images have a longer acquisition time allowing for degradation of the images by fetal movements. Nevertheless, this sequence is very useful to evaluate the presence of blood products, protein or fat in fetal organs or organic lesions. T1-WI sequences are recommended in the assessment of normal structures with a physiological T1 hypersignal such as: meconium, myelin, thyroid, pituitary gland and T1 hyperintense pathological lesions such as haemorrhage and some types of ischaemia.

Diffusion-Weighted MRI (DWI)

DWI uses the physical process of diffusion based on the random intercellular movement of water molecules (i.e. Brownian movements). Therefore this technique will not only display cerebral white matter and early cortical maturation, but it can also indicate oedema, ischaemia and acute haemorrhage in lesions or tumour masses. DWI is not only used in brain but has been shown to give potential information about abnormal developing kidneys during fetal life. A more specific and direction-encoded form of DWI, so-called diffusion tensor imaging (DTI) is applied to demonstrate major fibre tracts in the fetal brain. Obviously the use of DTI is still the subject of research and its clinical use in the future is yet to be determined.

Spectroscopy

MRI spectroscopy (MRS) is a technique that is currently used in brain imaging. It is able to indirectly measure brain chemistry, by measuring the levels of N-acetylaspartate, (a marker for neuronal damage and chronic brain damage), creatine, choline, lactate and so on, thus allowing evaluation of the biochemical and metabolic status of the brain parenchyma. It has its clinical applications in monitoring biochemical changes in tumours, stroke, epilepsy, metabolic disorders, infections and neurodegenerative diseases.

Many research projects are ongoing to evaluate the use of MRS in the fetal brain and lung, however long acquisition times and thus increased fetal motion-related artefacts are currently preventing the clinical use of this technique.

Main Applications of Fetal MRI

Analysis of Fetal Abnormalities

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