A significant portion of the patient population has difficulty performing the necessary breath-holds for conventional segmented cine MRI. Overall image quality (temporal and spatial resolution, SNR, or CNR) could be sacrificed to reduce the breath-hold duration. In this work we present the respiratory self-gating technique as an alternative that does not require such trade-offs, ideally decoupling the choice of relative imaging parameters from breath-hold duration.
A study comparing breath-holding to free-breathing with respiratory self-gating in normal volunteers demonstrated no significant difference in quantitative assessments of image sharpness while respiratory self-gating demonstrated significant improvements over simple k-space averaging. With the exception of the long-axis orientation scores for Reviewer 2, comparisons between breath-holding and respiratory self-gating qualitative image sharpness scores demonstrated no significant differences between breath-holding and respiratory self-gating with significant improvements over simple k-space averaging in both orientations. However, ranking of image sharpness resulted in statistically significant superior rankings for the breath-hold image series relative to the respiratory self-gated image series. Most of the respiratory self-gated image series exhibited a subtle background temporal intensity variation that was only discernable when replaying each image stack as a cine loop. These resulted from the combination of views at different cardiac phases that were sampled while the imaged tissue was at slightly different positions in the respiratory cycle. Though not affecting the quantitative measures, these subtle intensity variations may have influenced the absolute qualitative scoring or ranking by the reviewers when viewing the series side by side as a dynamic cine display. Future patient studies are necessary to evaluate whether such subtle variations affect the clinical utility of the respiratory self-gating technique.
The overall imaging time required for the free-breathing respiratory self-gated acquisitions were much longer than those required by comparable breath-hold strategies. Even while taking into consideration the recovery time necessary between breath-held acquisitions, such increases in scan time could potentially hinder patient comfort by increasing the overall examination time. To reduce acquisition time, the scan efficiency could potentially be increased by utilizing some of the motion correction techniques already described by Hardy et al. (10
). Alternatively, a reduction in the total number of views acquired for each image should be possible. In an effort to focus this work on the derivation of gating signals rather than on the salient properties of undersampled projection reconstruction (22
), we chose to acquire 160 total views/image, effectively 20% less undersampled than previously reported radial TrueFISP segmented cine techniques (13
). Decreases of in scan time should be possible by simply incorporating similar imaging parameters. Small additional decreases in scan time can likely be achieved by reducing the number of dummy pulses applied to achieve steady state prior to data acquisition.
The particular choices of ROI position were based on previous observations of significant blurring in these regions when using averaging techniques while imaging during free-breathing. Although other ROI positions may have sufficed, we chose to use a single position for each orientation to maintain a consistent methodology. An ROI with dimensions smaller than those necessary to cover the entire heart were used for these studies to reduce correlation coefficient computation times allowing near real-time generation of the respiratory gating signal during the acquisition. However, the necessity to manually select a limited ROI, specifically located at an anatomic position sufficiently reflective of respiratory motion, could add undesirable procedural complications if utilizing respiratory self-gating techniques in a clinical setting. Initial off-line postprocessing investigations of simply using a much larger ROI covering the entire heart for respiratory self-gating have demonstrated that such a strategy produces gating signals very similar to those produced from a reduced ROI. An example comparing the self-gating signal produced using a smaller ROI (50 × 50 mm2) and larger ROI covering the entire heart (120 × 120 mm2) are shown in . However, more rigorous studies are necessary to confirm that the larger ROI strategy continues to produce signals sufficient for respiratory gating. Provided that sufficient gating signals can be produced and that algorithms and software can be improved to handle the added computational requirements necessary using a larger ROI, such a strategy could significantly reduce procedural burdens when utilizing the respiratory self-gating techniques.
FIG. 7 Comparison between the respiratory self-gating signal derived using a smaller ROI of 50 × 50 mm2 (a) and the respiratory self-gating signal derived using a larger ROI of 120 × 120 mm2 covering the entire heart (b). Notice the similarity (more ...)
The choice of 20 cardiac phases for the respiratory target image series was based on the common use of 15 to 20 cardiac phases for conventional cine imaging. Further investigations into optimum ROI choice in different imaging orientations and the optimum number of target cardiac phases could provide improvements to the respiratory self-gating technique.
For this initial respiratory self-gating work, low-resolution gating images were reconstructed as opposed to full-resolution gating images, including all 192 sampled readout points for each view. Reconstruction of full-resolution gating images would have resulted in significant under-sampling streak artifact. The position of these artifacts would change according to the particular k
-space segment sampled and thereby compromise the correlation comparisons between images reconstructed from imaging data acquired at different k
-space segments. Intuitively, one might expect that the use of these lower resolution images for gating signal derivation might result in an inability to detect in-plane rigid displacements of distances smaller than the low-resolution image voxel dimensions. However, as rigorously described by Sussman et al. (12
) who refers to the work of Hajnal et al. (25
), image resolution is actually not a limiting factor given sufficient image SNR. However, future studies are certainly necessary to determine (a) the minimum displacement discernable with the RSG technique given the use of a relatively high SNR TrueFISP pulse sequence implementation and (b) the optimal gating image spatial and temporal resolution for free-breathing cine MRI, particularly to measure displacement for motion correction.
Previous work has described the development of image correlation, complex echo-peak and center-of-mass (COM) cardiac self-gating strategies for “wire-less” segmented cine MRI (9
). Although each of these self-gating strategies can provide respiratory gating information, we chose the correlation approach for this initial feasibility study because it provides a gating signal derived from 2D image space, thereby avoiding corruption of the respiratory gating signal by tissues external to the selected ROI. Image-domain correlation-based approaches can provide respiratory gating information based on changes in the position of the heart rather than remote changes in diaphragm or chest wall position. This property should be useful while imaging free-breathing patients exhibiting erratic respiratory patterns; however, further comparison studies with bellows and NAV echo techniques are still necessary. The cardiac self-gating strategies can be combined with the respiratory self-gating strategy described by this work to allow free-breathing segmented cine MRI without the need for ECG triggering (26
). However, previous work combining the self-gating strategies utilized only retrospective respiratory gating, potentially much less efficient than the prospective respiratory gating approach described by this work. Furthermore, the accuracy of self-gated cardiac cycle synchronization during free-breathing has yet to be established. Ideally for combined cardiac and prospective respiratory self-gating, efficient signal processing techniques are necessary to allow real-time separation of the respiratory and cardiac gating signal components and real-time detection of cardiac phase and respiratory position based on these signals.
Although rigorous clinical validation of the respiratory self-gating technique has not been performed, wall motion abnormalities should not pose any complications for respiratory self-gating provided that heart motion remains constant relative to respiratory cycle position. However, particularly because of the long overall acquisition times for the respiratory self-gating scans, arrhythmias producing intermittent changes in cardiac motion could certainly pose problems for the proposed gating signal derivation strategy. Further investigations are necessary to evaluate the sensitivity of the respiratory self-gating technique to intermittent changes in cardiac motion patterns.
Derivation of the respiratory target image series, hRT
, based on the target acquisition strategy proposed by this work, relies on an assumption that the hm
set generated at expiration will produce a correlation signal, Cm
), having the greatest number samples above 0.9(Cmax
) + Cmin
. Although successful for these initial feasibility studies, such an assumption may not always hold. In these cases it may be necessary to include additional information for the derivation of the respiratory target image series, potentially provided by a COM signal derived from the same raw data (26
). Alternatively, a hybrid approach could be used that incorporates a single-heartbeat breath-hold to derive hRT
). Derivation of a gating signal representative of respiration depends on adequate respiratory motion within the chosen ROI. For this preliminary study, we investigated imaging in only two common cardiac cine imaging orientations. Furthermore, the duration of the target acquisition period was empirically chosen (20 s to sample 330 low-resolution images) based on initial failures of the strategy when using shorter durations of only 10 s. Further studies are necessary to optimize target acquisition duration and evaluate the respiratory self-gating technique in all clinically important orientations.
Parallel imaging techniques can allow significant reductions in breath-hold lengths for segmented cine MRI (27
) while continuing to provide sufficient spatial resolution, temporal resolution, and overall image quality. These accelerated imaging strategies may be sufficient to image patients who cannot maintain the breath-holds necessary for conventional segmented cine techniques. However, the continued necessity for even a very short breath-hold remains problematic for some patients. Future studies are needed to determine if parallel imaging techniques, free-breathing respiratory gating techniques as described by this work, or some combination provide the best practical solution for imaging patients having difficulties breath-holding.