The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform

Steven A. Julious - One of the best experts on this subject based on the ideXlab platform.

  • Designing clinical trials with uncertain estimates of variability
    Pharmaceutical Statistics, 2004
    Co-Authors: Steven A. Julious
    Abstract:

    An estimated sample size is a function of three components: the required power, the predetermined Type I error rate, and the specified effect size. For Normal data the Standardized effect size is taken as the difference between two means divided by an estimate of the Population Standard Deviation. However, in early phase trials one may not have a good estimate of the Population variance as it is often based on the results of a few relatively small trials. The imprecision of this estimate should be taken into account in sample size calculations. When estimating a trial sample size this paper recommends that one should investigate the sensitivity of the trial to the assumptions made about the variance and consider being adaptive in one's trial design. Copyright © 2004 John Wiley & Sons Ltd.

Allan Mackenzie-graham - One of the best experts on this subject based on the ideXlab platform.

  • In vivo vs. ex vivo Magnetic Resonance Imaging In Mice.
    Frontiers in Neuroinformatics, 2012
    Co-Authors: Allan Mackenzie-graham
    Abstract:

    General Commentary published: 17 May 2012 doi: 10.3389/fninf.2012.00019 NEUROINFORMATICS In vivo vs. ex vivo magnetic resonance imaging in mice Allan MacKenzie-Graham* Multiple Sclerosis Program, Department of Neurology, University of California Los Angeles, Los Angeles, CA, USA *Correspondence: amg@ucla.edu A commentary on Wanted dead or alive? The tradeoff between in vivo vs. ex vivo MR brain imaging in the mouse by Lerch, J. P., Gazdzinski, L., Germann, J., Sled, J. G., Henkelman, R. M., and Nieman, B. J. (2012). Front. Neuroinform. 6:6. doi: 10.3389/fninf.2012.00006 The use of high-resolution MRI for the eval- uation of structural changes in the mouse brain is rapidly gaining favor with research- ers. The decision to use either in vivo or ex vivo imaging is often a practical one. For example, technical limitations, such as the availability of appropriate equipment to image in vivo, may force an investigator to use ex vivo imaging. Conversely, animal availability may limit the number of sam- ples used at each time-point in an ex vivo experimental design, but not affect an in vivo design. But what if we were not lim- ited by these considerations? What if we could decide which approach to take based entirely on which approach would yield the best data? In “Wanted dead or alive? The trade- off between in vivo vs. ex vivo MR brain imaging in the mouse” presented by Lerch et al. (2012), this is exactly what is consid- ered. Ex vivo imaging benefits from greater resolution and sensitivity due to the lack Frontiers in Neuroinformatics of ­constraints on imaging time, the use of tighter fitting coils, high concentration contrast agents, and a lack of movement artifacts. In vivo imaging allows for the longitudinal analysis of structural change, a benefit that can not be underestimated. Longitudinal studies lend themselves to other forms of statistical analysis, such as repeated measures ANOVAs, which can increase the statistical power of the stud- ies. But beyond the obvious differences, how sensitive are these MRI measures to struc- tural changes on the order of 5% of the total volume of a neuroanatomical structure? The authors describe a series of statistical analy- ses (based on imaging data they acquired) used to evaluate the tradeoffs between the use of in vivo (longitudinal) analysis and ex vivo (cross sectional) analysis. For a remarkably complex problem, the results are surprisingly straightforward. Firstly, ex vivo imaging is more precise than in vivo imaging. In cases where precise time- course data is not required, ex vivo imaging provides better results. This is due to the lower within subject variability inherent to ex vivo imaging (higher resolution, no movement). However, if changes in absolute volumes or rates of change are required, in vivo imaging provides better information. Interestingly, in longitudinal experiments, the addition of more subjects, rather than more time-points, increases statistical power more rapidly. www.frontiersin.org Secondly, the kind of analysis used mat- ters. Spatial normalization to an unbiased consensus average, correcting for gross differences in brain size, decreases the Population Standard Deviation markedly (by approximately 50%). Before spa- tial normalization, Population Standard Deviations are about the same size as the effects to be observed. Clearly, spatial nor- malization is a crucial step in any form of volumetry. As in vivo imaging equipment and exper- tise become more common, the decision between the use of in vivo or ex vivo imag- ing begins to boil down to choosing the best tool for the job; ex vivo for precision, in vivo for time-course, and spatial normalization for all. Reference Lerch, J. P., Gazdzinski, L., Germann, J., Sled, J. G., Henkelman, R. M., and Nieman, B. J. (2012). Wanted dead or alive? The tradeoff between in-vivo versus ex-vivo MR brain imaging in the mouse. Front. Neuroinform. 6:6. doi: 10.3389/fninf.2012.00006 Received: 23 April 2012; accepted: 26 April 2012; published online: 17 May 2012. Citation: MacKenzie-Graham A (2012) In vivo vs. ex vivo magnetic resonance imaging in mice. Front. Neuroinform. 6:19. doi: 10.3389/fninf.2012.00019 Copyright © 2012 MacKenzie-Graham. This is an open- access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduc- tion in other forums, provided the original authors and source are credited. May 2012 | Volume 6 | Article 19 | 1

Peter H. Jarritt - One of the best experts on this subject based on the ideXlab platform.

  • Effect of attenuation correction on myocardial thallium-201 distribution in patients with a low likelihood of coronary artery disease
    European Journal of Nuclear Medicine, 1997
    Co-Authors: Elizabeth M. Prvulovich, Albert Hr Lonn, Jamshed B. Bomanji, Peter H. Jarritt
    Abstract:

    Regional variation of tracer distribution is seen in uncorrected thallium-201 images of normal hearts. This study evaluates the effect of attenuation correction on myocardial^201Tl distribution in patients with low risk of coronary artery disease. An L-shaped dualdetector single-photon emission tomographic system equipped with a pair gadolinium-153 scanning line sources was used for sequential emission/transmission imaging in 36 patients (14 men and 22 women) with less than 5% risk for coronary artery disease. Uncorrected emission images were reconstructed using filtered back-projection (FBP) whereas the attenuation corrected (AC) images were iteratively reconstructed using the attenuation map computed from the transmission data. Both sets of images were reorientated into short axis, vertical long axis and horizontal long axis images. For quantification data were reconstructed into polar plots and count density estimated in 17 myocardial segments. The Population % Standard Deviation for each segment of AC data was significantly smaller than that for FBP data, indicating improved homogeneity of tracer distribution. In men the anterior-basal inferior activity ratio improved from 1.20 for FBP to 0.96 for AC (stress) and from 1.23 for FBP to 0.98 for AC (delay) ( P

  • effect of attenuation correction on myocardial thallium 201 distribution in patients with a low likelihood of coronary artery disease
    European Journal of Nuclear Medicine and Molecular Imaging, 1997
    Co-Authors: Elizabeth M. Prvulovich, Albert Hr Lonn, Jamshed B. Bomanji, Peter H. Jarritt
    Abstract:

    Regional variation of tracer distribution is seen in uncorrected thallium-201 images of normal hearts. This study evaluates the effect of attenuation correction on myocardial201Tl distribution in patients with low risk of coronary artery disease. An L-shaped dualdetector single-photon emission tomographic system equipped with a pair gadolinium-153 scanning line sources was used for sequential emission/transmission imaging in 36 patients (14 men and 22 women) with less than 5% risk for coronary artery disease. Uncorrected emission images were reconstructed using filtered back-projection (FBP) whereas the attenuation corrected (AC) images were iteratively reconstructed using the attenuation map computed from the transmission data. Both sets of images were reorientated into short axis, vertical long axis and horizontal long axis images. For quantification data were reconstructed into polar plots and count density estimated in 17 myocardial segments. The Population % Standard Deviation for each segment of AC data was significantly smaller than that for FBP data, indicating improved homogeneity of tracer distribution. In men the anterior-basal inferior activity ratio improved from 1.20 for FBP to 0.96 for AC (stress) and from 1.23 for FBP to 0.98 for AC (delay) (P<0.0001). In women the anterior-basal inferior activity ratio changed from 1.08 for FBP to 0.94 for AC (stress) and from 1.08 for FBP to 0.93 for AC (delay) (P<0.001). These ratios reflect appropriate compensation for basal attenuation but a lack of scatter correction. The lateral—septal activity ratio in men changed from 1.05 for FBP to 0.99 for AC (stress) and from 1.02 for FBP to 0.96 for AC (delay), while in women it changed from 1.05 for FBP to 0.98 for AC (stress) and from 1.04 for FBP to 0.98 for AC (delay) (P<0.005 in all cases). The apex of AC images showed a decrease in activity consistent with wall thining at this site. It is concluded that the use of attenuation correction yields improved homogeneity of myocardial tracer distribution in patients with low risk of coronary artery disease. The diagnostic benefits of attenuation correction are yet to be fully assessed.

Richard H Browne - One of the best experts on this subject based on the ideXlab platform.

  • on the use of a pilot sample for sample size determination
    Statistics in Medicine, 1995
    Co-Authors: Richard H Browne
    Abstract:

    To compute the sample size needed to achieve the planned power for a t-test, one needs an estimate of the Population Standard Deviation δ. If one uses the sample Standard Deviation from a small pilot study as an estimate of δ, it is quite likely that the actual power for the planned study will be less than the planned power. Monte Carlo simulations indicate that using a 100(1 − γ) per cent upper one-sided confidence limit on δ will provide a sample size sufficient to achieve the planned power in at least 100(1 − γ) per cent of such trials.

Shilpa Nayyar - One of the best experts on this subject based on the ideXlab platform.

  • Determination of requisite sample size for surveys
    Journal of research, 2004
    Co-Authors: B S Bhullar, Shilpa Nayyar
    Abstract:

    The sample sizes required to estimate Population mean with per-specified margins of error were empirically determined. As expected, it was observed that the sample size required depend upon i) the Population Standard Deviation; and ii) the permissible margin of error. More the Population Standard Deviation, more will be the required sample size; and more the permissible margin of error, less the sample size. Also, the formulae for the estimation of required sample size as proposed by Sukhatme and Sukhatme (1970) and Murthy (1977) were found to be valid for the actual data on farmer's fields.