Magnetic Resonance Imaging Versus Ultrasound in Fetal Neuroimaging

Magnetic Resonance Imaging Versus Ultrasound in Fetal Neuroimaging
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Abstract
Antenatal imaging of the spinal cord and the brain has shown to be particularly important in the
identification of malfunctioning parts to direct intrauterine and patient counseling. However,
being aware of the difference between normal and abnormal fetus is critical since the brain of the
fetus changes often throughout the pregnancy period. This review paper discusses the use of
magnetic resonance Imaging and ultrasound to map the brain of the fetus to identify anomalies.
The first part discusses scanning methods for the two modalities while the second part
investigates the safety issues arising from both modalities. The information obtained from MRI
and ultrasound must be properly analyzed to avoid unintentional misinterpretation. This article
reviews magnetic resonance imaging and ultrasound imaging of antenatal fetal brain anatomy in
the second and third trimesters. Knowledge of the power of the cornea of the fetus is necessary to
verify normal growth. The preferred imaging modality for fetal neuroimaging is Ultrasound.
However, in situations where extra information concerning fetal physical structure is necessary,
an alternative modality is required. This is where Fast magnetic resonance imaging (MRI) comes
in. MRI is becoming more popular as a neuroimaging technique during pregnancy due to several
reasons including a wide view field, high contrast and the absence of ionizing radiation. Despite,
the popularity of MRI, ultrasonic imaging and analysis of the fetus is still critical for the
identification of the most suitable fetuses to undergo the magnetic resonance imaging procedure
and for directing the examination process. Quality ultrasound and magnetic resonance images as
well as their correct analysis and interpretation are necessary for providing helpful guidance to
the parent. Usually, magnetic resonance imaging provides extra information in addition to that
provided by ultrasound.

Introduction
The fetal brain differs considerably from the brain of an adult due to its higher water content [7].
Correspondingly, most of the neuroimaging techniques must be designed in such a way as to
obtain high image quality. In this consideration, T1 weighted sequences obtained during the
antenatal period are normally carried out using a shorter echo time and a shorter repetition time,
while T2 weighted sequences are obtained using longer repetition and longer echo times. The
inversion recovery sequences require long inversion times. The T1 weighted sequence and the
T2 weighted sequence are the two commonly used MRI sequences in antenatal fetal
neuroimaging [12]. Although ultrasound is still the main modality applied for the investigating of
pregnancy related malfunctions, fetal MRI is becoming the new favorite. Magnetic resonance
imaging is not significantly restricted by factors such as the position of the fetus and the
ossification of the skull as compared to ultrasonic techniques. Through its superior soft tissue
contrast resolution, MRI is capable of distinguishing separate fetal structures and displaying high
resolution images [2]. Furthermore, MRI provides multi-planar imaging as well a wide field of
view, which simplifies the analysis of complicated neurological abnormalities. A disadvantage of
MRI is that when applied to fetuses in their early stages of development the image information
may be limited [2]. In this review paper, Fetal MRI technique is compared with ultrasound
imaging technique. Recent advances in both fields are also highlighted.

Ultrasound
The application of ultrasound in the field of medicine began in the early twentieth century.
Initially, it was applied in the field of therapy rather than in clinical diagnosis. The history of
ultrasound goes back to the18th century when it was applied to measure distance and to detect
objects under water. From then it was gradually advanced and incorporated in the field of
medicine. Sonar is an abbreviation for ‘sound navigation and ranging’. An ultrasound imaging
machine is a type of medical sonar which utilizes sonar’s thermal and disruptive effects on the
tissues of living things. The first neurologist credited with using ultrasound in the field of
medicine is Karl Theo Dussik, from Austria, Vienna.
All sound waves with a frequency greater than the human hearing threshold of approximately
18000 Hz are defined as being ultrasonic. Most ultrasonic Image scanners operate within a
frequency band of about 2 to 15 MHz. Choosing the frequency of operation is a matter of
compromise between image quality and the depth of penetration of the waves. Short wavelength
ultrasonic waves which correspondingly have high frequency produce high resolution images.
However, short wavelength waves have a higher level of attenuation per unit distance hence they
are only useful for superficial imaging. On the contrary, low frequency waves have a
correspondingly large wavelength hence they can penetrate deeper into the body due to the lower
level of attenuation. However, the images produced are of a lower resolution.
The source of the ultrasonic sound wave is usually a piezoelectric transducer enclosed in a probe.
The majority of piezoelectric transducers are constructed from ceramic materials. Part of the
incident pressure wave is returned back towards the source at a different angle (angle of
reflection) by tissue layers of different thickness. The reflected pressure wave causes the