Standardized image interpretation and post processing in cardiovascular magnetic resonance: Society for Cardiovascular Magnetic Resonance (SCMR) Board of Trustees Task Force on Standardized Post Processing

With mounting data on its accuracy and prognostic value, cardiovascular magnetic resonance (CMR) is becoming an increasingly important diagnostic tool with growing utility in clinical routine. Given its versatility and wide range of quantitative parameters, however, agreement on specific standards for the interpretation and post-processing of CMR studies is required to ensure consistent quality and reproducibility of CMR reports. This document addresses this need by providing consensus recommendations developed by the Task Force for Post Processing of the Society for Cardiovascular MR (SCMR). The aim of the task force is to recommend requirements and standards for image interpretation and post processing enabling qualitative and quantitative evaluation of CMR images. Furthermore, pitfalls of CMR image analysis are discussed where appropriate.     

Aortic Distensibility in Type 1 Diabetes

OBJECTIVE

To evaluate the relationship between long-term glycemia, traditional cardiovascular disease (CVD) risk factors, and ascending aortic stiffness in type 1 diabetes.

RESEARCH DESIGN AND METHODS

Eight hundred seventy-nine subjects in the Diabetes Control and Complications Trial (DCCT)/Epidemiology of Diabetes Interventions and Complications (EDIC) study were evaluated. The stiffness/distensibility of the ascending thoracic aorta (AA) was measured with magnetic resonance imaging. Associations of AA distensibility and CVD risk factors, mean HbA_1c, and cardiovascular complications including macroalbuminuria were assessed using multivariate linear regression models.

RESULTS

The mean age of the subjects was 50 ± 7 years (47% women, mean diabetes duration of 28 years). Over 22 years of follow-up, 27% of participants had cardiovascular complications. After adjusting for gender and cohort, AA distensibility was lower with increasing age, mean systolic blood pressure, LDL, and HbA_1c measured over an average of 22 years (−26.3% per 10 years, −11.0% per 10 mmHg SBP, −1.8% per 10 mg/dL of LDL, and −9.3% per unit mean HbA_1c [%], respectively). Patients with macroalbuminuria had 25% lower AA distensibility compared with those without (P < 0.0001). Lower AA distensibility also was associated with greater ratio of left ventricular mass to volume (−3.4% per 0.1 g/mL; P < 0.0001).

CONCLUSIONS

Our findings indicate strong adverse effects of hypertension, chronic hyperglycemia and macroalbuminuria on AA stiffness in type 1 diabetes in the DCCT/EDIC cohort.Increased arterial stiffness is an important marker of increased left ventricular load and an independent predictor of cardiovascular morbidity and mortality both in asymptomatic humans (1) and in disease including renal failure (2), hypertension (3), and diabetes (4). Increased aortic stiffness has been shown to be an independent predictor of 10-year mortality in diabetic patients (5).Aortic stiffness is a marker of vascular age and is notably greater after the fifth decade of life in healthy men and women (6,7). The main mechanism for age-related aortic stiffening is fracture and fragmentation of elastin fibers with repetitive stretch, leading to the transfer of stress to less extensible collagenous fibers in the arterial wall (8). Arterial–ventricular coupling is an important determinant of circulatory function (9). Aortic stiffness corresponds to a chronic increased afterload leading to concentric left ventricular remodeling and hypertrophy and potentially heart failure (10). Age-related aortic changes are accelerated by cardiovascular disease (CVD) and potentially modifiable cardiovascular risk factors including hypertension (11) and glucose status (12). Age and diabetes have been shown to lead to aortic stiffness through arterial wall glycation processes (13) that potentiate accelerated aortic alterations in younger individuals (12).The adverse impact of type 1 diabetes on the stiffness/distensibility of large arteries may depend on a number of factors, such as concurrent CVD risk factors, the presence of macrovascular or microvascular complications, and duration of diabetes (5). Whether the degree of chronic glycemic control also affects distensibility is not known. A better understanding of factors that contribute to the development of a less distensible aorta in type 1 diabetes may provide a useful start point to formulate strategies designed to improve arterial health. In this study, we used magnetic resonance imaging (MRI) to characterize aortic distensibility (14) in the Diabetes Control and Complications Trial (DCCT)/Epidemiology of Diabetes Interventions and Complications (EDIC) cohort of patients with type 1 diabetes (15). The DCCT/EDIC cohort is uniquely suited to evaluate risk factors that contribute to decreased aortic distensibility in type 1 diabetes because of its large sample size and long period of close patient follow-up.     

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. © 2012 Wiley Periodicals, Inc.     

Left ventricular shape variation in asymptomatic populations: the multi-ethnic study of atherosclerosis

Background

Although left ventricular cardiac geometric indices such as size and sphericity characterize adverse remodeling and have prognostic value in symptomatic patients, little is known of shape distributions in subclinical populations. We sought to quantify shape variation across a large number of asymptomatic volunteers, and examine differences among sub-cohorts.

Methods

An atlas was constructed comprising 1,991 cardiovascular magnetic resonance (CMR) cases contributed from the Multi-Ethnic Study of Atherosclerosis baseline examination. A mathematical model describing regional wall motion and shape was used to establish a coordinate map registered to the cardiac anatomy. The model was automatically customized to left ventricular contours and anatomical landmarks, corrected for breath-hold mis-registration between image slices. Mathematical techniques were used to characterize global shape distributions, after removal of translations, rotations, and scale due to height. Differences were quantified among ethnicity, sex, smoking, hypertension and diabetes sub-cohorts.

Results

The atlas construction process yielded accurate representations of global shape (errors between manual and automatic surface points in 244 validation cases were less than the image pixel size). After correction for height, the dominant shape component was associated with heart size, explaining 32% of the total shape variance at end-diastole and 29% at end-systole. After size, the second dominant shape component was sphericity at end-diastole (13%), and concentricity at end-systole (10%). The resulting shape components distinguished differences due to ethnicity and risk factors with greater statistical power than traditional mass and volume indices.

Conclusions

We have quantified the dominant components of global shape variation in the adult asymptomatic population. The data and results are available at cardiacatlas.org. Shape distributions were principally explained by size, sphericity and concentricity, which are known correlates of adverse outcomes. Atlas-based global shape analysis provides a powerful method for quantifying left ventricular shape differences in asymptomatic populations.

Trial registration

ClinicalTrials.gov {"type":"clinical-trial","attrs":{"text":"NCT00005487","term_id":"NCT00005487"}}NCT00005487     

Progression of diffuse myocardial fibrosis assessed by cardiac magnetic resonance T1 mapping

To evaluate long-term changes in diffuse myocardial fibrosis using cardiac magnetic resonance (CMR) with late gadolinium enhancement (LGE) and T₁ mapping. Patients with chronic stable cardiomyopathy and stable clinical status (n = 52) underwent repeat CMR at a 6 month or greater follow up interval and had LGE and left ventricular (LV) T₁ mapping CMR. Diffuse myocardial fibrosis (excluding areas of focal myocardial scar) was assessed by post gadolinium myocardial T₁ times. Mean baseline age of 52 patients (66 % male) was 35 ± 19 years with a mean interval between CMR examinations of 2.0 ± 0.8 years. CMR parameters, including LV mass and ejection fraction, showed no change at follow-up CMR (p > 0.05). LVT₁ times (excluding focal scar) decreased over the study interval (from 468 ± 106 to 434 ± 82 ms, p = 0.049). 38 Patients had no visual LGE?, while 14 were LGE+. For LGE? patients, greater change in LV mass and end systolic volume index were associated with change in T₁ time (β = ?2.03 ms/g/m², p = 0.035 and β = 2.1 ms/mL/m², p = 0.029, respectively). For LGE+ patients, scar size was stable between CMR1 and CMR2 (10.7 ± 13.8 and 11.5 ± 13.9 g, respectively, p = 0.32). These results suggest that diffuse myocardial fibrosis, as assessed by T₁ mapping, progresses over time in patients with chronic stable cardiomyopathy.