The Relationship between MRI Radiofrequency Energy and Function of Nonconditional Implanted Cardiac Devices: A Prospective Evaluation

Background The risks associated with MRI in individuals who have implanted cardiac devices are thought to arise from the interaction between the implanted device and static, gradient, and radiofrequency magnetic fields. Purpose To determine the relationship between the peak whole-body averaged specific absorption rate (SAR) and change in magnetic field per unit time (dB/dt), maximum specific energy dose, imaging region, and implanted cardiac device characteristics and their function in patients undergoing MRI. Materials and Methods This prospective observational cohort study was conducted from October 16, 2003, to January 22, 2015 (https://ClinicalTrials.gov, NCT01130896). Any individual with an implanted cardiac device who was referred for MRI was included. Clinical MRI protocols without SAR restriction were used. Exclusion criteria were newly implanted leads, abandoned or epicardial leads, and dependence on a pacemaker with an implantable cardioverter defibrillator without asynchronous pacing capability. For each MRI pulse sequence, the calculated whole-body values for SAR, dB/dt, and scan duration were collected. Atrial and ventricular sensing, lead impedance, and capture threshold were evaluated before and immediately after (within 10 minutes) completion of each MRI examination. Generalized estimating equations with Gaussian family, identity link, and an exchangeable working correlation matrix were used for statistical analysis. Results A total of 2028 MRI examinations were performed in 1464 study participants with 2755 device leads (mean age, 67 years ± 15 [standard deviation]; 930 men [64%]). There was no evidence of an association between radiofrequency energy deposition, dB/dt, or scan duration and changes in device parameters. Thoracic MRI was associated with decreased battery voltage immediately after MRI (β = -0.008 V, P < .001). Additionally, right ventricular (RV) lead length was associated with decreased RV sensing (β = -0.012 mV, P = .05) and reduced RV capture threshold (β = -0.002 V, P < .01) immediately after MRI. Conclusion There was no evidence of an association between MRI parameters that characterize patient exposure to radiofrequency energy and changes in device and lead parameters immediately after MRI. Nevertheless, device interrogation before and after MRI remains mandatory due to the potential for device reset and changes in lead or generator parameters. © RSNA, 2020 See also the editorial by Shellock in this issue.

Acute enhancement of necrotic radio-frequency ablation lesions in left atrium and pulmonary vein ostia in swine model with non-contrast-enhanced T1 -weighted MRI

PURPOSE

To evaluate non-contrast-enhanced MRI of acute radio-frequency ablation (RFA) lesions in the left atrium (LA) and pulmonary vein (PV) ostia. The goal is to provide a method for discrimination between necrotic (permanent) lesions and reversible injury, which is associated with recurrence after treatment of atrial fibrillation.

METHODS

Fifteen normal swine underwent RFA around the right-superior PV ostia. Electrical pulmonary vein isolation (PVI) was verified by electro-anatomic mapping (EAM) and pacing. MRI was carried out using a 3D respiratory-gated T1 -weighted long inversion time (TWILITE) sequence without contrast agent. Key settings were: inversion time 700 ms, triggering over 2 cardiac cycles, pixel size 1.1 mm3 . Contrast-enhanced imaging and T2 -weighted imaging were carried out for comparison. Six animals were sacrificed on ablation day for TTC-stained gross pathology, 9 animals were sacrificed after 2-3 mo after repeat EAM and MRI. Image intensity ratio (IIR) was used to measure lesion enhancement, and gross pathology was used to validate image enhancement patterns and compare lesion widths.

RESULTS

RFA lesions exhibited unambiguous enhancement in acute TWILITE imaging (IIR = 2.34 ± 0.49 at 1.5T), and the enhancement patterns corresponded well with gross pathology. Lesion widths in MRI correlated well with gross pathology (R2 = 0.84), with slight underestimation by 0.9 ± 0.5 mm. Lesion enhancement subsided chronically.

CONCLUSION

TWILITE imaging allowed acute detection of permanent RFA lesions in swine LA and PV ostia, without the need for contrast agent. Lesion enhancement pattern showed good correspondence to gross pathology and was well visualized by volume rendering. This method may provide valuable intra- or post-procedural assessment of RFA treatment.

Applications of a deep learning method for anti-aliasing and super-resolution in MRI

Magnetic resonance (MR) images with both high resolutions and high signal-to-noise ratios (SNRs) are desired in many clinical and research applications. However, acquiring such images takes a long time, which is both costly and susceptible to motion artifacts. Acquiring MR images with good in-plane resolution and poor through-plane resolution is a common strategy that saves imaging time, preserves SNR, and provides one viewpoint with good resolution in two directions. Unfortunately, this strategy also creates orthogonal viewpoints that have poor resolution in one direction and, for 2D MR acquisition protocols, also creates aliasing artifacts. A deep learning approach called SMORE that carries out both anti-aliasing and super-resolution on these types of acquisitions using no external atlas or exemplars has been previously reported but not extensively validated. This paper reviews the SMORE algorithm and then demonstrates its performance in four applications with the goal to demonstrate its potential for use in both research and clinical scenarios. It is first shown to improve the visualization of brain white matter lesions in FLAIR images acquired from multiple sclerosis patients. Then it is shown to improve the visualization of scarring in cardiac left ventricular remodeling after myocardial infarction. Third, its performance on multi-view images of the tongue is demonstrated and finally it is shown to improve performance in parcellation of the brain ventricular system. Both visual and selected quantitative metrics of resolution enhancement are demonstrated.

Ablation Lesion Characterization in Scarred Substrate Assessed Using Cardiac Magnetic Resonance

OBJECTIVES

This study examined radiofrequency catheter ablation (RFCA) lesions within and around scar by cardiac magnetic resonance (CMR) imaging and histology.

BACKGROUND

Substrate modification by RFCA is the cornerstone therapy for ventricular arrhythmias. RFCA in scarred myocardium, however, is not well understood.

METHODS

We performed electroanatomic mapping and RFCA in the left ventricles of 8 swine with myocardial infarction. Non-contrast-enhanced T₁-weighted (T1w) and contrast-enhanced CMR after RFCA were compared with gross pathology and histology.

RESULTS

Of 59 lesions, 17 were in normal myocardium (voltage >1.5 mV), 21 in border zone (0.5 to 1.5 mV), and 21 in scar (<0.5 mV). All RFCA lesions were enhanced in T1w CMR, whereas scar was hypointense, allowing discrimination among normal myocardium, scar, and RFCA lesions. With contrast-enhancement, lesions and scar were similarly enhanced and not distinguishable. Lesion width and depth in T1w CMR correlated with necrosis in pathology (both; r² = 0.94, p < 0.001). CMR lesion volume was significantly different in normal myocardium, border zone, and scar (median: 397 [interquartile range (IQR): 301 to 474] mm³, 121 [IQR: 87 to 201] mm³, 66 [IQR: 33 to 123] mm³, respectively). RFCA force-time integral, impedance, and voltage changes did not correlate with lesion volume in border zone or scar. Histology showed that ablation necrosis extended into fibrotic tissue in 26 lesions and beyond in 14 lesions. In 7 lesions, necrosis expansion was blocked and redirected by fat.

CONCLUSIONS

T1w CMR can selectively enhance necrotic tissue in and around scar and may allow determination of the completeness of ablation intra- and post-procedure. Lesion formation in scar is affected by tissue characteristics, with fibrosis and fat acting as thermal insulators.

Non-contrast-enhanced T1 -weighted MRI of myocardial radiofrequency ablation lesions

PURPOSE

To demonstrate imaging of radiofrequency ablation lesions with non-contrast-enhanced T1 -weighted (T1w) MRI.

METHODS

Fifteen swine underwent left ventricular ablation followed by MRI using different preparations: endocardial or epicardial ablation of naïve animal, or endocardial ablation of animal with myocardial infarction. Lesion imaging was performed using free-breathing, non-contrast-enhanced, T1w sequence with long inversion time (TI). Also acquired were T1 maps and delayed contrast-enhanced (DCE) imaging. Hearts were excised for ex vivo imaging, and sliced for gross pathology and histology.

RESULTS

All ablations were visibly enhanced in non-contrast-enhanced T1w imaging using TI = 700 ms. T1w enhancement agreed with regions of necrosis in gross pathology and histology. Enhanced lesion cores were surrounded by dark bands containing contraction band necrosis, hematoma, and edema. In animals with myocardial infarction, chronic scar was hypointense in T1w, whereas acute ablations were enhanced, allowing discrimination between chronic scar and acute lesions, unlike DCE. Contrast was sufficient to create 3D volume renderings of lesions after minor postprocessing.

CONCLUSIONS

Non-contrast-enhanced T1w imaging with long TI promises to be an effective method for visualizing necrosis within radiofrequency ablation lesions. Enhancement is more specific and stationary than that from DCE. The imaging can be repeated as needed, unlike DCE, and may be especially useful for assessing ablations during or after a procedure. Magn Reson Med 79:879-889, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A Magnetic Resonance Imaging-Conditional External Cardiac Defibrillator for Resuscitation Within the Magnetic Resonance Imaging Scanner Bore

BACKGROUND

Subjects undergoing cardiac arrest within a magnetic resonance imaging (MRI) scanner are currently removed from the bore and then from the MRI suite, before the delivery of cardiopulmonary resuscitation and defibrillation, potentially increasing the risk of mortality. This precludes many higher-risk (acute ischemic and acute stroke) patients from undergoing MRI and MRI-guided intervention. An MRI-conditional cardiac defibrillator should enable scanning with defibrillation pads attached and the generator ON, enabling application of defibrillation within the seconds of MRI after a cardiac event. An MRI-conditional external defibrillator may improve patient acceptance for MRI procedures.

METHODS AND RESULTS

A commercial external defibrillator was rendered 1.5 Tesla MRI-conditional by the addition of novel radiofrequency filters between the generator and commercial disposable surface pads. The radiofrequency filters reduced emission into the MRI scanner and prevented cable/surface pad heating during imaging, while preserving all the defibrillator monitoring and delivery functions. Human volunteers were imaged using high specific absorption rate sequences to validate MRI image quality and lack of heating. Swine were electrically fibrillated (n=4) and thereafter defibrillated both outside and inside the MRI bore. MRI image quality was reduced by 0.8 or 1.6 dB, with the generator in monitoring mode and operating on battery or AC power, respectively. Commercial surface pads did not create artifacts deeper than 6 mm below the skin surface. Radiofrequency heating was within US Food and Drug Administration guidelines. Defibrillation was completely successful inside and outside the MRI bore.

CONCLUSIONS

A prototype MRI-conditional defibrillation system successfully defibrillated in the MRI without degrading the image quality or increasing the time needed for defibrillation. It can increase patient acceptance for MRI procedures.

Rectus femoris knee muscle moment arms measured in vivo during dynamic motion with real-time magnetic resonance imaging

Moment arms represent a muscle's ability to generate a moment about a joint for a given muscle force. The goal of this study was to develop a method to measure muscle moment arms in vivo over a large range of motion using real-time magnetic resonance (MR) imaging. Rectus femoris muscle-tendon lengths and knee joint angles of healthy subjects (N = 4) were measured during dynamic knee joint flexion and extension in a large-bore magnetic resonance imaging (MRI) scanner. Muscle-tendon moment arms were determined at the knee using the tendon-excursion method by differentiating measured muscle-tendon length with respect to joint angle. Rectus femoris moment arms were averaged across a group of healthy subjects and were found to vary similarly during knee joint flexion (mean: 3.0 (SD 0.5) cm, maximum: 3.5 cm) and extension (mean: 2.8 (SD 0.4) cm, maximum: 3.6 cm). These moment arms compare favorably with previously published dynamic tendon-excursion measurements in cadaveric specimens but were relatively smaller than moment arms from center-of-rotation studies. The method presented here provides a new approach to measure muscle-tendon moment arms in vivo and has the potential to be a powerful resource for characterizing musculoskeletal geometry during dynamic joint motion.

Interventional MRI using multiple 3D angiography roadmaps with real-time imaging

PURPOSE

To enhance real-time magnetic resonance (MR)-guided catheter navigation by overlaying colorized multiphase MR angiography (MRA) and cholangiopancreatography (MRCP) roadmaps in an anatomic context.

MATERIALS AND METHODS

Time-resolved MRA and respiratory-gated MRCP were acquired prior to real-time imaging in a pig model. MRA and MRCP data were loaded into a custom real-time MRI reconstruction and visualization workstation where they were displayed as maximum intensity projections (MIPs) in distinct colors. The MIPs were rendered in 3D together with real-time multislice imaging data using alpha blending. Interactive rotation allowed different views of the combined data.

RESULTS

Fused display of the previously acquired MIP angiography data with real-time imaging added anatomical context during endovascular interventions in swine. The use of multiple MIPs rendered in different colors facilitated differentiation of vascular structures, improving visual feedback during device navigation.

CONCLUSION

Interventional real-time MRI may be enhanced by combining with previously acquired multiphase angiograms. Rendered as 3D MIPs together with 2D slice data, this technique provided useful anatomical context that enhanced MRI-guided interventional applications.

Visualization of active devices and automatic slice repositioning ("SnapTo") for MRI-guided interventions

The accurate visualization of interventional devices is crucial for the safety and effectiveness of MRI-guided interventional procedures. In this paper, we introduce an improvement to the visualization of active devices. The key component is a fast, robust method ("CurveFind") that reconstructs the three-dimensional trajectory of the device from projection images in a fraction of a second. CurveFind is an iterative prediction-correction algorithm that acts on a product of orthogonal projection images. By varying step size and search direction, it is robust to signal inhomogeneities. At the touch of a key, the imaged slice is repositioned to contain the relevant section of the device ("SnapTo"), the curve of the device is plotted in a three-dimensional display, and the point on a target slice, which the device will intersect, is displayed. These features have been incorporated into a real-time MRI system. Experiments in vitro and in vivo (in a pig) have produced successful results using a variety of single- and multichannel devices designed to produce both spatially continuous and discrete signals. CurveFind is typically able to reconstruct the device curve, with an average error of approximately 2 mm, even in the case of complex geometries.

Multimodality cardiovascular molecular imaging technology

Cardiovascular molecular imaging is a new discipline that integrates scientific advances in both functional imaging and molecular probes to improve our understanding of the molecular basis of the cardiovascular system. These advances are driven by in vivo imaging of molecular processes in animals, usually small animals, and are rapidly moving toward clinical applications. Molecular imaging has the potential to revolutionize the diagnosis and treatment of cardiovascular disease. The 2 key components of all molecular imaging systems are the molecular contrast agents and the imaging system providing spatial and temporal localization of these agents within the body. They must deliver images with the appropriate sensitivity and specificity to drive clinical applications. As work in molecular contrast agents matures and highly sensitive and specific probes are developed, these systems will provide the imaging technologies required for translation into clinical tools. This is the promise of molecular medicine.

Active two-channel 0.035'' guidewire for interventional cardiovascular MRI

PURPOSE

To develop an "active" (receiver-coil) clinical-grade guidewire with enhanced visibility for magnetic resonance imaging (MRI) and favorable mechanical characteristics for interventional MRI procedures that require conspicuous intravascular instruments distinguishable from surrounding tissues.

MATERIALS AND METHODS

We designed a 0.035-inch guidewire combining two antenna designs on separate channels. A loop antenna visualizes the tip and a dipole antenna visualizes the whole shaft. We compared mechanical characteristics of this guidewire with x-ray alternatives and tested MRI performance at 1.5T in vitro and in vivo in swine.

RESULTS

Images reflected tip position within 0.97 +/- 0.42 mm and afforded whole-shaft visibility under expected conditions without sacrificing device size or handling. We report tip stiffness, torquability, and pushability comparable to commercial interventional guidewires.

CONCLUSION

Our clinical-grade 0.035-inch active guidewire is conspicuous under MRI and has mechanical performance comparable to x-ray interventional guidewires. This may enable a range of interventional procedures using real-time MRI.

Whole shaft visibility and mechanical performance for active MR catheters using copper-nitinol braided polymer tubes

BACKGROUND

Catheter visualization and tracking remains a challenge in interventional MR.Active guidewires can be made conspicuous in "profile" along their whole shaft exploiting metallic core wire and hypotube components that are intrinsic to their mechanical performance. Polymer-based catheters, on the other hand, offer no conductive medium to carry radio frequency waves. We developed a new "active" catheter design for interventional MR with mechanical performance resembling braided X-ray devices. Our 75 cm long hybrid catheter shaft incorporates a wire lattice in a polymer matrix, and contains three distal loop coils in a flexible and torquable 7Fr device. We explored the impact of braid material designs on radiofrequency and mechanical performance.

RESULTS

The incorporation of copper wire into in a superelastic nitinol braided loopless antenna allowed good visualization of the whole shaft (70 cm) in vitro and in vivo in swine during real-time MR with 1.5 T scanner. Additional distal tip coils enhanced tip visibility. Increasing the copper:nitinol ratio in braiding configurations improved flexibility at the expense of torquability. We found a 16-wire braid of 1:1 copper:nitinol to have the optimum balance of mechanical (trackability, flexibility, torquability) and antenna (signal attenuation) properties. With this configuration, the temperature increase remained less than 2 degrees C during real-time MR within 10 cm horizontal from the isocenter. The design was conspicuous in vitro and in vivo.

CONCLUSION

We have engineered a new loopless antenna configuration that imparts interventional MR catheters with satisfactory mechanical and imaging characteristics. This compact loopless antenna design can be generalized to visualize the whole shaft of any general-purpose polymer catheter to perform safe interventional procedures.

Interventional cardiovascular magnetic resonance: still tantalizing

The often touted advantages of MR guidance remain largely unrealized for cardiovascular interventional procedures in patients. Many procedures have been simulated in animal models. We argue these opportunities for clinical interventional MR will be met in the near future. This paper reviews technical and clinical considerations and offers advice on how to implement a clinical-grade interventional cardiovascular MR (iCMR) laboratory. We caution that this reflects our personal view of the "state of the art."

Real-time catheter-directed MRA with effective background suppression and persistent rendering

PURPOSE

To develop an imaging and visualization technique for real-time magnetic resonance angiography (rtMRA) fully integrated with a real-time interactive imaging environment on a clinical MR scanner.

MATERIALS AND METHODS

Intraarterial injections of contrast agent and imaging processing techniques were employed for rapid catheter-directed assessment of vessel patency and regional tissue perfusion. Operators can image multiple thin slices to maximize anatomic detail or use thick slice or projection imaging to maximize vessel coverage. Techniques in both pulse sequence and image processing were employed to ensure background suppression. Accumulation of maximum pixel values allows persistent display of bolus signal as it passes through the vessels and into tissues. Automatic brightness adjustment was used to ensure visibility at all stages of bolus passage.

RESULTS

Experimental intraarterial rtMRA of coronary, renal, and carotid arteries show that vessel trajectories and perfusion territories are well visualized in swine. Switching between standard real-time imaging and rtMRA imaging after contrast injection was easy to perform during a procedure without stopping the scanner.

CONCLUSION

The proposed technique facilitates visualization of intraarterial contrast injections using real-time MRI. Although designed for rapid deployment during rtMRI-guided interventional procedures, the technique may also be useful to supplement the study of vessel anatomy, flow, or perfusion.

Real-time magnetic resonance imaging guidance for cardiovascular procedures

Magnetic resonance imaging (MRI) of the cardiovascular system has proven to be an invaluable diagnostic tool. Given the ability to allow for real-time imaging, MRI guidance of intraoperative procedures can provide superb visualization, which can facilitate a variety of interventions and minimize the trauma of the operations as well. In addition to the anatomic detail, MRI can provide intraoperative assessment of organ and device function. Instruments and devices can be marked to enhance visualization and tracking, all of which is an advance over standard X-ray or ultrasonic imaging.

Interventional cardiovascular procedures guided by real-time MR imaging: an interactive interface using multiple slices, adaptive projection modes and live 3D renderings

PURPOSE

To develop and test a novel interactive real-time MRI environment that facilitates image-guided cardiovascular interventions.

MATERIALS AND METHODS

Color highlighting of device-mounted receiver coils, accelerated imaging of multiple slices, adaptive projection modes, live three-dimensional (3D) renderings and other interactive features were utilized to enhance navigation of devices and targeting of tissue.

RESULTS

Images are shown from several catheter-based interventional procedures performed in swine that benefit from this custom interventional MRI interface. These include endograft repair of aortic aneurysm, balloon septostomy of the cardiac interatrial septum, angioplasty and stenting, and endomyocardial cell injection, all using active catheters containing MRI receiver coils.

CONCLUSION

Interactive features not available on standard clinical scanners enhance real-time MRI for guiding cardiovascular interventional procedures.

Parallelized hybrid TGRAPPA reconstruction for real-time interactive MRI

Real-time parallel MRI reconstruction was demonstrated using a hybrid implementation of the TGRAPPA algorithm. The GRAPPA coefficients were calculated in k-space and applied in the image domain after appropriate transformation, thereby achieving improved speed and excellent image quality. Adaptive B1-weighted combining of the per coil images permitted use of pre-calculated composite image domain weights providing significant decrease in computation. The weight calculation was decoupled from the real-time image reconstruction as a parallel processing thread which was updated in an adaptive manner to speed convergence in the event of interactive change in scan plane. The computation was parallelized and implemented on a general purpose multi-core architecture. Reconstruction speeds of 65-70 frames per second were achieved with a matrix of 192 x 144 with 15 coils.

Endoventricular patch plasty for dyskinetic anteroapical left ventricular aneurysm increases systolic circumferential shortening in sheep

OBJECTIVE

Endoventricular patch plasty (Dor procedure) has gained favor as a surgical treatment for heart failure associated with large anteroapical myocardial infarction. We tested the hypotheses that the Dor procedure increases systolic circumferential shortening and longitudinal shortening in noninfarcted left ventricular regions in sheep.

METHODS

In 6 male Dorsett sheep, the left anterior descending coronary artery and its second diagonal branch were ligated 40% of the distance from the apex to the base. Sixteen weeks after myocardial infarction, a Dor procedure was performed with a Dacron patch that was 50% of the infarct neck dimension. Two weeks before and 2 and 6 weeks after the Dor procedure, animals underwent magnetic resonance imaging with tissue tagging in multiple short-axis and long-axis slices. Fully three-dimensional strain analyses were performed. All 6 end-systolic strain components were compared in regions 1 cm, 2 cm, 3 cm, and 4 cm below the valves, as well as in the anterior, posterior, and lateral left ventricular walls and the interventricular septum.

RESULTS

Circumferential shortening increased from before the Dor procedure to 6 weeks after repair in nearly every left ventricular region (13/16). The greatest regional change in circumferential shortening was found in the equatorial region or 2 cm below the base and in the posterior wall (from 9.0% to 18.4%; P < .0001). Longitudinal shortening increased 2 weeks after the Dor procedure but then returned near baseline by 6 weeks after the Dor procedure.

CONCLUSION

The Dor procedure significantly increases systolic circumferential shortening in nearly all noninfarcted left ventricular regions in sheep.

Real-time interactive MRI-guided cardiac surgery: aortic valve replacement using a direct apical approach

Minimally invasive cardiac surgery requires arresting and emptying of the heart, which compromises visualization of the surgical field. In this feasibility study a novel surgical procedure is demonstrated in which real-time MRI is used to guide the placement of a prosthetic aortic valve in the beating heart via direct apical access in eight porcine hearts. A clinical stentless bioprosthetic valve affixed to a platinum stent was compressed onto a balloon-tipped catheter. This was fed through a 15-18-mm delivery port inserted into the left ventricular (LV) apex via a minimally invasive subxyphoid incision. Using interactive real-time MRI, the surgeon implanted the prosthetic valve in the correct location at the aortic annulus within 90 s. In four of the animals immediately after implantation, ventricular function, blood flow through the valve, and myocardial perfusion were evaluated with MRI. MRI-guided beating-heart surgery may provide patients with a less morbid and more durable solution to structural heart disease.

Real-time MRI guided atrial septal puncture and balloon septostomy in swine

Cardiac perforation during atrial septal puncture (ASP) might be avoided by improved image guidance. X-ray fluoroscopy (XRF), which guides ASP, visualizes tissue poorly and does not convey depth information. Ultrasound is limited by device shadows and constrained imaging windows. Alternatively, real-time MRI (rtMRI) provides excellent tissue contrast in any orientation and may enable ASP and balloon atrial septostomy (BAS) in swine. Custom MRI catheters incorporated "active" (receiver antenna) and "passive" (iron or gadolinium) elements. Wholly rtMRI-guided transfemoral ASP and BAS were performed in 10 swine in a 1.5T interventional suite. Hemodynamic results were measured with catheters and velocity encoded MRI. Successful ASP was performed in all 10 animals. Necropsy confirmed septostomy confined within the fossa ovalis in all. BAS was successful in 9/10 animals. Antenna failure in a re-used needle led to inadvertent vena cava tear prior to BAS in 1 animal. ASP in the same animal was easily performed using a new needle. rtMRI illustrated clear device-tissue-lumen relationships in multiple orientations, and facilitated simple ASP and BAS. The mean procedure time was 19 +/- 10 minutes. Septostomy achieved a mean left to right shunt ratio of 1.3:1 in these healthy animals. Interactive rtMRI permits rapid transcatheter ASP and BAS in swine. Further technical development may enable novel applications.

The effect of anteroapical aneurysm plication on end-systolic three-dimensional strain in the sheep: a magnetic resonance imaging tagging study

OBJECTIVES

Although repair of left ventricular aneurysm has been extensively studied, its effect on regional ventricular function remains unclear. The primary goal of this study was to quantify the effect of anteroapical aneurysm plication on systolic deformation in noninfarcted adjacent (border zone) and remote left ventricular regions in sheep.

METHODS

Eight sheep underwent anteroapical myocardial infarction (25% of left ventricular mass). Ten weeks later, animals underwent aneurysm plication. Two and 6 weeks after this operation, animals underwent magnetic resonance imaging with tissue tagging in multiple short-axis and long-axis slices. Fully 3-dimensional strain analyses were performed. All 6 end-systolic strain components were compared at midwall in the border zone of the aneurysm or repair and in regions 1 cm, 2 cm, and 3 cm below the valves.

RESULTS

Circumferential shortening progressively increases from before plication to 2 weeks after plication to 6 weeks after plication toward the border zone. The effect on circumferential shortening is most pronounced in the anterior wall and septum. The biggest change is from 2 to 6 weeks after plication (from 4.3% to 11.3% in anterior wall, P < .0001; from 3.5% to 6.5% in septum, P < .0007). Longitudinal shortening is decreased at 2 weeks after plication but then returns to baseline (with slight improvement in the border zone) at 6 weeks after plication.

CONCLUSIONS

Repair of left ventricular aneurysm significantly increases systolic circumferential shortening at the border zone in sheep.

Real-time magnetic resonance imaging-guided endovascular recanalization of chronic total arterial occlusion in a swine model

BACKGROUND

Endovascular recanalization (guidewire traversal) of peripheral artery chronic total occlusion (CTO) can be challenging. X-ray angiography resolves CTO poorly. Virtually "blind" device advancement during x-ray-guided interventions can lead to procedure failure, perforation, and hemorrhage. Alternatively, MRI may delineate the artery within the occluded segment to enhance procedural safety and success. We hypothesized that real-time MRI (rtMRI)-guided CTO recanalization can be accomplished in an animal model.

METHODS AND RESULTS

Carotid artery CTO was created by balloon injury in 19 lipid-overfed swine. After 6 to 8 weeks, 2 underwent direct necropsy analysis for histology, 3 underwent primary x-ray-guided CTO recanalization attempts, and the remaining 14 underwent rtMRI-guided recanalization attempts in a 1.5-T interventional MRI system. Real-time MRI intervention used custom CTO catheters and guidewires that incorporated MRI receiver antennae to enhance device visibility. The mean length of the occluded segments was 13.3+/-1.6 cm. The rtMRI-guided CTO recanalization was successful in 11 of 14 swine and in only 1 of 3 swine with the use of x-ray alone. After unsuccessful rtMRI (n=3), x-ray-guided attempts were also unsuccessful.

CONCLUSIONS

Recanalization of long CTO is entirely feasible with the use of rtMRI guidance. Low-profile clinical-grade devices will be required to translate this experience to humans.

Real-time magnetic resonance imaging-guided stenting of aortic coarctation with commercially available catheter devices in Swine

BACKGROUND

Real-time MR imaging (rtMRI) is now technically capable of guiding catheter-based cardiovascular interventions. Compared with x-ray, rtMRI offers superior tissue imaging in any orientation without ionizing radiation. Translation to clinical trials has awaited the availability of clinical-grade catheter devices that are both MRI visible and safe. We report a preclinical safety and feasibility study of rtMRI-guided stenting in a porcine model of aortic coarctation using only commercially available catheter devices.

METHOD AND RESULTS

Coarctation stenting was performed wholly under rtMRI guidance in 13 swine. rtMRI permitted procedure planning, device tracking, and accurate stent deployment. "Active" guidewires, incorporating MRI antennas, improved device visualization compared with unmodified "passive" nitinol guidewires and shortened procedure time (26+/-11 versus 106+/-42 minutes; P=0.008). Follow-up catheterization and necropsy showed accurate stent deployment, durable gradient reduction, and appropriate neointimal formation. MRI immediately identified aortic rupture when oversized devices were tested.

CONCLUSIONS

This experience demonstrates preclinical safety and feasibility of rtMRI-guided aortic coarctation stenting using commercially available catheter devices. Patients may benefit from rtMRI in the future because of combined device and tissue imaging, freedom from ionizing radiation, and the ability to identify serious complications promptly.

Real-time magnetic resonance-guided endovascular repair of experimental abdominal aortic aneurysm in swine

OBJECTIVES

This study tested the hypotheses that endografts can be visualized and navigated in vivo solely under real-time magnetic resonance imaging (rtMRI) guidance to repair experimental abdominal aortic aneurysms (AAA) in swine, and that MRI can provide immediate assessment of endograft apposition and aneurysm exclusion.

BACKGROUND

Endovascular repair for AAA is limited by endoleak caused by inflow or outflow malapposition. The ability of rtMRI to image soft tissue and flow may improve on X-ray guidance of this procedure.

METHODS

Infrarenal AAA was created in swine by balloon overstretch. We used one passive commercial endograft, imaged based on metal-induced MRI artifacts, and several types of homemade active endografts, incorporating MRI receiver coils (antennae). Custom interactive rtMRI features included color coding the catheter-antenna signals individually, simultaneous multislice imaging, and real-time three-dimensional rendering.

RESULTS

Eleven repairs were performed solely using rtMRI, simultaneously depicting the device and soft-tissue pathology during endograft deployment. Active devices proved most useful. Intraprocedural MRI provided anatomic confirmation of stent strut apposition and functional corroboration of aneurysm exclusion and restoration of laminar flow in successful cases. In two cases, there was clear evidence of contrast accumulation in the aneurysm sac, denoting endoleak.

CONCLUSIONS

Endovascular AAA repair is feasible under rtMRI guidance. Active endografts facilitate device visualization and complement the soft tissue contrast afforded by MRI for precise positioning and deployment. Magnetic resonance imaging also permits immediate post-procedural anatomic and functional evaluation of successful aneurysm exclusion.

Invasive human magnetic resonance imaging: feasibility during revascularization in a combined XMR suite

We tested the feasibility and safety of invasive magnetic resonance imaging (MRI) during peripheral angioplasty. Real-time MRI can image soft tissue and may potentially guide therapeutic procedures without ionizing radiation or nephrotoxic contrast. MRI-guided diagnostic catheterization has been described recently, but safe and conspicuous catheter devices are not widely available. An active guidewire, which serves as an MRI receiver antenna, might be useful to guide catheterization or even to image atheroma. We describe a combined interventional suite offering both X-ray fluoroscopy and real-time MRI. We used a 0.030'' active guidewire receiver coil for invasive MRI after X-ray lesion traversal in patients undergoing percutaneous iliofemoral artery revascularization. Intravascular MRI was compared with noninvasive MRI, X-ray angiography, and intravascular ultrasound (IVUS). Seven eligible patients consented to participate, but three were excluded because of lengthy revascularization procedures. Four remaining patients safely underwent combined X-ray fluoroscopy and real-time magnetic resonance imaging (XMR) transport, continuous monitoring, and all imaging modalities. There was no device dislodgment, contamination or evidence of heating. The intravascular MRI coil was well visualized except at the tip, but did not provide superior mural imaging compared with IVUS. Therefore, because an adequate safety and workflow experience was obtained, enrollment was terminated after only four subjects. Invasive MRI is feasible and apparently safe during peripheral angioplasty. Patients can safely be transported and monitored in an XMR interventional suite. An active quarter-wavelength guidewire coil does not provide superior imaging compared with IVUS, but provides satisfactory guidewire visualization. These tools may prove useful for advanced therapeutic procedures in the future.

Imaging of myocardial infarction for diagnosis and intervention using real-time interactive MRI without ECG-gating or breath-holding

Current methods for MRI of infarcted myocardium require ECG-gating and breath-holding during contrast-enhanced segmented k-space inversion-recovery (IR) imaging. However, ECG-gating can be problematic in MRI, and breath-holding can be difficult for some patients. This work demonstrates that infarcted tissue can be visualized without ECG-gating or breath-holding with the use of intermittent inversion pulses during real-time (RT) interactive imaging with steady-state free precession (SSFP). The sequence generates a RT image stream containing a myocardium-nulled image every few frames, which allows nearly simultaneous observation of both infarcted regions and wall motion. First-pass perfusion and wall motion can be simultaneously observed with minor parameter modifications. This method may reduce diagnostic scan time, expand the target population, improve patient comfort, and facilitate targeted, interventional treatment of infarcted myocardium. Supplementary material for this article can be found on the MRM website at http://www.interscience.wiley.com/jpages/0740-3194/suppmat/index.html.

Reduced field of view and undersampled PR combined for interventional imaging of a fully dynamic field of view

Active catheter imaging was investigated using real-time undersampled projection reconstruction (PR) combined with the temporal filtering technique of reduced field of view (rFOV). Real-time rFOV processing was interactively enabled during highly undersampled catheter imaging, resulting in improved artifact suppression with better temporal resolution than that obtained by view-sharing. Imaging with 64 to 32 projections provided a resolution of 2 x 2 x 8 mm, and four to eight true frames per second. Image artifacts were reduced when rFOV processing was applied to the undersampled images. A comparison with Cartesian rFOV showed that PR image quality is less susceptible to aliasing that results from rFOV imaging with a wholly dynamic outer FOV. Simulations and MRI experiments demonstrated that PR rFOV provides significant artifact suppression, even for a fully dynamic FOV. The near doubling of temporal resolution that is possible with PR rFOV permits accurate monitoring of highly dynamic events, such as catheter movements, and arrhythmias, such as ventricular ectopy.

Magnetic resonance fluoroscopy allows targeted delivery of mesenchymal stem cells to infarct borders in Swine

BACKGROUND

The local environment of delivered mesenchymal stem cells (MSCs) may affect their ultimate phenotype. MR fluoroscopy has the potential to guide intramyocardial MSC injection to desirable targets, such as the border between infarcted and normal tissue. We tested the ability to (1) identify infarcts, (2) navigate injection catheters to preselected targets, (3) inject safely even into fresh infarcts, and (4) confirm injection success immediately.

METHODS AND RESULTS

A 1.5-T MRI scanner was customized for interventional use, with rapid imaging, independent color highlighting of catheter channels, multiple-slice 3D rendering, catheter-only viewing mode, and infarct-enhanced imaging. MRI receiver coils were incorporated into guiding catheters and injection needles. These devices were tested for heating and used for targeted MSC delivery. In infarcted pigs, myocardium was targeted by MR fluoroscopy. Infarct-enhanced imaging included both saturation preparation MRI after intravenous gadolinium and wall motion. Porcine MSCs were MRI-labeled with iron-fluorescent particles. Catheter navigation and multiple cell injections were performed entirely with MR fluoroscopy at 8 frames/s with 1.7x3.3x8-mm voxels. Infarct-enhanced MR fluoroscopy permitted excellent delineation of infarct borders. All injections were safely and successfully delivered to their preselected targets, including infarct borders. Iron-fluorescent particle-labeled MSCs were readily visible on delivery in vivo and post mortem.

CONCLUSIONS

Precise targeted delivery of potentially regenerative cellular treatments to recent myocardial infarction borders is feasible with an MR catheter delivery system. MR fluoroscopy permits visualization of catheter navigation, myocardial function, infarct borders, and labeled cells after injection.

Serial cardiac magnetic resonance imaging of injected mesenchymal stem cells

BACKGROUND

Delivery and tracking of endomyocardial stem cells are limited by the inability to image transplanted cells noninvasively in the beating heart. We hypothesized that mesenchymal stem cells (MSCs) could be labeled with a iron fluorophore particle (IFP) to provide MRI contrast in vivo to assess immediate and long-term localization.

METHODS AND RESULTS

MSCs were isolated from swine. Short-term incubation of MSCs with IFP resulted in dose-dependent and efficient labeling. Labeled cells remained viable for multiple passages and retained in vitro proliferation and differentiation capacity. Labeled MSCs (10(4) to 10(6) cells/150 microL) were injected percutaneously into normal and freshly infarcted myocardium in swine. One, 3, and 1 animals underwent serial cardiac MRI (1.5T) for 4, 8, and 21 days, respectively. MRI contrast properties were measured both in vivo and in vitro for cells embedded in agar. Injection sites containing as few as 10(5) MSCs could be detected and contained intact IFP-bearing MSCs on histology.

CONCLUSIONS

IFP labeling of MSCs imparts useful MRI contrast, enabling ready detection in the beating heart on a conventional cardiac MR scanner after transplantation into normal and infarcted myocardium. The dual-labeled MSCs can be identified at locations corresponding to injection sites, both ex vivo using fluorescence microscopy and in vivo using susceptibility contrast on MRI. This technology may permit effective in vivo study of stem cell retention, engraftment, and migration.

Real-time accelerated interactive MRI with adaptive TSENSE and UNFOLD

Reduced field-of-view (FOV) acceleration using time-adaptive sensitivity encoding (TSENSE) or unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD) can improve the depiction of motion in real-time MRI. However, increased computational resources are required to maintain a high frame rate and low latency in image reconstruction and display. A high-performance software system has been implemented to perform TSENSE and UNFOLD reconstructions for real-time MRI with interactive, on-line display. Images were displayed in the scanner room to investigate image-guided procedures. Examples are shown for normal volunteers and cardiac interventional experiments in animals using a steady-state free precession (SSFP) sequence. In order to maintain adequate image quality for interventional procedures, the imaging rate was limited to seven frames per second after an acceleration factor of 2 with a voxel size of 1.8 x 3.5 x 8 mm. Initial experiences suggest that TSENSE and UNFOLD can each improve the compromise between spatial and temporal resolution in real-time imaging, and can function well in interactive imaging. UNFOLD places no additional constraints on receiver coils, and is therefore more flexible than SENSE methods; however, the temporal image filtering can blur motion and reduce the effective acceleration. Methods are proposed to overcome the challenges presented by the use of TSENSE in interactive imaging. TSENSE may be temporarily disabled after changing the imaging plane to avoid transient artifacts as the sensitivity coefficients adapt. For imaging with a combination of surface and interventional coils, a hybrid reconstruction approach is proposed whereby UNFOLD is used for the interventional coils, and TSENSE with or without UNFOLD is used for the surface coils.

Novel technique for cardiac electromechanical mapping with magnetic resonance imaging tagging and an epicardial electrode sock

Near-simultaneous measurements of electrical and mechanical activation over the entire ventricular surface are now possible using magnetic resonance imaging tagging and a multielectrode epicardial sock. This new electromechanical mapping technique is demonstrated in the ventricularly paced canine heart. A 128-electrode epicardial sock and pacing electrodes were placed on the hearts of four anesthetized dogs. In the magnetic resonance scanner, tagged cine images (8-15 ms/frame) and sock electrode recordings (1000 Hz) were acquired under right-ventricular pacing and temporally referenced to the pacing stimulus. Electrical recordings were obtained during intermittent breaks in image acquisition, so that both data sets represented the same physiologic state. Since the electrodes were not visible in the images, electrode recordings and cine images were spatially registered with Gd-DTPA markers attached to the sock. Circumferential strain was calculated at locations corresponding to electrodes. For each electrode location, electrical and mechanical activation times were calculated and relationships between the two activation patterns were demonstrated. This method holds promise for improving understanding of the relationships between the patterns of electrical activation and contraction in the heart.

Undersampled projection reconstruction for active catheter imaging with adaptable temporal resolution and catheter-only views

In this study undersampled projection reconstruction (PR) was used for rapid catheter imaging in the heart, employing steady-state free precession (SSFP) contrast. Active catheters and phased-array coils were used for combined imaging of anatomy and catheter position in swine. Real-time imaging of catheter position was performed with relatively high spatial and temporal resolution, providing 2 x 2 x 8 mm spatial resolution and four to eight frames per second. Two interactive features were introduced. The number of projections (Np) was adjusted interactively to trade off imaging speed and artifact reduction, allowing acquisition of high-quality or high-frame-rate images. Thin-slice imaging was performed, with interactive requests for thick-slab projection images of the signal received solely from the active catheter. Briefly toggling on catheter-only projection images was valuable for verifying that the catheter tip was contained within the selected slice, or for locating the catheter when part of it was outside the selected slice.

Myocardial tagging with SSFP

This work presents the first implementation of myocardial tagging with refocused steady-state free precession (SSFP) and magnetization preparation. The combination of myocardial tagging (a noninvasive method for quantitative measurement of regional and global cardiac function) with the high tissue signal-to-noise ratio (SNR) obtained with SSFP is shown to yield improvements in terms of the myocardium-tag contrast-to-noise ratio (CNR) and tag persistence when compared to the current standard fast gradient-echo (FGRE) tagging protocol. Myocardium-tag CNR and tag persistence were studied using numerical simulations as well as phantom and human experiments. Both quantities were found to decrease with increasing imaging flip angle (alpha) due to an increased tag decay rate and a decrease in myocardial steady-state signal. However, higher alpha yielded better blood-myocardium contrast, indicating that optimal alpha is dependent on the application: higher alpha for better blood-myocardium boundary visualization, and lower alpha for better tag persistence. SSFP tagging provided the same myocardium-tag CNR as FGRE tagging when acquired at four times the bandwidth and better tag- and blood-myocardium CNRs than FGRE tagging when acquired at equal or twice the receiver bandwidth (RBW). The increased acquisition efficiency of SSFP allowed decreases in breath-hold duration, or increases in temporal resolution, as compared to FGRE.

Phased array ghost elimination (PAGE) for segmented SSFP imaging with interrupted steady-state

Steady-state free precession (SSFP) has recently proven to be valuable for cardiac imaging due to its high signal-to-noise ratio and blood-myocardium contrast. Data acquired using ECG-triggered, segmented sequences during the approach to steady-state, or return to steady-state after interruption, may have ghost artifacts due to periodic k-space distortion. Schemes involving several preparatory RF pulses have been proposed to restore steady-state, but these consume imaging time during early systole. Alternatively, the phased-array ghost elimination (PAGE) method may be used to remove ghost artifacts from the first several frames. PAGE was demonstrated for cardiac cine SSFP imaging with interrupted steady-state using a simple alpha/2 magnetization preparation and storage scheme and a spatial tagging preparation.

Multishot EPI-SSFP in the heart

Refocused steady-state free precession (SSFP), or fast imaging with steady precession (FISP or TrueFISP), has recently proven valuable for cardiac imaging because of its high signal-to-noise ratio (SNR) and excellent blood-myocardium contrast. In this study, various implementations of multiecho SSFP or EPI-SSFP for imaging in the heart are presented. EPI-SSFP has higher scan-time efficiency than single-echo SSFP, as two or more phase-encode lines are acquired per repetition time (TR) at the cost of a modest increase in TR. To minimize TR, a noninterleaved phase-encode order in conjunction with a phased-array ghost elimination (PAGE) technique was employed, removing the need for echo time shifting (ETS). The multishot implementation of EPI-SSFP was used to decrease the breath-hold duration for cine acquisitions or to increase the temporal or spatial resolution for a fixed breath-hold duration. The greatest gain in efficiency was obtained with the use of a three-echo acquisition. Image quality for cardiac cine applications using multishot EPI-SSFP was comparable to that of single-echo SSFP in terms of blood-myocardium contrast and contrast-to-noise ratio (CNR). The PAGE method considerably reduced flow artifacts due to both the inherent ghost suppression and the concomitant reduction in phase-encode blip size. The increased TR of multishot EPI-SSFP led to a reduced specific absorption rate (SAR) for a fixed RF flip angle, and allowed the use of a larger flip angle without increasing the SAR above the FDA-approved limits.

Catheter-based endomyocardial injection with real-time magnetic resonance imaging

BACKGROUND

We tested the feasibility of targeted left ventricular (LV) mural injection using real-time MRI (rtMRI).

METHODS AND RESULTS

A 1.5T MRI scanner was customized with a fast reconstruction engine, transfemoral guiding catheter-receiver coil (GCC), MRI-compatible needle, and tableside consoles. Commercial real-time imaging software was customized to facilitate catheter navigation and visualization of injections at 4 completely refreshed frames per second. The aorta was traversed and the left ventricular cavity was entered under direct rtMRI guidance. Pigs underwent multiple injections with dilute gadolinium-DTPA. All myocardial segments were readily accessed. The active GCC and the passive Stiletto needle injector were readily visualized. More than 50 endomyocardial injections were performed with the aid of rtMRI; 81% were successful with this first-generation prototype.

CONCLUSION

Percutaneous endomyocardial drug delivery is feasible with the aid of rtMRI, which permits precise 3-dimensional localization of injection within the LV wall.

Real-time volume rendered MRI for interventional guidance

Volume renderings from magnetic resonance imaging data can be created and displayed in real-time with user interactivity. This can provide continuous 3D feedback to assist in guiding an interventional procedure. A system is presented which can produce real-time volume renderings from 2D multi-slice or 3D MR pulse sequences. Imaging frame rates up to 30 per second have been demonstrated with a latency of approximately one-third of a second, depending on the image matrix size. Several interactive capabilities have been implemented to enhance visualization such as cut planes, individual channel scaling and color highlighting, view sharing, saturation preparation, complex subtraction, gating control, and choice of alpha blending or MIP rendering. The system is described and some interventional application examples are shown. To view movies of some of the examples, enter the following address into a web browser: http://nhlbi.nih.gov/labs/papers/lce/guttman/rtvolmri/index/htm.

Techniques for fast stereoscopic MRI

Stereoscopic MRI can impart 3D perception with only two image acquisitions. This economy over standard multiplanar 3D volume renderings allows faster frame rates, which are needed for real-time imaging applications. Real-time 3D perception may enhance the appreciation of complex anatomical structures, and may improve hand-eye coordination while manipulating a medical device during an image-guided interventional procedure. To this goal, a system is being developed to acquire and display stereoscopic MR images in real-time. A clinically used, fast gradient-recalled echo-train sequence has been modified to produce stereo image pairs. Features have been added for depth cueing, view sharing, and bulk signal suppression. A workstation was attached to a clinical MR scanner for fast data extraction, image reconstruction and stereoscopic image display.

Catheter-tracking FOV MR fluoroscopy

Recent improvements in intravascular magnetic resonance imaging techniques mandate an accurate method of monitoring the introduction of MR catheter probes into the vessel of interest. For this purpose, a novel imaging protocol and a display method have been designed. First, a roadmap 3D image data set with standard pulse sequences is obtained using an external imaging coil. Subsequently, using very narrow rectangular-FOV fast-spoiled gradient recalled (SPGR), a movie of the percutaneous placement procedure of an MR catheter probe is acquired at a rate of 7.3 frames/second. In this protocol, the probe is used to transmit RF pulses and receive MR signal. A computer program was written for image unwrapping and for displaying the unwrapped movie frames on the roadmap image. In an alternative protocol, the movie frames in two projection angles were acquired in an interleaved fashion. Frames were unwrapped and combined with a 3D roadmap and displayed on a Silicon Graphics workstation equipped with stereovision goggles. Using these methods, percutaneous catheter placement in a phantom and a dog was examined. In conclusion, a new visualization technique for MR catheter placement is proposed. Combining this technique with high resolution intravascular MRI techniques may result in a very useful diagnostic tool for the evaluation of atherosclerosis and other vessel diseases.

Precision of myocardial contour estimation from tagged MR images with a "black-blood" technique

RATIONALE AND OBJECTIVES

The authors determined whether blood presaturation of tagged magnetic resonance (MR) images affects identification of left ventricular endocardial borders.

MATERIALS AND METHODS

Three healthy volunteers underwent MR imaging performed with a breath-hold segmented spoiled gradient-recalled-echo sequence with tissue tagging. Two saturation pulses (in the basal and apical regions) were used to generate black-blood images. Manual segmentation of endocardial contours on black-blood and white-blood images was performed independently by five observers.

RESULTS

Endocardial borders were better identified on black-blood images compared with white-blood images, especially in the early systolic phases. Interobserver variability in contour estimation was significantly higher for white-blood images (P < .001) and was twice that for corresponding black-blood images during early systole. Contour variability appeared to be affected mainly by tag-to-myocardium contrast (P = .009) and myocardium-to-chamber contrast (P = .05).

CONCLUSION

Blood presaturation of tagged MR images improves reliability of contour segmentation.

Impact of semiautomated versus manual image segmentation errors on myocardial strain calculation by magnetic resonance tagging

RATIONALE AND OBJECTIVES

Accurate image segmentation is essential for the study of mechanical properties of the myocardium by tagged magnetic resonance imaging (MRI). The relative accuracy of three methods of segmentation of myocardial borders and tags and their impact on myocardial strain calculations were evaluated.

METHODS

Radially tagged, spin-echo magnetic resonance (MR) images of dog hearts were segmented manually, automatically, and semiautomatically. The variability of segmentation methods was separately determined for myocardial contours and tags. Error propagation assessment for strain calculation was estimated.

RESULTS

The variability of the segmentation of the contours was five times greater than that of the tags. The error propagation is nonuniform in all directions, maximal for the radial component of strain.

CONCLUSION

Errors in the segmentation of myocardial contours are significantly greater than those of the tags. Strain calculations should be based solely on segmentation of MR tags to avoid significant errors, particularly in radial strain estimates.

Tag and contour detection in tagged MR images of the left ventricle

Tracking magnetic resonance tags in myocardial tissue promises to be an effective tool for the assessment of myocardial motion. The authors describe a hierarchy of image processing steps which rapidly detects both the contours of the myocardial boundaries of the left ventricle and the tags within the myocardium. The method works on both short axis and long axis images containing radial and parallel tag patterns, respectively. Left ventricular boundaries are detected by first removing the tags using morphological closing and then selecting candidate edge points. The best inner and outer boundaries are found using a dynamic program that minimizes a nonlinear combination of several local cost functions. Tags are tracked by matching a template of their expected profile using a least squares estimate. Since blood pooling, contiguous and adjacent tissue, and motion artifacts sometimes cause detection errors, a graphical user interface was developed to allow user correction of anomalous points. The authors present results on several tagged images of a human. A fully automated run generally finds the endocardial boundary and the tag lines extremely well, requiring very little manual correction. The epicardial boundary sometimes requires more intervention to obtain an acceptable result. These methods are currently being used in the analysis of cardiac strain and as a basis for the analysis of alternate tag geometries.