Clinical Importance Of Myocardial T2 Mapping And Texture Analysis

Late gadolinium enhancement (LGE) magnetic resonance imaging (MRI) is valuable for diagnosis and assessment of the severity of various myocardial diseases owing to its potential to visualize myocardial scars. T1 mapping is complementary to LGE because it can quantify the degree of myocardial fibrosis or edema. As such, T1-weighted imaging techniques, including LGE using an inversion recovery sequence, contribute to cardiac MRI. T2-weighted imaging is widely used to characterize the tissue of many organs. T2-weighted imaging is used in cardiac MRI to identify myocardial edema related to chest pain, acute myocardial diseases, or severe myocardial injuries. However, it is difficult to determine the presence and extent of myocardial edema because of the low contrast between normal and diseased myocardium and image artifacts of T2-weighted images and the lack of an established method to quantify the images. T2 mapping quantifies myocardial T2 values and helps identify myocardial edema. The T2 values are significantly related to the clinical symptoms or severity of nonischemic cardiomyopathy. Texture analysis is a postprocessing method to quantify tissue alterations that are reflected in the T2-weighted images. Texture analysis provides a variety of parameters, such as skewness, entropy, and grey-scale non-uniformity, without the need for additional sequences. The abnormal signal intensity on T2-weighted images or T2 values may correspond to not only myocardial edema but also other tissue alterations. In this review, the techniques of cardiac T2 mapping and texture analysis and their clinical relevance are described. 

Keywords: cardiac MRI, T2-weighted imaging, myocardium, T2 mapping, texture analysis

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Introduction

Late gadolinium enhancement (LGE) magnetic resonance imaging (MRI) is valuable for the diagnosis and assessment of the severity of various myocardial diseases owing to its potential to visualize myocardial scars.1–3 LGE MRI provides binary contrast between scarred and unscarred tissues, while T1 mapping is complementary to LGE because it can quantify the degree of myocardial fibrosis or edema.4–6 As such, T1-weighted imaging techniques, including LGE using an inversion recovery sequence, contribute to cardiac MRI. Conversely, T2-weighted imaging is widely used for identification of the pathological status in many organs, because T2 values are correlated with edema, cellular proliferation, and vessel densities.7,8 Because the scarring tissues or fibrosis are more prominent than neoplasms and inflammatory diseases in the myocardium, T1-weighted imaging is more popular than T2-weighted imaging in the field of cardiac MRI. Recently, T2-weighted imaging has been used to identify myocardial edema related to chest pain, acute myocardial diseases, or severe myocardial injuries.9–13 However, it is difficult to evaluate the presence and extent of myocardial edema because of the low contrast between normal and diseased myocardium, image artifacts, and the lack of an established method to quantify the T2-weighted images; therefore, some quantitative methods of myocardial T2 are required to evaluate myocardial edema or injuries. T2 mapping quantifies myocardial T2 values and helps identify myocardial edema.14,15 Texture analysis is a postprocessing method to quantify the tissue alterations that are reflected in any medical image. Texture analysis provides a lot of parameters, such as skewness, entropy, and grey-scale nonuniformity, without the need for additional sequences.16–18 The texture analysis has been applied to cardiac T2-weighted images, which identify diffuse myocardial tissue abnormalities associated with hypertrophic cardiomyopathy.16 These MRI and post-processing tools may be valuable for evaluating myocardial diseases in clinical practice. In this review, the T2-weighted cardiac MRI techniques, cardiac T2 mapping, and texture analysis, and their clinical relevance for several myocardial diseases are described. We showed T2-weighted images, T2 mapping, or other imaging acquired by a 1.5T imager (Ingenia, Philips Healthcare, Best, The Netherlands).

Imaging Techniques Used in Cardiac T2-weighted MRI 

T2-weighted imaging is widely used for tissue characterization and identification of pathologies in many organs, but cardiac and respiratory motion, high intensity of the pericardial fat and intraventricular blood, and blood flow artifacts prevent the identification of myocardial injuries.19 Therefore, electrocardiogram (ECG) gating and black-blood technique using double inversion recovery (IR), chemical shift suppresssion or short inversion time IR (STIR or triple IR) and breathholding is required for cardiac T2-weighted MR images with sufficient imaging quality.19–22 Breath-holding is used to eliminate respiratory artifacts, and turbo spin-echo and parallel imaging techniques are used to reduce the breath-holding time.19,22–24 Black-blood imaging technique combined with ECG gating reduces the blood signals, flow artifacts, and cardiac motion.20 The stagnant blood flow adjacent to hypoxianetic myocardium can show high intensity (Fig. 1a), and the changes in the RF thickness of the IR pulse may reduce the signals in our experience. Spectrally selective fat suppression may provide a higher signal-to-noise ratio than triple IR in T2-weighted imaging, but it is more sensitive to magnetic inhomogeneity.21 Therefore, either technique can be applied according to the magnetic field strength, MR images used, and shimming methods, to reduce fat signals from the pericardium and chest wall. Fat suppression improves the dynamic range identification of myocardial edema.

Quantitative Techniques of Myocardial T2 Signal ratio measurement on T2-weighted images 

Measurement of the signal ratio between the myocardium and skeletal muscle is useful for the detection of myocardial edema associated with acute myocarditis.10,25 This quantitative method is easy and fast in clinical practice. A gantry coil has been used to measure the signal intensities of the myocardium and skeletal muscle to avoid the geometrical factor or signal correction associated with the use of a multichannel receiver coil and associated parallel imaging techniques. However, the multichannel coil is commonly used to improve image quality and throughput of the cardiac MRI examinations. Skeletal myositis can be associated with myocarditis.26 As other quantitative methods, including T2 mapping and texture analysis, emerge, the signal ratio measurement is becoming obsolete.

T2 mapping 

T2 mapping is a quantitative method for identifying and estimating myocardial injuries. T2-prepared steady-state free precession or multi-echo gradient- and spin-echo imaging sequences are used for T2 mapping.27–29 ECG gating, fat and blood signal suppression, and fast data acquisition techniques are commonly applied to T2 mapping to measure myocardial T2 values accurately during a single breath-holding.16,27–29 Otherwise, navigator gating is used to minimize respiratory artifacts.5 The advantages of T2 mapping over T1 mapping are the fewer selection of MRI sequences, the reduced variability of myocardial T2 values (i.e., 45–55 ms) despite magnetic field strength, imaging sequences, and MR machine vendors, its high sensitivity to myocardial edema, and the ability of visual comparison between T2 mapping and T2-weighted images.29 These allow us to refer to previous reports about myocardial T2 mapping, although the range of normal myocardial T2 values should be determined in each institution.30 We are also able to determine the imaging planes of the T2 mapping appropriately by referring to the T2-weighted images. By contrast, myocardial T1 values are greatly affected by magnetic field strength, and many T1 mapping sequences have been reported.4,30 No comparison has been made between T1 mapping and non-contrast-enhanced T1-weighted images. A limitation of T2 mapping is its inability to quantify myocardial fibrosis, which is a common pathology associated with various myocardial diseases.

Fig. 1 Myocardial infraction. T2-weighted imaging visualizes only acute myocardial infarction (a, arrow), while both acute (arrow) and chronic infarction (arrowhead) show late gadolinium enhancement (b). The dotted arrow shows the stagnant flow artifact adjacent to chronic myocardial infarction (a).

Texture analysis 

Texture analysis is a quantitative postprocessing method based on statistical analyses.17,18 The histogram is a well-known quantitative analysis that gives the grey-level value of each pixel. From the histogram, the mean value, variance, skewness, and 90% percentile of a certain area are derived, which can characterize the signal intensity pattern of the area reflecting the corresponding tissues in the body. The spatial variation and correlation between the grey-level value of one pixel and that of its neighbor may reflect the texture of tissues.17,18 The neighboring pixels can be defined in any direction in the medical images. If many pixels have the same grey level in a certain direction, for example, the region of interest may consist of uniform biological tissues. The degree of grey-level changes, randomness, or inhomogeneity of the pixel distribution can be calculated, and these texture features may reflect the degeneration, necrosis, and mixture of several tissues in the pathology. As such, texture analysis provides a variety of parameters, such as entropy and grey-scale nonuniformity, and can be applied to any imaging modality, sequence, and pathology (Fig. 2).17,18,31,32 Texture analysis has been already used in the field of cardiac MRI, resulting in the identification of myocardial tissue alterations.16,31,32 A combination of T2 mapping and texture analysis has been also performed to evaluate myocarditis showing acute-onset symptoms.33 In this case, texture analysis is applied to the grey level reflecting myocardial T2 values. The advantages of texture analysis are its abundant parameters, the lack of necessity for additional imaging sequences, which allows for retrospective analysis of the past image series, and the existence of open-access software.17,18 There is a possible demerit to texture analysis: too many parameters are difficult to use in clinical routines and may overfit the quantitative data. Thus, we should select several parameters from more than 200 provided by texture analysis with artificial intelligence or empirically.32 It is also difficult to determine pathological alterations of the myocardium that are consistent with abnormal variables given by the texture analysis.

Clinical Application and Relevance of Quantitative Myocardial T2 Myocardial infarction 

T2-weighted cardiac MRI is useful for differentiating between acute and chronic myocardial infarction because of its potential to identify "acute" myocardial injury, and myocardial edema (Fig. 1).9 The discrepancy between T2-weighted and LGE imaging indicates the area at risk that can be salvaged by intervention, although there are some controversies about the ability of cardiac MRI to identify the area at risk.34 In addition, myocardial edema in acute myocardial infarction may suggest a poor prognosis for patients even without myocardial scarring.35 T2 mapping and texture analysis have been used to identify acute myocardial infarction and to differentiate between acute and chronic infarction (Fig. 3).31,36 These techniques provide quantitative and precise identification of the myocardial edema associated with coronary artery diseases (Fig. 4).