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Biomedical subjects

D Holz

Publications and source records attributed to D Holz.

15 recordsLinked to original sources
PubMed1997

Catheter tracking using continuous radial MRI.

The guidance of minimally invasive procedures may become a very important future application of MRI. The guidance of interventions requires images of the anatomy as well as the information of the position of invasive devices used. This paper introduces continuous radial MRI for the simultaneous acquisition of the anatomic MR image and the position of one or more small RF-coils (mu-coils), which can be mounted on invasive devices such as catheters or biopsy needles. This approach allows the in-plane tracking of an invasive device without any prolongation of the overall acquisition time. The extension to three-dimensional position tracking is described. Phantom studies are presented demonstrating the capability of this technique for real-time automatic adjustment of the slice position to the current catheter position with a temporal resolution of 100 ms. Simultaneously the in-plane catheter position is depicted in the actually acquired MR image during continuous scanning.

Catheterization↗
PubMed1995

Continuous radial data acquisition for dynamic MRI.

Since image acquisition times in MRI have been reduced considerably over recent years, several new important application areas of MRI have appeared. In addition to pure static anatomic information, the evolution of a dynamic process may be visualized by a sequence of temporal snapshots of the process acquired within a short time period. This makes applications like interactive or interventional MRI as well as the acquisition of additional functional information feasible. For high temporal resolution, all these applications require a quasi real-time image acquisition during the time the interaction or dynamic process evolves. We present an approach to real-time imaging using a continuous radial acquisition scheme. The intrinsic advantages of radial or projection reconstruction (PR) techniques are used to minimize motion-related image distortions. Modifications of the acquisition scheme as well as dedicated reconstruction techniques are used to further reduce the temporal blurring due to the finite acquisition time of one entire data set in our approach. So far we have used this technique for the visualization of active joint motion.

Artifacts↗
PubMed1994

Radial turbo spin echo imaging.

Fast MR imaging methods should provide a familiar contrast behavior at a reduced scan time. The multi-spin echo approach (TSE) is one of the most promising techniques satisfying this condition. Although the data acquisition time is significantly reduced, image quality may still suffer from artifacts due to patient motion and flow. The radial turbo spin echo (rTSE) approach combines TSE methods and projection reconstruction (PR) techniques. In PR images, artifacts induced by patient motion or flow are known to have a different appearance with lower level of intensity. The contrast and artifact behavior of the rTSE approach has been investigated. The new technique has been applied to abdominal imaging with acquisition times shorter than 30 s and to heart imaging in combination with cardiac triggering.

Abdomen↗
PubMed1991

[New therapeutic approach in the celiac trunk compression syndrome].

Etiology and treatment of the coeliac compression syndrome remains controversial. The external compression, near the origin of the truncus coeliacus and the poststenotic dilatation is well demonstrated by angiography on a lateral projection. The development of collaterals like the pancreaticoduodenal and gastroduodenal artery can be assessed on a frontal view. The grading of the stenosis as well as the direction of the flow in these artery can be measured with the duplex sonography. These investigations show a steal syndrome: blood coming from the mesenteric artery irrigate the common hepatic artery through the collaterals. The flow in the common hepatic artery is therefore retrograde. We used an original technique to correct this flow pattern in the case of a 52-year-old patient: after dividing the arcuate ligament, we reduced the vascular territory of the truncus coeliacus with the proximal ligation of the splenic artery. The duplex sonography shows postoperatively the suppression of the steal syndrome with a normal flow pattern in the hepatic artery despite minimal change in the stenosis of the truncus coeliacus. The patient remained asymptomatic since the operation. We concluded that an angioplasty or reimplantation of the truncus coeliacus is unnecessary to correct the steal syndrome. A normal flow can be provided by a stenotic truncus coeliacus once the splenic artery has been ligated.

Blood Flow Velocity↗
PubMed1986

The importance of SH-containing substances for red blood cells in acute myeloic leukemia.

Electron spin resonance (ESR) spectra of lyophilized erythrocytes obtained from patients with acute myeloid leukemia (AML) show, in comparison to controls, a characteristic change especially in the low-field region of the spectrum concomitant with a reduction of the spin concentration. This effect can be simulated by addition of SH-containing substances (e.g. reduced glutathione or cysteine) to healthy erythrocytes. S-S containing compounds exhibit no effect. Since SH-containing substances can hardly permeate plasma membranes, the membrane surface seems to be defective in the case of "AML" erythrocytes. Furthermore, it can be concluded that the concentration of SH-containing substances, such as cysteine, is increased in the plasma of AML-patients, which could be confirmed by HPLC-measurements. In the case of a successful treatment of the patients with alexan, daunoblastin, and thioguanine the spin concentration increased again and the resulting ESR spectrum is very similar to the control spectrum. It should be pointed out, that the ascorbic acid concentration is very low in both plasma and erythrocytes of AML patients.

Ascorbic Acid↗
PubMed1986

Influence of metal ions on the transport of ascorbate across membranes.

The effect of Fe2+, Cu2+, Ca2+, and Mg2+ on the permeation of ascorbate across membranes of dipalmitoyllecithin (DPPC) vesicles was investigated. As in previous experiments, the spin label I (1,14) was used for monitoring this transport. While Mg2+ and Ca2+ exerted a small effect only, Fe2+ and Cu2+ increased considerably the reduction of the spin label. In the case of Fe2+, this is not only due to a higher permeation rate of ascorbate but is also caused by a permeation of Fe2+. As a matter of fact, the largest reduction of the spin label was obtained if Fe2+ alone was added to the spin labelled DPPC solution. The reduction of the spin label seems to depend on the redox potentials of the constituents.

Ascorbic Acid↗
PubMed1984

Structure of ascorbic acid and its biological function. I. ESR determination of the ascorbyl radical in biological samples and in model systems.

ESR investigations on lyophilized systems have shown that the signal at g = 2.005 can be explained by an interaction between Na+ or K+ and the anionic ascorbyl radical. The unpaired electron is probably localized near the C(4) region and is produced by a cleavage of an H atom belonging to a water molecule bound tightly to C(4). Experiments on aqueous samples revealed that ascorbic acid in its radical configuration and in its highest concentration exists only at physiological pH and temperature. An additional splitting is obtained by the ring formation between C(3) and C(6)-OH. The coupling constants of the triplets produced by the CH2-6 protons differ between ascorbic acid and isoascorbic acid. Thus, the ESR technique can be applied for an easy distinction between these two epimers.

Ascorbic Acid↗
PubMedDate not supplied

Automatic shimming for localized spectroscopy.

Localized in vivo nuclear magnetic resonance studies often require a high spectral resolution not achievable with the basic shim of a whole-body magnetic resonance magnet. Therefore, the magnetic field homogeneity needs to be optimized in the selected volume of interest within a reasonable time. For this purpose, a method of automatic shimming was developed and tested on phantoms and volunteers. The volume selection is performed by means of a surface coil or by using a localization method which generates a stimulated echo from the volume of interest. The optimization procedure uses the time integral over the magnitude of the free induction decay or echo signal as homogeneity criterion. A complete shimming process generally requires only 80 transients. Test experiments were conducted on various volume sizes ranging from 2 X 2 X 2 cm3 to 15 X 15 X 15 cm3 inside a large phantom. The resulting linewidth in small volumes at the magnet center compared well with the natural linewidth determined by means of the Carr-Purcell-Meiboom-Gill sequence. As expected, shimming in selected volumes at off center positions led to somewhat broader lines. Results obtained on volunteers demonstrate the practical value of this rapid, automatic shimming method for in vivo studies.

Algorithms↗
PubMedDate not supplied

On the origin of the non-haemic iron transferrin ESR signal: ESR investigations on histidine-iron-ascorbic acid systems.

The nature of the ferric high spin iron complex located at g = 4.3 has been investigated by means of electron spin resonance spectroscopy. It could be shown that the iron is bound to two histidines, three ascorbic acids, and one bicarbonate. This agrees well with previous findings according to which the ligand field of iron is composed mainly of oxygen and nitrogen atoms. Another low-field signal located at g = 9.5 appears always concomitantly with the g = 4.3 signal. It should be due, therefore, to a transition between the two sublevels of the low-lying Kramers doublet in one principal direction.

Ascorbic Acid↗
PubMedDate not supplied

On the origin of the non-haemic iron transferrin ESR signal: ESR investigations on histidine-iron-citric acid systems.

The nature of the high spin ferric iron complex located at g = 4.3 has been investigated by means of electron spin resonance spectroscopy and polarography. It could be shown that two complexes each exist in the acid and alkaline pH region, and that the iron is bound to two histidines, three citric acids, and probably to one bicarbonate. These results agree well with previous findings according to which the ligand field of iron should be composed mainly of oxygen and nitrogen atoms. Another low-field signal located at g = 3.6 appears in the pH range from 2 to 7 only and exhibits its maximum where the g = 4.3 signal has its minimum. Its exact nature is still unknown but it seems to represent some intermediate state of the ternary Fe3+-histidine-citric acid complex. When citric acid is used, the spin concentration seems to be always larger than in the case of ascorbic acid. Since the effect obtained with ascorbic acid and citric acid seems to be similar, it may be concluded that the biological function of both of the acids might be somehow related to each other.

Ascorbic Acid↗
PubMedDate not supplied

ESR investigations on the modification of the ascorbic acid-erythrocyte interaction by ferrous ions.

An ascorbic acid-erythrocyte interaction results in the formation of the semidehydroascorbate (SDA) radical and an increase in spin concentration. Addition of a more than ten-fold concentration of Fe2+ (as FeSO4) to such a system compensates the effect produced by ascorbic acid, that is the electron spin resonance spectrum obtained resembles very closely that of erythrocytes only. With the disappearance of the SDA radical a high spin ferric ion signal appears concomitantly at g = 4.3.

Ascorbic Acid↗
PubMedDate not supplied

The effect of alcohol oxidase on the ascorbic acid-erythrocyte interaction: ESR investigations.

Addition of alcohol oxidase to erythrocytes treated with ascorbic acid reverses the vitamin C effect, as expressed by the appearance of the semidehydroascorbate signal, and the increase in spin concentration. The original erythrocyte electron spin resonance (ESR) signal is, however, never restored completely, as it is in the case if ascorbate oxidase is used. Additional interaction between the enzyme alcohol oxidase and haemoglobin iron must occur, since both of the iron ESR signals at g = 4.3 and g = 6 disappear and the erythrocyte ESR spectrum at g approximately equal to 2 is changed characteristically. The spin concentration of the latter spectrum increases again above a certain concentration of alcohol oxidase.

Alcohol Oxidoreductases↗