A monitoring, feedback, and triggering system for reproducible breath-hold MR imaging

Distortionless, free-breathing, and respiratory resolved 3D diffusion weighted imaging of the abdomen

PURPOSE

Abdominal imaging is frequently performed with breath holds or respiratory triggering to reduce the effects of respiratory motion. Diffusion weighted sequences provide a useful clinical contrast but have prolonged scan times due to low signal-to-noise ratio (SNR), and cannot be completed in a single breath hold. Echo-planar imaging (EPI) is the most commonly used trajectory for diffusion weighted imaging but it is susceptible to off-resonance artifacts. A respiratory resolved, three-dimensional (3D) diffusion prepared sequence that obtains distortionless diffusion weighted images during free-breathing is presented. Techniques to address the myriad of challenges including: 3D shot-to-shot phase correction, respiratory binning, diffusion encoding during free-breathing, and robustness to off-resonance are described.

METHODS

A twice-refocused, M1-nulled diffusion preparation was combined with an RF-spoiled gradient echo readout and respiratory resolved reconstruction to obtain free-breathing diffusion weighted images in the abdomen. Cartesian sampling permits a sampling density that enables 3D shot-to-shot phase navigation and reduction of transient fat artifacts. Theoretical properties of a region-based shot rejection are described. The region-based shot rejection method was evaluated with free-breathing (normal and exaggerated breathing), and respiratory triggering. The proposed sequence was compared in vivo with multishot DW-EPI.

RESULTS

The proposed sequence exhibits no evident distortion in vivo when compared to multishot DW-EPI, robustness to B0 and B1 field inhomogeneities, and robustness to motion from different respiratory patterns.

CONCLUSION

Acquisition of distortionless, diffusion weighted images is feasible during free-breathing with a b-value of 500 s/mm2, scan time of 6 min, and a clinically viable reconstruction time.

Magnetic resonance myocardial T1ρ mapping : Technical overview, challenges, emerging developments, and clinical applications

The potential of cardiac magnetic resonance to improve cardiovascular care and patient management is considerable. Myocardial T1-rho (T1ρ) mapping, in particular, has emerged as a promising biomarker for quantifying myocardial injuries without exogenous contrast agents. Its potential as a contrast-agent-free ("needle-free") and cost-effective diagnostic marker promises high impact both in terms of clinical outcomes and patient comfort. However, myocardial T1ρ mapping is still at a nascent stage of development and the evidence supporting its diagnostic performance and clinical effectiveness is scant, though likely to change with technological improvements. The present review aims at providing a primer on the essentials of myocardial T1ρ mapping, and to describe the current range of clinical applications of the technique to detect and quantify myocardial injuries. We also delineate the important limitations and challenges for clinical deployment, including the urgent need for standardization, the evaluation of bias, and the critical importance of clinical testing. We conclude by outlining technical developments to be expected in the future. If needle-free myocardial T1ρ mapping is shown to improve patient diagnosis and prognosis, and can be effectively integrated in cardiovascular practice, it will fulfill its potential as an essential component of a cardiac magnetic resonance examination.

Ultrasound-based sensors to monitor physiological motion

PURPOSE

Medical procedures can be difficult to perform on anatomy that is constantly moving. Respiration displaces internal organs by up to several centimeters with respect to the surface of the body, and patients often have limited ability to hold their breath. Strategies to compensate for motion during diagnostic and therapeutic procedures require reliable information to be available. However, current devices often monitor respiration indirectly, through changes on the outline of the body, and they may be fixed to floors or ceilings, and thus unable to follow a given patient through different locations. Here we show that small ultrasound-based sensors referred to as "organ configuration motion" (OCM) sensors can be fixed to the abdomen and/or chest and provide information-rich, breathing-related signals.

METHODS

By design, the proposed sensors are relatively inexpensive. Breathing waveforms were obtained from tissues at varying depths and/or using different sensor placements. Validation was performed against breathing waveforms derived from magnetic resonance imaging (MRI) and optical tracking signals in five and eight volunteers, respectively.

RESULTS

Breathing waveforms from different modalities were scaled so they could be directly compared. Differences between waveforms were expressed in the form of a percentage, as compared to the amplitude of a typical breath. Expressed in this manner, for shallow tissues, OCM-derived waveforms on average differed from MRI and optical tracking results by 13.1% and 15.5%, respectively.

CONCLUSION

The present results suggest that the proposed sensors provide measurements that properly characterize breathing states. While OCM-based waveforms from shallow tissues proved similar in terms of information content to those derived from MRI or optical tracking, OCM further captured depth-dependent and position-dependent (i.e., chest and abdomen) information. In time, the richer information content of OCM-based waveforms may enable better respiratory gating to be performed, to allow diagnostic and therapeutic equipment to perform at their best.

Cardiac and Respiratory Self-Gating in Radial MRI Using an Adapted Singular Spectrum Analysis (SSA-FARY)

Cardiac Magnetic Resonance Imaging (MRI) is time-consuming and error-prone. To ease the patient's burden and to increase the efficiency and robustness of cardiac exams, interest in methods based on continuous steady-state acquisition and self-gating has been growing in recent years. Self-gating methods extract the cardiac and respiratory signals from the measurement data and then retrospectively sort the data into cardiac and respiratory phases. Repeated breathholds and synchronization with the heart beat using some external device as required in conventional MRI are then not necessary. In this work, we introduce a novel self-gating method for radially acquired data based on a dimensionality reduction technique for time-series analysis (SSA-FARY). Building on Singular Spectrum Analysis, a zero-padded, time-delayed embedding of the auto-calibration data is analyzed using Principle Component Analysis. We demonstrate the basic functionality of SSA-FARY using numerical simulations and apply it to in-vivo cardiac radial single-slice bSSFP and Simultaneous Multi-Slice radiofrequency-spoiled gradient-echo measurements, as well as to Stack-of-Stars bSSFP measurements. SSA-FARY reliably detects the cardiac and respiratory motion and separates it from noise. We utilize the generated signals for high-dimensional image reconstruction using parallel imaging and compressed sensing with in-plane wavelet and (spatio-)temporal total-variation regularization.

Sorted Golden-step phase encoding: an improved Golden-step imaging technique for cardiac and respiratory self-gated cine cardiovascular magnetic resonance imaging

BACKGROUND

Numerous self-gated cardiac imaging techniques have been reported in the literature. Most can track either cardiac or respiratory motion, and many incur some overhead to imaging data acquisition. We previously described a Cartesian cine imaging technique, pseudo-projection motion tracking with golden-step phase encoding, capable of tracking both cardiac and respiratory motion at no cost to imaging data acquisition. In this work, we describe improvements to the technique by dramatically reducing its vulnerability to eddy current and flow artifacts and demonstrating its effectiveness in expanded cardiovascular applications.

METHODS

As with our previous golden-step technique, the Cartesian phase encodes over time were arranged based on the integer golden step, and readouts near ky = 0 (pseudo-projections) were used to derive motion. In this work, however, the readouts were divided into equal and consecutive temporal segments, within which the readouts were sorted according to ky. The sorting reduces the phase encode jump between consecutive readouts while maintaining the pseudo-randomness of ky to sample both cardiac and respiratory motion without comprising the ability to retrospectively set the temporal resolution of the original technique. On human volunteers, free-breathing, electrocardiographic (ECG)-free cine scans were acquired for all slices of the short axis stack and the 4-chamber view of the long axis. Retrospectively, cardiac motion and respiratory motion were automatically extracted from the pseudo-projections to guide cine reconstruction. The resultant image quality in terms of sharpness and cardiac functional metrics was compared against breath-hold ECG-gated reference cines.

RESULTS

With sorting, motion tracking of both cardiac and respiratory motion was effective for all slices orientations imaged, and artifact occurrence due to eddy current and flow was efficiently eliminated. The image sharpness derived from the self-gated cines was found to be comparable to the reference cines (mean difference less than 0.05 mm- 1 for short-axis images and 0.075 mm- 1 for long-axis images), and the functional metrics (mean difference < 4 ml) were found not to be statistically different from those from the reference.

CONCLUSIONS

This technique dramatically reduced the eddy current and flow artifacts while preserving the ability of cost-free motion tracking and the flexibility of choosing arbitrary navigator zone width, number of cardiac phases, and duration of scanning. With the restriction of the artifacts removed, the Cartesian golden-step cine imaging can now be applied to cardiac imaging slices of more diverse orientation and anatomy at greater reliability.

Using a respiratory navigator significantly reduces variability when quantifying left ventricular torsion with cardiovascular magnetic resonance

BACKGROUND

Left ventricular (LV) torsion is an important indicator of cardiac function that is limited by high inter-test variability (50% of the mean value). We hypothesized that this high inter-test variability is partly due to inconsistent breath-hold positions during serial image acquisitions, which could be significantly improved by using a respiratory navigator for cardiovascular magnetic resonance (CMR) based quantification of LV torsion.

METHODS

We assessed respiratory-related variability in measured LV torsion with two distinct experimental protocols. First, 17 volunteers were recruited for CMR with cine displacement encoding with stimulated echoes (DENSE) in which a respiratory navigator was used to measure and then enforce variability in end-expiratory position between all LV basal and apical acquisitions. From these data, we quantified the inter-test variability of torsion in the absence and presence of enforced end-expiratory position variability, which established an upper bound for the expected torsion variability. For the second experiment (in 20 new, healthy volunteers), 10 pairs of cine DENSE basal and apical images were each acquired from consecutive breath-holds and consecutive navigator-gated scans (with a single acceptance position). Inter-test variability of torsion was compared between the breath-hold and navigator-gated scans to quantify the variability due to natural breath-hold variation. To demonstrate the importance of these variability reductions, we quantified the reduction in sample size required to detect a clinically meaningful change in LV torsion with the use of a respiratory navigator.

RESULTS

The mean torsion was 3.4 ± 0.2°/cm. From the first experiment, enforced variability in end-expiratory position translated to considerable variability in measured torsion (0.56 ± 0.34°/cm), whereas inter-test variability with consistent end-expiratory position was 57% lower (0.24 ± 0.16°/cm, p < 0.001). From the second experiment, natural respiratory variability from consecutive breath-holds translated to a variability in torsion of 0.24 ± 0.10°/cm, which was significantly higher than the variability from navigator-gated scans (0.18 ± 0.06°/cm, p = 0.02). By using a respiratory navigator with DENSE, theoretical sample sizes were reduced from 66 to 16 and 26 to 15 as calculated from the two experiments.

CONCLUSIONS

A substantial portion (22-57%) of the inter-test variability of LV torsion can be reduced by using a respiratory navigator to ensure a consistent breath-hold position between image acquisitions.

An interactive videogame designed to improve respiratory navigator efficiency in children undergoing cardiovascular magnetic resonance

BACKGROUND

Advanced cardiovascular magnetic resonance (CMR) acquisitions often require long scan durations that necessitate respiratory navigator gating. The tradeoff of navigator gating is reduced scan efficiency, particularly when the patient's breathing patterns are inconsistent, as is commonly seen in children. We hypothesized that engaging pediatric participants with a navigator-controlled videogame to help control breathing patterns would improve navigator efficiency and maintain image quality.

METHODS

We developed custom software that processed the Siemens respiratory navigator image in real-time during CMR and represented diaphragm position using a cartoon avatar, which was projected to the participant in the scanner as visual feedback. The game incentivized children to breathe such that the avatar was positioned within the navigator acceptance window (±3 mm) throughout image acquisition. Using a 3T Siemens Tim Trio, 50 children (Age: 14 ± 3 years, 48 % female) with no significant past medical history underwent a respiratory navigator-gated 2D spiral cine displacement encoding with stimulated echoes (DENSE) CMR acquisition first with no feedback (NF) and then with the feedback game (FG). Thirty of the 50 children were randomized to undergo extensive off-scanner training with the FG using a MRI simulator, or no off-scanner training. Navigator efficiency, signal-to-noise ratio (SNR), and global left-ventricular strains were determined for each participant and compared.

RESULTS

Using the FG improved average navigator efficiency from 33 ± 15 to 58 ± 13 % (p < 0.001) and improved SNR by 5 % (p = 0.01) compared to acquisitions with NF. There was no difference in navigator efficiency (p = 0.90) or SNR (p = 0.77) between untrained and trained participants for FG acquisitions. Circumferential and radial strains derived from FG acquisitions were slightly reduced compared to NF acquisitions (-16 ± 2 % vs -17 ± 2 %, p < 0.001; 40 ± 10 % vs 44 ± 11 %, p = 0.005, respectively). There were no differences in longitudinal strain (p = 0.38).

CONCLUSIONS

Use of a respiratory navigator feedback game during navigator-gated CMR improved navigator efficiency in children from 33 to 58 %. This improved efficiency was associated with a 5 % increase in SNR for spiral cine DENSE. Extensive off-scanner training was not required to achieve the improvement in navigator efficiency.

Adaptive online self-gating (ADIOS) for free-breathing noncontrast renal MR angiography

PURPOSE

To develop a respiratory self-gating method, adaptive online self-gating (ADIOS), for noncontrast MR angiography (NC MRA) of renal arteries to overcome some limitations of current free-breathing methods.

METHODS

A NC MRA pulse sequence for online respiratory self-gating was developed based on three-dimensional balanced steady-state free precession (bSSFP) and slab-selective inversion-recovery. Motion information was derived directly from the slab being imaged for online gating. Scan efficiency was maintained by an automatic adaptive online algorithm. Qualitative and quantitative assessments of image quality were performed and results were compared with conventional diaphragm navigator (NAV).

RESULTS

NC MRA imaging was successfully completed in all subjects (n = 15). Similarly good image quality was observed in the proximal-middle renal arteries with ADIOS compared with NAV. Superior image quality was observed in the middle-distal renal arteries in the right kidneys with no NAV-induced artifacts. Maximal visible artery length was significantly longer with ADIOS versus NAV in the right kidneys. NAV setup was completely eliminated and scan time was significantly shorter with ADIOS on average compared with NAV.

CONCLUSION

The proposed ADIOS technique for noncontrast MRA provides high-quality visualization of renal arteries with no diaphragm navigator-induced artifacts, simplified setup, and shorter scan time.

In vivo diffusion tensor MRI of the human heart: reproducibility of breath-hold and navigator-based approaches

The aim of this study was to implement a quantitative in vivo cardiac diffusion tensor imaging (DTI) technique that was robust, reproducible, and feasible to perform in patients with cardiovascular disease. A stimulated-echo single-shot echo-planar imaging (EPI) sequence with zonal excitation and parallel imaging was implemented, together with a novel modification of the prospective navigator (NAV) technique combined with a biofeedback mechanism. Ten volunteers were scanned on two different days, each time with both multiple breath-hold (MBH) and NAV multislice protocols. Fractional anisotropy (FA), mean diffusivity (MD), and helix angle (HA) fiber maps were created. Comparison of initial and repeat scans showed good reproducibility for both MBH and NAV techniques for FA (P > 0.22), MD (P > 0.15), and HA (P > 0.28). Comparison of MBH and NAV FA (FAMBHday1 = 0.60 ± 0.04, FANAVday1 = 0.60 ± 0.03, P = 0.57) and MD (MDMBHday1 = 0.8 ± 0.2 × 10(-3) mm(2) /s, MDNAVday1 = 0.9 ± 0.2 × 10(-3) mm(2) /s, P = 0.07) values showed no significant differences, while HA values (HAMBHday1Endo = 22 ± 10°, HAMBHday1Mid-Endo = 20 ± 6°, HAMBHday1Mid-Epi = -1 ± 6°, HAMBHday1Epi = -17 ± 6°, HANAVday1Endo = 7 ± 7°, HANAVday1Mid-Endo = 13 ± 8°, HANAVday1Mid-Epi = -2 ± 7°, HANAVday1Epi = -14 ± 6°) were significantly different. The scan duration was 20% longer with the NAV approach. Currently, the MBH approach is the more robust in normal volunteers. While the NAV technique still requires resolution of some bulk motion sensitivity issues, these preliminary experiments show its potential for in vivo clinical cardiac diffusion tensor imaging and for delivering high-resolution in vivo 3D DTI tractography of the heart.

Use of respiratory biofeedback and CLAWS for increased navigator efficiency for imaging the thoracic aorta

A novel technique to guide a subjects' breathing pattern using a respiratory biofeedback (rBF) "game" to improve respiratory efficiency is presented. The continuously adaptive windowing strategy, a fully automatic and highly efficient free-breathing navigator gated technique, is used to acquire the data as it ensures that all potential navigator acceptance windows are possible. This enables the rBF to be fully adaptable to a subject's respiratory pattern. Images of the thoracic aorta acquired using balanced steady-state free precession with continuously adaptive windowing strategy respiratory motion control, with and without rBF, were compared in 10 healthy subjects. Total scan time was reduced by using rBF. The mean scan time was reduced from 7 min 44 s (463 cardiac cycles, ± 127 cc) without rBF to 5 min 43 s (380 cardiac cycles, ± 118 cc) with the use of rBF (P < 0.05). Respiratory efficiency was increased from 45% without rBF to 56% with rBF (P < 0.01). Image quality was the same for both techniques (P = ns). In conclusion, rBF significantly improved respiratory efficiency and reduced acquisition duration without affecting image quality.

Correction of breathing-induced errors in magnetic resonance thermometry of hyperthermia using multiecho field fitting techniques

PURPOSE

Breathing motion can create large errors when performing magnetic resonance (MR) thermometry of the breast. Breath holds can be used to minimize these errors, but not eliminate them. Between breath holds, the referenceless method can be used to further reduce errors by relying on regions of nonheated fatty tissue surrounding the heated region. When the surrounding tissue is heated (i.e., for a hyperthermia treatment), errors can result due to phase changes of the small amounts of water in the tissue. Therefore, an extension of the referenceless method is proposed which fits for the field in fatty tissue independent of temperature change and extrapolates it to the water-rich regions.

METHODS

Nonheating experiments were performed with male volunteers performing breath holds on top of a phantom mimicking a breast with a tumor. Heating experiments were also conducted with the same phantom while mechanically simulated breath holds were performed. A nonheating experiment was also performed with a healthy female breast. For each experiment, a nonlinear fitting algorithm was used to fit for temperature change and B0 field inside of the fatty tissue. The field changes were then extrapolated into water-rich (tumor) portions of the image using a least-squares fit to a fifth-order equation, to correct for field changes due to breath hold changes. Similar results were calculated using the image phase, to mimic the use of the referenceless method.

RESULTS

Phantom results showed large reduction of mean error and standard deviation. In the non-heating experiments, the traditional referenceless method and our extended method both corrected by similar amounts. However, in the heating experiments, the average deviation of the temperature calculated with the extended method from a fiber optic probe temperature was approximately 50% less than the deviation with the referenceless method. The in vivo breast results demonstrated reduced standard deviation and mean.

CONCLUSIONS

In this paper, we have developed an extension of the referenceless method to correct for breathing errors using multiecho fitting methods to fit for the B0 field in the fatty tissue and using measured field changes as references to extrapolate field corrections into a water-only (tumor) region. This technique has been validated in a number of situations, and in all cases, the correction method has been shown to greatly reduce temperature error in water-rich regions. The method has also been shown to be an improvement over similar methods that use image phase changes instead of field changes, particularly when temperature changes are induced.

Strategies for reducing respiratory motion artifacts in renal perfusion imaging with arterial spin labeling

Arterial spin labeling (ASL) perfusion measurements may have many applications outside the brain. In the abdomen, severe image artifacts can arise from motions between acquisitions of multiple signal averages in ASL, even with single-shot image acquisition. Background suppression and respiratory motion synchronization techniques can be used to ameliorate these artifacts. Two separate in vivo studies of renal perfusion imaging using pulsed continuous ASL (pCASL) were performed. The first study assessed various combinations of background suppression and breathing strategies. The second investigated the retrospective sorting of images acquired during free breathing based on respiratory position. Quantitative assessments of the test-retest repeatability of perfusion measurements and the image quality scored by two radiologists were made. Image quality was most significantly improved by using background suppression schemes and controlled breathing when compared to other combinations without background suppression or with free breathing, assessed by test-retests (5% level, F-test), and by radiologists' scores (5% level, Mann-Whitney U-test). Under free breathing, retrospectively sorting images based on respiratory position showed significant improvement. Both radiologists found 100% of the images had preferable image sharpness after sorting. High-quality renal perfusion measurements with reduced respiratory motion artifacts have been demonstrated using ASL when appropriate background suppression and breathing strategies are applied.

Motion in cardiovascular MR imaging

Modern rapid magnetic resonance (MR) imaging techniques have led to widespread use of the modality in cardiac imaging. Despite this progress, many MR studies suffer from image degradation due to involuntary motion during the acquisition. This review describes the type and extent of the motion of the heart due to the cardiac and respiratory cycles, which create image artifacts. Methods of eliminating or reducing the problems caused by the cardiac cycle are discussed, including electrocardiogram gating, subject-specific acquisition windows, and section tracking. Similarly, for respiratory motion of the heart, techniques such as breath holding, respiratory gating, section tracking, phase-encoding ordering, subject-specific translational models, and a range of new techniques are considered.

Coronary MR imaging: navigator echo biofeedback increases navigator efficiency--initial experience

RATIONALE AND OBJECTIVES

The aim of this study was to investigate whether a respiratory biofeedback system could increase navigator efficiency and maintain image quality compared to conventional respiratory-gated magnetic resonance coronary angiography (MRCA).

MATERIALS AND METHODS

Eighteen healthy volunteers underwent MRCA using three different respiratory-gating protocols. A conventional expiratory free-breathing (FB) sequence was compared to two approaches using navigator echo biofeedback (NEB), a midinspiratory approach (NEBin) and an expiratory approach (NEBex). Navigator data reflecting the position of the diaphragm relative to a 3-mm gating window were made available to the subject using a video projector in combination with a Plexiglas screen and mirror goggles. Image quality was graded by two radiologists in consensus using a visual score ranging from 1 (not visible) to 4 (excellent vessel depiction).

RESULTS

The NEB approaches improved navigator efficiency (71.1% with NEBex and 68.0% with NEBin vs 42.2% with FB), thus reducing total imaging time. This difference was statistically significant (P(NEBin)=.007; P(NEBex)=.001). Image quality in the NEBex group was comparable to that in the FB group (median score, 2.44 vs 2.52), but it proved to be significantly lower (median score, 1.94 vs 2.52) for the right coronary artery and the left anterior descending coronary artery in the NEBin group.

CONCLUSION

NEB maintains image quality and significantly increases navigator efficiency, thereby decreasing total imaging time by about 40% compared to a conventional FB acquisition strategy.

Aortic vessel wall magnetic resonance imaging at 3.0 Tesla: a reproducibility study of respiratory navigator gated free-breathing 3D black blood magnetic resonance imaging

The purpose of this study was to evaluate a free-breathing three-dimensional (3D) dual inversion-recovery (DIR) segmented k-space gradient-echo (turbo field echo [TFE]) imaging sequence at 3T for the quantification of aortic vessel wall dimensions. The effect of respiratory motion suppression on image quality was tested. Furthermore, the reproducibility of the aortic vessel wall measurements was investigated. Seven healthy subjects underwent 3D DIR TFE imaging of the aortic vessel wall with and without respiratory navigator. Subsequently, this sequence with respiratory navigator was performed twice in 10 healthy subjects to test its reproducibility. The signal-to-noise (SNR), contrast-to-noise ratio (CNR), vessel wall sharpness, and vessel wall volume (VWV) were assessed. Data were compared using the paired t-test, and the reproducibility of VWV measurements was evaluated using intraclass correlation coefficients (ICCs). SNR, CNR, and vessel wall sharpness were superior in scans performed with respiratory navigator compared to scans performed without. The ICCs concerning intraobserver, interobserver, and interscan reproducibility were excellent (0.99, 0.94, and 0.95, respectively). In conclusion, respiratory motion suppression substantially improves image quality of 3D DIR TFE imaging of the aortic vessel wall at 3T. Furthermore, this optimized technique with respiratory motion suppression enables assessment of aortic vessel wall dimensions with high reproducibility.

Accelerated whole-heart 3D CSPAMM for myocardial motion quantification

Myocardial tissue tagging using complementary spatial modulation of magnetization (CSPAMM) allows detailed assessment of myocardial motion. To capture the complex 3D cardiac motion pattern, multiple 2D tagged slices are usually acquired in different orientations. These approaches are prone to slice misregistration and associated with long acquisition times. In this work, a fast method for acquiring 3D CSPAMM data is proposed that allows measuring deformation of the whole heart in three breath-holds of 18 heartbeats duration each. Three acquisitions are sequentially performed with line tag preparation in each orthogonal direction. Measurement acceleration is achieved by applying localized tagging preparation and a hybrid multishot, segmented echo-planar imaging sequence. Five healthy volunteers and five patients with myocardial infarction were measured. Midwall contours were tracked throughout the cardiac cycle with an enhanced variant of the harmonic phase (HARP) technique. Circumferential shortening at end-systole ranged from 14.1% (base) to 20.1% (apex) in healthy subjects. Hypokinetic regions in patients corresponded well with regions exhibiting hyperenhancement after contrast injection. Time to maximum circumferential shortening varied more significantly over the left ventricle in patients than in volunteers (P<0.01). The proposed measurement scheme was well tolerated by patients and holds considerable potential to investigate cardiac mechanics in various diseases.

Coronary magnetic resonance angiography

Coronary magnetic resonance angiography (MRA) is a powerful noninvasive technique with high soft-tissue contrast for the visualization of the coronary anatomy without X-ray exposure. Due to the small dimensions and tortuous nature of the coronary arteries, a high spatial resolution and sufficient volumetric coverage have to be obtained. However, this necessitates scanning times that are typically much longer than one cardiac cycle. By collecting image data during multiple RR intervals, one can successfully acquire coronary MR angiograms. However, constant cardiac contraction and relaxation, as well as respiratory motion, adversely affect image quality. Therefore, sophisticated motion-compensation strategies are needed. Furthermore, a high contrast between the coronary arteries and the surrounding tissue is mandatory. In the present article, challenges and solutions of coronary imaging are discussed, and results obtained in both healthy and diseased states are reviewed. This includes preliminary data obtained with state-of-the-art techniques such as steady-state free precession (SSFP), whole-heart imaging, intravascular contrast agents, coronary vessel wall imaging, and high-field imaging. Simultaneously, the utility of electron beam computed tomography (EBCT) and multidetector computed tomography (MDCT) for the visualization of the coronary arteries is discussed.

CT Fluoroscopy–guided Biopsy of Small Pulmonary and Upper Abdominal Lesions: Efficacy with a Modified Breathing Technique

PURPOSE

To characterize a new protocol of computed tomographic (CT) fluoroscopy-guided biopsy of the lung and upper abdomen to minimize the intervention time, complication rate, and exposure to ionizing radiation for both the patient and the radiologist.

MATERIALS AND METHODS

Fifty patients (23 women, 27 men; mean age, 64.3 years; age range, 36-83 years) with lung (n = 41) or upper abdomen (n = 9) nodules 15 mm or smaller underwent CT fluoroscopy-guided biopsy from November 2005 to October 2006. The mean nodule diameter was 12.6 mm (range, 8-15 mm), the mean depth to skin was 57.3 mm (range, 20-114 mm), and the mean depth of nodules from pleura and/or peritoneum was 18.9 mm (range, 1-77 mm). Histopathologic evaluation of samples was performed on the day of the procedure. A CT fluoroscopy-guided biopsy protocol was established as follows: (a) native CT with breath-holding at an intermediate respiration level, (b) selection of section position with target nodule and insertion of an 18-gauge coaxial biopsy needle extrapleurally and/or extraperitoneally virtually targeting at nodule, (c) start of CT fluoroscopy (130 kVp, 30 mAs, 5-mm-thick sections) at inspiration level with the patient expiring, (d) stop of CT fluoroscopy when the target nodule reaches the section position, short breath-hold, needle advancement to the target nodule, (e) control of needle position with CT fluoroscopy, and (f) biopsy.

RESULTS

The mean total table time was 23.8 minutes (range, 15-41 minutes), the mean duration of CT fluoroscopy was 8.2 seconds (range, 4-23 seconds), and the mean duration of breath-holding--including needle insertion to target nodule and control CT fluoroscopy--was 10.3 seconds (range, 5-15 seconds). There were three minor pneumothoraces that required no further intervention, seven minor pulmonary hemorrhages, three moderate pulmonary hemorrhages with hemoptysis, and one moderate liver hematoma. There were no major complications. The diagnostic accuracy of biopsy samples was 96%.

CONCLUSIONS

The presented modification of CT fluoroscopy-guided biopsy of mobile pulmonary and upper abdominal lesions is a rapid and safe procedure, requiring only short exposure to ionizing radiation.

Feasibility and diagnostic accuracy of whole heart coronary MR angiography using free-breathing 3D balanced turbo-field-echo with SENSE and the half-fourier acquisition technique

OBJECTIVE

We wanted to assess the feasibility and diagnostic accuracy of whole heart coronary magnetic resonance angiography (MRA) with using 3D balanced turbo-field-echo (b-TFE) with SENSE and the half-Fourier acquisition technique for identifying stenoses of the coronary artery.

MATERIALS AND METHODS

Twenty-one patients who underwent both whole heart coronary MRA examinations and conventional catheter coronary angiography examinations were enrolled in the study. The whole heart coronary MRA images were acquired using a navigator gated 3D b-TFE sequence with SENSE and the half-Fourier acquisition technique to reduce the acquisition time. The imaging slab covered the whole heart (80 contiguous slices with a reconstructed slice thickness of 1.5 mm) along the transverse axis. The quality of the images was evaluated by using a 5-point scale (0 - uninterpretable, 1 - poor, 2 - fair, 3 - good, 4 - excellent). Ten coronary segments of the heart were evaluated in each case; the left main coronary artery (LM), and the proximal, middle and distal segments of the left anterior descending (LAD), the left circumflex (LCX) and the right coronary artery (RCA). The diagnostic accuracy of whole heart coronary MRA for detecting significant coronary artery stenosis was determined on the segment-by-segment basis, and it was compared with the results obtained by conventional catheter angiography, which is the gold standard.

RESULTS

The mean image quality was 3.7 in the LM, 3.2 in the LAD, 2.5 in the LCX, and 3.3 in the RCA, respectively (the overall image quality was 3.0 +/- 0.1). 168 (84%) of the 201 segments had an acceptable image quality (> or =grade 2). The sensitivity, specificity, accuracy, negative predictive value and positive predictive value of the whole heart coronary MRA images for detecting significant stenosis were 81.3%, 92.1%, 91.1%, 97.9%, and 52.0%, respectively. The mean coronary MRA acquisition time was 9 min 22 sec (+/-125 sec).

CONCLUSION

Whole heart coronary MRA is a feasible technique, and it has good potential to evaluate the major portions of the coronary arteries with an acceptable image quality within a reasonable scan time.

Dual-contrast single breath-hold 3D abdominal MR imaging

OBJECT

Multiple contrasts are often helpful for a comprehensive diagnosis. In 3D abdominal MRI, breath-hold techniques are preferred for single contrast acquisitions to avoid respiratory artifacts. In this paper, highly accelerated parallel MRI is used to acquire large 3D abdominal volumes with two different contrasts within a single breath-hold.

MATERIAL AND METHODS

In vivo studies have been performed on six healthy volunteers, combining T (1)- and T (2)-weighted, gradient- or spin-echo based scans, as well as water/fat resolved imaging in a single breath-hold. These 3D scans were acquired with an acceleration factor of six, using a prototype 32-element receive array.

RESULTS

The presented approach was tested successfully on all volunteers. The whole liver area was covered by a FOV of 350 x 250 x 200 mm(3) for all scans with reasonable spatial resolution. Arbitrary scan protocols generating different contrasts have been shown to be combinable in this single breath-hold approach. Good spatial correspondence with negligible spatial offset was achieved for all different scan combinations acquired in overall breath-hold times between 15 and 25 s.

CONCLUSION

Enabled by highly parallel imaging technology, this study demonstrates the technical feasibility and the promising image quality of single breath-hold dual contrast MRI.

Coronary vessel-wall and lumen imaging using radial k-space acquisition with MRI at 3 Tesla

This study investigates the feasibility of imaging the coronary lumen and vessel-wall, using MRI with a radial k-space trajectory at 3 T. Such radial trajectories offer the advantage of greater vessel sharpness than traditional Cartesian trajectories. This field strength offers an increased signal-to-noise ratio (SNR) compared with 1.5 T, which compensates for the slight SNR reduction due to the radial sequence. Images of the coronary lumen were acquired for seven healthy volunteers. In ten volunteers the vessel wall was scanned, with blood suppression using oblique-slab adiabatic re-inversion. Scans were performed during free breathing, using prospective respiratory navigator-gating. Coronary lumen scans had SNR of 16.0+/-1.9 and contrast-to-noise ratio (CNR) of 10.3+/-2.1, showing acceptable image quality. Vessel wall images showed good image quality, with mean SNR of 16.6+/-2.0/5.8+/-2.8/10.1+/-2.2 for vessel wall/lumen/epicardial fat. The wall-blood CNR was 10.7+/-2.7, and wall-fat CNR was 6.5+/-2.5. It is concluded that radial gradient-echo imaging at 3 T is a promising method for coronary vessel-wall imaging, and is also feasible for imaging the coronary lumen.

Characterizing coronary motion and its effect on MR coronary angiography—Initial experience

PURPOSE

To characterize coronary artery motion as a prescan procedure to select the optimum scan setting that will produce high-resolution images.

MATERIALS AND METHODS

A 2D real-time scan was used to image the major coronary arteries during breath-holding and free-breathing conditions. With the use of the 2D images, motion displacement of each artery was measured along three axes. Motion data obtained from a computer simulation were used to estimate point-spread functions (PSFs) associated with different high-resolution spiral acquisition strategies, including real-time, cardiac-gated, and respiratory-gated acquisitions. The simulation output determined the optimum acquisition and scan parameters that would produce the highest-spatial-resolution images of the coronary arteries. The effects of heart rate (HR), extended breath-holding, and number of slices per heart cycle were also investigated.

RESULTS

Substantial variations in coronary motion occur among individuals, which directly influences the optimum parameters for a high-resolution scan. Lower HRs and longer breath-holds yield substantially increased spatial resolution. The maximum number of slices per heart cycle can also be determined to minimize slice-to-slice distortion.

CONCLUSION

The results suggest that to obtain high-resolution coronary images, one should perform a prescan coronary-motion characterization for each individual so that the scan parameters can be optimized before data acquisition.

A new strategy for respiration compensation, applied toward 3D free-breathing cardiac MRI

In thorax and abdomen imaging, image quality may be affected by breathing motion. Cardiac MR images are typically obtained while the patient holds his or her breath, to avoid respiration-related artifacts. Although useful, breath-holding imposes constraints on scan duration, which in turn limits the achievable resolution and SNR. Longer scan times would be required to improve image quality, and effective strategies are needed to compensate for respiratory motion. A novel approach at respiratory compensation, targeted toward 3D free-breathing cardiac MRI, is presented here. The method aims at suppressing the negative effects of respiratory-induced cardiac motion while capturing the heart's beating motion. The method is designed so that the acquired data can be reconstructed in two different ways: First, a time series of images is reconstructed to quantify and correct for respiratory motion. Then, the corrected data are reconstructed a final time into a cardiac-phase series of images to capture the heart's beating motion. The method was implemented, and initial results are presented. A cardiac-phase series of 3D images, covering the entire heart, was obtained for two free-breathing volunteers. The present method may prove especially useful in situations where breath-holding is not an option, for example, for very sick, mentally impaired or infant patients.

Imaging of atherosclerosis using magnetic resonance: state of the art and future directions

Atherosclerosis is the leading cause of morbidity and mortality in industrialized societies, and its incidence is projected to increase in the future. Because the atherosclerotic process begins in the vessel wall, the focus of cardiovascular imaging is shifting from the arterial lumen to imaging of the vessel wall, with the goal of detecting preclinical atherosclerosis. MRI, because of its high resolution, three-dimensional capabilities, noninvasive nature, and capacity for soft tissue characterization, is emerging as an important modality to assess the atherosclerotic plaque burden in the arterial wall and can monitor atherosclerosis in different arterial beds, including the carotid arteries, aorta, and more recently, the coronary arteries. Furthermore, it has also been successfully utilized to monitor plaque regression following therapeutic interventions. Finally, the emergence of high-resolution MRI and development of sophisticated contrast agents offers tremendous promise for in vivo molecular imaging of the atherosclerotic plaque.

New 4-D imaging for real-time intraoperative MRI: adaptive 4-D scan

Aiming at real-time 3-D visualization of organ motion to navigate surgical procedures in MRI-guided surgery, a new 4-D MR imaging technique called "Adaptive 4-D Scan" has been proposed. The technique is designed to acquire a time series of volumetric 3-D images (4-D image) of cyclically moving organ, even in a low-field open-configuration MR scanner. A pre-operative 4-D image is acquired with respiratory phase parameter, which is monitored by using navigator-echo-based real-time tracking of the liver and diaphragm. During operation, the respiratory phase is again monitored in real-time, and a 3-D image, reflecting the current state of the target organ, is extracted from the pre-operative 4-D image and provided to physicians as a pseudo real-time 3-D image. We implemented Adaptive 4-D Scan into a 0.5 Tesla open-configuration clinical MRI system for intervention. Phantom and volunteer studies were performed to assess feasibility of this technique, in terms of image quality, imaging time and position accuracy of the imaged subject. A 4-D image (matrix: 256 x 128 x 10 x 8) of cyclically moving phantom was acquired in 719 s, and RMS position error between the imaged subject and the real subject was 2.3 mm, where the range of motion was 50 mm. 4-D image of the moving liver was also successfully acquired under near clinical condition. In conclusion, the study shows that the proposed method is feasible and has capability to provide real-time dynamic 3-D atlas for surgical navigation.

CT Fluoroscopy–guided Biopsy of the Lung or Upper Abdomen with a Breath-hold Monitoring and Feedback System: A Prospective Randomized Controlled Clinical Trial

PURPOSE

To prospectively determine the clinical effectiveness of a breath-hold monitoring and feedback system in computed tomographic (CT) fluoroscopy-guided biopsies in which respiratory motion is a problem.

MATERIALS AND METHODS

Institutional review board approval and oral and written informed consent were obtained. This study was HIPAA compliant. A bellows-based system was used to monitor respiration and provide patient feedback. A randomized controlled clinical trial compared intermittent mode CT fluoroscopy-guided biopsies of the lung or upper abdomen performed with (n = 56) and without (n = 57) the bellows system. Inclusion criteria for 113 patients were lesions 6 cm or smaller in maximum dimension that were not affixed to the chest or abdominal wall. Primary outcome measurements were CT fluoroscopy exposure time and patient dose. Wilcoxon rank sum, chi(2), and Fisher exact tests were used for statistical analysis.

RESULTS

Median CT fluoroscopy exposure time was 12.6 seconds (range, 2.4-44.4 seconds) for the bellows group and 18.0 seconds (range, 6.0-118.0 seconds) for the nonbellows group (P = .004). Patient dose was decreased in the bellows group (median dose, 29.5 mGy; range, 4.7-135.8 mGy) versus the nonbellows group (median, 41.3 mGy; range, 11.8-155.9 mGy) (P = .01). Lesions were accessed successfully with one needle puncture attempt in 43 of 56 patients (77%) in the bellows group and 30 of 57 patients (53%) in the nonbellows group (P = .007). Pneumothorax developed in 11 of 50 patients (22%) in the bellows group who underwent lung biopsy compared with 16 of 50 (32%) patients in the nonbellows group.

CONCLUSION

A breath-hold monitoring and feedback system allows depiction of mobile target lesions throughout CT fluoroscopy-guided biopsy of the lung and upper abdomen.

Cardiovascular magnetic resonance of anomalous coronary arteries

Cardiovascular magnetic resonance of anomalous coronary arteries is a class I indication. The term anomalous coronary artery encompasses those with an abnormal origin (from the incorrect sinus, too-high or too-low from the correct sinus, or from the pulmonary artery) and/or number of ostia. Their clinical significance results from the increased risk of myocardial infarction and sudden cardiac death associated with those traversing an interarterial course between the aorta and main pulmonary artery/right ventricular outflow tract. In this article, we review the role and practice of cardiovascular magnetic resonance in this field.

3D motion adapted gating (3D MAG): a new navigator technique for accelerated acquisition of free breathing navigator gated 3D coronary MR-angiography

This study aimed to evaluate the influence of a new navigator technique (3D MAG) on navigator efficiency, total acquisition time, image quality and diagnostic accuracy. Fifty-six patients with suspected coronary artery disease underwent free breathing navigator gated coronary MRA (Intera, Philips Medical Systems, 1.5 T, spatial resolution 0.9x0.9x3 mm3) with and without 3D MAG. Evaluation of both sequences included: 1) navigator scan efficiency, 2) total acquisition time, 3) assessment of image quality and 4) detection of stenoses >50%. Average navigator efficiencies of the LCA and RCA were 43+/-12% and 42+/-12% with and 36+/-16% and 35+/-16% without 3D MAG (P<0.01). Scan time was reduced from 12 min 7 s without to 8 min 55 s with 3D MAG for the LCA and from 12 min 19 s to 9 min 7 s with 3D MAG for the RCA (P<0.01). The average scores of image quality of the coronary MRAs with and without 3D MAG were 3.5+/-0.79 and 3.46+/-0.84 (P>0.05). There was no significant difference in the sensitivity and specificity in the detection of coronary artery stenoses between coronary MRAs with and without 3D MAG (P>0.05). 3D MAG provides accelerated acquisition of navigator gated coronary MRA by about 19% while maintaining image quality and diagnostic accuracy.

Magnetic resonance imaging of the coronary vessel wall at 3 T using an obliquely oriented reinversion slab with adiabatic pulses

Three-dimensional methods offer volumetric coverage in coronary vessel wall imaging, in addition to high signal-to-noise ratios (SNR). To increase SNR further, it is desirable to implement such 3D methods at 3 T. At this field strength, the pulse sequence must be robust to main field and RF inhomogeneities. To achieve this, the double inversion-recovery (DIR) preparation was adapted to use adiabatic pulses, with a slab-selective reinversion replacing the previously used 2D pencil-beam. The slab was oriented obliquely, in order to avoid upstream blood (e.g., left ventricle) or the navigator beam. Phantom experiments suggest that at 3 T, this approach improves both the net profile of the DIR pulse pair and the restoration of magnetization in the navigator region. Using this method, the feasibility of 3D coronary vessel wall imaging was demonstrated at 3 T. Fourteen healthy subjects were scanned using a segmented gradient-echo sequence with prospective navigator gating. Good-quality images of left and right coronary arteries were obtained, with SNR values of 29.7 +/- 7.5 (vessel wall); 10.5 +/- 4.4 (blood); 14.3 +/- 5.2 (fat); and 45.6 +/- 18.0 (myocardium). No problems occurred with ECG-gating or power deposition (SAR) limits.

MR coronary angiography and late-enhancement myocardial MR in children who underwent arterial switch surgery for transposition of great arteries

PURPOSE

To prospectively evaluate the feasibility of magnetic resonance (MR) coronary artery imaging and to define myocardial damage with late-enhancement myocardial MR imaging in children who underwent arterial switch surgery for transposition of the great arteries.

MATERIALS AND METHODS

The local research ethics committee approved this study, and the subjects and/or a parent or guardian gave informed consent. Sixteen asymptomatic subjects who had undergone arterial switch surgery for transposition of the great arteries were studied (mean age, 10.8 years +/- 1.3; 11 male subjects, five female subjects). MR coronary angiography, late-enhancement MR imaging, global ventricular function, and regional wall motion were assessed. Fifteen children were awake during imaging; one was imaged with the use of general anesthetic.

RESULTS

In 23 (72%) of 32 coronary arteries imaged, diagnostic-quality images of the coronary ostium and proximal coronary artery course were acquired; this increased to 100% in subjects older than 11 years. No coronary ostial stenoses were seen. In all subjects, the proximal course of the coronary arteries was visualized. Two subendocardial viability defects were detected, which corresponded to known compromise of the artery that supplied that territory at the time of surgery. Global left and right ventricular function were preserved, with no regional wall abnormalities.

CONCLUSION

Diagnostic-quality MR coronary angiography is feasible in subjects who have undergone arterial switch surgery for transposition of the great arteries, with no unexpected areas of myocardial infarction detected.

MRI monitoring of the effect of tissue interfaces in the penetration of high intensity focused ultrasound in kidney in vivo

In this paper, we studied the effect of interfaces during the application of high intensity focused ultrasound (HIFU) ablation in rabbit kidney in vivo. In kidney ablation, mainly two types of interfaces are encountered: these are muscle-kidney and fat-kidney. It was observed that the intensity for which the probability of cavitation (POC) is one was decreased when HIFU penetrated through interfaces, meaning that an interface is a potential site of cavitation. We utilized the concept of scanning the area to be treated in two dimensions (rectangular grid) by applying low intensity ultrasound (diagnostic scan). When all the points of the grid show decrease of signal in T1-weighted fast spoiled gradient (FSPGR) which indicated heating, complete necrosis was observed in the targeted area during the application of HIFU (therapeutic scan). If ultrasound goes through an interface that includes air spaces, the diagnostic scan indicates spaces with poor ultrasound penetration and as a result, during the application of the therapeutic scan, some sites remain untreated. The muscle-kidney and fat-kidney interfaces cause reflection of ultrasound, which prevents the penetration of ultrasound. Microscopic bubbles in the interface may initiate cavitation, especially at high intensities. However, sometimes these types of interfaces do not include any bubbles and therefore the propagation of ultrasound is not inhibited.

Multiprocessor scheduling implementation of the simultaneous multiple volume (SMV) navigator method

The simultaneous multiple volume (SMV) approach in navigator-gated MRI allows the use of the whole motion range or the entire scan time for the reconstruction of final images by simultaneously acquiring different image volumes at different motion states. The motion tolerance range for each volume is kept small, thus SMV substantially increases the scan efficiency of navigator methods while maintaining the effectiveness of motion suppression. This article reports a general implementation of the SMV approach using a multiprocessor scheduling algorithm. Each motion state is regarded as a processor and each volume is regarded as a job. An efficient scheduling that completes all jobs in minimal time is maintained even when the motion pattern changes. Initial experiments demonstrated that SMV significantly increased the scan efficiency of navigator-gated MRI.

MR imaging in ischemic heart disease

This article reviews the current MR imaging literature with respect to ischemic heart disease and focuses on the clinical practicalities of cardiac MR imaging today.

Current Concepts and Advances in Clinical Parallel Magnetic Resonance Imaging

Parallel imaging (PI) is one of the most promising recent advances in MRI technology and has, similar to the introduction of multidetector helical scanning in CT, revolutionized MR imaging. The speed of all conventional MRI methods has been limited by either gradient strength or their switching times. The basic idea in PI is to use some of the spatial information contained in the individual elements of a radiofrequency (RF) receiver coil array to increase imaging speed. These PI techniques are removing some of the previous limitations in speed of MRI scanners and set the basis for accelerated image formation. Initially, PI was motivated by the wish to accelerate image acquisition without reducing the spatial resolution of the image. However, depending on the application, it turned out that PI harbors several other advantages. Among those is the possibility for higher spatial resolution, shorter breath-holds or multiple averaging to diminish motion artifacts, reduced image blurring and geometric distortions, better temporal resolution, and means for navigator correction. This overview focuses on technical aspects, clinical applications, and ongoing research in different areas of the human body. The critical review demonstrates PI's great versatility as well as the current trends to use this unique technique in the majority of clinical scan protocols.

RingTag: ring-shaped tagging for myocardial centerline assessment

RATIONALE AND OBJECTIVES

Although endocardial ejection indexes lead to overestimation of contractility in hypertrophied hearts, circumferential fiber shortening at the mid wall (cFS) is less affected by wall thickness. In this study magnetic resonance tagging is exploited to assess directly cFS in normal and hypertrophied hearts.

METHODS

A novel tagging procedure generates freely definable, convex ring saturation bands. Data acquisition during the cardiac cycle is achieved with a fast, single breath-hold echo-planar imaging measurement that is combined with a slice-following approach and a navigator-guided breath-holding technique to improve reproducibility of breath hold positions.

RESULTS

The procedure is able to create variably shaped convex saturation structures on the myocardium that can be tracked automatically throughout the cardiac cycle. Circumferential shortening at the endocardial border (FSendo) obtained in 6 healthy volunteers and in 6 patients with hypertensive cardiomyopathy suggested hypercontractility of hypertrophied hearts (30.7 +/- 4.1% vs. 43.9 +/- 4.4% respectively; P < 0.002), whereas shortening at the level of the myofibers assessed as cFS was not different (17.2 +/- 1.4% vs. 18.1 +/- 2.8% respectively; P = 0.49).

CONCLUSIONS

The presented approach allows for assessment of midwall myocardial mechanics and may become a useful tool to study contractile function in hypertrophied hearts.

High-resolution MR-imaging of the liver with T2-weighted sequences using integrated parallel imaging: Comparison of prospective motion correction and respiratory triggering

PURPOSE

To compare high-resolution T2-weighted images of the liver with and without integrated parallel acquisition techniques (iPAT) using either breath-hold sequences in combination with prospective acquisition motion correction (PACE) or respiratory triggering.

MATERIALS AND METHODS

Ten volunteers and 10 patients underwent each four different high-resolution fast spin echo (FSE) T2-weighted sequences with 5 mm slice thickness and a full 320 matrix: a multi-breath-hold FSE sequence with and without iPAT and PACE and a respiratory-triggered FSE sequence with and without iPAT. Image quality was rated with a five-point scale by two independent readers. Signal intensity measurements were performed on a water phantom.

RESULTS

The sequences with iPAT required a substantially shorter acquisition time without loss of image quality. Overall image quality was rated equal for all sequences by both readers. Image time for nine slices with iPAT was 13 seconds (19 seconds without iPAT) with multi-breath-hold and on average 4:00 minutes (7:02 minutes without iPAT) with respiratory triggering. Imaging with the PACE technique resulted in more correct positioning of the image stacks.

CONCLUSION

T2-weighted fast imaging with iPAT is feasible and results in high-quality images within a short acquisition time. Overall image quality is not negatively affected by iPAT.

Intermittent-Mode CT Fluoroscopy–guided Biopsy of the Lung or Upper Abdomen with Breath-hold Monitoring and Feedback: System Development and Feasibility

A bellows-based breath-hold monitoring and feedback system was developed and evaluated for use in intermittent-mode computed tomographic (CT) fluoroscopy-guided biopsy procedures in the lung or upper abdomen. The bellows system is described, and its feasibility is demonstrated in studies with a respiratory phantom and human volunteers. Results are reported for seven patients who underwent bellows-assisted biopsy. Breath-hold monitoring and feedback with the bellows system allow the patient to perform reliable breath holding at a preselected level. This optimizes intermittent-mode CT fluoroscopy-guided biopsies by allowing consistent visualization of the target lesion throughout the procedure.

Cardiac MR imaging with external respirator: synchronizing cardiac and respiratory motion--feasibility study

The feasibility of electrocardiography (ECG)-synchronized respiration with an external cuirass-type respirator in cardiac magnetic resonance (MR) imaging was evaluated. Cardiac MR imaging was performed in 10 nonsedated healthy volunteers with an ECG-triggered external respirator that was modified for use in the MR environment. Coronary MR angiograms and multiphase gradient-echo cine images were acquired with one respiratory cycle performed per cardiac cycle. The technique was feasible and in this group of volunteers resulted in equivalent image quality but shorter acquisition times than those of conventional free-breathing and breath-holding techniques.

Coronary artery anomalies: assessment with free-breathing three-dimensional coronary MR angiography

PURPOSE

To evaluate a simplified protocol by using free-breathing three-dimensional (3D) coronary magnetic resonance (MR) angiography to determine the anatomy of anomalous coronary arteries, in particular the relationship of the vessels to the aortic root.

MATERIALS AND METHODS

Twenty-six patients (18 men, eight women; mean age, 50 years; age range, 18-77 years) who had a history of chest pain, palpitations, or syncope and who were suspected of having coronary artery anomalies were examined with free-breathing MR angiography. Multiple 3D volume slabs were acquired at the level of the sinuses of Valsalva by using diaphragmatic navigators for respiratory artifact suppression. The proximal anatomy of the coronary arteries was determined.

RESULTS

Six anomalous circumflex arteries originated from the right sinus of Valsalva and passed behind the aortic root. Six right coronary arteries arose from the left sinus of Valsalva and coursed between the aortic root and the right ventricular outflow tract (RVOT). Nine left coronary arteries arose from the right sinus of Valsalva; seven of nine coursed between the aortic root and the RVOT. Five patients had minor anomalies. Overall, in eight patients with anomalous arteries that coursed between the aortic root and the RVOT, conventional coronary angiography could not be used confidently to identify the proximal course.

CONCLUSION

Free-breathing 3D coronary MR angiography can be used to identify the proximal anatomy of anomalous coronary arteries.

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.

Magnetic resonance coronary angiography

Magnetic resonance coronary angiography (MRCA) has witnessed tremendous technical advances over the past decade. Although high-quality images of the coronary arteries have been demonstrated, this imaging modality is not performed routinely today. The fundamental properties of the coronary arteries deterring noninvasive imaging are well known. This article provides an overview of the developmental efforts to overcome these challenges, and highlights key technical and clinical advances. The future prospect of MRCA depends on clinical implementation of the technique. In order to meet this challenge, the following issues must be addressed: contrast- and signal-to-noise ratio, temporal and spatial resolution, and scan protocol.