A MRI gradient waveform model for automated sequence calibration.

In order to set up magnetic resonance imaging (MRI) procedures of arbitrary voxel dimensions, slice orientation, and sequence timing in a reasonable time, some form of automatic gradient pulse calibration is required. One such method, involving simulation of gradient waveforms, is presented. Waveforms are modeled based on measurements of the step response. The model used divides each transition into three time regions: a "start" region in the first 0.3 ms, a "slew" region, and a "tail" region representing decay of the eddy current compensation error. In the "slew" region, the time derivative of the gradient, G' (t), is expressed as a function of G(t). The first two regions are nonlinear with respect to demand. The mean error in the simulated gradient is generally less than 0.04 mT m-1 in spin echo sequences. Image signal/noise ratios resulting from sequences calibrated using the model are within 5% of those of empirically calibrated sequences.     

Dietary behaviour and health in Northern Ireland: an exploration of biochemical and haematological associations.

STUDY OBJECTIVE--The aim was to determine the relationships between dietary behaviour and biochemical and haematological measures. DESIGN--This was a cross sectional population study. SETTING--The study took place in the general community within Northern Ireland. SUBJECTS--522 randomly selected adults aged 18-64 years took part (65% of the eligible sample). MEASUREMENTS AND MAIN RESULTS--Four dietary behaviours were identified using principal components analysis from 7 d weighed dietary records described in terms of mean intake of 41 food groups. Haematological and biochemical analyses were carried out on non-fasting blood samples. Social, personal, and lifestyle information was ascertained through interviewer administered questionnaires. Partial correlations controlled for age and smoking behaviour were calculated. There were significant negative associations between iron status measures and the "traditional" behaviour, while the "meat and two veg" behaviour showed positive associations. There were positive associations between the "cosmopolitan" and "convenience" behaviours and folate status in women. White cell count and platelet levels in women were negatively associated with the "traditional" and "cosmopolitan" behaviours. Total cholesterol levels showed significant negative associations with the "cosmopolitan" behaviour in women and HDL cholesterol levels were positively associated with the "cosmopolitan", "convenience" and "meat and two veg" behaviours. CONCLUSIONS--Through a multivariate approach to dietary assessment it is possible to identify food combinations that cluster and interact to influence biochemical and haematological indices of health status.     

Variations in T1 and T2 relaxation times of normal appearing white matter and lesions in multiple sclerosis

OBJECTIVE: To investigate the variation in T1 and T2 relaxation times of normal appearing white matter (NAWM) and lesions in multiple sclerosis (MS) throughout the brain. BACKGROUND: The magnetic resonance imaging (MRI) sequence fast FLAIR (fluid attenuated inversion recovery) has demonstrated overall increased lesion detection when compared to conventional or fast spin echo (FSE) but fewer lesions in the posterior fossa and spinal cord. The reasons for this are unknown, but may be due to variations in the T1 and T2 relaxation times within NAWM and MS lesions. METHOD: Ten patients and 10 controls underwent MRI of the brain which involved FSE, fast FLAIR and the measurement of T1 and T2 relaxation times. RESULTS: Of 151 lesions analysed (22 infra-tentorial, 129 supra-tentorial), eight were missed by the fast FLAIR sequence. T1 and T2 relaxation times in normal controls were longer in the infra-tentorial, than supra-tentorial, region. Patient NAWM relaxation times were prolonged compared with control values in both regions. Lesions demonstrated longer relaxation times than either control white matter or patient NAWM in both regions, however this difference was less marked infra-tentorially. The eight posterior fossa lesions not visible on the fast FLAIR sequence were characterised by short T1 and T2 relaxation times which overlapped with the patient NAWM for both T1 and T2 and with control values for T2 relaxation times. CONCLUSION: Both lesion and NAWM relaxation time characteristics vary throughout the brain. The T1 and T2 relaxation times of infra-tentorial lesions are closer to the relaxation times of local NAWM than supra-tentorial lesions, resulting in reduced contrast between posterior fossa lesions and the background NAWM. Consequently the characteristics of some lesions overlap with those of NAWM resulting in reduced conspicuity. By utilising this information, it may be possible to optimise fast FLAIR sequences to improve infra-tentorial lesion detection.     

D-dimer assay for deep vein thrombosis: its role with colour Doppler sonography

AIM: To evaluate the role of a negative D-dimer assay in the initial management of patients with clinically suspected deep venous thrombosis (DVT), using colour Doppler ultrasound as the primary diagnostic technique. MATERIALS AND METHODS: A double-blind prospective trial was performed on 143 patients with clinically suspected DVT. All patients underwent a D-dimer assay prior to anticoagulant therapy. DVT was confirmed or excluded by diagnostic colour Doppler ultrasound within 24 h of presentation. RESULTS: In nearly one-third of the cases (31.8%), Doppler ultrasound was positive. The D-dimer assay demonstrated a sensitivity of 97.7% with only one false-negative, but the specificity was low at 48.9% with 45 false-positive results. The positive predictive value for D-dimer assay was 48.8%, whilst the important negative predictive value was 98%. CONCLUSION: If D-dimer was used to screen for DVT, and patients with negative results were not imaged, then the imaging workload could be reduced by 35%. In this study one small calf vein thrombus would have been missed by adopting this practice.Bradley, M. (2000). Clinical Radiology 55, 525-527.     

Atopy, lung function, and obstructive airways disease after prenatal exposure to famine

BACKGROUND: Associations have been found between a large head size at birth and atopy, and between low birth weight and obstructive airways disease. A study was undertaken of people born around the time of the Dutch famine in 1944-5 to determine the effects of maternal malnutrition during specific periods of gestation on the prevalence of obstructive airways disease and atopy. METHODS: Nine hundred and twelve people aged about 50, born at term between November 1943 and February 1947 in Amsterdam, were asked about their medical history. Lung function was measured in 733 and serum concentrations of total IgE and specific IgE against mite, pollen and cat were measured in 726. Those exposed in late, mid, and early gestation (exposed participants) were compared with those born before or conceived after the famine (non-exposed participants). RESULTS: Exposure to famine during gestation affected neither the concentrations of total or specific IgE nor lung function values. The prevalence of obstructive airways disease was increased in people exposed to famine in mid gestation (odds ratio adjusted for sex 1.7, 95% confidence interval (CI) 1.1 to 2.6) and tended to be higher in those exposed in early gestation (odds ratio 1.5, 95% CI 0. 9 to 2.6). CONCLUSIONS: The observed increase in the prevalence of obstructive airways disease in people exposed to famine in mid and early gestation was not parallelled by effects on IgE concentrations or lung function. The link between exposure to famine in mid and early gestation and obstructive airways disease in adulthood suggests that fetal lungs can be permanently affected by nutritional challenges during periods of rapid growth.