Generalized self-navigated motion detection technique: Preliminary investigation in abdominal imaging.

Patient motion remains a primary obstacle to diagnostic image quality, especially in the abdomen, despite the existence of various motion artifact reduction techniques. This work presents a self-navigated motion detection method that can be generalized for most pulse sequences and k-space trajectories. Motion information is extracted directly from raw MR data, requiring no additional gradient or RF pulses, no physiologic monitoring equipment, and minimal--if any--impact on scan time. Initial feasibility results with a two-dimensional gradient echo sequence are shown in phantom studies and in vivo volunteer abdominal studies, demonstrating the sensitivity of the method to both respiratory motion and cardiovascular pulsatility. Prospectively gated images were acquired using the self-navigated data to synchronize image acquisition with motion. These preliminary results suggest that the self-navigated method is a promising technique for reducing motion artifacts in clinical abdominal and cardiac applications.     

Time-resolved contrast-enhanced carotid imaging using undersampled projection reconstruction acquisition

PURPOSE: To investigate the utility of nonuniform angular spacing of projections in a three-dimensional (3D) hybrid undersampled projection reconstruction (PR) acquisition for contrast-enhanced (CE) time-resolved carotid imaging. MATERIALS AND METHODS: Carotid CE magnetic resonance angiography (CE-MRA) was performed on seven healthy volunteers using a time-resolved hybrid sequence that combined undersampled PR acquisition in-plane and Cartesian slice encoding through-plane. The undersampling streak artifact comes mainly from the superior-inferior (S/I) direction in carotid imaging and is suppressed by nonuniform distribution of the projections. Phantom and volunteer studies were performed to demonstrate its efficacy. RESULTS: The undersampling streak artifact was significantly suppressed through a nonuniform distribution of the projection angles with more projections aligned along the S/I direction. The hybrid PR sequence combined with nonuniform distribution of the projection angles provided time-resolved images of the carotid arteries with high temporal resolution (two seconds per frame) and high spatial resolution (1.0 x 1.0 x 1.5 mm(3)) simultaneously. CONCLUSION: High-resolution dynamic imaging of the carotid arteries is feasible with the use of a hybrid undersampled PR acquisition. Undersampling streak artifact can be suppressed significantly through nonuniform distribution of the projections.     

A modified projection reconstruction trajectory for reduction of undersampling artifacts

PURPOSE: To reduce undersampling artifacts for a given number of repetitions of the projection reconstruction (PR) sequence by modifying its k-space trajectory to sample more mid-frequencies while reducing the sampling coverage of the peripheral spatial frequencies. MATERIALS AND METHODS: The single k-space spoke measured per repetition in the standard PR was modified so that one complete and two partial spokes were measured per repetition but with decreased k-space extent. The point spread functions (PSFs) and undersampling artifacts of the modified PR were compared with those of the standard PR for various numbers of projections. Phantom and in vivo images were used to assess the relative performance. RESULTS: PSF analysis indicated that the modified PR method provided reduced undersampling artifacts with somewhat reduced spatial resolution. The phantom and in vivo images corroborated this. CONCLUSION: The modified PR trajectory provides reduced undersampling artifact vs. the standard PR, particularly when the number of projections is limited and the artifact level is high.     

Reduced field of view and undersampled PR combined for interventional imaging of a fully dynamic field of view.

Active catheter imaging was investigated using real-time undersampled projection reconstruction (PR) combined with the temporal filtering technique of reduced field of view (rFOV). Real-time rFOV processing was interactively enabled during highly undersampled catheter imaging, resulting in improved artifact suppression with better temporal resolution than that obtained by view-sharing. Imaging with 64 to 32 projections provided a resolution of 2 x 2 x 8 mm, and four to eight true frames per second. Image artifacts were reduced when rFOV processing was applied to the undersampled images. A comparison with Cartesian rFOV showed that PR image quality is less susceptible to aliasing that results from rFOV imaging with a wholly dynamic outer FOV. Simulations and MRI experiments demonstrated that PR rFOV provides significant artifact suppression, even for a fully dynamic FOV. The near doubling of temporal resolution that is possible with PR rFOV permits accurate monitoring of highly dynamic events, such as catheter movements, and arrhythmias, such as ventricular ectopy.     

Projection reconstruction balanced fast field echo for interactive real-time cardiac imaging.

A balanced fast field echo (FFE) sequence (also referred to as true fast imaging with steady precession (true FISP)), based on projection reconstruction (PR) is evaluated in combination with real-time reconstruction and interactive scanning capabilities for cardiac function studies. Cardiac image sequences obtained with the balanced PR-FFE method are compared with images obtained with a spin-warp (2D Fourier transform (2DFT)) technique. In particular, the representation of motion artifacts in both techniques is investigated. Balanced PR-FFE provides a similar contrast to spin-warp-related techniques, but is less sensitive to motion artifacts. The use of angular undersampling within balanced PR-FFE is examined as a means to increase temporal resolution while causing only minor artifacts. Furthermore, a modification of the profile order allows the reconstruction of PR images at different spatial and temporal resolution levels from the same data. This study shows that balanced PR-FFE is a robust tool for cardiac function studies.     

MR microscopy of the rat lung using projection reconstruction

Projection reconstruction has been implemented with self-refocused selection pulses on a small bore, 2.0 T MR microscope, to allow imaging of lung parenchyma. Scan synchronous ventilation and cardiac gating have been integrated with the sequence to minimize motion artifacts. A systematic survey of the pulse sequence parameters has been undertaken in conjunction with the biological gating parameters to optimize resolution and signal-to-noise (SNR). The resulting projection images with effective echo time of <300 usec allow definition of lung parenchyma with an SNR improvement of ～15 x over a more conventional 2DFT short echo gradient sequence.     

Magnetization spoiling in radial FLASH contrast-enhanced MR digital subtraction angiography

Purpose:

To increase the in‐plane spatial resolution and image update rates of 2D magnetic resonance (MR) digital subtraction angiography (DSA) pulse sequences to 0.57 × 0.57 mm and 6 frames/sec, respectively, for intracranial vascular disease applications by developing a radial FLASH protocol and to characterize a new artifact, not previously described in the literature, which arises in the presence of such pulse sequences.

Materials and Methods:

The pulse sequence was optimized and artifacts were characterized using simulation and phantom studies. With Institutional Review Board (IRB) approval, the pulse sequence was used to acquire time‐resolved images from healthy human volunteers and patients with x‐ray DSA‐confirmed intracranial vascular disease.

Results:

Artifacts were shown to derive from inhomogeneous spoiling due to the nature of radial waveforms. Gradient spoiling strategies were proposed to eliminate the observed artifact by balancing gradient moments across TR intervals. The resulting radial 2D MR DSA sequence (2.6 sec temporal footprint, 6 frames/sec with sliding window factor 16, 0.57 × 0.57 mm in‐plane) demonstrated small vessel detail and corroborated x‐ray DSA findings in intracranial vascular imaging studies.

Conclusion:

Appropriate gradient spoiling in radial 2D MR DSA pulse sequences improves intracranial vascular depiction by eliminating circular banding artifacts. The proposed pulse sequence may provide a useful addition to clinically applied 2D MR DSA scans. J. Magn. Reson. Imaging 2012;36:249–258. © 2012 Wiley Periodicals, Inc.     

In vivo MR imaging of pulmonary arteries of normal and experimental emboli in small animals

PURPOSE: To demonstrate the feasibility of pulmonary MRA in living rodents. MATERIALS AND METHODS: A three-dimensional (3D) gradient echo sequence was adapted to perform a time-of-flight (TOF) angiography of rat lung. Angiogram with a spatial resolution of 195 x 228 x 228 microm(3) was acquired in around 33 minutes. The method was then applied in animals before and after pulmonary embolism (PE) induction. Section of the proximal right pulmonary artery was measured and compared between the two populations. RESULTS: Good quality images were obtained with a contrast-to-noise ratio (CNR) of 9 +/- 3 in the proximal part of the pulmonary artery. Cross-section areas of the right main artery are statistically different before (3.45 +/- 0.69 mm(2)) and after induction of PE (4.3 +/- 0.86 mm(2)). CONCLUSION: This noninvasive tool permits angiogram acquisition at around 200 microm spatial resolution and objective distinction between healthy and embolized arteries.     

Fast imaging of phosphocreatine in the normal human myocardium using a three-dimensional RARE pulse sequence at 4 Tesla

PURPOSE: To investigate the use of a three-dimensional rapid acquisition with relaxation enhancement (RARE) pulse sequence for direct acquisition of phosphocreatine (PCr) images of the human myocardium. MATERIALS AND METHODS: A short elliptical birdcage radiofrequency (RF) body coil was constructed to produce a uniform flip angle throughout the chest cavity. In vivo images using a spectrally-selective RARE sequence with a spatial resolution of 1.2 cm x 1.2 cm x 2.5 cm (4 cm(3)) were acquired in nine minutes and 40 seconds. RESULTS: Scans of phantoms demonstrated excellent spectral selectivity. The signal-to-noise ratio in the myocardium ranged from 12.6 in the anterior wall to 5.3 in the mid septum. CONCLUSION: This study demonstrates that PCr data can be acquired using a three-dimensional RARE sequence with greater spatial and temporal resolution than spectroscopic techniques.     

Quantification of left ventricular function with magnetic resonance images acquired in real time

The application of real-time magnetic resonance imaging (MRI) techniques to cardiac imaging is particularly attractive because current MR examinations of left ventricular (LV) function can be prohibitively long and are dependent on electrocardiographic triggering. We conducted a study of the minimum spatial and temporal resolution requirements necessary for real-time ventricular function MR imaging to quantify LV volumes accurately, both at resting conditions and during cardiac stress tests. In addition, we implemented a real-time segmented echoplanar imaging pulse sequence and used it to quantify LV volume in 10 healthy volunteers. We compared these results with those obtained using conventional gradient-echo cine imaging and found good agreement throughout the cardiac cycle (mean difference -0.8 +/- 10.6 ml). In conclusion, real-time cardiac MR imaging can be used to quantify LV volumes accurately throughout the cardiac cycle, over the physiologic range of heart rates, thereby decreasing the time required for a complete functional cardiac examination. J. Magn. Reson. Imaging 2000;12:430-438.     

Mice with a deletion in the first intron of the Col1a1 gene develop dissection and rupture of aorta in the absence of aneurysms: high-resolution magnetic resonance imaging, at 4.7 T, of the aorta and cerebral arteries.

Deletion of the majority of the first intron of the Col1a1 gene in mice leads to decreased type I collagen synthesis and content in the aortic wall. In 54% of cases, mice homozygous for the Col1a1 mutation die of thoracic hemorrhage by the age of 18 months. It is unknown whether the fatal bleeding results from an acute dissection of the aortic wall or a gradually developing dilatation of the medial layer prior to rupture. We optimized high-resolution MRI methods using a 4.7 T MR scanner to obtain in vivo images of the entire mouse aorta. The MR images were acquired in three imaging planes using gradient echo, spin echo, and spin echo with inversion recovery pulse sequences with a maximum in-plane resolution of 68 x 68 microm and acquisition times less than 10 min. In five Col1a1 mutated mice aged 16 months, the MR images showed no signs of aneurysmal dilatation, wall defects, or former dissection, suggesting that the mechanism for aortic rupture is an acute dissection of the aortic medial layer. Cerebral arteries were imaged using a three-dimensional time of fight pulse sequence. The resolution of 73 x 73 x 94 microm showed normal cerebral arteries. Histology showed a 22% thinner cerebral artery wall in Col1a1 mutated mice.     

Influence of physiologic motion on the appearance of tissue in MR images

Studies were performed to determine the possible influence of physiologic motion on the parenchymal intensity of organs in magnetic resonance (MR) images. It is known that periodic motion associated with respiration and cardiac function causes characteristic artifacts in spin-warp images. The present study shows that bulk motion can also cause striking intensity changes at velocities equivalent to the craniocaudal respiratory excursion of organs in the upper abdomen. The magnitude of the effect depends on the velocity and direction of motion with respect to the three orthogonal axes of the imager and on the technical details of the imager and pulse sequence. Large systematic errors in calculated tissue relaxation times are possible due to this phenomenon. The findings have important implications for clinical imaging because motion can cause artifactual changes in the gray-scale relationships among tissues. Some pulse sequences are much less sensitive to these effects. These results provide guidance for selecting MR techniques that reduce the detrimental effect of respiratory and other physiologic motion on examinations of the upper abdomen and thorax.     

Single breath-hold whole-heart MRA using variable-density spirals at 3T.

Multislice breath-held coronary imaging techniques conventionally lack the coverage of free-breathing 3D acquisitions but use a considerably shorter acquisition window during the cardiac cycle. This produces images with significantly less motion artifact but a lower signal-to-noise ratio (SNR). By using the extra SNR available at 3 T and undersampling k-space without introducing significant aliasing artifacts, we were able to acquire high-resolution fat-suppressed images of the whole heart in 17 heartbeats (a single breath-hold). The basic pulse sequence consists of a spectral-spatial excitation followed by a variable-density spiral readout. This is combined with real-time localization and a real-time prospective shim correction. Images are reconstructed with the use of gridding, and advanced techniques are used to reduce aliasing artifacts.     

Coronary MRA with 3D undersampled projection reconstruction TrueFISP.

Undersampled projection reconstruction (PR) offers improved imaging efficiency allowing a relative tradeoff between signal-to-noise ratio (SNR) or streak artifact and the number of acquired k-space views rather than the tradeoff between resolution or aliasing artifact and the number of acquired k-space lines inherent to Fourier imaging techniques. TrueFISP (true fast imaging with steady state precession) is ideally suited for undersampled PR imaging because of its inherently high SNR. The purpose of this work was to investigate the feasibility of using undersampled three-dimensional (3D) PR TrueFISP for breathhold coronary artery imaging. Phantom studies and a preliminary in vivo comparison demonstrated the improved spatial resolution of PR over Fourier TrueFISP with the same imaging time. In a 24-heartbeat coronary imaging scheme, segmented 3D PR TrueFISP provided a 1.0 x 1.0 mm(2) isotropic in-plane voxel size while acquiring between 93 and 153 views per partition. Streak artifacts were present in some images but were not found to impede coronary artery delineation. In conclusion, 3D undersampled PR TrueFISP provides isotropic in-plane voxel size within a single breathhold and is a promising technique for coronary artery imaging.     

High-resolution magnetic resonance imaging at 2 Tesla: potential for atherosclerotic lesions exploration in the apolipoprotein E knockout mouse

INTRODUCTION: The aim of the present study was to evaluate the potential of high-resolution MRI at 2 Tesla (T) for direct noninvasive imaging of the aortic wall in a mouse model of atherosclerosis. MATERIAL AND METHODS: A specific mouse antenna was developed and sequence parameters were adjusted. T(1)- and T2-weighted images of abdominal aorta were obtained at 2 T with a spatial resolution of 86 x 86 x 800 microm3 in vivo. With a dedicated small coil, ex vivo MRI of the aorta was performed with a spatial resolution of 54 x 54 x 520 microm3. RESULTS: In vivo, the aortic wall was clearly defined on T(2)-weighted images in 15 of 16 mice: along the aorta the lumen circumference ranged from 1.07 to 3.61 mm and mean wall thickness from 0.11 to 0.67 mm. In vivo measurements of plaque distribution were confirmed by ex vivo MR imaging and by histology, with a good correlation with histology regarding lumen circumference (r = 0.94) and wall thickness (r = 0.97). CONCLUSION: Magnetic resonance imaging at 2 T to analyze in vivo atherosclerotic lesions in mice is possible with a spatial resolution of 86 x 86 x 800 microm3 and thus can be used for noninvasive follow-up in evaluation of new drugs.     

Resolution improvement in thick-slab magnetic resonance digital subtraction angiography using SENSE at 3T

PURPOSE: To evaluate the use of parallel imaging (sensitivity encoding [SENSE]) to improve spatial resolution and achieve sub-second temporal resolution in fluoroscopic contrast-enhanced, magnetic resonance digital subtraction angiography (MR-DSA). MATERIALS AND METHODS: A MR-DSA sequence was optimized on a 3-T scanner with respect to sampling bandwidth and SENSE acceleration factor subject to the constraints of half-second acquisition time and 0.6 x 1.2 mm in-plane resolution. MR-DSA with and without SENSE acceleration was then evaluated in patients with arterio-venous malformations (AVMs). RESULTS: Consistent with previously reported results and theory, SENSE factors greater than two and increasing sampling bandwidth both led to increasing image noise. Compared to lower resolution MR-DSA images with similar temporal resolution, the SENSE accelerated sequence provided better spatial resolution without notable changes in the contrast enhancement of the vascular territories of the AVMs but was hampered somewhat in the late venous phases by a reconstruction artifact. CONCLUSION: SENSE acceleration of MR-DSA by a factor of two allows improved temporal or spatial resolution without significant loss of image quality. Signal-to-noise degradation associated with higher SENSE acceleration factors are likely to necessitate other approaches to further improving resolution in MR-DSA. Clinically, SENSE accelerated MR-DSA improves the non-invasive pre- and postoperative depiction of AVM flow dynamics.     

Three-dimensional time-of-flight MR angiography: applications in the abdomen and thorax

The application of three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiographic techniques to the vasculature of the abdomen and thorax has not, to the authors' knowledge, been previously reported; this is possibly due to the large amount of physiologic motion in these regions along with the anticipated sensitivity of the 3D acquisition scheme to image degradation caused by motion artifact. The authors describe an asymmetric short-echo velocity-compensated 3D TOF technique that minimized the effects of physiologic motion on image quality and provided rapid high-resolution 3D MR arteriograms of the abdomen and thorax. Contiguous 3D volumes were often combined to provide sufficient anatomic coverage. Benefits include high spatial resolution and minimization of signal loss; limitations include sensitivity to motion artifact and progressive spin saturation.     

Myocardial wall tagging with undersampled projection reconstruction.

Azimuthally undersampled projection reconstruction (PR) acquisition is investigated for use in myocardial wall tagging with MR using grid tags to provide increased temporal and spatial resolution. PR can provide the high-resolution images required for tagging with very few projections, at the expense of artifact. Insight is provided into the PR undersampling artifact, in the context of measuring myocardial motion with tags. For Fourier transform imaging, at least 112 phase-encodings must be collected to image tagging grids spaced 7 pixels apart. PR requires about 80 projections, a 1.4-fold reduction in scan time. Magn Reson Med 45:562-567, 2001. Published 2001 Wiley-Liss, Inc.     

Techniques for high-speed cardiac magnetic resonance imaging in rats and rabbits

Progress in research on hypertension, heart failure, aging, post-infarct remodeling, and the molecular basis of cardiovascular diseases in general has been greatly facilitated in recent years by the development of specialized small-mam-mal models by selective breeding and/or genetic alteration. Routine noninvasive evaluation of cardiac function and perfusion in these animals models, however, is difficult using existing methods. In principle, MRI can be used for this purpose, but in practice this is difficult because of problems related to RF coils, cardiac gating, and imaging pulse sequences. In this article, solutions to these problems are described that have allowed us to use MRI to routinely image the hearts of rats and rabbits. Specifically described are four RF coils, cardiac gating schemes, and an imaging pulse sequence specially designed for cardiac imaging in these animals on a 4.7 T Omega chemical-shift imaging (CSI) spectrometer. These techniques can be used to obtain, within 2 min, eight double-oblique short-axis images of the rat at different cardiac phases with 200 × 400μm in-plane resolution and a slice thickness of 2 mm. Moreover, myocardial tissue tagging can be performed with tag thicknesses and separations comparable to those used routinely in humans. The technical information is presented in sufficient detail to allow researchers at other sites to reproduce the results. This information should facilitate the use of MRI for the noninvasive examination of cardiac function and perfusion, which can be combined with other established techniques for the study of cardiovascular disease in specialized animal models.     

Quantitative evaluation of metal artifact reduction techniques

PURPOSE: To develop a technique to quantify artifact, and to use it to compare the effectiveness of several approaches to metal artifact reduction, including view angle tilting and increasing the slice select and image bandwidths (BWs), in terms of metal artifact reduction, noise, and blur. MATERIALS AND METHODS: Nonmetallic replicas of two metal implants (stainless steel and titanium/chromium-cobalt femoral prostheses) were fabricated from wax, and MR images were obtained of each component immersed in water. The differences between the images of each metal prosthesis and its wax counterpart were measured. The contributions from noise and blur were isolated, resulting in a measure of the metal artifact. Several off-resonance artifact reduction techniques were assessed in terms of metal artifact reduction capability, as well as signal to noise ratio and blur. RESULTS: Increasing the image BW from +/-16 kHz to +/-64 kHz was found to reduce the artifact by an average of 60%, while employing view angle tilting (VAT) alone was found to reduce the artifact by an average of 63%. The metal artifact reduction sequence (MARS), which combines several susceptibility artifact reduction techniques, resulted in the least amount of image distortion, reducing the artifact by an average of 79%. CONCLUSION: The results indicate that while VAT alone (with an image BW of +/-16 kHz) resulted in the smallest amount of total energy and no reduction in the signal-to-noise ratio compared to a conventional spin-echo pulse sequence, MARS resulted in significantly less artifact and dramatically less blur.