Infrared thermographic SAR measurements of interstitial hyperthermia applicators: errors due to thermal conduction and convection.

Thermal conduction and convection were examined as sources of error in thermographically measured SAR patterns of an interstitial microwave hyperthermia applicator. Measurements were performed in a layered block of muscle-equivalent phantom material using an infrared thermographic technique with varying heating duration. There was a 52.7% reduction in maximum SAR and 75.5% increase in 50% iso-SAR contour area for a 60-s heating duration relative to a 10-s heating duration. A finite element model of heat transfer in an homogeneous medium was used to model conductive and convective heat transfer during the thermographic measurement. Thermal conduction artefacts were found to significantly distort thermographically measured SAR patterns. Convective cooling, which occurs when phantom layers are exposed for thermal image acquisition, was found to significantly affect the magnitude, but not the spatial distribution, of thermographically measured SAR patterns. Results from this investigation suggest that the thermal diffusion artefacts can be minimized if the duration of the applied power pulse is restricted to 10 s or less.

Defibrillation current density imaging: comparison of in-vivo and post-mortem measurements in a pig.

Current density imaging (CDI) is an MRI technique used to measure electrical current density vectors throughout a volume of tissue. Previous work used CDI to measure current pathways through the heart and chest of a post-mortem pig when current is applied using external flexible defibrillation electrodes with typical anterior-anterior positioning. In these post-mortem studies, current pathways were probably influenced by the anisotropic conductivity of the tissues. This work aims to compare post-mortem ( approximately 15 min. and approximately 1 hour after death) results with new in vivo CDI measurements. These measurements indicate that the macroscopic (i.e. across the whole body) current pathways remain similar before and after death, however, at a smaller scale (i.e. distances of a few cm) current pathways are different, particularly in the heart. This comparison demonstrates the influence on current pathways of rapidly changing electric properties of tissue following death.

A new approach to current density impedance imaging.

Current density impedance imaging (CDII) is a new impedance imaging technique that utilizes current density vector measurements made using magnetic resonance imager (MRI). CDII provides a simple mathematical expression for the gradient of the logarithm of conductivity, nablaln(sigma), at each point in a region where two current density vector has been measured. From the images of the gradient of the logarithm of conductivity, ln(sigma) can be reconstructed through integration and of sigma by a priori knowledge of the conductivity at a single point in the object. The CDII technique was tested on a conductivity phantom made from tissue mimicking gel. The results showed accurate reconstruction of the gel conductivity from two current density measurements. This study, for the first time, has demonstrated a local reconstruction technique to calculate sample conductivity inside the phantom noninvasively.

Changes in the complex permittivity during spreading depression in rat cortex.

With recent developments in current density imaging (CDI), it is feasible to utilize this new technique in brain imaging applications. Since CDI's ability to measure changes in current density depends on a concomitant activity-dependent change in the conductivity of the brain tissue, we have examined the changes in complex conductivity during spreading depression (SD) in rodent neocortex using a coaxial probe. SD was chosen because it is often referred to as an animal model of cerebral ischemia and migraine with aura. The conductivity measurements revealed a change with short latency (30-60 s) followed by a change with a longer latency (200-300 s). This change in conductivity with short latency has not been reported before, and we conjecture that it may be the priming or triggering mechanism prior to the main SD episode. A 20% change in conductivity during SD is sufficiently large to be measured by CDI. Therefore, the ability to measure changes in the conductivity, as opposed to metabolic changes, makes CDI a viable approach to the study of ischemia and migraine with aura.

Imaging of current density and current pathways in rabbit brain during transcranial electrostimulation.

A magnetic resonance imaging (MRI) method was used for a noninvasive study of current density (CD) and current pathways (CP's) inside the skull during transcranial electrostimulation in rabbits. The transcranial impulse current directions studied were those previously used in transcranial electric treatment either sagittally or bilaterally. MRI data were collected from slices perpendicular to the direction of current application. In these slices, only the perpendicular component of the CD was measured. Computer methods for accurate topographic mapping of the main areas with high CD and for reconstruction of CP's are described. It was revealed that current applied on the head sagittally passed mostly through the cerebrospinal fluid in the basal brain cisternas connected in series, and through the anterior horns of the lateral ventricles, foramina of Monro, ventrocaudal part of the third ventricle, aqueductus, and fourth ventricle. Possible connections between these CP's are suggested. Bilaterally applied current passed through the brain and skull core more diffusely without concentrations in cisternas and ventricles. The results of the present study suggest an explanation for the observation that sagittally applied current more effectively stimulates brain structures with antinociceptive function and elicits more pronounced analgesic effect.

Thalamic stimulation and functional magnetic resonance imaging: localization of cortical and subcortical activation with implanted electrodes. Technical note.

The utility of functional magnetic resonance (fMR) imaging in patients with implanted thalamic electrodes has not yet been determined. The aim of this study was to establish the safety of performing fMR imaging in patients with thalamic deep brain stimulators and to determine the value of fMR imaging in detecting cortical and subcortical activity during stimulation. Functional MR imaging was performed in three patients suffering from chronic pain and two patients with essential tremor. Two of the three patients with pain had undergone electrode implantation in the thalamic sensory ventralis caudalis (Vc) nucleus and the other had undergone electrode implantation in both the Vc and the periventricular gray (PVG) matter. Patients with tremor underwent electrode implantation in the ventralis intermedius (Vim) nucleus. Functional MR imaging was performed during stimulation by using a pulse generator connected to a transcutaneous extension lead. Clinically, Vc stimulation evoked paresthesias in the contralateral body, PVG stimulation evoked a sensation of diffuse internal body warmth, and Vim stimulation caused tremor arrest. Functional images were acquired using a 1.5-tesla MR imaging system. The Vc stimulation at intensities provoking paresthesias resulted in activation of the primary somatosensory cortex (SI). Stimulation at subthreshold intensities failed to activate the SI. Additional stimulation-coupled activation was observed in the thalamus, the secondary somatosensory cortex (SII), and the insula. In contrast, stimulation of the PVG electrode did not evoke paresthesias or activate the SI, but resulted in medial thalamic and cingulate cortex activation. Stimulation in the Vim resulted in thalamic, basal ganglia, and SI activation. An evaluation of the safety of the procedure indicated that significant current could be induced within the electrode if a faulty connecting cable (defective insulation) came in contact with the patient. Simple precautions, such as inspection of wires for fraying and prevention of their contact with the patient, enabled the procedure to be conducted safely. Clinical safety was further corroborated by performing 86 MR studies in patients in whom electrodes had been implanted with no adverse clinical effects. This is the first report of the use of fMR imaging during stimulation with implanted thalamic electrodes. The authors' findings demonstrate that fMR imaging can safely detect the activation of cortical and subcortical neuronal pathways during stimulation and that stimulation does not interfere with imaging. This approach offers great potential for understanding the mechanisms of action of deep brain stimulation and those underlying pain and tremor generation.

Rotating frame RF current density imaging.

RF current density imaging (RF-CDI) is a new MRI technique for imaging the Larmor frequency current density parallel to B0 in electrolytic media. To extend the use of RF-CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a large B1 field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three-compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time and B1, whereas the avoidance of phase artifacts imposes a lower bound on B1.

RF current density imaging in homogeneous media.

MRI has proven capable of imaging quasistatic volume current densities in electrolytic and biological media. In this paper, the feasibility of extending the method to image RF current density at the Larmor frequency is studied. RF current imaging could be relevant to MR power absorption and safety and to hyperthermia analysis, as well as creating dielectric and conductivity-dependent tissue contrast. The approach is to deliberately induce or inject RF currents in a sample synchronous with an MR pulse sequence and measure the resulting transverse RF magnetic field components. Current density is extracted by computing the curl of the magnetic fields. The preliminary theory has been developed for uniform media where both displacement and conduction currents exist while skin effects or eddy currents are absent. If the derivative in the B0 direction of the RF magnetic field component parallel to B0 is negligible, then sufficient information exists to reconstruct the RF current density component that is parallel to B0 without rotating the sample. The relative phase of the current can also be estimated. The method has been proven feasible by successfully imaging a uniform 85.6-MHz current density in a salt water phantom. The experiment conforms closely to capacitively coupled hyperthermia heating.

Quantitative two-dimensional time correlation relaxometry.

An experimental and analytical method is presented for obtaining two-dimensional NMR time correlation spectra of T1 (or T1p) and T2 exponential relaxation in heterogeneous samples. The numerical algorithms used in this study do not bias the solution by any a priori assumptions as to the number of required components. The constraints used to overcome the ill-posed and ill-conditioned nature of this inverse problem are also described. With this method, the correlations of T1 (or T1p) and T2 exponential components in systems of nonexchanging standards and rat muscle tissues are obtained.

Pulsed NMR relaxometry of striated muscle fibers.

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation of fast- and slow-twitch muscle fibers are examined using rat muscle tissues in which one fiber type predominates. Both continuum and discrete exponential component fits are made to Carr-Purcell-Meiboom-Gill (CPMG) and inversion recovery pulse sequence measurements. In addition, experiments which illustrate the large sources of variability that have led to apparent conflicts in the literature are presented. Based on the results of this study, unique NMR features that distinguish fast- and slow-twitch muscle fibers are presented. The feasibility of differentiating fast- and slow-twitch muscle fibers using magnetic resonance (MR) imaging is briefly discussed.

Quantum noise measurements for fourier multiaperture emission tomography.

The mean and variance of an image reconstructed by Fourier multiaperture emission tomography are estimated by a physical experiment and by simulations. Results correspond closely to those obtained from a stochastic model of the noise process. A new scheme to enhance the statistical performance is proposed which improves the signal-to-noise ratio by 35 percent.

Endotracheal artificial larynx -- preliminary report.

In experimental animals a one stage surgical procedure has been developed which at laryngectomy allows for the creation of a wide and patent tracheopharyngeal fistula. An endotracheal artificial larynx has been developed which has been found effective in prevention of aspiration in experimental animals. Bench studies indicate that an adequate voice probably can be obtained with this device.

Resolution of pinhole collimators used with gamma cameras.

The line spread function (LSF) of an imaging system incorporating a pinhole of 0.275 cm radius and a scintillation camera has been measured and its full width at half maximum (FWHM) compared to the values predicted by an expression based on the resolution index of Anger. In most cases, the actual resolution of the system is approximately 20% superior to that predicted by the above expression. However, good agreement is observed between our measured LSFs and those computed using a frequency domain method. Based on these computed LSFs, an empirical expression relating the FWHM of the total system to pinhole and camera parameters with an accuracy of +/- 3% is presented. This expression is applicable to most pinhole imaging systems in use today.

Fourier multiaperture emission tomography (FMET).

We describe a new emission tomographic technique, using an Anger scintillation camera and a new family of coded apertures, in which a noniterative computer algorithm is used to reconstruct multiplanar images. The novel feature is a set of spatial narrow band filters that allow the depth information to be separated in the frequency domain. The position of the reconstructed planes can be selected at will. Using this technique, a planar resolution of 0.75 cm and a depth resolution of 1.6 cm have been measured at a depth of 10 cm and a magnification of 1. To demonstrate the potential of this system, the results of a phantom study are reported.