B. Issa

In vivo intravoxel incoherent motion measurements in the human placenta using echo-planar imaging at 0.5 T

This paper presents the first in vivo measurements of intravoxel incoherent motion in the human placenta, obtained using the pulsed gradient spin echo (PGSE) sequence. The aims of this study were two‐fold. The first was to provide an initial estimate of the values of the IVIM parameters in this organ, which are currently unknown. The second aim was then to use these results to optimize the sequence timings for future studies. The moving blood fraction (f), diffusion coefficient (D), and pseudo‐diffusion coefficient (D*) were measured. The average value of f was 26 ± 6 % (mean ± SD), D was 1.7 ± 0.5 × 10−3 mm2/sec, and D* was 57 ± 41 × 10−3 mm2/sec. For the optimized values of b, the expected percentage uncertainty in the fitted values of f, D, and D* for the placenta were σf/f = 14.9%, σD/D = 14.3%, σD*/D* = 44.9%, for an image signal‐to‐noise of 20:1, and a total imaging time of 800 sec. Magn Reson Med 43:295–302, 2000.

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

Objectivesu2003 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.

IN VIVO RELAXATION TIME MEASUREMENTS IN THE HUMAN PLACENTA USING ECHO PLANAR IMAGING AT 0.5 T

This paper presents the first in vivo measurements of the nuclear magnetic resonance relaxation times T1 and T2 at 0.5 T in the human placenta from 20 weeks gestational age until term, in both normal and compromised pregnancies. T1 measurements were performed by using both an inversion recovery sequence and the Look-Locher echo planar imaging (EPI) sequence on a total of 41 women with normal pregnancies and 11 women with compromised pregnancies. T2 measurements were performed by using a spin-echo EPI sequence on 36 women with normal pregnancies and 14 women with compromised pregnancies. In normal pregnancies, both the T1 values measured with the inversion recovery sequence and the T2 values were found to decrease with gestational age, the linear regression results gave T1 = -9.1t + 1538 r2 = 0.23 p = 0.03. T2 = -4.0t + 338 r2=0.47 p =410(-6) where t is the gestational age in weeks, and T1 and T2 are the relaxation times in milliseconds. T1 values measured very rapidly with the Look-Locher EPI sequence, but, therefore, with a much lower signal-to-noise ratio, showed no significant trends. The T1 values measured in the abnormal group were significantly lower than those measured in the normal group. Four out of eight patients with compromised pregnancies had placental T1 values lying outside the 90% confidence limits for the normal population based about the regression line, significantly more than expected by chance (p = 0.005). Ten out of fourteen of the T2 measurements in the abnormal group were below the regression line established for the normal group, with 4 lying below the 90% confidence interval, although these trends were only just significant (p = 0.06 and p = 0.03).

Demonstration of changes in fetal liver erythropoiesis using echo-planar magnetic resonance imaging

This study investigated the variation in magnetic resonance characteristics of the fetal liver during a time of changing erythropoietic function. Echo-planar imaging was carried out in 25 normal pregnant women at 20 and 26 wk gestation. The signal intensity from regions of the fetal liver, background image, and maternal back muscle and the highest signal intensity from the maternal spinal cord were measured and compared with the signal intensity of amniotic fluid. Data are expressed as ratios, in arbitrary units (median pixel values; interquartile range shown in parentheses), and analyzed with the use of Wilcoxons signed-rank test. At 20 wk, the signal intensity ratio of liver to amniotic fluid was 0.309 (0.231-0.365). At 26 wk, the ratio was 0.544 (0.429-0.616). The change was highly significant (P < 0.0001). No change in the signal intensity ratios of amniotic fluid compared with other measured parameters was noted. These data are consistent with known changes in fetal liver erythropoiesis occurring between 20 and 26 wk gestation and have potential use in early noninvasive physiological assessment of the fetus.

Studies of fluid transport in porous rocks by echo-planar MRI

Attempts to quantify the permeability constant that appears in Darcys equation in terms of the porosity, tortuosity and surface area have been made by Kozeny and Carman. These results in turn have been translated by Seevers into expressions involving spin-lattice relaxation times. When this work was done originally in the mid-1960s, NMR imaging and spatially localised T1 maps had not been invented. Therefore, bulk T1 average values were used and it is not surprising to see substantial deviations from predicted permeabilities in bore core material. In this paper we re-examine the Kozeny-Carman and Seevers theories in the light of new techniques development for the localised measurement of both T1 maps and fluid velocity maps. New results show spatially correlated T1 and flow maps obtained from Bentheimer sandstone samples. These are discussed in terms of a localised permeability constant. By this means we show that the relationship between T1 and permeability does not hold at the local level for isolated pixels within a larger specimen.

Permeability estimation from T1 mapping

Fluids permeating a solid matrix can interact strongly with the pores in which they are contained. Enhanced relaxation behaviour over that of the bulk water can ensue due to the presence of paramagnetic centres or to exchange with bound water at the pore surface. In the latter case, the ratio of pore surface to the pore volume is effective in the relaxation behaviour. NMR provides an effective tool for investigating the solid matrix properties (e.g., pore size distribution) via indirect measurement of the fluid NMR parameters. In addition to spin density, we use T1 mapping to provide an estimate of the specific permeability of sandstone for a few samples.

Gastric motility by tagged EPI

The high speed echo-planar NMR imaging technique is particularly suitable for visualizing human gut motility and transit. To enable a quantitative study of the rheology of the stomach contents, the magnetization can be prepared using a SPAMM (SPAtial Modulation of Magnetization) tagging sequence prior to imaging. Movement of stomach contents has been measured and analyzed temporally and in the Fourier domain and correlated with stomach motility. Increased stomach activity combined with retropulsive motion were observed at about 25 min following a porridge meal, indicating mixing of the meal and inhibition of gastric emptying.

Quantification of blood velocity and flow rates in the uterine vessels using echo planar imaging at 0.5 Tesla.

To measure uterine artery and vein blood velocity and flow rate profiles using MRI during normal pregnancy.

A microscopic model of fluid transport in porous rocks

NMR measurements of water velocity flowing through two different porous rocks, Bentheimer and Clashach, have been made using the PEPI sequence. Random changes of localised velocity occur in both materials even though the average velocities across the core cross-sections follow Darcys Law. The velocity distributions are approximately Gaussian and the velocity variance follows a linear relationship with mean velocity as predicted in the stochastic theory of Mansfield and Issa. The origin of the large fluctuations in localised velocity is thought to be hydrodynamic and a detailed theory of intervoxel coupling is presented, which accounts for such behaviour in a coupled voxel array. In this work the intervoxel coupling is restricted to isolated voxel pairs.

Echo-Planar Magnetic Resonance Imaging to Assess Water Volume in the Distal Small Bowel

AbstractPurpose. Assessment of fluid volumes and flow through the small intestine has in the past only been possible by means of invasive intubation studies on human volunteers. Intubation very likely disturbs gut motility and stimulates secretion.nMethods. The aim of this study was to utilise the new technique of echo-planar magnetic resonance imaging in order to non-invasively visualise the changing volume of water in the small intestinal lumen. 200mls of test solution was ingested and water volume assessed using a multi-slice scanning technique on 3 separate days. The solutions were pure water, pure water plus 2.264 or l0g of mannitol. These were taken on separate days by 8 healthy male volunteers. Regions of interest were constructed in the area of the lower pelvis excluding retroperitoneal structures.nResults. The water content of the lower small intestine did not change significantly over the 4 hours after the control solution. By contrast after both mannitol solutions there was an increase in the amount of water in the distal intestine as assessed by the area under the curve of the volume time profile (Control 51 ml.h (SD ±47); mannitol 2.264g/200ml 72ml.h (SD ±57); 10g/200ml mannitol 115ml.h (SD ±56)). Pages L Trend test showed that the trend for the volume to increase with increasing mannitol concentration to be statistically significant at the 1% level (L= 108).nConclusions. The study highlights the potential of echo-planar magnetic resonance imaging to visualise changes in gastrointestinal physiology in a non-invasive manner.