Real-time MR-guided retrieval of inferior vena cava filters: an in vitro and animal model study.

PURPOSE: To develop interventional magnetic resonance (MR) guidance techniques for inferior vena cava (IVC) filter retrieval in vitro and demonstrate feasibility in vivo. MATERIALS AND METHODS: Three optional IVC filters and their retrieval systems were investigated. Experiments were performed on a 1.5-T MR system. Real-time MR imaging was optimized by using a custom-built IVC phantom. A three-dimensional (3D) contrast-enhanced MR venography sequence was optimized in vitro for improved detection of thrombus trapped within the filters. Filters were then retrieved in vitro and in vivo in a swine model under MR guidance. In-vivo retrieval procedure time was measured. RESULTS: The combination of one of the nitinol filters and a loop snare was suitable for real-time MR procedures. With a 90° flip angle, 3D MR venography allowed detection of simulated thrombus within the filter. A radial true fast imaging sequence with steady-state precession allowed visualization of the loop snare and IVC filter hook and successful retrieval of the filter in vivo and in vitro. In-vivo MR fluoroscopy time for retrieval was 97 seconds ± 51 (mean ± SD). CONCLUSIONS: MR-guided retrieval of a nitinol-based IVC filter by using a loop snare is feasible with the use of optimized sequences and passive device tracking.

Magnetic resonance imaging of carotid atherosclerotic plaque in clinically suspected acute transient ischemic attack and acute ischemic stroke.

BACKGROUND: Carotid atherosclerotic plaque rupture is thought to cause transient ischemic attack (TIA) and ischemic stroke (IS). Pathological hallmarks of these plaques have been identified through observational studies. Although generally accepted, the relationship between cerebral thromboembolism and in situ atherosclerotic plaque morphology has never been directly observed noninvasively in the acute setting. METHODS AND RESULTS: Consecutive acutely symptomatic patients referred for stroke protocol magnetic resonance imaging/angiography underwent additional T1- and T2-weighted carotid bifurcation imaging with the use of a 3-dimensional technique with blood signal suppression. Two blinded reviewers performed plaque gradings according to the American Heart Association classification system. Discharge outcomes and brain magnetic resonance imaging results were obtained. Image quality for plaque characterization was adequate in 86 of 106 patients (81%). Eight TIA/IS patients with noncarotid pathogenesis were excluded, yielding 78 study patients (38 men and 40 women with a mean age of 64.3 years, SD 14.7) with 156 paired watershed vessel/cerebral hemisphere observations. Thirty-seven patients had 40 TIA/IS events. There was a significant association between type VI plaque (demonstrating cap rupture, hemorrhage, and/or thrombosis) and ipsilateral TIA/IS (P<0.001). A multiple logistic regression model including standard Framingham risk factors and type VI plaque was constructed. Type VI plaque was the dominant outcome-associated observation achieving significance (P<0.0001; odds ratio, 11.66; 95% confidence interval, 5.31 to 25.60). CONCLUSIONS: In situ type VI carotid bifurcation region plaque identified by magnetic resonance imaging is associated with ipsilateral acute TIA/IS as an independent identifier of events, thereby supporting the dominant disease pathophysiology.

Calf muscle perfusion at peak exercise in peripheral arterial disease: measurement by first-pass contrast-enhanced magnetic resonance imaging.

PURPOSE: To develop a contrast-enhanced magnetic resonance (MR) technique to measure skeletal muscle perfusion in peripheral arterial disease (PAD). MATERIALS AND METHODS: A total of 11 patients (age = 61 +/- 11 years) with mild to moderate symptomatic PAD (ankle-brachial index [ABI] = 0.75 +/- 0.08) and 22 normals were studied using an MR-compatible ergometer. PAD and normal(max) (Nl(max); N = 11) exercised to exhaustion. Nl(low) (N = 11) exercised to the same workload achieved by PAD. At peak exercise, 0.1 mm/kg of gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) was infused at 3-4 cm(3)/second followed by a saline flush at the same rate. A dual-contrast gradient echo (GRE) sequence enabled simultaneous acquisition of muscle perfusion and arterial input function (AIF). The perfusion index (PI) was defined as the slope of the time-intensity curve (TIC) in muscle divided by the arterial TIC slope. RESULTS: Median workload was 120 Joules in PAD, 210 Joules in Nl(low), and 698 Joules in Nl(max) (P < 0.001 vs. Nl(low) and PAD). Median PI was 0.29 in PAD (25th and 75th percentiles [%] = 0.20, 0.40), 0.48 in Nl(low) (25th, 75th % = 0.36, 0.62; P < 0.02 vs. PAD), and 0.69 in Nl(max) (25th, 75th % = 0.5, 0.77; P < 0.001 vs. PAD). Area under the ROC-curve for PI differentiating patients from Nl(max) was 0.95 (95% confidence interval [CI] = 0.77-0.99). CONCLUSION: Peak-exercise measurement of lower limb perfusion with dual-contrast, first-pass MR distinguishes PAD from normals. This method may be useful in the study of novel therapies for PAD.

Reproducibility and reliability of atherosclerotic plaque volume measurements in peripheral arterial disease with cardiovascular magnetic resonance.

A high resolution, noninvasive approach to quantify atherosclerotic plaque in the peripheral vasculature could have significant clinical and research utility. Seventeen patients with peripheral arterial disease (PAD) were studied in a 1.5T CMR scanner. Atherosclerotic plaque volume in the superficial femoral artery was measured and interobserver, intraobserver, and test-retest variability determined. Nineteen vessels were studied with mean acquisition time of 13.1 minutes per vessel. Mean plaque volume was 7.27 +/- 3.73 cm3. Intra-observer intraclass correlation was R = 0.997, inter-observer was R = 0.987, and test-retest reproducibility was R = 0.996. Thus, high resolution measurement of plaque volume in PAD is reliable and reproducible.

Technology insight: in vivo cell tracking by use of MRI.

Animal studies have shown some success in the use of stem cells of diverse origins to treat heart failure and ventricular dysfunction secondary to ischemic injury. The clinical use of these cells is, therefore, promising. In order to develop effective cell therapies, the location, distribution and long-term viability of these cells must be evaluated in a noninvasive manner. MRI of cells labeled with magnetically visible contrast agents after either direct injection or local or intravenous infusion has the potential to fulfill this goal. In this Review, techniques for labeling and imaging a variety of cells will be discussed. Particular attention will be given to the advantages and limitations of various contrast agents and passive and facilitated cell-labeling methods, as well as to imaging techniques that produce negative and positive contrast, and the effect on image quantification of compartmentalization of contrast agents within the cell.

Delayed calf muscle phosphocreatine recovery after exercise identifies peripheral arterial disease.

OBJECTIVES: In this study we intend to characterize phosphocreatine (PCr) recovery kinetics with phosphorus-31 ((31)P) magnetic resonance spectroscopy in symptomatic peripheral arterial disease (PAD) patients compared with control subjects and determine the diagnostic value and reproducibility of this parameter. BACKGROUND: Due to the inconsistent relationship between flow and function in PAD, novel techniques focused on the end-organ are needed to assess disease severity and measure therapeutic response. METHODS: Fourteen normal subjects (5 men, age 45 +/- 14 years) and 20 patients with mild-to-moderate symptomatic PAD (12 men, age 67 +/- 10 years, mean ankle brachial index 0.62 +/- 0.13) were studied. Subjects exercised one leg to exhaustion while supine in a 1.5-T magnetic resonance scanner using a custom-built plantar flexion device. Surface coil-localized, free induction decay acquisition localized to the mid-calf was used. Each 31P spectrum consisted of 25 signal averages at a repetition time of 550 ms. The PCr recovery time constant was calculated by monoexponential fit of PCr versus time, beginning at exercise completion. RESULTS: Median exercise time was 195.0 s in normal subjects and 162.5 s in PAD patients (p = 0.06). Despite shorter exercise times in patients, the median recovery time constant of PCr was 34.7 s in normal subjects and 91.0 s in PAD patients. Area under the receiver-operating characteristic curve was 0.925 +/- 0.045. Test-retest reliability was excellent. CONCLUSIONS: The PCr recovery time constant is prolonged in patients with symptomatic PAD compared with normal subjects. The method is reproducible and may be useful in the identification of disease. Further study of this parameter's ability to track response to therapy as well as its prognostic capability is warranted.

Real-time magnetic resonance-guided placement of retrievable inferior vena cava filters: comparison with fluoroscopic guidance with use of in vitro and animal models.

PURPOSE: To compare the precision of magnetic resonance (MR)-guided versus fluoroscopy-guided placement of retrievable inferior vena cava (IVC) filters with use of real-time MR imaging strategies optimized for each device in an in vitro model and in an animal model. MATERIALS AND METHODS: Three different retrievable IVC filters were used in this study, including the Recovery, Günther Tulip, and OptEase devices. Experiments were performed on a 1.5 T-MR system with pre-release interactive MR software. For each device, high-resolution real-time MR imaging was optimized with use of steady-state free precession and fast low-angle shot sequences with radial and cartesian trajectories and varying flip angles (10 degrees -70 degrees ) and a frame rate of 2 per second. A custom-built IVC phantom was filled with dilute gadolinium contrast agent at a concentration of 0.05 mmol/L simulating a blood T1 of 8 msec and T2 of 6 msec. Signal intensities were measured in regions of interest at the filter, the IVC lumen, and the background. The contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) were calculated. The sequence suited best for each device was chosen for in vitro filter placement in a custom-made IVC phantom. Each device was deployed five times each under MR and fluoroscopic guidance with use of identical techniques. Accuracy was measured as absolute deviation of the filter tip in millimeters from a target landing zone. Differences were assessed statistically with use of the paired t test. Each device was also placed in vivo in a swine model under MR guidance. RESULTS: All three IVC filters could be clearly identified and positioned under fluoroscopic and MR imaging control. A cartesian true fast imaging sequence with steady-state precession with a flip angle of 30 degrees or 50 degrees resulted in optimal SNR and CNR for all three filters. The Tulip filter created more susceptibility artifacts than the other two. Filter placement accuracy was similar with MR and fluoroscopy whether comparing devices individually (P=NS) or as a group (P=NS). The mean absolute differences between MR and fluoroscopy were 0.088 mm for the OptEase filter, 0.41 mm for the Bard Recovery filter, and 0.34 mm for the Günther Tulip filter. CONCLUSIONS: MR-guided placement of retrievable IVC filters is feasible and as accurate as fluoroscopy-guided placement in an in vitro model. With optimized sequences, real-time MR has the potential to develop as a reasonable alternative to fluoroscopy.

Factors regulating macrophage endocytosis of nanoparticles: implications for targeted magnetic resonance plaque imaging.

BACKGROUND: The presence of activated macrophages (Mphi) is an early and consistent marker of the inflammatory nature of atherosclerotic disease. Dextran-coated superparamagnetic iron oxide particles (SPIO) are avidly endocytosed. These particles have a strong effect on magnetic resonance signal and have been proposed as a non-invasive probe for the presence of early non-occlusive atherosclerotic disease. We describe the extent to which endogenous and exogenous factors regulate Mphi uptake of SPIO particles. METHODS AND RESULTS: Cultured murine Mphi-like cells (J744A.1) incubated with SPIO (0, 11.2, 112.0 and 1120 microg Fe/ml) demonstrated significantly reduced SPIO uptake when pretreated with lovastatin to 61% (P < 0.001) and 43% (P = 0.02) of control at 1.0 microM and 17.5 microM lovastatin respectively. Interferon-gamma (IFN-gamma, 1000 U/ml) increased SPIO uptake to 163% of control, P < 0.05. Interleukin-4 (IL-4, 40 ng/ml) also increased uptake (178% of control, P < 0.04). In cells incubated with SPIO in the absence of serum proteins, SPIO uptake fell to 57% of control (P < 0.001). CONCLUSIONS: Uptake of SPIO by activated Mphi is regulated by endogenous cytokines and serum components as well and exogenous lovastatin. Thus, MRI signal changes after SPIO administration may reflect Mphi phagocytic capacity as well as Mphi presence.

Infarct involution and improved function during healing of acute myocardial infarction: the role of microvascular obstruction.

Delayed contrast-enhanced cardiac magnetic resonance imaging (ceCMR) delineates infarct size. The presence of hypoenhancement consistent with microvascular obstruction (MO) signifies larger infarcts with a worse prognosis. We hypothesized that the size of the contrast defect (CD) on ceCMR in acutely infarcted myocardium may change during infarct healing and depend upon the presence of MO. Twenty-five patients underwent CMR on weeks 1 and 8 after reperfused myocardial infarction. After short-axis cine CMR was performed, gadolinium was infused and ceCMR images and matched tagged cine MR images were obtained in the three most dysfunctional short-axis slices on cine CMR. The area and transmural extent of hyperenhancement (HE) with or without MO representing total CD size were planimetered. Between week 1 and week 8, the CD area fell from 1729+/-970 mm2 at week 1 to 1270+/-706 mm2 (p<0.001), as did the transmural extent of infarction (71+/-22% to 63+/-24%, p<0.001). The decline in CD trended to be higher in patients with MO (840+/-807 mm2) than in HE (312+/-485 mm2, p<0.07). In the patient group as a whole, ejection fraction (EF) improved (56+/-9% to 60+/-10%, p=0.002) between weeks 1 and 8, but patients with MO showed no increase in EF. Segments with some HE demonstrated partial functional improvement whereas no improvement was seen in HE+MO segments. In patients 8 weeks after reperfused myocardial infarction (MI), the size of infarction by ceCMR decreases compared to week 1 post-MI, especially in those with microvascular obstruction in whom there is little improvement in regional or global function.

Magnetic resonance tagging and echocardiographic response to dobutamine and functional improvement after reperfused myocardial infarction.

OBJECTIVE: Our objective was to compare the qualitative response to low-dose dobutamine by echocardiography (DSE) with the quantitative response of magnetic resonance myocardial tagging (DMRT) in the prediction and evaluation of functional improvement after reperfused myocardial infarction (MI). METHODS: Twenty-two patients with a reperfused first MI (aged 51 +/- 2 years, 20 male, 13 anterior MI) were studied. On day 3 +/- 1 after MI, patients underwent both DSE and DMRT at baseline and during infusion of 5 microg/kg/min and 10 microg/kg/min of dobutamine. The patients returned at week 8 +/- 1 for follow-up echocardiogram and MRT at rest. Two experienced observers interpreted the DSE for the presence of contractile reserve and functional improvement in dysfunctional segments. By DMRT, a 5% increase in percent intramyocardial circumferential shortening at peak response to dobutamine was defined as evidence of contractile reserve. Functional improvement by echocardiography was defined as the gold standard. RESULTS: Ejection fraction improved from 46% +/- 10% at week 1 to 51% +/- 12% at week 8 (P <.001) in the patients. Sixty-seven transmural segments with baseline dysfunction matched between imaging modalities by location were studied. For 51 (76%) of the segments, echocardiography and MR tagging were concordant in the assessment of functional improvement (kappa value 0.52). Twenty-nine segments (43%) demonstrated improvement by echocardiography, whereas 33 segments (49%) improved by MR tagging. With improvement of function by echocardiography as gold standard, the sensitivity and specificity of DMRT for prediction of functional improvement was 86% and 69%, respectively, with an overall accuracy of 76%. The sensitivity, specificity, and accuracy of DSE was 86%, 87%, and 85%, respectively. Overall accuracy was similar between techniques. CONCLUSIONS: Both DSMRT and DSE are sensitive and accurate techniques for predicting functional improvement after reperfused MI.

Reverse remodeling and improved regional function after repair of left ventricular aneurysm.

BACKGROUND: Changes in regional left ventricular mechanics after anteroapical aneurysm repair in human subjects can be studied noninvasively by means of magnetic resonance tagging. We hypothesized that left ventricular intramyocardial function would improve throughout the left ventricle after repair. METHODS: We studied 6 male patients with a left ventricular anteroapical aneurysm (mean age +/- SD, 63 +/- 5 years) using magnetic resonance tagging 3 +/- 1 weeks before and 6 +/- 1 weeks after aneurysm repair, coronary artery bypass grafting, and mitral valve repair (n = 2). Breath-hold tagged imaging spanned the left ventricle in the short axis from apex to base. Left ventricular mass, end-diastolic and end-systolic volume, and ejection fraction were measured. Two-dimensional strain analysis was applied; averaged for the apical, middle, and basal left ventricle and the whole left ventricle; and expressed as greatest lengthening (similar to wall thickening), greatest shortening, and angular deviation of the lengthening strain from the radial direction. RESULTS: After aneurysm repair, left ventricular mass decreased from 373 +/- 27 to 333 +/- 25 g (P <.05), end-diastolic volume from 212 +/- 22 to 168 +/- 18 mL (P <.005), and end-systolic volume from 188 +/- 26 to 113 +/- 18 mL (P <.005); ejection fraction improved from 13% +/- 4% to 23% +/- 4% (P <.005). For the whole left ventricle, lengthening strain increased from before to after the operation (8% +/- 1% to 10% +/- 1%, P <.01). Most of the improved lengthening occurred at the middle left ventricle (8% +/- 1% to 11% +/- 1%, P <.01), in the base (8% +/- 1% to 10% +/- 1%, P <.05), and in the inferior wall (9% +/- 1% to 12% +/- 1%, P <.05). Lengthening tended to become more radially oriented, decreasing from 31 degrees +/- 3 degrees to 27 degrees +/- 3 degrees (P =.10). Shortening strain did not change (10% +/- 1% to 11% +/- 1%, P = not significant). CONCLUSIONS: Left ventricular aneurysm repair is associated with reverse remodeling and an improvement in the extent and orientation of intramyocardial function, especially at the middle and basal left ventricle and inferior wall.

Fully automated registration and warping of contrast-enhanced first-pass perfusion images.

Respiratory motion during acquisition of first-pass myocardial perfusion images results in translation, distortion from out-of-plane motion, and changes in left ventricular geometry. Together these effects make visual image analysis more difficult and limit methods of quantitative analysis of contrast kinetics. We present a fully automated registration and warping algorithm for correcting translation and geometric distortions using a statistically based image registration method. Twelve patients (mean age 51 +/- 12 years) were studied 3 +/- 1 days after reperfused first myocardial infarction. Perfusion images were acquired during bolus administration of nonionic Gd-DTPA. Pixel intensity statistics were computed for each image in the neighborhood of high spatial frequencies. These statistics were then used to register and warp each target image (image to be registered and warped) to a common template image. Average image-to-image vertical translation was 2.6 +/- 0.8 pixels (3.4 +/- 1.0 mm) prior to processing and 0.9 +/- 0.3 pixels (1.2 +/- 0.4 mm) post-processing (P < 0.0001). Mean image-to-image horizontal translation was 1.7 +/- 1.2 pixels (1.8 +/- 1.2 mm) before and 1.3 +/- 0.7 pixels (1.4 +/- 0.7 mm) after processing (P = 0.05). Left ventricular endocardial area varied an average of 105 +/- 55 pixels (140.7 +/- 53.7 mm2) between images prior to processing vs. 51 +/- 15 pixels (68.3 +/- 20.1 mm2) after processing (P < 0.001). Thus automated, statistically based registration and warping of perfusion images is effective in reducing image-to-image translation. This method may permit more sensitive qualitative and quantitative evaluation of myocardial contrast-enhanced first-pass images.