Biomedical subjects

G Ehnholm

Publications and source records attributed to G Ehnholm.

4 recordsLinked to original sources

PubMed2001

Registration in interventional procedures with optical navigator.

Performing interventional procedures in the close proximity to an MR scanner widens the range of operations available for an optical tracking system. In order to gain the full benefits from both unrestricted use of surgical instruments outside the magnet and intraoperative imaging, a method for transferring the registration data of the optical navigator between two locations is required. An optical tracking system, which provides such a transfer method and tracks patient position during a surgical procedure, has been developed, tested, and demonstrated with two patient cases. J. Magn. Reson. Imaging 2001;13:93-98.

Adult↗

PubMed2001

Use of multicoil arrays for separation of signal from multiple slices simultaneously excited.

Increased acquisition efficiency has been achieved by exciting several slices simultaneously. The mixed data were unfolded to produce separate slices using the spatial encoding information inherent in a multicoil receiver system. Each coil yields a linear combination of signals from all excited slices weighted by the sensitivity of each coil. A matrix inversion provides a solution to unfold these images.

Equipment Design↗

PubMed2000

Dynamic in vivo oxymetry using overhauser enhanced MR imaging.

A noninvasive method for in vivo measurement of the oxygen concentration has been developed. By introducing a novel contrast medium (CM) based on a single electron substance, it is possible to enhance the proton signal through the Overhauser effect. A low-field magnetic resonance scanner is used to image the proton nuclei of the object. The electron spin transition of the CM is saturated using rf irradiation. As a consequence, the nuclear polarization becomes enhanced through dipole-dipole interaction. The signal enhancement is a function of rf power and of the EPR line width of the substance, which is influenced by the oxygen concentration. The maximum in vivo enhancement has been measured to 60. Image data, generated with different scanning parameters, is used in a postprocessing method to generate images showing pO(2) and the contrast medium concentration, respectively. The mathematical foundation of the postprocessing algorithm is outlined. The results from phantom experiments and animal experiments, in which the oxygen content of the inspired gas was varied, are presented. The potential for human imaging is discussed. J. Magn. Reson. Imaging 2000;12:929-938.

Algorithms↗

PubMed1998

EPR and DNP properties of certain novel single electron contrast agents intended for oximetric imaging.

Parameters of relevance to oximetry with Overhauser magnetic resonance imaging (OMRI) have been measured for three single electron contrast agents of the triphenylmethyl type. The single electron contrast agents are stable and water soluble. Magnetic resonance properties of the agents have been examined with electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and dynamic nuclear polarization (DNP) at 9.5 mT in water, isotonic saline, plasma, and blood at 23 and 37 degreesC. The relaxivities of the agents are about 0.2-0.4 mM-1s-1 and the DNP enhancements extrapolate close to the dipolar limit. The agents have a single, narrow EPR line, which is analyzed as a Voigt function. The linewidth is measured as a function of the agent concentration and the oxygen concentration. The concentration broadenings are about 1-3 microT/mM and the Lorentzian linewidths at infinite dilution are less than 1 microT in water at room temperature. The longitudinal electron spin relaxation rate is calculated from the DNP enhancement curves. The oxygen broadening in water is about 50 microT/mM O2 at 37 degreesC. These agents have good properties for oximetry with OMRI.

Algorithms↗