In vivo relaxation time measurements in the human placenta using echo planar imaging at 0.5 T

Comparison of 2D and 3D oxygen-enhanced MRI of the placenta

Oxygen-Enhanced Magnetic Resonance Imaging (OE-MRI) of the human placenta is potentially a sensitive marker of in vivo oxygenation. This methodological study shows that full coverage of the placenta is possible using 3D mapping of the change in longitudinal relaxation rate (ΔR1), in a group of healthy pregnant subjects breathing elevated levels of oxygen. Twelve pregnant subjects underwent a comparison of 2D and 3D OE-MRI. ΔR1 was mapped for a single 2D slice (ss-2D), a single matched-slice from the 3D volume (ss-3D) and the full 3D volume (vol-3D). The group-average median ΔR1 values for ss-3D (0.023 s-1) and vol-3D (0.022 s-1) do not differ significantly from ss-2D (0.020 s-1), when compared using a two-tailed paired t-test (ss-3D (p = 0.58) and vol-3D (p = 0.70)). However, median baseline T1 (T1b) for ss-2D was higher (1603 ms) than T1b for ss-3D (1540 ms, p = 0.07) and significantly higher than vol-3D (1515 ms, p = 0.02), when compared using a two-tailed paired t-test. In contrast with previous studies, no correlation of median ΔR1 with gestation age at scan for the normal group (N = 10) was observed for ss-2D, likely due to the smaller gestational range. Full volume OE-MRI maps reveal sensitivity to changes in ΔR1, with some participants showing an enhanced gradient in the intermediate space between the fetal and maternal sides of the placenta in the 3D data. This study shows that it is feasible to acquire whole placental volume OE-MRI data in women with healthy pregnancy.

The effect of maternal position on placental blood flow and fetoplacental oxygenation in late gestation fetal growth restriction: a magnetic resonance imaging study

Fetal growth restriction (FGR) and maternal supine going-to-sleep position are both risk factors for late stillbirth. This study aimed to use magnetic resonance imaging (MRI) to quantify the effect of maternal supine position on maternal-placental and fetoplacental blood flow, placental oxygen transfer and fetal oxygenation in FGR and healthy pregnancies. Twelve women with FGR and 27 women with healthy pregnancies at 34-38 weeks' gestation underwent MRI in both left lateral and supine positions. Phase-contrast MRI and a functional MRI technique (DECIDE) were used to measure blood flow in the maternal internal iliac arteries (IIAs) and umbilical vein (UV), placental oxygen transfer (placental flux), fetal oxygen saturation (FO2 ), and fetal oxygen delivery (delivery flux). The presence of FGR, compared to healthy pregnancies, was associated with a 7.8% lower FO2 (P = 0.02), reduced placental flux, and reduced delivery flux. Maternal supine positioning caused a 3.8% reduction in FO2 (P = 0.001), and significant reductions in total IIA flow, placental flux, UV flow and delivery flux compared to maternal left lateral position. The effect of maternal supine position on fetal oxygen delivery was independent of FGR pregnancy, meaning that supine positioning has an additive effect of reducing fetal oxygenation further in women with FGR, compared to women with appropriately grown for age pregnancies. Meanwhile, the effect of maternal supine positioning on placental oxygen transfer was not independent of the effect of FGR. Therefore, growth-restricted fetuses, which are chronically hypoxaemic, experience a relatively greater decline in oxygen transfer when mothers lie supine in late gestation compared to appropriately growing fetuses. KEY POINTS: Fetal growth restriction (FGR) is the most common risk factor associated with stillbirth, and early recognition and timely delivery is vital to reduce this risk. Maternal supine going-to-sleep position is found to increase the risk of late stillbirth but when combined with having a FGR pregnancy, maternal supine position leads to 15 times greater odds of stillbirth compared to supine sleeping with appropriately grown for age (AGA) pregnancies. Using MRI, this study quantifies the chronic hypoxaemia experienced by growth-restricted fetuses due to 13.5% lower placental oxygen transfer and 26% lower fetal oxygen delivery compared to AGA fetuses. With maternal supine positioning, there is a 23% reduction in maternal-placental blood flow and a further 14% reduction in fetal oxygen delivery for both FGR and AGA pregnancies, but this effect is proportionally greater for growth-restricted fetuses. This knowledge emphasises the importance of avoiding supine positioning in late pregnancy, particularly for vulnerable FGR pregnancies.

Reliability and Feasibility of Low-Field-Strength Fetal MRI at 0.55 T during Pregnancy

Background The benefits of using low-field-strength fetal MRI to evaluate antenatal development include reduced image artifacts, increased comfort, larger bore size, and potentially reduced costs, but studies about fetal low-field-strength MRI are lacking. Purpose To evaluate the reliability and feasibility of low-field-strength fetal MRI to assess anatomic and functional measures in pregnant participants using a commercially available 0.55-T MRI scanner and a comprehensive 20-minute protocol. Materials and Methods This prospective study was performed at a large teaching hospital (St Thomas' Hospital; London, England) from May to November 2022 in healthy pregnant participants and participants with pregnancy-related abnormalities using a commercially available 0.55-T MRI scanner. A 20-minute protocol was acquired including anatomic T2-weighted fast-spin-echo, quantitative T2*, and diffusion sequences. Key measures like biparietal diameter, transcerebellar diameter, lung volume, and cervical length were evaluated by two radiologists and an MRI-experienced obstetrician. Functional organ-specific mean values were given. Comparison was performed with existing published values and higher-field MRI using linear regression, interobserver correlation, and Bland-Altman plots. Results A total of 79 fetal MRI examinations were performed (mean gestational age, 29.4 weeks ± 5.5 [SD] [age range, 17.6-39.3 weeks]; maternal age, 34.4 years ± 5.3 [age range, 18.4-45.5 years]) in 47 healthy pregnant participants (control participants) and in 32 participants with pregnancy-related abnormalities. The key anatomic two-dimensional measures for the 47 healthy participants agreed with large cross-sectional 1.5-T and 3-T control studies. The interobserver correlations for the biparietal diameter in the first 40 consecutive scans were 0.96 (95% CI: 0.7, 0.99; P = .002) for abnormalities and 0.93 (95% CI: 0.86, 0.97; P < .001) for control participants. Functional features, including placental and brain T2* and placental apparent diffusion coefficient values, strongly correlated with gestational age (mean placental T2* in the control participants: 5.2 msec of decay per week; R2 = 0.66; mean T2* at 30 weeks, 176.6 msec; P < .001). Conclusion The 20-minute low-field-strength fetal MRI examination protocol was capable of producing reliable structural and functional measures of the fetus and placenta in pregnancy. Clinical trial registration no. REC 21/LO/0742 © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Gowland in this issue.

Low-Field Combined Diffusion-Relaxation MRI for Mapping Placenta Structure and Function

Purpose

Demonstrating quantitative multi-parametric mapping in the placenta with combined T 2 ∗ -diffusion MRI at low-field (0.55T).

Methods

We present 57 placental MRI scans performed on a commercially available 0.55T scanner. We acquired the images using a combined T 2 ∗ -diffusion technique scan that simultaneously acquires multiple diffusion preparations and echo times. We processed the data to produce quantitative T 2 ∗ and diffusivity maps using a combined T 2 ∗ -ADC model. We compared the derived quantitative parameters across gestation in healthy controls and a cohort of clinical cases.

Results

Quantitative parameter maps closely resemble those from previous experiments at higher field strength, with similar trends in T 2 ∗ and ADC against gestational age observed.

Conclusion

Combined T 2 ∗ -diffusion placental MRI is reliably achievable at 0.55T. The advantages of lower field strength - such as cost, ease of deployment, increased accessibility and patient comfort due to the wider bore, and increased T 2 ∗ for larger dynamic ranges - can support the widespread roll out of placental MRI as an adjunct to ultrasound during pregnancy.

Placental volume, thickness and transverse relaxation time (T2*) estimated by magnetic resonance imaging in relation to small for gestational age at birth

INTRODUCTION

Placental magnetic resonance imaging (MRI) may be a valuable tool in the prediction of small for gestational age (SGA) at birth. MRI provides reliable estimates of placental volume and thickness. In addition, placental transverse relaxation time (T2*) may be directly related to placental function. This study aimed to explore and compare the predictive performance of three placental MRI parameters - volume, thickness and T2* - in relation to SGA at birth.

METHODS

A mixed cohort of 85 pregnancies was retrieved from the placental MRI database at the study hospital. MRI was performed in a 1.5 T system at gestational weeks 15-41. In normal birthweight (BW) pregnancies [BW > -22 % of expected for gestational age (GA)], the correlation between each of the MRI parameters and GA was investigated by linear regression. The prediction of SGA was investigated by logistic regression analysis adjusted for GA at MRI.

RESULTS

In normal BW pregnancies, a significant linear correlation was found between GA and each of the MRI parameters. Univariate analysis demonstrated that placental volume [odds ratio (OR) 0.97, p = 0.001] and placental T2* (OR 0.79, p = 0.003), but not placental thickness (OR 0.92, p = 0.862) were significant predictors of SGA. A multi-variate model including all three MRI parameters found that placental T2* was the only independent predictor of SGA (OR 0.81, p = 0.04).

CONCLUSION

Among the MRI parameters investigated in this study, placental T2* was the only independent predictor of SGA in a multi-variate model. This finding underlines the strong position of T2*-weighted placental MRI in the prediction of SGA.

T2* weighted fetal MRI and the correlation with placental dysfunction

INTRODUCTION

Transverse relaxation time (T2*) is related to tissue oxygenation and morphology. We aimed to describe T2* weighted MRI in selected fetal organs in normal pregnancies, and to investigate the correlation between fetal organ T2* and placental T2*, birthweight (BW) deviation, and redistribution of fetal blood flow.

METHODS

T2*-weighted MRI was performed in 126 singleton pregnancies between 23+6- and 41+3-weeks' gestation. The T2* value was obtained from the placenta and fetal organs (brain, lungs, heart, liver, kidneys, and spleen). In normal BW pregnancies (BW > 10th centile), the correlation between the T2* value and gestational age (GA) at MRI was estimated by linear regression. The correlation between fetal organ Z-score and BW group was demonstrated by boxplots and investigated by analysis of variance (ANOVA) for each organ.

RESULTS

In normal BW pregnancies fetal organ T2* was negatively correlated with GA. We found a significant correlation between BW group and fetal organ T2* z-score in the fetal heart, kidney, lung and spleen. A positive linear correlation was demonstrated between fetal organ T2* and outcomes related to placental function such as BW deviation and placenta T2* in all investigated fetal organs except for the fetal liver. In the fetal heart, kidneys, and spleen the T2* value showed a significant correlation with fetal redistribution of blood flow (Middle cerebral artery Pulsatility Index) before delivery.

DISCUSSION

Fetal T2* is correlated with BW, placental function, and redistribution of fetal blood flow, suggesting that fetal organ T2* reflects fetal oxygenation and morphological changes related to placental dysfunction.

Risk Stratification for Pregnancies Diagnosed With Fetal Growth Restriction Based on Placental MRI

BACKGROUND

Diagnosis of fetal growth restriction (FGR) entails difficulties with differentiating fetuses not fulfilling their growth potential because of pathologic conditions, such as placental insufficiency, from constitutionally small fetuses. The feasibility of placental MRI for risk stratification among pregnancies diagnosed with FGR remains unexplored.

PURPOSE

To explore quantitative MRI features useful to identify pregnancies with unfavorable outcomes and to assess the diagnostic performance of visual analysis of MRI to detect pregnancies with unfavorable outcomes, among pregnancies diagnosed with FGR.

STUDY TYPE

Retrospective.

POPULATION

Thirteen pregnancies with unfavorable outcomes (preterm emergency cesarean section or intrauterine fetal death) and 11 pregnancies with favorable outcomes performed MRI at gestational weeks 21-36.

FIELD STRENGTH/SEQUENCE

A 5-T, half-Fourier-acquired single-shot turbo spin echo (HASTE), spin-echo echo-planar imaging (SE-EPI) and T2 map derived from SE-EPI.

ASSESSMENT

Placental size on HASTE sequences and T2 mapping-based histogram features were extracted. Three radiologists qualitatively evaluated the visibility of maternal cotyledon on HASTE and SE-EPI sequences with echo times (TEs) = 60, 90, and 120 msec using 3-point Likert scales: 0, absent; 1, equivocal; and 2, present.

STATISTICAL TESTS

Welch's t-test or Mann-Whitney U test for quantitative features between the favorable and unfavorable outcome groups. Areas under the receiver operating curves (AUCs) of the three readers' visual analyses to detect pregnancies with unfavorable outcomes. A P value of <0.05 was inferred as statistically significant.

RESULTS

Placental size (major and minor axis, estimated area of placental bed, and volume of placenta) and T2 mapping-based histogram features (mean, skewness, and kurtosis) were statistically significantly different between the two groups. Visual analysis of HASTE and SE-EPI with TE = 60 msec showed AUCs of 0.80-0.86 to detect pregnancies with unfavorable outcomes.

DATA CONCLUSION

Placental size, histogram features, and visual analysis of placental MRI may allow for risk stratification regarding outcomes among pregnancies diagnosed with FGR.

EVIDENCE LEVEL

3 TECHNICAL EFFICACY: Stage 5.

Visual assessment of the placenta in antenatal magnetic resonance imaging across gestation in normal and compromised pregnancies: Observations from a large cohort study

INTRODUCTION

Visual assessment of the placenta in antenatal magnetic resonance imaging is important to confirm healthy appearances or to identify pathology complicating fetal anomaly or maternal disease.

METHODS

We assessed the placenta in a large cohort of 228 women with low and high risk pregnancies across gestation. All women gave written informed consent and were imaged using either a 3T Philips Achieva or 1.5T Philips Ingenia scanner. Images were acquired with a T2-weighted single shot turbo spin echo sequence of the whole uterus (thereby including placenta) for anatomical information.

RESULTS

A structured approach to visual assessment of the placenta on T2-weighted imaging has been provided including determination of key anatomical landmarks to aid orientation, placental shape, signal intensity, lobularity and granularity. Transient factors affecting imaging are shown including the effect of fetal movement, gross fetal motion and contractions. Placental appearances across gestation in low risk pregnancies are shown and compared to pregnancies complicated by preeclampsia and chronic hypertension. The utility of other magnetic resonance techniques (T2* mapping as an indirect marker for quantifying oxygenation) and histological assessment alongside visual assessment of placental T2-weighted imaging are demonstrated.

DISCUSSION

A systematic approach with qualitative descriptors for placental visual assessment using T2-weighted imaging allows confirmation of normal placental development and can detect placental abnormalities in pregnancy complications. T2-weighted imaging can be visually assessed alongside functional imaging (such as T2* maps) in order to further probe the visual characteristics seen.

Emerging placental biomarkers of health and disease through advanced magnetic resonance imaging (MRI)

Placental dysfunction is a major cause of fetal demise, fetal growth restriction, and preterm birth, as well as significant maternal morbidity and mortality. Infant survivors of placental dysfunction are at elevatedrisk for lifelong neuropsychiatric morbidity. However, despite the significant consequences of placental disease, there are no clinical tools to directly and non-invasively assess and measure placental function in pregnancy. In this work, we will review advanced MRI techniques applied to the study of the in vivo human placenta in order to better detail placental structure, architecture, and function. We will discuss the potential of these measures to serve as optimal biomarkers of placental dysfunction and review the evidence of these tools in the discrimination of health and disease in pregnancy. Efforts to advance our understanding of in vivo placental development are necessary if we are to optimize healthy pregnancy outcomes and prevent brain injury in successive generations. Current management of many high-risk pregnancies cannot address placental maldevelopment or injury, given the standard tools available to clinicians. Once accurate biomarkers of placental development and function are constructed, the subsequent steps will be to introduce maternal and fetal therapeutics targeting at optimizing placental function. Applying these biomarkers in future studies will allow for real-time assessments of safety and efficacy of novel interventions aimed at improving maternal-fetal well-being.

Quantitative T1 and T2 mapping by magnetic resonance fingerprinting (MRF) of the placenta before and after maternal hyperoxia

INTRODUCTION

MR relaxometry has been used to assess placental exchange function, but methods to date are not sufficiently fast to be robust to placental motion. Magnetic resonance fingerprinting (MRF) permits rapid, voxel-wise, intrinsically co-registered T₁ and T₂ mapping. After characterizing measurement error, we scanned pregnant women during air and oxygen breathing to demonstrate MRF's ability to detect placental oxygenation changes.

METHODS

The accuracy of FISP-based, sliding-window reconstructed MRF was tested on phantoms. MRF scans in 9-s breath holds were acquired at 3T in 31 pregnant women during air and oxygen breathing. A mixed effects model was used to test for changes in placenta relaxation times between physiological states, to assess the dependency on gestational age (GA), and the impact of placental motion.

RESULTS

MRF estimates of known phantom relaxation times resulted in mean absolute errors for T₁ of 92 ms (4.8%), but T₂ was less accurate at 16 ms (13.6%). During normoxia, placental T₁ = 1825 ± 141 ms (avg ± standard deviation) and T₂ = 60 ± 16 ms (gestational age range 24.3-36.7, median 32.6 weeks). In the statistical model, placental T₂ rose and T₁ remained contant after hyperoxia, and no GA dependency was observed for T₁ or T₂.

DISCUSSION

Well-characterized, motion-robust MRF was used to acquire T₁ and T₂ maps of the placenta. Changes with hyperoxia are consistent with a net increase in oxygen saturation. Toward the goal of whole-placenta quantitative oxygenation imaging over time, we aim to implement 3D MRF with integrated motion correction to improve T₂ accuracy.

MRI based morphological examination of the placenta

Ultrasound is widely used as the initial diagnostic imaging modality during pregnancy with both high spatial and temporal resolution. Although MRI in pregnancy has long focused on the fetus, its use in placental imaging has greatly increased over recent years. In addition to the possibilities of evaluating function, MRI with a wide field of view and high contrast resolution allows characterization of placental anatomy, particularly in situations that are difficult to specify with ultrasound, especially for suspected placenta accreta. MRI also appears to be a particularly useful examination for the anatomical evaluation of the placenta independent of maternal body habitus or fetal position. Indeed, surprisingly little attention is paid to the placenta in MRI when the indication for the examination is fetal. Thus, some aspects of the placenta seem to us to be important to be recognized by the radiologist and to be described on the MRI report. In this review, we will describe MRI sequences used for, and common features seen in, imaging of i) the normal placenta, ii) abnormal aspects of the placenta that should be identified on MRI performed for fetal reason, and iii) placental anomalies for which placental MRI may be indicated.

T2* weighted placental MRI in relation to placental histology and birth weight

INTRODUCTION

Placental dysfunction may be found among normal birth weight (BW) pregnancies, as indicated by abnormal histological findings in postnatal placental examination in some of these pregnancies. T2* weighted placental MRI provides non-invasive information on placental oxygenation and thereby placental function. The aim of this study was to investigate the correlation between placental T2*, BW and placental histology.

METHODS

A total of 63 pregnant women underwent T2* weighted placental MRI at 15-40 week's gestation and a standardized placental histological examination (PHE). Abnormal PHE was defined by vascular malperfusion according to the Amsterdam workshop consensus. The correlation between PHE, BW z-score and T2* z-score was analyzed by logistic regression.

RESULTS

Abnormal PHE was revealed in 28 pregnancies. Multiple logistic regression revealed a significant correlation between abnormal PHE and T2* z-score (OR = 0.34, p = 0.008), whereas BW z-score did not add significantly to the correlation of placental histology (OR = 0.52, p = 0.115). In BW z-score≥0, PHE was normal in 100% of pregnancies. In BW z-score ≤ -2, PHE was abnormal in 89% of pregnancies. In intermediate BW (z-score between -2 and 0), PPE was abnormal in 35% of pregnancies. In this intermediate group, placental T2* z-score was reduced (-1.52 ± 1.22 (mean SD)) when compared to normal PHE pregnancies (-0.28 ± 1.17), p = 0.006.

DISCUSSION

This study demonstrates a correlation between abnormal placental histology and low placental T2* value regardless of fetal size. This indicates that T2* provides information of placental function in vivo even when fetal size is normal. This finding highlights that fetal size alone is not a valid marker of placental dysfunction.

Placental transverse relaxation time (T2) estimated by MRI: Normal values and the correlation with birthweight

INTRODUCTION

Placental transverse relaxation time (T2) assessed by MRI may have the potential to improve the antenatal identification of small for gestational age. The aims of this study were to provide normal values of placental T2 in relation to gestational age at the time of MRI and to explore the correlation between placental T2 and birthweight.

MATERIAL AND METHODS

A mixed cohort of 112 singleton pregnancies was retrieved from our placental MRI research database. MRI was performed at 23.6-41.3 weeks of gestation in a 1.5T system (TE (8): 50-440 ms, TR: 4000 ms). Normal pregnancies were defined by uncomplicated pregnancies with normal obstetric outcome and birthweight deviation within ±1 SD of the expected for gestational age. The correlation between placental T2 and birthweight was investigated using the following outcomes; small for gestational age (birthweight ≤-2 SD of the expected for gestational age) and birthweight deviation (birthweight Z-scores).

RESULTS

In normal pregnancies (n = 27), placenta T2 showed a significant negative linear correlation with gestational age (r = -.91, P = .0001) being 184 ms ± 15.94 ms (mean ± SD) at 20 weeks of gestation and 89 ms ± 15.94 ms at 40 weeks of gestation. Placental T2 was significantly reduced among small-for-gestational-age pregnancies (mean Z-score -1.95, P < .001). Moreover, we found a significant positive correlation between placenta T2 deviation (Z-score) and birthweight deviation (Z-score) (R²  = .26, P = .0001).

CONCLUSIONS

This study provides normal values of placental T2 to be used in future studies on placental MRI. Placental T2 is closely related to birthweight and may improve the antenatal identification of small-for-gestational-age pregnancies.

The application of in utero magnetic resonance imaging in the study of the metabolic and cardiovascular consequences of the developmental origins of health and disease

Observing fetal development in utero is vital to further the understanding of later-life diseases. Magnetic resonance imaging (MRI) offers a tool for obtaining a wealth of information about fetal growth, development, and programming not previously available using other methods. This review provides an overview of MRI techniques used to investigate the metabolic and cardiovascular consequences of the developmental origins of health and disease (DOHaD) hypothesis. These methods add to the understanding of the developing fetus by examining fetal growth and organ development, adipose tissue and body composition, fetal oximetry, placental microstructure, diffusion, perfusion, flow, and metabolism. MRI assessment of fetal growth, organ development, metabolism, and the amount of fetal adipose tissue could give early indicators of abnormal fetal development. Noninvasive fetal oximetry can accurately measure placental and fetal oxygenation, which improves current knowledge on placental function. Additionally, measuring deficiencies in the placenta’s transport of nutrients and oxygen is critical for optimizing treatment. Overall, the detailed structural and functional information provided by MRI is valuable in guiding future investigations of DOHaD.

Placental Magnetic Resonance Imaging: A Method to Evaluate Placental Function In Vivo

This article describes the use of placental magnetic resonance imaging (MRI) relaxation times in the in vivo assessment of placental function. It focuses on T2*-weighted placental MRI, the main area of the authors' research over the past decade. The rationale behind T2*-weighted placental MRI, the main findings reported in the literature, and directions for future research and clinical applications of this method are discussed. The article concludes that placental T2* relaxation time is an easily obtained and robust measurement, which can discriminate between normal and dysfunctional placenta. Placenta T2* is a promising tool for in vivo assessment of placental function.

Nonfetal Imaging During Pregnancy: Placental Disease

Placenta is a vital organ that connects the maternal and fetal circulations, allowing exchange of nutrients and gases between the two. In addition to the fetus, placenta is a key component to evaluate during any imaging performed during pregnancy. The most common disease processes involving the placenta include placenta accreta spectrum disorders and placental masses. Several systemic processes such as infection and fetal hydrops can too affect the placenta; however, their imaging features are nonspecific such as placental thickening, heterogeneity, and calcifications. Ultrasound is the first line of imaging during pregnancy, and MR imaging is reserved for problem solving, when there is need for higher anatomic resolution.

A time domain signal equation for multi-echo spin-echo sequences with arbitrary excitation and refocusing angle and phase

Accurate T₂ mapping using multi-echo spin-echo is usually impaired by non-ideal refocusing due to B₁+ inhomogeneities and slice profile effects. Incomplete refocusing gives rise to stimulated echo and so called "T₁-mixing" and consequently a non-exponential signal decay. Here we present a time domain formula that incorporates all relaxation and pulse parameters and enables accurate and realistic modelling of the magnetization decay curve. By pulse parameters here we specifically mean the actual refocusing angle and axis, and phase angle of both the excitation and refocusing pulse. The method used for derivation comprises the so called Generating functions approach with subsequent back-transformation to the time domain. The proposed approach was validated by simulations using realistic RF pulse shapes as well as by comparison to phantom measurements. Excellent agreement between simulations and measurements underpin the validity of the presented approach. Conclusively, we here present a complete time domain formula ready to use for accurate T₂ mapping with multi-echo spin-echo sequences.

Placental MRI: Developing Accurate Quantitative Measures of Oxygenation

The Human Placenta Project has focused attention on the need for noninvasive magnetic resonance imaging (MRI)-based techniques to diagnose and monitor placental function throughout pregnancy. The hope is that the management of placenta-related pathologies would be improved if physicians had more direct, real-time measures of placental health to guide clinical decision making. As oxygen alters signal intensity on MRI and oxygen transport is a key function of the placenta, many of the MRI methods under development are focused on quantifying oxygen transport or oxygen content of the placenta. For example, measurements from blood oxygen level-dependent imaging of the placenta during maternal hyperoxia correspond to outcomes in twin pregnancies, suggesting that some aspects of placental oxygen transport can be monitored by MRI. Additional methods are being developed to accurately quantify baseline placental oxygenation by MRI relaxometry. However, direct validation of placental MRI methods is challenging and therefore animal studies and ex vivo studies of human placentas are needed. Here we provide an overview of the current state of the art of oxygen transport and quantification with MRI. We suggest that as these techniques are being developed, increased focus be placed on ensuring they are robust and reliable across individuals and standardized to enable predictive diagnostic models to be generated from the data. The field is still several years away from establishing the clinical benefit of monitoring placental function in real time with MRI, but the promise of individual personalized diagnosis and monitoring of placental disease in real time continues to motivate this effort.

Exploring early human brain development with structural and physiological neuroimaging

Early brain development, from the embryonic period to infancy, is characterized by rapid structural and functional changes. These changes can be studied using structural and physiological neuroimaging methods. In order to optimally acquire and accurately interpret this data, concepts from adult neuroimaging cannot be directly transferred. Instead, one must have a basic understanding of fetal and neonatal structural and physiological brain development, and the important modulators of this process. Here, we first review the major developmental milestones of transient cerebral structures and structural connectivity (axonal connectivity) followed by a summary of the contributions from ex vivo and in vivo MRI. Next, we discuss the basic biology of neuronal circuitry development (synaptic connectivity, i.e. ensemble of direct chemical and electrical connections between neurons), physiology of neurovascular coupling, baseline metabolic needs of the fetus and the infant, and functional connectivity (defined as statistical dependence of low-frequency spontaneous fluctuations seen with functional magnetic resonance imaging (fMRI)). The complementary roles of magnetic resonance imaging (MRI), electroencephalography (EEG), magnetoencephalography (MEG), and near-infrared spectroscopy (NIRS) are discussed. We include a section on modulators of brain development where we focus on the placenta and emerging placental MRI approaches. In each section we discuss key technical limitations of the imaging modalities and some of the limitations arising due to the biology of the system. Although neuroimaging approaches have contributed significantly to our understanding of early brain development, there is much yet to be done and a dire need for technical innovations and scientific discoveries to realize the future potential of early fetal and infant interventions to avert long term disease.

MR Imaging Measurements of Altered Placental Oxygenation in Pregnancies Complicated by Fetal Growth Restriction

Purpose To evaluate oxygen-enhanced and blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging parameters in normal pregnancies and those complicated by fetal growth restriction (FGR). Materials and Methods This case-control study was approved by the local research ethics committee. Informed consent was obtained from all subjects. From October 2010 to October 2015, 28 women with uncomplicated pregnancies (individualized birthweight ratio [IBR] >20th percentile and delivery >37 weeks) and 23 with pregnancies complicated by FGR (IBR <5th percentile and abnormal Doppler ultrasonography [US] studies) underwent MR imaging. Differences in placental longitudinal R1 (1/T1) and transverse R2* (1/T2*) were quantified, with subjects breathing either air or oxygen. The difference in R1 (ΔR1) after hyperoxia was converted to change in partial pressure of oxygen (ΔPo₂). Data were acquired prospectively, with retrospective interpretation of group differences (unpaired t tests). Diagnostic models were developed by using logistic regression analysis with gestational age as a covariate. Results The mean baseline R1 and R2* for normal pregnancies (R1: 0.59 sec^-1, 95% confidence interval [CI]: 0.58 sec^-1, 0.60 sec^-1; R2*: 17 sec^-1, 95% CI: 14 sec^-1, 20 sec^-1) were significantly different from those of pregnancies complicated by FGR (R1: 0.63 sec^-1, 95% CI: 0.62 sec^-1, 0.65 sec^-1; R2*: 26 sec^-1, 95% CI: 22 sec^-1, 32 sec^-1) (P < .0001). The ΔR1 showed a significant negative association with gestational age (P < .0001) in the combined cohort, with the FGR group having a ΔR1 that was generally 61.5% lower than that in the normal pregnancy group (P = .003). The area under the receiver operating characteristic curve for the differentiation between pregnancy complicated by FGR and normal pregnancy by using ΔPo₂, baseline R1, and baseline R2* was 0.91 (95% CI: 0.82, 0.99). Conclusion R1, R2*, and ΔPo₂ were significantly different between normal pregnancies and those complicated by severe FGR. MR imaging parameters have the potential to help identify placental dysfunction associated with FGR and may have clinical utility in correctly identifying FGR among fetuses that are small for gestational age. A larger prospective study is needed to assess the incremental benefit beyond that offered by US. ^© RSNA, 2017.

Placental magnetic resonance imaging T2* measurements in normal pregnancies and in those complicated by fetal growth restriction

OBJECTIVES

The magnetic resonance imaging (MRI) variable transverse relaxation time (T2*) depends on multiple factors, one important one being the presence of deoxyhemoglobin. We aimed to describe placental T2* measurements in normal pregnancies and in those with fetal growth restriction (FGR).

METHODS

We included 24 normal pregnancies at 24-40 weeks' gestation and four FGR cases with an estimated fetal weight below the 1(st) centile. Prior to MRI, an ultrasound examination, including Doppler flow measurements, was performed. The T2* value was calculated using a gradient echo MRI sequence with readout at 16 different echo times. In normal pregnancies, repeat T2* measurements were performed and interobserver reproducibility was assessed in order to estimate the reproducibility of the method. Placental histological examination was performed in the FGR cases.

RESULTS

The method was robust regarding the technical and interobserver reproducibility. However, some slice-to-slice variation existed owing to the heterogeneous nature of the normal placenta. We therefore based T2* estimations on the average of two slices from each placenta. In normal pregnancies, the placental T2* value decreased significantly with increasing gestational age, with mean ± SD values of 120 ± 17 ms at 24 weeks' gestation, 84 ± 16 ms at 32 weeks and 47 ± 17 ms at 40 weeks. Three FGR cases had abnormal Doppler flow, histological signs of maternal hypoperfusion and a reduced T2* value (Z-score < -3.5). In the fourth FGR case, Doppler flow, placental histology and T2* value (Z-score, -0.34) were normal.

CONCLUSIONS

The established reference values for placental T2* may be clinically useful, as T2* values were significantly lower in FGR cases with histological signs of maternal hypoperfusion. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.

Longitudinal Changes in Placental Magnetic Resonance Imaging Relaxation Parameter in Murine Pregnancy: Compartmental Analysis

OBJECTIVE

To quantify gestation-dependent longitudinal changes in the magnetic resonance transverse relaxation time (T2) parameter of the major constituent regions of the mouse placenta and to evaluate their relative contributions to changes in overall placental T2.

METHODS

Timed-pregnant CD-1 mice underwent magnetic resonance imaging at 7.0 T field strength, on gestational day 13 (GD13), GD15 and GD17. T2 of the placenta and its constituent high and low blood perfusion regions were quantified. A linear mixed-effects model was used to fit the T2 across gestation, and the significance of coefficients was tested.

RESULTS

A decrease in the T2 values of the placenta and its constituent regions was observed across gestation. The temporal change in T2 was estimated to be -1.85 ms/GD (p < 0.0001) for the placenta, -1.00 ms/GD (p < 0.001) for the high-perfusion zones (HPZs) and -1.66 ms/GD (p < 0.0001) for the low-perfusion zones (LPZs).

CONCLUSION

T2 of the constituent zones of the murine placenta decreases with advancing gestation. While the T2 of the LPZ is smaller than that of the HPZ, there is no difference in their decrease rate relative to that of the whole placenta (p = 0.24). The results suggest an increased role of constituent volume fractions in affecting overall gestation-dependent placental T2 decrease in mice.

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.

R1 and R2 * changes in the human placenta in response to maternal oxygen challenge

PURPOSE

Pregnancy complications such as preeclampsia and fetal growth restriction are sometimes thought to be caused by placental abnormalities associated with reduced oxygenation. Oxygen-enhanced MRI (R1 contrast) and BOLD MRI (R2 * contrast) have the potential to noninvasively investigate this oxygen environment at a range of gestational ages.

METHODS

Scanning was carried out at 1.5 T under maternal air and oxygen breathing in a single placental slice in 14 healthy pregnant subjects of gestational ages 21-37 weeks. We report R1 changes using a respiratory-triggered inversion recovery-turbo spin-echo sequence, which is sensitive to changes in PO2 , and R2 * changes using a breathhold multiple gradient-recalled echo sequence sensitive to changes in oxygen saturation.

RESULTS

Significant R1 increases (P < 0.005, paired t-test) and R2 * decreases (P < 0.0001, paired t-test) between air and oxygen breathing were demonstrated. ΔR1 decreased with gestational age (P < 0.0005, r = -0.835, Pearson correlation test). No significant effect of gestational age on R2 * change was observed.

CONCLUSION

The results demonstrate the feasibility of non-invasive investigation of placental oxygenation using MRI and the sensitivity of R1 oxygen-enhanced MRI to gestational age. The techniques have the potential to provide unique noninvasive biomarkers in compromised pregnancies.

Magnetic resonance imaging relaxation time measurements of the placenta at 1.5 T

Placental insufficiency is a major cause of fetal growth restriction (FGR) and accumulating evidence indicates several aspects of placental morphology are altered in this condition. MRI provides quantitative indices that may be used in non-invasive assessment of the human placenta, such as relaxation time measurements, T1 and T2. We hypothesised that placental relaxation times relate to alterations in placental tissue morphology and hence may be useful in identifying the changes associated with FGR. We report on the first phase of testing this hypothesis, in a study of women in normal pregnancy.

AIMS

To assess relaxation time measurements in the placenta in normal pregnancy and correlate these with gestational age and stereological analyses of placental morphology following delivery.

METHODS

30 women underwent MRI examination (1.5 T) between 20 and 41 weeks gestation. Placental T1 and T2 measurements were acquired from a mid-depth placental region, co-localised to a structural scan. Fixed, wax-embedded sections of these placentas collected at delivery were stained with hematoxylin/eosin and subjected to stereological analysis.

RESULTS

Placental T1 and T2 show a significant negative correlation with gestation, (Pearson correlation p=0.01, 0.03 respectively). 17 placentas were analysed stereologically. In the group as a whole there was no significant correlation between T1 and T2 and morphological features. However, in a subset of 7 pregnancies scanned within a week of delivery, a significant positive correlation was observed between the fibrin volume density and the ratio of fibrin: villous volume densities and T2 (Spearman correlation p=0.02, 0.03 respectively).

DISCUSSION

The correlations between placental T1 and T2 and gestation show that these variables are clearly influenced by changes in placental structure. Fibrin might be a key component but further work is needed to fully elucidate the major structural influences on placental T1 and T2.

Quantitative MR imaging: physical principles and sequence design in abdominal imaging

Quantitative magnetic resonance (MR) imaging seeks to quantify fundamental biologic and MR-inducible tissue properties, in contrast to the routine application of MR imaging in the clinic, in which differences in MR parameters are used to generate contrast for subsequent subjective image analysis. Fundamental parameters that are commonly quantified by using MR imaging include proton density, diffusion, T1 relaxation, T2 and T2* relaxation, and magnetization transfer. Applications of these MR imaging-quantifiable parameters to abdominal imaging include oncologic imaging, evaluation of diffuse liver disease, and assessment of splenic, renal, and pancreatic disease. An understanding of the inherent physical principles underlying the basic quantitative parameters as well as the commonly used pulse sequences requisite to their derivation is critical, as this field is rapidly growing and its use will likely continue to expand in the clinic. The full potential of quantitative MR imaging applied to abdominal imaging has yet to be realized, but the myriad applications reported to date will undoubtedly continue to grow.

Dynamic contrast-enhanced magnetic resonance imaging: definitive imaging of placental function?

The placenta constitutes a complex circulatory interface between the mother and fetus, but the relationship between the maternal and fetal circulation is still very difficult to study in vivo. There is growing evidence that magnetic resonance imaging (MRI) is useful and safe during pregnancy, and MRI is increasingly used for fetal and placental anatomical imaging. MRI functional imaging is now a modern obstetric tool and has the potential to provide new insights into the physiology of the human placenta. Placental perfusion has been studied during the first pass of an MR contrast agent, by arterial spin labeling, diffusion imaging, T1 and T2 relaxation time measurement using echo-planar imaging, and by a combination of magnetization transfer with established stereological methods. The BOLD (blood oxygen level-dependent) effect offers new perspectives for functional MRI evaluation of the placenta.

Placental MRI

Placental function is of fundamental importance for normal fetal growth and development. The movement of blood within the placenta ensures adequate transfer of nutrients and waste products across the feto-maternal barrier. The placenta is a relatively easy organ to study with magnetic resonance imaging (MRI) as it has a very high blood volume. MRI can be used to assess both the growth and function of the normal placenta and can distinguish differences from normal in placentas from pregnancies compromised by fetal growth restriction and pre-eclampsia.

Rapid quantitation of magnetization transfer using pulsed off-resonance irradiation and echo planar imaging

A technique for producing a quantitative measure of magnetization transfer parameters in a clinically feasible time scale is proposed. The combination of pulsed off-resonance irradiation and echo planar imaging has produced an imaging sequence that negates the need for continuous wave irradiation and allows the approach to steady-state conditions to be studied. Data analysis involves the step-by-step numerical solution of the modified Bloch equations to generate a quantitative model of the measured signal intensity based on the relative size of the bound proton pool and the bound proton pool transverse relaxation time. The sequence and model are applied to the study of a series of agar gels of varying concentrations and the results are compared to those from the literature.

Arterial spin labeling blood flow magnetic resonance imaging for the characterization of metastatic renal cell carcinoma(1)

RATIONALE AND OBJECTIVE

This study sought to assess the feasibility of arterial spin labeling (ASL) blood flow (BF) magnetic resonance imaging (MRI) for the study of metastatic renal cell carcinoma (RCC) in the body, where the respiratory, cardiac, and peristaltic motions present challenges when applying ASL.

MATERIALS AND METHODS

ASL was performed using a background-suppressed single-section flow-alternating inversion recovery (FAIR) preparation and a single-shot fast spin-echo imaging sequence on a 3.0-T whole body imager. Tumor BF was evaluated for 26 patients with RCC metastatic to the liver, bone, lung, or lymph nodes before VEGF receptor inhibitor therapy. Two cases with tumor size change after treatment were also scanned 1 month after therapy. For validation, kidney cortex BF in five normal volunteers was measured with the same technique and compared with literature values.

RESULTS

ASL was successfully performed in all normal volunteers and in 20 of 26 patients. The six failures resulted from a systematic error, which can be avoided in future studies. For normal volunteers, measured kidney cortex BF was 275 +/- 14 mL/min/100 g, a value consistent with the literature. ASL determined tumor BF averaged across tumor volume and subjects was 194 mL/min/100 g (intersubject SD = 100), resulting in high perfusion signal and conspicuity of lesions. Bright signal was also seen in large vessels and occasionally in bowel. In the two cases studied 1 month after therapy, ASL perfusion changes were consistent with tumor size changes.

CONCLUSION

With background suppression, ASL MRI is a feasible method for quantifying BF in patients with renal cell carcinoma. This technique may be useful for evaluating tumor response to antiangiogenic agents.

A review of the current use of magnetic resonance imaging in pregnancy and safety implications for the fetus

This paper presents an overview of the application of and risks of exposure to Magnetic Resonance Imaging (MRI) in pregnancy. It reviews the risks to the fetus by considering the hazards in terms of the three main components of an MRI system. These are the static magnetic field, the time-varying magnetic gradient fields and the pulsed radio frequency fields. The hazards discussed are biological effects, miscarriage, heating effects and acoustic noise exposure. This paper also presents a survey of MRI sites within the United Kingdom to ascertain the extent of MRI usage in pregnancy. To validate the situation of MRI in pregnancy a survey was sent to 352 MR units throughout the United Kingdom. The questions were grouped to assess (a) maternal MRI diagnosis (b) fetal MRI and (c) work practices for pregnant MRI staff. The results showed that 91% of sites were imaging pregnant women in need of diagnosis in the second and third trimester. This paper highlights that MRI can add information for fetal central nervous system abnormalities identified by ultrasound screening, however within the UK direct fetal imaging was only performed in 8% of sites. This paper indicates the need for research to be undertaken for specific MRI clinical conditions. It also advises that risk assessment for pregnant staff working in MRI is performed, and that there is a clear need for further research into the effects of MRI in pregnancy as there is a need for clear authoritive advice.

A comparison of fetal organ measurements by echo-planar magnetic resonance imaging and ultrasound

OBJECTIVES

To compare fetal organ size measured using echo-planar magnetic resonance imaging and 2D ultrasound. To determine the relative accuracy with which each technique can predict fetal growth restriction.

DESIGN

A cross sectional, observational study comparing two different measurement techniques against a gold standard, in a normal clinical population and an abnormal population.

SETTING AND POPULATION

Seventy-four pregnant women (33 who were ultimately found to be normal and 37 with fetal growth restricted fetuses) were recruited from the City Hospital Nottingham UK to be scanned once (at various gestations).

METHODS

Each fetus had a standard ultrasound biometry assessment followed by magnetic resonance imaging measurement of organ volumes.

MAIN OUTCOME MEASURES

For each measurement for both techniques, the normal population was plotted with 90% confidence intervals. Fetal growth restricted subjects were compared with the normal population using this plot; 2 x 2 tables were created for each measurement. This was used to calculate the relative sensitivities and positive predictive value of the different measurements. A Bland-Altman plot was used to compare the ultrasound and magnetic resonance imaging measurements of fetal weight.

RESULTS

Brain sparing was seen in ultrasonic head circumference measurements, but an overall reduction in fetal growth restriction brain volume was apparent using magnetic resonance imaging at late gestations. Across the whole range of gestational ages, ultrasound assessment of fetal weight was the best predictor of fetal growth restriction.

CONCLUSION

Ultrasound fetal weight assessment appears to identify more fetuses with fetal growth restriction than abdominal circumference. The brain sparing apparent in ultrasonic head circumference measurements of fetuses with fetal growth restriction masks a reduction in brain volume observed with magnetic resonance imaging.

Rapid and accurate measurement of transverse relaxation times using a single shot multi-echo echo-planar imaging sequence

Methods for making rapid and accurate measurements and maps of the transverse relaxation time from a single free induction decay (FID) are proposed. The methods use a multi-echo sequence in combination with B1 insensitive (hyperbolic secant or BIREF2b) refocusing pulses and rapid echo-planar imaging techniques. The results were calibrated against a single spin echo echo-planar imaging sequence using a phantom containing a range of CuSO4 concentrations. The mean percentage absolute difference between the multi-echo and single-echo results was 3% for the multi-echo sequence using the hyperbolic secant refocusing pulse, and 7% for the multi-echo sequence using the BIREF2b refocusing pulse, compared to 13% for a multi-echo sequence using a nonselective sinc refocusing pulse. The use of the sequences in vivo has been demonstrated in studies of gastric function, i.e., the measurement of gastric dilution and monitoring of formation of a raft of alginate polysaccharide within the stomach.

Echo-planar magnetic resonance imaging of Gaviscon alginate rafts in-vivo

Liquid Gaviscon and Gaviscon Advance are established reflux suppressant formulations. This study describes the use of echo-planar magnetic resonance imaging (EPI) to visualise non-invasively intragastric alginate rafts of Liquid Gaviscon and Gaviscon Advance in healthy subjects. Secondly, the feasibility of using relaxation rate (T2(-1)) measurements to monitor changes in the physicochemical properties of the rafts in-vivo is evaluated. Six subjects ingested 500 mL of a liquid meal and received a single dose of 20 mL Liquid Gaviscon or 10 mL Gaviscon Advance on 2 separate visits each and were imaged every 15 min. An alginate raft was observed in the stomach for all subjects and both treatments. The raft was observed to consist of a few large fragments on the majority of the scans for both products. At t = 60 min a raft was still present in all cases. Three-dimensional volume reconstructions showed, for the first time, the spatial distribution of the rafts within the gastric lumen. The T2(-1) data showed potential for assessment of dynamic changes in the physicochemical properties of the alginate rafts in-vivo. We conclude that EPI shows great potential in assessing alginate rafts formation in-vivo.

Fast fetal magnetic resonance imaging techniques

New hardware and software allow for increased magnetic resonance imaging (MRI) speed. This speed is crucial for MRI of the fetus, where motion degrades image quality. Single-shot sequences such as echo-planar and single-shot fast spin echo now are practical with most clinical scanners. This article reviews and illustrates the physics of fast magnetic resonance sequences.

Fetal and placental volumetric and functional analysis using echo-planar imaging

Recent and past work using echo-planar imaging (EPI) in pregnancy has allowed important anatomic and physiological information to be obtained, giving advantages over conventional radiological methods such as ultrasound. EPI is a quick, convenient method of measuring organ volumes. The volumetric estimates throughout gestation correlate well with known fetal weight at these gestations. Relaxation time measurements also can be made in the placenta and lungs. By combining the changes in relaxation and volume with gestation in the future, it may be possible to develop an "index of maturity." This could be used to accurately reflect lung maturation. T1 and T2 parameters in the placenta decreased with gestational age and with abnormal placentation. EPI can be used to assess perfusion in the placenta and flow in the uterine arteries because of its rapid acquisition times. These techniques have been applied to assess perfusion within the fetal brain.

Angiogenesis and placental growth in normal and compromised pregnancies

Research on the subject of pre-eclampsia has revolved around placental growth and angiogenesis, as both are central to the aetiology of the disease. Vascular angiogenic growth factor (VEGF) is elevated in pre-eclampsia and correlates with the severity of disease. Its actions in vitro mimic the actions of plasma from women with pre-eclampsia. This chapter examines the available evidence that implicates VEGF in the maternal systemic effects seen in pre-eclampsia, and discusses how an understanding of this growth factor could lead to diagnostic and therapeutic options. Oxygenation status is the unifying concept that surrounds the discussion of placental growth and angiogenesis. The concept that 'hypoxia' is too simplistic a notion to describe pre-eclampsia is discussed. Maldevelopment of the angiogenic process can be assessed by Doppler ultrasound. The future may see a role for magnetic resonance imaging in the identification of poorly perfused placenta.

Gastric response to increased meal viscosity assessed by echo-planar magnetic resonance imaging in humans

Normal meals are highly viscous, and viscosity is a key factor in influencing gastric emptying of food. However, the process of meal dilution and mixing is difficult to assess with the use of conventional methods. The aim of this study was to validate an in vivo, novel, noninvasive, echo-planar magnetic resonance imaging (EPI) technique, capable of monitoring the viscosity of a model meal, and to use this to investigate the effects of viscosity on gastric emptying, meal dilution and satiety. Healthy volunteers (n = 8) ingested 500 mL of locust bean gum (0.25, 0.5, 1.0 or 1.5 g/100 g), nonnutrient, liquid meals of varying viscosities, and labeled with a nonabsorbable marker, phenol red. Meal viscosity was calibrated against the water proton transverse relaxation rate (T(2)(-1)) in vitro before ingestion, thus viscosity was measured in vivo via EPI measurements of T(2)(-1). Viscosity and dilution were also measured directly using nasogastric aspirates. Gastric volumes as measured by EPI, fullness, appetite and hunger were also assessed serially. Before ingestion, the log of initial meal viscosity was linearly related to T(2)(-1) (n = 8, r(2) = 0.95). Similarly, T(2)(-1) measured in vivo was also linearly related to the viscosity of the aspirates (r(2) = 0.88). All meals underwent rapid dilution, leading to a reduction in viscosity, which was greatest for the most viscous meal (P < 0.01). Surprisingly, despite the fact that the initial meal viscosity varied 1000-fold, there was only a small delay in gastric emptying (P for trend < 0.05). The area under the curve for satiety increased with initial meal viscosity, whereas that for hunger decreased (P < 0.05). In conclusion, the viscosity of a meal in vivo can be measured noninvasively using EPI. The stomach responds to meal ingestion by rapid intragastric dilution, causing a reduction of meal viscosity, and gastric emptying is minimally delayed. However, increased viscosity is associated with more prolonged satiety.

Optimization of the ultrafast Look-Locker echo-planar imaging T1 mapping sequence

The Look-Locker echo-planar imaging (LL-EPI) sequence has been numerically optimized in terms of the signal-to-noise ratio in the measured value of T1, for both single-shot (repetition time (TR) = infinity), and dynamically repeated T1 measurements. The sequence is optimized for the normal biologic range of T1 (0.2 s to 2.0 s) and for a range of sequence parameters found on most magnetic resonance (MR) scanners. Both linearly and geometrically spaced magnetization sample pulse intervals were considered. For single-shot measurements, the sequence with 24 linearly spaced sample pulses, an inversion time of 0.01 s, an inter-sample pulse delay of 0.10 s, and a sample radiofrequency (RF) pulse flip angle of 25 degrees was found to be optimum. When the number of sample pulses was limited due to hardware limitations, different pulse sequence parameters were indicated. The optimization procedures used are appropriate for any single-shot T1 mapping sequence variant and for any rapid T1 mapping application. The use of an optimized Look-Locker echo-planar imaging sequence is demonstrated by an example of dynamic contrast-enhanced scanning in the brain using fast T1 mapping.

In vivo perfusion measurements in the human placenta using echo planar imaging at 0.5 T

This paper presents the first in vivo measurements of perfusion in the human placenta from 20 weeks gestational age until term, using the non-selective/selective inversion recovery echo-planar imaging sequence, in which data is alternately acquired following a selective and non-selective inversion pulse. Twenty pairs of images were collected, two each at the following inversion times: 20, 310, 610, 910, 1110, 1410, 1910, 2810, 3310, and 4510 ms with the sequence being repeated with a repetition time (TR) of 10 s. The results of these measurements were used to suggest the optimum sequence for future work in terms of the signal to noise ratio in the measured perfusion rate in a given measurement time. The sequence was also analyzed to determine the expected variability in the measurements. In normal pregnancies the average value of perfusion rate was found to be 176 (standard error = +/-24) ml/100 mg/min. (n = 16, standard deviation = 96 ml/100 mg/min). The expected variability in the measured parameters due to signal to noise ratio considerations alone was calculated to be 71%. For a maximum scanning time of 400 s, the optimum sequence for measuring placental perfusion was found to require 8 repetitions at each of 10 inversion times which were geometrically spaced (given by a(o), a(o)r, a(o)r2, a(o)r3, . . .), with a(o) = 850 ms, r = 1.073 and TR = 5 s, giving a pixel variability of 38%. Other timing schemes are recommended for measuring perfusion in other anatomical regions with different values of perfusion rate and longitudinal relaxation time.