Thomas K. F. Foo

Infarct Tissue Heterogeneity by Magnetic Resonance Imaging Identifies Enhanced Cardiac Arrhythmia Susceptibility in Patients With Left Ventricular Dysfunction

Background— The extent of the peri-infarct zone by magnetic resonance imaging (MRI) has been related to all-cause mortality in patients with coronary artery disease. This relationship may result from arrhythmogenesis in the infarct border. However, the relationship between tissue heterogeneity in the infarct periphery and arrhythmic substrate has not been investigated. In the present study, we quantify myocardial infarct heterogeneity by contrast-enhanced MRI and relate it to an electrophysiological marker of arrhythmic substrate in patients with left ventricular (LV) systolic dysfunction undergoing prophylactic implantable cardioverter defibrillator placement. Methods and Results— Before implantable cardioverter defibrillator implantation for primary prevention of sudden cardiac death, 47 patients underwent cine and contrast-enhanced MRI to measure LV function, volumes, mass, and infarct size. A method for quantifying the heterogeneous infarct periphery and the denser infarct core is described. MRI indices were related to inducibility of sustained monomorphic ventricular tachycardia during electrophysiological or device testing. For the noninducible versus inducible patients, LV ejection fraction (30±10% versus 29±7%, P=0.79), LV end-diastolic volume (220±70 versus 228±57 mL, P=0.68), and infarct size by standard contrast-enhanced MRI definitions (P=NS) were similar. Quantification of tissue heterogeneity at the infarct periphery was strongly associated with inducibility for monomorphic ventricular tachycardia (noninducible versus inducible: 13±9 versus 19±8 g, P=0.015) and was the single significant factor in a stepwise logistic regression. Conclusions— Tissue heterogeneity is present and quantifiable within human infarcts. More extensive tissue heterogeneity correlates with increased ventricular irritability by programmed electrical stimulation. These findings support the hypothesis that anatomic tissue heterogeneity increases susceptibility to ventricular arrhythmias in patients with prior myocardial infarction and LV dysfunction.

Cardiac MRI: Recent progress and continued challenges

Cardiac MRI continues to develop and advance. MRI accurately depicts cardiac structure, function, perfusion, and myocardial viability with an overall capacity unmatched by any other single imaging modality. MRI is an accepted and widely utilized tool for cardiovascular research. Its clinical use has been limited, but is increasing because of its proven clinical efficacy, the proliferation of cardiac‐capable MRI systems, and the development of improved pulse sequences. The following article reviews the landmark developments in this field, with an emphasis on recent progress in the evaluation of ischemic or acquired heart disease. J. Magn. Reson. Imaging 2002;16:111–127. Published 2002 Wiley‐Liss, Inc.

Optimization of Gadolinium-Enhanced Magnetic Resonance Angiography Using an Automated Bolus-Detection Algorithm (MR SmartPrep)y

Gadolinium (Gd)-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) is a quick method for performing noninvasive diagnostic angiography. A major technical obstacle to the successful implementation of this technique, however, is the proper coordination of the data acquisition with th

Fast, three-dimensional free-breathing MR imaging of myocardial infarction: A feasibility study

Imaging delayed hyperenhancement of myocardial infarction is most commonly performed using an inversion recovery (IR) prepared 2D breathhold segmented k‐space gradient echo (FGRE) sequence. Since only one slice is acquired per breathhold in this technique, 12–16 successive breathholds are required for complete anatomical coverage of the heart. This prolongs the overall scan time and may be exhausting for patients. A navigator‐echo gated, free‐breathing, 3D FGRE sequence is proposed that can be used to acquire a single slab covering the entire heart with high spatial resolution. The use of a new variable sampling in time (VAST) acquisition scheme enables the entire 3D volume to be acquired in 1.5–2 min, minimizing artifacts from bulk motion and diaphragmatic drift and contrast variations due to contrast media washout. Magn Reson Med 51:1055–1060, 2004.

Fast method for correcting image misregistration due to organ motion in time-series MRI data

Time‐series MRI data often suffers from image misalignment due to patient movement and respiratory and other physiologic motion during the acquisition process. It is necessary that this misalignment be corrected prior to any automated quantitative analysis. In this article a fast and automated technique for removing in‐plane misalignment from time‐series MRI data is presented. The method is computationally efficient, robust, and fine‐tuned for the clinical setting. The method was implemented and tested on data from 21 human subjects, including myocardial perfusion imaging, renal perfusion imaging, and blood‐oxygen level‐dependent cardiac T  2* imaging. In these applications 10‐fold or better reduction in image misalignment is reported. The improvement after registration on representative time–intensity curves is shown. Although the method currently corrects translation motion using image center of mass, the mathematical framework of our approach may be extended to correct rotation and other higher‐order displacements. Magn Reson Med 49:506–514, 2003.

Measurement of Coronary Blood Flow and Flow Reserve Using Magnetic Resonance Imaging

PURPOSE It was the purpose of this study to demonstrate the feasibility of performing coronary artery flow and coronary flow reserve (CFR) measurements in normal human volunteers using a magnetic resonance (MR) phase contrast technique. MATERIALS AND METHODS Coronary flow rate, flow velocity, peak flow and CFR were determined at rest and during pharmacologically induced hyperemia in 10 healthy volunteers. The flow measurements were obtained during a single breath-hold by using a fast, prospectively gated, segmented k-space gradient-echo phase contrast acquisition with view sharing (FASTCARD PC) that was modified to improve sampling of the diastolic flow. Data were processed using the standard phase difference (PD) processing techniques as well as a new complex difference (CD) flow measurement method intended to improve the accuracy of flow measurements in small vessels. RESULTS Mean hyperemic flow velocity (40 +/- 16 cm/s) and blood flow (3.9 +/- 1.5 ml/s) rates differed significantly from resting velocity (13 +/- 6.6 cm/s) and flow (1.1 +/- 0.4 ml/s) measurements (p < 0.0001). PD methods consistently measured larger flow rates at rest (24% larger, p < 0.0005) and stress (29% larger, p < 0.0001). CFR, calculated as the ratio of the mean PD flows (4.7 +/- 2.8), was higher than CFR calculated as the ratio of mean CD flows (4.2 +/- 1.8); however, the differences did not reach statistical significance (p = 0.07). Flow measurements performed in adjacent slices of the same vessel correlated well (r = 0.88). CONCLUSIONS Coronary flow and CFR measurements using the MR techniques are feasible and are similar to those reported in the literature for healthy volunteers.

The apparent inversion time for optimal delayed enhancement magnetic resonance imaging differs between the right and left ventricles.

BACKGROUND Delayed post-contrast magnetic resonance (MR) imaging involves suppression of signal from myocardium using inversion times (TI) between 150-225 ms, when the myocardium appears dark and fibrotic scar appears bright. We noticed that at a TI optimized for signal suppression of the left ventricle (LV), the right ventricle (RV) appeared brighter. PURPOSE The purpose of this study was to evaluate the TI for signal suppression in RV compared to LV, and to try and identify the cause of this observation. Methods. We studied 31 patients (ages ranged from 17-79 years, 11 females) who had an MR scan on a 1.5 T GE scanner. Delayed post-contrast short-axis images were obtained 20 minutes after injection of 0.2 mmol/kg of intravenous gadolinium chelate. TI optimization was performed by acquiring a range of TI times within a single breath hold, in increments of 25 msec. The TI time that resulted in lowest signal for the RV arid LV was recorded. RESULTS With the imaging sequence employed, the TI leading to LV signal suppression ranged from 150-225 ms. At the TI that resulted in LV signal suppression, the corrected signal from the RV was significantly higher as compared to the LV (29 +/- 13 au vs. 15 +/- 8 au, p < 0.001). The findings were similar using only the body coil. The TI required to suppress the RV was usually < or =150 msec. The observation persisted before and after gadolinium infusion. CONCLUSION The TI for myocardial signal suppression appears to be different between LV and RV. Potential mechanisms include partial volume averaging with fat or blood pool (related to increased trabeculation) in the RV. Alternatively, increased blood pool signal (within Thebesian veins or arterioluminal communications) in RV compared to LV leads to altered TI times due to similar partial volume effects.

Short breath-hold, volumetric coronary MR angiography employing steady-state free precession in conjunction with parallel imaging

An ECG‐gated, 3D steady‐state free precession (SSFP) technique in conjunction with sensitivity encoding (SENSE)‐based parallel imaging was implemented for short breath‐hold, volumetric coronary MR angiograpy (CMRA). Two parallel imaging acquisition strategies (employing 1 R‐R and 2 R‐R intervals, respectively) were developed to achieve 1) very short breath‐hold times (12 s for a heart rate of 60 bpm), and 2) small acquisition windows to minimize sensitivity to physiologic motion. Both strategies were examined in CMRA applications over a range of heart rates. A four‐point scale blinded reading (with 4 indicating the most desirable features) revealed substantial image quality improvements for the accelerated data as compared to the nonaccelerated approach. The 1 R‐R interval scheme yielded an image score of 3.39 ± 0.60, and was found to be particularly suitable for low heart rates (P = 0.0008). The 2 R‐R interval strategy yielded an image score of 3.35 ± 0.64, and was more appropriate for higher heart rates (P = 0.03). The results demonstrate that 3D SSFP combined with parallel imaging is a versatile method for short breath‐hold CMRA while maintaining high spatial resolution. This strategy permits imaging of the major coronary artery distributions in two to three breath‐holds using targeted slabs, and offers the potential for single breath‐hold, large‐volume CMRA. Magn Reson Med 53:885–894, 2005.

High-resolution gadolinium-enhanced 3D MRA of the infrapopliteal arteries: Lessons for improving bolus-chase peripheral MRA

Peripheral magnetic resonance angiography (MRA) is growing in use. However, methods of performing peripheral MRA vary widely and continue to be optimized, especially for improvement in illustration of infrapopliteal arteries. The main purpose of this project was to identify imaging factors that can improve arterial visualization in the lower leg using bolus chase peripheral MRA. Eighteen healthy adults were imaged on a 1.5T MR scanner. The calf was imaged using conventional three-station bolus chase three-dimensional (3D) MRA, two dimensional (2D) time-of-flight (TOF) MRA and single-station Gadolinium (Gd)-enhanced 3D MRA. Observer comparisons of vessel visualization, signal to noise ratios (SNR), contrast to noise ratios (CNR) and spatial resolution comparisons were performed. Arterial SNR and CNR were similar for all three techniques. However, arterial visualization was dramatically improved on dedicated, arterial-phase Gd-enhanced 3D MRA compared with the multi-station bolus chase MRA and 2D TOF MRA. This improvement was related to optimization of Gd-enhanced 3D MRA parameters (fast injection rate of 2 mL/sec, high spatial resolution imaging, the use of dedicated phased array coils, elliptical centric k-space sampling and accurate arterial phase timing for image acquisition). The visualization of the infrapopliteal arteries can be substantially improved in bolus chase peripheral MRA if voxel size, contrast delivery, and central k-space data acquisition for arterial enhancement are optimized. Improvements in peripheral MRA should be directed at these parameters.

Detection of pulmonary embolism: comparison of contrast-enhanced spiral CT and time-of-flight MR techniques.

We compared the conspicuity of acute pulmonary emboli with contrast-enhanced spiral computed tomography (CT) and two- and three-dimensional time-of-flight magnetic resonance (MR) techniques. Seven dogs who received experimental pulmonary emboli and one control were imaged with spiral CT and with 2-D (FMPVAS and FASTCARD) and 3-D time-of-flight MR. Blinded, independent, prospective evaluations of the CT and MR images by two MR radiologists and two chest radiologists were then compared to the location of the emboli as determined by subsequent pathologic evaluation of the excised lungs. Embolus/blood contrast-to-noise ratios (CNRs) were calculated on both MR and CT images for pulmonary emboli that could be identified. Fifty emboli ranging from 1.0 to 5.5 mm (mean, 2.7, ±0.14 SEM) in diameter and from 3.0 to 60 mm (mean, 28.1, ±1.9 SEM) in length were found in the seven embolized dogs on pathologic examination. Three of the four radiologists identified more thrombi on CT images than they did on their best MR pulse sequence (FASTCARD) and with greater confidence. The fourth radiologist identified an equal percentage of clot on CT and FASTCARD images with confidence slightly greater on FASTCARD MR than on spiral CT. Mean CNR for the best MR technique was 43.4 (±3.9 SEM) and for CT was 20.7 (± 1.3 SEM). In general, pulmonary emboli were detected more accurately on contrast-enhanced spiral CT than on MR. This occurred although the embolus/blood CNR was higher on MR than on CT. Better pulmonary embolus conspicuity on CT images was attributed to better spatial resolution and fewer artifacts on CT than on MR. One MR radiologist performed equally well with both spiral CT and FASTCARD techniques, suggesting that experience may be a factor in performance.