DOI:10.2214/AJR.07.2063
AJR 2007; 189:W303-W308
© American Roentgen Ray Society
VIBE MRI for Evaluating the Normal and Abnormal Gastrointestinal Tract in Fetuses
Tsutomu Inaoka1,
Hiroyuki Sugimori1,
Yoshihito Sasaki2,
Koji Takahashi1,
Kazuo Sengoku2,
Nobuhisa Takada1 and
Tamio Aburano1
1 Department of Radiology, Asahikawa Medical College, 2-1-1-1
Midorigaoka-Higashi, Asahikawa City, Hokkaido, 078-8510, Japan.
2 Department of Obstetrics and Gynecology, Asahikawa Medical College, Asahikawa
City, Hokkaido, Japan.
Received February 17, 2007;
accepted after revision June 24, 2007.
WEB This is a Web exclusive article.
Address correspondence to T. Inaoka
(tinaoka{at}asahikawa-med.ac.jp).
Abstract
OBJECTIVE. The great potential of MRI for assessing gastrointestinal
abnormalities in fetuses has been described. T1-weighted images may add
additional information to T2-weighted images in diagnosing fetal
gastrointestinal abnormalities. The objective of this study was to assess the
performance of a 3D volumetric interpolated breath-hold sequence (VIBE) in
evaluating the normal and abnormal fetal gastrointestinal tract.
CONCLUSION. VIBE provides high-quality T1-weighted and 3D MR
colonography images for the evaluation of the normal and abnormal
gastrointestinal tract in fetuses, and 3D MR colonography provides excellent
delineation of the meconium.
Keywords: fetus • gastrointestinal tract • meconium • MR colonography • MRI • obstetrics • pediatrics • volume interpolated breath-hold imaging • women's imaging
Introduction
Prenatal diagnosis of fetal gastrointestinal tract malformations,
including bowel dilatation, polyhydramnios, hyperechoic bowel, and ascites, is
commonly made with sonographic findings. However, these findings are not
specific and may relate to transient normal variants
[1-3].
In addition, sonography has some disadvantages, such as operator dependence,
low image contrast, and a small field of view
[1-3].
Therefore, it is occasionally challenging for obstetric sonographers to assess
fetal gastrointestinal abnormalities when accurate recognition of the bowel
condition is required to determine fetal and neonatal management
[1,
2].
The usefulness of fast MRI, including single-shot fast spin-echo sequences,
for evaluating fetal abdominal disorders has been reported with increasing
frequency. The great potential of MRI for the assessment of the fetal
gastrointestinal tract has been described; in diagnosing abnormal cases, it is
important to assess whether meconium is present in the fetal bowel
[2-5].
MRI is more sensitive than sonography for detecting the presence of meconium
[2]. The location and amounts
of meconium in the fetal bowel depend on gestational age; however, the
accumulation of meconium may steadily advance from the anal canal in a normal
fetus after 20 weeks' gestation
[2-6].
Meconium exhibits an intermediate or low signal intensity on T2-weighted
images and a high signal intensity on T1-weighted images because of its high
protein and mineral content
[2-4].
T1-weighted images may add additional information to T2-weighted images in
diagnosing fetal gastrointestinal abnormalities because meconium is more
apparent on T1-weighted than on T2-weighted images
[5].
Three-dimensional MR images generated from T1-weighted images, which were
obtained using 2D or 3D fast gradient-echo MR sequences, are useful for 3D
understanding in the diagnosis and monitoring of fetal gastrointestinal tract
malformations [4,
7]. For conventional
T1-weighted sequences, a 4-mm or greater section thickness is required to
perform imaging through the fetus during a limited acquisition time
[2-7].
However, additional thin sections of 3 mm or less are needed to better
characterize abnormal findings and to make 3D MR images of sufficient quality
because a mean diameter of the intestine in fetuses after 20 weeks' gestation
is 3 mm or greater [4,
6,
7]. Conventional sequences are
still limited in temporal resolution for obtaining T1-weighted images of such
thin sections with adequate anatomic coverage for evaluating fetal
gastrointestinal abnormalities.
Recently, a 3D volumetric interpolated breath-hold sequence (VIBE), which
is a modified fast 3D gradient-echo sequence, has been applied to T1-weighted
images in clinical body MRI
[8-10].
This sequence may provide isotropic or nearly isotropic resolution in three
dimensions while preserving wide anatomic coverage in a short acquisition time
[8-10].
Motion artifacts are also reduced by the rapid data acquisition
[10]. Therefore, 3D MR images
of high quality can be available because of the resultant minimization of
partial volume averaging effects and motion artifacts
[8,
10]. We attempted to obtain
thin-section T1-weighted images through the fetus using VIBE and to generate
3D MR colonography images of the fetus from the VIBE data sets. The purpose of
this study was to assess the performance of VIBE in evaluating the normal and
abnormal fetal gastrointestinal tract.
Materials and Methods
Subjects
Between June 2004 and September 2006, 45 patients underwent fetal MRI after
sonography in our institute. Fetal MRI was limited to cases in which complex
anomalies were suspected at second- or third-trimester fetal sonography
screening or in which the sonography results were equivocal or undetermined.
Before the examination, all subjects were informed about the procedure and the
safety of the technique by obstetricians, and all provided their informed
consent. Of the 45 participants, 35 who fulfilled the following inclusion
criteria were enrolled in this retrospective study. The first criterion was
that HASTE T2-weighted and VIBE T1-weighted images were both obtained through
the fetus. The second criterion was that diagnoses were confirmed after
delivery in our institute. Although there was one twin pregnancy, we examined
only the indicated fetus.
On the basis of the medical records of postnatal diagnoses, 28 of the 35
fetuses had a normal gastrointestinal tract. The gestational ages of the
fetuses with a normal gastrointestinal tract ranged from 19 weeks 4 days to 40
weeks 5 days (mean, 30 weeks 5 days). These fetuses had other abnormalities:
CNS abnormalities (n = 10), urogenital abnormalities (n =
7), musculoskeletal abnormalities (n = 6), and hydrops fetalis
(n = 1). Four of those fetuses had normal findings. The remaining
seven fetuses were confirmed as having gastrointestinal abnormalities, which
included duodenal atresia (n = 3), congenital diaphragmatic hernia
(n = 2), gastroschisis (n = 1), and ileal atresia with
meconium peritonitis (n = 1).
MRI Protocol
MRI was performed on a 1.5-T MR unit (Magnetom Sonata, Siemens Medical
Solutions) using a phased-array body coil. In the fetal MR examinations, HASTE
T2-weighted sequences (TR/TEeff, 1,100/81; flip angle, 150°;
field of view, 400 mm; slice thickness, 5.0 mm; section gap, 1.0 mm; slice
number, 18; image matrix, 256; bandwidth, 476 Hz/pixel; turbo factor, 164; 1
signal acquisition; scanning time, 20 seconds) were obtained through the fetus
in the transverse, sagittal, and coronal planes. T1-weighted images were
obtained through the fetus in the coronal plane using VIBE (TR/TE, 4.45/1.34;
flip angle, 15°; field of view, 400 mm; slice thickness, 3 mm; section
gap, 0-0.6 mm; number of slices, 32-40; image matrix, 256 [interpolation,
512]; bandwidth, 490 Hz/pixel; 1 signal acquisition; scanning time, 24-28
seconds). Although the images were basically obtained during a single
breath-hold, these were sometimes acquired during quiet respiration. Neither
sedatives nor IV gadolinium-based contrast material was used. The pregnant
patients were instructed to walk for 10 minutes before the MR examination.
MR Image Processing and Evaluation
We generated 3D MR colonography images from the VIBE data sets using a
volume-rendering algorithm at a computer workstation (Aquarius, version 3.2,
Elk Corporation). T2-weighted images, T1-weighted images, and 3D MR
colonography images were simultaneously assessed on an image viewer. In the
fetuses with a normal gastrointestinal tract, signal characteristics of the
stomach, the duodenum, the proximal small intestine, the distal small
intestine, the ascending colon, the transverse colon, the descending colon,
the sigmoid colon, and the rectum were assessed. The small intestine was
subdivided into the duodenum, the proximal small intestine, and the distal
small intestine because a distinction between the jejunum and the ileum was
not feasible. The identification of the respective intestine was made on the
basis of the anatomic location, the continuity, and the presence or absence of
haustra. The liver was used as a landmark of the anatomic position in the
fetal abdomen. Signal intensity (high or low) of the precise intestinal
segments was subjectively determined on both T2- and T1-weighted images.
Particularly on T1-weighted images, we compared the signal intensity with that
of muscle because the T1-weighted signal in the small and large intestine is
greater than that of muscle
[4]. On 3D MR colonography
images, visualization of the intestine was judged to be satisfactory when its
signal intensity was distinguished from that of background and when the
continuity of the intestine was confirmed.
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Fig. 1A —MR images of fetus at 35 weeks 4 days' gestation show normal
gastrointestinal tract. St = stomach, pSm = proximal small intestine, dSm =
distal small intestine, As = ascending colon, Tr = transverse colon, Ds =
descending colon, Sg = sigmoid colon, R = rectum, L = liver, B = urinary
bladder. Coronal T2-weighted image shows high signal intensity from stomach to
small intestine but low signal intensity from transverse colon to sigmoid
colon.
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Fig. 1B —MR images of fetus at 35 weeks 4 days' gestation show normal
gastrointestinal tract. St = stomach, pSm = proximal small intestine, dSm =
distal small intestine, As = ascending colon, Tr = transverse colon, Ds =
descending colon, Sg = sigmoid colon, R = rectum, L = liver, B = urinary
bladder. Coronal 3D T1-weighted gradient-echo image shows low signal intensity
in stomach and high signal intensity from distal small intestine to colon and
rectum. Proximal small intestine shows higher signal intensity than liver
does.
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Fig. 1C —MR images of fetus at 35 weeks 4 days' gestation show normal
gastrointestinal tract. St = stomach, pSm = proximal small intestine, dSm =
distal small intestine, As = ascending colon, Tr = transverse colon, Ds =
descending colon, Sg = sigmoid colon, R = rectum, L = liver, B = urinary
bladder. Volume-rendered image in anteroposterior view visualizes through
distal small intestine to rectum and also shows liver.
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We divided the fetuses with a normal gastrointestinal tract into two age
groups of less than 32 weeks' gestation (n = 12) and 32 weeks' or
more gestation (n = 16) on the basis of a previous report
[2]. Whether there was
herniation, obstruction, dilatation, or narrowing of intestine in the fetal
gastrointestinal abnormalities was qualitatively assessed. The diagnosis of
herniation was made when the intestine was seen outside the abdomen, whereas
the diagnosis of obstruction was made when the intestine was constricted with
a resultant dilatation of the bowel proximal to the site of obstruction.
Results
In all 28 normal and seven abnormal cases, HASTE T2-weighted imaging, VIBE
T1-weighted imaging, and 3D MR colonography were performed. A summary of the
normal findings is presented in Table
1; abnormal findings are listed in
Table 2. In the three cases of
duodenal atresia, VIBE imaging and 3D MR colonography did not add to the
diagnosis. In the two diaphragmatic hernias and one gastroschisis case, VIBE
imaging and 3D MR colonography clearly showed colon involvement in the defect
but did not provide additional information. In the case of ileal atresia with
meconium peritonitis, the ileal atresia was confirmed (Figs.
1A,
1B,
1C,
2A,
2B,
2C,
2D,
3A,
3B,
3C,
3D,
3E,
3F,
3G,
3H).
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TABLE 1: Normal Gastrointestinal Tract Features Having High Signal Intensity in
Fetuses 19 Weeks 4 Days' to 40 Weeks 5 Days' Gestational Age
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Fig. 2A —MR images of fetus with congenital diaphragmatic hernia at 37
weeks 5 days' gestation. R = rectum, L = liver. Coronal T2-weighted (A)
and 3D T1-weighted gradient-echo (B) images show herniated intestine in
left thoracic space. Extent of colon into left thoracic space
(arrows) is clearly visualized.
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Fig. 2B —MR images of fetus with congenital diaphragmatic hernia at 37
weeks 5 days' gestation. R = rectum, L = liver. Coronal T2-weighted (A)
and 3D T1-weighted gradient-echo (B) images show herniated intestine in
left thoracic space. Extent of colon into left thoracic space
(arrows) is clearly visualized.
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Fig. 2C —MR images of fetus with congenital diaphragmatic hernia at 37
weeks 5 days' gestation. R = rectum, L = liver. Volume-rendered images in
anteroposterior (C) and posteroanterior (D) views show anatomic
relationship between herniated colon and liver. Colon beyond diaphragm
(arrows, C), is clearly shown. Liver has normal shape. Small
intestine is not visualized.
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Fig. 2D —MR images of fetus with congenital diaphragmatic hernia at 37
weeks 5 days' gestation. R = rectum, L = liver. Volume-rendered images in
anteroposterior (C) and posteroanterior (D) views show anatomic
relationship between herniated colon and liver. Colon beyond diaphragm
(arrows, C), is clearly shown. Liver has normal shape. Small
intestine is not visualized.
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Fig. 3A —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Sagittal
T2-weighted image shows extraabdominal bowel (arrows) has low signal
intensity.
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Fig. 3B —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Coronal 3D
T1-weighted gradient-echo image shows absence of colon in abdominal
cavity.
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Fig. 3C —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Sagittal
multiplanar reformatted (MPR) T1-weighted images show eviscerated bowel has
high signal intensity, which is consistent with normal colon. Small intestine
appears normal. When coronal image sections cannot show abnormal findings, MPR
images are useful for evaluation.
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Fig. 3D —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Sagittal
multiplanar reformatted (MPR) T1-weighted images show eviscerated bowel has
high signal intensity, which is consistent with normal colon. Small intestine
appears normal. When coronal image sections cannot show abnormal findings, MPR
images are useful for evaluation.
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Fig. 3E —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Volume-rendered
images in anteroposterior (E), oblique left-to-right (F),
left-to-right (G), and right-to-left (H) views show condition of
eviscerated bowel segments of gastroschisis. No volvulus or bowel atresia is
seen. Site of abdominal wall defect is predictable because intestine becomes
constricted just at defect site (arrows). Small intestine is not
visualized.
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Fig. 3F —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Volume-rendered
images in anteroposterior (E), oblique left-to-right (F),
left-to-right (G), and right-to-left (H) views show condition of
eviscerated bowel segments of gastroschisis. No volvulus or bowel atresia is
seen. Site of abdominal wall defect is predictable because intestine becomes
constricted just at defect site (arrows). Small intestine is not
visualized.
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Fig. 3G —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Volume-rendered
images in anteroposterior (E), oblique left-to-right (F),
left-to-right (G), and right-to-left (H) views show condition of
eviscerated bowel segments of gastroschisis. No volvulus or bowel atresia is
seen. Site of abdominal wall defect is predictable because intestine becomes
constricted just at defect site (arrows). Small intestine is not
visualized.
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Fig. 3H —MR images of fetus with gastroschisis at 28 weeks 6 days'
gestation. Sm = small intestine, As = ascending colon, Tr = transverse colon,
Ds = descending colon, R = rectum, L = liver, St = stomach. Volume-rendered
images in anteroposterior (E), oblique left-to-right (F),
left-to-right (G), and right-to-left (H) views show condition of
eviscerated bowel segments of gastroschisis. No volvulus or bowel atresia is
seen. Site of abdominal wall defect is predictable because intestine becomes
constricted just at defect site (arrows). Small intestine is not
visualized.
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Discussion
In fetal MRI, signal intensity of the gastrointestinal tract is basically
determined by the location and amounts of swallowed amniotic fluid and
meconium
[2-4,
6]. Meconium is formed from
secretions of the liver and intestinal glands, desquamated intestinal
epithelium, and some amniotic fluid after 13 weeks and slowly migrates from
the small intestine to the colon and rectum
[4]. Therefore, signal
intensity of the intestine greatly varies depending on gestational age.
According to previous reports
[2-4],
signal intensity of the stomach through the proximal small intestine is
hyperintense on T2-weighted images after 26-27 weeks' gestation, whereas that
of the distal small intestine through the colon is intermediate to low signal
intensity. On T1-weighted images, signal intensity from the sigmoid colon to
the rectum is always bright after 23 weeks' gestation
[2,
3]. The distal small intestine
is hyperintense in more than half of cases before 32 weeks' gestation;
thereafter, it remains hyperintense in almost 40% of cases
[3]. The visualization of the
proximal small intestine on T1-weighted images is limited
[2-4].
In our results, the percentages of the respective intestine segments showing a
high signal intensity on HASTE T2-weighted and VIBE T1-weighted images were
equal or superior to the previously published data; however, the percentage of
the proximal small intestine having a high T1 signal was much higher than in
previous reports. The rate of the distal small intestine showing a high signal
intensity on HASTE T2-weighted images in fetuses at 32 weeks' gestation or
longer was higher than that in fetuses at less than 32 weeks' gestation. The
rate of the proximal small intestine showing a high signal intensity on
T1-weighted images in fetuses at less than 32 weeks' gestation is higher than
that in fetuses at 32 weeks' gestation or longer.
The VIBE sequence, which is a modified 3D fast gradient-echo sequence, uses
a symmetric echo in the read direction, partial inplane Fourier sampling in
the phase-encoding direction, and asymmetric echo sampling and sinc
interpolation along the partition direction
[8-11].
This sequence has the ability to provide thinner sections, higher
signal-to-noise ratio, higher image contrast, and a shorter acquisition time
than conventional sequences while preserving adequate anatomic coverage
[8-11].
In fetal MRI, rapid data acquisition is important because it may reduce motion
artifacts of both mothers and fetuses. Indeed, VIBE enabled us to obtain 32-40
sections of 3-mm section thickness through the fetuses with a short
acquisition time, and it provided high-quality T1-weighted imaging and 3D MR
colonography of the fetuses.
Several investigators have suggested that 3D MR images were useful in
surgical simulation and treatment planning before birth
[7,
12-14].
Rotation of 3D MR colonography images on an image viewer aids in understanding
the anatomic position of the intestine and the relation between the intestine
and the liver. We have actually presented 3D MR colonography images, with
their excellent delineation of the meconium, at family counseling and at
conferences with neonatologists and pediatric surgeons. In our results,
T1-weighted and 3D MR colonography images of sufficient quality were generated
from the same VIBE data sets. The percentages of the recognition of the
proximal and distal small intestine on 3D MR colonography images were lower
than those on VIBE images. On 3D MR colonography images, the visualization of
the small intestine in fetuses at less than 32 weeks' gestation was inferior
to that in fetuses at more than 32 weeks' gestation. We thought that the
difference may have been related to the small diameter of the small intestine
in fetuses at earlier gestational ages.
Our study has some limitations. Because we enrolled a limited number of the
fetuses with normal and abnormal gastrointestinal tracts, the sample size is
relatively small. Most fetuses were in the third trimester, and the
gastrointestinal tract has a different appearance at earlier gestational ages.
A large series is required to precisely determine using VIBE the visualization
of the respective segments of the fetal gastrointestinal tract at different
gestational ages. In addition, we could not determine signal characteristics
of fetal gastrointestinal abnormalities, although it has been reported that
signal characteristics are more conspicuous in abnormal cases
[5]. The impact of VIBE on
findings and diagnosis in the abnormal cases is not yet clear.
In conclusion, we report the appearances of normal and abnormal fetal
gastrointestinal tracts on HASTE T2-weighted imaging, VIBE T1-weighted
imaging, and 3D MR colonography. VIBE allowed better visualization of the
fetal gastrointestinal tract than techniques used in previous reports
[2-4]
despite the fact that this sequence provided thinner slice sections through
the fetus. We believe that the routine use of VIBE for fetal MRI may offer
T1-weighted imaging and 3D MR colonography of high quality for the evaluation
of fetal gastrointestinal abnormalities.
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Abstract
Introduction
