31P NMR saturation transfer study of the creatine kinase reaction in human skeletal muscle at rest and during exercise

The creatine kinase reaction has been studied by 31P NMR in exercising human calf muscle. Quantitative analysis of high energy phosphates and saturation transfer study of the creatine kinase flux in the direction of ATP synthesis (Vfor) were performed at rest and during exercise. As expected, exercise induced a [PCr] decrease (from 28.5 +/- 0.9 to 21.9 +/- 1.5 mM, P < 0.01) matched by a Pi increase (from 4.5 +/- 0.2 to 8.9 +/- 1.8 mM, P = 0.06). pHi and [ATP] remained unchanged. Vfor did not change from rest (12.4 +/- 0.9 mM s(-1)) to moderate exercise and decreased at the highest exercise level (8.4 +/- 1.4 mM s(-1), P = 0.006). This observation differs from the prediction of the creatine kinase rate equation, showing an increase in the flux with exercise intensity. Computations suggest that this discrepancy arises from metabolite compartmentalization and/or from the reaction kinetics of a dead end complex stabilized by planar anions

Bias Reduction for Low-Statistics PET: Maximum Likelihood Reconstruction With a Modified Poisson Distribution

Positron emission tomography data are typically reconstructed with maximum likelihood expectation maximization (MLEM). However, MLEM suffers from positive bias due to the non-negativity constraint. This is particularly problematic for tracer kinetic modeling. Two reconstruction methods with bias reduction properties that do not use strict Poisson optimization are presented and compared to each other, to filtered backprojection (FBP), and to MLEM. The first method is an extension of NEGML, where the Poisson distribution is replaced by a Gaussian distribution for low count data points. The transition point between the Gaussian and the Poisson regime is a parameter of the model. The second method is a simplification of ABML. ABML has a lower and upper bound for the reconstructed image whereas AML has the upper bound set to infinity. AML uses a negative lower bound to obtain bias reduction properties. Different choices of the lower bound are studied. The parameter of both algorithms determines the effectiveness of the bias reduction and should be chosen large enough to ensure bias-free images. This means that both algorithms become more similar to least squares algorithms, which turned out to be necessary to obtain bias-free reconstructions. This comes at the cost of increased variance. Nevertheless, NEGML and AML have lower variance than FBP. Furthermore, randoms handling has a large influence on the bias. Reconstruction with smoothed randoms results in lower bias compared to reconstruction with unsmoothed randoms or randoms precorrected data. However, NEGML and AML yield both bias-free images for large values of their parameter.status: publishe

Interactive delineation of brain sulci and their merging into functional PET images.

A set of software tools has been developed to assist the neuro(physio)logist in the analysis of a series of cerebral images from single subjects by fusing sulci manually delineated on MRI brain surface into PET functional data. The procedure requires coregistered datasets and involves 4 steps: (i) segmentation of anatomical MRI data in order to extract the brain surface, (ii) generate the brain surface views by parallel ray casting, (iii) manual delineation of the relevant sulci from the surface views and (iv) fusion of the landmarks into any coregistered dataset from the same subject. The brain surface is segmented automatically from 3D MRI data using a new “Directional Watershed Transform” algorithm. From the segmented brain surface, 8 orthogonal surface views are calculated as visual support for interactive stereo definition of the major brain sulci. Each sulcus is built as a 3D trace line using a few vertices which are manually defined on one or several surface views. This technique allows one to follow the brain surface curvature rather independently of the number and the position of the vertices. The sulci are saved in an individual file for further use. The brain surface viewer is linked (via the 3D cursor position) to an independent volume viewer containing a coregistered (anatomical or functional) volume. Sulcal landmarks are finally projected onto this volume allowing further volume of interest definition. The use of the tool set is illustrated by a single subject brain activation study after 15O water injection

Volumic image analysis application bundle for UNIX workstations.

A set of applications has been developed for volumic medical image display and analysis in PET and MRI. Although the set may appear at first glance, as a implementation in X-Windows of a package previously developed at CTI under SunView, major improvements have been added to its functionality. All graphical user interfaces have been developed with InterViews, a public domain C++ library on top of X-Windows, in an effort to standardize the look and feel of all applications. They run on SUN/Spare and Digital UNIX workstations and support CTI ECAT and Interfile image formats. They are presently used on a daily basis for multimodal image display and analysis in the authors' laboratory. These applications are: (i) iv-colortool: a color lookup table server for multimodality image display which can be loaded with external tables when displaying fusion data, (ii) iv-volumetool: an orthogonal viewer which allows visualization of multiframe volumes as large as whole body studies. Multiple instances of the application can be created with linked cursors, a useful feature for visualizing coregistered studies, (iii) iv-heartool: an orthogonal viewer which allows heart volume reorientation along the long axis and generation of short axis slices on multiframe studies and (iv) ivbrain- align: an orthogonal viewer which allows rigid transformation of dynamic multi-tracer brain data. All applications can be controlled through a command language interface using UNIX keywords to control input/output and parameters. This feature allows their direct use from acquisitions scripts on CTI PET scanners controlled with System 7