Normal fetal anatomy as visualized with fast magnetic resonance imaging

Comparison of the colon with T1 breath-hold vs T1 free-breathing-A retrospective fetal MRI study

OBJECTIVES

Fetal magnetic resonance imaging (MRI) plays an increasingly important role in the prenatal diagnosis of gastrointestinal abnormalities. During gestation, the bowel develops T1-weighted hyperintensity due to meconium formation. Currently used T1-weighted sequences are performed in maternal breath-hold (BH) technique, which may take up to 20 s. The free-breathing (FB) T1-weighted 3D radial VIBE (volumetric interpolated breath-hold examination) sequence requires no breath-hold, improving patient comfort. This study aimed to address how well the FB acquisition technique can visualize large bowel structures compared to the routinely performed breath-hold sequence.

METHODS

Forty-seven fetal MRI studies between 21 and 36 weeks of gestation without abdominal pathologies on prenatal MRI and ultrasound were included. All fetal scans were performed using a Philips Ingenia 1.5 T MRI. Coronal T1-weighted BH and FB sequences without fat suppression were compared. The following acquisition parameters were used (T1, FB): resolution 1.137 mm, 1.004 mm; matrix size 288 × 288, 448 × 448; FOV 328 mm, 450 mm; TR 81-132 ms, 3.47 ms; TE 4.6 ms, 1.47 ms. Due to the necessity of the breath-hold the duration of the sequence could not exceed 20 s (mean duration of the T1-weighted BH sequence 15.17 s, and mean duration of the FB sequence 26.42 s). In all examined fetuses the following structures were evaluated with respect to their visibility (0-not visible, 1-partially visible, 2-clearly visible): rectum, sigmoid, descending, transverse and ascending colon, cecum. Furthermore, motion artifacts were assessed (0-none, 1-intermediate, 2-severe motion artifacts), and the signal intensity (SI) ratio between maternal fat and fetal rectum SI was calculated.

RESULTS

No significant differences in the visibility of sigmoid and colon between BH and FB were detected, only the cecum could be seen slightly better (in 29.8 % of cases) using BH technique. Motion artifacts were similar between BH and FB. There was a non-significant SI difference (p = 0.68) in the rectum, with a higher SI in the BH sequence.

CONCLUSIONS

The FB acquisition technique compared to T1 using BH is equal regarding visibility of bowel structures and artifacts. Due to non-inferiority to the BH technique, the FB sequence is a good alternative in cases where BH cannot be performed. As the FB sequence further allows for thinner slices with a good signal, even small bowel loops may be visualized.

Antenatal Transabdominal Ultrasound Detection of Cleft Lip and Palate in Western Australia from 1996 to 2003

Objective

To determine the frequency of detection of cleft lip with or without cleft palate and isolated cleft palate from antenatal ultrasound examinations conducted on mothers of infants born with cleft lip and/or palate and isolated cleft palate in Western Australia from 1996 to 2003.

Design

Review of patient records and purpose-designed questionnaire sent to parents of children born with cleft lip and/or palate and isolated cleft palate.

Results

There were 308 infants born with cleft lip and/or palate and isolated cleft palate in the study period. Of the 293 parents, 218 responses were available for the study (70.7%), and 2.9 ± 1.8 SD antenatal ultrasound scans were performed on 216 women. No such scans were performed on two women. Cleft lip and/or palate was detected in 22.2% of cases. There was no detection prior to 15 weeks gestational age in the 137 women screened. Between 15 and 19 weeks gestational age, 174 scans detected 30 cases. Between 20 and 29 weeks gestational age, 84 scans detected 11 cases. Between 30 and 40 weeks gestational age, 66 scans detected 7 cases. The detection rate for bilateral cleft lip and/or palate was 44.4% and for unilateral cleft lip and/or palate, 40.6%. Detection rate for isolated cleft lip was 33.3%. Antenatal ultrasound failed to detect any infants with an isolated cleft palate (n = 95). The rate of detection of cleft lip and/or palate increased through the study period.

Conclusions

The rate of detection of cleft lip and/or palate in Western Australia is comparable to that for referral centers worldwide and is increasing. The rate of detection of the various types of cleft anomalies using antenatal ultrasound ranged from 0% to 44%.

MR imaging of the fetal brain at 1.5T and 3.0T field strengths: comparing specific absorption rate (SAR) and image quality

OBJECTIVES

Our two objectives were to evaluate the feasibility of fetal brain magnetic resonance imaging (MRI) using a fast spin echo sequence at 3.0T field strength with low radio frequency (rf) energy deposition (as measured by specific absorption rate: SAR) and to compare image quality, tissue contrast and conspicuity between 1.5T and 3.0T MRI.

METHODS

T2 weighted images of the fetal brain at 1.5T were compared to similar data obtained in the same fetus using a modified sequence at 3.0T. Quantitative whole-body SAR and normalized image signal to noise ratio (SNR), a nominal scoring scheme based evaluation of diagnostic image quality, and tissue contrast and conspicuity for specific anatomical structures in the brain were compared between 1.5T and 3.0T.

RESULTS

Twelve pregnant women underwent both 1.5T and 3.0T MRI examinations. The image SNR was significantly higher (P=0.03) and whole-body SAR was significantly lower (P<0.0001) for images obtained at 3.0T compared to 1.5T. All cases at both field strengths were scored as having diagnostic image quality. Images from 3.0T MRI (compared to 1.5T) were equal (57%; 21/37) or superior (35%; 13/37) for tissue contrast and equal (61%; 20/33) or superior (33%, 11/33) for conspicuity.

CONCLUSIONS

It is possible to obtain fetal brain images with higher resolution and better SNR at 3.0T with simultaneous reduction in SAR compared to 1.5T. Images of the fetal brain obtained at 3.0T demonstrated superior tissue contrast and conspicuity compared to 1.5T.

Magnetic resonance imaging of the normal fetal esophagus

OBJECTIVES

To establish the appearance on magnetic resonance imaging (MRI) of the normal fetal esophagus and to provide biometric data from 20 to 39 gestational weeks.

METHODS

In this retrospective study of 250 fetal MRI examinations (20-39 gestational weeks) the visibility of the upper (E1, cervical to tracheal bifurcation) and lower (E2, tracheal bifurcation to diaphragm) esophageal segments was assessed separately in each of three section planes. Segments were scored as being visible or not, and whether they were fluid-filled was noted. Maximum esophageal diameters were recorded. To study the age dependency of esophageal visualization, fetuses were divided into three age groups: 20-25, 26-31 and 32-39 weeks. In 19 cases, there were dynamic studies and these were analyzed for the duration of fluid-filling.

RESULTS

Segment E1 was visualized only when it was fluid-filled and it was visible in 16.8% of axial, 4% of frontal and 22.4% of sagittal acquisitions. Segment E2 could also be detected in the unfilled state, when it appeared solid; it was seen in 96.4% of axial, 74% of frontal and 39.6% of sagittal acquisitions, being fluid-filled in 62.7-88.9% of these. Depending on the amount of fluid-filling and the gestational age, E2 segments had a variable appearance. Dynamic scans showed E1 segments to be fluid-filled for periods of 0.5-1.5 s and E2 segments for periods of 3-35 s. Both the frequency of visualization and the maximum diameter increased linearly with gestational age.

CONCLUSION

Visualization of the fetal esophagus by MRI depends on section plane, segment and gestational age, with considerable differences between the two segments: while E1 requires fluid-filling related to fetal swallowing, the E2 portion can usually be demonstrated independent of fluid-filling, as a consequence of its topographical relationships. The frequently fluid-filled state of E2 may reflect the immature nature of esophageal neuromuscular activity, and the lack of an anti-reflux mechanism due to the short length of the abdominal esophagus.

The value of 3D T1-weighted gradient-echo MR imaging for evaluation of the appendix during pregnancy: preliminary results

BACKGROUND

The use of oral contrast has been essential for the identification of a normal appendix on MR imaging during pregnancy. However, stool could be used as a positive oral contrast as it is characterized by a relatively high signal on T1-weighted imaging, and 3D T1-weighted gradient-echo (T1W-GRE) MR imaging has been used to evaluate 3 mm diameter intestines in fetuses.

PURPOSE

To evaluate the added value of 3D T1W-GRE MR imaging in combination with T2-weighted imaging (T2WI) compared to T2WI alone for evaluating the appendix during pregnancy.

MATERIAL AND METHODS

Eighteen consecutive pregnant patients who were clinically suspected of having acute appendicitis underwent appendix MR imaging which included T2WI with or without spectral presaturation attenuated inversion-recovery (SPAIR) fat suppression, and 3D T1W-GRE with SPAIR fat suppression. Two radiologists reviewed the two image sets (the T2WI set and the combined set of T2WI and 3D T1W-GRE images). Pathologic and clinical results served as the reference standard. The differences in the degree of visibility of the appendix and confidence scale for diagnosing acute appendicitis between two image sets were compared by using the paired Wilcoxon signed rank test.

RESULTS

For both reviewers, the degree of visibility of the appendix using the combined T2WI and 3D T1W-GRE images was significantly higher than using T2WI alone (P < 0.01), and the confidence levels for acute appendicitis using combined T2WI and 3D T1W-GRE images were significantly different from those using T2WI alone (P < 0.01). In the 13 patients with a normal appendix, both reviewers showed improved confidence levels for appendicitis using combined T2WI and 3D T1W-GRE images than T2WI alone.

CONCLUSION

Adding 3D T1W-GRE images to T2WI is helpful for identification of the appendix, as compared to T2WI alone in pregnant women without ingestion of oral contrast material. This may improve diagnostic confidence for acute appendicitis in pregnant patients.

Fetal Magnetic Resonance Imaging as a Complement to Fetal Ultrasonography

Ultrasonography is the screening method of choice for the evaluation of the fetus. It is safe, inexpensive, and easily performed. However, it is operator dependent, and evaluation may be limited because of fetal position, maternal obesity, overlying bone, and/or oligohydramnios. Magnetic resonance imaging is an alternative modality that uses no ionizing radiation, has excellent tissue contrast and a large field of view, is not limited by obesity or overlying bone, and can image the fetus in multiple planes, no matter the fetal lie. Faster scanning techniques allow studies to be performed without sedation in the second and third trimester with minimal motion artifact.

A new look at the fetus: Thick-slab T2-weighted sequences in fetal MRI

Although magnetic resonance imaging (MRI) of the fetus is considered an established adjunct to fetal ultrasound, stacks of images alone cannot provide an overall impression of the fetus. The present study evaluates the use of thick-slab T2-weighted MR images to obtain a three-dimensional impression of the fetus using MRI. A thick-slab T2-weighted sequence was added to the routine protocol in 100 fetal MRIs obtained for various indications (19th to 37th gestational weeks) on a 1.5 T magnet using a five-element phased-array surface coil. Slice thickness adapted to fetal size and uterine geometry varied between 25 and 50mm, as did the field of view (250-350 mm). Acquisition of one image took less than 1s. The pictorial essay shows that these images visualize fetal anatomy in a more comprehensive way than is possible with a series of 3-4mm thick slices. These thick-slab images facilitate the assessment of the whole fetus, fetal proportions, surface structures, and extremities. Fetal pathology may be captured in one image. Thick-slab T2-weighted images provide additional information that cannot be gathered from a series of images and are considered a valuable adjunct to conventional 2D MR images.

Fetal abdominal magnetic resonance imaging

This review deals with the in vivo magnetic resonance imaging (MRI) appearance of the human fetal abdomen. Imaging findings are correlated with current knowledge of human fetal anatomy and physiology, which are crucial to understand and interpret fetal abdominal MRI scans. As fetal MRI covers a period of more than 20 weeks, which is characterized not only by organ growth, but also by changes and maturation of organ function, a different MR appearance of the fetal abdomen results. This not only applies to the fetal intestines, but also to the fetal liver, spleen, and adrenal glands. Choosing the appropriate sequences, various aspects of age-related and organ-specific function can be visualized with fetal MRI, as these are mirrored by changes in signal intensities. Knowledge of normal development is essential to delineate normal from pathological findings in the respective developmental stages.

Fetal MRI of Urine and Meconium by Gestational Age for the Diagnosis of Genitourinary and Gastrointestinal Abnormalities

OBJECTIVE

The purpose of our study was to assess the appearance of the colon and genitourinary tract in fetuses with respect to gestational age with T1- and T2-weighted MRI acquisitions and their applications to abnormalities in these systems.

MATERIALS AND METHODS

Retrospective review of the fetal MRI database was performed to select studies in which both T1- and T2-weighted acquisitions were obtained. The signal characteristics of fluid in the fetal colon and urine in the fetal bladder were evaluated, and gestational age and fetal MRI diagnosis were recorded. A Mantel-Haenszel chi-square analysis was performed to evaluate the relationship of gestational age to MRI signal intensity. In fetuses with suspected colonic and genitourinary abnormalities, an assessment was made about whether the T1-weighted findings added information to the T2-weighted findings.

RESULTS

Eighty fetal MRI studies were reviewed. Forty-three studies showed normal findings, and 37 depicted genitourinary or gastrointestinal abnormalities. The mean gestational age was 27 weeks 6 days. The MRI signal characteristics of urine and meconium became significantly more conspicuous with increasing gestational age (urine bright on T2, p < 0.001; urine dark on T1, p < 0.001; meconium bright on T1, p < 0.001; meconium dark on T2, p < 0.001). Of the 37 cases with suspected problems of the gastrointestinal or genitourinary systems, the T1-weighted images added additional information in 23 cases.

CONCLUSION

The appearance of urine and meconium on T1- and T2-weighted images is significantly more apparent with increasing gestational age. T1-weighted images identified meconium in the colon beyond 24 weeks' gestation and aided in the diagnosis of complex abnormalities.

Assessment of cortical maturation with prenatal MRI. Part I: Normal cortical maturation

Cortical maturation, especially gyral formation, follows a temporospatial schedule and is a good marker of fetal maturation. Although ultrasonography is still the imaging method of choice to evaluate fetal anatomy, MRI has an increasingly important role in the detection of brain abnormalities, especially of cortical development. Knowledge of MRI techniques in utero with the advantages and disadvantages of some sequences is necessary, in order to try to optimize the different magnetic resonance sequences to be able to make an early diagnosis. The different steps of cortical maturation known from histology represent the background necessary for the understanding of maturation in order to be then able to evaluate brain maturation through neuroimaging. Illustrations of the normal cortical maturation are given for each step accessible to MRI for both the cerebral hemispheres and the posterior fossa.

The fetus cannot exercise like an astronaut: gravity loading is necessary for the physiological development during second half of pregnancy

On the basis of published Magnetic Resonance Images and the values of the specific fetal and amniotic fluid weights, apparent weight of the fetus from the 18th week of gestation until term was determined. Up to the 21-22nd gestation week the fetus is in conditions similar to neutral floating, while after the 26th gestation week the apparent weight of the fetus is 60-80% of the actual weight. Decreased effect of the buoyant forces that affect the fetus in human species during the last trimester has a number of implications for the colonization of the solar system. During space flight it is impossible to apply the existing countermeasures against microgravity deconditioning of the muscular and cardiovascular systems to the fetus. Absence of gravitational loading during the last trimester of gestation would cause hypotrophy of the spinal extensors and lower extremities muscles, reduction in the amount of myosin heavy chain type I in the extensor muscles of the trunk and legs, hypoplasy and osteopeny of the vertebras and lower extremities long bones, and hypotrophy of the left ventricle of the heart muscle. Because of decreased capacity of postural and locomotor stability, acquisition of the gross developmental milestones such as sitting, standing and walking could be delayed. In the authors' opinion, only artificial gravity (rotating platform) during space flight will allow physiological development of the human fetus. Independency of offspring's of the guinea pig as regards locomotion and nursing increases probability of successful breeding in microgravity compared with rat offspring's, and make this species a candidate for future experiments under conditions of microgravity and hypergravity. Examining the gestation of this species in different gravities requires first the experimental determination of the amount of buoyant force to which the fetus is exposed in physiological conditions.

Gender- and age-specific impairment of rat performance in the Morris water maze following prenatal exposure to an MRI magnetic field

We examined the effects of prenatal exposure (40 min/day, gestation days 12-18) of rats to a magnetic resonance imaging (MRI) magnetic field (MF) on their performance in the Morris water maze. At 2 months of age, female rats showed impaired performance. The animals spent longer time swimming and used inefficient strategies. However, no significant effects on maze performance were observed at 1 and 5 months of age. No evident maze performance deficit was detected in male rats prenatally exposed to the magnetic field. Thus, we conclude that prenatal exposure to MRI magnetic field induces cognitive/behavioral deficits in female rats at a specific age.