Single heartbeat cardiac tagging for the evaluation of transient phenomena - PubMed (original) (raw)
Clinical Trial
Single heartbeat cardiac tagging for the evaluation of transient phenomena
Daniel A Herzka et al. Magn Reson Med. 2005 Dec.
Abstract
Many cardiac abnormalities are of a transient nature, creating a beat-to-beat variation in myocardial function. This work presents the cardiac imaging technique for the measurement of regional function during transient cardiac phenomena. All information necessary for the reconstruction of a cine loop is acquired within a single heartbeat, avoiding the temporal blurring introduced by segmented imaging due to the assumption of cardiac cycle periodicity. This method incorporates a gradient-optimized, high-efficiency EPI-SSFP sequence and TSENSE parallel imaging. For acquisitions with readout resolutions of 128,160, 192, and 256 points, the technique produced images with average temporal resolution of 35, 39, 43, and 52 ms and average spatial resolutions of 2.65, 2.12, 1.77, and 1.32 mm in the readout direction, respectively, and 2.88 and 2.08 mm in the phase encode direction for acceleration rates of 3 and 4, respectively. Local apparent strains in the single slice and measurements of ventricular end-systolic and end-diastolic areas were used as quantitative measures to validate the single heartbeat technique. To demonstrate the utility of the sequence, movie loops were acquired for multiple heartbeats in non-breath-held acquisitions as well as during a Valsalva maneuver. A heartbeat-interleaved acquisition allowed for the reconstruction of nonaccelerated images from R contiguous heartbeats. Images reconstructed from such data displayed tag blurring and reduced tag persistence due to motion and inter-heartbeat variability. Images acquired during the Valsalva maneuver demonstrated apparent beat-to-beat variability, visible both in the images and as changing strain patterns and ventricular volumes.
Figures
FIG. 1
(a) Acquisition schematic: Upon detection of the ECG trigger, the myocardial tagging sequence and a magnetization-restoring sequence were played. Ten TRs were acquired for each cardiac phase using the pulse sequence shown in (b), yielding 30 lines of _k_-space per cardiac phase before TSENSE processing. At the end of the imaging period, typically covering 85–95% of the cardiac cycle, a magnetization-storing α/2 pulse followed by a fat saturation sequence was applied. (b) displays the three-echo EPISSFP sequence used for the single-heartbeat cine acquisitions. The use of gradient optimized waveforms results in the shortest possible TRs and highest efficiency allowed by a given slew rate.
FIG. 2
(a) Complete set of single heart beat tagged cine images obtained using R = 3 and a 192 × 90 image matrix compose of 21 cardiac phases with a temporal resolution of 42 ms. Tags are clearly visible for all of systole and part of diastole although they disappear by diastasis. Numbers denote time (in milliseconds) after the QRS trigger is detected. (b) Example of single heartbeat tagged cine images obtained with R = 4 and a 256 × 120 matrix. Eighteen cardiac phases were acquired with a temporal resolution of 51 ms.
FIG. 3
Comparison of accelerated (R = 4, left column) single heartbeat image reconstruction and 4-heartbeat (R = 1, right column) reconstruction of the same raw data acquired during breathing. Tag blurring is obvious both in the heart and in other moving organs such as the liver for the standard nonaccelerated reconstruction. Note that the blurring also affects tag visibility in the later cardiac phases. Numbers denote time in milliseconds after ECG trigger. Images window/leveled differently: (38/15 versus 120/49).
FIG. 4
The end-diastolic cardiac phase for a 25-heartbeat acquisition during a Valsalva maneuver. LV chamber contours are displayed in white and heartbeat number is displayed in the bottom right-hand corner of each image. The effects of the Valsalva maneuver are evident in the diastolic images, where the heart's conformational change is obvious. After breath-holding for 5 heartbeats, the maneuver begins reducing the size of both the LV and the RV. When viewed in movie mode, the flattening of the posterior wall becomes obvious. The increased intrathoracic pressure persists until the 21st heartbeat, when breathing resumes. The data from each heartbeat consisted of 21 cardiac phases with a temporal resolution of 42 ms and a 192 × 120 matrix acquired over a 36 × 27 cm FOV.
FIG. 5
Strains (% fractional shortening) along the tag direction in lateral (left) and septal (right) segments derived from the images showcased in Fig. 4. The vertical axes correspond to cardiac phase for each heartbeat; the horizontal axes denote heartbeat number creating a temporal plane that progresses from bottom to top and from left to right. Although the maneuver begins after the 5th heartbeat, strains do not appreciably change until later heartbeats. Breathing resumes during the 21st beat, again changing strain patterns. Note that beat 12 displays a different strain pattern than other neighboring beats, indicating beat-to-beat differences that would lead to image artifact in a segmented acquisition. These patterns are seen as changes in conformation when viewed in cine mode.
FIG. 6
Plots of relative (a) end-systolic right ventricular area, (b) end-systolic left ventricular area, and (c) end-diastolic left ventricular area measured over the initial 20 heartbeats displayed in Fig. 4. Upon initiation of the Valsalva maneuver (beat 5, dotted line), the right ventricular volume decreases, and after 2–3 beats the left ventricular volumes follow. Overall, the right ventricle (a low pressure cavity) decreases much more than either of the left-ventricular volumes.
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