Whole-heart T₁-mapping with single breath-hold

Background

T₁-mapping, directly measuring the underlying longitudinal relaxation times (T₁), and extracellular volume (ECV) quantification, are emerging techniques for myocardial fibrosis quantification. Recent studies have reported significant T₁ differences in fibrotic and normal tissue, but whole-heart T₁-mapping is rarely performed in clinical practice, due to the associated time-consuming data acquisition; this can lead to sampling error when the fibrotic process is not homogenous. Signal acquisition over multiple heart beats can also be problematic, due to the potential for motion artifacts. In this study, we present a rapid whole-heart T₁-mapping in a single breath-hold of, e.g., 6 heartbeats (typically, 5-7 seconds for total 9 T₁ maps).

Methods

To achieve rapid whole-heart T₁-mapping, we modified a turboFLASH pulse sequence to acquire multiple T₁-weighted (T₁w) images, with increasing sequential time delays (TD), after a non-selective saturation pulse. Whole-heart T₁-mapping was performed using a 1.5T MRI scanner (Avanto, Siemens). Within three heart beats, we acquired 9 T₁w images at different levels, with increasing sequential time delays TD=200 (for slices 1,4,7), 397 (for slices 2,5,8) and 594 ms (for slices 3,6,9) after a non-selective saturation pulse (Fig. 1). Centric k-space acquisition ordering is used to minimize the sensitivity to inflow effects and to reduce the sensitivity to B₁⁺ profile after image normalization. In the first three heartbeats, 9 corresponding proton density-weighted (PDw) images are acquired, in order to correct for the B₁^- and the unknown equilibrium magnetization, and normalize the signal. Post-contrast T₁ maps were acquired 37 minutes after contrast injection (0.15mmol/kg of gadolinium-DTPA). Using the Bloch equation, T₁ is obtained from the normalized signal, S^norm (=S_T1w/S_PDw), and TD: T₁ = -TD/ln(1-S^norm).

Figure 1

(left) 4CH cine image including 9 imaging locations. (right) Schematic pulse sequence diagram of the whole-heart T₁-mapping method.

Results

Figure 2 shows the results from a representative patient with hypertrophic obstructive cardiomyopathy (56 years old; male; EF=75%; maximum myocardial thickness=15mm; no focal late gadolinium enhancement (LGE)). Total scan time for this representative patient was 5.4s for 9 slice locations; and pre-contrast myocardial T₁ values of slice 3-8 were 1421±158ms and ECV values (assuming hematocrit of 0.4) were 0.24±0.05. Although there is no focal LGE, this patient shows a higher pre-contrast T₁ than in normal controls (T₁ of ~1s at 1.5T), suggesting a higher degree of diffuse myocardial fibrosis.

Figure 2

(top) T₁w images at different levels. (middle) pre- and post-contrast (37 minutes after contrast injection) T₁ maps. (bottom) ECV maps.

Conclusions

While conventional T₁-mapping methods are very time-consuming for routine clinical application, this whole-heart T₁-mapping method can be performed well in patients with cardiac disease-related problems, including arrhythmia or difficulty with breath-holding, due to its short acquisition time of, e.g., 6 heartbeats. Better characterizing whole-heart fibrosis may allow for more accurate and earlier diagnosis. Further studies are warranted.

Funding

None.