Undersampled elliptical centric view-order for improved spatial resolution in contrast-enhanced MR angiography.

Although contrast-enhanced MR angiography (CE-MRA) has been successfully developed into a routine clinical imaging technique, there is still need for improved spatial resolution in a given acquisition time. Undersampled projection reconstruction (PR) techniques maintain spatial resolution with reduced scan times, and the elliptical centric (EC) view order provides high quality arterial phase images without venous contamination. In this work, we present a hybrid elliptical centric-projection reconstruction (EC-PR) technique to provide spatial resolution improvement over standard EC in a given time. The k-space sampling was performed by undersampling the periphery of the k(Y)-k(Z) phase encoding plane of an EC view order in a PR like manner. The sampled views were maintained on a rectilinear grid, and thus reconstructed by standard 3DFT. The non-sampled views were compensated either by zero-filling or performing a 2D homodyne reconstruction. Compared to a fully sampled k-space, the EC-PR sequence acquired in the same scan time provides a resolution improvement of about two, as shown by point spread function analysis and phantom experiments. The hypothesis that EC-PR provides improved resolution while retaining diagnostically adequate SNR was tested in 11 CE-MRA studies of the popliteal and carotid arteries and shown to be true (P < 0.03).     

Accelerating MR diffusion tensor imaging via filtered reduced-encoding projection-reconstruction.

MR diffusion tensor imaging (DTI) is a promising tool for characterizing the microstructure of ordered tissues. However, its practical applications are hampered by relatively low signal-to-noise-ratio and spatial and temporal resolution. Reduced-encoding imaging (REI) via k-space sharing with constrained reconstruction has previously been shown to be effective for accelerating DTI, although the implementation was based on rectilinear k-space sampling. Due to the intrinsic oversampling of central k-space and allowance for isotropic downsampling, projection-reconstruction (PR) imaging may be better suited for REI. In this study, regularization procedures, including radial filtering and baseline signal correction to adequately reconstruct reduced encoded PR imaging data, are investigated. The proposed filtered reduced-encoding projection-reconstruction (FREPR) technique is applied to DTI tissue fiber orientation and fractional anisotropy (FA) measurements. Results show that FREPR offers improved reconstructions of the reduced encoded images and on an equal total scan-time basis provides more accurate fiber orientation and FA measurements compared to rectilinear k-space sampling-based REI methods or a control experiment consisting of only fully encoded images. These findings suggest a potentially significant role of FREPR in accelerating repeated imaging and improving the data acquisition-time efficiency of DTI experiments.     

Breath-hold contrast-enhanced three-dimensional MR angiography of the abdomen: time-resolved imaging versus single-phase imaging

PURPOSE: To evaluate a technique for time-resolved breath-hold contrast material-enhanced three-dimensional magnetic resonance (MR) angiography of the abdomen. MATERIALS AND METHODS: In a prospective study, 43 patients underwent time-resolved MR angiography (acquisition time per data set, 7 seconds). The patients also underwent single-phase high-spatial-resolution MR angiography (acquisition time, 27 seconds) (n = 6), conventional angiography (n = 7), or both (n = 30). No bolus timing study was performed for time-resolved MR angiography. Image quality (presence of artifacts, ability to prevent venous overlap on arterial phase images, contrast enhancement) and demonstration of anatomic variants (renal arterial and venous variants, vena caval anomaly, visceral arterial variants) and vascular diseases were assessed. RESULTS: Time-resolved MR angiographic images were characterized by fewer and less severe artifacts, less overlap of enhancing veins, and better contrast enhancement than were single-phase MR angiographic images (P < .05). The mean sensitivity and specificity were 90% (nine of 10) and 100% (1 73 of 1 73), respectively, for detection of arterial anatomic variants and 93% (28 of 30) and 100% (324 of 325), respectively, for detection of disease. The technique also proved to be reliable for demonstration of venous disease. CONCLUSION: In comparison with current non-time-resolved MR angiographic techniques, time-resolved MR angiography is more robust and easier to perform and allows simultaneous evaluation of arterial and venous disease.     

Postprocessing techniques for time-resolved contrast-enhanced MR angiography

The purpose of this study was to improve dynamic two-dimensional projection magnetic resonance digital subtraction angiography by using remasking and filtering postprocessing techniques. Four methods were evaluated in 50 patients: default mask subtraction, remasked subtraction, filtering based on the SD, and linear filtering. The results demonstrated that postprocessing techniques such as linear filtering can reduce background motion artifacts and improve arterial contrast-to-noise ratio.     

时间分辨的三维增强磁共振血管成像的发展现状

时间分辨的三维增强磁共振血管成像是一种类似DSA成像效果的新型MRA技术,可以清晰显示血管从动脉早期到平衡期的完整的动态充盈过程,且无需预先监测及设置对比剂的延迟时间,对头颈、四肢血管疾病的诊断有十分重要的价值。     

Whole-heart coronary magnetic resonance angiography: contrast-enhanced high-resolution, time-resolved 3D imaging

OBJECTIVES: To test the feasibility and performance of a 4D magnetic resonance coronary angiography sequence compared with conventional inversion recovery (IR) prepared gradient echo imaging. MATERIALS AND METHODS: A 4D sequence with 100 milliseconds temporal resolution was implemented on a 1.5 T system. Five minipigs were examined after administration of very small superparamagnetic iron oxide particles. Coronary angiographies with an isotropic resolution of 0.82 mm were performed in the pigs using 4D and IR sequences. RESULTS: The 4D sequence allowed visualization of the coronary arteries, the effect of their movement and that of the entire heart without prolonging scan time. The contrast-to-noise ratio of the IR images was on average 38% higher than that of the corresponding 4D phase. CONCLUSIONS: 4D magnetic resonance imaging is superior in that no trigger delay time needs to be determined and an additional whole-heart cine study can be obtained.     

时间分辨的三维增强磁共振血管成像的发展现状

时间分辨的三维增强磁共振血管成像是一种类似DSA成像效果的新型MRA技术,可以清晰显示血管从动脉早期到平衡期的完整的动态充盈过程,且无需预先监测及设置对比剂的延迟时间,对头颈、四肢血管疾病的诊断有十分重要的价值.     

Undersampled projection reconstruction for active catheter imaging with adaptable temporal resolution and catheter-only views.

In this study undersampled projection reconstruction (PR) was used for rapid catheter imaging in the heart, employing steady-state free precession (SSFP) contrast. Active catheters and phased-array coils were used for combined imaging of anatomy and catheter position in swine. Real-time imaging of catheter position was performed with relatively high spatial and temporal resolution, providing 2 x 2 x 8 mm spatial resolution and four to eight frames per second. Two interactive features were introduced. The number of projections (Np) was adjusted interactively to trade off imaging speed and artifact reduction, allowing acquisition of high-quality or high-frame-rate images. Thin-slice imaging was performed, with interactive requests for thick-slab projection images of the signal received solely from the active catheter. Briefly toggling on catheter-only projection images was valuable for verifying that the catheter tip was contained within the selected slice, or for locating the catheter when part of it was outside the selected slice.     

Postoperative assessment of extracranial-intracranial bypass by time-resolved 3D contrast-enhanced MR angiography using parallel imaging

PURPOSE: Our goals were to assess image quality of time-resolved contrast-enhanced MR angiography (CE MRA), by using 3D data acquisition along with a parallel imaging technique that can improve temporal resolution and to compare this technique with 3D-time-of-flight (TOF) MRA in the postoperative assessment of extracranial (EC)-intracranial (IC) bypass surgery. METHODS: On a 1.5T imaging system, we performed CE MRA by using a 3D fast field-echo sequence in combination with a parallel imaging technique, to obtain images in the coronal plane centered at the postoperative site. Our patient group comprised 17 patients, including 13 after superficial temporal artery-middle cerebral artery (MCA) anastomosis, 3 after external carotid artery-MCA anastomosis, and one after extracranial vertebral artery-posterior cerebral artery anastomosis. Visualization of the anastomosis and the distal flow on the CE-MRA images was assessed comparatively with that on 3D-TOF MR angiograms obtained at the same time. In 6 patients, we also compared the efficiency of visualization on CE-MRA images with that on conventional angiograms. RESULTS: A temporal resolution of 0.8 s/frame could be achieved with the technique employed. The bypass was better demonstrated postoperatively on CE-MRA images than on 3D-TOF MR angiograms in 13 patients (76%), whereas the 2 methods were equivalent in 4 patients (24%). Good correspondence of results was observed in the 6 patients for whom CE MRA and conventional digital subtraction angiography (DSA) images were compared. CONCLUSION: CE MRA by using the parallel imaging technique can increase image acquisition speed with sufficient image quality. This technique is at least equivalent to 3D-TOF MRA to evaluate the postoperative status of EC-IC bypass.     

Intracranial time-resolved contrast-enhanced MR angiography at 3T

BACKGROUND AND PURPOSE: A method is presented for high-temporal-resolution MR angiography (MRA) using a combination of undersampling strategies and a high-field (3T) scanner. Currently, the evaluation of cerebrovascular disorders involving arteriovenous shunting or retrograde flow is accomplished with conventional radiographic digital subtraction angiography, because of its high spatial and temporal resolutions. Multiphase MRA could potentially provide the same diagnostic information noninvasively, though this is technically challenging because of the inherent trade-off between signal intensity-to-noise ratio (S/N), spatial resolution, and temporal resolution in MR imaging. METHODS: Numerical simulations addressed the choice of imaging parameters at 3T to maximize S/N and the data acquisition rate while staying within specific absorption rate limits. The increase in S/N at 3T was verified in vivo. An imaging protocol was developed with S/N, spatial resolution, and temporal resolution suitable for intracranial angiography. Partial Fourier imaging, parallel imaging, and the time-resolved echo-shared acquisition technique (TREAT) were all used to achieve sufficient undersampling. RESULTS: In 40 volunteers and 10 patients exhibiting arteriovenous malformations or fistulas, intracranial time-resolved contrast-enhanced MRA with high acceleration at high field produced diagnostic-quality images suitable for assessment of pathologies involving arteriovenous shunting or retrograde flow. The technique provided spatial resolution of 1.1 x 1.1 x 2.5 mm and temporal resolution of 2.5 seconds/frame. The combination of several acceleration methods, each with modest acceleration, can provide a high overall acceleration without the artifacts of any one technique becoming too pronounced. CONCLUSION: By taking advantage of the increased S/N provided by 3T magnets over conventional 1.5T magnets and converting this additional S/N into higher temporal resolution through acceleration strategies, intracranial time-resolved MRA becomes feasible.     

Combining time-resolved and single-phase 3D techniques in contrast-enhanced carotid MR angiography.

We established an easy-to-use technique for performing contrast-enhanced carotid MR angiography (MRA) with a commercial scanner. Twenty-three patients with suspected carotid or vertebral arterial lesions were prospectively studied. Two techniques were applied in the study. After performing sagittal time-resolved acquisitions, we undertook a coronal single-phase 3D acquisition, in which the injection timing was estimated from the preceding images. In each case, we obtained multidirectional images with sufficient venous suppression. The combined use of time-resolved and single-phase 3D MRA is a feasible technique for obtaining selective arterial images without the use of special applications or hardware.     

Assessment of time-resolved, dynamic, contrast-enhanced MRDSA using radial sliding-window reconstruction.

Sliding-window reconstruction (SWR) has been recently introduced for rapid imaging that improves temporal resolution while maintaining signal-to-noise ratio (SNR) and spatial resolution. We assessed the quantity of 2-dimensional contrast-enhanced magnetic resonance digital subtraction angiography (2D CE-MRDSA) with non-Cartesian radial SWR in phantom and clinical studies. In phantoms, we compared the quantitative properties of time-intensity curves (TIC) obtained with dynamic 2D CE-MRDSA using SWR in a radial acquisition with those obtained by Cartesian acquisition. We calculated the mean variance and standard deviation among signal intensities in TICs and used SWR to study 2D CE-MRDSA in 5 patients with angiographically proven arteriovenous malformations. Using a 3-point grading scale, we individually scored vascular visualization capability and calculated time delay (TD) from the TIC in the feeding artery (FA), nidus, and draining vein (DV). The maximum signal intensity variance in Cartesian SWR was 6.58 +/- 2.27% among time-intensity curves and was 0.87 +/- 0.77% radial SWR. Signal intensity in radial SWR decreased significantly (P<0.001) compared with the Cartesian SWR. In clinical study, the mean rating on 2D CE-MRA of the feeding artery was 2.3, nidus, 2.6, and draining vein, 2.6. Mean delay time between DV and FA was 1.8 s. The radial SWR technique is useful for demonstrating the hemodynamic features of vascular malformations in the head with 2D CE-MRDSA.     

High-resolution time-resolved contrast-enhanced MR abdominal and pulmonary angiography using a spiral-TRICKS sequence.

Both high spatial resolution and high temporal resolution are desirable for contrast-enhanced magnetic resonance angiography (CE-MRA) in order to depict the arterial vasculature. In this work a fast MR pulse sequence (spiral time-resolved imaging with contrast kinetics (Spiral-TRICKS)) with spiral readout in-plane and Cartesian slice encoding was developed whereby the slices are partitioned into multiple regions and acquired in the order used with the TRICKS sequence. The combination of highly efficient spiral sampling with TRICKS acquisition significantly reduced imaging time requirements. A unit second temporal reconstructed frame rate could be achieved for three-dimensional (3D) CE-MRA without undersampling of the spiral trajectories. Image quality was improved through spiral trajectory measurement and field-map correction. Phantom and volunteer studies were performed to demonstrate the feasibility of this technique.     

Pulse sequences for contrast-enhanced magnetic resonance imaging

The theory and application of magnetic resonance imaging (MRI) pulse sequences following the administration of an exogenous contrast agent are discussed. Pulse sequences are categorised according to the contrast agent mechanism: changes in proton density, relaxivity, magnetic susceptibility and resonant frequency shift. Applications in morphological imaging, magnetic resonance angiography, dynamic imaging and cell labelling are described. The importance of optimising the pulse sequence for each application is emphasised.     

Equipment requirements to facilitate contrast-enhanced MR imaging.

During the past decade, MR equipment and imaging techniques have experienced unprecedented development. Significant improvements have been made in image quality, enhancing conventional contrast agent studies. In addition, the development of new applications, such as MR angiography, has expanded the role of existing agents. A major benefit for contrast agent research has been the development of ultrafast MR imaging. This ability to provide information on contrast agent dynamics will aid in the detection and characterization of neoplastic and other disease states. While the gradient-echo techniques may provide adequate temporal resolution for many applications, echo planar imaging will enable the assessment of perfusion for an entire organ, which is critical in the heart or when the location of the pathology is unknown a priori. It is also likely that continued development in MR contrast agents will have a synergistic impact on pulse sequences, e.g., MR diffusion imaging. The growth in functional MR imaging will be based in large measure on the continued interactive development of imaging strategies and magnetopharmaceutials.