An interleaved T1-T2* imaging sequence for assessing myocardial injury

Artifact-suppressed optimal three-dimensional T1 - and T2 *-weighted dual-echo imaging

PURPOSE

To develop a new artifact-suppressed optimal three-dimensional (3D) T₁ - and T₂ *-weighted dual-echo imaging.

METHODS

We optimized flip angles for 3D T₁ - and T₂ *-weighted imaging by conventional dual-echo in vivo experiments and computer simulations, and then implemented a dual-echo sequence with an echo-specific k-space reordering scheme to satisfy the optimal flip angles for both T₁ and T₂ * contrast. We also proposed two strategies to suppress ringing artifacts induced by the abrupt flip angle jumps in the proposed dual echo sequence: (i) implementing smooth transition regions and (ii) discarding the k-space regions of the abrupt flip angle jumps as dummy phase-encoding steps.

RESULTS

The optimal flip angles measured from experiments were different between T₁ - and T₂ *-weighted contrast, in agreement with simulations. The echo-specific k-space reordered dual-echo sequence showed optimal T₁ and T₂ * contrast simultaneously, but also showed ringing artifacts because of high flip-angle changes between k-space regions. The two proposed strategies effectively suppressed the ringing artifacts.

CONCLUSION

The proposed 3D dual-echo sequence provided optimal T₁ and T₂ * contrast simultaneously with no artifacts and thus is potentially applicable to routine clinical applications for simultaneous high resolution T₁ - and T₂ *-weighted imaging. Magn Reson Med 76:1504-1511, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Implementation of a rapid inversion-prepared dual-contrast gradient echo sequence for quantitative dynamic contrast-enhanced magnetic resonance imaging of the human prostate

The first step in quantitative pharmacokinetic modeling is to determine the arterial input function (AIF) by deriving the contrast medium (CM) concentration from an appropriate imaging sequence by monitoring changes in either the amplitude or the phase signal of an accommodative artery. The bolus passage is best detected on T2- or T2*-weighted images, while extravasation is best assessed on T1-weighted images. Here, an imaging sequence is used that employs a parallel acquisition technique for the interleaved acquisition of an inversion-prepared T1-weighted image and a T1/T2*-mixed-weighted image for determination of the AIF. The sequence was applied in six patients with prostate cancer. A method is presented for quantifying the AIF derived from the signal intensity-time courses of both the T1/T2*-mixed-weighted and the T1-weighted image. Furthermore, in some patients the signal intensity-time course of the T1-weighted image exhibits flow-induced signal modulations. To reduce the effect of this flow-related signal enhancement the corresponding phase information was used. The sequence presented here has the potential to improve the quantification of the AIF at all time points and pharmacokinetic modeling of the CM dynamics of the prostate.

Retrograde cardioplegia

OBJECTIVE

This study was undertaken to compare the efficacy of retrograde cardioplegia for myocardial perfusion with that of antegrade cardioplegia at the same flow rate.

METHODS

Colored microspheres were used in rat hearts to assess the capillary flow of cardioplegia solution. Myocardial perfusion was evaluated with magnetic resonance imaging in pig hearts. Phosphorus 31 magnetic resonance spectroscopy was used to determine the efficacies of the cardioplegic techniques in sustaining myocardial energy metabolism.

RESULTS

At the same flow rate, the number of colored microspheres delivered to the capillaries by retrograde cardioplegia (15 +/- 1 microspheres/mm2) was significantly lower than that delivered by antegrade cardioplegia (29 +/- 2 microspheres/mm2). Furthermore, only 19% +/- 3% of the colored microspheres delivered to the capillaries by retrograde cardioplegia were found in the arteriolar portions of the capillaries, whereas most (80% +/- 3%) remained in the venular portions. Moreover, magnetic resonance images showed that contrast-enhanced signal-time courses obtained from different regions of the myocardium during retrograde cardioplegia varied significantly. Localized phosphorus 31 spectra showed that retrograde cardioplegia required a higher flow rate than did antegrade cardioplegia to sustain normal myocardial energy metabolism.

CONCLUSIONS

We conclude that retrograde cardioplegia provides significantly less capillary flow than does antegrade cardioplegia. Its microvascular perfusion varies significantly among the various small areas of the myocardium. As a result, its efficacy in sustaining normal myocardial energy metabolism is lower than that of antegrade cardioplegia.

Simultaneously monitoring both T(1) and T(2)* signal intensities on a bolus injection of Gd-DTPA may distinguish infarcted myocardium

PURPOSE

To determine whether injured myocardium may be identified by simultaneously monitoring contrast-induced T(1) and T(2)* signal intensity time-course changes with an interleaved T(1)-T(2)* imaging sequence.

MATERIALS AND METHODS

Gadolinium-diethylene triamine pentaacetic acid (0.05 mmol/ kg) was injected as a bolus into ex vivo pig hearts, and simultaneous T(1) and T(2)* time-courses were obtained during the first pass.

RESULTS

Observing contrast-enhanced R(1) or R(2)* rates (1/T(1) or 1/T(2)* times, respectively) early after contrast injection did not fully differentiate viable from nonviable myocardium. T(2)* recovery at maximal T(1) signal intensity, measured using simultaneous T(1) and T(2)* imaging, displayed a significantly different percentage recovery (P < 0.05) among normal (30.5 +/- 2.4% of baseline value), reperfused infarcted (63 +/- 7.2%), and low-reflow infarcted (90 +/- 2.8%) myocardium.

CONCLUSION

Simultaneously monitoring both T(1) and T(2)* signal intensities may help in the assessment of myocardial injury.