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

Improving the robustness of MOLLI T1 maps with a dedicated motion correction algorithm

Myocardial tissue T1 constitutes a reliable indicator of several heart diseases related to extracellular changes (e.g. edema, fibrosis) as well as fat, iron and amyloid content. Magnetic resonance (MR) T1-mapping is typically achieved by pixel-wise exponential fitting of a series of inversion or saturation recovery measurements. Good anatomical alignment between these measurements is essential for accurate T1 estimation. Motion correction is recommended to improve alignment. However, in the case of inversion recovery sequences, this correction is compromised by the intrinsic contrast variation between frames. A model-based, non-rigid motion correction method for MOLLI series was implemented and validated on a large database of cardiac clinical cases (n = 186). The method relies on a dedicated similarity metric that accounts for the intensity changes caused by T1 magnetization relaxation. The results were compared to uncorrected series and to the standard motion correction included in the scanner. To automate the quantitative analysis of results, a custom data alignment metric was defined. Qualitative evaluation was performed on a subset of cases to confirm the validity of the new metric. Motion correction caused noticeable (i.e. > 5%) performance degradation in 12% of cases with the standard method, compared to 0.3% with the new dedicated method. The average alignment quality was 85% ± 9% with the default correction and 90% ± 7% with the new method. The results of the qualitative evaluation were found to correlate with the quantitative metric. In conclusion, a dedicated motion correction method for T1 mapping MOLLI series has been evaluated on a large database of clinical cardiac MR cases, confirming its increased robustness with respect to the standard method implemented in the scanner.

Preliminary study: myocardial T1 relaxation time in patients with ischemic findings and normal findings on coronary angiography

OBJECTIVE

The aim of this study is to evaluate the myocardium structure in patients with chest pain who were determined to have moderate and/or high risk for cardiac ischemic heart disease (IHD) but who had normal findings on conventional coronary angiography by using native cardiac magnetic resonance imaging (CMRI) T1 mapping and comparing with healthy volunteers.

METHODS

A total of 50 patients and 30 healthy volunteers who underwent CMRI were included in our prospective study. Patients whose clinical findings were compatible with stable angina pectoris, with moderate and/or high risk for IHD, but whose conventional coronary angiography was normal, were our patient group. Native T1 values were measured for 17 myocardial segments (segmented based on American Heart Association recommendations) by two radiologists independently. The data obtained were statistically compared with the sample t-test.

RESULTS

Myocardial native T1 values were found to be significantly prolonged in the patient group compared with the control group (p<0.05). Inter-observer reliability for native T1 value measurements of groups was high for both patient and control groups (α = 0.92 for the patient group and 0.96 for the control group).

CONCLUSION

Findings suggestive of ischemia were detected by T1 mapping in the myocardium of our patients. For this reason, it is recommended that this patient group should be included in early diagnosis and close follow-up assessments for IHD.

Transthyretin amyloidosis: an under-recognized neuropathy and cardiomyopathy

Transthyretin (TTR) amyloidosis (ATTR amyloidosis) is an underdiagnosed and important type of cardiomyopathy and/or polyneuropathy that requires increased awareness within the medical community. Raising awareness among clinicians about this type of neuropathy and lethal form of heart disease is critical for improving earlier diagnosis and the identification of patients for treatment. The following review summarizes current criteria used to diagnose both hereditary and wild-type ATTR (ATTRwt) amyloidosis, tools available to clinicians to improve diagnostic accuracy, available and newly developing therapeutics, as well as a brief biochemical and biophysical background of TTR amyloidogenesis.

Native T1 mapping of the heart - a pictorial review

T1 mapping is now a clinically feasible method, providing pixel-wise quantification of the cardiac structure’s T1 values. Beyond focal lesions, well depicted by late gadolinium enhancement sequences, it has become possible to discriminate diffuse myocardial alterations, previously not assessable by noninvasive means. The strength of this method includes the high reproducibility and immediate clinical applicability, even without the use of contrast media injection (native or pre-contrast T1). The two most important determinants of native T1 augmentation are (1) edema related to tissue water increase (recent infarction or inflammation) and (2) interstitial space increase related to fibrosis (infarction scar, cardiomyopathy) or to amyloidosis. Conversely, lipid (Anderson–Fabry) or iron overload diseases are responsible for T1 reduction. In this pictorial review, the main features provided by native T1 mapping are discussed and illustrated, with a special focus on the awaited clinical purpose of this unique, promising new method.

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.

The echocardiographic paradox index in patients with a repaired tetralogy of Fallot

OBJECTIVES

Right ventricular (RV) volume is very important for pulmonary valve replacement after the total correction of tetralogy of Fallot (TOF), and we attempted to identify a convenient echocardiographic index that is well correlated with the volumetric data obtained through magnetic resonance imaging (MRI).

DESIGN

All patients who underwent cardiac MRI and echocardiography at Sejong General Hospital for evaluating pulmonary regurgitation after TOF total correction were included. The paradox index is the amount of paradoxical motion of the interventricular septum on the short-axis echocardiographic view. The paradox index was compared to several cardiac MRI indices.

RESULTS

Fifty-four patients were included. The paradox index for all patients was 1.22 ± 0.12 (1.06-1.67), and the index of the operation group was significantly higher than that of the non-operation group (1.26 ± 1.12 vs 1.16 ± 1.12, P = 0.009). The paradox index was well correlated with the RV systolic and diastolic volumes, as measured by cardiac MRI (P = 0.002 and 0.003). Using a simple linear regression analysis, a paradox index of 1.24 corresponded to a RV diastolic volume of 160 ml/m(2).

CONCLUSIONS

The paradox index could help to indicate the time for an MRI analysis of the RV volume in patients after TOF total correction.

[Cardiac MRI: technology, clinical applications, and future directions]

The field of cardiovascular MRI has evolved rapidly over the past decade, feeding new applications across a broad spectrum of clinical and research areas. Advances in magnet hardware technology, and key developments such as segmented k-space acquisitions, advanced motion encoding techniques, ultra-rapid perfusion imaging and delayed myocardial enhancement imaging have all contributed to a revolution in how patients with ischemic and non-ischemic heart disease are diagnosed and treated. Actually, cardiac MRI is a widely accepted method as the "gold standard" for detection and characterization of many forms of cardiac diseases. The aim of this review is to present an overview of cardiac MRI technology, advances in clinical applications, and future directions.