Displacement encoding with stimulated echoes (DENSE) is an accurate and reproducible^1,2 technique for measuring myocardial strain throughout the cardiac cycle. The strain maps provided could be valuable in dilated cardiomyopathy (DCM) patients where strain has a prognostic value³. However, DENSE acquisitions typically require long breath holds which are difficult for DCM patients or navigator gating which is time consuming and prone to failure. In this work we accelerate DENSE acquisitions by selectively exciting a volume of tissue around the heart. This allows fewer spiral interleaves to be acquired without aliasing and, therefore a shorter breath hold. We compare accelerated and unaccelerated DENSE in healthy volunteers and demonstrate the utility of accelerated DENSE in an example patient.

A cine spiral DENSE sequence⁴ was modified to selectively excite a reduced field of view. A slice selective gradient was added to the first and second RF pulses on the read and phase axes respectively (figure 1). An improved excitation profile without increasing TE was achieved by making the 1st pulse asymmetric (peak at 81% of duration) and the 2nd pulse a time-reversed copy of the first.

In-vivo 2D cine DENSE was performed in 8 normal subjects (Siemens Skyra 3T) in a mid short-axis slice. Images were acquired with variable flip angle (20^o max), TE=1ms, fat suppression, 3.5x3.5mm² spatial resolution, 128² reconstruction matrix, 8mm slice thickness, TR=15ms, 30ms temporal resolution, 2 spirals/frame, 6ms spiral duration, 2 direction simple encoding (+reference) at 0.06cycles/mm, CSPAMM and through-plane dephasing artifact suppression⁵ (0.08cycles/mm). Long, medium and short acquisitions were performed with square fields of view/breath hold durations of 360mm²/20RR-intervals (RR), 224mm²/14RR and 120mm²/8RR, respectively, including 2RR-intervals for a B₀ field map in each case. The long acquisition used a similar field of view to previous work performed without the zonal excitation and was used as a reference⁴. Images were processed using the DENSE analysis tool from the University of Virginia⁶. Peak and time-to-peak radial and circumferential strains measured globally over the left ventricle were compared between protocols using a Wilcoxon signed rank test (p<0.05 threshold) and by calculating the root mean square error between the strain-time curves.

Mid-ventricular short axis spiral cine DENSE using the medium breath hold protocol was also performed in one patient diagnosed with dilated cardiomyopathy.

Figure 2 shows example magnitude images in one example volunteer acquired using long, medium and short acquisitions and the corresponding strain curves are shown in figure 3. There is good agreement between the strain curves from all three acquisitions. Global peak strain, time to peak strain and the differences between acquisitions (long acquisition as reference, expressed as bias and root mean square error) are summarized in the table for radial and circumferential strain for all 8 volunteers. While there is a significant under-estimation (p<0.01) of peak radial strain using the medium acquisition, the magnitude of the difference is small (0.01). There were no other significant differences and the medium and short breath hold acquisitions appear to be equally accurate.

Upon review of the patient data, a wall motion abnormality and late gadolinium enhancement (LGE) indicated that the diagnosis was incorrect and the patient had an extensive infarct. Figure 4 shows the strain curves derived from DENSE with the corresponding LGE images. Radial and circumferential strain is reduced with abnormal strain patterns in the infarcted regions of myocardium, while the remote regions show relatively normal strains.

Spiral cine DENSE imaging can be accelerated by up to a factor of 2.5 by selectively exciting and imaging a small field of view around the heart. The associated loss of signal to noise ratio is partly compensated for by imaging at 3T. This improvement allows 2D acquisitions to be performed in a medium (~14s) or even short breath hold (~8s) which most patients could sustain. The technique was demonstrated in a patient with myocardial infarction. Segments of the heart with reduced strain were found to correspond to regions of the heart with late gadolinium enhancement. In future the same approach may be used to accelerate 2D long axis and 3D spiral cine DENSE acquisitions.