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5. When Characteristics of Clinical Trials Require Per-Protocol as Well as Intention-to-Treat Outcomes to Draw Reliable Conclusions: Three Examples

Author

Scheim, David E., Aldous, Colleen, Osimani, Barbara, Fordham, Edmund J., Hoy, Wendy E., Scheim, David E., Aldous, Colleen, Osimani, Barbara, Fordham, Edmund J., and Hoy, Wendy E.

Abstract

Under exceptional circumstances, including high rates of protocol non-compliance, per-protocol (PP) analysis can better indicate the real-world benefits of a medical intervention than intention-to-treat (ITT) analysis. Exemplifying this, the first randomized clinical trial (RCT) considered found that colonoscopy screenings were marginally beneficial, based upon ITT analysis, with only 42% of the intervention group actually undergoing the procedure. However, the study authors themselves concluded that the medical efficacy of that screening was a 50% reduction in colorectal cancer deaths among that 42% PP group. The second RCT found a ten-fold reduction in mortality for a COVID-19 treatment drug vs. placebo by PP analysis, but only a minor benefit by ITT analysis. The third RCT, conducted as an arm of the same platform trial as the second RCT, tested another COVID-19 treatment drug and reported no significant benefit by ITT analysis. Inconsistencies and irregularities in the reporting of protocol compliance for this study required consideration of PP outcomes for deaths and hospitalizations, yet the study coauthors refused to disclose them, instead directing inquiring scientists to a data repository which never held the study’s data. These three RCTs illustrate conditions under which PP outcomes may differ significantly from ITT outcomes and the need for data transparency when these reported or indicated discrepancies arise.

Published
2023

8. A finite element approach to forward modeling of nuclear magnetic resonance measurements in coupled pore systems.

Author

Mitchell, Jonathan, Souza, Andre, Fordham, Edmund, and Boyd, Austin

Subjects
NUCLEAR magnetic resonance, NUCLEAR magnetic resonance spectroscopy, MAGNETIC measurements, NUCLEAR models, RANDOM walks, POROUS materials
Abstract

Porous media characterized by a hierarchy of length scales are ubiquitous in industry and nature, and include carbonate rocks, cements, heterogeneous catalysts, and biological cells. Nuclear magnetic resonance (NMR) is a popular tool for studying liquid-saturated porous materials, where the spin relaxation rate is generally considered proportional to pore size. However, in porous granular media, the relaxation rate is modified by diffusion between the intraparticle and interparticle pores. The observed relaxation rates do not reflect the pore size under such conditions. Deconvolving the various contributions of surface relaxation, geometry, and diffusion is nontrivial, and forward models are a powerful technique for elucidating the underlying pore structure. Various forward models have been proposed previously, including analytic solutions and random walk simulations. Here, a finite element method is adopted to simulate the diffusion of nuclear magnetization in a coupled pore geometry. We validate our model against existing solutions and use the simulations to determine the surface relaxivity of powdered silica by matching experimental results. The finite element approach is more versatile than other modeling methods, allowing direct visualization of the diffusing magnetization and being trivially extensible to multidimensional NMR exchange experiments. [ABSTRACT FROM AUTHOR]

Published
2019
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23. What are, and what are not, Inverse Laplace Transforms

Author

Fordham, Edmund J., Venkataramanan, Lalitha, Mitchell, Jonathan, and Valori, Andrea

Subjects
T2-distribution, Laplace Transform, Inverse Laplace Transform, Fredholm Integral Equation
Abstract

Time-domain NMR, in one and higher dimensionalities, makes routine use of inversion algorithms to generate results called \T2-distributions' or joint distributions in two (or higher) dimensions of other NMR parameters, T1, diffusivity D, pore size a, etc. These are frequently referred to as \Inverse Laplace Transforms' although the standard inversion of the Laplace Transform long-established in many textbooks of mathematical physics does not perform (and cannot perform) the calculation of such distributions. The operations performed in the estimation of a \T2-distribution' are the estimation of solutions to a Fredholm Integral Equation (of the First Kind), a different and more general object whose discretization results in a standard problem in linear algebra, albeit suffering from well-known problems of ill-conditioning and computational limits for large problem sizes. The Fredholm Integral Equation is not restricted to exponential kernels; the same solution algorithms can be used with kernels of completely different form. On the other hand, (true) Inverse Laplace Transforms, treated analytically, can be of real utility in solving the diffusion problems highly relevant in the subject of NMR in porous media.

Published
2017

24. Combined High-Resolution Solid-State 1H/13C NMR Spectroscopy and 1H NMR Relaxometry for the Characterization of Kerogen Thermal Maturation

Author

Panattoni, Francesco, Mitchell, Jonathan, Fordham, Edmund J., Kausik, Ravinath, Grey, Clare P., and Magusin, Pieter C. M. M.

Abstract

A key factor for the petroleum potential of source rock is the degree of chemical and physical structure evolution of its kerogen fraction through a range of maturation processes. In this study, various high-field, solid-state NMR methods have been applied to a series of kerogen isolates (type I) over a defined maturity range (vitrinite reflectance R0from 0.98 to 1.86%). Results obtained from 13C MAS NMR show that the sp2/sp3-hybridized carbon ratio of kerogen, here defined as the aromatic/aliphatic ratio, increases with increasing maturity. 1H MAS NMR spectra contain partly overlapping aliphatic and aromatic resonances with distinct transverse relaxation behavior. In Hahn-echo experiments, the aromatic signal decays more slowly than the aliphatic signal, indicating that for these systems, transverse 1H relaxation is rather controlled by local distances between hydrogen atoms than by molecular mobility. Similar relaxation differences are also found in static (nonspinning) 1H Hahn-echo NMR experiments, here used to discriminate between phases with different proton mobilities and/or densities in the kerogen samples and, ultimately, between aromatic and aliphatic fractions. The distributions of the static transverse relaxation time (T2), extracted from the Hahn-echo decays, are characterized by a short-T2peak (∼10 μs) and a long-T2peak (∼100 μs). The ratio between these two peaks correlates well with the aliphatic-to-aromatic signal intensity ratios in MAS NMR spectra of the corresponding kerogen samples, suggesting that a net decrease in kerogen proton density—occurring during maturation—is also reflected by 1H NMR relaxation. For the investigated kerogen isolates, the long-T2peak in the T2distribution can be considered an indicator of aromatic content, which can be directly detected by measuring 1H T2relaxation.

Published
2021
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27. Chemical resolution in T 2 - T 1 correlations

Author

University of Cambridge, New Museums Site, Schlumberger Cambridge Research, Universität Leipzig, Mitchell, Jonathan, Chandrasekera, Thusara C., Fordham, Edmund J., Crawshaw, John, Staniland, John, Johns, Mike L., Gladden, Lynn F., University of Cambridge, New Museums Site, Schlumberger Cambridge Research, Universität Leipzig, Mitchell, Jonathan, Chandrasekera, Thusara C., Fordham, Edmund J., Crawshaw, John, Staniland, John, Johns, Mike L., and Gladden, Lynn F.

Abstract

Oil and water fractions have been identified in fluid saturated carbonate rock cores using a novel T2-T1-d pulse sequence. The inclusion of the chemical shift dimension d allows T2-T1 plots to be generated independently for the oil and water. The T2-T1-d pulse sequence utilises a “double-shot” T1 measurement that provides free induction decays (FIDs) as a function of both relaxation times for suitably broad line samples. The T2-T1-d data set is acquired in the same experimental time as a conventional T1-T2 measurement (without chemical resolution) of equivalent data density. Here we demonstrate that different behaviour can be observed between the oil and water fractions in water wet and preferentially oil wet cores, and that saturation states can be determined. This technique could provide a quantitative NMR measure of wettability.

Published
2015

28. Chemical resolution in T 2 - T 1 correlations

Author

Mitchell, Jonathan, Chandrasekera, Thusara C., Fordham, Edmund J., Crawshaw, John, Staniland, John, Johns, Mike L., Gladden, Lynn F., University of Cambridge, New Museums Site, Schlumberger Cambridge Research, and Universität Leipzig

Subjects
diffusion, transport, ddc:530
Abstract

Oil and water fractions have been identified in fluid saturated carbonate rock cores using a novel T2-T1-d pulse sequence. The inclusion of the chemical shift dimension d allows T2-T1 plots to be generated independently for the oil and water. The T2-T1-d pulse sequence utilises a “double-shot” T1 measurement that provides free induction decays (FIDs) as a function of both relaxation times for suitably broad line samples. The T2-T1-d data set is acquired in the same experimental time as a conventional T1-T2 measurement (without chemical resolution) of equivalent data density. Here we demonstrate that different behaviour can be observed between the oil and water fractions in water wet and preferentially oil wet cores, and that saturation states can be determined. This technique could provide a quantitative NMR measure of wettability.

Published
2009

30. Monitoring Chemical EOR Processes

Author

Mitchell, Jonathan, additional, Staniland, John, additional, Wilson, Alex, additional, Howe, Andrew, additional, Clarke, Andrew, additional, Fordham, Edmund J., additional, Edwards, John, additional, Faber, Rien, additional, and Bouwmeester, Ron, additional

Published
2014
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31. Time-Lapse Diffusion Logs to Discriminate Reservoir Oil from Miscible Gas

Author

Edwards, John, additional, Mutina, Albina, additional, Cheng, Yuesheng, additional, Gao, Jinglin, additional, Fordham, Edmund, additional, Freed, Denise, additional, Bachman, Nate, additional, Dolan, Sean, additional, Al Rashdi, Yaqoob, additional, Wilson, Ove Bjorn, additional, Haynes, Byron, additional, and Looyestijn, Wi m, additional

Published
2012
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38. Letters to the editor

Author

Lorino, Ing. Francesco, primary, Bittleston, Simon H., additional, Fordham, Edmund J., additional, Tehrani, M. Ahmadi, additional, Mehmeto??lu, Tanju, additional, and Gücüyener, Hakki, additional

Published
1994
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