Cardiac T2 * mapping: Techniques and clinical applications

Abstract

Cardiac T₂ * mapping is a noninvasive MRI method that is used to identify myocardial iron accumulation in several iron storage diseases such as hereditary hemochromatosis, sickle cell disease, and β-thalassemia major. The method has improved over the years in terms of MR acquisition, focus on relative artifact-free myocardium regions, and T₂ * quantification. Several improvement factors involved include blood pool signal suppression, the reproducibility of T₂ * measurement as affected by scanner hardware, and acquisition software. Regarding the T₂ * quantification, improvement factors include the applied curve-fitting method with or without truncation of the signals acquired at longer echo times and whether or not T₂ * measurement focuses on multiple segmental regions or the midventricular septum only. Although already widely applied in clinical practice, data processing still differs between centers, contributing to measurement outcome variations. State of the art T₂ * measurement involves pixelwise quantification providing better spatial iron loading information than region of interest-based quantification. Improvements have been proposed, such as on MR acquisition for free-breathing mapping, the generation of fast mapping, noise reduction, automatic myocardial contour delineation, and different T₂ * quantification methods. This review deals with the pro and cons of different methods used to quantify T₂ * and generate T₂ * maps. The purpose is to recommend a combination of MR acquisition and T₂ * mapping quantification techniques for reliable outcomes in measuring and follow-up of myocardial iron overload. The clinical application of cardiac T₂ * mapping for iron overload's early detection, monitoring, and treatment is addressed. The prospects of T₂ * mapping combined with different MR acquisition methods, such as cardiac T₁ mapping, are also described. Level of Evidence: 4 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2019.

Keywords: T2* techniques; cardiac T2* mapping; cardiac iron overload; magnetic resonance imaging.

FIGURE 1

Bright‐blood multigradient echo image series (a–h) of midventricular short‐axis myocardium. Endocardial (green line) and epicardial (red line) contours are drawn (i) to represent the myocardial region (black dash lines) on the T₂* map (j).

FIGURE 2

Black‐blood multigradient echo image series (a–h) of midventricular short‐axis myocardium. Endocardial (green line) and epicardial (red line) contours are drawn (i) to represent the myocardial region (black dash lines) on the T₂* map (j).

FIGURE 3

Artifacts appearance on a bright‐blood multigradient echo image series. At midventricular short‐axis myocardium (a), a specific window level and window width setting of Fig. 1 enhances the presence of motion artifacts propagated in the phase‐encode direction (located by dash lines) and susceptibility artifact progression at inferior and inferolateral segments of the left ventricle (b–i).

FIGURE 4

Bull's‐eye plots showing global myocardium using AHA 16‐segments model where midventricular septum approximates the sum of segments 8 and 9.

FIGURE 5

Pixelwise monoexponential T₂* fitting of full left midventricular myocardium image (a) without (b) and with an offset (c) or truncation based on R² (d) or SNR (e), with the same T₂* range, performed differently by handling the presence of motion artifacts as described in Fig. 3 yielding different segmental T₂* values.

FIGURE 6

Pixelwise T₂* measurement depicted at inferoseptal (a), inferior (b) and inferolateral (c) on left midventricular short‐axis myocardia as shown by arrowheads. The monoexponential fitting plots (red line) of the four methods correspond to the locations in the presence of motion artifacts at the phase‐encode direction and susceptibility artifact highlighted in Fig. 3. The dashed line on the fitting plots represents classic monoexponential fitting of the respective alternative methods.