Prospective adaptive navigator correction for breath-hold MR coronary angiography

Current MR coronary angiography (MRCA) methods use breath-holding to minimize respiratory motion. A major limitation to this technique is misregistration between imaging slices due to breath-hold variability. Prospective adaptive correction of image location using real-time navigator measurement of diaphragm position is a potential method for improving slice registration in breath-hold MRCA. Ten subjects underwent MRCA using an ECG-gated, fat-suppressed, segmented k-space, gradient-echo sequence. Transverse and coronal images were acquired using standard breath-holding with and without prospective navigator correction. Breath-hold MRCA with prospective navigator correction resulted in a 47% reduction in craniocaudal slice registration error compared to standard breath-holding (0.9 ± 0.2 mm versus 1.7 2 0.4 mm, P = 0.04). Prospective adaptive navigator correction of image location significantly improves slice registration for breath-hold MRCA and is a promising motion correction technique for cardiac MR.     

Retrospective adaptive motion correction for navigator-gated 3D coronary MR angiography

Retrospective adaptive motion correction (AMC) was developed for reducing effects of residual respiration in real-time navigator-gated three-dimensional (3D) coronary magnetic resonance (MR) angiography. In both motion phantom and in vivo experiments, AMC improved image sharpness of coronary arteries. This navigator-based technique combining adaptive correction and real-time gating is potentially an efficient and effective motion reduction method for 3D coronary MR angiography.     

MR angiography with oblique gradient-recalled echo technique

Magnetic resonance (MR) angiography with use of an oblique gradient-recalled echo sequence is discussed. The technique was developed for the efficient acquisition of angiographic data when the desired projection direction is already known. The raw data set is acquired directly at the projection angle; thus, a high-resolution projection is created, despite a decrease in the number of phase-encoding views acquired per axial image. Excessive reduction in the number of views acquired causes a loss of contrast in the projection images rather than a loss of resolution. High-resolution, high-contrast MR projection angiograms can be obtained in 2 1/2 minutes, with 50 3-mm sections and 48 phase encodings per section, a repetition time of 50 msec, an echo time of 15 msec, and a flip angle of 45 degrees. This represents one-fifth the time required for a conventional angiographic image. Venous blood is successfully saturated when the saturation band is placed at a fixed distance from the current imaging section. The method is demonstrated in the imaging of the carotid bifurcation in healthy volunteers and of a patient who had undergone carotid endarterectomy.     

A three-coil comparison for MR angiography

The purpose of this work was to compare intracranial magnetic resonance angiography (MRA) image quality using three different radiofrequency coils. The three coil types included a reduced volume quadrature birdcage coil with endcap, a commercially available quadrature birdcage head coil, and a four-element phased-array coil. Signal-to-noise ratio (SNR) measurements were obtained from comparison studies performed on a uniform cylindrical phantom. MRA comparisons were performed using data acquired from 15 volunteers and applying a thick-slab three-dimensional time-of-flight sequence. Analysis was performed using the signal difference-to-noise ratio, a quantitative measure of the relative vascular signal. The reduced-volume endcap and phased-array coils, which were designed specifically for imaging the intracranial volume of the head, improved the image SNR and vascular detail considerably over that obtained using the commercially available head coil. The endcap coil configuration provided the best vascular signal overall, while the phased-array coil provided the best results for arteries close to the coil elements.     

A novel MR angiography technique: SPEED acquisition using half-Fourier RARE

A novel MR angiography (MRA) method, swap phase encode extended data (SPEED), was developed. Two one-shot images with the phase-encode directions swapped were collected within a single breath-hold period and processed with a maximum intensity projection (MIP) to obtain an image. In this study, a long echo train two-dimensional rapid acquisition with relaxation enhancement (RARE) sequence with half-Fourier (half-RARE) was used to obtain the pulmonary MRA images. The MIP image obtained using the SPEED technique presented promising results for pulmonary vessels.     

MR angiography fusion technique for treatment planning of intracranial arteriovenous malformations

PURPOSE: To develop an image fusion technique using elliptical centric contrast-enhanced (CE) MR angiography (MRA) and three-dimensional (3D) time-of-flight (TOF) acquisitions for radiosurgery treatment planning of arteriovenous malformations (AVMs). MATERIALS AND METHODS: CE and 3D-TOF MR angiograms with disparate in-plane fields of view (FOVs) were acquired, followed by k-space reformatting to provide equal voxel dimensions. Spatial domain addition was performed to provide a third, fused data volume. Spatial distortion was evaluated on an MRA phantom and provided slice-dependent and global distortion along the three physical dimensions of the MR scanner. In vivo validation was performed on 10 patients with intracranial AVMs prior to their conventional angiogram on the day of gamma knife radiosurgery. RESULTS: Spatial distortion in the phantom within a volume of 14 x 14 x 3.2 cm(3) was less than +/-1 mm (+/-1 standard deviation (SD)) for CE and 3D-TOF data sets. Fused data volumes were successfully generated for all 10 patients. CONCLUSION: Image fusion can be used to obtain high-resolution CE-MRA images of intracranial AVMs while keeping the fiducial markers needed for gamma knife radiosurgery planning. The spatial fidelity of these data is within the tolerance acceptable for daily quality control (QC) purposes and gamma knife treatment planning.     

高分辨率增强MR血管成像中的零点充填技术

目的在解剖学仿真动脉模型上对2种零点充填(zero-filling interpolation,ZIP)技术进行比较研究。方法 8个解剖学仿真动脉模型,腔内径为2～10 mm,在1.5 T MR 扫描仪上用头线圈进行钆喷替酸葡甲胺增强 MR 血管成像。快速扰相位梯度回波(FSPGR)序列的参数设置如下:反转角45°,TR6 ms,TE 1.4 ms,带宽31.2 kHz,层厚1.4 mm。研究过程中上述参数保持不变,而 ZIP 则选择1024×1024或512×512。重组最大信号强度投影(MIP)图像后,原始单层图像和 MIP 图像均用于图像质量的评估。在 ZIP 1024×1024和 ZIP 512×512图像上均测量了信噪比(SNR)。利用宽度中点(FWHM)法确定血管边缘后计算腔内直径。结果 8个解剖学仿真动脉模型在2种 ZIP 技术中均得到了很好的显示。在所有仿真模型图像中,ZIP 1024×1024技术获得的图像血管边缘均较 ZIP512×512技术的更清晰;而在 FWHM 结果中,8个仿真模型腔内直径的计算结果准确性均相同。虽然ZIP 1024×1024技术的平均 SNR(26.7±3.8)比 ZIP 512×512的平均 SNR(31.6±4.1)低(t=4.018,P<0.01),但 ZIP 1024×1024图像的总体质量均比 ZIP 512×512的更好。结论 ZIP 1024×1024技术的图像总体质量优于 ZIP 512×512技术。研究方向可着重于扫描序列的修改及参数的调整,以期同时获得较高的分辨率和 SNR。     

Displays for MR angiography

There are several MR imaging procedures that result in three-dimensional data of moving spins. One way to evaluate these data is a sequential observation of 2D slices. A more common and efficient approach is to retrospectively calculate two-dimensional projection images at different angles representing individual views of the tomographic volume. The display of different views from the same data set will allow a realistic visualization of the blood vessel anatomy.     

Selective contrast-enhanced MR angiography.

In this study the feasibility of intraarterial contrast administration was investigated. Its use for navigation and treatment evaluation during MR-guided intravascular interventions was explored in phantom and animal experiments. An injection protocol was developed, which accounts for sequence parameters and vessel flow rate. Tracking a bolus of contrast agent was useful to verify the catheter tip position and to assess flow conditions. Compared to intravenous contrast-enhanced magnetic resonance angiography (CE-MRA), selective contrast administration permitted a strongly reduced dose. In two-dimensional (2D) acquisitions overlap of vessels was prevented. Injection and acquisition were easily and accurately synchronized in selective 3D CE-MRA, and a high contrast concentration could be maintained during the entire acquisition. Selective injection is useful in the course of an intervention, to facilitate navigation, provide information on flow conditions, and to evaluate treatment progress repeatedly.     

Velocity-coded color MR angiography.

We developed a method of velocity-coded color MR angiography using a color code from the data obtained from velocity-phase images of phase-contrast MR angiography in order to add flow direction information to MR angiograms. Phase-contrast MR angiography with reconstruction of velocity-phase images was performed in 30 patients. Two projection images from velocity-phase images of each phase-contrast MR angiogram were obtained and assigned color according to flow direction. We then superimposed the two color images onto the maximum intensity projection image of the MR angiogram. The velocity-coded color MR angiogram clearly showed flow direction from the data on the phase-contrast MR angiogram of the neck. Veins were readily distinguishable from arteries, and flow changes, such as a subclavian steal, were also identified.     

Use of MR angiography for stereotactic planning.

With the introduction of MR angiography (MRA) into clinical routine MR protocols, it has become possible now to image flowing as well as stationary tissue with excellent contrast using a single modality. This has opened up new perspectives for planning stereotactic approaches, which are characterized by high risks for damaging intracerebral vessels or vital brain structures. In this article we present an MRA based planning method for the treatment of arteriovenous malformations by stereotactic radiosurgery. It includes flow compensated gradient echo pulse sequences for the acquisition of angiographic MR datasets, a stereotactic MR marker system, an algorithm for the correction of geometric distortion of MR image data, and a three-dimensional workstation system for the creation and evaluation of treatment plans. The latter is based on the concept of simultaneously displaying both MR slice and angiographic projection images. This allows the evaluation of intracerebral vasculature together with brain anatomy. The MRA guided planning approach was tested and compared to a conventional X-ray angiographic technique in a clinical study. Our satisfactory results suggest that MRA is a technique that can be used advantageously for stereotactic planning.     

Timing adjustment in gadolinium MR angiography by raw data recombination. Technical note.

To solve the problem of injection timing in gadolinium MR angiography, a simple procedure is proposed which allows the acquisition interval to be chosen after injection. Starting simultaneously with the injection, several consecutive acquisitions are made, after which raw data acquired in a contiguous interval with a variable starting time are recombined to one data set, which is then used for delayed image reconstruction.     

Time-resolved MR angiography of the intracranial venous system: an alternative MR venography technique

Objectives  

To compare time-resolved imaging of contrast kinetics (TRICKS) magnetic resonance angiography (MRA) with two-dimensional time-of-flight (TOF) magnetic resonance venography (MRV), and three-dimensional contrast-enhanced (CE) MRV in the visualisation of normal cerebral veins and dural venous sinuses.     

Novel technique for evaluating complex pelvic arteriovenous malformations with catheter-directed subtracted MR angiography

Residual or recurrent arteriovenous malformation (AVM) nidal filling is not uncommon after embolization of complex peripheral AVMs. The major reason for this is incomplete embolization of the nidus owing to failure to detect all feeder vessels other than the dominant one. Herein, the authors describe a novel "intravenous minus intraarterial" subtraction magnetic resonance (MR) angiography technique that enabled the delineation of additional feeding branches not clearly identifiable at intravenous contrast-enhanced MR angiography or nonselective pelvic angiography. This technique may be particularly useful in the evaluation of complex AVMs and their subsequent management with embolization.     

Lumen definition in MR angiography.

Factors affecting blood vessel lumen definition for two-dimensional and three-dimensional inflow magnetic resonance (MR) imaging methods are considered. Vessel definition is affected (a) by the amount of dephasing of the blood in the vessels, both for uncompensated and velocity-compensated gradients; (b) by the image reconstruction technique (normal Fourier reconstruction when asymmetric echoes are collected or a maximum-intensity projection technique in post-processing); (c) by loss of signal due to T2* dephasing; (d) by misregistration; (e) by vessel wall motion; and (f) by partial-volume effects. The first two factors were found to dominate for resolution on the order of 1 mm3. To overcome these dephasing problems, the authors developed asymmetric echo, velocity-compensated sequences with TEs as short as 4.8 msec. The data were then reconstructed with an iterative partial Fourier algorithm, enabling improved lumen definition to be obtained in phantoms and in vivo.     

MR angiography of Takayasu arteritis.

We describe MR angiographic findings utilizing a three-dimensional time-of-flight technique and compare the results with angiography in a case of suspected Takayasu arteritis involving vessels to the neck and upper extremities.     

A method of coronary MR angiography

Procedures for the in vivo detection of coronary (and pulmonary) vessels using MR angiographic techniques were investigated. The most successful technique used gradient-recalled thin slice acquisitions that were gated to the cardiac cycle. The resulting data sets consist of three spatial dimensions and one time dimension. Acquisition of four dimensions of data proved necessary to obtain useful images of small vessels located on the moving myocardium.