Balloon sizing and transcatheter closure of acute atrial septal defects guided by magnetic resonance fluoroscopy: assessment and validation in a large animal model

PURPOSE: To quantitatively assess atrial septal defects (ASDs) with small shunts using MRI followed by transcatheter closure monitored by MR fluoroscopy. MATERIALS AND METHODS: Acute ASDs were created in 14 pigs under x-ray fluoroscopy. Six animals were studied in order to select MR-compatible delivery systems and imaging strategies. ASDs in eight animals were examined with balloon sizing under MR fluoroscopy, flow measurements, and contrast media injections, after which transcatheter closure was performed under MR fluoroscopy. The delivery system was assembled from commercially available materials. RESULTS: The ratio of pulmonary to systemic flow (Qp/Qs) was reduced from 1.23 +/- 0.15 before ASD closure to 1.07 +/- 0.11 after ASD closure (P < 0.001). In two out of eight animals Qp/Qs was close to 1.0 before closure despite the presence of defects >15 mm. The ASDs were measurable with MR balloon sizing in all of the animals. Balloon sizing was identical with MR (16.9 +/- 2.3 mm) and x-ray fluoroscopy (17.1 +/- 1.3 mm). The in-house-assembled delivery system allowed successful placement of closure devices under MR guidance. CONCLUSION: Assessment and closure of small shunts with MR fluoroscopy is feasible. A barrier to the rapid implementation of transcatheter closure in patients is uncertainty about the MR safety of guidewires and device delivery systems.     

Hepatocellular carcinoma: regional therapy with a magnetic targeted carrier bound to doxorubicin in a dual MR imaging/ conventional angiography suite--initial experience with four patients

Four patients with inoperable hepatocellular carcinoma were treated with a magnetic targeted carrier bound to doxorubicin (MTC-DOX) by using a joint magnetic resonance (MR) imaging/conventional angiography system consisting of a 1.5-T short-bore magnet connected to a C-arm angiography unit by a sliding tabletop. Selective transcatheter delivery of the MTC-DOX to the hepatic artery was monitored by using intraprocedural MR imaging, and interim catheter manipulation was performed with fluoroscopic guidance to optimize agent delivery to the tumor and minimize delivery to normal tissue. The final fraction of treated tumor volume ranged from 0.64 to 0.91. The fraction of affected normal liver volume ranged from 0.07 to 0.30. The dual MR imaging/conventional angiography system shows promise for directing magnetically targeted tumor therapies.     

Truly Hybrid Interventional MR/X-Ray System: Investigation of in Vivo Applications

RATIONALE AND OBJECTIVES: The purpose of this study was to provide in vivo demonstrations of the functionality of a truly hybrid interventional x-ray/magnetic resonance (MR) system. MATERIALS AND METHODS: A digital flat-panel x-ray system (1,024(2) array of 200 microm pixels, 30 frames per second) was integrated into an interventional 0.5-T magnet. The hybrid system is capable of MR and x-ray imaging of the same field of view without patient movement. Two intravascular procedures were performed in a 22-kg porcine model: placement of a transjugular intrahepatic portosystemic shunt (TIPS) (x-ray-guided catheterization of the hepatic vein, MR fluoroscopy-guided portal puncture, and x-ray-guided stent placement) and mock chemoembolization (x-ray-guided subselective catheterization of a renal artery branch and MR evaluation of perfused volume). RESULTS: The resolution and frame rate of the x-ray fluoroscopy images were sufficient to visualize and place devices, including nitinol guidewires (0.016-0.035-inch diameter) and stents and a 2.3-F catheter. Fifth-order branches of the renal artery could be seen. The quality of both real-time (3.5 frames per second) and standard MR images was not affected by the x-ray system. During MR-guided TIPS placement, the trocar and the portal vein could be easily visualized, allowing successful puncture from hepatic to portal vein. CONCLUSION: Switching back and forth between x-ray and MR imaging modalities without requiring movement of the patient was demonstrated. The integrated nature of the system could be especially beneficial when x-ray and MR image guidance are used iteratively.     

MR fluoroscopy-guided transthoracic fine-needle aspiration biopsy: feasibility

The purpose of this study was to evaluate the feasibility of using an open-configuration magnetic resonance (MR) imaging system with MR fluoroscopic guidance to perform percutaneous transthoracic fine-needle aspiration biopsy in patients with lung masses. Percutaneous transthoracic aspiration biopsies were performed with MR fluoroscopic guidance in 14 patients. The masses were 2-7 cm in diameter (mean, 4.1 cm). The needle was positioned by using a free-hand technique with MR fluoroscopic guidance. The needle tip reached the target lesion, and biopsy was performed. Analysis of the biopsy specimens facilitated a specific diagnosis in all patients. Pneumothorax was noted in two patients (14%) with chronic obstructive pulmonary disease. Study results showed that the described MR fluoroscopy-guided transthoracic biopsy technique can be used safely and successfully for lung masses. MR fluoroscopy can be used to reach the target lesion easily and accurately.     

Real-time MR Imaging Guidance for Percutaneous Core Biopsy of US- and CT-negative Lesion

Biopsies traditionally are performed under ultrasound (US), computed tomography (CT), or fluoroscopic guidance. In situations in which lesions are difficult to visualize with US or CT guidance, magnetic resonance (MR) imaging often can provide better imaging results. The authors describe a case in which a recurrent calf mass not well visualized under fluoroscopy, CT, or US was identified on MR imaging. In the absence of real-time needle visualization, percutaneous interventions under MR guidance have been limited by prohibitively long imaging times. A novel guidance system providing real-time MR guidance of needle position was used to procure a core biopsy specimen of the lesion.     

Female pelvic organ prolapse: a comparison of triphasic dynamic MR imaging and triphasic fluoroscopic cystocolpoproctography

OBJECTIVE: This study compared dynamic MR imaging with fluoroscopic cystocolpoproctography for the detection and measurement of prolapse of pelvic organs. SUBJECTS AND METHODS: Ten patients underwent triphasic dynamic MR imaging and triphasic fluoroscopic cystocolpoproctography with identical amounts of contrast material to opacify the bladder, vagina, and rectum. The dynamic MR imaging procedure included cine-loop presentation. Each examination was analyzed to determine the presence and extent of prolapse of pelvic organs based on specific measurements. RESULTS: Both dynamic MR imaging and fluoroscopic cystocolpoproctography revealed 10 rectoceles (mean extents, 2.85 and 2.45 cm, respectively). Nine cystoceles were revealed by both dynamic MR imaging (mean extent, 4.05 cm) and fluoroscopy (mean extent, 4.55 cm). Seven enteroceles were revealed, one of which was initially not seen on dynamic MR imaging. Two sigmoidoceles were revealed, one of which was not seen on fluoroscopy. The mean extent of the enteroceles and sigmoidoceles on dynamic MR imaging was 3.50 cm, and the mean extent on fluoroscopy was 4.25 cm. Nine of the 10 patients were able to defecate in the supine position on the MR imaging table. Patients were divided equally in their preference for dynamic MR imaging or fluoroscopic cystocolpoproctography. CONCLUSION: Triphasic dynamic MR imaging and triphasic fluoroscopic cystocolpoproctograpy show similar detection rates for prolapse of pelvic organs. Although dynamic MR imaging underestimates the extent of cystoceles and enteroceles, it has the advantage of revealing all pelvic organs and the pelvic floor musculature in a multiplanar cine-loop presentation.     

Assessing cervical spine stability in obtunded blunt trauma patients: review of medical literature

PURPOSE: To review the current medical literature on dynamic fluoroscopic and magnetic resonance (MR) imaging assessment of cervical spine stability in obtunded patients who sustained blunt trauma. MATERIALS AND METHODS: The English-language literature within the Swetswise and Medline databases was searched for articles describing dynamic fluoroscopic or MR imaging assessment of cervical spine stability in patients who sustained blunt trauma. Patients with fractures or radiographic signs of injury were excluded. The frequencies of purely ligamentous injuries, injuries requiring immobilization, and other clinically important nonligamentous abnormalities were determined. RESULTS: The frequency of isolated cervical ligamentous injuries diagnosed with dynamic fluoroscopy, as reported in the literature, was 0.9% (11 of 1166 patients), whereas the reported frequency of these injuries diagnosed with MR imaging was 22.7% (125 of 550 patients). All injuries diagnosed with dynamic fluoroscopy and 101 (80.8%) of those diagnosed with MR imaging required continued cervical immobilization. Six (60%) of 10 injuries diagnosed with dynamic fluoroscopy and seven (5.6%) of 125 injuries diagnosed with MR imaging required surgical or halo stabilization. Five (2.5%) of the 200 obtunded patients assessed with MR imaging and six (0.5%) of the 1166 obtunded patients evaluated with dynamic fluoroscopy required surgery. CONCLUSION: Review of the current medical literature provided no clear evidence of the superiority of either MR imaging or dynamic fluoroscopy in the diagnosis of unstable ligamentous injury, although other relative advantages of MR imaging indicate that it is preferred for assessing cervical spine stability in obtunded blunt trauma patients.     

Gadomer-enhanced MR imaging in the detection of microvascular obstruction: alleviation with nicorandil therapy

PURPOSE: To evaluate Gadomer-enhanced magnetic resonance (MR) imaging in the quantification of small microvascular obstruction regions and determine if nicroandil alleviates the formation of microvascular obstruction. MATERIALS AND METHODS: Approval of the institutional committee on animal research was obtained, and this study complied with guidelines for care and use of animals. Rats underwent coronary artery occlusion and reperfusion. After 24 hours, Gadomer-enhanced T1-weighted spin-echo MR imaging was used to define microvascular obstruction in animals in control and nicorandil groups. Sequential MR images obtained at two midventricular levels were acquired to measure microvascular obstruction and ischemically injured regions and monitor diffusive and/or convective transport of Gadomer in microvascular obstruction regions. Two investigators working in consensus and using threshold signal intensity measured differentially enhanced regions. Left-ventricular (LV) end-systolic and end-diastolic MR images obtained at the same two midventricular levels were used to measure regional wall thickening and systolic reduction in LV relative volumes. Agreement and correlation between MR imaging and postmortem data were determined with Bland-Altman and linear regression analyses. Animals were sacrificed 3 minutes after intravenous injection of blue dye. RESULTS: On Gadomer-enhanced MR images, two differentially enhanced regions were observed in ischemically injured myocardium, namely, the hypoenhanced region and the surrounding hyperenhanced region. Hypoenhanced regions at MR imaging and unstained regions at blue dye administration were identical 3 minutes after administration (17% +/- 1 and 17% +/- 2; P = .6; r = 0.98). In the control group, Gadomer provided a prolonged imaging window (eg, 6 minutes) for accurately quantifying small microvascular obstruction regions. Microvascular obstruction was observed in all animals in the control group and 27% of animals in the nicorandil group. Microvascular obstruction regions were smaller in the nicorandil group (eg, 3% +/- 1) than in the control group (eg, 17% +/- 2) (P < .001). Hyper- and hypoenhanced regions were also smaller (eg, 20% +/- 2) in rats in the nicorandil group than in those in the control group (37% +/- 4, P < .001). Improvement in LV function in the nicorandil group is likely related to alleviation and reduction in infarct size. CONCLUSION: Gadomer-enhanced MR imaging can be used to quantify small microvascular obstruction regions 24 hours after reperfusion. Intravenous therapy with nicorandil reduces formation of microvascular obstruction regions.     

A vascular stent as an active component for locally enhanced magnetic resonance imaging: initial in vivo imaging results after catheter-guided placement in rabbits

RATIONALE AND OBJECTIVE: A vascular stent constructed as a high frequency resonator improves the local signal-to-noise ratio at magnetic resonance (MR) imaging. After catheter placement and intravascular expansion, the stent can be used as an inductively coupled coil for MRI. The imaging properties of this balloon-expandable active MRI stent (AMRIS) were evaluated after x-ray fluoroscopy guided placement in the abdominal aorta of five rabbits using MR angiography (MRA) and flow measurements. METHODS: The AMRIS was implanted in the abdominal aorta of five rabbits using a balloon catheter inserted through the common carotid artery. The rabbits were examined by MRA (3D fast low-angle shot) at 1.5 tesla before and after intravenous injection of an iron-oxide-based blood pool contrast medium (dose 50 micro mol Fe/kg) and flow measurements (ECG-triggered phase contrast cine gradient-echo sequence). Signal-to-noise ratios (SNR) were calculated and flow volume curves were generated. The in-stent increase in temperature was measured in vitro using a fiberoptic thermometry system. RESULTS: The SNR was 5.0 +/- 0.6 outside the stent and 23.2 +/- 14.1 within the stent ( < 0.0 5) in plain MRA, 19.5 +/- 5.0 outside and 30.7 +/- 8.2 within the stent ( < 0.05) in contrast enhanced MRA, and 5.8 +/- 1.6 and 13.9 +/- 5.9, respectively ( < 0.05) in the magnitude images of the flow measurements. Flow volume curves within and distal to the stent were comparable. CONCLUSIONS: The expandable active MRI stent produces local signal enhancement in MRA and MR flow measurements after catheter placement and thus may improve assessment of the stented vessel segment by MR imaging.     

MR-guided percutaneous intramyocardial injection with an MR-compatible catheter: feasibility and changes in T1 values after injection of extracellular contrast medium in pigs

PURPOSE: To assess the feasibility of percutaneous magnetic resonance (MR)-guided intramyocardial injection of gadodiamide by using real-time imaging and to quantify T1 values and the size of the enhanced region for different concentrations of contrast agent for 30 minutes after injection. MATERIALS AND METHODS: Animal care committee approval was obtained. A catheter with a needle tip was advanced into the left ventricle in seven pigs by using real-time imaging with radial steady-state free precession. After intramyocardial injection of 2 mL of solution at concentrations of 0.05 or 0.10 mmol/mL gadodiamide, local changes in T1 values and size of the contrast material-enhanced region were sequentially measured at 3, 15, and 30 minutes after injection by using the Look-Locker sequence. Two-tailed paired Student t tests were used for statistical analysis. RESULTS: Catheter guidance and visualization of contrast agent distribution were feasible in all animals. Regional changes in T1 values were significantly different for different contrast agent concentrations (for 0.05 mmol/mL, 456 msec +/- 5 [+/- standard error of the mean]; for 0.10 mmol/mL, 228 msec +/- 4; P < .001) measured 3 minutes after injection. T1 values increased significantly (P < .05) to 720 msec +/- 7 for 0.05 mmol/mL gadodiamide and 445 msec +/- 6 for 0.10 mmol/mL gadodiamide 30 minutes after injection but remained significantly lower than those of remote myocardium (879 msec +/- 8). The size of the contrast-enhanced region increased from 13 mm(2) +/- 2 at 3 minutes to 30 mm(2) +/- 3 at 30 minutes (P < .05). CONCLUSION: Catheter MR-guided percutaneous intramyocardial injection is feasible; after intramyocardial injection of gadodiamide at concentrations of 0.05 and 0.10 mmol/mL, T1 values decreased over the observation time.     

Gadodiamide administration causes spurious hypocalcemia

PURPOSE: To evaluate the prevalence of spurious hypocalcemia after gadodiamide-enhanced magnetic resonance (MR) imaging. MATERIALS AND METHODS: Eight hundred ninety-six inpatients with available serum calcium data obtained before and after gadodiamide-enhanced MR imaging were identified. Changes in serum calcium measurements following gadodiamide administration in 1,049 MR imaging examinations performed in these patients were correlated with gadodiamide dose, renal function, and time between gadodiamide administration and phlebotomy. RESULTS: Following 42 gadodiamide-enhanced examinations, serum calcium measurements spuriously decreased by more than 2 mg/dL (0.5 mmol/L), resulting in laboratory reports of "critical" hypocalcemia (ie, calcium level < 6 mg/dL [1.5 mmol/L]) in 25 examinations. These reduced calcium measurements were correlated with serum creatinine level (r = 0.39, P <.001), gadodiamide dose (r = 0.37, P <.001), and time between gadodiamide injection and phlebotomy (r = -0.28, P <.001). Spurious reductions in calcium measurements after administration of 0.1 mmol of gadodiamide per kilogram of body weight were greater in patients with renal insufficiency (0.6 mg/dL [0.15 mmol/L] +/- 0.5 [0.125, SD]) than in those with normal renal function (0.14 mg/dL [0.035 mmol/L] +/- 0.4 [0.1]) (P <.001). After administration of more than 0.2 mmol/kg of gadodiamide, spurious calcium measurement decreases were greater in patients with renal insufficiency (2.4 mg/dL [0.6 mmol/L] +/- 3.6 [0.9]) than in those with normal renal function (0.4 mg/dL [0.1 mmol/L] +/- 0.7 [0.175]) (P <.001). Patients with renal insufficiency had spuriously low calcium measurements up to 4(1/2) days after gadodiamide administration. Seven patients were inappropriately treated with intravenous calcium and eleven with oral calcium in response to false-positive laboratory reports of critical hypocalcemia. No patient had characteristic symptoms of hypocalcemia or injuries attributed to the inappropriate medical treatment. CONCLUSION: Gadodiamide administration causes spurious hypocalcemia, particularly at doses of 0.2 mmol/kg or higher and in patients with renal insufficiency.     

MR versus fluoroscopic guidance of a catheter/guidewire system: In vitro comparison of steerability

Our purpose in this study was to evaluate the steerability of a combined catheter guide-wire system with MR tracking in an open-configuration .5T MR system and compare it with fluoroscopic guidance. Experiments were performed with an aorta-shaped glass phantom with different-size branches connected to a roller flow pump to simulate pulsatile flow. A .035″ guidewire was used in conjunction with a 5F Cobra 2-shaped catheter. For active MR guidance, a small RF coil was incorporated into the tips of all devices. In addition to fluoroscopic guidance, we used MR tracking devices to selectively catheterize all branches in the .5T open magnet (Signa SP, GEMS). Time requirements for the latter were compared with those needed under conventional angiographic conditions using standard catheters and guidewires. Active MR tracking permitted the simultaneous real time (4 updates/sec) guidance of both guide-wire and catheter. Under MR guidance, all branch vessels were successfully catheterized. We found no significant differences in time requirements between guidance with the MR tracking device and fluoroscopic guidance. However, cannulation under fluoroscopic guidance with standard angiography devices required significantly less time (P < .05). Selective catheterization of small branch vessels is possible with active MR tracking of a combined cath-eter/guidewire system. Limitations of MR tracking device material and design result in a considerable lengthening of the cannulation procedures.     

Comparison of Gd [DTPA-bis (2-aminoethoxy) ethane] polymeric contrast agent with gadodiamide injection for interstitial MR lymphography: experimental study with rabbits

PURPOSE: To compare the differences between macromolecular and small molecular contrast agents in interstitial magnetic resonance (MR) lymphography. MATERIALS AND METHODS: In two groups of rabbits (N = 12 each), 0.5-mL volumes of 0.5 M and 0.1 M gadodiamide, respectively, were injected subcutaneously into the dorsal feet of both hind legs. After a 24-hour interval, 0.5 mL of Gd [DTPA-bis (2-aminoethoxy) ethane] polymeric contrast agent (Gd-poly-DTPA-EOEA) injection (0.1 M) was injected into the same sites in each rabbit. T1-weighted three-dimensional gradient-echo and maximum intensity projection (MIP) images were obtained before and after the administration of each agent. The nodal maximum short-axis diameter (MSAD) and signal-to-noise ratio (SNR) at each time point were measured. Postmortem measurements of nodal MSADs were also performed. The nodal MSAD measurements obtained by MR lymphography were compared with the postmortem measurements. As an additional control experiment, six rabbits received a subcutaneous injection of 0.5 mL of 0.1 M gadodiamide at the same sites as those described above. This procedure was repeated after a 24-hour interval to determine whether the first injected gadodiamide had any effect on the performance of the subsequently injected agent, by comparing the differences in nodal MSADs obtained by MR lymphography after each injection. RESULTS: Gd-poly-DTPA-EOEA showed a significantly longer enhancement plateau compared to gadodiamide. In comparison with postmortem measurements, nodal measurements by MR lymphography with Gd-poly-DTPA-EOEA at the peak-enhancement phase revealed no significantly different MSADs (P > 0.05), whereas nodal measurements by MR lymphography with gadodiamide showed significantly smaller MSADs (P < 0.05). A comparison of the nodal MSADs measured in two gadodiamide-enhanced MR examinations after the 24-hour interval showed no significant difference (P > 0.05). CONCLUSION: Interstitial MR lymphography with polymeric Gd-poly-DTPA-EOEA enhancer enables more sufficient and reliable visualization of lymph nodes compared to small molecular gadodiamide.     

Truly hybrid X-ray/MR imaging: toward a streamlined clinical system

RATIONALE AND OBJECTIVE: We have installed an improved X-ray/MR (XMR) truly hybrid system with higher imaging signal-to-noise ratio (SNR) and versatility than our first prototype. In our XMR design, a fixed anode X-ray fluoroscopy system is positioned between the two donut-shaped magnetic poles of a 0.5T GE Signa-SP magnet (SP-XMR). This paper describes the methods for increased compatibility between the upgraded x-ray and MR systems that have helped improve patient management. MATERIALS AND METHODS: A GE OEC 9800 system (GE OEC Salt Lake City, UT) was specially reconfigured for permitting X-ray fluoroscopy inside the interventional magnet. A higher power X-ray tube, a new permanent tube mounting system, automatic exposure control (AEC), remote controlled collimators, choice of multiple frame rates, DICOM image compatibility, magnetically shimmed X-ray detector, X-ray compatible MR coil, and better RF shielding are the highlights of the new system. A total of 23 clinical procedures have been conducted with SP-XMR guidance of which five were performed using the new system. RESULTS: The 70% increased power for fluoroscopy, and a new 6 times higher power single frame imaging mode, has improved imaging capability. The choice of multiple imaging frame rates, AEC, and collimator control allow reduction in X-ray exposure to the patient. The DICOM formatting has permitted easy transfer of clinical images over the hospital PACS network. The increased MR compatibility of the detector and the X-ray transparent MR coil has enabled faster switching between X-ray and MR imaging modes. CONCLUSION: The improvements introduced in our SP-XMR system have further streamlined X-ray/MR hybrid imaging. Additional clinical procedures could benefit from the new SP-XMR imaging.     

Endovascular treatment of experimental canine aneurysms: feasibility with MR imaging guidance

PURPOSE: To evaluate the feasibility of using magnetic resonance (MR) imaging to guide and monitor endovascular therapeutic procedures. MATERIALS AND METHODS: Endovascular therapeutic procedures were performed with MR imaging guidance in eight dogs by using a 1.5-T MR unit with echo-planar imaging capabilities. Carotid arterial aneurysms were surgically created in four dogs. The ability to depict, track, and position catheters, guide wires, and Guglielmi detachable coils was assessed. Catheters were first positioned with fluoroscopic guidance. Tracking and depiction were achieved with MR imaging by using commercially available catheters filled with a gadopentetate dimeglumine solution and a fast, two-dimensional, time-resolved, variable-rate k-space sampling technique. RESULTS: When either a catheter or the coaxial space between a catheter and a guide wire was filled with a solution of gadopentetate dimeglumine, catheter movement was always depicted. In the animals with aneurysms, it was possible to depict movement of a catheter into and out of the aneurysm. This was achieved by superimposing reconstructed images obtained during catheter movement onto a previously acquired MR angiogram ("road map"). Prototype Guglielmi detachable coils were successfully positioned and detached. Aneurysm obliteration was monitored with the acquisition of new road map images. CONCLUSION: The results demonstrate the feasibility of using MR imaging to guide endovascular therapeutic procedures.     

Delivery and assessment of endovascular stents to repair aortic coarctation using MR and X-ray imaging

PURPOSE: To investigate the utility of MR and X-ray imaging for characterizing aortic coarctation and flow, and guiding the endovascular catheter to place a stent to repair the coarctation. MATERIALS AND METHODS: The descending aorta in eight dogs was looped with elastic band and tightened distal to the subclavian artery. Balanced fast field echo (bFFE) and velocity-encoded cine (VEC) MRI sequences were used for device tracking and measuring aortic flow. A T1-weighted fast-field echo sequence (T1-FFE) was used to visualize the coarctation and roadmap the aorta. Nitinol stents were guided by a nitinol guidewire and placed under MR guidance. RESULTS: Aortic coarctation was visible on MR and X-ray imaging. The procedure success rate was 88%. VEC MRI measured the changes in aortic flow (baseline = 1.3 +/- 0.2, coarctation = 0.2 +/- 0.02, and stent placement = 0.8 +/- 0.1 liters/minute). A significant reduction in iliac blood pressure was measured after coarctation, but it was reversed by stent placement. The stent lumen was visible on X-ray fluoroscopy, but not on MRI. CONCLUSION: Stent deployment to repair aortic coarctation is feasible under MR guidance. The combined use of MR and X-ray imaging is effective for anatomic and functional evaluation of aortic coarctation dilation, which may be crucial for optimal therapy.     

Gadolinium-enhanced T1-weighted renal and abdominal MR imaging: quantitative discrepancy between clinical and in vitro findings

RATIONALE AND OBJECTIVES: This study was conducted to compare the magnetic resonance (MR) contrast medium enhancement of abdominal organs in vivo with the signal intensity (SI) values of known in vitro gadolinium solutions. MATERIALS AND METHODS: A phantom was imaged with the MR contrast medium gadodiamide (Omniscan; Nycomed, Princeton, NJ) of solutions at full-strength (0.5 mmol/mL), one-third, 1/10, and 1/100 concentrations. A fat-suppressed fast spoiled gradient-echo pulse sequence with flip angles ranging from 10 degrees to 170 degrees (at 20 degrees increments) was performed with a 1.5-T magnet. In 12 subjects, the SIs of abdominal organs were determined with identical imaging parameters, before and after administration of gadodiamide injection at 0.1 mmol/kg. RESULTS: As anticipated, the plot of SI in relation to gadodiamide concentration is nonlinear, with a decrease in SI due to T2 effects at concentrations above 0.05 mmol/mL. The kidney showed the highest SI after gadodiamide enhancement (125.2 +/- 11.6 [standard error] at 2.5 minutes), followed by the liver (76.5 +/- 11.5 at 1 minute) and spleen (57.26 +/- 9.35 at 30 seconds). The SI of the renal medulla (114.2 +/- 9.8 at 4.5 minutes) was approximately one-third that in phantom observations. CONCLUSION: The authors observed a marked discrepancy between empirical contrast medium performance in abdominal organs and SI values for comparable gadodiamide concentrations in vitro. One possible reason is the intracellular compartmentalization of water molecules in vivo. These results suggest a need for a better understanding of MR contrast medium performance in vivo.     

MR guidance of targeted injections into border and core of scarred myocardium in pigs

PURPOSE: To use (a) dysprosium-based contrast agent (sprodiamide) to confirm the site of myocardial injection and (b) T1-enhancing magnetic resonance (MR) contrast media to mark the myocardial target and T2*-enhancing contrast media to demonstrate injection sites in the margins or core of infarction on delayed contrast-enhanced images. MATERIALS AND METHODS: Approval of the institutional committee on animal research was obtained. A phantom and six pigs subjected to chronic infarction (8 weeks) underwent MR-guided experiments. At inversion-recovery gradient-echo imaging, gadoterate meglumine (0.1 mmol/kg) was intravenously administered to delineate scar tissue. A catheter fitted with multiple receiver coils was used to visualize catheter navigation and injection sites. A steady-state free precession (balanced fast field-echo) sequence was used for MR fluoroscopy. A high-resolution multiphase balanced gradient-echo cine MR sequence was used after intramyocardial deposition of sprodiamide. The border and core of scarred myocardium were characterized histopathologically. The 95% confidence interval (CI) was used to demonstrate the range, extent of hyperenhanced and hypoenhanced regions after contrast media administration. RESULTS: In the phantom and in vivo, the actively guided catheter produced a high signal intensity at the terminal portion of the shaft and tip. Scarred myocardium was recognized as a bright region on gadoterate meglumine-enhanced images. Intramyocardial injection of sprodiamide caused local and persistent signal intensity loss, and the extent was volume dependent on balanced fast field-echo and T2-weighted turbo spin-echo images. At 5 minutes after administration of 0.2, 0.4, and 0.6 mL of sprodiamide, the 95% CIs of the extents of the hypoenhanced regions were 0.08%, 0.23%; 0.27%, 0.51%; and 0.46%, 0.70%, respectively, of left ventricular (LV) surface area (P < .05, paired t test). Failure of intramyocardial injection was confirmed by a brief signal loss of LV chamber blood. CONCLUSION: Sprodiamide allows visualization of injection sites within enhanced infarction. A catheter with integrated receiver coils aided in effective catheter guidance and precise intramyocardial injection.     

Endovascular stents in pulmonary valve and artery in swine: feasibility study of MR imaging-guided deployment and postinterventional assessment

PURPOSE: To assess the feasibility of using magnetic resonance (MR) imaging to guide stent deployment in the pulmonary valve and artery and evaluate, after stent deployment, the position and morphology of and blood flow through the stent. MATERIALS AND METHODS: Angiography and 1.5-T MR imaging were performed in a dual-imaging suite. Nitinol stents were placed in the pulmonary valve and main pulmonary artery in five pigs by using MR imaging guidance. For interactive MR imaging monitoring of catheter manipulation and stent delivery, balanced fast field-echo and T1-weighted turbo field-echo sequences were used. Visualization of the delivery system was based on T2* (with air as the contrast material) or T1 (with gadodiamide as the contrast material). After stent deployment, the position and morphology of and flow through the stent were verified with multiphase multisection balanced fast field-echo and velocity-encoded cine MR imaging. Findings at angiography and postmortem examination also helped verify stent placement. The paired Student t test was used for data analysis. RESULTS: The stent was successfully deployed in all animals. The stent was placed distal to the pulmonary valve in four animals and across the pulmonary valve in one animal. The position and morphology of the stent were clearly depicted on balanced fast field-echo images. In the animal with the stent placed across the pulmonary valve, the pulmonary regurgitant fraction was 37%; this was not seen in the animals with stents placed distal to the pulmonary valve. No complication (eg, stent migration, intramural injury, or vascular perforation) was noted during the intervention. Findings at angiography and postmortem examination confirmed the position of the stents. CONCLUSION: MR imaging has the potential to guide stent placement in the pulmonary valve or artery and to evaluate flow volume within the stent lumen after the intervention.     

Image-guided and -monitored renal artery stenting using only MRI

PURPOSE: To demonstrate the ability of a unique interventional MR system to be used safely and effectively as the only imaging modality for all phases of MR-guided stent-supported angioplasty. MATERIALS AND METHODS: An experimental disease model of renal stenosis was created in six pigs. An interventional MR system, which employed previously reported tools for real-time catheter tracking with automated scan-plane positioning, adaptive image parameters, and radial true-FISP imaging with steady-state precession (True-FISP) imaging coupled with a high-speed reconstruction technique, was then used to guide all phases of the intervention, including: guidewire and catheter insertion, stent deployment, and confirmation of therapeutic success. Pre- and postprocedural X-ray imaging was used as a gold standard to validate the experimental results. RESULTS: All of the stent-supported angioplasty interventions were a technical success and were performed without complications. The average postoperative residual stenosis was 14.9%. The image guidance enabled the stents to be deployed with an accuracy of 0.98 +/- 0.69 mm. Additionally, using this interventional MRI system to guide renal artery stenting significantly reduces the procedure time, as compared to using X-ray fluoroscopy. CONCLUSION: This study has clearly demonstrated the first successful treatment of renal artery stenting in an experimental animal model solely under MRI guidance and monitoring.