A comparison of prospective and retrospective respiratory navigator gating in 3D MR coronary angiography

(1)H-MR imaging of the lungs at 3.0 T

BACKGROUND

One disadvantage of magnetic resonance imaging (MRI) is the inability to adequately image the lungs. Recent advances in hyperpolarized gas technology [e.g., helium-3 ((3)He) and xenon-129 ((129)Xe)] have changed this. However, the required technology is expensive and often needing extra physics or engineering staff. Hence there is considerable interest in developing (1)H (proton)-based MRI approaches that can be readily implemented on standard clinical systems. Thus, the purpose of this work was to compare a newly developed free breathing proton-based MR lung imaging method to that of a standard gadolinium (Gd) based perfusion approach.

METHODS

Healthy volunteers [10] were scanned using a 3-T MRI with 8 parallel receivers, and a cardiac gated fast spin echo (FSE) sequence. Acquisition was cardiac triggered, with different time delays incremented to cover the entire cardiac cycle. Image k-space was filled rectilinearly. But to reduce motion artefacts k-space was retrospectively sorted using the minimal variance algorithm (MVA), based on physiologic data recorded from both the respiratory bellows and electrocardiogram (ECG). Resorted and reconstructed FSE images were compared to contrast enhanced lung images, obtained following intravenous injection of Gd-DTPA-BMA.

RESULTS

Biphasic variation in FSE lung signal intensity was observed across the cardiac cycle with a maximal signal change following rapid cardiac ejection (between S and T waves), and following rapid isovolumetric relaxation. A difference image between systolic and diastolic states in the cardiac cycle resulted in images with improved lung contrast to noise ratio (CNR). FSE image intensity was uniform over lung parenchyma while Gd-based enhancement of spoiled gradient recalled echo (SPGR) images showed gravitational dependence.

CONCLUSIONS

Here we show how 1H-MR images of lung can be obtained during free breathing. The image contrast obtained during this approach is likely the result of flow and oxygen modulation during the cardiac cycle. This free breathing method provides lung images comparable to those obtained using Gd-enhancement. Besides having the advantage of free breathing, this approach doesn't require any Gd-contrast or suffer from methodological problems associated with perfusion (e.g., poor bolus timing). However, as gravitational differences typically observed in lung perfusion are not visible with this method it is not providing exclusive microvascular perfusion information.

Direct coronary motion extraction from a 2D fat image navigator for prospectively gated coronary MR angiography

PURPOSE

Direct 2D tracking of cardiac motion may provide superior respiratory navigator gating for coronary magnetic resonance angiography compared to conventional liver-diaphragm navigators. However, additional 2D processing for motion extraction is unsuitable for real-time prospective gating. In this work, a 2D fat-selective image navigator, which delineates the epicardial fat surrounding coronary arteries, is developed to directly monitor epicardial fat motion at every heartbeat in real-time for prospective gating.

METHODS

The proposed navigator is incorporated into a real-time interactive software that allows rapid setup and efficient motion extraction, and runs on standard clinical hardware without any additional dedicated components for processing. The proposed 2D cardiac fat image navigator was compared with the conventional 1D diaphragm navigator in free-breathing b-SSFP coronary MRAs in 12 healthy volunteers at 1.5T.

RESULTS

Real-time motion extraction from 2D cardiac fat navigator images was feasible within 20 ms, enabling successful prospectively gated coronary magnetic resonance angiographies in all subjects. Compared to 1D diaphragmatic navigator, 2D fat image navigator reduced scan time by 38% (P < 0.0005), and significantly improved vessel sharpness, myocardial suppression, and image quality (P < 0.05).

CONCLUSION

This demonstrates the feasibility of a 3D SSFP coronary magnetic resonance angiography sequence using a 2D epicardial fat image as a navigator for real-time prospective motion tracking.

DC-gated high resolution three-dimensional lung imaging during free-breathing

PURPOSE

To use the acquisition of the k-space center signal (DC signal) implemented into a Cartesian three-dimensional (3D) FLASH sequence for retrospective respiratory self-gating and, thus, for the examination of the whole human lung in high spatial resolution during free breathing.

MATERIALS AND METHODS

Volunteer as well as patient measurements were performed under free breathing conditions. The DC signal is acquired after the actual image data acquisition within each excitation of a 3D FLASH sequence. The DC signal is then used to track respiratory motion for retrospective respiratory gating.

RESULTS

It is shown that the acquisition of the DC signal after the imaging module can be used in a 3D FLASH sequence to extract respiratory motion information for retrospective respiratory self-gating and allows for shorter echo times (TE) and therefore increased lung parenchyma SNR.

CONCLUSION

The acquisition of the DC signal after image signal acquisition allows successful retrospective gating, enabling the reconstruction of high resolution images of the whole human lung under free breathing conditions.

Acceleration of tissue phase mapping with sensitivity encoding at 3T

BACKGROUND

The objective of this study was to investigate the impact of sensitivity encoding on the quantitative assessment of cardiac motion in black blood cine tissue phase mapping (TPM) sequences. Up to now whole volume coverage of the heart is still limited by the long acquisition times. Therefore, a significant increase in imaging speed without deterioration of quantitative motion information is indispensable.

METHODS

20 volunteers were enrolled in this study. Each volunteer underwent myocardial short-axis TPM scans with different SENSE acceleration factors. The influence of SENSE acceleration on the measured motion curves was investigated.

RESULTS

It is demonstrated that all TPM sequences with SENSE acceleration have only minimum influence on the motion curves. Even with a SENSE factor of four, the decrease in the amplitude of the motion curve was less than 3%. No significant difference was observed for the global correlation coefficient and deviation between the motion curves obtained by the reproducibility and the SENSE accelerated measurements.

CONCLUSIONS

It is feasible to accelerate myocardial TPM measurements with SENSE factors up to 4 without losing substantial information of the motion pattern.

Z intensity-weighted position self-respiratory gating method for free-breathing 3D cardiac CINE imaging

A free-breathing 3D cine steady-state free precession (SSFP) technique was developed using the z intensity-weighted position (ZIP) which is the center of mass of a projection along the slice direction as a respiratory gating signal. The ZIP signal was continuously acquired using a slice encoded k-space center sampling in every TR. The performance of this gating method was compared with a method using the k-space center signal (KC) and with conventional 2D breath-hold cine SSFP in healthy subjects by measuring image quality and left ventricular function. The preliminary data obtained here demonstrated that the ZIP gating method provided superior respiratory motion artifact suppression when compared to the KC gating and provided left ventricular ejection fractions, and end-diastolic and end-systolic volumes similar to those obtained with the breath-hold 2D cine SSFP acquisition.

Free-breathing 3-dimensional steady-state free precession coronary magnetic resonance angiography: comparison of four navigator gating techniques

This work compared the performance of four navigator gating algorithms [accept/reject (A/R), diminishing variance algorithm (DVA), phase ordering with automatic window selection (PAWS) and retrospective gating (RETRO)] in suppressing respiratory motion artifacts in free-breathing 3D balanced steady-state free precession coronary MRA. In 10 volunteers, the right coronary artery (RCA) or the left anterior descending artery (LAD) was imaged (both if time permitted) at 1.5 T with the four gating techniques in random order. Vessel signal, vessel contrast and motion suppression were scored by the consensus of two blinded readers. In 15 imaged vessels (nine RCA and six LAD), PAWS provided significantly better image quality than A/R (P<.05), DVA (P=.02) and RETRO (P=.002). While the quality difference between A/R and DVA was not statistically significant, both algorithms yielded significantly better image quality than RETRO. PAWS and DVA were the most efficient algorithms, providing an approximately 20% and 40% relative increase in average navigator efficiency compared to A/R and RETRO, respectively.

High temporal resolution OCT using image-based retrospective gating

High temporal resolution OCT imaging is very advantageous for analyzing cardiac mechanics in the developing embryonic heart of small animals. An image-based retrospective gating technique is presented to increase the effective temporal resolution of an OCT system and to allow visualization of systolic dynamics in 3D. The gating technique employs image similarity measures for rearranging asynchronously acquired input data consisting of a time series of 2D images at each z position along the heart volume, to produce a time sequence of 3D volumes of the beating heart. The study includes a novel robust validation technique, which quantitatively evaluates the accuracy of the gating technique, in addition to visual evaluations by 2D multiplanar reformatting (MPR) and 3D volume rendering. The retrospective gating and validation is demonstrated on a stage 14 embryonic quail heart data set. Using the validation scheme, it is shown that the gating is accurate within a standard deviation of 4.7 ms, which is an order of magnitude shorter than the time interval during which systolic contraction (approximately 50 ms) occurs in the developing embryo. This gating method has allowed, for the first time, clear visualization of systolic dynamics of the looping embryonic heart in 3D.

Respiratory self-gating for free-breathing abdominal phase-contrast blood flow measurements

PURPOSE

To demonstrate the feasibility of using a free-breathing (FB) respiratory self-gated (RSG) approach for abdominal phase-contrast (PC) blood flow measurements.

MATERIALS AND METHODS

PC-magnetic resonance imaging (MRI) flow measurements were performed within the right renal artery, common hepatic artery, and main portal vein during breath-hold (BH) and FB with both signal averaging and RSG in eight healthy volunteers. The resultant images were qualitatively scored by two independent reviewers blinded to acquisition techniques. Blood flow volume and cross-sectional vessel size measurements were compared for three techniques.

RESULTS

The overall efficiency for the RSG-PC sequence was 38.9% +/- 4.7%. Images acquired with RSG effectively mitigated respiratory motion artifacts, which were clearly evident within FB signal-averaged images. RSG produced similar image quality to that of BH techniques (P > 0.146) and resulted in similar vessel size measurements (P = 0.694). Flow results for both FB RSG and signal-averaged reconstructions correlated well with BH flow measurements (r = 0.97 and 0.92, P < 0.001). However, only the RSG methods demonstrated excellent absolute agreement with BH-PC flow measurements (P = 0.600), with signal-averaged methods resulting in significant overestimations.

CONCLUSION

RSG methods can limit respiratory motion artifacts to reduce flow measurement inaccuracies during free-breathing PC measurements in the abdomen.

A dual-projection respiratory self-gating technique for whole-heart coronary MRA

PURPOSE

To investigate the accuracy of a dual-projection respiratory self-gating (DP-RSG) technique in dynamic heart position measurement and its feasibility for free-breathing whole-heart coronary MR angiography (MRA).

MATERIALS AND METHODS

A DP-RSG method is proposed to enable accurate direct measurement of heart position by acquiring two whole-heart projections. On 14 volunteers we quantitatively evaluated the efficacy of DP-RSG by comparison with diaphragmatic navigator (NAV) and single-projection-based respiratory self-gating (SP-RSG) methods. For DP-RSG we also compared center-of-mass and two profile-matching algorithms in deriving heart motion. Coronary imaging was conducted on eight volunteers based on retrospective gating to preliminarily validate the effectiveness of DP-RSG for whole-heart coronary MRA. Comparison of vessel delineation was performed between images reconstructed using different gating methods.

RESULTS

The quantitative evaluation shows that DP-RSG more accurately tracks heart motion than NAV with all gating window (GW) values and SP-RSG approaches with GW>or=2.5 mm and profile-matching algorithms are more reliable for motion derivation than center-of-mass calculations with GW>or=1.0 mm. Whole-heart coronary MRA studies demonstrate the feasibility of using DP-RSG to improve overall delineation of the coronary arteries.

CONCLUSION

DP-RSG is a promising approach to better resolve respiratory motion for whole-heart coronary MRA compared to conventional NAV and SP-RSG.

Respiratory self-gated four-dimensional coronary MR angiography: a feasibility study

The four-dimensional (4D) coronary MR angiography (MRA) approach has been developed to eliminate the need for accurate determination of the acquisition window and trigger delay time. Diaphragm navigator (NAV) has been the conventional respiratory gating method for free-breathing coronary MRA. However, NAV echo acquisition interrupts the continuous radiofrequency pulse application required for 4D steady-state free precession coronary MRA. The objective of this work was to investigate the feasibility of a respiratory self-gating (RSG) technique for 4D coronary MRA and its effectiveness by comparing with retrospective NAV gating. Data were acquired continuously throughout the cardiac cycle and retrospectively remapped to cardiac phases based on the electrocardiogram signal simultaneously recorded. An RSG signal extracted from a direct measurement of the heart position was used for retrospective respiratory gating and motion correction. In seven healthy volunteers, 4D MRA images were reconstructed, allowing retrospective assessment of the cardiac motion of the coronary artery and selection of the images with the best vessel delineation. Statistical analysis shows that 4D RSG provides coronary artery delineation comparable to mid-diastole images acquired using NAV. Respiratory self-gating is an effective method for eliminating respiratory motion artifacts and allows 4D coronary MRA during free breathing.

3D MR coronary angiography: optimization of the technique and preliminary results

OBJECTIVE

Current clinical full MR angiography with multiple breathhold multiple thin slab acquisition (MTS) is difficult and arduous. This study describes the optimisation of the whole heart free - breathing balanced turbo field echo (B-TFE) protocol. A high-resolution image of the whole heart is produced in less or comparable time to MTS acquisition and allows for reconstruction afterwards to visualise the individual coronary arteries. The scan is easily performed because the volume has to be targeted only once.

DESIGN AND SETTING

Eighteen healthy adults without a history of cardiovascular disease underwent free-breathing 3D MR angiography with the B-TFE protocol. The whole-heart data set was reformatted in identical orientations in all subjects to visualise the major coronary arteries.

MAIN OUTCOME MEASURES

Vessel length, signal and contrast to noise ratio were determined and compared for each vessel.

RESULTS

Mean visible vessel lengths were 116 mm for the right, 102 mm for the left main and left descending and 76 mm for the left circumflex coronary artery. The average signal to noise ratio was 7.5 and contrast to noise ratio was 4.9. Because of the need for synchronised cardiac and respiratory triggering the coronaries could not be judged in 25% of the subjects.

CONCLUSIONS

The optimised B-TFE protocol had equal judgeability and vessels could be judged over longer contiguous distances compared to earlier implementations of the B-TFE protocol. We conclude whole heart free breathing navigator-gated and slice-tracked 3D coronary MR angiography with use of the adjusted B-TFE protocol is possible, but still suboptimal for clinical use.

Navigator-gated three-dimensional MR angiography of the pulmonary arteries using steady-state free precession

PURPOSE

To assess the quality of a navigator-gated, free breathing, steady-state free precession (SSFP) technique in comparison to a single breathhold for pulmonary artery imaging in normal volunteers.

MATERIALS AND METHODS

Sagittal sections of the left pulmonary arteries of 10 volunteers were obtained with a three-dimensional SSFP sequence using both a single breathhold of 30 seconds and a navigator-gated version of the same sequence. The images were compared and rated by a blinded cardiovascular radiologist for image quality, sharpness, and artifact.

RESULTS

On a scale ranging from -2 to 2, in which positive numbers denote that the navigator method was favorable compared to the single breathhold method, image quality was rated 0.7+/-1.4, sharpness 0.6+/-1.5, and artifact 0.1+/-1.4. Thus, there was no statistical difference between the two methods.

CONCLUSION

The navigator-gated SSFP sequence is able to acquire images equal in quality to the breathhold sequence. This may be of clinical importance for pulmonary imaging in patients who are unable to sustain a long breathhold.

Using the axis of rotation of polar navigator echoes to rapidly measure 3D rigid-body motion

An improved technique to prospectively correct three-dimensional rigid-body motion using polar spherical navigator (pNAV) echoes is presented. The technique is based on acquiring pNAVs of an object in a baseline and rotated position and determining the axis of rotation (AOR) between data sets, thereby reducing 3D rotations to a 2D, planar rotation. Finding the AOR is simplified by prerotating the baseline trajectory, which forces the axis to lie within a specific polar region of a spherical shell in k-space. Orbital navigator echoes are interpolated from the pNAV data in planes orthogonal to the AOR and cross-correlated to determine the 2D rotation. The rotation about the AOR is used in conjunction with its orientation to calculate the overall 3D rotation. The pNAV-AOR technique was tested for its precision, accuracy, and processing speed in detecting compound rotations and translations of varying magnitude. In comparison to the spherical navigator echo technique, the pNAV-AOR technique is noniterative, fast, and independent of rotation magnitude and direction. At low SNR, the technique can detect compound rotations to 0.5 degrees accuracy in an estimated 100 msec, indicating that prospective 3D rigid-body motion correction may be feasible with this technique.

Simultaneous multiple volume (SMV) acquisition algorithm for real-time navigator gating

Navigator gating techniques can effectively reduce motion effects in MRI by accepting data only when the object is in a small range of positions at the cost of significantly prolonging scan time. A simultaneous multiple volume (SMV) algorithm is reported here that can substantially increase the scan efficiency while maintaining the effectiveness of motion suppression. This is achieved by acquiring different image volumes at different motion states. Initial experiments demonstrate that SMV can significantly increase the scan efficiency of navigator MRI.

Prospective navigator gating with a dual acceptance window technique to reduce respiratory motion artifacts in 3D MR coronary angiography

A prospective navigator algorithm with a dual acceptance window (DAW) technique was developed to reduce image artifacts induced by respiratory motion without increasing imaging time. A phantom study shows that the ghost level measured by ghost-to-image ratio was reduced by 27.1 % (p < 0.005) using the DAW technique compared to the conventional single acceptance window technique. This DAW technique can also be used to reduce imaging time while maintaining comparable ghosting level.