Cyclic variation of T1 in the myocardium at 3 T

Blood correction reduces variability and gender differences in native myocardial T1 values at 1.5 T cardiovascular magnetic resonance - a derivation/validation approach

BACKGROUND

Myocardial native T1 measurements are likely influenced by intramyocardial blood. Since blood T1 is both variable and longer compared to myocardial T1, this will degrade the precision of myocardial T1 measurements. Precision could be improved by correction, but the amount of correction and the optimal blood T1 variables to correct with are unknown. We hypothesized that an appropriate correction would reduce the standard deviation (SD) of native myocardial T1.

METHODS

Consecutive patients (n = 400) referred for CMR with known or suspected heart disease were split into a derivation cohort for model construction (n = 200, age 51 ± 18 years, 50% male) and a validation cohort for assessing model performance (n = 200, age 48 ± 17 years, 50% male). Exclusion criteria included focal septal abnormalities. A Modified Look-Locker inversion recovery sequence (MOLLI, 1.5 T Siemens Aera) was used to acquire T1 and T1* maps. T1 and T1* maps were used to measure native myocardial T1, and blood T1 and T1*. A multivariate linear regression correction model was implemented using blood measurement of R1 (1/T1), R1* (1/T1*) or hematocrit. The correction model from the derivation cohort was applied to the validation cohort, and assessed for reduction in variability with the F-test.

RESULTS

Blood [LV + RV] mean R1, mean R1* and hematocrit correlated with myocardial T1 (Pearson's r, range 0.37 to 0.45, p < 0.05 for all) in both the derivation and validation cohorts respectively, suggesting that myocardial T1 measurements are influenced by intramyocardial blood. Mean myocardial native T1 did not differ between the derivation and validation cohorts (1030 ± 42.6 ms and 1023 ± 45.2 ms respectively, p = 0.07). In the derivation cohort, correction using blood mean R1 and mean R1* yielded a decrease in myocardial T1 SD (45.2 ms to 36.6 ms, p = 0.03). When the model from the derivation cohort was applied to the validation cohort, the SD reduction was maintained (39.3 ms, p = 0.049). This 13% reduction in measurement variability leads to a 23% reduction in sample size to detect a 50 ms difference in native myocardial T1.

CONCLUSIONS

Correcting native myocardial T1 for R1 and R1* of blood improves the precision of myocardial T1 measurement by ~13%, and could consequently improve disease detection and reduce sample size needs for clinical research.

Manganese-enhanced MRI detection of impaired calcium regulation in a mouse model of cardiac hypertrophy

The aim of this study was to use manganese (Mn)-enhanced MRI (MEMRI) to detect changes in calcium handling associated with cardiac hypertrophy in a mouse model, and to determine whether the impact of creatine kinase ablation is detectable using this method. Male C57BL/6 (C57, n = 11) and male creatine kinase double-knockout (CK-M/Mito(-/-) , DBKO, n = 12) mice were imaged using the saturation recovery Look-Locker T1 mapping sequence before and after the development of cardiac hypertrophy. Hypertrophy was induced via subcutaneous continuous 3-day infusion of isoproterenol, and sham mice not subjected to cardiac hypertrophy were also imaged. During each scan, the contrast agent Mn was administered and the resulting change in R1 (=1/T1) was calculated. Two anatomical regions of interest (ROIs) were considered, the left-ventricular free wall (LVFW) and the septum, and one ROI in an Mn-containing standard placed next to the mouse. We found statistically significant (p < 0.05) decreases in the uptake of Mn in both the LVFW and septum following the induction of cardiac hypertrophy. No statistically significant decreases were detected in the standard, and no statistically significant differences were found among the sham mice. Using a murine model, we successfully demonstrated that changes in Mn uptake as a result of cardiac hypertrophy are detectable using the functional contrast agent and calcium mimetic Mn. Our measurements showed a decrease in the relaxivity (R1) of the myocardium following cardiac hypertrophy compared with normal control mice.

Influence of the cardiac cycle on time-intensity curves using multislice dynamic magnetic resonance perfusion

Flow and pressure variations cause potential changes in magnetic resonance imaging (MRI) signal intensity across the cardiac cycle. Nevertheless, cardiac dynamic contrast-enhanced (perfusion) MRI is performed and analyzed regardless of the cardiac phase. We investigate whether the cardiac phase impacts myocardial and left ventricle (LV) cavity time intensity curves (TICs) at rest and during vasodilatation. Fifteen healthy volunteers (seven females, eight males; mean age: 32.5 ± 9.3 years; age range: 19-49 years) were included in this prospective study. They underwent four separate short-axis multislice (apical, mid and basal) LV perfusion MRI, with different electrocardiogram-triggering during normal vasotone and adenosine-stress. TIC parameters were extracted from the myocardium and the LV cavity. General linear mixed model analyses were used to evaluate their variability according to vasotone, cardiac phase and slice-position. Maximal enhancement and normalized Steepest slopes were higher at stress than at rest (p values <0.001). A similar trend towards higher inflow was shown on systole versus diastole in the LV cavity and diastole versus systole in the myocardium (p < 0.05).These TIC parameters were slice-position dependent, as the inflow decreased from the base to the apex in the LV, and peaked on the mid-slice for the myocardium. There are significant variability of both the LV and the myocardial TICs, with respect to the cardiac cycle phase and the slice position where imaging actually takes place. These appeal to measurement standardization for a better intra- and inter-study reproducibility.

Myocardial T1 mapping: modalities and clinical applications

Myocardial fibrosis appears to be linked to myocardial dysfunction in a multitude of non-ischemic cardiomyopathies. Accurate non-invasive quantitation of this extra-cellular matrix has the potential for widespread clinical benefit in both diagnosis and guiding therapeutic intervention. T1 mapping is a cardiac magnetic resonance (CMR) imaging technique, which shows early clinical promise particularly in the setting of diffuse fibrosis. This review will outline the evolution of T1 mapping and the various techniques available with their inherent advantages and limitations. Histological validation of this technique remains somewhat limited, however clinical application in a range of pathologies suggests strong potential for future development.

Normal diastolic and systolic myocardial T1 values at 1.5-T MR imaging: correlations and blood normalization

PURPOSE

To introduce blood normalization for myocardial T1 values at magnetic resonance (MR) imaging and to evaluate regional differences between systolic and diastolic myocardial T1 values in healthy subjects.

MATERIALS AND METHODS

This prospective study (ClinicalTrials.gov identification number, NCT01728597) was approved by the institutional review board, and volunteer informed consent was obtained. Forty healthy subjects (20 women; age range, 20-35 years) underwent electrocardiographically gated 1.5-T MR imaging. A modified Look-Locker inversion recovery sequence was used to acquire myocardial T1 maps in systole and diastole. Regional T1 values were evaluated in 16 myocardial segments; blood T1 was derived from the blood pool in the center of the left ventricular cavity. Linear regression slopes between myocardial and blood T1 values were used to normalize myocardial T1 to the mean blood T1 of the study population. Mean T1 values were compared by using the t test, with P < .05 considered to indicate a significant difference.

RESULTS

Mean myocardial T1 (984 msec ± 28 [standard deviation] in diastole, 959 msec ± 21 in systole) and all segmental T1 values between diastole and systole differed significantly (P < .001). Blood T1 correlated well with segmental myocardial T1 (R = 0.73 for diastole, R = 0.72 for systole). After normalization to blood T1, significant sex differences in myocardial T1 disappeared and variances in mean myocardial T1 decreased. Blood-normalized diastolic and systolic myocardial T1 values correlated strongly with each other on segmental (r = 0.72) and global (r = 0.89) levels. Subregional myocardial T1 distribution characteristics in diastole were similar to those in systole.

CONCLUSION

In normal myocardium, diastolic and systolic myocardial T1 values differ significantly but correlate strongly. Blood normalization eliminates sex differences in myocardial T1 values and reduces their variability.

Advanced Cardiac MR Imaging for Myocardial Characterization and Quantification: T1 Mapping

Magnetic resonance as an imaging modality provides an excellent soft tissue differentiation, which is an ideal choice for cardiac imaging. Cardiac magnetic resonance (CMR) allows myocardial tissue characterization, as well as comprehensive evaluation of the structures. Although late gadolinium enhancement after injection of the gadolinium extracellular contrast agent has further extended our ability to characterize the myocardial tissue, it also has limitations in the quantification of enhanced myocardial tissue pathology, and the detection of diffuse myocardial disease, which is not easily recognized by enhancement contrast. Recently, the remarkable advances in CMR technique, such as T1 mapping, which can quantitatively evaluate myocardial status, showed potentials to overcome limitations of existing CMR sequences and to expand the application of CMR. This article will review the technical and clinical points to be considered in the practical use of pre- and post-contrast T1 mapping.

Detecting myocardial ischemia at rest with cardiac phase-resolved blood oxygen level-dependent cardiovascular magnetic resonance

BACKGROUND

Fast noninvasive identification of ischemic territories at rest (before tissue-specific changes) and assessment of functional status can be valuable in the management of severe coronary artery disease. This study investigated the use of cardiac phase-resolved blood oxygen level-dependent (CP-BOLD) cardiovascular magnetic resonance in detecting myocardial ischemia at rest secondary to severe coronary artery stenosis.

METHODS AND RESULTS

CP-BOLD, standard cine, and T2-weighted images were acquired in canines (n=11) at baseline and within 20 minutes of ischemia induction (severe left anterior descending stenosis) at rest. After 3 hours of ischemia, left anterior descending stenosis was removed, and T2-weighted and late-gadolinium-enhancement images were acquired. From standard cine and CP-BOLD images, end-systolic and end-diastolic myocardium was segmented. Affected and remote sections of the myocardium were identified from postreperfusion late-gadolinium-enhancement images. Systolic-to-diastolic ratio (S/D), quotient of mean end-systolic and end-diastolic signal intensities (on CP-BOLD and standard cine), was computed for affected and remote segments at baseline and ischemia. Ejection fraction and segmental wall thickening were derived from CP-BOLD images at baseline and ischemia. On CP-BOLD images, S/D was >1 (remote and affected territories) at baseline; S/D was diminished only in affected territories during ischemia, and the findings were statistically significant (ANOVA, post hoc P<0.01). The dependence of S/D on ischemia was not observed in standard cine images. Computer simulations confirmed the experimental findings. Receiver-operating characteristic analysis showed that S/D identifies affected regions with performance (area under the curve, 0.87) similar to ejection fraction (area under the curve, 0.89) and segmental wall thickening (area under the curve, 0.75).

CONCLUSIONS

Preclinical studies and computer simulations showed that CP-BOLD cardiovascular magnetic resonance could be useful in detecting myocardial ischemia at rest. Patient studies are needed for clinical translation.

T1 mapping of the myocardium: intra-individual assessment of the effect of field strength, cardiac cycle and variation by myocardial region

BACKGROUND

Myocardial T1 relaxation time (T1 time) and extracellular volume fraction (ECV) are altered in the presence of myocardial fibrosis. The purpose of this study was to evaluate acquisition factors that may result in variation of measured T1 time and ECV including magnetic field strength, cardiac phase and myocardial region.

METHODS

31 study subjects were enrolled and underwent one cardiovascular MR exam at 1.5 T and two exams at 3 T, each on separate days. A Modified Look-Locker Inversion Recovery (MOLLI) sequence was acquired before and 5, 10, 12, 20, 25 and 30 min after administration of 0.15 mmol/kg gadopentetate dimeglumine (Gd-DTPA; Magnevist) at 1.5 T (exam 1). For exam 2, MOLLI sequences were acquired at 3 T both during diastole and systole, before and after administration of Gd-DTPA (0.15 mmol/kg Magnevist).Exam 3 was identical to exam 2 except gadobenate dimeglumine was administered (Gd-BOPTA; 0.1 mmol/kg Multihance). T1 times were measured in myocardium and blood. ECV was calculated by (ΔR1myocardium/ΔR1blood)*(1-hematocrit).

RESULTS

Before gadolinium, T1 times of myocardium and blood were significantly greater at 3 T versus 1.5 T (28% and 31% greater, respectively, p < 0.001); after gadolinium, 3 T values remained greater than those at 1.5 T (14% and 12% greater for myocardium and blood at 3 T with Gd-DTPA, respectively, p < 0.0001 and 18% and 15% greater at 3 T with Gd-BOPTA, respectively, p < 0.0001). However, ECV did not vary significantly with field strength when using the same contrast agent at equimolar dose (p = 0.2). Myocardial T1 time was 1% shorter at systole compared to diastole pre-contrast and 2% shorter at diastole compared to systole post-contrast (p < 0.01). ECV values were greater during diastole compared to systole on average by 0.01 (p < 0.01 to p < 0.0001). ECV was significantly higher for the septum compared to the non-septal myocardium for all three exams (p < 0.0001-0.01) with mean absolute differences of 0.01, 0.004, and 0.07, respectively, for exams 1, 2 and 3.

CONCLUSION

ECV is similar at field strengths of 1.5 T and 3 T. Due to minor variations in T1 time and ECV during the cardiac cycle and in different myocardial regions, T1 measurements should be obtained at the same cardiac phase and myocardial region in order to obtain consistent results.

Rapid T1 quantification based on 3D phase sensitive inversion recovery

Background

In Contrast Enhanced Magnetic Resonance Imaging fibrotic myocardium can be distinguished from healthy tissue using the difference in the longitudinal T₁ relaxation after administration of Gadolinium, the so-called Late Gd Enhancement. The purpose of this work was to measure the myocardial absolute T₁ post-Gd from a single breath-hold 3D Phase Sensitivity Inversion Recovery sequence (PSIR). Equations were derived to take the acquisition and saturation effects on the magnetization into account.

Methods

The accuracy of the method was investigated on phantoms and using simulations. The method was applied to a group of patients with suspected myocardial infarction where the absolute difference in relaxation of healthy and fibrotic myocardium was measured at about 15 minutes post-contrast. The evolution of the absolute R₁ relaxation rate (1/T₁) over time after contrast injection was followed for one patient and compared to T₁ mapping using Look-Locker. Based on the T₁ maps synthetic LGE images were reconstructed and compared to the conventional LGE images.

Results

The fitting algorithm is robust against variation in acquisition flip angle, the inversion delay time and cardiac arrhythmia. The observed relaxation rate of the myocardium is 1.2 s^-1, increasing to 6 - 7 s^-1 after contrast injection and decreasing to 2 - 2.5 s^-1 for healthy myocardium and to 3.5 - 4 s^-1 for fibrotic myocardium. Synthesized images based on the T₁ maps correspond very well to actual LGE images.

Conclusions

The method provides a robust quantification of post-Gd T₁ relaxation for a complete cardiac volume within a single breath-hold.