Molecular Magnetic Resonance Imaging of Pulmonary Emboli with a Fibrin-specific Contrast Agent
http://www.100md.com
美国呼吸和危急护理医学 2005年第8期
Department of Diagnostic Radiology, Aachen Technical University, Aachen, Germany
Epix Pharmaceuticals, Cambridge
Department of Medicine (Cardiovascular Division), Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts
ABSTRACT
Rationale and Objectives: The detection of pulmonary embolism is still challenging due to the often nonspecific clinical findings. The aim of this study was to investigate the potential of molecular targeted magnetic resonance imaging (MRI) of pulmonary emboli using low-dose application of a fibrin-specific contrast agent (EP-2104R; Epix Pharmaceuticals, Cambridge, MA). Methods: Fresh clots from human blood were engineered ex vivo and delivered in the lungs of 11 swine. Subsequently, a T1-weighted breath-hold three-dimensional gradient-echo sequence was performed before as well as 5 minutes, 1 hour, and 2 hours after systemic administration of 7.5 (n = 5) or 4 (n = 5) eol/kg EP-2104R. One swine that did not receive any contrast agent served as a control. MR images were analyzed by two investigators and contrast-to-noise ratio between the thrombus and the surrounding tissue (blood pool and lung parenchyma) was assessed. Localization of thrombi was compared with 16-row multislice computed tomography. Finally, the animals were killed and thrombi were removed for assessment of gadolinium concentration. Main Results: Before contrast media application, thrombi were not visible on MR images. At 1 and 2 hours after contrast media injection, pulmonary emboli were selectively visualized with high-signal intensity foci, independent of the dosage used. A high gadolinium concentration in thrombi was found after both dosages (83 ± 41 e for 4 eol/kg and 130 ± 57e for 7.5 eol/kg), resulting in a similar high contrast-to-noise ratio on MR images (between 11 and 13). Conclusion: Systemic low-dose application of EP-2104R allows for selective molecular MRI of fresh pulmonary thromboembolism in a swine model.
Key Words: contrast; embolism; fibrin; magnetic resonance imaging; molecular imaging; pulmonary vessel
Pulmonary embolism is a common disease, with an incidence of more than 1 in 1,000 and a high risk of death (1). Early diagnosis is crucial because, with effective anticoagulation, recurrence of embolism is increasingly rare (2, 3). However, clinical findings are often nonspecific, resulting in a diagnostic dilemma (2eC5). Because x-ray angiography (the gold standard for detection of pulmonary embolism) is invasive, variable noninvasive tests have been implemented, such as perfusion/ventilation scintigraphy, computed tomographic (CT) scanning, or magnetic resonance angiography (MRA) (2, 3, 6eC13). Scintigraphy is frequently used in routine clinical settings, although image quality is often limited (9, 14). Spiral CT after bolus contrast media injection has been implemented as an alternative clinical tool for noninvasive imaging of the pulmonary vasculature (2, 8, 10, 15). Although this technique allows for pulmonary embolism visualization in an animal model (15eC17), several pitfalls have been described in patient studies, such as in planeeCvessel orientations or false-negative findings if small vessels demonstrate only signal break-off (2, 7, 12, 15, 17). MRA is a recently implemented noninvasive imaging modality without the need of iodinated contrast agents and without the need of x-ray exposure (6). Using this technique, a fast MRA sequence is performed after bolus contrast media injection during the first pass. However, timing of data acquisition is crucial, and because only one very brief dataset can be acquired, spatial resolution is limited. Furthermore, similar to CT scanning, pulmonary emboli are only seen as a filling defect of the vessel lumen, which also may limit visualization of smaller (subsegmental) or occlusive emboli because the embolus and the surrounding lung tissue both appear as areas with low signal. This may result in potentially false-negative findings. In addition, tumor masses involving the pulmonary vessels or lymph nodes may mimic embolism (7, 12, 18). Advantages of routine CT scanning include the simultaneous information about parenchymal abnormalities, which are not visible on MRA. On the other hand, CT requires intravenous administration of iodinated contrast medium, which is a limitation in patients with renal insufficiency.
Currently, no single noninvasive imaging modality reliably rules out pulmonary embolism, and in approximately 30% of cases, patients only suffer from subsegmental pulmonary embolism, which is a pitfall for all currently noninvasive imaging modalities (2). For improved noninvasive imaging, selective visualization of the embolus itself would be of great value. Recently, fibrin-specific contrast agents have been developed, allowing for high local gadolinium (Gd) concentration on the thrombus surface (19eC22). With such agents, selective and high-contrast visualization of pulmonary emboli may be enabled. Hence, the purpose of this study was the investigation of a fibrin-specific contrast agent (EP-2104R; Epix Pharmaceuticals, Cambridge, MA) for selective molecular MR imaging (MRI) of pulmonary emboli in a swine model. Two low doses (7.5 and 4 eol/kg), which represent only 7.5 and 4% of the dosage of clinically used extracellular contrast agents, are compared.
METHODS
Animals
Molecular MRI of pulmonary emboli was performed in 11 healthy swine (48eC52 kg) as approved by the German government committee on animal investigations. After premedication with 0.5 ml intramuscular atropine, 0.2 ml intramuscular azaperone/kg bodyweight, and 0.1 ml ketamine/kg bodyweight, an aqueous solution of pentobarbital (1:3) was administered intravenously via an ear vein as needed. The animals were intubated, and mechanical ventilation was maintained throughout the entire study. A 16-F sheath (Cordis, Roden, The Netherlands) was placed in the right iliac vein.
Clot Engineering
Thrombi were formed ex vivo for injection into the lungs. For each 1 ml of thrombus, 10 mg human fibrinogen, 10 NIH units of thrombin, 25 mM CaCl2, and blood were mixed in a 1-ml syringe and allowed to sit for at least 30 minutes. Segments of approximately 0.5 to 0.8 ml were delivered for pulmonary embolism.
Fibrin-specific Contrast Agent
The novel fibrin-specific MR contrast agent EP-2104R is composed of a small peptide with 4 Gd-chelate moieties (23). Similar to EP-1873, it binds to fibrin without binding to circulating fibrinogen (19). EP-2104R (15 mM in saline) was infused over 3 minutes for a dose of 4 and 7.5 eol/kg [0.004 and 0.0075 mmol/kg, representing 7.5 and 4% of a clinical dose of standard Gd diethylenetriamine pentaacetic acid (DTPA)].
MR Scanner
All studies were performed on a 1.5-T Gyroscan Intera whole body MR system (Philips Medical Systems, Best, The Netherlands; 23 mT/m-meter, 219-microsecond rise time). A four-element body wrap around phase-array coil was used for signal reception. All subjects were examined in supine position.
Pulmonary MRI Sequences
MRI of the lung consisted of a radio-frequency (RF) spoiled breath-hold three-dimensional (3D) gradient-echo sequence (3D fast-field echo [3D FFE]). In contrast to earlier studies, no magnetization prepulses were used to allow for more reliable signal comparison of the two doses used (signal intensity and contrast in magnetization-prepared sequences may depend on prepulse efficacy) (23, 24). Sequence parameters of the 3D FFE sequence included the following: repetition time (TR) = 5.5 milliseconds, echo time (TE) = 1.5 milliseconds, flip angle = 40°, field-of-view = 450 x 270 mm, 512 x 205 matrix, resulting in a 0.9 x 1.7 x 1.7eCmm voxel size. One hundred coronal slices covering the entire thorax were acquired, avoiding any inflow effects in the outer slices as seen in smaller volume coverage gradient-echo imaging. Using a loweChigh k-space sampling order, central k-space lines, which are more sensitive to motion artifacts, were acquired first. Thus, the breath-hold length could be limited to approximately 25 seconds, whereas the remaining scanning time ( 1 minute) was acquired during free breathing, maintaining suppressions of respiratory motion artifacts as previously described by Maki and colleagues (25).
Description of Experiments
Four to six thrombi per swine (in total, 52 thrombi in 10 swine, 26 in each group) were loaded into a 12-F sheath and then delivered via the 16-F sheath in the iliac vein by floating the sheath with saline. Five swine received 4 eol/kg and the other five received 7.5 eol/kg EP-2104R via an ear vein. As a control in a further pig, seven pulmonary emboli were delivered and underwent imaging without application of any extrinsic contrast medium.
After delivery of the clots, the swine were positioned in the MR unit and the 3D FFE sequence was performed before contrast media application. After injection of the contrast medium over 3 minutes via an ear vein, MRI was repeated 5 minutes, about 1 hour after contrast media administration (between 60 and 90 minutes after contrast injection), and again approximately 2 hours after contrast media administration (between 2 and 2.5 hours after contrast injection). This allowed for studying of any impact of the washout of the contrast medium from the blood pool. More prolonged, delayed MRI was not performed to allow for Gd concentration analyses in the clots without any major time delay from contrast media injection. Further MRI was performed only in the control swine, which did not receive any contrast medium, 4 and 6 hours after clot delivery to exclude any signal changes of the clot itself during the time of experiments.
Finally, the animals were transferred to a 16-row multislice CT unit. Multislice CT was performed for comparison because it can detect pulmonary embolism in a swine model (11, 15eC17) and is superior when compared with single-slice CT (15, 17). CT is also a tomographic imaging modality that allows for easier comparison to MRI when compared with projection techniques like x-ray pulmonary angiography. Furthermore, it is fast to perform, avoiding any major time delay between contrast media application and autopsy in our study. Sixteen-row multidetector CT scanning of the lung was performed with 16 x 0.75eCmm collimation (Somatom Sensation; Siemens, Erlangen, Germany), 120-kV tube voltage, 300-mm reconstruction field-of-view, 15-mm table feed/rotation after bolus application of 90 ml of nonionic contrast material (Ultravist 370; Schering, Berlin, Germany) at a flow rate of 3.5 ml/second. Axial images with a 2-mm reconstruction increment and coronal multiplanar reconstructions (MPRs) from 1.0/0.6 mm reconstruction were used as described in the literature (15).
After multidetector CT, animals were immediately killed and the clots were removed from the pulmonary vascular bed for the assessment of Gd concentration. Only clots that could be clearly detected and that could be entirely removed were used for quantitative analyses.
Data analyses.
Molecular MR images of pulmonary emboli were visually analyzed by two radiologists before CT scanning and autopsy. Hence, they were not aware of findings seen on subsequent CT examination and autopsy. However, they knew the range of the number of clots injected (4eC6 clots/swine as described in the study protocol). Number and localization of the clots was recorded and then compared with CT and autopsy. CT was analyzed by two investigators who were aware of the range of clots seen on MR images. In case of disagreement, they could discuss with a third investigator.
Signal analyses included determination of contrast-to-noise ratio (CNR) between the clot and the surrounding blood pool/lung parenchyma over time. Regions of interest were placed manually in the era of bright signal (clots), the blood pool, and lung parenchyma on 1- and 2-hour postcontrast images. Because identical geometric data of MR images were used, regions of interest were copied to the precontrast images. CNR was calculated as follows:
where SDair refers to the SD of the signal in air outside the swine.
CNR pre- and postcontrast and for both dosages was averaged for each pig and compared using two-tailed Students t test. A p value of less than 0.05 was considered significant.
Gd concentrations were measured by inductively coupled plasma-mass spectrometry. Thrombi were digested overnight with concentrated nitric acid. Digested samples were then diluted with 5% nitric acid containing terbium (Tb) as an internal standard. The Gd concentration was determined against a calibration curve of the Gd to internal standard ratio versus concentration. Gd concentration measurements were available in 17 clots in the 4-eol/kg group and 26 clots in the 7.5-eol/kg group.
RESULTS
Pulmonary clot embolism and MRI were successfully completed in all animals. Before contrast media application, the lung vessels and lung tissue were signal suppressed in the entire dataset. No inflow effects were visible. Pulmonary emboli could not be observed on precontrast MR images (Figures 1eC4).
Five minutes after contrast media application, with both dosages, the pulmonary vessels were seen with a bright signal due to T1-shortening of the blood pool. Also, slight signal enhancement of the lung tissue was seen; hence, thrombi could not be detected. Inconsistently, however, some defects of signal enhancement of lung parenchyma could be seen (dotted arrow in Figure 2A). On MR images performed 1 and 2 hours after contrast media injection, signal of the blood pool went clearly down and pulmonary emboli could be seen with a high signal when compared with the blood pool and the surrounding lung tissue (Figures 1eC4, Movie E1 in the online supplement) using both dosages (7.5 eol/kg to n = 32 emboli, 4 eol/kg to n = 29 emboli), independent of the size and localization of the emboli (some thrombi broke in smaller sized portions). However, visually slightly better contrast was achieved on the 2-hour postcontrast images when compared with the 1-hour postcontrast images (Figures 1eC4).
CNR measurements yielded for both dosages used a highly significant increase of CNR 1 and 2 hours after contrast media application (CNR clot/lung between 11 and 13, CNR clot/blood between 9 and 11) when compared with the precontrast images (CNR < 1.5; Table 1). Objective CNR increase did not differ significantly between both dosages nor between the 1- and 2-hour postcontrast images (Table 1). High contrast in the postcontrast images was in concordance with Gd concentration in the clots measured ex vivo (n = 26 and 17). For both dosages, high Gd concentrations (7.5 eol/kg, 130 ± 57 e; 4 eol/kg, 83 ± 41 e; p < 0.03) were found. In the swine that did not receive any contrast medium, no emboli were seen and no Gd was detected.
Pulmonary emboli seen on MR images were proved by multislice CT or macroscopically. One clot seen on MRI was only retrospectively visible on CT, and two clots next to the heart and mediastinum were missed on CT due to motion artifacts; however, these clots were proved macroscopically. Three clots were interpreted as separate, adjacent clots on MRI, whereas these clots appeared on CT as a longer clot.
DISCUSSION
Pulmonary embolism is a frequent and life-threatening disease that requires early detection and treatment because, with anticoagulation, recurrent embolism and potential collapse of the circulation can be avoided. Chronic smaller embolism may result in pulmonary hypertension (3, 26). In addition, in some cases, tumor may mimic embolism.
MRI is the preferred noninvasive imaging modality because it does not need x-ray exposure and no potentially nephrotoxic, iodinated contrast agents. However, currently used MRA is of limited value because data have to be acquired during the first pass after bolus contrast media application. Hence, only one dataset with limited spatial resolution can be acquired. Furthermore, smaller, more subsegmental emboli result in a break of the vessel on MRA. However, this may be difficult to detect, because the embolus appears with similar low-signal intensity when compared with the surrounding lung tissue. Molecular imaging with targeted imaging of the clot itself using a fibrin-specific contrast agent may overcome this limitation, since only the embolus appears bright, resulting in a high contrast to the surrounding blood pool and lung tissue. Furthermore, in contrast to MRA, multiple datasets can be acquired because, with molecular binding of a contrast agent, longer scanning times may become possible.
The present study demonstrated selective and high-contrast imaging of pulmonary emboli in a swine model after low-dose administration of a molecular targeted (fibrin-specific) contrast agent, EP2104R. This agent results in a high accumulation of Gd in the clots.
For molecular MRI of pulmonary emboli, we used a standard 3D FFE sequence available on a clinical MR scanner. High spatial resolution scanning (0.9 x 1.7 x 1.7eCmm measured voxel size) was used because, in our model, relatively small emboli were delivered. A thick 3D imaging slab with 100 slices was acquired covering the entire chest, allowing for detection of all (also more peripheral) emboli. Furthermore, coverage of the entire chest avoids any signal alterations based on blood inflow in gradient-echo imaging. On the other hand, for larger volume imaging, longer measurement time when compared with standard contrast enhanced pulmonary MRA was necessary (6). However, because EP-2104R binds to fibrin and remains bound for several hours, data acquisition time is not crucial. This may eliminate any time constraints for the sequence used and several subsequent datasets can be acquired, which is not possible with standard MRA approaches. To achieve respiratory motion artifact reduction in the 3D gradient-echo imaging, maintaining a breath-hold length close to a clinical setting, only the first 25 seconds with the more central k-space lines were acquired during suspended breathing, as previously described (25). With such a regime, no respiratory motion artifacts were observed, allowing for visualization of even very small emboli as shown in Figures 1 through 4. Furthermore, due to the very high contrast obtained, detection of even very small emboli may become possible.
In our study, two low dosages (4 and 7.5 eol/kg, representing only 4 and 7.5% of the dosage of standard extracellular contrast agent) were compared. With both doses, emboli could be clearly seen on the 1- and 2-hour images. In addition to cost reduction, lower dosages are preferred because the blood pool signal will drop faster to a lower level, which allows for sufficient suppression of the surrounding tissue while maintaining high concentration in the clot due to specific binding. In addition, our results show that, with 4 eol/kg, high signal and high contrast could be obtained. Because detection of clots using molecular targeted MRI does not necessarily need precontrast scanning, one regime for patient examination may include contrast injection before MR scanning. However, the optimal time delay between contrast media injection and molecular MRI remains to be defined in patients.
In a recently published study, using molecular imaging of clots in a rabbit model with a similar agent (EP-1873), a prolonged binding (up to 24 hours) was shown (19). This prolonged binding may also be helpful for MR follow-up control after treatment of pulmonary embolism with anticoagulation or lysis (preferably without the need of any further contrast media application).
The present study focused on pulmonary embolism. However, for routine clinical use a direct combination with imaging of the lower extremities for detection of deep vein thrombosis would be preferable. Deep vein thrombosis can be visualized by MR using noncontrast enhanced MR sequences (7, 27eC29) or pedal contrast media injection (30). However, with systemic injection of a fibrin-specific contrast agent, selective clot visualization for detection of deep vein thrombosis may also be enabled. However, the potential of EP2104R for visualization of deep vein thrombosis remains to be investigated.
EP-2104R is a fibrin-specific contrast agent. Because binding may vary between different species, we used engineered clot from human blood, allowing for easier transfer of our data to a clinical setting. However, further studies are needed to demonstrate the potential of this new agent for selective visualization of in situeCdeveloped deep vein thrombosis and pulmonary embolism in patients as well. Furthermore, in vivo in humans, clots may demonstrate continuous modifications of the surface and the configuration, which also may have an impact on the binding and thus MRI. Our results warrant clinical investigations in patients with pulmonary embolism to show the clinical value of this new agent for molecular MRI of pulmonary embolism.
In the last couple of years, variable fibrin-specific contrast agents have been developed. Mostly, Gd-loaded nanoparticles or iron particles were used (20eC22). D-dimereCspecific antibodies have been used for nuclear medicine studies (31). However, these agents may be less likely to come into clinical use due to biodistribution and safety issues (32). EP-2104R is a small peptide, which binds to fibrin without antibodies, and thus such agents have a higher chance to reach clinical use without safety concerns.
In addition to molecular imaging with fibrin-specific contrast agents or contrast-enhanced MRA, so-called direct thrombus imaging has also been proposed. This technique uses the T1-shortening of clots based on the breakdown products of hemoglobin (33, 34). However, the signal properties of clots change over time, reducing the potential imaging window (35), which is limited to between 8 hours and 6 months (36).
Because fibrin is seen in acute as well as older clots (37), fibrin-targeted molecular imaging may allow for selective clot imaging over a long time. Thus, further studies investigating the potential of EP-2104R for detection of acute as well as chronic thrombi are needed.
Conclusions
Systemic low-dose application of EP-2104R allows for selective molecular MRI of acute pulmonary thromboembolism in a swine model.
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
REFERENCES
Goldhaber SZ. Pulmonary embolism. Lancet 2004;363:1295eC1305.
Fedullo PF, Tapson VF. Clinical practice: the evaluation of suspected pulmonary embolism. N Engl J Med 2003;349:1247eC1256.
Olin JW. Pulmonary embolism. Rev Cardiovasc Med 2002;3:S68eCS75.
Miniati M, Prediletto R, Formichi B, Marini C, Di Ricco G, Tonelli L, Allescia G, Pistolesi M. Accuracy of clinical assessment in the diagnosis of pulmonary embolism. Am J Respir Crit Care Med 1999;159:864eC871.
Girard P, Decousus M, Laporte S, Buchmuller A, Herve P, Lamer C, Parent F, Tardy B. Diagnosis of pulmonary embolism in patients with proximal deep vein thrombosis: specificity of symptoms and perfusion defects at baseline and during anticoagulant therapy. Am J Respir Crit Care Med 2001;164:1033eC1037.
Meaney JF, Weg JG, Chenevert TL, Stafford-Johnson D, Hamilton BH, Prince MR. Diagnosis of pulmonary embolism with magnetic resonance angiography. N Engl J Med 1997;336:1422eC1427.
Tapson VF. Pulmonary embolismeCnew diagnostic approaches. N Engl J Med 1997;336:1449eC1451.
Ghaye B, Remy J, Remy-Jardin M. Non-traumatic thoracic emergencies: CT diagnosis of acute pulmonary embolism: the first 10 years. Eur Radiol 2002;12:1886eC1905.
Prospective Investigation of Pulmonary Embolism Diagnosis Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism: results of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED). JAMA 1990;263:2753eC2759.
Schoepf UJ, Goldhaber SZ, Costello P. Spiral computed tomography for acute pulmonary embolism. Circulation 2004;109:2160eC2167.
Haage P, Piroth W, Krombach G, Karaagac S, Schaffter T, Gunther RW, Bucker A. Pulmonary embolism: comparison of angiography with spiral computed tomography, magnetic resonance angiography, and real-time magnetic resonance imaging. Am J Respir Crit Care Med 2003;167:729eC734.
Han D, Lee KS, Franquet T, Muller NL, Kim TS, Kim H, Kwon OJ, Byun HS. Thrombotic and nonthrombotic pulmonary arterial embolism: spectrum of imaging findings. Radiographics 2003;23:1521eC1539.
Lorut C, Ghossains M, Horellou MH, Achkar A, Fretault J, Laaban JP. A noninvasive diagnostic strategy including spiral computed tomography in patients with suspected pulmonary embolism. Am J Respir Crit Care Med 2000;162:1413eC1418.
Hartmann IJ, Hagen PJ, Melissant CF, Postmus PE, Prins MH. Diagnosing acute pulmonary embolism: effect of chronic obstructive pulmonary disease on the performance of D-dimer testing, ventilation/perfusion scintigraphy, spiral computed tomographic angiography, and conventional angiography. ANTELOPE Study Group (Advances in New Technologies Evaluating the Localization of Pulmonary Embolism). Am J Respir Crit Care Med 2000;162:2232eC2237.
Remy-Jardin M, Mastora I, Remy J. Pulmonary embolus imaging with multislice CT. Radiol Clin North Am 2003;41:507eC519.
Baile EM, King GG, Muller NL, D'Yachkova Y, Coche EE, Pare PD, Mayo JR. Spiral computed tomography is comparable to angiography for the diagnosis of pulmonary embolism. Am J Respir Crit Care Med 2000;161:1010eC1015.
Ghaye B, Szapiro D, Mastora I, Delannoy V, Duhamel A, Remy J, Remy-Jardin M. Peripheral pulmonary arteries: how far in the lung does multi-detector row spiral CT allow analysis Radiology 2001;219:629eC636.
Gupta A, Frazer CK, Ferguson JM, Kumar AB, Davis SJ, Fallon MJ, Morris IT, Drury PJ, Cala LA. Acute pulmonary embolism: diagnosis with MR angiography. Radiology 1999;210:353eC359.
Botnar RM, Perez AS, Witte S, Wiethoff AJ, Laredo J, Hamilton J, Quist W, Parsons EC Jr, Vaidya A, Kolodziej A, et al. In vivo molecular imaging of acute and subacute thrombosis using a fibrin-binding magnetic resonance imaging contrast agent. Circulation 2004;109:2023eC2029.
Yu X, Song SK, Chen J, Scott MJ, Fuhrhop RJ, Hall CS, Gaffney PJ, Wickline SA, Lanza GM. High-resolution MRI characterization of human thrombus using a novel fibrin-targeted paramagnetic nanoparticle contrast agent. Magn Reson Med 2000;44:867eC872.
Flacke S, Fischer S, Scott MJ, Fuhrhop RJ, Allen JS, McLean M, Winter P, Sicard GA, Gaffney PJ, Wickline SA, et al. Novel MRI contrast agent for molecular imaging of fibrin: implications for detecting vulnerable plaques. Circulation 2001;104:1280eC1285.
Choudhury RP, Fuster V, Badimon JJ, Fisher EA, Fayad ZA. MRI and characterization of atherosclerotic plaque: emerging applications and molecular imaging. Arterioscler Thromb Vasc Biol 2002;22:1065eC1074.
Botnar R, Buecker A, Wiethoff AJ, Parsons EC Jr, Katoh M, Katsimaglis G, Weisskoff RM, Lauffer RB, Graham PB, Gunther RW, et al. In vivo magnetic resonance imaging of coronary thrombosis using a fibrin-binding molecular magnetic resonance contrast agent. Circulation 2004;110:1463eC1466.
Spuentrup E, Buecker A, Katoh M, Wiethoff AJ, Parsons EC Jr, Botnar RM, Weisskoff RM, Graham PB, Manning WJ, Gunther RW. Molecular magnetic resonance imaging of coronary thrombosis and pulmonary emboli with a novel fibrin-targeted contrast agent. Circulation 2005;22:1377eC1382.
Maki JH, Chenevert TL, Prince MR. The effects of incomplete breath-holding on 3D MR image quality. J Magn Reson Imaging 1997;7:1132eC1139.
Fedullo PF, Auger WR, Kerr KM, Rubin LJ. Chronic thrombembolic pulmonary hypertension. N Engl J Med 2001;345:1465eC1472.
Evans AJ, Sostman HD, Knelson MH, Spritzer CE, Newman GE, Paine SS, Beam CA. 1992 ARRS Executive Council Award. Detection of deep venous thrombosis: prospective comparison of MR imaging with contrast venography. AJR Am J Roentgenol 1993;161:131eC139.
Spuentrup E, Buecker A, Stuber M, Gunther RW. MR-venography using high resolution true-FISP. Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr 2001;173:686eC690.
Moody AR, Pollock JG, O'Connor AR, Bagnall M. Lower-limb deep venous thrombosis: direct MR imaging of the thrombus. Radiology 1998;209:349eC355.
Ruehm SG, Zimny K, Debatin JF. Direct contrast-enhanced 3D MR venography. Eur Radiol 2001;11:102eC112.
Morris TA, Marsh JJ, Chiles PG, Konopka RG, Pedersen CA, Schmidt PF, Gerometta M. Single photon emission computed tomography of pulmonary emboli and venous thrombi using anti-D-dimer. Am J Respir Crit Care Med. 2004;169:987eC93.
Weinmann HJ, Ebert W, Misselwitz B, Schmitt-Willich H. Tissue-specific MR contrast agents. Eur J Radiol 2003;46:33eC44.
Moody AR, Liddicoat A, Krarup K. Magnetic resonance pulmonary angiography and direct imaging of embolus for the detection of pulmonary emboli. Invest Radiol 1997;32:431eC440.
Moody AR, Murphy RE, Morgan PS, Martel AL, Delay GS, Allder S, MacSweeney ST, Tennant WG, Gladman J, Lowe J, et al. Characterization of complicated carotid plaque with magnetic resonance direct thrombus imaging in patients with cerebral ischemia. Circulation 2003;107:3047eC3052.
Corti R, Osende JI, Fayad ZA, Fallon JT, Fuster V, Mizsei G, Dickstein E, Drayer B, Badimon JJ. In vivo noninvasive detection and age definition of arterial thrombus by MRI. J Am Coll Cardiol 2002;39:1366eC1373.
Fraser DG, Moody AR, Morgan PS, Martel AL, Davidson I. Diagnosis of lower-limb deep venous thrombosis: a prospective blinded study of magnetic resonance direct thrombus imaging. Ann Intern Med 2002;136:89eC98.
Davies MJ. Stability and instability: two faces of coronary atherosclerosis. The Paul Dudley White Lecture 1995. Circulation 1996;94:2013eC2020.(Elmar Spuentrup, Marcus K)
Epix Pharmaceuticals, Cambridge
Department of Medicine (Cardiovascular Division), Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts
ABSTRACT
Rationale and Objectives: The detection of pulmonary embolism is still challenging due to the often nonspecific clinical findings. The aim of this study was to investigate the potential of molecular targeted magnetic resonance imaging (MRI) of pulmonary emboli using low-dose application of a fibrin-specific contrast agent (EP-2104R; Epix Pharmaceuticals, Cambridge, MA). Methods: Fresh clots from human blood were engineered ex vivo and delivered in the lungs of 11 swine. Subsequently, a T1-weighted breath-hold three-dimensional gradient-echo sequence was performed before as well as 5 minutes, 1 hour, and 2 hours after systemic administration of 7.5 (n = 5) or 4 (n = 5) eol/kg EP-2104R. One swine that did not receive any contrast agent served as a control. MR images were analyzed by two investigators and contrast-to-noise ratio between the thrombus and the surrounding tissue (blood pool and lung parenchyma) was assessed. Localization of thrombi was compared with 16-row multislice computed tomography. Finally, the animals were killed and thrombi were removed for assessment of gadolinium concentration. Main Results: Before contrast media application, thrombi were not visible on MR images. At 1 and 2 hours after contrast media injection, pulmonary emboli were selectively visualized with high-signal intensity foci, independent of the dosage used. A high gadolinium concentration in thrombi was found after both dosages (83 ± 41 e for 4 eol/kg and 130 ± 57e for 7.5 eol/kg), resulting in a similar high contrast-to-noise ratio on MR images (between 11 and 13). Conclusion: Systemic low-dose application of EP-2104R allows for selective molecular MRI of fresh pulmonary thromboembolism in a swine model.
Key Words: contrast; embolism; fibrin; magnetic resonance imaging; molecular imaging; pulmonary vessel
Pulmonary embolism is a common disease, with an incidence of more than 1 in 1,000 and a high risk of death (1). Early diagnosis is crucial because, with effective anticoagulation, recurrence of embolism is increasingly rare (2, 3). However, clinical findings are often nonspecific, resulting in a diagnostic dilemma (2eC5). Because x-ray angiography (the gold standard for detection of pulmonary embolism) is invasive, variable noninvasive tests have been implemented, such as perfusion/ventilation scintigraphy, computed tomographic (CT) scanning, or magnetic resonance angiography (MRA) (2, 3, 6eC13). Scintigraphy is frequently used in routine clinical settings, although image quality is often limited (9, 14). Spiral CT after bolus contrast media injection has been implemented as an alternative clinical tool for noninvasive imaging of the pulmonary vasculature (2, 8, 10, 15). Although this technique allows for pulmonary embolism visualization in an animal model (15eC17), several pitfalls have been described in patient studies, such as in planeeCvessel orientations or false-negative findings if small vessels demonstrate only signal break-off (2, 7, 12, 15, 17). MRA is a recently implemented noninvasive imaging modality without the need of iodinated contrast agents and without the need of x-ray exposure (6). Using this technique, a fast MRA sequence is performed after bolus contrast media injection during the first pass. However, timing of data acquisition is crucial, and because only one very brief dataset can be acquired, spatial resolution is limited. Furthermore, similar to CT scanning, pulmonary emboli are only seen as a filling defect of the vessel lumen, which also may limit visualization of smaller (subsegmental) or occlusive emboli because the embolus and the surrounding lung tissue both appear as areas with low signal. This may result in potentially false-negative findings. In addition, tumor masses involving the pulmonary vessels or lymph nodes may mimic embolism (7, 12, 18). Advantages of routine CT scanning include the simultaneous information about parenchymal abnormalities, which are not visible on MRA. On the other hand, CT requires intravenous administration of iodinated contrast medium, which is a limitation in patients with renal insufficiency.
Currently, no single noninvasive imaging modality reliably rules out pulmonary embolism, and in approximately 30% of cases, patients only suffer from subsegmental pulmonary embolism, which is a pitfall for all currently noninvasive imaging modalities (2). For improved noninvasive imaging, selective visualization of the embolus itself would be of great value. Recently, fibrin-specific contrast agents have been developed, allowing for high local gadolinium (Gd) concentration on the thrombus surface (19eC22). With such agents, selective and high-contrast visualization of pulmonary emboli may be enabled. Hence, the purpose of this study was the investigation of a fibrin-specific contrast agent (EP-2104R; Epix Pharmaceuticals, Cambridge, MA) for selective molecular MR imaging (MRI) of pulmonary emboli in a swine model. Two low doses (7.5 and 4 eol/kg), which represent only 7.5 and 4% of the dosage of clinically used extracellular contrast agents, are compared.
METHODS
Animals
Molecular MRI of pulmonary emboli was performed in 11 healthy swine (48eC52 kg) as approved by the German government committee on animal investigations. After premedication with 0.5 ml intramuscular atropine, 0.2 ml intramuscular azaperone/kg bodyweight, and 0.1 ml ketamine/kg bodyweight, an aqueous solution of pentobarbital (1:3) was administered intravenously via an ear vein as needed. The animals were intubated, and mechanical ventilation was maintained throughout the entire study. A 16-F sheath (Cordis, Roden, The Netherlands) was placed in the right iliac vein.
Clot Engineering
Thrombi were formed ex vivo for injection into the lungs. For each 1 ml of thrombus, 10 mg human fibrinogen, 10 NIH units of thrombin, 25 mM CaCl2, and blood were mixed in a 1-ml syringe and allowed to sit for at least 30 minutes. Segments of approximately 0.5 to 0.8 ml were delivered for pulmonary embolism.
Fibrin-specific Contrast Agent
The novel fibrin-specific MR contrast agent EP-2104R is composed of a small peptide with 4 Gd-chelate moieties (23). Similar to EP-1873, it binds to fibrin without binding to circulating fibrinogen (19). EP-2104R (15 mM in saline) was infused over 3 minutes for a dose of 4 and 7.5 eol/kg [0.004 and 0.0075 mmol/kg, representing 7.5 and 4% of a clinical dose of standard Gd diethylenetriamine pentaacetic acid (DTPA)].
MR Scanner
All studies were performed on a 1.5-T Gyroscan Intera whole body MR system (Philips Medical Systems, Best, The Netherlands; 23 mT/m-meter, 219-microsecond rise time). A four-element body wrap around phase-array coil was used for signal reception. All subjects were examined in supine position.
Pulmonary MRI Sequences
MRI of the lung consisted of a radio-frequency (RF) spoiled breath-hold three-dimensional (3D) gradient-echo sequence (3D fast-field echo [3D FFE]). In contrast to earlier studies, no magnetization prepulses were used to allow for more reliable signal comparison of the two doses used (signal intensity and contrast in magnetization-prepared sequences may depend on prepulse efficacy) (23, 24). Sequence parameters of the 3D FFE sequence included the following: repetition time (TR) = 5.5 milliseconds, echo time (TE) = 1.5 milliseconds, flip angle = 40°, field-of-view = 450 x 270 mm, 512 x 205 matrix, resulting in a 0.9 x 1.7 x 1.7eCmm voxel size. One hundred coronal slices covering the entire thorax were acquired, avoiding any inflow effects in the outer slices as seen in smaller volume coverage gradient-echo imaging. Using a loweChigh k-space sampling order, central k-space lines, which are more sensitive to motion artifacts, were acquired first. Thus, the breath-hold length could be limited to approximately 25 seconds, whereas the remaining scanning time ( 1 minute) was acquired during free breathing, maintaining suppressions of respiratory motion artifacts as previously described by Maki and colleagues (25).
Description of Experiments
Four to six thrombi per swine (in total, 52 thrombi in 10 swine, 26 in each group) were loaded into a 12-F sheath and then delivered via the 16-F sheath in the iliac vein by floating the sheath with saline. Five swine received 4 eol/kg and the other five received 7.5 eol/kg EP-2104R via an ear vein. As a control in a further pig, seven pulmonary emboli were delivered and underwent imaging without application of any extrinsic contrast medium.
After delivery of the clots, the swine were positioned in the MR unit and the 3D FFE sequence was performed before contrast media application. After injection of the contrast medium over 3 minutes via an ear vein, MRI was repeated 5 minutes, about 1 hour after contrast media administration (between 60 and 90 minutes after contrast injection), and again approximately 2 hours after contrast media administration (between 2 and 2.5 hours after contrast injection). This allowed for studying of any impact of the washout of the contrast medium from the blood pool. More prolonged, delayed MRI was not performed to allow for Gd concentration analyses in the clots without any major time delay from contrast media injection. Further MRI was performed only in the control swine, which did not receive any contrast medium, 4 and 6 hours after clot delivery to exclude any signal changes of the clot itself during the time of experiments.
Finally, the animals were transferred to a 16-row multislice CT unit. Multislice CT was performed for comparison because it can detect pulmonary embolism in a swine model (11, 15eC17) and is superior when compared with single-slice CT (15, 17). CT is also a tomographic imaging modality that allows for easier comparison to MRI when compared with projection techniques like x-ray pulmonary angiography. Furthermore, it is fast to perform, avoiding any major time delay between contrast media application and autopsy in our study. Sixteen-row multidetector CT scanning of the lung was performed with 16 x 0.75eCmm collimation (Somatom Sensation; Siemens, Erlangen, Germany), 120-kV tube voltage, 300-mm reconstruction field-of-view, 15-mm table feed/rotation after bolus application of 90 ml of nonionic contrast material (Ultravist 370; Schering, Berlin, Germany) at a flow rate of 3.5 ml/second. Axial images with a 2-mm reconstruction increment and coronal multiplanar reconstructions (MPRs) from 1.0/0.6 mm reconstruction were used as described in the literature (15).
After multidetector CT, animals were immediately killed and the clots were removed from the pulmonary vascular bed for the assessment of Gd concentration. Only clots that could be clearly detected and that could be entirely removed were used for quantitative analyses.
Data analyses.
Molecular MR images of pulmonary emboli were visually analyzed by two radiologists before CT scanning and autopsy. Hence, they were not aware of findings seen on subsequent CT examination and autopsy. However, they knew the range of the number of clots injected (4eC6 clots/swine as described in the study protocol). Number and localization of the clots was recorded and then compared with CT and autopsy. CT was analyzed by two investigators who were aware of the range of clots seen on MR images. In case of disagreement, they could discuss with a third investigator.
Signal analyses included determination of contrast-to-noise ratio (CNR) between the clot and the surrounding blood pool/lung parenchyma over time. Regions of interest were placed manually in the era of bright signal (clots), the blood pool, and lung parenchyma on 1- and 2-hour postcontrast images. Because identical geometric data of MR images were used, regions of interest were copied to the precontrast images. CNR was calculated as follows:
where SDair refers to the SD of the signal in air outside the swine.
CNR pre- and postcontrast and for both dosages was averaged for each pig and compared using two-tailed Students t test. A p value of less than 0.05 was considered significant.
Gd concentrations were measured by inductively coupled plasma-mass spectrometry. Thrombi were digested overnight with concentrated nitric acid. Digested samples were then diluted with 5% nitric acid containing terbium (Tb) as an internal standard. The Gd concentration was determined against a calibration curve of the Gd to internal standard ratio versus concentration. Gd concentration measurements were available in 17 clots in the 4-eol/kg group and 26 clots in the 7.5-eol/kg group.
RESULTS
Pulmonary clot embolism and MRI were successfully completed in all animals. Before contrast media application, the lung vessels and lung tissue were signal suppressed in the entire dataset. No inflow effects were visible. Pulmonary emboli could not be observed on precontrast MR images (Figures 1eC4).
Five minutes after contrast media application, with both dosages, the pulmonary vessels were seen with a bright signal due to T1-shortening of the blood pool. Also, slight signal enhancement of the lung tissue was seen; hence, thrombi could not be detected. Inconsistently, however, some defects of signal enhancement of lung parenchyma could be seen (dotted arrow in Figure 2A). On MR images performed 1 and 2 hours after contrast media injection, signal of the blood pool went clearly down and pulmonary emboli could be seen with a high signal when compared with the blood pool and the surrounding lung tissue (Figures 1eC4, Movie E1 in the online supplement) using both dosages (7.5 eol/kg to n = 32 emboli, 4 eol/kg to n = 29 emboli), independent of the size and localization of the emboli (some thrombi broke in smaller sized portions). However, visually slightly better contrast was achieved on the 2-hour postcontrast images when compared with the 1-hour postcontrast images (Figures 1eC4).
CNR measurements yielded for both dosages used a highly significant increase of CNR 1 and 2 hours after contrast media application (CNR clot/lung between 11 and 13, CNR clot/blood between 9 and 11) when compared with the precontrast images (CNR < 1.5; Table 1). Objective CNR increase did not differ significantly between both dosages nor between the 1- and 2-hour postcontrast images (Table 1). High contrast in the postcontrast images was in concordance with Gd concentration in the clots measured ex vivo (n = 26 and 17). For both dosages, high Gd concentrations (7.5 eol/kg, 130 ± 57 e; 4 eol/kg, 83 ± 41 e; p < 0.03) were found. In the swine that did not receive any contrast medium, no emboli were seen and no Gd was detected.
Pulmonary emboli seen on MR images were proved by multislice CT or macroscopically. One clot seen on MRI was only retrospectively visible on CT, and two clots next to the heart and mediastinum were missed on CT due to motion artifacts; however, these clots were proved macroscopically. Three clots were interpreted as separate, adjacent clots on MRI, whereas these clots appeared on CT as a longer clot.
DISCUSSION
Pulmonary embolism is a frequent and life-threatening disease that requires early detection and treatment because, with anticoagulation, recurrent embolism and potential collapse of the circulation can be avoided. Chronic smaller embolism may result in pulmonary hypertension (3, 26). In addition, in some cases, tumor may mimic embolism.
MRI is the preferred noninvasive imaging modality because it does not need x-ray exposure and no potentially nephrotoxic, iodinated contrast agents. However, currently used MRA is of limited value because data have to be acquired during the first pass after bolus contrast media application. Hence, only one dataset with limited spatial resolution can be acquired. Furthermore, smaller, more subsegmental emboli result in a break of the vessel on MRA. However, this may be difficult to detect, because the embolus appears with similar low-signal intensity when compared with the surrounding lung tissue. Molecular imaging with targeted imaging of the clot itself using a fibrin-specific contrast agent may overcome this limitation, since only the embolus appears bright, resulting in a high contrast to the surrounding blood pool and lung tissue. Furthermore, in contrast to MRA, multiple datasets can be acquired because, with molecular binding of a contrast agent, longer scanning times may become possible.
The present study demonstrated selective and high-contrast imaging of pulmonary emboli in a swine model after low-dose administration of a molecular targeted (fibrin-specific) contrast agent, EP2104R. This agent results in a high accumulation of Gd in the clots.
For molecular MRI of pulmonary emboli, we used a standard 3D FFE sequence available on a clinical MR scanner. High spatial resolution scanning (0.9 x 1.7 x 1.7eCmm measured voxel size) was used because, in our model, relatively small emboli were delivered. A thick 3D imaging slab with 100 slices was acquired covering the entire chest, allowing for detection of all (also more peripheral) emboli. Furthermore, coverage of the entire chest avoids any signal alterations based on blood inflow in gradient-echo imaging. On the other hand, for larger volume imaging, longer measurement time when compared with standard contrast enhanced pulmonary MRA was necessary (6). However, because EP-2104R binds to fibrin and remains bound for several hours, data acquisition time is not crucial. This may eliminate any time constraints for the sequence used and several subsequent datasets can be acquired, which is not possible with standard MRA approaches. To achieve respiratory motion artifact reduction in the 3D gradient-echo imaging, maintaining a breath-hold length close to a clinical setting, only the first 25 seconds with the more central k-space lines were acquired during suspended breathing, as previously described (25). With such a regime, no respiratory motion artifacts were observed, allowing for visualization of even very small emboli as shown in Figures 1 through 4. Furthermore, due to the very high contrast obtained, detection of even very small emboli may become possible.
In our study, two low dosages (4 and 7.5 eol/kg, representing only 4 and 7.5% of the dosage of standard extracellular contrast agent) were compared. With both doses, emboli could be clearly seen on the 1- and 2-hour images. In addition to cost reduction, lower dosages are preferred because the blood pool signal will drop faster to a lower level, which allows for sufficient suppression of the surrounding tissue while maintaining high concentration in the clot due to specific binding. In addition, our results show that, with 4 eol/kg, high signal and high contrast could be obtained. Because detection of clots using molecular targeted MRI does not necessarily need precontrast scanning, one regime for patient examination may include contrast injection before MR scanning. However, the optimal time delay between contrast media injection and molecular MRI remains to be defined in patients.
In a recently published study, using molecular imaging of clots in a rabbit model with a similar agent (EP-1873), a prolonged binding (up to 24 hours) was shown (19). This prolonged binding may also be helpful for MR follow-up control after treatment of pulmonary embolism with anticoagulation or lysis (preferably without the need of any further contrast media application).
The present study focused on pulmonary embolism. However, for routine clinical use a direct combination with imaging of the lower extremities for detection of deep vein thrombosis would be preferable. Deep vein thrombosis can be visualized by MR using noncontrast enhanced MR sequences (7, 27eC29) or pedal contrast media injection (30). However, with systemic injection of a fibrin-specific contrast agent, selective clot visualization for detection of deep vein thrombosis may also be enabled. However, the potential of EP2104R for visualization of deep vein thrombosis remains to be investigated.
EP-2104R is a fibrin-specific contrast agent. Because binding may vary between different species, we used engineered clot from human blood, allowing for easier transfer of our data to a clinical setting. However, further studies are needed to demonstrate the potential of this new agent for selective visualization of in situeCdeveloped deep vein thrombosis and pulmonary embolism in patients as well. Furthermore, in vivo in humans, clots may demonstrate continuous modifications of the surface and the configuration, which also may have an impact on the binding and thus MRI. Our results warrant clinical investigations in patients with pulmonary embolism to show the clinical value of this new agent for molecular MRI of pulmonary embolism.
In the last couple of years, variable fibrin-specific contrast agents have been developed. Mostly, Gd-loaded nanoparticles or iron particles were used (20eC22). D-dimereCspecific antibodies have been used for nuclear medicine studies (31). However, these agents may be less likely to come into clinical use due to biodistribution and safety issues (32). EP-2104R is a small peptide, which binds to fibrin without antibodies, and thus such agents have a higher chance to reach clinical use without safety concerns.
In addition to molecular imaging with fibrin-specific contrast agents or contrast-enhanced MRA, so-called direct thrombus imaging has also been proposed. This technique uses the T1-shortening of clots based on the breakdown products of hemoglobin (33, 34). However, the signal properties of clots change over time, reducing the potential imaging window (35), which is limited to between 8 hours and 6 months (36).
Because fibrin is seen in acute as well as older clots (37), fibrin-targeted molecular imaging may allow for selective clot imaging over a long time. Thus, further studies investigating the potential of EP-2104R for detection of acute as well as chronic thrombi are needed.
Conclusions
Systemic low-dose application of EP-2104R allows for selective molecular MRI of acute pulmonary thromboembolism in a swine model.
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
REFERENCES
Goldhaber SZ. Pulmonary embolism. Lancet 2004;363:1295eC1305.
Fedullo PF, Tapson VF. Clinical practice: the evaluation of suspected pulmonary embolism. N Engl J Med 2003;349:1247eC1256.
Olin JW. Pulmonary embolism. Rev Cardiovasc Med 2002;3:S68eCS75.
Miniati M, Prediletto R, Formichi B, Marini C, Di Ricco G, Tonelli L, Allescia G, Pistolesi M. Accuracy of clinical assessment in the diagnosis of pulmonary embolism. Am J Respir Crit Care Med 1999;159:864eC871.
Girard P, Decousus M, Laporte S, Buchmuller A, Herve P, Lamer C, Parent F, Tardy B. Diagnosis of pulmonary embolism in patients with proximal deep vein thrombosis: specificity of symptoms and perfusion defects at baseline and during anticoagulant therapy. Am J Respir Crit Care Med 2001;164:1033eC1037.
Meaney JF, Weg JG, Chenevert TL, Stafford-Johnson D, Hamilton BH, Prince MR. Diagnosis of pulmonary embolism with magnetic resonance angiography. N Engl J Med 1997;336:1422eC1427.
Tapson VF. Pulmonary embolismeCnew diagnostic approaches. N Engl J Med 1997;336:1449eC1451.
Ghaye B, Remy J, Remy-Jardin M. Non-traumatic thoracic emergencies: CT diagnosis of acute pulmonary embolism: the first 10 years. Eur Radiol 2002;12:1886eC1905.
Prospective Investigation of Pulmonary Embolism Diagnosis Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism: results of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED). JAMA 1990;263:2753eC2759.
Schoepf UJ, Goldhaber SZ, Costello P. Spiral computed tomography for acute pulmonary embolism. Circulation 2004;109:2160eC2167.
Haage P, Piroth W, Krombach G, Karaagac S, Schaffter T, Gunther RW, Bucker A. Pulmonary embolism: comparison of angiography with spiral computed tomography, magnetic resonance angiography, and real-time magnetic resonance imaging. Am J Respir Crit Care Med 2003;167:729eC734.
Han D, Lee KS, Franquet T, Muller NL, Kim TS, Kim H, Kwon OJ, Byun HS. Thrombotic and nonthrombotic pulmonary arterial embolism: spectrum of imaging findings. Radiographics 2003;23:1521eC1539.
Lorut C, Ghossains M, Horellou MH, Achkar A, Fretault J, Laaban JP. A noninvasive diagnostic strategy including spiral computed tomography in patients with suspected pulmonary embolism. Am J Respir Crit Care Med 2000;162:1413eC1418.
Hartmann IJ, Hagen PJ, Melissant CF, Postmus PE, Prins MH. Diagnosing acute pulmonary embolism: effect of chronic obstructive pulmonary disease on the performance of D-dimer testing, ventilation/perfusion scintigraphy, spiral computed tomographic angiography, and conventional angiography. ANTELOPE Study Group (Advances in New Technologies Evaluating the Localization of Pulmonary Embolism). Am J Respir Crit Care Med 2000;162:2232eC2237.
Remy-Jardin M, Mastora I, Remy J. Pulmonary embolus imaging with multislice CT. Radiol Clin North Am 2003;41:507eC519.
Baile EM, King GG, Muller NL, D'Yachkova Y, Coche EE, Pare PD, Mayo JR. Spiral computed tomography is comparable to angiography for the diagnosis of pulmonary embolism. Am J Respir Crit Care Med 2000;161:1010eC1015.
Ghaye B, Szapiro D, Mastora I, Delannoy V, Duhamel A, Remy J, Remy-Jardin M. Peripheral pulmonary arteries: how far in the lung does multi-detector row spiral CT allow analysis Radiology 2001;219:629eC636.
Gupta A, Frazer CK, Ferguson JM, Kumar AB, Davis SJ, Fallon MJ, Morris IT, Drury PJ, Cala LA. Acute pulmonary embolism: diagnosis with MR angiography. Radiology 1999;210:353eC359.
Botnar RM, Perez AS, Witte S, Wiethoff AJ, Laredo J, Hamilton J, Quist W, Parsons EC Jr, Vaidya A, Kolodziej A, et al. In vivo molecular imaging of acute and subacute thrombosis using a fibrin-binding magnetic resonance imaging contrast agent. Circulation 2004;109:2023eC2029.
Yu X, Song SK, Chen J, Scott MJ, Fuhrhop RJ, Hall CS, Gaffney PJ, Wickline SA, Lanza GM. High-resolution MRI characterization of human thrombus using a novel fibrin-targeted paramagnetic nanoparticle contrast agent. Magn Reson Med 2000;44:867eC872.
Flacke S, Fischer S, Scott MJ, Fuhrhop RJ, Allen JS, McLean M, Winter P, Sicard GA, Gaffney PJ, Wickline SA, et al. Novel MRI contrast agent for molecular imaging of fibrin: implications for detecting vulnerable plaques. Circulation 2001;104:1280eC1285.
Choudhury RP, Fuster V, Badimon JJ, Fisher EA, Fayad ZA. MRI and characterization of atherosclerotic plaque: emerging applications and molecular imaging. Arterioscler Thromb Vasc Biol 2002;22:1065eC1074.
Botnar R, Buecker A, Wiethoff AJ, Parsons EC Jr, Katoh M, Katsimaglis G, Weisskoff RM, Lauffer RB, Graham PB, Gunther RW, et al. In vivo magnetic resonance imaging of coronary thrombosis using a fibrin-binding molecular magnetic resonance contrast agent. Circulation 2004;110:1463eC1466.
Spuentrup E, Buecker A, Katoh M, Wiethoff AJ, Parsons EC Jr, Botnar RM, Weisskoff RM, Graham PB, Manning WJ, Gunther RW. Molecular magnetic resonance imaging of coronary thrombosis and pulmonary emboli with a novel fibrin-targeted contrast agent. Circulation 2005;22:1377eC1382.
Maki JH, Chenevert TL, Prince MR. The effects of incomplete breath-holding on 3D MR image quality. J Magn Reson Imaging 1997;7:1132eC1139.
Fedullo PF, Auger WR, Kerr KM, Rubin LJ. Chronic thrombembolic pulmonary hypertension. N Engl J Med 2001;345:1465eC1472.
Evans AJ, Sostman HD, Knelson MH, Spritzer CE, Newman GE, Paine SS, Beam CA. 1992 ARRS Executive Council Award. Detection of deep venous thrombosis: prospective comparison of MR imaging with contrast venography. AJR Am J Roentgenol 1993;161:131eC139.
Spuentrup E, Buecker A, Stuber M, Gunther RW. MR-venography using high resolution true-FISP. Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr 2001;173:686eC690.
Moody AR, Pollock JG, O'Connor AR, Bagnall M. Lower-limb deep venous thrombosis: direct MR imaging of the thrombus. Radiology 1998;209:349eC355.
Ruehm SG, Zimny K, Debatin JF. Direct contrast-enhanced 3D MR venography. Eur Radiol 2001;11:102eC112.
Morris TA, Marsh JJ, Chiles PG, Konopka RG, Pedersen CA, Schmidt PF, Gerometta M. Single photon emission computed tomography of pulmonary emboli and venous thrombi using anti-D-dimer. Am J Respir Crit Care Med. 2004;169:987eC93.
Weinmann HJ, Ebert W, Misselwitz B, Schmitt-Willich H. Tissue-specific MR contrast agents. Eur J Radiol 2003;46:33eC44.
Moody AR, Liddicoat A, Krarup K. Magnetic resonance pulmonary angiography and direct imaging of embolus for the detection of pulmonary emboli. Invest Radiol 1997;32:431eC440.
Moody AR, Murphy RE, Morgan PS, Martel AL, Delay GS, Allder S, MacSweeney ST, Tennant WG, Gladman J, Lowe J, et al. Characterization of complicated carotid plaque with magnetic resonance direct thrombus imaging in patients with cerebral ischemia. Circulation 2003;107:3047eC3052.
Corti R, Osende JI, Fayad ZA, Fallon JT, Fuster V, Mizsei G, Dickstein E, Drayer B, Badimon JJ. In vivo noninvasive detection and age definition of arterial thrombus by MRI. J Am Coll Cardiol 2002;39:1366eC1373.
Fraser DG, Moody AR, Morgan PS, Martel AL, Davidson I. Diagnosis of lower-limb deep venous thrombosis: a prospective blinded study of magnetic resonance direct thrombus imaging. Ann Intern Med 2002;136:89eC98.
Davies MJ. Stability and instability: two faces of coronary atherosclerosis. The Paul Dudley White Lecture 1995. Circulation 1996;94:2013eC2020.(Elmar Spuentrup, Marcus K)