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Biomedical subjects

I R Johnson

Publications and source records attributed to I R Johnson.

At least 37 records · Page 2Linked to original sources
PubMed1998

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.

Adult↗
PubMed1998

The investigation of placental relaxation and estimation of placental perfusion using echo-planar magnetic resonance imaging.

Echo-planar imaging (EPI) is a form of magnetic resonance imaging (MRI) which acquires images in milliseconds rather than minutes as with conventional MRI. The images produced using EPI are affected by the physiological environment in which the hydrogen atoms producing the signals are found, a process referred to as relaxation. Also by producing images a matter of milliseconds apart, quantification of perfusion within the tissue being imaged is feasible. The objective of this study was to investigate T1 and T2 relaxation times along with perfusion in placentae from normal pregnancies at different gestations and also to compare these to pregnancies complicated by abnormal placental function. A cross-sectional study of normal and compromised pregnancies from 20 weeks to term and a longitudinal study of normal pregnancy were performed. Placental T1, T2 relaxation times, and perfusion were measured using echo-planar magnetic resonance imaging. Placental T1 and T2 relaxation times decreased in normal pregnancy (P<0.001). Relaxation times in pregnancies associated with placental pathology appeared to be reduced for that gestation although the numbers were too small to allow any statistical validation. No differences in placental perfusion with gestation or between normal and compromised pregnancy were demonstrated using this technique. This is the first demonstration of placental magnetic resonance relaxation and perfusion measurements in normal pregnancy using echo-planar magnetic resonance imaging. In the future it may be possible to identify compromised pregnancies by differences in placental T1 and T2 relaxation times, using this novel non-invasive technique.

Adult↗
PubMed1998

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).

Adult↗
PubMed1998

Plasma from pre-eclamptic women and functional change in myometrial resistance arteries.

This study assesses the ability of plasma from women with pre-eclampsia to induce altered endothelial function in myometrial resistance vessels from normotensive women. Vessels from normotensive pregnant women (n = 7) were incubated with plasma from other pregnant women who were normotensive (n = 6) or had pre-eclampsia (n = 7). A wire myograph was used to test the endothelium-dependent relaxatory response to bradykinin of preconstricted vessels. The relaxation in vessels incubated with plasma from women with pre-eclampsia was markedly less than in vessels incubated with plasma from normotensive pregnant women (P = 0.039). This supports the theory that a plasma-borne factor contributes to the endothelial changes seen in pre-eclampsia.

Arteries↗
PubMed1998

Failure to detect intrauterine growth restriction following in utero exposure to MRI.

Echo planar imaging is a form of MRI with short image acquisition times, which permits in utero fetal imaging without motion artefacts. Echo planar imaging has been used to measure accurately fetal organ volume and to assess placental function. Two small animal studies have suggested the possibility of intrauterine growth restriction consequent upon MRI. We thus performed a prospective study of pregnancies in which fetuses were exposed to echo planar imaging, compared with a control group in which there was no in utero echo planar imaging exposure. There were no significant differences between the groups when maternal age, parity, proportion of smokers and proportion of Caucasian women were compared. Although the gestational age of delivery was lower in the echo planar imaging group, the proportion of women delivering prematurely was not significantly different. Although infant birthweights were significantly lower in the MRI group, the corrected birthweight for gestational age centiles (individualized birthweight ratio) was not significantly different between the two groups. In utero exposure to echo planar imaging thus did not have a marked effect on intrauterine fetal growth. A 10 year follow-up study of all infants imaged in utero is being performed.

Birth Weight↗
PubMed1997

Angiogenesis is associated with vascular endothelial growth factor expression in cervical intraepithelial neoplasia.

Squamous cell carcinoma of the cervix (SCC) is preceded by a premalignant condition known as cervical intraepithelial neoplasia (CIN). The majority of cases of CIN regress spontaneously; however, methods are needed to identify those lesions likely to progress. Increased blood vessel density, signifying angiogenesis, is an independent prognostic indicator in a number of cancers, although little is known about its significance in premalignant lesions. The aim of the present study was to determine the relationship between vessel density, expression of the potent angiogenic factor vascular endothelial growth factor (VEGF) and CIN grade. Using immunohistochemistry, mean vessel density (MVD) and VEGF expression were assessed in samples from 54 patients who had undergone cone biopsy for CIN or hysterectomy for SCC and from 16 patients with no cervical pathology. There were significant increases in MVD and VEGF expression from normal cervix through CIN I to CIN III to invasive SCC, but no difference in mean vessel diameter between groups. There was a strong correlation between mean vessel density and VEGF expression, and both were associated with histological grade of CIN. The original MVDs for a small group of patients later presenting with recurrent disease were found to be equal to or greater than the mean for their histological grade. We conclude that the onset of angiogenesis is an early event in premalignant changes of the cervix due, in part, to enhanced expression of VEGF by the abnormal epithelium.

Adult↗
PubMed1997

Loss of endothelium-dependent relaxation in myometrial resistance arteries in pre-eclampsia.

OBJECTIVE: To measure endothelium-dependent relaxation in myometrial resistance arteries and to compare this parameter in nonpregnant and normotensive pregnant women and those with pregnancies complicated by pre-eclampsia. DESIGN: A prospective study. SETTING: Nottingham City Hospital. SAMPLE: Thirty-seven nonpregnant women undergoing hysterectomy and 51 pregnant women undergoing caesarean section, of whom there were 39 normotensive and 12 with pre-eclampsia. METHODS: Resistance arteries, dissected from myometrial biopsies, were mounted on a wire myograph and preconstricted with vasopressin then subjected to incremental doses of bradykinin. RESULTS: Endothelium-dependent relaxation to bradykinin was seen in the vessels of nonpregnant and normotensive pregnant women. Markedly reduced endothelium-dependent relaxation was found in the myometrial arteries from women with pre-eclampsia when compared with both nonpregnant (P < 0.0001) and normotensive pregnant (P < 0.0001) women. CONCLUSION: A significant loss of endothelium-dependent relaxation in myometrial resistance arteries in pre-eclampsia may contribute to the altered vasoreactivity seen in pre-eclampsia, and particularly to the decreased placental perfusion and fetal growth retardation commonly associated with this condition.

Adult↗
PubMed1997

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 Wilcoxon's 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.

Adult↗
PubMed1996

Modelling canopy photosynthesis in response to environmental conditions.

Physiological models in the plant and crop sciences provide a means of integrating different aspects of the system, in particular the interaction between plant processes and environmental factors. This paper focuses on the response of canopy photosynthesis, including adaptation, to environmental conditions. Adaptation is likely to be important when considering controlled ecological life support systems since physiological characteristics are affected by past as well as current environmental conditions. In particular, the level of photosynthetic enzymes in a plant is generally greater for plants grown in high irradiance levels than for similar plants grown in low irradiance. The models have been developed to apply to 'normal' growing conditions, although the principles will apply to closed bioregenerative systems.

Adaptation, Physiological↗
PubMed1996

A comparison of endothelium-dependent relaxation in omental and myometrial resistance arteries in pregnant and nonpregnant women.

OBJECTIVES: The study aimed to compare endothelium-dependent relaxation in the resistance arteries of the uterine vascular bed by use of a systemic comparison and to investigate the role of the endothelium in adaptation to pregnancy in these vascular beds. STUDY DESIGN: Myometrial and omental resistance arteries were collected from 22 normotensive pregnant women and 27 nonpregnant women of reproductive age. On a wire myograph and preconstricted with vasopressin, the arteries were relaxed with the endothelium-dependent vasodilator bradykinin. RESULTS: The omental arteries showed a consistently greater endothelium-dependent relaxation than the myometrial arteries, both in the nonpregnant state (p < 0.0001) and in pregnancy (p = 0.008); pregnancy did not significantly alter this relaxation in arteries from either the myometrial (p = 0.2075) or omental (p = 0.372) vascular beds. CONCLUSIONS: The endothelium-dependent relaxation to bradykinin is less in myometrial resistance arteries than in omental resistance arteries. There appears to be no intrinsic difference between vessels from nonpregnant and pregnant women in endothelium-dependent response to bradykinin.

Adolescent↗
PubMed1996

Angiotensin receptors in myometrium and myometrial vessels from uteri of women during the follicular and luteal phases of the menstrual cycle and in late pregnancy.

1. Receptors for angiotensin II were identified and characterized in membrane fractions from myometrial samples obtained at non-pregnant hysterectomy in women of reproductive age. Specific binding was also measured in paired samples of vascular and "vessel-free' myometrium. 2. A single class of high-affinity receptor sites was identified in the total myometrial preparations (n = 10), with a median equilibrium dissociation constant (Kd) of 0.122 nmol/l (range 0.065-0.465 nmol/l) and concentration of receptor sites (Bmax) of 149 fmol/mg protein (range 105-435 fmol/mg). Receptor characterization studies using losartan (an angiotensin type 1 receptor antagonist) and PD123177 (an angiotensin type 2 receptor antagonist) showed the myometrium to contain almost entirely type 2 receptors. 3. The median Kd and Bmax for specific angiotensin II binding in isolated myometrial vessel homogenates were 0.086 nmol/l (range 0.060-0.433) and 260 fmol/mg protein (range 197-737 fmol/mg) respectively. Vascular specific binding density was always higher in dissected out myometrial vessels than in paired vessel-free myometrium (median 372 compared with 120 fmol/mg protein; n = 20; P < 0.001). The specific binding in the paired samples was strongly correlated (r = 0.8904, P < 0.0001). 4. The specific binding of 125I-labelled angiotensin II in "vessel-free' myometrium was higher in samples from the follicular phase than in samples from the luteal phase (median 144 compared with 37 fmol/mg; P < 0.02). A similar trend was found in the vessels themselves, but this failed to reach statistical significance (459 compared with 225 fmol/mg; P = 0.051). 5. It is suggested that the down-regulation of the angiotensin type 2 receptors in the luteal (secretory) phase is a preparation for pregnancy, removing an inhibitory factor to uterine growth and vascularization and allowing greater prostacyclin synthesis.

Adult↗
PubMed1996

The effects of parity on birthweight using successive pregnancies.

OBJECTIVE: Accurate assessment of the difference in birthweight between first and second live-births to the same woman having excluded the effects of physiological factors known to affect birthweight. DESIGN: Retrospective longitudinal observational study. SETTING: Three large obstetric units in the East Midlands of the United Kingdom. SUBJECTS AND METHODS: Women in whom data were recorded for their first two pregnancies on the UK East Midlands Obstetric database which resulted in the delivery of a liveborn, singleton and congenitally normal baby. Six thousand five hundred and thirty such cases were identified, of which 3457 had complete datasets and delivered both babies at term (259 to 300 days). An analysis was performed of changes between the paired pregnancies of physiological factors known to affect birthweight. Regression analyses were used to enable prediction of the second birthweight with the knowledge of the first birthweight. RESULTS: The mean crude birthweight difference between first and second pregnancies was an increase of 138 g. Significant differences between the paired pregnancies were found in maternal booking visit weight, blood pressure, maternal age and gestation at delivery. Independent factors affecting difference in birthweight were gestation at delivery, maternal booking weight and baby's sex. Regression towards the mean was demonstrated which meant that a woman delivering a first baby weighing more than 3720 g could expect a lighter baby for her second delivery provided that all other factors remained constant. CONCLUSIONS: In general terms a woman is more likely to deliver a heavier baby in her second pregnancy than in her first pregnancy. However, maternal physiological factors differ in the two pregnancies and these differences have additional effects on birthweight. The effects of both these observations are tempered by regression towards the mean which has a profound influence in predicting the likely change in birthweight between first and second pregnancies. Clinical decisions should not be based on the assumption that a second baby will inevitably be heavier than the first baby.

Birth Order↗
PubMed1995

Measurement of fetal liver, brain and placental volumes with echo-planar magnetic resonance imaging.

OBJECTIVE: To quantify accurately in utero fetal liver, brain and placental volumes using echo planar imaging, and to assess whether the technique has the potential to enhance intrauterine fetal assessment. DESIGN: Thirty-two singleton, complicated pregnancies were scanned using echo planar imaging, a form of magnetic resonance imaging. Pregnancies were subdivided on the basis of whether the fetus was found subsequently to have an individualised birthweight ratio above (n = 21) or below (n = 11) the 10th centile. Comparisons of the organ volumes of these two groups were made. RESULTS: The first quantitative in utero measurement of fetal liver volume showed a linear relation between liver volume and gestational age in fetuses where the individualised birthweight ratio was above the 10th centile (the normal growth group). Ten of the 11 liver volume measurements of fetuses subsequently found to have an individualised birthweight ratio below the 10th centile fell on or outside the 95% confidence limits established for the normal growth group. In contrast, no such differences were demonstrated when the brain and placental volumes were considered, with 10 of the 11 brain measurements and all of the 11 placental measurements falling within the 95% confidence limits of the normal growth group. CONCLUSIONS: A single measurement of fetal liver volume using echo planar imaging enabled accurate identification of fetuses subsequently found to have individualised birthweight ratios below the 10th centile. If these findings are repeated in larger, more representative studies, this suggests that the technique has the potential to contribute to intrauterine fetal assessment.

Adult↗
PubMed1995

The effect of social deprivation on birthweight, excluding physiological and pathological effects.

OBJECTIVE: To study the effect of social deprivation on birthweight, excluding the effect of known physiological factors and exploring the effect of possible pathological factors. DESIGN: Retrospective analysis of computerised obstetric database. SETTING: Two teaching hospitals and an associated district general hospital which provided a defined catchment area in the East Midlands. SUBJECTS: The final analysis included 7493 women with complete datasets and gestations of between 259 and 300 days at delivery, dated by ultrasound scan. MAIN OUTCOME MEASURES: Smoking habit, alcohol consumption, weight gain during pregnancy, systolic and diastolic blood pressures at booking, bleeding during pregnancy and Jarman score; also, the effect of these variables on birthweight, adjusted for the effects of physiological factors using the individualised birthweight ratio. RESULTS: Smoking during pregnancy reduced birthweight but the effect is not linear, becoming less marked as the number of cigarettes smoked increases. Alcohol intake, diastolic and systolic blood pressures at the booking visit and vaginal bleeding during early pregnancy were not significantly related to birthweight. Pregnancy weight gain was significantly positively related to birthweight especially in the normal weight range (60-99 kg). A multivariate analysis including physiological and pathological factors found increasing Jarman score to be negatively related to birthweight. CONCLUSIONS: In this central British population social deprivation is correlated negatively with birthweight: the most socially deprived mothers have the smallest babies. This association cannot be explained in terms of physiological differences in the population nor in a higher prevalence of known pathological factors.

Alcohol Drinking↗
PubMed1995

Paternal influences on birthweight.

OBJECTIVE: To assess the influence of paternal size on birthweight after suitable control for maternal and fetal factors. DESIGN: Prospective observational study. SETTING: Delivery suite, City Hospital, Nottingham. SUBJECTS: 571 husbands/partners of unselected women delivering August 1992 to February 1993. MAIN OUTCOME METHODS: Individualised birthweight ratio and thereby an adjusted birthweight for a typical mother. The results of a multiple regression analysis with the individualised birthweight ratio as the dependent variable. RESULTS: When considered in isolation both paternal height and weight are significantly positively associated with crude and adjusted birthweight (p < 0.01, analysis of variance). Due to correlations of paternal size with maternal size and smoking habit, only paternal height is significant in the multiple regression analysis (p = 0.01). CONCLUSION: If the partner of an average woman is short (mean-2s.d.) then the baby will be 183 g lighter than if he is tall (mean + 2s.d.). This effect of paternal height on birthweight must be genetic and therefore should be taken into account when defining intra-uterine growth retardation and macrosomia.

Birth Weight↗