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K Wårdell

Publications and source records attributed to K Wårdell.

15 recordsLinked to original sources
PubMed2001

Analysis of temperature measurement for monitoring radio-frequency brain lesioning.

During ablative neurosurgery of movement disorders, for instance therapy of Parkinson's disease, temperature monitoring is crucial. This study aims at a quantitative comparison of measurement deviations between the maximum temperature located outside the lesioning electrode and two possible thermocouple locations inside the electrode. In order to obtain the detailed temperature field necessary for the analysis, four finite element models associated with different surroundings and with different power supplies are studied. The results from the simulations show that both the power level and the power density as well as the surrounding medium affect the temperature measurement and the temperature field in general. Since the maximum temperature is located outside the electrode there will always be a deviation in time and level between the measured and the maximum temperature. The deviation is usually 2-7 s and 3-12 degrees C, depending on, for example, the thermocouple location and surrounding medium. Therefore, not only the measured temperature but also the relation between measured and maximum temperature must be accounted for during therapy and device design.

Catheter Ablation↗
PubMed2001

Phototesting based on a divergent beam--a study on normal subjects.

In a previous publication from our group, phototesting based on a single exposure to a divergent UVB beam with radially decreasing irradiance values was suggested. The aim of the present study was to evaluate technical, practical and biological aspects of the suggested method in normal subjects. Twenty healthy volunteers were provoked on the back with both a collimated beam (four fixed doses, in circular areas with a diameter of 1.5 cm) and the divergent beam (a continuous, radially attenuating dose spectrum covering an area with a diameter of 4.5 cm). Eleven of the subjects were subjected to double provocation with the divergent beam. Assessment was carried out at 6 and 24 h after exposure by measuring the diameter of the reactions both visually and by mapping the skin blood flow change with laser Doppler perfusion imaging (LDPI). Minimal erythemal dose (MED) was determined for both the collimated and the divergent provocation. The reaction diameters were used to decide MED by combination to a mm for mm mapped dose spectrum of the divergent beam profile. Dose-response curves were plotted using the quantitative response data of the LDPI-images against the corresponding dosimetry data. No systematic difference could be proven between LDPI and visual diameters and a 95% confidence interval for the mean difference was calculated to (-0.8, 2.0). Slightly greater diameters were found at the visual assessment performed at 6 h compared to 24 h (95% confidence interval (-0.1, 2.8)). Double provocation showed a good reproducibility both for the visual and the LDPI assessment (P<0.05). The divergent beam provocation allowed a more detailed discrimination of MED compared to the collimated beam provocation. The MED values determined with the divergent beam were, however, generally higher, especially in the lower range of MED values. Technical factors related to the beam divergence and the correct measurement of erythemal effective irradiance are believed to be the explanation for this phenomenon, which is thus correctable. In conclusion, the results from this study support our belief that the phototesting protocol based on a divergent beam constitutes a good opportunity for improved phototesting, since MED and dose-response characteristics may be extracted in more detail from a single UV exposure.

Adult↗
PubMed1999

A comparison between in vitro studies of protein lesions generated by brain electrodes and finite element model simulations.

The aim of this study was to develop a finite element model for simulation of the thermal characteristics of brain electrodes and to compare its performances with an in vitro experimental albumin model. Ten lesions were created in albumin using a monopolar electrode connected to a Leksell Neuro Generator and a computer-assisted video system was used to determine the size of the generated lesions. A finite element model was set up of the in vitro experiments using the same thermal properties. With a very simple heat source applied to the finite element model in the proximity of the upper part of the tip, a good agreement (no deviations in width and distance from tip but a deviation in length of -1.6 mm) with the in vitro experiments (width 4.6 +/- 0.1 mm and length 7.4 +/- 0.1 mm) was achieved when comparing the outline of the lesion. In addition, a gelatinous albumin-model was set up and compared to computer simulations resulting in deviations in width of -0.4 mm, length of -2.2 mm and distance from the tip of -0.1 mm. Hence, the utilisation of finite element model simulations may be a useful complement to in-vitro experiments.

Albumins↗
PubMed1999

In vitro evaluation of brain lesioning electrodes (Leksell) using a computer-assisted video system.

Radiofrequency (RF) generated thermal brain lesions are widely used in functional neurosurgery. The size, shape and development of the lesions depends on system parameter settings and the electrode configuration. Difficulties in studying the effect of these factors in vivo stimulated us to develop an in vitro system for standardized comparison between different electrodes and physical parameters. A computer-assisted video system was set-up allowing continuous video recording of RF-generated coagulations in either a standard albumin solution or in the fresh white of a hen's egg as transparent test substrates. Ten lesions were made with each test electrode (two bipolar and three monopolar) in each of the two substrates at 70 degrees, 80 degrees and 90 degrees C (t = 60 sec). Due to the better homogeneity the lesions in the albumin solution were much more regular and reproducible. This made it possible to calculate the size (width 2.2 +/- 0.1 to 5.3 +/- 0.1 mm and length 3.0 +/- 0.1 to 8.7 +/- 0.3 mm) as well as the volume (8.5 +/- 1.4 mm3 to 133.5 +/- 26.8 mm3). It is concluded that this in vitro system offers a reproducible way to study and document the effect of different electrode configurations and RF-generator settings on the formation of a heat lesion. Even if the results are not directly applicable to the living human brain they give an estimate of the form and size of a coagulation lesion and can be of value for standardized comparisons between different electrodes.

Brain↗
PubMed1997

Superficial blood flow following photodynamic therapy of malignant non-melanoma skin tumours measured by laser Doppler perfusion imaging.

Laser Doppler perfusion imaging offers a new modality for in vivo monitoring of the superficial blood perfusion in biological tissue. In this study, the superficial blood perfusion of malignant non-melanoma skin tumours and the surrounding normal skin was measured in conjunction with photodynamic therapy (PDT) using topical delta-aminolaevulinic acid (ALA)-induced protoporphyrin IX as a photosensitizer. The results clearly show that, in contradiction to PDT with the intravenously administered photosensitizer Photofrin, no direct vascular damage can be seen. With the topical sensitization the blood perfusion is increased immediately after the treatment irradiation. The increased blood flow is seen up to a week after treatment, in a similar way as for an inflammatory reaction. Despite this, all basal cell carcinoma and squamous cell carcinoma in situ lesions in this study healed without any sign of residual tumour after the treatment, suggesting an efficient direct tumour cell destruction induced by PDT.

Adult↗
PubMed1996

Duplex laser Doppler perfusion imaging.

A duplex mode for recoding of both spatial and temporal blood perfusion components has been developed and evaluated. This modality, which has been implemented as a software module in the laser Doppler perfusion imager, consists of various local area scan (LAS) configurations. These include single-point recording or multipoint recording after repeated movements of the laser beam in quadratic patterns including 2 x 2 or 4 x 4 measurement sites. For the 2 x 2 and 4 x 4 LAS, the output value constitutes the average perfusion of all values captured within the actual region of interest. The 2 x 2 local area scan is corrected by time shifting the sequentially recorded measurement values at consecutive tissue sites, while the 4 x 4 LAS is presented as a vector of the individual subimages. With the standard setting of 65 msec for the signal integration time at each measurement site, the 1 x 1 and 2 x 2 LAS configurations can capture and reproduce perfusion signals with maximal bandwidths of 7.7 and 1.9 Hz, respectively. System evaluation showed that the signal integration time can be reduced to 45 msec without impaired signal quality, thereby further increasing the system bandwidth with a factor of about 1.5. Skin recordings showed that averaged time traces of adjacent measurement sites improve the signal-to-noise ration and allow for a more reliable analysis of, for example, the reactive hyperemic response. Individual time-trace extraction, however, showed reperfusion patterns that differed markedly between sites.

Humans↗
PubMed1995

Human in vivo cutaneous microdialysis: estimation of histamine release in cold urticaria.

A novel bioanalytical in vivo sampling technique, cutaneous microdialysis, was used to follow the chronology of skin histamine release in 3 patients with cold urticaria and in 2 healthy volunteers. Laser Doppler perfusion imaging was used simultaneously to monitor the skin circulatory response. Microdialysis samples were collected at 10-min intervals and analysed by radioimmunoassay technique. Fifty minutes after probe insertion, the ventral forearm skin in the area of the dialysis membrane was provoked for 5-15 min with a 25 x 40 mm ice cube covered with plastic foil. In the cold urticaria patients, an up to 80-fold increase of histamine was observed, with peak levels 20-30 min after challenge. Histamine levels then fell to reach "baseline" levels within 50 min. In the healthy subjects, the histamine increase was earlier, less pronounced and of shorter duration. Cutaneous microdialysis and laser Doppler imaging offer new possibilities for the chronological multiparameter assessment of inflammatory skin disorders in vivo.

Adult↗
PubMed1994

Spatial heterogeneity in normal skin perfusion recorded with laser Doppler imaging and flowmetry.

Spatial and temporal variations in forearm skin perfusion captured by laser Doppler perfusion imaging (LDI) have been compared with topographic maps recorded by laser Doppler flowmetry. In order to determine the shortest LDI sampling time required at each measurement site, with an adequate signal-to-noise ratio and with the ability to display the heterogeneity in skin perfusion, the noise-limited resolution of the LDI system as well as various sampling times were tested. The noise-limited resolution for medium and high light intensities were less than 0.5% (temporal) and 0.3% (spatial) of full scale. A sampling time of 1 sec was selected and image presentation was made by performing bilinear interpolation between perfusion values. The same area (10 x 10 mm) was mapped with LDI and topographic mapping at seven different sites. In addition, a larger area covering the surrounding skin was recorded with LDI. The small area recordings with LDI and topographic mapping could be identified in the larger LDI image. High-and low-perfusion spots coincided between the two systems. Temporal variations were studied by repeated LDI recordings of the same areas as above. Small spots were selected in the areas and plotted versus time. Without provocation, the total perfusion changes at each spot showed large variations, but the relative perfusion levels between neighboring spots persisted. Provocation with heat increased the perfusion in all spots.

Adult↗
PubMed1994

Changes in skin circulation after insertion of a microdialysis probe visualized by laser Doppler perfusion imaging.

Microdialysis makes possible in vivo estimation of endogenous and exogenous substances in the dermal extracellular space. Insertion of the microdialysis probe and its subsequent presence in the skin may affect both the reactivity of the skin test site and the measurement of target substances. Laser Doppler flowmetry is a non-invasive method for estimating cutaneous blood flow. A further development of this technique, laser Doppler perfusion imaging, has been used to study the time course of the circulatory changes caused in the area of microdialysis probe insertion. Laser Doppler perfusion imaging was performed prior to, during, and after microdialysis probe insertion in the skin of the ventral forearm in three subjects. Probe insertion caused an increase in skin blood perfusion in the whole test area. About 15 min after probe insertion, the flare, which is presumed to be of chiefly axon reflex origin, began to subside and the circulatory response could be seen to center around the site of insertion and the tip of the probe. Skin perfusion levels had returned to near normal levels within 60 min. Local anesthesia of the point of guide insertion inhibited the flare, but did not affect circulatory reactivity in the skin nearby. Both microdialysis and laser Doppler perfusion imaging seem to be promising new methods in dermatologic research.

Adult↗
PubMed1993

Laser Doppler perfusion imaging by dynamic light scattering.

Imaging of tissue perfusion is important in assessing the influence of peripheral vascular disease on microcirculation. This paper reports on a laser Doppler perfusion imaging technique based on dynamic light scattering in tissue. When a laser beam sequentially scans the tissue (maximal area approximately 12 cm *12 cm), moving blood cells generate Doppler components in the back-scattered light. A fraction of this light is detected by a remote photodiode and converted into an electrical signal. In the signal processor, a signal proportional to the tissue perfusion at each measurement point is calculated and stored. When the scanning procedure is completed, the system generates a color-coded perfusion image on a monitor. A perfusion image is typically built up of data from 4,096 measurement sites, recorded during a time period of 4 min. This image has a spatial resolution of about 2 mm * 2 mm. A theory for the system inherent amplification factor dependence on the distance between individual measurement points and detector is proposed and correction measures are presented. The performance of the laser Doppler perfusion imager was evaluated using a flow simulator. The correlation coefficient between the estimated flow parameter and the perfusion through a mechanical flow simulator was calculated to r = 0.996. To assess the sampling depth of the laser beam, light scattering in tissue was simulated by a Monte Carlo technique. The average sampling depth for skin tissue was calculated to 200-240 microns, depending on the blood content.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1993

The cutaneous vascular axon reflex in humans characterized by laser Doppler perfusion imaging.

1. Laser Doppler perfusion imaging was used to map the cutaneous vascular axon response induced by trains of electrical skin stimuli (1 ms, 2 Hz) on the dorsum of the hand, finger and foot in twenty-four healthy subjects. Conduction anaesthesia was applied to nerves supplying the stimulated skin areas. Subtraction of images recorded before and after stimulation was used for data analysis of the intensity and area of the response. 2. The stimulation evoked a localized perfusion increase around the stimulating electrode which lasted approximately 30 min and increased in intensity and area with increasing stimulation strength to a maximum at 20 pulses and 20 mA. The intensity and area of the response was greater on the hand than on the foot. 3. Approximating the response area as a circle, the maximal perfusion increase in the hand extended 9 +/- 3 mm (mean +/- S.D.) outside the perimeter of the stimulating electrode. When stimulating within skin which had been subjected to surface anaesthesia, no response occurred, but when stimulating at the border of surface-anaesthetized skin, the perfusion increase extended 2 +/- 1 mm (mean +/- S.D.) into anaesthetized skin. 4. The results show that the perfusion increase must have been due in part to impulse conduction to, and release of transmitters from, axon endings terminating in skin outside the contact area of the probe. It is concluded that the area of perfusion increase corresponds to the size of the receptive fields of afferent polymodal C fibres.

Adult↗
PubMed1992

Evaluation of port wine stain perfusion by laser Doppler imaging and thermography before and after argon laser treatment.

Thirteen patients with port wine stains (PWS) were treated with argon laser therapy. Before and at different points in time following treatment, skin blood perfusion and temperature were mapped with laser Doppler imaging and thermography. In nine patients no elevation in blood perfusion was observed in the PWS in comparison with the surrounding normal skin before treatment. In the remaining four patients a significantly (p less than 0.01) higher blood flow was recorded within the PWS. Immediately after treatment nine patients showed elevated perfusion within the PWS. During the first two days following treatment, all patients showed a gradually decreasing hyperperfusion in the borderline between the PWS lesion and surrounding skin. Immediately after treatment 10 patients had a significantly (p less than 0.01) higher temperature in the PWS than in normal skin. During the first 24 h following treatment, an elevated perfusion was in general accompanied by a tissue temperature increase. Three and a half months after argon laser treatment, three patients showed excellent clinical results with no remaining PWS spots or scarring. Two of these patients had had both elevated perfusion and temperature in the PWS prior to treatment.

Adolescent↗
PubMedDate not supplied

Differential effect on vasodilatation and pain after intradermal capsaicin in humans during decay of intravenous regional anesthesia with mepivacaine.

BACKGROUND AND OBJECTIVES: When given intracutaneously, capsaicin can cause burning pain by central propagation in thin afferents, as well as neurogenic vasodilatation, reflecting antidromic conduction in the same fibers. We wanted to test the hypothesis that an intravenous regional block (IVRA) inhibits these two phenomena to a similar degree. METHODS: Sixteen healthy volunteers participated. A bilateral IVRA was performed by simultaneously injecting mepivacaine in one arm and normal saline in the other in a randomized, double-blind manner. Ten minutes after release of the tourniquet, neurogenic inflammation was inflicted in each forearm by intracutaneous capsaicin. Microvascular skin blood flow was measured with a laser Doppler perfusion imager. The area of the flare and the flow therein were measured, taking into account the change in baseline caused by mepivacaine treatment and the postischemic hyperemia. Pain was repeatedly evaluated by visual analog scale. RESULTS: The reactive hyperemia following arterial occlusion was less in the mepivacaine-treated arm 10 minutes after tourniquet release (P=.026). Intracutaneous capsaicin elicited a flare in both arms. The area of the flare was smaller 10 minutes after capsaicin (P=.009) in the mepivacaine-treated arm. There was no difference between the arms concerning the mean blood flow within the flare or in ischemic or capsaicin-induced pain. CONCLUSIONS: Mepivacaine, given as an IVRA, had no effect on the post-IVRA sensory function of thin afferents but differentially decreased the spread of the capsaicin-induced flare.

Adult↗