Neville Gai

T1 Mapping in Cardiomyopathy at Cardiac MR: Comparison with Endomyocardial Biopsy

PURPOSE To determine the utility of cardiac magnetic resonance (MR) T1 mapping for quantification of diffuse myocardial fibrosis compared with the standard of endomyocardial biopsy. MATERIALS AND METHODS This HIPAA-compliant study was approved by the institutional review board. Cardiomyopathy patients were retrospectively identified who had undergone endomyocardial biopsy and cardiac MR at one institution during a 5-year period. Forty-seven patients (53% male; mean age, 46.8 years) had undergone diagnostic cardiac MR and endomyocardial biopsy. Thirteen healthy volunteers (54% male; mean age, 38.1 years) underwent cardiac MR as a reference. Myocardial T1 mapping was performed 10.7 minutes ± 2.7 (standard deviation) after bolus injection of 0.2 mmol/kg gadolinium chelate by using an inversion-recovery Look-Locker sequence on a 1.5-T MR imager. Late gadolinium enhancement was assessed by using gradient-echo inversion-recovery sequences. Cardiac MR results were the consensus of two radiologists who were blinded to histopathologic findings. Endomyocardial biopsy fibrosis was quantitatively measured by using automated image analysis software with digital images of specimens stained with Masson trichrome. Histopathologic findings were reported by two pathologists blinded to cardiac MR findings. Statistical analyses included Mann-Whitney U test, analysis of variance, and linear regression. RESULTS Median myocardial fibrosis was 8.5% (interquartile range, 5.7-14.4). T1 times were greater in control subjects than in patients without and in patients with evident late gadolinium enhancement (466 msec ± 14, 406 msec ± 59, and 303 msec ± 53, respectively; P < .001). T1 time and histologic fibrosis were inversely correlated (r = -0.57; 95% confidence interval: -0.74, -0.34; P < .0001). The area under the curve for myocardial T1 time to detect fibrosis of greater than 5% was 0.84 at a cutoff of 383 msec. CONCLUSION Cardiac MR with T1 mapping can provide noninvasive evidence of diffuse myocardial fibrosis in patients referred for evaluation of cardiomyopathy.

T1 Mapping of the Gadolinium-Enhanced Myocardium: Adjustment for Factors Affecting Interpatient Comparison

Quantitative T1 mapping of delayed gadolinium‐enhanced cardiac magnetic resonance imaging has shown promise in identifying diffuse myocardial fibrosis. Despite careful control of magnetic resonance imaging parameters, comparison of T1 times between different patients may be problematic because of patient specific factors such as gadolinium dose, differing glomerular filtration rates, and patient specific delay times. In this work, a model driven approach to account for variations between patients to allow for comparison of T1 data is provided. Kinetic model parameter values were derived from healthy volunteer time‐contrast curves. Correction values for the factors described above were used to normalize T1 values to a matched state. Examples of pre‐ and postcorrected values for a pool of normal subjects and in a patient cohort of type 1 diabetic patients shows tighter clustering and improved discrimination of disease state. Magn Reson Med, 2011.

Myocardial T1 Mapping with MRI: comparison of Look-Locker and MOLLI sequences

To evaluate the relationship between “Look‐Locker” (LL) and modified Look‐Locker Inversion recovery (MOLLI) approaches for T1 mapping of the myocardium.

Cardiac magnetic resonance T1 mapping of left atrial myocardium.

BACKGROUND Cardiac magnetic resonance (CMR) T1 mapping is an emerging tool for objective quantification of myocardial fibrosis. OBJECTIVES To (a) establish the feasibility of left atrial (LA) T1 measurements, (b) determine the range of LA T1 values in patients with atrial fibrillation (AF) vs healthy volunteers, and (c) validate T1 mapping vs LA intracardiac electrogram voltage amplitude measures. METHODS CMR imaging at 1.5 T was performed in 51 consecutive patients before AF ablation and in 16 healthy volunteers. T1 measurements were obtained from the posterior LA myocardium by using the modified Look-Locker inversion-recovery sequence. Given the established association of reduced electrogram amplitude with fibrosis, intracardiac point-by-point bipolar LA voltage measures were recorded for the validation of T1 measurements. RESULTS The median LA T1 relaxation time was shorter in patients with AF (387 [interquartile range 364-428] ms) compared to healthy volunteers (459 [interquartile range 418-532] ms; P < .001) and was shorter in patients with AF with prior ablation compared to patients without prior ablation (P = .035). In a generalized estimating equations model, adjusting for data clusters per participant, age, rhythm during CMR, prior ablation, AF type, hypertension, and diabetes, each 100-ms increase in T1 relaxation time was associated with 0.1 mV increase in intracardiac bipolar LA voltage (P = .025). CONCLUSIONS Measurement of the LA myocardium T1 relaxation time is feasible and strongly associated with invasive voltage measures. This methodology may improve the quantification of fibrotic changes in thin-walled myocardial tissues.

Modified Look-Locker T1 evaluation using Bloch simulations: human and phantom validation.

Modified Look‐Locker imaging is frequently used for T1 mapping of the myocardium. However, the specific effect of various MRI parameters (e.g., encoding scheme, modifications of flip angle, heart rate, T2, and inversion times) on the accuracy of T1 measurement has not been studied through Bloch simulations. In this work, modified Look‐Locker imaging was characterized through a numerical solution for Bloch equations. MRI sequence parameters that may affect T1 accuracy were systematically varied in the simulation. For validation, phantoms were constructed with various T2 and T1 times and compared with Bloch equation simulations. Human volunteers were also evaluated with various pulse sequences parameters to assess the validity of the numerical simulations. There was close agreement between simulated T1 times and T1 times measured in phantoms and volunteers. Lower T2 times (i.e., <30 ms) resulted in errors greater than 5% for T1 determination. Increasing maximum inversion time value improved T1 accuracy particularly for precontrast myocardial T1. Balanced steady‐state free precession k space centric encoding improved accuracy for short T1 times (post gadolinium), but linear encoding provided improved accuracy for precontrast T1 values. Lower flip angles are preferred if the signal‐to‐noise ratio is sufficiently high. Bloch simulations for modified Look‐Locker imaging provide an accurate method to comprehensively quantify the effect of pulse sequence parameters on T1 accuracy. As an alternative to otherwise lengthy phantom studies or human studies, such simulations may be useful to optimize the modified Look‐Locker imaging sequence and compare differences in T1‐derived measurements from different scanners or institutions. Magn Reson Med, 2013.

Modified look-locker inversion recovery T1 mapping indices: assessment of accuracy and reproducibility between magnetic resonance scanners.

BackgroundCardiovascular magnetic resonance (CMR) T1 mapping indices, such as T1 time and partition coefficient (λ), have shown potential to assess diffuse myocardial fibrosis. The purpose of this study was to investigate how scanner and field strength variation affect the accuracy and precision/reproducibility of T1 mapping indices.MethodsCMR studies were performed on two 1.5T and three 3T scanners. Eight phantoms were made to mimic the T1/T2 of pre- and post-contrast myocardium and blood at 1.5T and 3T. T1 mapping using MOLLI was performed with simulated heart rate of 40-100 bpm. Inversion recovery spin echo (IR-SE) was the reference standard for T1 determination. Accuracy was defined as the percent error between MOLLI and IR-SE, and scan/re-scan reproducibility was defined as the relative percent mean difference between repeat MOLLI scans. Partition coefficient was estimated by ΔR1myocardium phantom/ΔR1blood phantom. Generalized linear mixed model was used to compare the accuracy and precision/reproducibility of T1 and λ across field strength, scanners, and protocols.ResultsField strength significantly affected MOLLI T1 accuracy (6.3% error for 1.5T vs. 10.8% error for 3T, p<0.001) but not λ accuracy (8.8% error for 1.5T vs. 8.0% error for 3T, p=0.11). Partition coefficients of MOLLI were not different between two 1.5T scanners (47.2% vs. 47.9%, p=0.13), and showed only slight variation across three 3T scanners (49.2% vs. 49.8% vs. 49.9%, p=0.016). Partition coefficient also had significantly lower percent error for precision (better scan/re-scan reproducibility) than measurement of individual T1 values (3.6% for λ vs. 4.3%-4.8% for T1 values, approximately, for pre/post blood and myocardium values).ConclusionBased on phantom studies, T1 errors using MOLLI ranged from 6-14% across various MR scanners while errors for partition coefficient were less (6-10%). Compared with absolute T1 times, partition coefficient showed less variability across platforms and field strengths as well as higher precision.

Assessment of Cardiac Involvement in Myotonic Muscular Dystrophy by T1 Mapping on Magnetic Resonance Imaging

BACKGROUND Patients with myotonic muscular dystrophy (DM) are at risk for atrioventricular block and left ventricular (LV) dysfunction. Noninvasive detection of diffuse myocardial fibrosis may improve disease management in this population. OBJECTIVE To define functional and postcontrast myocardial T1 time cardiac magnetic resonance characteristics in patients with DM. METHODS Thirty-three patients with DM (24 with type 1 and 9 with type 2) and 13 healthy volunteers underwent cardiac magnetic resonance for the assessment of LV indices and the evaluation of diffuse myocardial fibrosis by T1 mapping. The association of myocardial T1 time with electrocardiogram abnormalities and LV indices was examined among patients with DM. RESULTS Patients with DM had lower end-diastolic volume index (68.9 mL/m(2) vs 60.3 mL/m(2); P = .045) and cardiac index (2.7 L/min/m(2) vs 2.33 L/min/m(2); P = .005) and shorter myocardial T1 time (394.5 ms vs 441.4 ms; P < .0001) than did control subjects. Among patients with DM, there was a positive association between higher T1 time and LV mass index (2.2 ms longer per g/m(2); P = .006), LV end-diastolic volume index (1.3 ms longer per mL/m(2); P = .026), filtered QRS duration (1.2 ms longer per unit; P = .005), and low-amplitude (<40 mcV) late-potential duration (0.9 ms longer per unit; P = .01). Using multivariate random effects regression, each 10-ms increase in myocardial T1 time of patients with type 1 DM was independently associated with 1.3-ms increase in longitudinal PR and QRS intervals during follow-up. CONCLUSIONS DM is associated with structural alterations on cardiac magnetic resonance. Postcontrast myocardial T1 time was shorter in patients with DM than in controls, likely reflecting the presence of diffuse myocardial fibrosis.

Myofiber Architecture of the Human Atria as Revealed by Submillimeter Diffusion Tensor Imaging

Background—Accurate knowledge of the human atrial fibrous structure is paramount in understanding the mechanisms of atrial electric function in health and disease. Thus far, such knowledge has been acquired from destructive sectioning, and there is a paucity of data about atrial fiber architecture variability in the human population. Methods and Results—In this study, we have developed a customized 3-dimensional diffusion tensor magnetic resonance imaging sequence on a clinical scanner that makes it possible to image an entire intact human heart specimen ex vivo at submillimeter resolution. The data from 8 human atrial specimens obtained with this technique present complete maps of the fibrous organization of the human atria. The findings demonstrate that the main features of atrial anatomy are mostly preserved across subjects although the exact location and orientation of atrial bundles vary. Using the full tractography data, we were able to cluster, visualize, and characterize the distinct major bundles in the human atria. Furthermore, quantitative characterization of the fiber angles across the atrial wall revealed that the transmural fiber angle distribution is heterogeneous throughout different regions of the atria. Conclusions—The application of submillimeter diffusion tensor magnetic resonance imaging provides an unprecedented level of information on both human atrial structure, as well as its intersubject variability. The high resolution and fidelity of this data could enhance our understanding of structural contributions to atrial rhythm and pump disorders and lead to improvements in their targeted treatment.

Fat-Corrected T2 Measurement as a Marker of Active Muscle Disease in Inflammatory Myopathy

OBJECTIVE We sought to improve the utility of T2 measurement as a marker of active muscle disease in patients with idiopathic inflammatory myopathy by correcting for T2 prolongations caused by fatty replacement of muscle that accompnaies chronic muscle damage. SUBJECTS AND METHODS Twenty-one patients with idiopathic inflammatory myopathy underwent a standardized MRI evaluation of the thighs. Fat fraction maps were calculated from dual-echo gradient-echo images. Fat-corrected T2 maps were generated from multiecho spin-echo images on the basis of a biexponential model that incorporated voxelwise fat fraction estimates. Semiautomated summaries of conventional and fat-corrected muscle T2 values were compared with one another and with standardized visual scores of muscle disease based on T1-weighted spin-echo and STIR images. RESULTS Fat-corrected muscle T2 maps showed lower mean values and greater histogram entropy than conventional T2 maps, as analyzed over a standardized portion of the thigh muscles. Conventional and fat-corrected T2 values correlated with visual scores of active muscle disease on STIR images and with the varying intensity of disease depicted with STIR in focal muscle regions. CONCLUSION MRI T2 maps of muscle can be corrected for varying fat content by combining the information from chemical shift-sensitive gradient-echo and multiecho spin-echo images. Use of this strategy may prove useful in the study of idiopathic inflammatory myopathy and other diseases characterized by both muscle inflammation and atrophy.

Axial Scan Orientation and the Tibial Tubercle-Trochlear Groove Distance: Error Analysis and Correction

OBJECTIVE The tibial tubercle (TT)-trochlear groove (TG) distance is an important metric in the assessment of patellofemoral dysfunction and is routinely measured on axial MRI and CT. This study examines error in measurements of the TT-TG distance related to variance in axial MRI scan orientation. SUBJECTS AND METHODS Isotropic 3D turbo spin-echo MRI of the extended knee was performed in 12 healthy subjects. The z-axis of the scanner defines the perpendicular to a routine axial plane, and the anatomic axial plane is parallel to the knee joint. Isotropic MRI was reformatted into routine and anatomic axial planes and in axial planes simulating 5° of femoral adduction and abduction relative to the anatomic plane. A method for correcting the TT-TG distance to account for variable axial scan orientation is presented. RESULTS Five degrees of simulated femoral abduction is associated with a mean increase in the TT-TG distance of 38% (SD = 17%), whereas 5° of simulated femoral adduction is associated with a mean decrease in the TT-TG distance of 51% (SD = 39%). The average deviation of the routine axial plane from the anatomic axial plane was 5.0° abduction (SD = 2.3°). The simplest correction method reduced the mean discrepancy in the observed TT-TG distance by 68% and 72% in simulated femoral abduction and adduction, respectively. CONCLUSION The TT-TG distance is sensitive to small changes in femoral alignment and should be interpreted with caution if axial image acquisition is not standardized. Knowing the vertical separation of the TT from the TG facilitates a simplified correction of the TT-TG distance, which is as effective as more complex corrections.