Joseph J. H. Ackerman

Bio: Joseph J. H. Ackerman is an academic researcher from Washington University in St. Louis. The author has contributed to research in topics: Magnetic resonance imaging & Intracellular. The author has an hindex of 47, co-authored 159 publications receiving 7545 citations. Previous affiliations of Joseph J. H. Ackerman include University of Washington.

Water proton MR properties of human blood at 1.5 Tesla: Magnetic susceptibility, T1, T2, T *2, and non‐Lorentzian signal behavior

Abstract: Accurate knowledge of the magnetic properties of human blood is required for the precise modeling of functional and vascular flow-related MRI. Herein are reported determinations of the relaxation parameters of blood, employing in vitro samples that are well representative of human blood in situ. The envelope of the blood (1)H(2)O free-induction decay signal magnitude during the first 100 msec following a spin echo at time TE is well- described empirically by an expression of the form, S(t) = S(o). exp[-R(*)(2). (t - TE) - AR*. (t - TE)(2)]. The relaxation parameters AR* and R(*)(2) increase as a function of the square of the susceptibility difference between red blood cell and plasma and depend on the spin-echo time. The Gaussian component, AR*, should be recognized in accurate modeling of MRI phenomena that depend upon the magnetic state of blood. The magnetic susceptibility difference between fully deoxygenated and fully oxygenated red blood cells at 37 degrees C is 0.27 ppm, as determined independently by MR and superconducting quantum interference device (SQUID) measurements. This value agrees well with the 1936 report of Pauling and Coryell (Proc Natl Acad Sci USA 1936;22:210-216), but is substantially larger than that frequently used in MRI literature. Magn Reson Med 45:533-542, 2001.

Rapid vacuolar sequestration: the horseweed glyphosate resistance mechanism

Abstract: BACKGROUND: Glyphosate-resistant (GR) weed species are now found with increasing frequency and threaten the critically important GR weed management system. RESULTS: The reported 31P NMR experiments on glyphosate-sensitive (S) and glyphosate-resistant (R) horseweed, Conyza canadensis (L.) Cronq., show significantly more accumulation of glyphosate within the R biotype vacuole. CONCLUSIONS: Selective sequestration of glyphosate into the vacuole confers the observed horseweed resistance to glyphosate. This observation represents the first clear evidence for the glyphosate resistance mechanism in C. canadensis. Copyright © 2010 Society of Chemical Industry

Statistical model for diffusion attenuated MR signal

Abstract: Growing interest in diffusion MRI stems from numerous clinical and research applications (for example, see recent reviews in special issues of NMR in Biomedicine (1,2)). Most applications rely on a Stejskal-Tanner (3) pulsed gradient spin echo (PGSE) experiment and an assumption that the diffusion-attenuated MR signal can be expressed as a monoexponential function given by (3): S=S0exp(−b ADC). [1] Here ADC is the apparent diffusion coefficient for tissue-water or other MR active species under consideration; the so-called b-value depends on the shape of the diffusion-sensitizing gradient pulse waveform G(t) (4): b=γ2∫0tdt′[∫0t′dt″ G(t″)]2, [2] where γ is the magnetogyric (late gyromagnetic) ratio of the nuclide under consideration. For example (3), for the case of bipolar rectangular gradient pulses with amplitude G, duration δ and interval between pulse centers Δ: b=(γGδ)2(Δ−13δ). [3] However, numerous studies of the diffusion of water and/or other metabolites in brain tissue and other biological systems have documented a non-monoexponential behavior of the MR signal S as a function of the b-value at fixed diffusion times (e.g., see Refs. 5–17). Most authors report that their data can be fit well by a biexponential function with two different diffusion coefficients (large/fast and small/slow) and suggest ascribing the two exponential components to two physical compartments (extra-and intracellular) in a tissue. However, as noted by Le Bihan and van Zijl (18), the origin of fast- and slow-diffusion pools is “still mysterious.” Moreover, Sehy et al. (19) have directly observed biexponential diffusion MR signal behavior within the intracellular space of a single cell, the frog oocyte. Generally speaking, Eqs. [1] and [3] describe the diffusion-attenuated MR signal in a PGSE experiment only for unrestricted diffusion in homogeneous media. However, in most in vivo experiments each imaging or spectroscopic voxel contains numerous cells with different cell types, sizes, geometries, orientations, membrane permeabilities, and presumably different T2 and T1 relaxation time constants. Hence, the expectation that such a system can be described in terms of a simple unrestricted diffusion (monoexponential) model Eq. [1] is not reasonable. Practically any of the above-mentioned issues can lead to a deviation from monoexponential behavior—a point clearly illustrated in a recent article by Chin et al. (20). These authors carried out numerical simulations for a realistic geometrical structure of a rat spinal cord and found that under a variety of conditions the signal behavior is not monoexponential and can be generally described in terms of biexponential diffusion attenuation. Theoretical modeling of diffusion in multicompartment systems with cylindrical geometry by van der Weerd et al. (21) also demonstrated that “multi-exponential analysis of diffusion signal behavior cannot be freely related to the geometrical parameters of the system.” Moreover, for short diffusion times this laboratory has shown that biexponential signal behavior is predicted theoretically even for a single homogeneous compartment (22). A number of theoretical models based on the complex structure of biological objects have also been developed to describe non-monoexponential PGSE MR signal behavior (10,16, 23–27,32). While in some cases a biexponential behavior of an MR signal can obviously be related to the tissue compartmentalization or geometrical structure, in most cases it simply reflects the fact that a biexponential function describes signal behavior better than does a monoexponential function without a simple obvious relationship between the biexponential model’s parameters and the physical parameters of the system under investigation. It would seem reasonable to develop a well-defined physical model that recognizes the presence of variable length scales for restrictions and hindrances to water diffusion in biological systems. Such systems, which by their very nature represent highly complicated structures, present an architectural arrangement characterized by an extraordinary range of length scales. The MR diffusion experiment is sensitive to a subset of the full range of possible length scales, dependent on experimental parameters and the practical constraints of the measurement (SNR, etc.). The MR relevant range of length scales for restrictions and hindrances to water diffusion should be reflected in any model of MR diffusion data from biological systems. The critical issue is how best to do this. Below we propose a statistical method/model to reflect in a well-defined manner the consequences of a distribution of length scales for restrictions and hindrances to water diffusion (i.e., by a resulting diffusion coefficient probability distribution characterized by a mean value and a distribution width). This general phenomenological model can describe a rather large number of experimental results related to the structure of PGSE diffusion-attenuated MR signal in biological systems. We apply this model to describe MR data in a human brain and demonstrate good agreement with data obtained from practically all regions of brain for a b-value interval from zero to 2 ms/μm2.

Evaluation of extra- and intracellular apparent diffusion in normal and globally ischemic rat brain via 19F NMR

Abstract: The biophysical mechanism(s) underlying diffusion-weighted MRI contrast following brain injury remains to be elucidated. Although it is generally accepted that water apparent diffusion coefficient (ADC) decreases after brain injury, it is unknown whether this is associated with a decrease in intracellular or extracellular water displacement, or both. To address this question, 2-[ 19 F]luoro-2-deoxyglucose-6-phosphate (2FDG-6P) was employed as a compartment-specific marker in normal and globally ischemic rat brain. Through judicious choice of routes of administration, 2FDG-6P was confined to the intra- or extracellular space. There was no statistical difference between intra- and extracellular 2FDG-6P ADCs in normal or in globally ischemic brain (p > 0.16), suggesting that water ADCs in both compartments are similar. However, ischemia did result in a 40% ADC decrease in both compartments (P < 0.001). Assuming that 2FDG-6P reflects water motion, this study shows that water ADC decreases in both spaces after ischemia, with the reduction of intracellular water motion being the primary source of diffusion-weighted contrast.

The basis of anisotropic water diffusion in the nervous system – a technical review

Abstract: Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system.

Blood Flow, Oxygen and Nutrient Supply, and Metabolic Microenvironment of Human Tumors: A Review

Abstract: The objective of this review article is to summarize current knowledge of blood flow and perfusion-related parameters, which usually go hand in hand and in turn define the cellular metabolic microenvironment of human malignancies. A compilation of available data from the literature on blood flow, oxygen and nutrient supply, and tissue oxygen and pH distribution in human tumors is presented. Whenever possible, data obtained for human tumors are compared with the respective parameters in normal tissues, isotransplanted or spontaneous rodent tumors, and xenografted human tumors. Although data on human tumors in situ are scarce and there may be significant errors associated with the techniques used for measurements, experimental evidence is provided for the existence of a compromised and anisotropic blood supply to many tumors. As a result, O2-depleted areas develop in human malignancies which coincide with nutrient and energy deprivation and with a hostile metabolic microenvironment (e.g., existence of severe tissue acidosis). Significant variations in these relevant parameters must be expected between different locations within the same tumor, at the same location at different times, and between individual tumors of the same grading and staging. Furthermore, this synopsis will attempt to identify relevant pathophysiological parameters and other related areas future research of which might be most beneficial for designing individually tailored treatment protocols with the goal of predicting the acute and/or long-term response of tumors to therapy.

Creatine and Creatinine Metabolism

Abstract: The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis an...

Molecular imaging in living subjects: seeing fundamental biological processes in a new light

Abstract: References http://genesdev.cshlp.org/content/17/5/545.full.html#related-urls Article cited in: http://genesdev.cshlp.org/content/17/5/545.full.html#ref-list-1 This article cites 228 articles, 79 of which can be accessed free at: service Email alerting click here top right corner of the article or Receive free email alerts when new articles cite this article sign up in the box at the Collections Topic (33 articles) Molecular Physiology and Metabolism • (98 articles) Cancer and Disease Models • Articles on similar topics can be found in the following collections