MR imaging of thrombi using EP-2104R, a fibrin-specific contrast agent: initial results in patients

This study was an initial phase II trial in humans of molecular magnetic resonance (MR) imaging for improved visualization of thrombi in vessel territories potentially responsible for stroke using a new fibrin-specific contrast agent (EP-2104R). Eleven patients with thrombus in the left ventricle (n = 2), left or right atrium (n = 4), thoracic aorta (n = 4) or carotid artery (n = 1) as verified by an index examination (ultrasound, computed tomograpy, or conventional MR) were enrolled. All MR imaging was performed on 1.5 T whole-body MR-system using an inversion-recovery black-blood gradient-echo sequence. The same sequence was performed before and 2-6 h after low-dose intravenous administration of 4 mumol/kg EP-2104R. Two investigators assessed image quality and signal amplification. Furthermore, contrast-to-noise ratios (CNR) between the clot and the blood pool/surrounding soft tissue before and after administration of the contrast agent were compared using Student's t-test. MR imaging and data analysis were successfully completed in 10 patients. No major adverse effects occurred. On enhanced images, thrombi demonstrated high signal amplification, typically at the clot surface, with a significantly increased contrast in comparison to the surrounding blood pool and soft tissue (CNR for clot vs. blood pool, unenhanced and enhanced: 6 +/- 8 and 29 +/- 14; CNR for clot vs. soft tissue, unenhanced and enhanced: 0 +/- 4 and 21 +/- 13; P < 0.01 for both comparisons). EP-2104R allows for molecular MR imaging of thrombi potentially responsible for stroke. High contrast between thrombus and surrounding blood and soft tissues can be achieved with enhanced imaging.     

Cine gradient refocused echo (GRE) imaging of intravascular masses: differentiation between tumor and nontumor thrombus.

Spin echo MR imaging has not permitted reliable differentiation between intraluminal blood clot and tumor thrombus. This study assessed the role of ECG referenced repetitive gradient refocused echo (cine GRE) imaging for the differentiation of intravascular tumor from blood clot. Cine GRE images were reviewed in 23 patients, 11 of whom had intravascular tumor and 12 of whom had intravascular blood clots. Percentage contrast between the lesion and skeletal muscle as the reference tissue was determined from a subjective review of the images and objective signal intensity measurements. Intravascular clots were found to be lower in signal intensity than muscle (mean -55 +/- 29%). Intravascular tumors showed higher signal intensity relative to muscle (mean +17 +/- 9%) with the exception of myxomas (n = 2), which had signal intensity values relative to muscle as low as clots (mean -41 +/- 17%). Three masses in the inferior vena cava were composed of central tumor and peripheral clot; the two components could be differentiated with cine GRE imaging. Cine GRE imaging provides adequate signal intensity differences to visualize intravascular masses and helps to differentiate intravascular clot from tumor thrombus. However, if the tumor contains substantial amounts of iron, then the signal is also low and consequently clot and thrombus may not be distinguishable. This can occur in some atrial myxomas.     

Radioimmunoimaging of experimental thrombi in dogs using technetium-99m-labeled monoclonal antibody fragments reactive with human platelets

Monoclonal antibody 50H.19, which reacts with human platelets, was converted to fragments, pretinned, and made into kits for subsequent radiolabeling with 99mTc. The antibody, which cross-reacts with dog platelets, was used to evaluate in vitro binding to blood clots and in vivo in experimental thrombi in dogs. After radiolabeling, 97.4 +/- 6.4% of the 99mTc was antibody-associated. The preparations retained immunoreactivity, as determined by: binding studies using whole blood and determining the ratio of cell-to-plasma radioactivity (ratios of 57.6-61.2) and binding of the antibody to clots (clot/serum ratios were 57.2-74.6%). Approximately 50% of the radioactivity was cleared from the blood in 3-6 min and 18-24% was excreted in urine within 3 hr. Experimental thrombi in dogs could be visualized consistently within 2-3 hr postinjection in peripheral veins and arteries, pulmonary arteries, and the right ventricle. In addition, damage to blood vessel intima without visible thrombi could also be detected. This method has the following advantages: short and simple pre-imaging preparation, and rapid visualization of thrombi with no need for blood-pool subtraction or delayed imaging.     

Detection of experimental thrombi in rabbits with an 131I-labelled fibrin-specific monoclonal antibody

The detection of thrombi in rabbits has been investigated with 131I-labelled DD-3B6/22, a monoclonal antibody (Mab) reactive at high affinity (Kd = 2.68 x 10(-10) M) with human D Dimer (DD). DD-3B6/22 bound well to both "fresh" and "aged" human clots in an in vitro assay but showed poor binding to rabbit clots. However, reactivity was restored to rabbit blood if it was seeded, before clotting, with human DD covalently coupled to Sepharose beads. Thus, a rabbit model was developed in which blood was allowed to clot around DD-Sepharose beads introduced into the jugular vein. Gamma camera imaging showed that intact 131I-labelled DD-3B6/22 localised to these clots within 24 h. Uptake at this time was 0.202 +/- 0.012% injected dose per gram (%ID/g) compared with 0.086 +/- 0.018%ID/g after injection of control antibody. 131I-labelled F(ab')2 fragments of DD-3B6/22 allowed earlier scintigraphic detection of the clot which was evident 4 h after injection. Uptake in the clot at 24 h was 0.154 +/- 0.038 %ID/g compared with 0.109 +/- 0.027 %ID/g for a control F(ab')2. As antigen levels in the clot are estimated to be less than 300 micrograms DD, thus representing a very small human clot, the DD-3B6/22 Mab would appear to have a good potential for the sensitive detection of thrombi in a clinical setting.     

A targeted contrast agent for magnetic resonance imaging of thrombus: implications of spatial resolution

A preparation of ultra-small superparamagnetic iron oxide (USPIO) particles coupled to an RGD peptide (RGD-USPIO) was investigated as an MR contrast agent, targeted to activated platelets, in both ex vivo and in vivo thrombus models. Thrombus visualization ex vivo was compared using RGD-USPIO and a non-targeted UPSIO. The influence of thrombus visualization on thrombus exposure time to RGD-USPIO (ex vivo) and on the spatial resolution of the MR image (ex vivo and in vivo) was assessed. RGD-USPIO resulted in better thrombus visualization than non-targeted USPIO ex vivo, and maximum enhancement was achieved after approximately one hour exposure time of the thrombus to RGD-USPIO. The ability to visualize the clots was highly dependent on the spatial resolution of the image. In vivo, an in-plane resolution of less than 0.2 x 0.2 mm(2) was required for good clot visualization after contrast enhancement. It is concluded that the achievable resolution and sensitivity is a potential limitation to the usefulness of active vascular targeting in MRI.     

Localization and visualization of pulmonary emboli with radiolabeled fibrin-specific monoclonal antibody

Indium-111-labeled monoclonal antibody 64C5 specific for the beta-chain of fibrin monomer was used to image canine (n = 6) experimental pulmonary emboli (at least one barium-thrombin and one copper-coil induced clot per dog). Uptake of 111In-64C5 and 125I-control-DIG26-11 were compared in 10 clots (7 barium-thrombin and 3 copper-coil) identified in the lungs. There was no difference in the blood clearance of 111In-64C5 and 125I-DIG26-11. Uptake of 111In-64C5 (0.183 +/- 0.105, mean %ID/g) was greater than 125I-DIG26-11 (0.024 +/- 0.025) in pulmonary clots (p less than 0.001). Mean thrombus to blood ratios at 24 hr were 6.78:1 for 64C5 and 0.57:1 for DIG26-11. The clots visualized in vivo were larger (0.315 +/- 0.381 g) than clots not visualized (0.089 +/- 0.098). Negative images were recorded in three dogs with pulmonary emboli, injected with 111In-labeled control monoclonal antibody 3H3. These data suggest that 111In-labeled antifibrin can detect large pulmonary emboli in vivo.     

Magnetic resonance characterization of blood coagulation in vitro. Effect of platelet depletion

To optimize magnetic resonance (MR) methods of characterizing thrombi, further studies of biologic determinants of spinlattice (T1) and spin-spin (T2) relaxation times of thrombi are needed. As a step toward evaluating the influence of thrombus cellular composition on MR properties, the authors evaluated the effect of platelet depletion on MR relaxation times of in vitro blood clots. Blood from 13 fasting normal men was collected into sodium citrate (3.8%) and centrifuged ( [10,000 X g] X 5-10 minutes). Platelet-poor specimens (less than 20,000 per mm3) were reconstituted from the plasma and packed erythrocytes to match precentrifugation hematocrit levels. T1 and T2 measurements were made at 20 MHz within three to six hours of initiating clotting. The mean T1 value for platelet-rich (normal) specimens was 1117 +/- 86 mseconds versus 1119 +/- 68 mseconds for the platelet-poor specimens (P greater than .90). The mean T2 value for platelet-rich (normal) specimens was 616 +/- 130 mseconds versus 434 +/- 79 mseconds for the platelet-poor specimens (P less than .001). The mean water content in the platelet-rich (normal) specimens was 79.5% +/- 1.2% versus 80.0% +/- 1.2% in the platelet-poor specimens (P greater than .50). In summary, platelet depletion by buffy coat removal significantly shortens MR T2 values of in vitro clot. These data suggest that thrombus cellular composition, other than erythrocytes, alters MR relaxation times of clotted blood.     

Venous thrombosis: clinical and experimental MR imaging

Five venous thrombi were induced in the external jugular veins of three laboratory dogs, and were repeatedly imaged over 3 weeks using a 0.35-T magnetic resonance (MR) imager. MR magnitude and phase images, T1 and T2 relaxation times, venography, and histologic sections of these thrombi were evaluated to determine the changes in appearance on MR images with time. Venous thrombi appeared hyperintense compared with muscle on both relatively T1- and T2-weighted spin-echo sequences regardless of the age of the clot. Organization of the thrombus beyond 1 week was manifested as increased prominence of flow signal void in and around the clot. Distinction between intraluminal thrombus and flow-related artifacts was aided by phase image reconstruction. Nineteen venous thrombi locations in 13 patients revealed an MR appearance similar to that of the experimental animal model. Three patients (six thrombi locations) had serial examinations over 4 weeks. No significant change in thrombus signal characteristics was noted with time. It is concluded that MR imaging at 0.35 T cannot be used to predict the age of thrombus (up to 3 weeks) but may be helpful in following its resolution.     

MR imaging of pulmonary emboli: an experimental study in dogs

Experimental pulmonary emboli, consisting of tantalum-labeled autologous blood clots and barium-labeled 3-mm plastic spheres that did not produce an MR signal, were introduced through the femoral vein into each of five dogs. The sensitivity of MR to detect autologous clots of known size was assessed, and the size of the clot on MR was compared with its actual size. Emboli were localized by using chest radiographs made in multiple projections. Cardiac-synchronized MR imaging was performed on a 0.35-T superconducting magnet and was followed by a 99mTc macroaggregated lung scan. All 12 blood clots larger than 3 mm, and three of 20 clots less than 3 mm in transverse diameter were correctly visualized with MR. Five of the 15 clots seen on MR were larger than 150% of the actual size. There were nine false-positive emboli on MR. Two of six plastic spheres resulted in an abnormal signal on MR. MR signal from pulmonary emboli results from the thrombus itself and probably also from slow blood flow proximal to the obstruction. MR may be of value in detecting pulmonary emboli; clinical trials to evaluate its usefulness should be carried out.     

Measurement of carotid wall volume and maximum area with contrast-enhanced 3D MR imaging: initial observations

PURPOSE: To investigate whether postcontrast three-dimensional (3D) magnetic resonance (MR) imaging would yield more accurate measurement of carotid artery wall volume and maximum wall area, which are both measures of plaque burden, than precontrast 3D MR imaging. MATERIALS AND METHODS: Eleven consecutive patients scheduled to undergo carotid endarterectomy were recruited for the study. A 3D fast gradient-recalled-echo sequence was applied to acquire both precontrast and postcontrast images of the carotid artery wall. The same sequence was used to image the ex vivo excised plaque as a reference for measurement of carotid wall volume and maximum wall area. RESULTS: The mean difference in maximum wall area between the precontrast in vivo measurements and the ex vivo measurements (mean +/- SD, 18.22 mm2 +/- 15.61) was significantly larger than that between the postcontrast in vivo measurements and the ex vivo measurements (12.33 mm2 +/- 14.49) (P =.02). The difference in wall volume between the precontrast in vivo measurements and the ex vivo measurements (41.81 mm3 +/- 36.51) was larger than that between the postcontrast in vivo measurements and the ex vivo measurements (32.73 mm3 +/- 35.00) (P =.004). Postcontrast images yielded better correlation with ex vivo images than did precontrast images, in both carotid luminal area (R = 0.88 for postcontrast images, R = 0.80 for precontrast images) and outer wall boundary area (R = 0.79 for postcontrast images, R = 0.71 for precontrast images) measurements. CONCLUSION: Postcontrast 3D MR imaging may be useful in the measurement of carotid artery plaque burden.     

Real-time MR imaging-guided passive catheter tracking with use of gadolinium-filled catheters

PURPOSE: To test the hypothesis that real-time magnetic resonance (MR) imaging-guided passive catheter tracking is feasible with use of dilute gadolinium (Gd)-filled catheters, to determine the optimal Gd concentration required for tracking, and to measure catheter tip tracking accuracy. MATERIALS AND METHODS: The authors tested a real-time, T1-weighted, two-dimensional, spoiled gradient-recalled echo MR imaging sequence suitable for tracking catheters. In a yogurt phantom, the authors placed 5-F catheters filled with 2%-12% Gd solutions. MR imaging was performed with and without use of a projection dephaser that suppressed background signal. The authors measured signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and enhancement ratio to determine the optimal Gd concentration for catheter depiction. Catheter tip tracking accuracy was measured in an acrylic phantom with use of linear regression analysis, with goodness of fit assessed statistically with the F test. RESULTS: Peak catheter SNR, CNR, and enhancement ratios were obtained with 4%-6% Gd concentrations. Tip tracking accuracy was determined to be +/- 0.41 mm (R2 = 0.99; P < .0001). MR imaging reconstructions were displayed up to 3.1 frames/sec. CONCLUSIONS: Accurate MR imaging-guided passive catheter tracking was feasible in real-time with use of dilute Gd-filled catheters. This technique may have application in MR imaging-guided endovascular procedures.     

Low-artifact intravascular devices: MR imaging evaluation

Flow-phantom magnetic resonance (MR) imaging, with use of both spin-echo (SE) and gradient-echo (GRE) techniques at 1.5 T, was performed on the percutaneous Greenfield (beta-III titanium alloy [TMA wire]), Amplatz (MP32-N alloy), and Simon nitinol filters and TMA wire facsimiles of the bird's nest, Gunther, new retrievable, and Amplatz vena caval filters. SE imaging allowed detection of thrombi as small as 5 X 5 mm trapped within the percutaneous Greenfield, Simon nitinol, and TMA-wire facsimile filters; with the MP32-N Amplatz filter, a larger volume of thrombus (10 X 20-mm clots) was necessary for clot detection. GRE imaging allowed detection of intraluminal tilting of the percutaneous Greenfield and facsimile Amplatz (TMA-wire) filters. GRE imaging was useful for demonstrating postfilter turbulence due to clots, which was greatest for the Amplatz filter. Imaging of facsimile vascular devices made of tantalum or TMA wire did not cause the severe "black-hole" MR artifacts typical of the stainless-steel devices. SE and GRE imaging were very useful for determining caval patency in two patients with previously placed Mobin-Uddin filters. Noninvasive MR evaluation of blood vessels in the presence of a variety of low-artifact intravascular devices appears feasible.     

Hepatocyte-targeted MR contrast agents: contrast enhanced detection of liver cancer in diffusely damaged liver.

The performance of hepatocyte-targeted magnetic resonance (MR) contrast agents in the detection of liver tumor was tested in rats with hepatitis. Hepatocyte-targeted MR contrast agents (paramagnetic hepatobiliary complex [manganese-DPDP] and superparamagnetic iron oxide coated with arabinogalactan [SPIO-AG]) were injected into normal rats and rats with carbon tetrachloride-induced hepatitis. Before and after injection of either contrast agent, ex vivo relaxometry (0.94T) or in vivo MR imaging (1.0T) were performed. The obtained liver and tumor T1 and T2 relaxation times, liver and tumor signal-to-noise ratios (SNR), and tumor-liver contrast-to-noise ratios (CNR) of control rats and rats with hepatitis were compared. Both relaxometry and MR imaging showed that MnDPDP and SPIO-AG selectively enhanced liver tissue in controls and in rats with hepatitis to the same degree, and little tumor enhancement was seen in either group. As a result, no significant difference between control rats and rats with hepatitis was observed in the postcontrast tumor-liver CNR. For a MnDPDP-enhanced CNR with spin echo (SE) of 310/15, the results were -10.4+/-3.6 in control rats vs. -11.5+/-1.4 in rats with hepatitis; for a SPIO-AG-enhanced CNR with SE 2000/45 and 2000/90, respectively, the results were 30.7+/-9.2 and 18.7+/-4.7 in control rats vs. 31.9+/-7.1 and 17.7+/-2.4 in rats with hepatitis. These results indicate that hepatocyte-targeted contrast agents effectively enhance liver tissue and enhance liver-tumor image contrast despite hepatocellular dysfunction.     

Detection and characterization of intracardiac thrombi on MR imaging

OBJECTIVE: The aim of our study was to compare the diagnostic accuracy achieved using different MR techniques with the diagnostic accuracy achieved using transthoracic and transesophageal echocardiography to detect intracardiac thrombi. MATERIALS AND METHODS: Twenty-four patients with known or suspected intracardiac thrombi were examined using MR imaging and echocardiography. All MR examinations were performed on a 1.5-T MR scanner using dark-blood-prepared half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences, fast imaging steady-state free precession (trueFISP) cine sequences, and inversion recovery gradient-echo fast low-angle-shot (inversion recovery turbo FLASH) sequences after injection of 0.2 mmol/kg of gadolinium diethylene triamine pentaacetic acid. RESULTS: MR imaging and echocardiography revealed 12 thrombi-two in the right atrium, one in the right ventricle, three in the left atrium, and six in the left ventricle. Compared with echocardiography, MR imaging revealed three additional thrombi in the left ventricle; these thrombi were confirmed at surgery. All 15 thrombi appeared as filling defects on early contrast-enhanced inversion recovery turbo FLASH MR images. Only seven thrombi were detected on HASTE images, and 10 thrombi were seen on trueFISP images. Four thrombi showed enhancement 10-20 min after contrast material injection and were characterized as organized clots. CONCLUSION: Contrast-enhanced inversion recovery turbo FLASH sequences were superior to dark-blood-prepared HASTE and trueFISP cine MR images in revealing intracardiac thrombi. Compared with transthoracic echocardiography, MR imaging was more sensitive for the detection of left ventricular thrombi. The characterization of thrombi may be used to predict the risk of embolism, which is higher for subacute clots than for organized thrombi.     

Glucose-receptor MR imaging of tumors: study in mice with PEGylated paramagnetic niosomes

PURPOSE: To evaluate a magnetic resonance (MR) imaging contrast agent for tumor detection based on paramagnetic nonionic vesicles (niosomes) bearing polyethylene glycol (PEG) and glucose conjugates for the targeting of overexpressed glucose receptors. MATERIALS AND METHODS: Four gadobenate dimeglumine-loaded niosome preparations including nonconjugated niosomes, niosomes bearing glucose conjugates (N-palmitoyl glucosamine [NPG]), niosomes bearing PEG 4400, and niosomes bearing both PEG and NPG were tested. In vitro cellular uptake was measured at electron paramagnetic resonance (EPR) after incubation with human prostate carcinoma, PC3, cells. In vivo distribution was studied at MR imaging 6, 12, and 24 hours after injection, with assessment of tumor, brain, liver, and muscle signal intensity (SI) in 49 mice bearing PC3 cells. Efficiency of targeted contrast agents was assessed with tumor-to-muscle contrast-to-noise ratio (CNR). Testing for differences was performed with analysis of variance followed by a posteriori Fisher test. RESULTS: In vitro, gadolinium could be detected at EPR only in cell pellets incubated with niosomes bearing glucose conjugates or niosomes bearing both glucose conjugates and PEG (4.9. 10(-15) and 4.5. 10(-15) mol gadolinium per PC3 cell). In vivo, marked predominant tumor enhancement was demonstrated 24 hours after injection of glycosylated PEG niosomes (P <.01); no significant differences were observed following injection of nonconjugated niosomes, glycosylated niosomes, or PEG 4400 niosomes. Twenty-four hours after injection, sole presence of NPG or PEG 4400 on the surface of the niosome led to higher tumor-to-muscle CNR than that observed after injection of nonconjugated niosomes (CNR of 3.3 +/- 0.7 [SD], 3.4 +/- 2.2, and 0 +/- 1.9). Combination of NPG and PEG led to even higher tumor-to-muscle CNR (6.3 +/- 2.2). CONCLUSION: Combination of PEG and glucose conjugates on the surface of niosomes significantly improved tumor targeting of an encapsulated paramagnetic agent assessed with MR imaging in a human carcinoma xenograft model.     

Cross-species pharmacologic evaluation of plasmin as a direct-acting thrombolytic agent: ex vivo evaluation for large animal model development

PURPOSE: Human plasma-derived plasmin has been developed for the treatment of thrombosed hemodialysis arteriovenous grafts and vascular occlusive diseases. To further investigate this drug in large animal models and derive preliminary dosing estimates, the authors compared plasmin's relative lytic potential in four species, including man. The goal was to find which species' whole blood clots best compared to human clots in terms of lysis with plasmin. The results from these studies will serve to guide species selection for large animal experimentation. MATERIALS AND METHODS: Clotted blood from human, pig, sheep, and bovine subjects were treated with saline solution control, plasmin, or tissue plasminogen activator. Electron microscopy (EM) techniques were used to investigate the effects of clot size and fragmentation on plasmin lysis, the effects of intrathrombic infusion by injection of plasmin directly into whole blood clots, and species fibrin structural differences. RESULTS: Under static conditions, plasmin efficiently lysed clots from all species studied at an optimal dose of 4-5 mg per 4-5 g of clot. With fragmented human clots, plasmin (5 mg)-induced lysis was 80% +/- 2% at 60 minutes. Porcine clots were more resistant to plasmin lysis compared with human, ovine, and bovine clots. Percent lysis at 60 minutes with plasmin for ovine clots was 72% +/- 3% (4-mg dose), compared with 50% +/- 4% for porcine clots (5-mg dose; P < .05). EM of porcine clots showed a compact fibrin network that appeared more dense than that in human or sheep clots, which may account for the decreased lytic rate. CONCLUSIONS: Human plasmin is an effective direct-acting thrombolytic agent that is capable of lysing fibrin from several species. Ex vivo lysis studies were used to investigate the most appropriate large animal model that best approximates plasmin lysis with human clots under certain conditions. It was determined that ovine clots treated with plasmin most closely resemble the lysis observed with human clots.     

Deep venous thrombosis evaluation with limited-flip-angle, gradient- refocused MR imaging: preliminary experience

Sixteen patients (17 lower extremities) were prospectively examined with venography and limited-flip-angle, gradient-refocused magnetic resonance (MR) imaging for the presence or absence of deep venous thrombosis. Thrombosed vessels showed decreased-to-absent signal intensity, while patent vessels had high signal intensity. In 16 of 17 extremities, MR images allowed accurate detection and localization of the thrombi found with venography. In the remaining extremity, MR imaging allowed correct identification of thrombus in the iliac and femoral veins but incorrectly demonstrated clot in the calf and popliteal veins. MR imaging with limited-flip-angle, gradient-refocused pulse sequences appears to be a sensitive, noninvasive means of detecting deep venous thrombosis.     

How thrombus model impacts the in vitro study of interventional thrombectomy procedures

RATIONALE AND OBJECTIVES: Numerous experimental models are used to investigate the effectiveness of thrombectomy devices. We aimed to study the systematic effects of different in vitro thrombus models on the results of experimental thrombectomy and examined how thrombi formed in vitro and ex vivo differ. METHODS: Three variables involved in human in vitro thrombogenesis were investigated: spontaneous or thrombin-induced clotting, age (1 or 5 days old), and storage temperature (4 degrees C or 21 degrees C). The fibrin content of in vitro and fresh or old ex vivo thrombi was measured by histologic studies. Ten experiments were performed with each of 8 different in vitro thrombus types using (1) ultrasound thrombolysis, (2) Oasis thrombectomy, (3) Amplatz thrombectomy, and (4) Straub-Rotarex catheters. Thrombus weight was measured after standardized treatment. RESULTS: The fibrin content was markedly lower in all in vitro than in fresh and old ex vivo thrombi. In vitro thrombus type had no impact on the effectiveness of ultrasound thrombolysis and Amplatz thrombectomy. Thrombogenesis type affected Oasis and Straub-Rotarex catheter use. Storage temperature had a systematic impact on the outcome of Oasis thrombectomies. CONCLUSION: The fibrin content of in vitro thrombi differs substantially from that of fresh and old ex vivo human thrombi. Experimental conditions may systematically impact experimental evaluation of thrombectomy procedures. In vitro thrombi with thrombin-induced thrombogenesis should be favored for use in thrombectomy experiments.     

Evaluation of DNA aptamers directed to thrombin as potential thrombus imaging agents

Two DNA aptamers directed against two separate exosites on human alpha-thrombin were evaluated for thrombus-imaging potential. Aptamer ODN 1 is directed to the thrombin substrate binding site (exosite 1). Our finding that ODN 1 competes with fibrin for binding to exosite 1 on thrombin suggests that ODN 1 will not be useful for thrombus imaging. Aptamer ODN 2 is directed against the thrombin heparin binding site (exosite 2). ODN 2 bound to model thrombi that were formed either by clotting purified fibrinogen with thrombin, or by recalcifying citrated plasma. As the thrombin content of thrombi was increased the rate of ODN 2 uptake into preformed thrombi increased, whereas the rate of release of ODN 2 out of preformed thrombi decreased. This in vitro data suggested that ODN 2 might be useful for thrombus imaging because it can bind to exosite 2 on fibrin-bound thrombin. However, in a rabbit jugular vein model using thrombus supplemented with human thrombin, ODN 2 uptake was equal to the ovalbumin control, and did not reflect thrombin content. While the in vitro results with ODN 2 were consistent with thrombus imaging, the rapid clearance of ODN 2 from circulation, combined with slow mass transfer in the clot, seem to work against in vivo thrombin-dependent imaging or washout analysis.     

兔慢性肺动脉血栓栓塞模型的建立及其64层螺旋CT表现

目的：建立兔的慢性肺动脉血栓栓塞模型,研究其64层CT肺动脉成像表现。方法：健康家兔28只,体量2.0-2.5 kg。栓塞组20只,假栓塞组8只。栓塞组兔肺动脉多次注入自体血栓,血栓直径为1-2 mm,长3-5 mm,每次栓塞量约3-5条,栓塞前后和12周时64层螺旋CT平扫和CT肺动脉成像扫描,分析其影像学表现特点。结果：栓塞组20只动物,死亡4只,存活并符合实验要求动物16只,成功率80%;假栓塞组无动物死亡。兔肺动脉栓塞后血浆D-二聚体在急性期明显增高,约增加80%,在慢性期降至正常水平。栓塞组16例CT肺动脉成像均发现肺动脉栓塞直接征象并经过病理证实。慢性肺动脉栓塞的间接征象中,肺动脉直径增粗的发生率为87.5%,肺梗死发生率为18.8%,斑片状实变影和胸膜增厚的发生率为12.5%,肺少血的发生率为6%。结论：兔自体血栓反复注射法建立慢性肺血栓栓塞模型有较高的可行性,64层CT肺动脉成像能清楚显示肺动脉栓塞的直接征象和间接征象,有助于肺动脉栓塞的诊断。