Lysing patterns of retracted blood clots with diffusion or bulk flow transport of plasma with urokinase into clots--a magnetic resonance imaging study in vitro.

Fresh retracted clots are known to be poorly lysable by fibrinolytic agents. We have studied whether lysis of retracted clots could be enhanced by bulk transport in comparison to pure diffusion of plasma containing urokinase (400 IU/ml) into the clots. Cylindrical retracted blood clots were occlusively glued by a polyester into plastic tubes and put in contact with plasma through the clot bases. One group of clots (perfused clots, n = 10) was placed under a pressure difference of 6 kPa (60 cm H2O) which resulted in an average plasma flow of 0.97 +/- 0.34 microliters/min through the clot during the first hour. Another group of clots (non-perfused clots, n = 10) was incubated in the lytic plasma without a pressure difference. Clot sizes were measured during lysis by magnetic resonance imaging (MRI). Channels representing lysed areas penetrated into perfused clots with a velocity of 5.4 +/- 1.6 mm/h (n = 10), whereas the boundaries of non-perfused clots subsided with a velocity of less than 0.1 mm/h. Eight of the 10 perfused clots were recanalized after 8 h and the sizes of the perfused group were reduced to 64.0 +/- 10.7% of the initial values. The relative sizes of non-perfused clots after 8 h remained significantly higher: 95.0 +/- 1.3%, p < 0.005. In a separate experiment good agreement was obtained between the measured clot sizes by MRI and the residual radioactivity of 125I-fibrin in the clot.(ABSTRACT TRUNCATED AT 250 WORDS)

Dependence of blood clot lysis on the mode of transport of urokinase into the clot--a magnetic resonance imaging study in vitro.

Magnetic resonance imaging was employed to study the dependence of clot lysing patterns on two different modes of transport of urokinase into whole blood clots. In one group of clots (nonperfused clots, n1 = 10), access of urokinase to the fibrin network was possible by diffusion only, whereas in the other group (perfused clots, n2 = 10) bulk flow of plasma containing urokinase was instituted through occlusive clots by a pressure difference of 3.7 kPa (37 cm H2O) across 3 cm long clots with a diameter of 4 mm. It was determined separately that this pressure difference resulted in a volume flow rate of 5.05 +/- 2.4 x 10(-2) ml/min through occlusive clots. Perfused clots diminished in size significantly in comparison to nonperfused ones already after 20 min (p less than 0.005). Linear regression analysis of two-dimensional clot sizes measured by MRI showed that the rate of lysis was more than 50-times faster in the perfused group in comparison to the nonperfused group. It was concluded that penetration of the thrombolytic agent into clots by perfusion is much more effective than by diffusion. Our results might have some implications for understanding the differences in lysis of arterial and venous thrombi.

Prolonged T1 values of the crushed nerve are not exclusively attributable to Wallerian degeneration or endoneurial edema.

Axonotmesis of the left sciatic nerve was performed on adult male Wistar rats. T1 values of the crushed nerve samples were determined with proton magnetic resonance spectroscopy. Denervation changes of the dependent hind plantar flexor muscles were assessed with proton magnetic resonance imaging of their cross-sectional area and muscle force measurements. However, Tl relaxation time of the crushed nerve samples remained prolonged implying the absence of nerve regeneration. This would be in contradiction with what is known about regeneration after simple crash and the muscle force and magnetic resonance imaging measurements of the dependent plantar flexor muscles. These studies indicated that sufficient nerve regeneration had taken place to restore their muscle force and volume. A model is suggested to explain the persistence of prolonged Tl values in the presence of muscle force recovery.

Magnetic resonance current density imaging of chemical processes and reactions.

Electric current density imaging was used to image conductivity changes that occur as a chemical process or reaction progresses. Feasibility was assessed in two models representing the dissolving of an ionic solid and the formation of an insoluble precipitate. In both models, temporal and spatial changes in ionic concentrations were obtained on current density images. As expected, the images showed significant signal enhancement along the ionization/dissociation sites.

Force, fatigue, and the cross-sectional area of wrist extensor muscles after radial nerve grafting.

The force and fatigue of the wrist extensor muscles during maximal voluntary and tetanic contractions were measured and compared in the injured and noninjured extremities of 11 patients with radial nerve gap injury and in 9 normal volunteers. The cross-sectional area (CSA) of the wrist extensor muscles was determined by magnetic resonance imaging and was correlated with force. In the patient group, an average of three (range, 2-4) sural nerve cable grafts, measuring 11.5 +/- 5 cm (range, 5-20 cm) in length, were sutured to the nerve stumps at least 9 years before this study. Differences in the CSA values of the injured and noninjured arms were compared, and a ratio was established (CSAR). The mean CSAR was 82.9% +/- 14.3. These differences were not statistically significant (P > 0.10, paired t-test). Despite very well-recovered muscular mass, the maximal voluntary contraction force was found to be incompletely recovered by up to 62.7% +/- 23, when compared with the noninjured side (P < 0.05, paired t-test). The fraction maximal voluntary contraction force/CSA had decreased by up to 76.4% +/- 25.5 (P < 0.05, paired t-test). An increased fatigability of the affected muscles persisted in all patients. The patients' noninjured extremity behaved in the same way as that of the dominant extremity of normal volunteers with regard to force, lever, and CSA values.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrast enhanced Gd-DTPA magnetic resonance imaging in the evaluation of rheumatoid arthritis during a clinical trial with DMARDs. A prospective two-year follow-up study on hand joints in 31 patients.

OBJECTIVE: The aim of this prospective 24-month follow-up study was to compare clinical features with radiological and magnetic resonance imaging (MRI) findings in evaluating synovial proliferation in the hand joints of 31 patients with rheumatoid arthritis (RA). A single joint was used for the follow-up of each patient. METHODS: Thirty-one small hand joints were examined by conventional radiography and MRI before and after 24 months of treatment. MRI assessment of disease progression (volume and/or signal intensity of the synovial proliferation on T1 weighted precontrast, T1 weighted postcontrast and T2 weighted images) was compared with a clinical assessment of the chosen joints, and with a plain x-ray film evaluation (Larsen's score). RESULTS: Of 26 joints which clinically improved (14 markedly and 14 slightly) during the study, on MRI 16 showed improvement, 8 showed no change, and 2 showed deterioration. Four clinically unchanged joints appeared improved on MRI. One joint deteriorated clinically and on MRI. Overall, there was a 58% congruence between clinical and MRI findings. On x-ray 23 joints showed no change; nine of these were also unchanged on MRI, while 13 showed improvement and one deterioration. Only in 2 out of 8 joints showing deterioration on x-ray were the MRI findings in accordance. In the remaining six joints MRI showed improvement. The congruence between x-ray and MRI was therefore 36%. CONCLUSION: The long-term follow-up of rheumatoid synovial proliferation of the small joints in the hand using contrast enhanced MRI is feasible and may provide additional information regarding disease activity. Important advantages over conventional radiography methods are its ability to demonstrate qualitative differences of synovial proliferation within bone erosions, and demonstrate not only deterioration, but also the improvement of inflammatory disease.

Magnetic resonance imaging of alternating electric currents.

Electric Current Density Imaging (CDI) is a new modality of magnetic resonance imaging that enables electric current distribution imaging in conductive samples containing water. So far, two CDI techniques have been in use: DC-CDI operating at zero frequency and RF-CDI operating at the RF Larmor frequency. In this paper we present a new CDI technique, which extends the CDI frequency range to alternating electric currents (AC-CDI). First, a theoretical model for the electric current response to the alternating voltage is presented. Later, this model is used for the frequency analysis of the AC-CDI sequence. Additionally, the effect of off-resonance spins and imperfect refocusing RF pulses on the stability of the AC-CDI sequence and the echo formation is studied. The new theory is verified by experiments on a model system and compared to the other two methods: DC-CDI and RF-CDI. Finally, an application of the AC-CDI sequence to biological systems is demonstrated by an experiment on a wood twig in which an increase of approximately 30% was obtained at AC as compared to DC electric current.

Electric current density imaging of bone by MRI.

Current density imaging (CDI) has been shown to be a feasible method to map spatial distribution of electric currents through bone structures and for studying osteoporosis and bone fracture models. For the osteoporosis model, bone sample was moistened in a solution of a sodium salt of ethylendiamintetraacetic acid (EDTA) which causes chemical reaction with hydroxyapatite Ca2+ ions and lowers the mineralisation degree of the solid bone. This enables clear visualisation of conventional magnetic resonance imaging and CDI. Sensitivity of conventional magnetic resonance and CD images of bone was improved by immersing the bone samples into physiological saline containing contrast agent Gd-DTPA prior to imaging. To stimulate effects of bone fracture on electric current conductivity through bone, a transverse cut was made through the bone, and the resulting gap was filled with an insulator. Electric current density images under these conditions have shown that regions of strong conductivity can be distinguished from regions of no conductivity at the site where the insulator restricts electric current. Real bone fracture was imaged as well. To demonstrate influence of electrolyte concentration on electric current spatial distribution, the bone samples were imaged after being immersed in various saline concentrations. The same contrast in current density images was produced with the combinations of higher electrolyte concentrations and lower voltages. Our observations demonstrate the feasibility of the method in mapping current density in bone structures, which could have implications in understanding and monitoring the effects of the electrical stimulation.

Tumor angiography using high-resolution, three-dimensional magnetic resonance imaging: comparison of gadopentetate dimeglumine and a macromolecular blood-pool contrast agent.

RATIONALE AND OBJECTIVES: We compared the peritumoral vascular definition in rats using either a paramagnetic extracellular or a macromolecular contrast medium in combination with high-resolution magnetic resonance (MR) imaging. METHODS: High-resolution, three-dimensional spoiled gradient-refocused acquisition in a steady state (SPGR) images were acquired from tumor-bearing Fischer-344 rats before, immediately after, and again 40 min after administration of gadopentetate dimeglumine (0.1 mmol Gd/kg; n = 10) and albumin-(Gd-DTPA)30 (0.05 mmol Gd/kg; n = 5). Small peritumoral vessels were analyzed semiquantitatively on maximum intensity projection angiograms using a 4-point scoring system; quantitative analyses included signal-to-background ratios (SBRs) and signal-to-noise ratios. RESULTS: Gadopentetate dimeglumine caused a transient and low-scoring (0.2 +/- 0.1, SBR = 1.9 +/- 0.2) vessel definition but strong rim enhancement (score = 1.4 +/- 0.2). Albumin-(Gd-DTPA)30 produced persistent, high-quality angiograms (score = 2.6 +/- 0.2, SBR = 7.4 +/- 0.2) but minimal rim enhancement (score = 0.3 +/- 0.2). CONCLUSION: Albumin-(Gd-DTPA)30 combined with high-resolution MR imaging produces time-persistent, detailed angiographic definition of peritumoral vessels. Vascular maps obtained with gadopentetate dimeglumine enhancement are not time persistent or of equal quality.

Angiographic properties of Gd-DTPA-24-cascade-polymer--a new macromolecular MR contrast agent.

PURPOSE: A new macromolecular MR contrast agent, Gd-DTPA-24-cascade-polymer, was assessed for MR angiography of peritumoral vessels in rats. MATERIAL AND METHODS: High resolution 3D-SPGR (TR/TE 100/5ms, alpha = 90 degrees) angiograms were acquired in 10 Fischer rats bearing subcutaneous R3230 mammary adenocarcinomas. MRI was performed before, immediately and 40 min after administration of Gd-DTPA (0.1 mmol Gd/kg), and after either Gd-DTPA-cascade-polymer or albumin-(Gd-DTPA)30 (each 0.05 mmol Gd/kg). A semi-quantitative analysis of small peritumoral vessels and tumor rim enhancement was performed on maximum intensity projection (MIP) angiograms using a 4-point scoring system. A quantitative analysis compared vascular signal-to-background-(S/B), signal-to-noise-, and contrast-to-noise-ratio. RESULTS: Gd-DTPA produced a transient and low-scoring vessel definition (0.2 +/- 0.1), but strong rim enhancement (score 1.7 +/- 0.1). The cascade polymer resulted in better but submaximal vessel delineation (score 1.6 +/- 0.3, S/B 5.0 +/- 0.2) and strong rim enhancement (score 1.8 +/- 0.1). Albumin-(Gd-DTPA)30 produced the best and most time-persistent angiograms (score 2.6 +/- 0.2, S/B 7.4 +/- 0.2), but minimal rim enhancement (score 0.3 +/- 0.2). CONCLUSIONS: The Gd-DTPA-24-cascade-polymer demonstrated the useful combination of strong tumor rim enhancement and detailed angiographic definition of peritumoral vessels. These are advantages associated with extracellular and blood pool contrast media, respectively.

Cross-section areas of calf muscles in athletes of different sports.

Differences in cross-section areas of calf structures were studied in different sportsmen groups and controls: sprinters, long-distance runners, free-climbers, and sport non active persons. The cross-section areas of calf tissues were established by magnetic resonance imaging. There were no statistically significant differences between the groups in single muscle areas. The only statistically significant difference was found in subcutaneous fat between sportsmen and non active persons. Results show a general adaptive tendency of the muscle to greater activity, without exhibiting sport-group specificity differences.

Metal implant localizers: frontiers and diagnostic feasibility.

The paper describes and analyses a method for locating metal implants based on the interaction of a conductive object with a weak alternating magnetic field. Theoretical values for the boundary of detectability and the boundary of localability are introduced to show some technical characteristics, the diagnostic feasibility and limitations of the method. Clinical experiences are described in order to compare the theoretical expectations with practical results. Several advantages of the proposed diagnostic method in terms of inexpensive instrumentation, ease of operation and sensitivity are discussed.

Specific absorption rate study for radiofrequency current density imaging using a two-dimensional finite element model.

Radiofrequency current density imaging is an MR technique that images tissue conductivity contrast. Compared to conventional MRI, RF-CDI uses two additional sources of RF power to be absorbed and that must be evaluated in terms of proper parameter optimization to prevent excessive tissue heating and effects on the nervous system. In view of possible future clinical use of RF-CDI, a simple 2D finite element model of a rat brain was built to simulate current density distribution and distribution of absorbed RF power, i.e., SAR and related tissue heating. Current density in the rat brain was also evaluated qualitatively and quantitatively in an in vivo RF-CDI experiment. The results demonstrate that a numerical model can predict SAR and tissue temperature changes. The study also shows that substantial sensitivity and resolution of RF-CDI can be achieved using imaging parameters that produce SAR and temperature changes within allowed limits.

Assessment of kainate toxicity using contrast enhanced magnetic resonance imaging.

The purpose of this study was to test the capability of contrast enhanced magnetic resonance imaging (MRI) in assessing lesion formation in rat brain after systemic (i.v.) administration of kainate. MRI was performed with T1-weighted spin echo sequence before and after the administration of kainate and contrast media. Contrast media used were based on paramagnetic gadolinium (III) ion: Gd-DTPA (gadoliniumdiethylenetriaminepentaacetic acid) and prototype agents for blood-pool enhancement. Gadomer-17 and polylysine-Gd-DTPA. Enhancement of lesion rims and other brain tissue abnormalities due to kainate with Gd-DTPA, Gadomer-17 and polylysine-Gd-DTPA were observed mainly in the region of hippocampus and in the areas not protected by the blood-brain-barrier (BBB).

Assessment of liver regeneration by quantitative MRI analysis.

Liver regeneration after an extensive liver resection is a serious clinical problem, which is difficult to study in detail on patients. Therefore animal models were developed to study liver regeneration. Anabolic processes in the liver were assessed by measurements of the cholinesterase synthesis, and the regeneration of liver mass was monitored by use of magnetic resonance imaging. It has been shown that the liver mass reaches 90% of the control within the first week after the resection of 75% of the liver. This is partly due to the regeneration, and partly to the increased water content of the regenerated liver, shown by the magnetic resonance images. The results allow the conclusion that the magnetic resonance imaging is a reliable method to assess liver regeneration in vivo.