Characterization of radiofrequency ablated myocardium with optical coherence tomography

Certain types of cardiac arrhythmias are best treated with radiofrequency (RF) ablation, in which an electrode is inserted into the targeted area of the myocardium and then RF electrical current is applied to heat and destroy surrounding tissue. The resulting ablation lesion usually consists of a coagulative necrotic core surrounded by a rim region of mixed viable and non-viable cells. The characterization of the RF ablated lesion is of potential clinical importance. Here we aim to elaborate optical coherence tomography (OCT) imaging for the characterization of RF-ablated myocardial tissue. In particular, the underlying principles of OCT and its polarization-sensitive counterpart (PS-OCT) are presented, followed by the knowledge needed to interpret their optical images. Studies focused on real-time monitoring of RF lesion formation in the myocardium using OCT systems are summarized. The design and development of various hybrid probes incorporating both OCT guidance and RF ablation catheters are also discussed. Finally, the challenges related to the transmission of OCT imaging systems to cardiac clinics for real-time monitoring of RF lesions are outlined.     

Dual contrast TrueFISP imaging for left ventricular segmentation.

Based on varying tissue contrasts at different RF flip angles, a new TrueFISP imaging strategy for cardiac function measurement is presented. A single breath-hold dual RF flip angle cine multi-slice TrueFISP imaging sequence was implemented which provides a significant increase in signal contrast between blood and myocardium. The increase in image contrast combined with different characteristics in RF response facilitates the delineation of cardiovascular borders. Based on this imaging strategy it is demonstrated how a simple 2D histogram clustering algorithm can be used for the fully automatic segmentation of the left ventricular (LV) blood pool. The method is validated with data acquired from 10 asymptomatic subjects, and the results are shown to be comparable to that of manual delineation by experienced observers.     

Measurement of regional left ventricular function using labelled magnetic resonance imaging

A technique for assessing regional left ventricular function using magnetic resonance imaging is described. Spatial modulation of magnetization (SPAMM) is effected immediately before images are obtained at various intervals during the cardiac cycle using a modified field echo even rephasing technique (FEER). By performing such modulation in two planes, a grid pattern of labelling can be produced across the image. On the resulting labelled short axis images of the left ventricle, the systolic increase in thickness (thickening) and decrease in length (shortening) of different regions of myocardium can then be measured. The findings in five normal volunteers are presented. Radial shortening was twice as great in the endocardium (mean 20.4%, standard deviation (SD) 5.7) than in the epicardium (mean 10.2%, SD 5.5) and appears to offer more promise as a marker of regional function than simple thickening (mean 9.8%, SD 13.6).     

An efficient MR phosphorous spectroscopic localization technique for studying ischemic heart.

To obtain the spatially resolved (31)P spectroscopic image from myocardium during an acute myocardium ischemia at a high signal-to-noise ratio (SNR) in a very limited time window, we have exploited the spatial variation of the radiofrequency (RF) field produced by a single loop transmit/receive (TR) RF coil along its axis for spatial discrimination. By incrementally lengthening the duration of a square RF excitation pulse, the positional information can be systematically encoded as harmonics of various orders in MR signal. In the in vivo open-chest animal experiment, this RF coil was surgically sutured onto the epicardial surface of the left ventricular (LV) wall over the region perfused by the left anterior descending coronary artery. Using only 17 encoding steps, we have obtained one-dimensional (31)P spectroscopic images from both a multiple-layer phosphor phantom and an in vivo LV myocardium. In the animal study, the cardiac gating is used with respiratory synchronization. The MR data were only collected during the end diastole phase of the cardiac cycle (cardiac and respiratory synchronized) with an effective sequence repetition time (TR) of 6 seconds (to ensure the complete relaxation of the phosphorous magnetization). The total acquisition time for a complete experiment is about 10 minutes. Prior to the CSI reconstruction process, the raw data matrix was zero-filled in the spatial dimension. The spatially resolved metabolite map exhibited all the metabolite peaks including creatine phosphate and adenosine triphosphate. At the layer of endocardium, two peaks corresponding to 2, 3-diphosphoglycerate, which is contained in the erythrocytes, were clearly seen in the LV wall. Also, the method allows compensation in both volume and coil sensitivity variations for the resulting spectra. All results have demonstrated that it is an efficient nuclear magnetic resonance method capable of obtaining high-quality (31)P spectroscopic images with both excellent spatial localization and SNR in the research of cardiac ischemia. J. Magn. Reson. Imaging 1999;10:892-898.     

Compensation for spill-in and spill-out partial volume effects in cardiac PET imaging

Background

Partial volume effects (PVEs) in PET imaging result in incorrect regional activity estimates due to both spill-out and spill-in from activity in neighboring regions. It is important to compensate for both effects to achieve accurate quantification. In this study, an image-based partial volume compensation (PVC) method was developed and validated for cardiac PET.

Methods and Results

The method uses volume-of-interest (VOI) maps segmented from contrast-enhanced CTA images to compensate for both spill-in and spill-out in each VOI. The PVC method was validated with simulation studies and also applied to images of dog cardiac perfusion PET data. The PV effects resulting from cardiac motion and myocardial uptake defects were investigated and the efficacy of the proposed PVC method in compensating for these effects was evaluated.

Results

Results indicate that the magnitude and the direction of PVEs in cardiac imaging change over time. This affects the accuracy of activity distributions estimates obtained during dynamic studies. The defect regions have different PVEs as compared to the normal myocardium. Cardiac motion contributes around 10% to the PVEs. PVC effectively removed both spill-in and spill-out in cardiac imaging.

Conclusions

PVC improved left ventricular wall uniformity and quantitative accuracy. The best strategy for PVC was to compensate for the PVEs in each cardiac phase independently and treat severe uptake defects as independent regions from the normal myocardium.     

Differentiation of tumor from viable myocardium using cardiac tagging with MR imaging

We report the application of myocardial tagging by MR to define tissue planes and differentiate contractile from noncontractile tissue in a neonate with congenital cardiac rhabdomyoma. Using custom-written pulse programming software, six 2 mm thick radiofrequency (RF) slice-selective presaturation pulses (tags) were used to label the chest wall and myocardium in a star pattern in diastole, approximately 60 ms before the R-wave gating trigger. This method successfully delineated the myocardium from noncontractile tumor, providing information that influenced clinical management. This RF tagging technique allowed us to confirm the exact intramyocardial location of a congenital cardiac tumor.     

Mapping cyclic change of regional myocardial blood volume using steady-state susceptibility effect of iron oxide nanoparticles

PURPOSE: To demonstrate an in vivo magnetic resonance imaging (MRI) technique that maps the cyclic change of regional myocardial blood volume (MBV) during the cardiac cycle. MATERIALS AND METHODS: The method is based on the dominant T(2)* shortening effect of iron oxide nanoparticle-induced magnetic susceptibility perturbation in myocardium in the steady state. The technique was demonstrated in vivo with normal mouse hearts at 9.4 T. The regional MBV maps in left ventricular myocardium were computed from the steady-state pre- and post-monocrystalline iron oxide nanoparticle (MION) gradient echo (GE) cine images. Cyclic changes of MBV in normal mice were analyzed quantitatively in different transmural and angular locations. RESULTS: High-resolution MBV maps at various cardiac points were obtained. The study showed a general regional MBV decrease from end-diastole (ED) to end-systole (ES). Percentage reductions were 18.2 +/- 6.6%, P < 0.03 in the lateral wall and 24.7 +/- 3.1%, P < 0.0002 in the interventricular septum. The heterogeneous characteristics of MBV transmural distribution were also reported. CONCLUSION: The steady-state susceptibility effect of intravascular superparamagnetic contrast agent (CA) can be used to map the cyclic change of regional MBV. This imaging approach is relatively simple and may provide a new perspective for functional assessment of the microvasculature in myocardium.     

Cardiac SSFP imaging at 3 Tesla.

Balanced steady-state free precession (SSFP) techniques provide excellent contrast between myocardium and blood at a high signal-to-noise ratio (SNR). Hence, SSFP imaging has become the method of choice for assessing cardiac function at 1.5T. The expected improvement in SNR at higher field strength prompted us to implement SSFP at 3.0T. In this work, an optimized sequence protocol for cardiac SSFP imaging at 3.0T is derived, taking into account several partly adverse effects at higher field, such as increased field inhomogeneities, longer T(1), and power deposition limitations. SSFP contrast is established by optimizing the maximum amplitude of the radiofrequency (RF) field strength for shortest TR, as well as by localized linear or second-order shimming and local optimization of the resonance frequency. Given the increased SNR, sensitivity encoding (SENSE) can be employed to shorten breath-hold times. Short-axis, long-axis, and four-chamber cine views obtained in healthy adult subjects are presented, and three different types of artifacts are discussed along with potential methods for reducing them.     

True myocardial motion tracking

Myocardial tagging is a powerful tool for the assessment of in-plane cardiac motion. However, for previous myocardial tagging techniques, the imaged slice is fixed with respect to the magnet coordinate system. Thus, images acquired at different heart phases do not always represent the same slice of the myocardium. A new myocardial tagging technique is presented, which takes the through-plane motion into consideration. It involves tagging of the desired myocardial slice and applying a subtraction imaging technique to image just that part of the myocardium. The examination time can be reduced considerably by the acquisition of two one-dimensionally tagged images. To increase the signal-to-noise ratio especially at later heart phases, variable imaging RF excitation flip angles are applied. To reduce motion artifacts a repetitive breathhold scheme was applied. in vivo results demonstrate that the tags can be accurately tracked within the entire heart period with a temporal resolution of 35 ms, even at a top basal level of the heart and right ventricle.     

Optimized cardiac magnetic resonance imaging inversion recovery sequence for metal artifact reduction and accurate myocardial scar assessment in patients with cardiac implantable electronic devices

Late gadolinium enhancement (LGE) cardiovascular magnetic resonance (CMR) is the gold standard for imaging myocardial viability. An important application of LGE CMR is the assessment of the location and extent of the myocardial scar in patients with ventricular tachycardia (VT), which allows for more accurate identification of the ablation targets. However, a large percentage of patients with VT have cardiac implantable electronic devices (CIEDs), which is a relative contraindication for cardiac magnetic resonance imaging due to safety and image artifact concerns. Previous studies showed that these patients can be safely scanned on 1.5 T scanners provided that an adequate imaging protocol is adopted. Nevertheless, imaging patients with a CIED result in metal artifacts due to the strong frequency off-resonance effects near the device; therefore, the spins in the surrounding myocardium are not completely inverted, and thus give rise to hyperintensity artifacts. These artifacts obscure the myocardial scar tissue and limit the ability to study the correlation between the myocardial scar structure and the electro-anatomical map during catheter ablation. In this study, we developed a modified inversion recovery technique to alleviate the CIED-induced metal artifacts and improve the diagnostic image quality of LGE images in patients with CIEDs without increasing scan time or requiring additional hardware. The developed technique was tested in phantom experiments and in vivo scans, which showed its capability for suppressing the hyperintensity artifacts without compromising myocardium nulling in the resulting LGE images.     

Clinical utility of delayed-contrast computed tomography for tissue characterization of cardiac thrombus

Among patients that present with cardiac masses, thrombus is an important diagnostic consideration that affects both clinical management and prognosis. Although thrombus can occasionally be difficult to diagnose using structural criteria alone, it can be distinguished from other structures according to tissue characteristics. Because thrombus is inherently avascular, absence of contrast uptake was used as a highly specific identifying feature. Delayed-contrast cardiac computed tomography (CT) imaging has been previously used for myocardial tissue characterization, distinguishing between viable and infarcted myocardium based upon differences in contrast uptake. This technique also offers potential diagnostic utility for assessment of thrombus. In this report, we describe a case of a patient with a giant left atrial mass in whom delayed-contrast CT was employed as a useful diagnostic technique for identification of cardiac thrombus.     

Reduction in flow artifacts by using interleaved data acquisition in segmented balanced steady-state free precession cardiac MRI.

Balanced steady-state free precession (SSFP) magnetic resonance (MR) imaging is feasible for cine cardiac images because of the high contrast between myocardium and blood pool and robustness to rapid blood flow. Nonetheless, the flow artifacts are often observed because of off-resonance effects and to in-flow effects of the blood flow. Although reshimming the gradients or readjusting the center frequency reduces the artifacts, the technique can be susceptible for respiratory and cardiac motion and operator-dependent. The purpose of this study is to use another MR imaging technique for the reduction in the flow artifacts in the heart: odd-even interleaved data acquisition in segmented balanced SSFP imaging. The flow artifacts in the ventricle, ghost outside the heart, and visualization of the myocardial border were visually compared between sequential and odd-even interleaved k-space data acquisitions in cine balanced SSFP cardiac MR imaging. The odd-even interleaved k-space data acquisition significantly reduced dark flow artifacts in the left ventricle, improved the visualization of the myocardial border, and was easily installed. This imaging technique should be applied to cine segmented balanced SSFP cardiac MR imaging.     

Identification of viable myocardium in patients with chronic coronary artery disease: comparison of thallium-201 scintigraphy with reinjection and technetium-99m-methoxyisobutyl isonitrile.

We compared the results of 201Tl reinjection and those of 99mTc-methoxyisobutyl isonitrile (MIBI) in identifying viable myocardium in 20 male patients with angiographically proven coronary artery disease (CAD) and left ventricular dysfunction (ejection fraction 30% +/- 8%). All patients had irreversible defects on standard exercise-redistribution thallium imaging. Thallium was reinjected immediately after the redistribution study, and images were reacquired. The patients also underwent stress and rest 99mTc-MIBI myocardial scintigraphy (2-day protocol). A total of 300 myocardial regions were analyzed, of which 122 (41%) had irreversible thallium defects on redistribution images before reinjection. Of the 122 myocardial regions with irreversible defects on standard stress-redistribution thallium cardiac imaging, 65 (53%) did not change at reinjection and 57 (47%) demonstrated enhanced uptake of thallium after reinjection. Of the same 122 irreversible defects on stress-redistribution thallium, 100 (82%) appeared as fixed defects and 22 (18%) were reversible on 99mTc-MIBI myocardial scintigraphy. These data indicate that 201Tl cardiac imaging with rest reinjection is superior to 99mTc-MIBI myocardial scintigraphy in identifying viable myocardium in patients with chronic CAD, suggesting that regions with severe reduction of 99mTc-MIBI uptake both on stress and rest images may contain viable myocardium.     

Toward high-resolution myocardial tagging.

Magnetic resonance imaging with preceding tissue tagging is a robust method for assessing cardiac motion of the entire heartbeat cycle with a high degree of accuracy. One limitation of this technique, however, is the low resolution of the obtained displacement map of the labeled points within the myocardium. By a new tagging technique, which is based on the combination of two or more measurements of the same slice but with different grid positions, a highly improved resolution of cardiac motion data can be achieved. In combination with a multi-heart-phase echo-planar imaging sequence, such images with doubled grid frequency can be acquired in two short breath-hold periods.     

Thallium 201 for assessment of myocardial viability

Left ventricular (LV) performance is reduced in a large subset of patients with chronic coronary artery disease (CAD) and LV dysfunction on the basis of regionally ischemic or hibernating myocardium rather than irreversibly infarcted tissue. The detection of dysfunctional but viable myocardium is clinically relevant since regional and global LV function in such patients will improve after revascularization procedures; however, the identification of patients with such potentially reversible LV dysfunction is difficult. Although thallium 201 imaging may be of value in detecting viable myocardium if regions with perfusion defects during exercise demonstrate redistribution of thallium on a 3- to 4-hour resting image, thallium defects often appear persistently "fixed" within regions of severely ischemic or hibernating myocardium. It has been shown that up to 50% of regions with apparently irreversible thallium defects will improve in function after revascularization. Thus, standard exercise-redistribution thallium scintigraphy may not differentiate LV dysfunction arising from infarcted versus hibernating myocardium. The precision with which thallium imaging identifies viable myocardium can be improved greatly by additional studies once 4-hour redistribution imaging demonstrates an irreversible thallium defect. These additional studies include late (24-hour) redistribution imaging, repeat imaging after thallium reinjection, or a combination of thallium reinjection followed by late imaging. Several recent studies suggest that thallium reinjection techniques, by demonstrating thallium uptake in dysfunctional regions with apparently irreversible defects, predict improvement after revascularization with similar predictive accuracy as that achieved using metabolic imaging with positron emission tomography (PET). Studies directly comparing such thallium methods and PET, which thus far involve only small numbers of patients, suggest that the assessment of regional metabolic activity using PET and the assessment of regional thallium activity using single photon emission computed tomography provide concordant results. These findings, if confirmed by larger ongoing studies, suggest that thallium reinjection imaging is a convenient, clinically accurate, and relatively inexpensive method with which to identify viable myocardium in patients with chronic CAD and LV dysfunction.     

Proton nuclear magnetic resonance imaging of regionally ischemic canine hearts: effect of paramagnetic proton signal enhancement

In a study to evaluate the potential of proton nuclear magnetic resonance (NMR) imaging with and without manganese contrast with and without manganese contrast enhancement for detecting acute myocardial infarction, 12 dogs underwent 90-minute occlusion of the left circumflex coronary artery. Transverse-section NMR images of the excised, nonbeating heart were obtained at 1-cm intervals using the steady-state-free-precession (SSFP) technique. All NMR images revealed detailed structure of the heart. The three hearts without manganese showed no difference in intensity between the normal and the ischemic posterior regions, whereas those with manganese demonstrated a clearly demarcated zone of reduced signal intensity consistent with the ischemic zone. It is concluded that high-resolution tomograms of the excised canine myocardium can be obtained using proton NMR imaging. With the SSFP imaging technique, proton signal enhancement with manganese infusion is necessary to differentiate between ischemic and nonischemic myocardium after 90 minutes of coronary occlusion.     

Adenosine-stress dynamic myocardial volume perfusion imaging with second generation dual-source computed tomography: Concepts and first experiences

Recent research suggests that multidetector-row CT may have potential as a standalone modality for integrative imaging of coronary heart disease, including the assessment of the myocardial blood supply. However, the technical prerequisites for volumetric, time-resolved imaging of the passage of a contrast medium bolus through the myocardium have only been met with latest generation wide-detector CT scanners. Second-generation dual-source CT enables performing electrocardiographic (ECG)–synchronized dynamic myocardial perfusion imaging by a dedicated “shuttle” mode. With this acquisition mode, image data can be acquired during contrast medium infusion at 2 alternating table positions with the table shuttling back and forth between the 2 positions covering a 73-mm anatomic volume. We applied this acquisition technique for detecting differences in perfusion patterns between healthy and diseased myocardium and for quantifying myocardial blood flow under adenosine stress in 3 patients with coronary heart disease. According to our initial experience, the addition of adenosine stress volumetric dynamic CT perfusion to a cardiac CT protocol comprising coronary artery calcium quantification, prospectively ECG-triggered coronary CT angiography, and delayed acquisition appears promising for the comprehensive assessment of coronary artery luminal integrity, cardiac function, perfusion, and viability with a single modality.     

Design,evaluation and application of an eight channel transmit/receive coil array for cardiac MRI at 7.0 T

The objective of this work is to design, examine and apply an eight channel transmit/receive coil array tailored for cardiac magnetic resonance imaging at 7.0 T that provides image quality suitable for clinical use, patient comfort, and ease of use. The cardiac coil array was designed to consist of a planar posterior section and a modestly curved anterior section. For radio frequency (RF) safety validation, numerical computations of the electromagnetic field (EMF) and the specific absorption rate (SAR) distribution were conducted. In vivo cardiac imaging was performed using a 2D CINE FLASH technique. For signal-to-noise ratio (SNR) assessment reconstructed images were scaled in SNR units. The parallel imaging capabilities of the coil were examined using GRAPPA and SENSE reconstruction with reduction factors of up to R = 4. The assessment of the RF characteristics yielded a maximum noise correlation of 0.33. The baseline SNR advantage at 7.0 T was put to use to acquire 2D CINE images of the heart with a spatial resolution of 1 mm × 1 mm × 4 mm. The coil array supports 1D acceleration factors of up to R = 3 without impairing image quality significantly. For un-accelerated 2D CINE FLASH acquisitions the results revealed an SNR of approximately 140 for the left ventricular blood pool. Blood/myocardium contrast was found to be approximately 90 for un-accelerated 2D CINE FLASH acquisitions. The proposed 8 channel cardiac transceiver surface coil has the capability to acquire high contrast, high spatial and temporal resolution in vivo images of the heart at 7.0 T.     

Severity of aortic regurgitation: interstudy reproducibility of measurements with velocity-encoded cine MR imaging.

The interstudy reproducibility of velocity-encoded cine (VEC) magnetic resonance (MR) imaging for quantification of regurgitant volume (RV) and regurgitant fraction (RF) was studied in 10 patients with chronic aortic regurgitation. Each patient underwent two VEC MR imaging studies. RV and RF were measured on the aortic flow curve by quantifying antegrade and retrograde flow per cardiac cycle. VEC MR imaging measurements for RV and RF correlated closely with volumetric measurements for both studies (r greater than .97). Interstudy reproducibility for VEC MR imaging measurement of RV and RF was high (r greater than .97), and the interstudy variability for VEC MR imaging measurements was low. These results demonstrate a high accuracy of VEC MR imaging for measurement of RV and RF in patients with chronic aortic regurgitation. The level of interstudy reproducibility of VEC MR imaging for quantitative assessment of RV and RF indicates the potential of this technique for follow-up and monitoring of response to therapy.     

Single-breathhold 3D-trueFISP cine cardiac imaging.

Cardiac MRI function measurements are typically based on multiple breathhold 2D sequences to acquire images of the entire heart. In the present study, the feasibility of a cine 3D TrueFISP technique in which several complete volumetric measurements may be obtained during a single breathhold is demonstrated. In contrast to 3D FLASH, the TrueFISP sequence offers an excellent contrast between the myocardium and the intraventricular cavity without the use of contrast agent. An ECG-gated 3D cine TrueFISP sequence was implemented with a repetition time of 2.4-2.8 ms, which allows imaging of the complete heart within a single breathhold throughout 20-46 heartbeats with a 3D frame rate of 8-13 volumes per cardiac cycle and a spatial resolution of about 1.5 x 3.5 x 3.5 mm(3). Breathhold volumetric cine imaging with the 3D TrueFISP technique holds promise for rapid and accurate evaluation of the cardiac regional wall motion and the calculation of cardiac volume and ejection fraction.