Reduction of reconstruction time for time-resolved spiral 3D contrast-enhanced magnetic resonance angiography using parallel computing.

Time-resolved 3D MRI with high spatial and temporal resolution can be achieved using spiral sampling and sliding-window reconstruction. Image reconstruction is computationally intensive because of the need for data regridding, a large number of temporal phases, and multiple RF receiver coils. Inhomogeneity blurring correction for spiral sampling further increases the computational work load by an order of magnitude, hindering the clinical utility of spiral trajectories. In this work the reconstruction time is reduced by a factor of >40 compared to reconstruction using a single processor. This is achieved by using a cluster of 32 commercial off-the-shelf computers, commodity networking hardware, and readily available software. The reconstruction system is demonstrated for time-resolved spiral contrast-enhanced (CE) peripheral MR angiography (MRA), and a reduction of reconstruction time from 80 min to 1.8 min is achieved.     

Fast inversion recovery magnetic resonance angiography of the intracranial arteries

Inversion‐prepared pulse sequences can be used for noncontrast MR angiography (MRA) but suffer from long scan times when acquired using conventional nonaccelerated techniques. This work proposes a subtraction‐based spin‐labeling, three‐dimensional fast inversion recovery MRA (FIR‐MRA) method for imaging the intracranial arteries. FIR‐MRA uses alternating cycles of nonselective and slab‐selective inversions, leading to dark‐blood and bright‐blood images, respectively. The signal difference between these images eliminates static background tissue and generates the angiogram. To reduce scan time, segmented fast gradient recalled echo readout and parallel imaging are applied. The inversion recovery with embedded self‐calibration method used allows for parallel acceleration at factors of 2 and above. An off‐resonance selective inversion provides effective venous suppression, with no detriment to the depiction of arteries. FIR‐MRA was compared against conventional three‐dimensional time‐of‐flight angiography at 3 T in eight normal subjects. Results showed that FIR‐MRA had superior vessel conspicuity in the distal vessels (P < 0.05), and equal or better vessel continuity and venous suppression. However, FIR‐MRA had inferior vessel sharpness (P < 0.05) in four of nine vessel groups. The clinical utility of FIR‐MRA was demonstrated in three MRA patients. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Orbital navigator echoes for motion measurements in magnetic resonance imaging

A single “orbital” navigator echo, that has a circular k-space trajectory, is used to simultaneously measure in-plane rotational and multi-axis translational global motion. Rotation is determined from the shift in the magnitude profile of the echo with respect to a reference echo. Displacements are calculated from the phase difference between the current echo and a reference echo. Phantom studies show that this technique can accurately measure rotation and translations. Preliminary results from adaptive motion correction studies on phantom and human subjects indicate that the orbital navigator echo is an effective method for motion measurement in MRI.     

Soft tissue enhancement on time-resolved peripheral magnetic resonance angiography

PURPOSE: To evaluate the incidence and locations of soft tissue enhancement on time-resolved two-dimensional projection magnetic resonance angiography (MRA) of the calf and foot. MATERIALS AND METHODS: Time-resolved two-dimensional projection MRA of the knee, calf, and foot, performed at 1.5 Tesla using the head coil, was retrospectively reviewed in 326 patients. Soft tissue enhancement of the foot was identified and graded by blinded review. Subsequently, patient medical records were reviewed to determine the presence or absence of diabetes, cellulitis, gangrene, and ulceration of the foot and presence of neuropathic joints. RESULTS: Enhancing spots were identified in 228 patients. For those feet without clinical abnormalities, diabetic patients had an average of 1.2 enhancing spots per foot, while nondiabetics had only 0.6 (P < 0.001). Higher-grade lesions were more prevalent in diabetics and in heavier patients. Of 64 patients with follow-up, 8 (13%) developed cellulitis (N = 4) or ulceration (N = 4) at the location of an enhancing spot, including 5 diabetic and 3 nondiabetic patients. CONCLUSION: Pedal soft tissue enhancement frequently occurs on time-resolved gadolinium (Gd):MRA of the feet. The etiology is uncertain, but the high frequency in diabetic patients and observation of progression to cellulitis/ulceration suggest this soft tissue enhancement may identify sites of subclinical pedal soft tissue injury.     

Impact of navigator timing on free-breathing submillimeter 3D coronary magnetic resonance angiography.

The purpose of this study was to investigate the impact of navigator timing on image quality in navigator-gated and real-time motion-corrected, free-breathing, three-dimensional (3D) coronary MR angiography (MRA) with submillimeter spatial image resolution. Both phantom and in vivo investigations were performed. 3D coronary MRA with real-time navigator technology was applied using variable navigator time delays (time delay between the navigator and imaging sequences) and varying spatial resolutions. Quantitative objective and subjective image quality parameters were assessed. For high-resolution imaging, reduced image quality was found as a function of increasing navigator time delay. Lower spatial resolution coronary MRA showed only minor sensitivity to navigator timing. These findings were consistent among volunteers and phantom experiments. In conclusion, for submillimeter navigator-gated and real-time motion-corrected 3D coronary MRA, shortening the time delay between the navigator and the imaging portion of the sequence becomes increasingly important for improved spatial resolution.     

Correction of motion artifacts from cardiac cine magnetic resonance images

RATIONALE AND OBJECTIVES: An image registration method was developed to automatically correct motion artifacts, mostly from breathing, from cardiac cine magnetic resonance (MR) images. MATERIALS AND METHODS: The location of each slice in an image stack was optimized by maximizing a similarity measure of the slice with another image slice stack. The optimization was performed iteratively and both image stacks were corrected simultaneously. Two procedures to optimize the similarity were tested: standard gradient optimization and stochastic optimization in which one slice is chosen randomly from the image stacks and its location is optimized. In this work, cine short- and long-axis images were used. In addition to visual inspection results from real data, the performance of the algorithm was evaluated quantitatively by simulating the movements in four real MR data sets. The mean error and standard deviation were defined for 50 simulated movements as each slice was randomly displaced. The error rate, defined as the percentage of non-satisfactory registration results, was evaluated. The paired t-test was used to evaluate the statistical difference between the tested optimization methods. RESULTS: The algorithm developed was successfully applied to correct motion artifacts from real and simulated data. The results, where typical motion artifacts were simulated, indicated an error rate of about 3%. Subvoxel registration accuracy was also achieved. When different optimization methods were compared, the registration accuracy of the stochastic approach proved to be superior to the standard gradient technique (P < 10(-9)). CONCLUSIONS: The novel method was capable of robustly and accurately correcting motion artifacts from cardiac cine MR images.     

Blood velocity assessment using 3D bright-blood time-resolved magnetic resonance angiography.

Blood velocity is a functional parameter that is not easily assessed noninvasively, especially in small animals. A new noninvasive method that uses magnetic resonance angiography (MRA) to measure blood flows is proposed. This method is based on the time-of-flight (TOF) phenomenon. By initially suppressing the signal from the stationary spins in the area of interest, it is possible to sequentially visualize only the signal from the moving spins entering a given volume. With this method, 3D cine images of the blood flow can be generated by positive contrast, with unparalleled spatial (<200 microm) and temporal resolutions (<10 ms/image). As a result, it is possible to measure flow in sinuous paths. The present method was applied in vivo to measure the blood velocity in mouse carotid arteries. Because of its robustness and simplicity of implementation, this method has numerous potential applications for fundamental studies in small animal models.     

Respiratory motion in coronary magnetic resonance angiography: a comparison of different motion models

PURPOSE: To assess respiratory motion models for coronary magnetic resonance angiography (CMRA). In this study various motion models that describe the respiration-induced 3D displacements and deformations of the main coronary arteries were compared.MATERIALS AND METHODS: Multiple high-resolution 3D coronary MR images were acquired in healthy volunteers using navigator-based respiratory gating, each depicting the coronary vessels at different respiratory motion states. In the images representing the different inspiratory states the displacements and deformations of the main coronary vessels with respect to the end-expiratory state were determined, by means of elastic registration. Several correction models (superior-inferior (SI) translation, 3D translation, and 3D affine transformation) were tested and compared with respect to their ability to map a selected inspiratory to the end-expiratory motion state.RESULTS: 3D translation was found to be superior over SI translation, which is commonly used for prospective motion correction in CMRA. The 3D affine transformation was found to be the best correction model considered in this study. Furthermore, a large intersubject variability of the model parameters was observed.CONCLUSION: The results of this study indicate that a patient-adapted 3D correction model (3D translation or better 3D affine) will considerably improve prospective motion correction in CMRA.     

Three-dimensional reconstruction of limited-view projections for contrast-enhanced magnetic resonance angiography at high temporal and spatial resolution.

The feasibility of reconstructing three-dimensional (3D) MRI data sets from limited-view projections is investigated in phantom and in vivo animal studies to improve the temporal resolution of magnetic resonance angiography without sacrificing spatial resolution. Thirty-two pairs of orthogonal biplane projections are acquired in an interleaved manner during the first pass of a contrast agent. The full data set is reconstructed as a priori 3D information. Each pair of projections is then reconstructed into an individual 3D data set based on a correlation analysis with the a priori data set. In this way, time-resolved 3D data sets at 1- to 2-s time intervals are reconstructed with submillimeter spatial resolution. Artifacts are limited if the image is simply structured or sparse and if SNR is sufficient in the projection images. With this technique, both high temporal and spatial resolution can be achieved simultaneously.     

Need for background suppression in contrast-enhanced peripheral magnetic resonance angiography.

To determine if background suppression is beneficial for peripheral magnetic resonance angiography (pMRA), nonsubtracted, subtracted, and fat-saturated contrast-enhanced (CE) pMRA were compared in 10 patients with peripheral arterial disease. Signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs), as well as venous enhancement and subjective interpretability, were determined in a station-by-station fashion for each technique. In three patients X-ray angiography was available as a standard of reference. SNRs and CNRs were significantly higher for fat-saturated vs. the other two techniques (P = 0.005). Subjective interpretability was best for subtracted data sets in the lower-leg station. In the iliac station, fat-saturated data sets were considered to have significantly lower interpretability than subtracted data sets. Venous enhancement occurred significantly more often in the lower-leg station with the fat-saturated technique. The value of subtraction depends on the hardware one has available and is a useful tool if dedicated surface coils are used. Background suppression by means of magnitude subtraction leads to the best lower-leg image interpretability. Care must be taken to avoid venous enhancement in the lower-leg station when using fat saturation.     

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.     

Improved bulk myocardial motion suppression for navigator-gated coronary magnetic resonance imaging

PURPOSE: To evaluate the impact of a new, cross-correlation based method for compensation of respiratory induced motion of the heart using an individually adapted three-dimensional (3D) translation or affine transformation approach. MATERIALS AND METHODS: A total of 32 patients underwent a routine cardiac MR examination. In each patient, a calibration scan was performed during free-breathing to register breathing-related motion within a 3D ellipsoid registration kernel covering the entire heart. Three navigators were employed for all three spatial dimensions (feet-head, anterior-posterior, and left-right) and the optimal translatory correction factors for each spatial dimension were determined. In addition, the cross-correlations for different motion models (no compensation, fixed 1D-translation, adapted 3D-translation, and affine transformation) were calculated. RESULTS: The mean correction factor for the feet-head direction was 0.45 +/- 0.13. Though the mean correction factors for the anterior-posterior and left-right direction were nearly zero (-0.01 +/- 0.08 and 0.02 +/- 0.09, respectively), the correction factors exceeded the amount of 0.1 in 12 (19%) and in 19 patients (30%), respectively. All motion compensation models showed significantly higher cross-correlations when compared to "no compensation" (P < 0.05). In particular, the affine transformation algorithm achieved the highest cross-correlation values (88.3 +/- 5.1%) with a significant increase compared to fixed 1D translation (84.7 +/- 6.5%, P < 0.05). CONCLUSION: A considerable number of patients demonstrated relevant breathing-related movement of the heart in the anterior-posterior or left-right direction in addition to the predominant breathing-related movement in the feet-head direction. Thus, it is recommended to compensate for all three spatial dimensions. The affine transformation algorithm combined with three navigators significantly improved breathing-related cardiac motion compensation when compared to the conventionally applied 1D translation with a fixed correction factor.     

First-line investigation of acute intracerebral hemorrhage using dynamic magnetic resonance angiography

Purpose: To report the initial experience of magnetic resonance (MR) digital subtraction angiography (MR-DSA) in the dynamic assessment of the cerebral circulation in acute non-traumatic intracerebral hemorrhage (ICH).

Material and Methods: Twelve patients with acute ICH were investigated within 6 days of the ictus using a dynamic contrast-enhanced 2-D MR angiogram that produces subtracted images with a temporal resolution of 1-2 frame/s. The MR-DSA examinations were assessed for evidence of an intracranial vascular abnormality and were compared with (i) the routine MR sequences, (ii) non-dynamic time-of-flight MR angiography, and (iii) catheter angiogram performed during the same admission.

Results: All 12 MR-DSA examinations were considered to be technically satisfactory. MR-DSA detected an intracranial vascular abnormality in 7 patients (3 arteriovenous malformations, 2 aneurysms, 1 dural arteriovenous fistula, and 1 venous thrombosis). All abnormalities were confirmed by catheter angiography with the exception of one patient with venous sinus thrombosis found on MR imaging that did not undergo catheter angiography. All four arteriovenous shunts were detected by MR-DSA by virtue of early venous filling.

Conclusion: MR-DSA can be performed satisfactorily in the setting of acute ICH and provides an alternative method to catheter angiography for identifying shunting vascular abnormalities such as arteriovenous malformations and fistulae, as well as large aneurysms and venous occlusions. MR-DSA is a contrast-medium-based technique that does not suffer from the T1 shortening effects of acute hemorrhage that can obscure abnormalities on conventional flow-based non-dynamic techniques.     

Targeted ROTational magnetic resonance angiography (TROTA).

An MR angiographic method is presented in which a rotating 2D slice is centered on and targets a region or vessel of interest. Collecting a series of slices rotating about the center of the targeted region yields projection data sufficient for the calculation of 3D volumetric data of the region using conventional backprojection reconstruction techniques. These volumetric data depict the internal structure of the vessel and can be processed and displayed with multiplanar reformation, maximum intensity projections, and 3D rendering algorithms. The rotational angiographic acquisition preserves the high temporal resolution of 2D-MR digital subtraction angiography with the added benefit of 3D reformatting and display. The method is explained in detail and results from phantom and human experiments are presented.     

Whole-heart steady-state free precession coronary artery magnetic resonance angiography.

Current implementations of coronary artery magnetic resonance angiography (MRA) suffer from limited coverage of the coronary arterial system. Whole-heart coronary MRA was implemented based on a free-breathing steady-state free-precession (SSFP) technique with magnetization preparation. The technique was compared to a similar implementation of conventional, thin-slab coronary MRA in 12 normal volunteers. Three thin-slab volumes were prescribed: 1) a transverse slab, covering the left main (LM) artery and proximal segments of the left anterior ascending (LAD) and left circumflex (LCX) coronary arteries; 2) a double-oblique slab covering the right coronary artery (RCA); and 3) a double-oblique slab covering the proximal and distal segments of the LCX. The whole-heart data set was reformatted in identical orientations. Visible vessel length, vessel sharpness, and vessel diameter were determined and compared separately for each vessel. Whole-heart coronary MRA visualized LM/LAD (11.7 +/- 3.4 cm) and LCX (6.9 +/- 3.6 cm) over a significantly longer distance than the transverse volume (LM/LAD, 6.1 +/- 1.1 cm, P < 0.001; LCX, 4.2 +/- 1.2 cm, P < 0.05). Improvements in visible vessel length for RCA and LCX in the whole-heart approach vs. their respective targeted volumes were not significant. It is concluded that the whole-heart coronary MRA technique improves visible vessel length and facilitates high-quality coronary MRA of the complete coronary artery tree in a single measurement.     

大脑中动脉狭窄的磁共振血管成像与数字减影血管造影的对照研究

目的 探讨磁共振血管成像(MRA)在大脑中动脉狭窄或闭塞病变中的诊断价值.方法 35例缺血性脑血管病患者先后行MRA及DSA检查,以DSA为金标准,分析MRA对不同程度大脑中动脉狭窄的诊断价值.结果 35例患者70支大脑中动脉中,MRA显示正常血管28支,病变血管42支,病变部位47处,MRA与DSA诊断符合53处,符...