MRI of the fetal gastrointestinal tract

Is that bowel normal? Nomograms for fetal colon and rectum measurements by MRI from 20-36 weeks' gestation

BACKGROUND

MRI is being increasingly used as a supplemental tool to ultrasound for prenatal diagnosis of fetal anomalies. Evidence suggests fetal MRI may aid in diagnosis and management of gastrointestinal abnormalities, though it is not widely used for this indication at this time. There is a lack of well-established nomograms of fetal colonic measurements by gestational age (GA) with MRI.

OBJECTIVE

We aim to establish a nomogram of normal colonic and rectal size throughout the second and third trimesters by fetal MRI, starting at a younger GA than previously documented.

METHODS AND MATERIALS

A retrospective study was performed. Fetal MRI databases spanning October 2010 to June 2021 were searched for examinations performed from 19 weeks GA to term. Cases were excluded for poor image quality or if a gastrointestinal abnormality was noted. Coronal and sagittal T1-weighted images were used to measure the ascending, transverse, descending, and sigmoid colon; rectosigmoid colon (at the level of the iliac crest); and rectum (at the distal third in the coronal views and at the base of the bladder and largest dimension in the sagittal views, at the orthogonal plane of the colon). Reproducible measurement areas for fetal bowel were established between two readers, and two-way mixed single-measure intra-class correlation coefficients (ICCs) were developed to assess inter-rater reliability. Growth curves were developed using generalized additive models. The main outcomes were the measurements at each section of the bowel and the predictor was GA. The curves of the fitted models were plotted for the estimated 10th, 25th, 50th, 75th, and 90th percentiles.

RESULTS

A total of 315 patients had at least one measurable bowel segment. If 100 or more measurements were obtained at a bowel segment, a nomogram was created. Due to limited available data prior to 20 weeks and after 36 weeks GA, bowel nomograms were developed for 20-36 weeks GA. The three bowel segments with the most included patients and greatest number of measurements obtained were the rectosigmoid colon at the iliac crest, distal one-third of the rectum, and the sigmoid colon (n = 251, 234, 178, respectively) with excellent reader agreement (ICC = 0.96, 0.90, 0.94, respectively). The range of bowel diameter increased with advancing GA at the 10th percentile through the 90th percentile in a nearly linear distribution at the sigmoid colon, rectum at the base of the bladder, and distal rectum and in a non-linear distribution at the rectum at the level of the iliac crest and descending colon.

CONCLUSION

Our study provides normative data for fetal colon and rectum measurements from 20 to 36 weeks GA on MRI. Radiologists can reliably measure the normal fetal colon and rectum on fetal MRI. Our nomogram of normal fetal bowel measurements provides radiologists and clinicians with data that can aid in the early recognition of bowel anomalies on fetal MRI.

Standardised and structured reporting in fetal magnetic resonance imaging: recommendations from the Fetal Task Force of the European Society of Paediatric Radiology

Over the last decades, magnetic resonance imaging (MRI) has emerged as a valuable adjunct to prenatal ultrasound for evaluating fetal malformations. Several radiological societies advocate for standardised and structured reporting practices to enhance the uniformity of imaging language. Compared to narrative formats, standardised and structured reports offer enhanced content quality, minimise reader variability, have the potential to save reporting time, and streamline the communication between specialists by employing a shared lexicon. Structured reporting holds promise for mitigating medico-legal liability, while also facilitating rigorous scientific data analyses and the development of standardised databases. While structured reporting templates for fetal MRI are already in use in some centres, specific recommendations and/or guidelines from international societies are scarce in the literature. The purpose of this paper is to propose a standardised and structured reporting template for fetal MRI to assist radiologists, particularly those with less experience, in delivering systematic reports. Additionally, the paper aims to offer an overview of the anatomical structures that necessitate reporting and the prevalent normative values for fetal biometrics found in current literature.

Prenatal hydrocolpos: imaging findings and differential diagnosis

Prenatal hydrocolpos is characterized by fluid distension of the vagina. Hydrocolpos can be caused by multiple underlying etiologies and often demonstrates overlapping imaging features compared to other cystic abdominal and pelvic lesions. The purpose of the current pictorial essay is to provide a systematic prenatal magnetic resonance imaging (MRI) approach to differentiating the primary etiologies leading to hydrocolpos. After discussing the fundamental embryological processes involved in vaginal development, the current essay discusses the most common causes of hydrocolpos with their associated prenatal and postnatal imaging features. An approach to distinguishing the more common differential diagnoses is provided. Given the implications of parental counseling and postnatal management, this essay provides an important approach for narrowing differential diagnoses based on prenatal imaging.

MR Imaging of the Fetal Genitourinary Tract

Fetal MR imaging overcomes many of the technical barriers of ultrasound and is an important diagnostic tool for fetal genitourinary (GU) anomalies. It is suited for evaluation of GU anomalies because of the fluid-sensitive sequences and superior soft tissue contrast. Often GU malformations are part of a multisystem genetic or congenital condition, and imaging the entire fetus with MR adds additional clarity about the extent of disease. It adds confidence to diagnoses of renal agenesis, urinary tract dilation, cystic disease, and tumors. It is particularly useful to delineate anatomy in complex GU malformations. This additional information guides counseling.

Ultrasound and magnetic resonance imaging features of fetal urogenital anomalies: A pictorial essay

This pictorial essay focuses on ultrasound (US) and magnetic resonance imaging (MRI) features of fetal urogenital anomalies. Fetal urogenital malformations account for 30%-50% of all anomalies discovered during pregnancy or at birth. They are usually detected by fetal ultrasound exams. However, when ultrasound data on their characteristics is insufficient, MRI is the best option for detecting other associated anomalies. The prognosis highly depends on their type and whether they are associated with other fetal abnormalities.

Magnetic resonance imaging of fetal abdominal pathology: a complementary tool to prenatal ultrasound

Fetal magnetic resonance imaging (MRI) is increasingly being used worldwide as a complementary tool to prenatal ultrasound (US) for multiple fetal pathologies. The aim of this article is to describe and illustrate how MRI can help US to evaluate fetal abdominal anomalies, based on cases performed in a tertiary public university hospital. Prenatal US, fetal MRI and postnatal imaging of these cases will be shown side-by-side to describe and illustrate the added value of fetal MRI in the different organs/systems and its impact on clinical management.

A Spectrum of Ultrasound and MR Imaging of Fetal Gastrointestinal Abnormalities: Part 1 Esophagus to Colon

Ultrasound (US) and magnetic resonance imaging (MRI) are two modalities for diagnosing fetal gastrointestinal (GI) anomalies. Ultrasound (US) is the modality of choice. MRI can be used as a complementary method. Despite its expanding utilization in central nervous system (CNS) fetal malformation, MRI has not yet been established for evaluation of fetal GI abnormalities. Therefore, more attention should be paid to the clinical implications of MRI investigations following screening by US.

A Spectrum of Ultrasound and MR Imaging of Fetal Gastrointestinal Abnormalities: Part 2 Anorectal Malformation, Liver, and Abdominal Wall Anomalies

Ultrasound (US) and magnetic resonance imaging (MRI) are two modalities for diagnosing fetal gastrointestinal (GI) anomalies. Ultrasound (US) is the modality of choice. MRI can be used as a complementary method. Despite its expanding utilization in central nervous system (CNS) fetal malformation, MRI has not yet been established for evaluation of fetal GI abnormalities. Therefore, more attention should be paid to the clinical implications of MRI investigations following screening by US.

MRI in Pregnancy and Precision Medicine: A Review from Literature

Magnetic resonance imaging (MRI) offers excellent spatial and contrast resolution for evaluating a wide variety of pathologies, without exposing patients to ionizing radiations. Additionally, MRI offers reproducible diagnostic imaging results that are not operator-dependent, a major advantage over ultrasound. MRI is commonly used in pregnant women to evaluate, most frequently, acute abdominal and pelvic pain or placental abnormalities, as well as neurological or fetal abnormalities, infections, or neoplasms. However, to date, our knowledge about MRI safety during pregnancy, especially about the administration of gadolinium-based contrast agents, which are able to cross the placental barrier, is still limited, raising concerns about possible negative effects on both the mother and the health of the fetus. Contrast agents that are unable to cross the placenta in a way that is safe for the fetus are desirable. In recent years, some preclinical studies, carried out in rodent models, have evaluated the role of long circulating liposomal nanoparticle-based blood-pool gadolinium contrast agents that do not penetrate the placental barrier due to their size and therefore do not expose the fetus to the contrast agent during pregnancy, preserving it from any hypothetical risks. Hence, we performed a literature review focusing on contrast and non-contrast MRI use during pregnancy.

Ultrasound and Fetal MRI Complementary Contributions to Appropriate Counseling in Small Bowel Obstruction

OBJECTIVE

Bowel obstructions beyond the duodenum represent a heterogeneous group of congenital anomalies with a highly variable prognosis, the main issue being postnatal short bowel syndrome (SBS). The objective of our study was to evaluate the contributions of fetal MRI in cases of bowel obstruction.

MATERIALS AND METHODS

A retrospective analysis of all newborns, for whom both ante-natal ultrasound and fetal MRI were available, referred to our center for suspected bowel obstruction was performed. Examinations were reviewed blinded to the postnatal outcome. Key outcome measures included exact diagnosis and the existence of postoperative SBS. We evaluated the contribution of MRI in determining precise location and etiology of the bowel obstruction, dilatation of the proximal bowel loops, and assessment of the quality of the remaining distal bowel loops.

RESULTS

Twenty-five newborns were included. There were 19 single obstructions and 6 complex forms (4 apple peel syndromes and 2 multiple atresias). MRI correctly identified the affected segment of the small bowel in 59.1% of the cases. MRI identified the mechanism of obstruction in 72% of cases. MRI reliably predicted an abnormal appearance of the bowel distal to the obstruction in 100% of the severe cases (3/3) and in 66.7% of complex forms (4/6).

CONCLUSION

Our study suggests that fetal MRI, when done in addition to prenatal ultrasound, is contributory in the management of fetuses with suspected bowel obstruction. MRI may be particularly useful in determining the location and origin of the bowel obstruction and in assessing the quality of the bowel distal to the obstruction, which might assist in the prediction of SBS and more detailed prenatal counseling.

Prenatal diagnosis of intestinal nonrotation using magnetic resonance imaging: Is it possible?

BACKGROUND

Malrotation of the bowel refers to any variation in the rotation and fixation of the gastrointestinal tract during the first trimester and is most commonly detected postnatally. Nonrotation of the bowel and incomplete rotation of the bowel are subtypes of malrotation.

OBJECTIVE

To determine if the nonrotation subtype of malrotation of the bowel can be detected on prenatal magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Cases from 2012 to 2018 with nonrotation of the bowel without obstruction confirmed by imaging, surgery and/or autopsy were compared to prenatal imaging. Prenatal imaging was retrospectively reviewed to determine if prenatal diagnosis of malrotation could be made. Exclusion criteria included diaphragmatic hernia, omphalocele and gastroschisis.

RESULTS

Ten cases of nonrotation diagnosed postnatally by upper gastrointestinal series (upper GI)/small bowel follow-through (SBFT) or autopsy had prenatal MRI. Prenatal MR studies were performed for assessment of heterotaxy syndrome with congenital heart disease (6/10), congenital heart disease with additional anomalies (suspected VACTERL [vertebral, anorectal, cardiac, tracheoesophageal, renal, limb] and suspected lung agenesis, ventriculomegaly) (3/10) and skeletal dysplasia (1/10). Eight upper GI/SBFT cases demonstrated nonrotation of the bowel without obstruction with the small bowel completely on one side of the abdomen contralateral to the stomach and the colon ipsilateral to the stomach; four cases were confirmed by surgery. The small bowel in one upper GI/SBFT case was unilateral contralateral to the stomach with a meandering colon. One case had nonrotation diagnosed at autopsy. There were no cases of postnatal midgut volvulus. Retrospective review of the 10 cases had prenatal MRI performed between 23 and 37 weeks of gestation. The coronal plane was the most optimal plane to assess the position of the stomach, small bowel and colon in relationship to each other. The small bowel was best assessed on T2-weighted images while the colon was best assessed on T1-weighted images. A nonrotated position of the small bowel was present in all 10 fetal MRI cases mirroring postnatal findings, with the small bowel contralateral to the stomach in 9/10 cases and ipsilateral to the stomach (in the right abdomen) in 1/10 cases. The colon was visualized by prenatal MRI in 9/10 cases, with 1 case limited due to a lack of T1-weighted imaging. A nonrotated position of the colon contralateral to the small bowel was present in 7/9 cases. In 2/9 cases, the colon was wandering, positioned on both sides of the midline. Colonic position in all nine cases matched postnatal findings. No cases presented with prenatal bowel obstruction.

CONCLUSION

Detection of nonrotation of the bowel is possible on prenatal MRI.

Fetal body MRI for fetal and perinatal management

Fetal magnetic resonance imaging (MRI) has become a valuable adjunct to ultrasound (US) in diagnosing fetal abnormalities. This review is intended to highlight the contribution of MRI in parental counselling and perinatal treatment. A state-of-the-art fetal MRI protocol with experts of maternal-fetal medicine present in the MRI suite allows emphasis on patient-centred care and maximises therapeutic options.

Fetal sigmoid colon mesentery made visible by routine ultrasound in the first and second trimester of pregnancy

BACKGROUND

Ultrasound is a routine procedure performed during pregnancy to monitor the status of the human fetus, including the development of the digestive tract. The aim of this publication was to determine the shape of the fetal sigmoid colon mesentery during the first and second trimester of pregnancy by means of ultrasound.

METHODS

The study was performed in 45 pregnant women (age: 36.4 years on average, range 25-45) during a routine ultrasound examination. The fetuses were between 12 and 22 weeks of pregnancy. The shape of the fetal sigmoid colon mesentery was analyzed.

RESULTS

We visualized the triangular shape of the mesentery in all 45 cases. A prevalence of scalene or isosceles acute triangles was found in both trimesters. At the 12-13 weeks we observed 5 different forms of mesenteric triangles - the scalene or isosceles acute triangles appeared at 33.3% and 28.6%, respectively. The obtuse scalene was present in 23.8 %. In the 2nd trimester (20-22 weeks) 4 types were found, among them 37.5 % acute scalene and 33.3 % acute isosceles.

CONCLUSIONS

The fetal sigmoid colon mesentery can be visualized from the 12th week of pregnancy. The triangular shape of the sigmoid colon mesentery is easy to follow during routine ultrasound examinations.

Utility of Magnetic Resonance Imaging in Diagnosis of Prenatal Non-Visualization of the Fetal Gallbladder: A Case-Series Study

BACKGROUND This study aimed to assess the utility of magnetic resonance imaging (MRI) in the diagnosis of prenatal non-visualization of the fetal gallbladder (PNVGB). MATERIAL AND METHODS The clinical data of 32 pregnant women with PNVGB who underwent MRI examination during the second and third trimester of pregnancy were collected and their outcomes were analyzed. RESULTS MRI showed that 26 patients (81.3%) had isolated PNVGB and 6 (18.8%) had additional malformations. In 26 patients with isolated PNVGB, 7 were found in the gallbladder on MRI and 4 were found on subsequent ultrasonography. One patient had termination of pregnancy (TOP) and 1 patient was lost to follow-up; the remaining 24 patients were known to deliver a healthy child. Among the 6 patients with additional malformations, 3 terminated their pregnancies due to combined severe abnormalities: 1 patient with horseshoe kidney and 1 with fetal echogenic bowel both had a healthy child, while 1 with fetal growth restriction (FGR) delivered a child who walked on tiptoe. CONCLUSIONS MRI contributes to identifying PNVGB detected or suspected by ultrasonography.

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.

Magnetic resonance imaging of the fetal gastrointestinal system

In this paper the authors review the normal imaging appearance of the fetal gastrointestinal tract and patterns of fetal gastrointestinal tract obstruction. The authors include a detailed summary from esophagus to the rectum, including the expected pattern of meconium and small-bowel contents at different gestational ages. Common fetal gastrointestinal tract obstructions are reviewed with accompanying case examples, emphasizing the role of the meconium and bowel-caliber patterns in establishing differential diagnoses. This review also includes imaging pitfalls, complications of gastrointestinal tract obstruction, and implications for patient care.

Visualisation of fetal meconium on post-mortem magnetic resonance imaging scans: a retrospective observational study

Background

Less invasive techniques for fetal post-mortems are increasingly used to correlate with parental wishes. With the use of post-mortem magnetic resonance imaging (MRI), normal appearance of the organs must be established.

Purpose

To investigate the after death appearance of the fetal meconium throughout gestation using the hyperintense appearance of meconium on T1 weighted MRI.

Material and Methods

This was a retrospective study that took place in a tertiary referral centre radiology department. Sixty-two fetal body post-mortem MRI scans (January 2014 to May 2018) between 12 and 41 weeks gestation were reviewed. Signal intensity of meconium at the rectum, sigmoid colon, splenic flexure and hepatic flexure was evaluated and correlated with gestational age. Interrater reliability was calculated.

Results

Meconium did not consistently have high signal intensity on T1 scans and was not always obvious. Rectal meconium had the highest intensity, and the more proximal the bowel the lower the intensity. The meconium had higher intensity at earlier gestations. Interrater reliability for rectal meconium gradings was excellent.

Conclusion

This study provides the first published primary research on the appearance of fetal meconium on post-mortem MRI. Overall, results were variable and suggest an alteration of bowel contents after death, but further investigation is needed to effectively inform practice.

Cloacal malformation: A rare case report and review of prenatal imagings

RATIONALE

Cloacal malformation (CM) is a serious type of anorectal and urogenital tract malformation. However, prenatal ultrasound (US) detection of CM is challenging. In this paper, we reported a rare case of CM prenatally diagnosed by US and magnetic resonance imaging (MRI), as well as reviewed the prenatal US and MRI characteristics of CM in the literature.

PATIENT CONCERNS

A 30-year-old pregnant woman complained of cystic mass in the fetal abdomen detected by prenatal US.

DIAGNOSIS

Fetus CM.

INTERVENTIONS

The fetus was diagnosed as fetal CM by US and MRI, then the pregnant woman received a drug-induced labor treatment. After the neonate was delivered, the measurement was performed on the weight, length, head circumference, abdomen circumference, and bilateral thigh circumference.

OUTCOMES

A female dead neonate was delivered from the vagina of the gravida, showing congenital anus absence. Prenatal ultrasound demonstrated right kidney duplication, hydronephrosis, and right ureteral dilatation. Meanwhile, prenatal MRI showed a cystic cavity, double collecting systems of right kidney, right ureteral dilatation, and right rectum dilatation. In addition, general parameters are as follows: weight: 2280 g; length: 39 cm; head circumference: 26.3 cm; abdomen circumference: 31 cm; right thigh circumference: 17 cm, and left thigh circumference: 18 cm.

LESSONS

US combined with MRI can not only provide reliable evidence for fetal CM in the third trimester but also offer crucial information to the pregnant women to establish clinic treatment programs as early as possible.

Appearance of fetal intestinal obstruction on fetal MRI

OBJECTIVE

To retrospectively analyze the imaging findings of fetal intestinal obstruction diagnosed by MRI and compare with postnatal surgery findings.

METHODS

MRI data of 3346 pregnant women were retrospectively analyzed; we found 47 cases of suspected fetal small intestinal obstruction. Twenty-nine underwent postnatal surgery.

RESULTS

We identified one case of jejunal obstruction secondary to perforation, five annular pancreas, 10 duodenal stenoses, four jejunal stenoses, five jejunal atresias, two ileal atresias, four intestinal volvulus, and four intestinal malrotations. We further found four cases of duodenal stenosis with intestinal malrotation (two cases also showed volvulus). On fetal MRI, annular pancreas and duodenal obstruction manifested as a "double bubble." Jejunal stenosis appeared as a "triple bubble." Jejunal and ileal atresia mainly manifested as proximal dilatation with high signal on T1WI. Intestinal volvulus showed a sausage-like intestinal distortion and mixed signals on T1WI and DWI sequences. Intestinal malrotations were characterized as abnormal duodenal morphology. We missed two malrotations; one jejunal obstruction was misdiagnosed.

CONCLUSION

Prenatal MRI can accurately determine the degree of intestinal obstruction and help qualitatively diagnose its possible etiology according to changes in MRI signals in multiple sequences, providing guidance for prenatal counseling.

Biometric analysis of the foetal meconium pattern using T1 weighted 2D gradient echo MRI

Objectives:

Foetal MRI is used to assess abnormalities after ultrasonography. Bowel anomalies are a significant cause of neonatal morbidity, however there are little data concerning its normal appearance on antenatal MRI. This study aims to investigate the pattern of meconium accumulation throughout gestation using its hyperintense appearance on T₁ weighted scans and add to the current published data.

Methods:

This was a retrospective cohort study in a tertiary referral clinical MRI centre. Foetal body MRI scans of varying gestational ages were obtained dating between October 2011 and March 2018. The bowel was visualised on T₁ weighted images. The length of the meconium and the width of the meconium at the rectum, sigmoid colon, splenic flexure and hepatic flexure was measured. Presence or absence of meconium in the small bowel was noted. Inter- and intrarater reliability was assessed.

Results:

181 foetal body scans were reviewed. 52 were excluded and 129 analysed. Visualisation of the meconium in the large bowel became increasingly proximal with later gestations, and small bowel visualisation was greater at earlier gestations. There was statistically significant strong (r = 0.6–0.8) or very strong (r = 0.8–1.0) positive correlation of length and width with increasing gestation. Interrater reliability was moderate to excellent (r = 0.4–1.0).

Conclusion:

This study provides new information regarding the pattern of meconium accumulation throughout gestation. With care, the results can be used in clinical practice to aid diagnosis of bowel pathology.

Advances in knowledge:

The findings of this study provide further information concerning the normal accumulation of foetal meconium on MR imaging, an area where current research is limited.

Imaging of anorectal malformations in utero

PURPOSE

To document the imaging findings suggestive of anorectal malformation (ARMs) on prenatal US and MRI.

METHODS

Retrospective evaluation of the screening US and prenatal MRI exams of the rectum and ano-perineal region in normal fetuses and in patients with ARMs.

RESULTS

Examples showing the normal rectal and anoperineal anatomy on prenatal US and MRI exams and the imaging findings observed in different types of confirmed ARMS.

CONCLUSIONS

Prenatal diagnosis of ARMs requires both a systematic evaluation of the fetal pelvis and perineum and an appropriate knowledge of its suggestive imaging findings.

The spectrum of cloacal malformations: how to differentiate each entity prenatally with fetal MRI

The term cloacal malformation is commonly used to describe the classic cloacal malformation where there is a single common urogenital and intestinal channel located at the expected site of the urethra. There is, however, a spectrum of cloacal abnormalities that differ from this classic type and are less well discussed in the radiologic and surgical literature. The aim of this pictorial essay is to familiarize radiologists with the anatomy, appropriate terminology and key prenatal imaging findings that differentiate the six entities that constitute the spectrum of cloacal abnormalities.

A survey of pediatric diagnostic radiologists in North America: current practices in fetal magnetic resonance imaging

BACKGROUND

Fetal magnetic resonance imaging (MRI) is an imaging examination in evolution. Rapid developments over recent decades have led to better image quality, an increased number of examinations and greater impact on patient care.

OBJECTIVE

To gather data regarding current practices among established programs in North America and provide information to radiologists interested in implementing or growing a fetal MRI service.

MATERIALS AND METHODS

An electronic survey containing 15 questions relevant to the use of fetal MRI was submitted to pediatric radiologists and neuroradiologists. Items regarded scheduling and reporting logistics, magnet strength, patient positioning and patient preparation. Answers and comments were collected, and descriptive statistics were summarized.

RESULTS

One hundred and six survey responses were evaluated. Of the survey responses, 62/106 (58.5%) allow fetal MR scheduling any time during the day and 72/105 (68.6%) exclusively use 1.5-T strength platforms for fetal MRI, while only 7/105 (6.7%) use exclusively 3 T. Patient positioning is variable: supine, 40/106 (37.8%); left lateral decubitus, 22/106 (20.8%), and, patient's choice, 43/106 (40.6%). Of the centers responding, 51/104 (49.0%) require no particular fasting instructions, while 20/104 (19.2%) request the patient avoid caffeine before the scanning.

CONCLUSION

Logistical trends in performing fetal MRI may supplement the American College of Radiology's published technical standards and offer guidance to radiologists new to the field.

The development of the cloaca in the human embryo

Subdivision of cloaca into urogenital and anorectal passages has remained controversial because of disagreements about the identity and role of the septum developing between both passages. This study aimed to clarify the development of the cloaca using a quantitative 3D morphological approach in human embryos of 4–10 post‐fertilisation weeks. Embryos were visualised with Amira 3D‐reconstruction and Cinema 4D‐remodelling software. Distances between landmarks were computed with Amira3D software. Our main finding was a pronounced difference in growth between rapidly expanding central and ventral parts, and slowly or non‐growing cranial and dorsal parts. The entrance of the Wolffian duct into the cloaca proved a stable landmark that remained linked to the position of vertebra S3. Suppressed growth in the cranial cloaca resulted in an apparent craniodorsal migration of the entrance of the Wolffian duct, while suppressed growth in the dorsal cloaca changed the entrance of the hindgut from cranial to dorsal on the cloaca. Transformation of this ‘end‐to‐end’ into an ‘end‐to‐side’ junction produced temporary ‘lateral (Rathke's) folds’. The persistent difference in dorsoventral growth straightened the embryonic caudal body axis and concomitantly extended the frontally oriented ‘urorectal (Tourneux's) septum’ caudally between the ventral urogenital and dorsal anorectal parts of the cloaca. The dorsoventral growth difference also divided the cloacal membrane into a well‐developed ventral urethral plate and a thin dorsal cloacal membrane proper, which ruptured at 6.5 weeks. The expansion of the pericloacal mesenchyme followed the dorsoventral growth difference and produced the genital tubercle. Dysregulation of dorsal cloacal development is probably an important cause of anorectal malformations: too little regressive development may result in anorectal agenesis, and too much regression in stenosis or atresia of the remaining part of the dorsal cloaca.

Decreased rectal meconium signal on MRI in fetuses with open spinal dysraphism

OBJECTIVE

To evaluate rectal meconium signal in fetuses with open spinal dysraphism and correlate findings with postnatal exam.

METHODS

This is a single-institution Institutional Review Board-approved Health Insurance Portability and Accountability Act (HIPAA) compliant retrospective analysis of fetal MRIs of open spinal dysraphism from 2004 to 2016. Fetuses with diagnostic T1-weighted images and postnatal follow-up at our institution were included.

RESULTS

A total of 115 fetuses (average gestational age 23.9 ± 3.6 weeks) met inclusion criteria. Of these, 80% (92/115) had T1 hyperintense rectal meconium signal. Average height of the meconium column, measured from the base of the bladder to its most inferior extent, was 9.2 ± 4.3 mm in fetuses ≥20-week gestational age and 11.1 ± 4.4 mm in fetuses ≥23-week gestational age (n = 110) . None had bowel dilation. One of 115 fetuses had a simple form of anorectal malformation allowing complete repair in the neonatal period, but this fetus had a normal meconium column height on fetal MRI of 22 mm. The remaining 23/115 fetuses with lack of normal rectal meconium signal were born without evidence of anorectal malformation.

CONCLUSION

Decreased or absent T1-hyperintense rectal meconium signal in fetuses with open spinal dysraphism does not correlate with imperforate anus postnatal and may be a reflection of neurogenic bowel in this patient population.

The development of the dorsal mesentery in human embryos and fetuses

The vertebrate intestine has a continuous dorsal mesentery between pharynx and anus that facilitates intestinal mobility. Based on width and fate the dorsal mesentery can be subdivided into that of the caudal foregut, midgut, and hindgut. The dorsal mesentery of stomach and duodenum is wide and topographically complex due to strong and asymmetric growth of the stomach. The associated formation of the lesser sac partitions the dorsal mesentery into the right-sided "caval fold" that serves as conduit for the inferior caval vein and the left-sided mesogastrium. The thin dorsal mesentery of the midgut originates between the base of the superior and inferior mesenteric arteries, and follows the transient increase in intestinal growth that results in small-intestinal looping, intestinal herniation and, subsequently, return. The following fixation of a large portion of the abdominal dorsal mesentery to the dorsal peritoneal wall by adhesion and fusion is only seen in primates and is often incomplete. Adhesion and fusion of mesothelial surfaces in the lesser pelvis results in the formation of the "mesorectum". Whether Toldt's and Denonvilliers' "fasciae of fusion" identify the location of the original mesothelial surfaces or, alternatively, represent the effects of postnatal wear and tear due to intestinal motility and intra-abdominal pressure changes, remains to be shown. "Malrotations" are characterized by growth defects of the intestinal loops with an ischemic origin and a narrow mesenteric root due to insufficient adhesion and fusion.

How to Explore Fetal Sacral Agenesis Without Open Dysraphism: Key Prenatal Imaging and Clinical Implications

The estimated prevalence of fetal caudal dysgenesis is 1 per 100,000 births. The functional prognosis of sacral agenesis is dominated by the large spectrum of associated caudal malformations. Except for cases associated with hydrocephalus secondary to open spinal dysraphism or chromosomal anomalies, association with mental deficiency is rare. We propose a systematic prenatal approach to cases of fetal sacral agenesis based on 9 etiologic items: clinical context, type of sacral dysgenesis, associated spinal cord malformations, mobility of lower limbs, investigation of the presacral region, analysis of the gastrointestinal tract, analysis of the genitourinary tract, associated vertebral defects, and cytogenetic analysis.

Fetal MRI in management of complicated meconium ileus: Prenatal and surgical imaging

OBJECTIVE

To review fetal MRI cases surgically proven to have meconium ileus (MI) and obstruction, describe the common fetal MRI findings that distinguish cases of complicated MI, and to compare these findings with surgical images and perinatal outcomes.

METHOD

We performed a retrospective review of all fetal MRI examinations and the corresponding medical record from our tertiary care children's hospital over an 18-month period. Postnatal management and outcomes were reviewed for these patients, and those patients with surgical or postmortem diagnosis of complicated MI were included in the study.

RESULTS

Our analysis revealed 7 cases. In this cohort, 3 imaging features of the fetal bowel were repeatedly seen: gradient appearance of intraluminal bowel contents, abnormally localized meconium signal, and collapsed appearance of the colon on MRI. Surgical diagnoses confirmed MI. All live-born infants underwent surgical repair.

CONCLUSION

Fetal MRI should be included in the diagnostic algorithm of any pregnancy where fetal bowel obstruction is suspected to better risk stratify patients.

Fetal anterior abdominal wall defects: prenatal imaging by magnetic resonance imaging

Abdominal wall defects range from the mild umbilical cord hernia to the highly complex limb-body wall syndrome. The most common defects are gastroschisis and omphalocele, and the rarer ones include the exstrophy complex, pentalogy of Cantrell and limb-body wall syndrome. Although all have a common feature of viscera herniation through a defect in the anterior body wall, their imaging features and, more important, postnatal management, differ widely. Correct diagnosis of each entity is imperative in order to achieve appropriate and accurate prenatal counseling and postnatal management. In this paper, we discuss fetal abdominal wall defects and present diagnostic pearls to aid with diagnosis.

Growth of the colon and rectum throughout gestation: evaluation with fetal MRI

Background

Congenital abnormalities of the gastrointestinal tract are increasingly being evaluated by prenatal magnetic resonance imaging (MRI). However, there is a paucity of reports describing the normal quantitative development of the fetal colon and rectum on MRI.

Purpose

To provide growth curves of the MRI estimated diameter of the fetal colon and rectum as a function of gestational age.

Material and Methods

This is a retrospective review of 191 singleton fetal MRI studies at 25–39 weeks of gestation. Measurements included maximal diameter of the ascending, transverse, and descending colon on coronal and sagittal views, maximal diameter of the rectum on coronal and sagittal views, and maximal diameter of the rectum at the level of the bladder base on sagittal views. Median growth curves were built using a generalized additive model. Confidence regions were built for 10th, 25th, 75th, and 90th percentiles.

Results

Smoothed growth curves for the median, and one and three quartiles for each of the five sections as a function of gestational age were calculated. All graphs had a slightly exponential curve.

Conclusion

This study provides normal ranges of the prenatal colon and rectum as a function of gestational age. They may serve as reference values when interpreting fetal MRI.

Fetal Body Imaging: When is MRI Indicated?

Recent improvements in fetal therapies and perinatal care and the multidisciplinary involvement in fetal medicine have increased the demand for more accurate prenatal diagnosis. Fetal Magnetic Resonance Imaging (MRI) is a complementary imaging technique for the assessment of thoraco-abdominal anomalies for which Ultrasonography (US) is not conclusive. It is indicated in selected situations to precise the prognosis in diaphragmatic hernia, to characterise the nature and extension of a pulmonary malformation, to locate a bowel atresia or to better depict an abdominal cystic lesion or tumoural extension. It has become a mandatory complementary diagnostic tool and improves the management of the fetuses and newborns.

Fetal MRI compared with ultrasound for the diagnosis of obstructive genital malformations

OBJECTIVE

To compare the accuracy of magnetic resonance imaging (MRI) and ultrasound (US) to diagnose and characterize congenital obstructive genital abnormalities.

METHOD

Retrospective cohort of 20 fetuses who underwent a fetal MRI following a US diagnosis of obstructive urogenital malformation. We compared MRI and US findings and their correlation with the definitive diagnosis.

RESULT

The correct diagnosis was obtained in 6/20 (30%) cases and 19/20 cases (95%) with US and MRI, respectively. MRI revealed additional information to US in 15/20 cases (75%) and modified the prenatal management in 14 fetuses (70%). The identification rates of the most important anatomical landmarks for the diagnosis, using US and MRI, were compared. Bladder: US 17/20 (85%) vs MRI 20/20 (100%) P = 0.23; vagina: US 6/19 (31.5%) vs MRI 19/19 (100%) P < 10^-4 ; uterus: US 11/19 (57.8%) vs MRI 19/19 (100%) P = 0.003, kidneys: US: 40/40 (100%) MRI: 40/40 (100%) P = 1, ureters: US 14/40 (35%) vs MRI 30/40 (75%) P=0.001, rectum: US 6/20 (30%) MRI 20/20 (100%) P < 10^-4 , and sacrum: US 20/20 (100%) vs MRI 17/20 (85%) P = 0.23.

CONCLUSION

In fetuses with obstructive urogenital malformations, MRI facilitates assessment of major pelvic organs and provides significant information that may alter the prenatal management.

Prenatal magnetic resonance and ultrasonographic findings in small-bowel obstruction: imaging clues and postnatal outcomes

BACKGROUND

Prenatal small-bowel obstruction can result from single or multiple atresias, and it can be an isolated abnormality or part of a syndrome. It is sometimes the first manifestation of cystic fibrosis. Accurate prediction of the level of obstruction and length of bowel affected can be difficult, presenting a challenge for counseling families and planning perinatal management.

OBJECTIVE

To review the prenatal US and MRI findings of small-bowel obstruction and to assess whether fetal MRI adds information that could improve prenatal counseling and perinatal management.

MATERIALS AND METHODS

We retrospectively reviewed 12 prenatally diagnosed cases of small-bowel obstruction evaluated by both US and MRI from 2005 to 2015. We analyzed gestational age at evaluation, US and MRI findings, gestational age at delivery and postnatal outcomes.

RESULTS

The final diagnoses were jejunal atresia (7), ileal atresia (1), cystic fibrosis (3) and combined jejunal and anal atresia (1). Four of the eight with jejunal atresia were found to have multiple small-bowel atresias. Prenatal perforation was noted in three. We identified a trend of increasing complexity of bowel contents corresponding to progressively distal level of obstruction, as indicated by increasing US echogenicity and high T1 signal on MRI. Seven cases of jejunal atresia and one case of ileal atresia demonstrated small ascending, transverse and descending colon (microcolon) with filling of a normal-diameter rectum. In contrast, all three fetuses with cystic fibrosis and the fetus with jejunal-anal atresia demonstrated microcolon as well as abnormal paucity or absence of rectal meconium. Polyhydramnios was present in nine. Eight were delivered prematurely, of whom seven had polyhydramnios. The fetus with jejunal and anal atresia died in utero. Postnatally, three had short gut syndrome, all resulting from multiple jejunal atresias; these three were among a subset of four fetuses whose bowel diameter measured more than 3 cm. Eight infants had no further gastrointestinal complications. The presence of multiple atresias was not predicted by prenatal US or MRI.

CONCLUSION

MR provides useful additional information regarding meconium distribution in the small bowel, which helps to clarify the level of obstruction. MR was additionally useful in the assessment of colon and rectal contents, serving as a fetal enema. Abnormally diminished meconium in the rectum suggests cystic fibrosis or combined small-bowel and colonic obstruction, information that is useful in counseling and preparing for postnatal care.

[Fetal MRI practices in a university prenatal center]

OBJECTIVES

Fetal MRI is a third intention examination to prenatal diagnosis. If its diagnostic value is well known in many pathologies, its place in the management of pregnancies remains unclear.

METHODS

We collected retrospectively demographical, radiological (fetal MRI indications, fetal anatomical region and diagnostic information provided by fetal MRI) and obstetrical data of pregnant patients in university prenatal center during a 5 years' period.

RESULTS

Among 2439 patients of the prenatal center, 196 (8%) patients with fetal MRI were included. The main anatomical regions studied were the brain (n=132, 67%), the thorax (n=31, 16%) and the abdomen (n=25, 13%). No cardiac fetal MRI was performed. Ninety-five percent of fetal MRI was consecutively of an ultrasound sign. Fetal brain MRI was abnormal in 65% of cases, the thoracic and abdominopelvic MRI in 81.5%. The ultrasound diagnosis was unchanged in 42%, completed in 50% and redirected in 8% of cases. A termination of pregnancy was deemed admissible in 31% of patients with MRI versus 21% in patients without MRI (P=0.001).

CONCLUSION

Fetal MRI requires selective indications and provides additional diagnostic information with important implications for the future of the pregnancy, particularly in case of severe and incurable pathologies. Our results could be useful as a reference basis for the comparison with others prenatal center practices.

Fetal MR Imaging of Gastrointestinal Abnormalities

Fetal magnetic resonance (MR) imaging plays an increasing and valuable role in antenatal diagnosis and perinatal management of fetal gastrointestinal (GI) abnormalities. Advances in MR imaging data acquisition and use of motion-insensitive techniques have established MR imaging as an important adjunct to obstetric ultrasonography (US) for fetal diagnosis. In this regard, MR imaging provides high diagnostic accuracy for antenatal diagnosis of common and uncommon GI pathologic conditions. In the setting of fetal GI disease, T1-weighted images demonstrate the amount and distribution of meconium, which is crucial to the diagnostic capability of fetal MR imaging. Specifically, knowledge of the T1 signal intensity characteristics of fetal meconium, the normal pattern of meconium with advancing gestational age, and the expected caliber of small and large bowel in the fetus is key to diagnosis of abnormalities of the GI tract. Use of ultrafast T2-weighted sequences for evaluation of the expected location and morphology of fluid-containing structures, including the stomach and small bowel, in the fetal abdomen further aids in diagnostic confidence. Uncommonly encountered fetal GI pathologic conditions, especially cloacal dysmorphology, may demonstrate characteristic MR imaging patterns, which may add additional information to that from fetal US, allowing improved fetal and neonatal management. This article discusses common indications for fetal MR imaging of the GI tract, imaging protocols for fetal GI MR imaging, the normal appearance of the fetal GI tract with advancing gestational age, and the imaging appearances of common fetal GI abnormalities, as well as uncommon fetal GI conditions with characteristic appearances. (©)RSNA, 2016.

Magnetic resonance imaging of fetal pelvic cysts

The detection of fetal anomalies has improved in the last years as a result of the generalization of ultrasound pregnancy screening exams. The presence of a cystic imaging in the fetal pelvis is a relatively common finding, which can correspond to a real congenital cystic lesion or result from the anomalous liquid accumulation in a whole pelvic organ, mainly the urinary bladder, the uterus, or the vagina. In selected cases with poor prognosis and/or inconclusive echographic findings, magnetic resonance may bring additional information in terms of the characterization, anatomical location, and real extension of the pathology. This pictorial essay describes the normal pelvic fetal anatomy, as well as the most common pelvic cysts. It also describes the causes of an anomalous distension of the whole pelvic organs detected in utero, with emphasis on prenatal magnetic resonance imaging exams. Moreover, it proposes practical teaching points to reduce the differential diagnosis of these lesions based on the sex of the fetus, the division of the pelvis in anatomical spaces, and the imaging findings of the pathology. Finally, it discusses the real utility of complementary MRI.

Role of intraluminal bowel echogenicity on prenatal ultrasounds to determine the anatomical level of intestinal atresia

OBJECTIVE

To evaluate the correlation between different degrees of bowel intraluminal echogenicity showed by prenatal ultrasounds and the anatomic level of intestinal atresia.

METHODS

We report three cases of intestinal atresia at different intestinal levels verified during the neonatal surgery with specific ultrasonographic prenatal features. Intensity of sonolucency was analyzed using the image-processing program ImageJ for quantitative measurements based on the gray-scale intensity values.

RESULTS

A total of three cases are reported, a jejunal, an ileal and a colonic atresia. All cases showed intestinal dilatation. Both, jejunal and ileal atresia, showed two degrees of hypoechoic intestinal content, while colonic atresia showed hyperechogenic content dilated loop at prenatal ultrasound scan.

CONCLUSIONS

We propose the use of prenatal ultrasounds echogenicity of intestinal dilated loop fluid content to help in determining the level of obstruction in bowel atresia. These are initial results, to be confirmed by a multicentric research with more cases.

Prenatal diagnosis of bowel malposition using T2-weighted fetal MRI sequences

OBJECTIVE

The goal of this study was to investigate the capability of T2-weighted magnetic resonance imaging (MRI) in revealing fetal bowel malposition.

MATERIALS AND METHODS

All fetal MRI examinations (excluding central nervous system MRI examinations) performed in our department from January 2005 to January 2014 were retrospectively studied by 2 independent observers for situs, stomach and jejunum location on T2-weighted images. Patients data were also reviewed for results of ultrasound examinations, MRI indication, and gestational age. Abnormally positioned jejunums were classified into 3 groups: intrathoracic (A), extra-fetal (B) and abnormal intra-fetal (C). Prenatal data were compared to postnatal imaging, surgery or autopsy findings that served as standard of reference.

RESULTS

A total of 709 fetal MRI examinations were analyzed. In 64 fetus (9%), the jejunum was not present in the left subgastric area on T2-weighted MR images. In these 64 fetuses, proximal jejunum was intrathoracic (41/64, 64%, group A), extra-fetal (11/64, 17%, group B), or intra-abdominal but abnormally positioned (12/64, 19%, group C). Interobserver agreement was 100%. All diagnoses for fetuses in groups A and B (52 cases) were confirmed postnatally (41 cases) or at autopsy (11 cases). In group C, bowel malposition was suspected after ultrasound in only 2/12 fetuses (16.6%); it was confirmed postnatally in 1 fetus but not confirmed in the remaining one. In the 10 remaining fetuses (83%), malposition was confirmed postnatally although not initially suspected.

CONCLUSION

T2-weighted fetal MR images are useful for the prenatal diagnosis of bowel malposition, even when they are unsuspected on ultrasound examination.

Prenatal Imaging of the Gastrointestinal Tract with Postnatal Imaging Correlation

Prenatal detection of a wide variety of anomalies and masses of the gastrointestinal tract is now possible. Prenatal imaging with ultrasonography and in selected cases magnetic resonance imaging provides invaluable information to the referring obstetrician, the maternal fetal medicine specialist, the neonatologist and pediatrician who will care for the child after birth, the surgeons and pediatric specialists who will repair or manage a prenatally detected anomaly, and of course to the parents, allowing them to prepare psychologically and financially for the specific interventions that may be needed for their child. Additional screening for associated anomalies can take place, route of delivery can be decided, and arrangements for delivery in an appropriate setting can be made. Prenatal detection also allows for consideration for pregnancy termination. This article will give a broad overview of anomalies of the gastrointestinal tract that can be detected prenatally and their imaging appearance postnatally.

Imaging Evaluation of Fetal Megacystis: How Can Magnetic Resonance Imaging Help?

Evaluation of the kidneys, bladder, and amniotic fluid volume forms part of any standard obstetrical ultrasound. When a fetal genitourinary anomaly is suspected, a more detailed evaluation is necessary. This detailed imaging can be challenging in the setting of decreased or absent amniotic fluid or large maternal body habitus, and in complex malformations. In these situations, magnetic resonance imaging can help to better define the fetal anatomy and provide a more confident and specific prenatal diagnosis.

The clinical relevance of fetal MRI in the diagnosis of Type IV cystic sacrococcygeal teratoma--a review

The introduction of fetal magnetic resonance imaging (MRI) has improved the prenatal evaluation of uterine, placental and fetal anatomy. However, its utilization has mostly been restricted to fetal central nervous system anomalies. We review how adjunct fetal MRI was performed and diagnosis of cystic type IV sacrococcygeal teratoma was made. We also discuss the clinical relevance of fetal MRI in differentiating this lesion from other selected abdominal/pelvic cystic malformations and lesions.

Role of magnetic resonance imaging in the prenatal diagnosis of gastrointestinal fetal anomalies

PURPOSE

This study was done to evaluate the role of fetal magnetic resonance imaging (MRI) in the study of gastrointestinal malformations in comparison to prenatal ultrasound (US).

MATERIALS AND METHODS

A prospective (2010-2012) study of 38 fetal MRI scans was performed on 38 fetuses between 24 and 38 weeks of gestation. All the fetuses had a US diagnosis of gastrointestinal anomalies. T2-weighted HASTE, T1-weighted fast gradient echo, TrueFISP and diffusion-weighted images of the fetal abdomen were obtained on a 1.5-Tesla magnet. All fetal MRI diagnoses were compared with postnatal US findings, autopsy or surgical reports.

RESULTS

Fetal MRI was able to confirm the sonographic findings in nine of 38 fetuses (23.7%), to provide additional information in 23 of 38 fetuses (60.6%), to exclude the US diagnosis in five cases (5.2%) and to change it in two cases (5.2%). It was not able to characterize a case of gastric duplication and a case of abdominal cystic lymphangioma (5.2%).

CONCLUSIONS

Fetal MRI can be used as a complementary imaging modality to US in prenatal evaluation of gastrointestinal anomalies and can be considered a valuable tool not only for confirming or excluding but also for providing additional information to fetal ultrasonographic findings.

Fetal magnetic resonance imaging: jumping from 1.5 to 3 tesla (preliminary experience)

Several attempts have been made at imaging the fetus at 3 T as part of the continuous search for increased image signal and better anatomical delineation of the developing fetus. Until very recently, imaging of the fetus at 3 T has been disappointing, with numerous artifacts impeding image analysis. Better magnets and coils and improved technology now allow imaging of the fetus at greater magnetic strength, some hurdles in the shape of imaging artifacts notwithstanding. In this paper we present the preliminary experience of evaluating the developing fetus at 3 T and discuss several artifacts encountered and techniques to decrease them, as well as safety concerns associated with scanning the fetus at higher magnetic strength.