LITERATURE REVIEW OF OBSTETRICAL MAGNETIC RESONANCE.

Mena Olmedo Glenn , PhD; Alarcón Carlos , MD; Carrión Cesilia, MD

Departamento de Radiología del Hospital Eugenio Espejo, Quito , Ecuador.

ABSTRACT.

No doubt that ultrasound today is the main diagnostic method in obstetric pathology, it is the cheaper and grants greater access than magnetic resonance, the level of evidence for major diseases is very acceptable, the sensitivity of the method is high, although its specificity and PP, PN values are similar or lower to the MRI.

The Ultrasound evaluates the fetal heart in a better way than MRI, it`s safer and faster, despite the use of special fast sequences this has not exceeded the anatomical detail that can be obtained with High Resolution.

Tridimensional reconstruction, tri planes in real time and path physiological detail of the cardiac hemodynamic that are obtained with the Doppler and other elements of dynamic analysis. But the MRI is the method par excellence in anatomical pathology of the central nervous system. As the study of birth defects of limbs and body regions, in which the fetal position and

oligohydramnios are often present, these are not adequately assessed by the Ultrasound. MRI is also useful in the study of twin pregnancies, multiple pregnancies, placental acretism and concomitant maternal pathology.

Perhaps the cost and availability remains as an obstacle in many Latin American Countries, but that should not be an obstacle in special cases, there is always a solution.

We reviewed some publications on the MRI use of In obstetrics, the main objective is to present an organized guideline of benefits, indications and contraindications, with the purpose of serving as a guide for radiologists and radiology students interested in this medical area.

INTRODUCTION

MRI has been used for about 20 years to assess obstetric and fetal disease, numerous clinical and laboratory studies that show no evidence of harmful effects on the fetus, however, remains cautious regarding the effects of heat caused by RF pulses and the effects of acoustic noise( 22,24,25) .The RM also called "soft X-ray, " is based on the interaction of matter with magnetic fields and radio waves, resulting in a relaxation signal emitted by the tissues, from which volumetric images are generated, the basic element of the RM is hydrogen, being the most abundant in the body to constitute 60-90% of the tissue structure.

The use of MRI for early detection of malformations was first described by Smith in 1983.

In regard to cervical cancer in the pregnant patient, MRI is used for staging, although not to integrate the protocols suggested by the International Federation of Gynecology and Obstetrics (FIGO).

In 1991, the safety committee of the Society of Magnetic Resonance Imaging declares that MRI can be used in pregnant women if other non-ionizing forms of diagnostic imaging are inadequate.

With the use of ionizing radiation, the threshold dose produces a 3% risk of developing certain cancers, has reported a 6% chance of mental retardation and a 15% chance of microcephaly, however, the risk in relation to the time of exposure, for example, spontaneous abortion may occur in early gestation. The risk of fetal damage depends on three main factors: the absorbed dose, the distribution on time and gestational age. The threshold dose has been estimated at 150 mGy (15 rads).

In the stage of organogenesis in humans (up to 8 weeks) has maintained a conservative threshold of 100-200 mSv (10 to 20 rem). In the early stage (9-25 weeks) show that the most important effect is mental retardation, with a threshold dose between 120-200 mSv (10-20 rem).

MRI confirms the ultrasonographic diagnosis in doubtful cases, because of its ability to provide anatomical details in the fetus with ultrasound not come to see (Fig. 9-10), for example to identify the soft palate, or absence of fetal cerebellum is difficult to assess with US. (Fig. 1), MRI can easily evaluate these findings. The presence of cleft palate suspects the possibility of another malformation or

association of any chromosomal abnormality. Furthermore, the evaluation of fetuses prior to intrauterine surgery is a technique that is already underway in some hospitals.

One of the areas where fetal MRI has proved more useful is the assessment of the CNS (Fig. 1), which allows direct multiplanar display. The two most common indications for fetal MRI in CNS malformations are ventriculomegalies (Fig. 6) and dysgenesis the corpus callosum. (20,25)

Another use of MRI is the assessment of neural tube defects, as in the case of bifida spine , and to visualize the exact height of the lesion, especially at caudal level, where the ultrasound is more difficult. In Chile since 2000 is given folic acid in wheat flour, reducing the prevalence of births with bifida spine by 51% and 42% anencephaly. (16, 20, 24,25)

Outside the CNS, fetal MRI is useful to define abdominal, pulmonary and pelvic masses (Fig. 2). A typical case is the congenital diaphragmatic hernia (CDH), where improper fusion of structures gives rise to a hole in the diaphragm that allows abdominal contents of the thorax moves the pressure difference, the default is left in a 75-90 %. It is also useful in cases of fetal renal malformations or pulmonary disorders such as fetal pulmonary cystic adenomatoid malformation.

Further, adequately assessed and congenital heart disorders, provided as a complement to fetal echocardiography (4, 20, 24).

The three steps that overlap in the development of the cerebral cortex are prenatal cell proliferation, neuronal migration and cortical organization.

Lissencephaly (fig. 13-14) is a severe malformation of the cerebral cortex resulting from altered neuronal migration during the third and fourth month of gestation, the affected brain has resulting in a lack of development of folds and grooves .

Magnetic resonance imaging may allow assessment of the degree of fetal brain maturation, the absence or abnormal appearance of a groove in the fetus, particularly in the fetal age should raise suspicion about the possibilities of delayed or abnormal cortical development. (24,25)

Obstetric ultrasound was introduced in 1958 by Donald, considered one of the great achievements of modern medicine, is considered to be 60-100% of American mothers are performed ultrasound in the antenatal period. Teratogenic effects of ionizing radiation make CT and conventional radiography are not useful in the prenatal period. (19,20)

Ultrasound remains the initial study for the fetal malformations; however, there are special conditions and limitations that make MRI a useful method in these circumstances providing great detail of the fetal anatomy and pathology. (19, 21)

A third of the main embryological abnormalities affecting the CNS and can be diagnosed prenatally by ultrasound, is the technique of choice for routine examination of the fetal brain, whether performed through the vaginal or transabdominal throughout pregnancy, their ability to obtain real time images at low cost. (21)

Isolated ventriculomegaly has a better prognosis than those that are accompanied by other malformations of the CNS (Fig. 6). The incidence of isolated ventriculomegaly

by ultrasound is 84%, while the RM low figures to 42%.

The placental accreta and percreta associated with a history of placenta praevia and previous cesarean section, so its diagnosis is essential before delivery to reduce the risk of maternal mortality (Fig. 4-5). (17,20)

The ultrasound in search of placenta accreta has a sensitivity of 93% and a specificity of 79%, positive predictive value of 78%, whereas MRI increased the sensitivity and specificity close to 100% to assess the degree of invasion. The presence of the normal hypoechoic miometral area is consistent with a low probability of less than 10% of placenta accreta. The color Doppler can increase the level of reliability in identifying the area miometral with greater specificity in the diagnosis of placenta accreta and a better assessment of the depth of invasion or serosal miometral when identifying prominent vessels extending from the base the placenta to the myometrium or bladder wall (Fig. 3-4). (13, 14,20)

The autopsy is a valuable research tool for recording death Fig. 7-8 however, there is the problem of rejection by parents, for this reason has been gaining strength MRI study, which is useful as a diagnostic technique recognized as an alternative to fetal autopsy, allows noninvasive access to information about the anatomical defects that were previously only researched the autopsy, also allows in situ exploration, allowing the assessment orcomplementation to other techniques or repeating the observation by other examiner, with obvious applications in teaching, scientific research and the judiciary. (1, 23)

The journal "The Lancet" enhances the properties of this test image is but a step

on the way to create a true virtual medical examiner (fig. 7-8). (1) Much of the information can be obtained from normal autopsy can also be achieved through a high field magnetic resonance (1)

DISADVANTAGES, BENEFITS AND RISKS: The main drawbacks. - High cost. (19) - Maternal obesity. (19) - Polyhydramnios (19). - Fetal Movements (19) - Low availability in hospitals (19) - Claustrophobia (19) Main advantages - Lack of ionizing radiation. (19) - multiple plans. (8, 19) - Excellent soft tissue contrast, approximately 500% more than CT (18,19) - Study of the liver parenchyma. (19) - Independence of the fetal position (19) - Reduces the need for postnatal studies, it does not require sedation. (19) - The MRI can demonstrate high sensitivity of acute blood accumulation, hemosiderin, fat tissue. (19) - High sensitivity to blood flow. (11) - hematopoietic disorders, hemochromatosis (19) - Ability to obtain three-dimensional image reconstruction in any plane (24)

Potential risks: - Possibilities of fetal acoustic injury. (23). - Heat of the magnetic field, mainly in the first trimester of pregnancy (19) - allergic reactions to contrast (itching or difficulty breathing). (18) - Anaphylactic reactions are rare (18)

OBJECTIVES: 1.-Identify potential risks of the fetus with the use of MRI.

2. - To determine the value of MRI in the prenatal study of fetal pathologies. TECHNICAL: Materials and methods: The use of ultrafast sequences with less than 15 to 20 seconds long render the sedation of the fetus. (20) However, it recommends a short period of fasting (6 hours) to minimize fetal movements. Coils are used with computers that allow greater definition (also used the body coil, an additional one placed on the pelvis).

You can improve image sharpness saturation by adding two bands (one of the subcutaneous fat of the mother's abdomen and another in the lower back). The optimal parameters of image acquisition must be adapted to the composition of the fetal brain, which changes during the gestation period. You can perform the following sequence:

T1, T2 TSE, GRE, FLASH, HASTE, this abbreviation stands for varieties of image acquisition. In our environment we use sequences that correspond most closely to the anatomical preparations: T1 if necessary with IR, and ultrafast acquisition T2 TSE (13 seconds). As a routine, in addition, the brain slices is suitable to sagittal spine and lumbar axial level to detect defects in neural tube closure. (20)

CONTRAST AGENTS. - To our knowledge there are no well-controlled studies on the use of contrast material by mouth or intravenous iodinated intravenous gadolinium in pregnancy. Oral contrast material is not considered a threat to pregnant patients due to intraluminal administration, intraluminal barium actually can act as internal

shielding. The IV iodinated contrast material. It has been shown to cross the placenta and enters the fetus when administered in usual clinical doses, there is concern about damage related to fetal thyroid uptake of iodine, is believed to be generally inert and safe in pregnancy. Thyroid tests can be done in the first days of life if the mother received iodinated contrast material during pregnancy.

Gadolinium is a paramagnetic substance for intravenous use, the effect is a change in signal strength which improves tissue contrast in T1 sequences improves the ability to detect lesions. It is the most widely used dye in MRI, was used for the first time in 1997 as a method to compare the placenta, high vascularization provides a rapid enhancement after intravenous injection of contrast uptake by the myometrium takes a few seconds The difference between the two tissues is very noticeable to the 45-and 90 seconds of intravenous injection.

As cross the placental barrier, it is believed that gadolinium chelates may remain in the amniotic fluid and dissociate with time, the change occurs gadolinium signal intensity and contrast enhancement in tissues, but given the limited experience agent is not recommended for obstetrical use, especially in the first trimester of pregnancy, can be used in the second and third trimester of pregnancy, to investigate fetal and placental abnormalities that are not well defined with ultrasound. (5, 6.18, 24.25).

RESULTS: RM INDICATIONS: • Values cephalo-pelvic disproportion in the second and third trimester of pregnancy. (20) • Siamese fetuses. (19) • lung and pelvic masses (19) • Diagnosis of placenta previa. (3) • Assessment of pelvic diseases incidental to pregnancy. (20) • Existence of ventriculomegaly diagnosed by ultrasound. (20) • Suspicion of malformations diagnosed by ultrasound. (20) • Suspected infection, trauma (20) • Existence of multiple fetal malformations (20) • Suspicion of malformations in twin pregnancies (20) • Diagnosis of genetic disease known (20) • CNS lung maturity in the fetus. (20) • Supplement to ultrasound in diagnosis of fetal malformations (19)

CONTRAINDICATIONS: • Foreign matter - metal (copper T) (19) • Pacemakers. (19) • Metal staples (19) • Claustrophobia. (19) • Fragments intraorbital or intracranial ferromagnetic (19) • aneurysm clips. (19) • Otic or cochlear implants (19) • heart valves. (19) • stimulators (19) • internal defibrillator implanted (19) • Renal failure (19)

INTRAVENTRICULAR HEMATOMA, DANDY WALKER

A B

Fig 1. RM, intraventricular hematoma and Dandy Walker (absence of the cerebellum in sagittal section) (7)

From: Rev. Chil Ultrasonog. Utilidad de la Resonancia Magnética Cerebral fetal antenatal como método de exploración complementario a la ultrasonografía. 2003

CONGENITAL OBSTRUCTION OF UPPER AIRWAY:

FIG 2 MRI, 35 weeks, coronal, narrowing of the fetal larynx (thin arrow), tracheal dilatation (thick arrow) and enlarged pulmonary artery (*) (19)

From: Boletín de Pediatría 2009 .Resonancia magnética en el diagnóstico prenatal de malformaciones congénitas.

PLACENTA ACCRETA

Fig 3 US, Placenta previa total occlusion, shows disappearance of the hypoechoic area miometral retroplacental posterior segment of the uterus and presence of vascular lakes (13)

From: Rev. Med 2007. Acretismo placentario y su diagnostico antenatal.

PLACENTA ACCRETA

FIG. 4 -5 Fig 4. - US Transvaginal Doppler in a patient with placenta previa, shows vascular bridges between placenta and myometrium and adjacent vascular lakes (cesarean section and histopathology confirmed the diagnosis) (13) Figure 5 the addition of MRI increases the sensitivity and specificity close to 100%, to assess the degree of invasion, but not for diagnosis. (13) from: Rev. Med 2007 de Acretismo placentario y su diagnostico antenatal.

VENTRICULOMEGALY

Figure 6, US 24 weeks, atrophic and echogenic brain parenchyma (arrow) ventriculomegalies (V) enlargement of the subarachnoid space (12)

Taken from: Estudio de patológica fetal intracraneana demostrada primariamente en el embarazo tardío, desordenes de proliferación celular. Nov. 2003.

STILLBORN TWINS.TWIN 1

Fig. 7: RM, A-B coronal view.

Normal-appearing lungs, liver and kidneys normal (1)

From: Rev. Chil. 2006. Estudio por imagen de mortinatos; una alternativa a la autopsia

STILLBORN TWINS. TWIN 2

Fig. 8: RM, cortes coronales de G2.

Normal-appearing lungs, liver and abdominal shields without alterations in the retroperitoneum are visible both kidneys of normal appearance. (1)

From Rev. Chil. 2006. Estudio por imagen de mortinatos; una alternativa a la autopsia.

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