Reduction of artifacts in susceptibility-weighted MR venography of the brain

PURPOSE: To reduce the off-resonance artifact in susceptibility-weighted imaging (SWI)-based MR venography (MRV) in the brain regions with severe field inhomogeneity and to reduce the signal loss in the minimum-intensity projection (mIP) display of the 3D MRV. MATERIALS AND METHODS: A novel postprocessing approach was presented to map the local field gradients (LFGs) using the 3D SWI data without phase-unwrapping. LFG measurements were used to assess the severity of field inhomogeneity and suppress the residual phase in the phase mask induced by the off-resonance effect. Volume segmentation of brain tissue was used to reduce the signal loss in the peripheral regions of the brain in the through-plane mIP images and enable in-plane mIP display of MRV. RESULTS: Off-resonance artifact in the brain regions with severe field inhomogeneity was effectively reduced by the LFG-based phase suppression approach. Signal loss was reduced in the through-plane mIP of MRV using volume segmentation of brain tissue prior to projection. In-plane mIP of MRV also became feasible with volume segmentation. CONCLUSION: Off-resonance artifacts and signal loss in mIP display of MRV can be effectively reduced through postprocessing.     

ESWAN序列同次采集颅脑MRA及MRV

目的：探讨多回波采集T2＊WI的三维梯度回波序列（ESWAN）动脉成像（MRA）及静脉成像（MRV）的可行性。方法：11例健康志愿者均行回波数为11的ESWAN及3D-TOFMRA检查,其中7例还加做2D-TOFMRV扫描。仅保留ESWAN序列中动脉流入增强效应较强的前3个回波后处理获得的幅度图像,经最大强度投影重组Willis环图像,分析Willis环各段显示效果并与3D-TOFMRA比较;保留后7个回波的图像后处理得到的幅度图像,经最小强度投影显示颅内静脉,比较ESWAN与2D-TOFMRV对脑深部静脉的显示效果。结果：ESWAN与3D-TOFMRA对Willis环显示结果高度一致（P=1.00）,ESWAN对大脑深部静脉的显示明显优于2D-TOFMRV（P〈0.0001）。结论：合理设置ESWAN参数,可以实现同次扫描采集MRA及MRV。     

Dynamic MR venography: an intrinsic benefit of time-resolved MR angiography

PURPOSE: To investigate the possibility of obtaining dynamic contrast-enhanced magnetic resonance venography (DCE-MRV) images of the lower extremities. MATERIALS AND METHODS: Peripheral contrast-enhanced magnetic resonance angiography (CE-MRA) was performed on 20 patients using a time-resolved sequence that combined undersampled projection reconstruction (PR) in-plane and Cartesian slice encoding through-plane. The contrast dynamics of distal vessels were depicted. An automated segmentation algorithm based on a contrast arrival time (CAT) threshold was used to generate contrast dynamics in the venous system. The signal difference between the vein and artery was measured to evaluate the effectiveness of this technique in isolating the venous contrast dynamics. RESULTS: The automatically generated image series depicted the contrast dynamics of both the arterial and venous systems, including asymmetric venous enhancement and background tissue enhancement. Quantitative measurement showed a mean venous/arterial signal ratio increase from 1.58 to 4.82 for the peak venous frame after arterial signal suppression. CONCLUSION: DCE-MRV is a minimally invasive technique for evaluating the venous side of the systemic vascular anatomy. Time-resolved MRA has the potential clinical benefit of enabling both arterial and venous disease to be detected in patients undergoing CE-MRA.     

MR venography of the brain with enhanced vessel contrast using image-domain high-pass filtering of the susceptibility phase shift

Purpose:

To develop a postprocessing algorithm that enhances the visibility of intracranial venous vasculature and reduces the artifacts in the display of susceptibility‐weighted images (SWI).

Materials and Methods:

Image‐domain high‐pass filters based on second‐order phase difference were applied to the complex 3D SWI data to enhance the susceptibility phase shift of the veins and suppress background signal in SWI. A multivariant statistical parameter was used to suppress the noise in air.

Results:

Magnetic resonance (MR) venography with enhanced susceptibility phase shift and reduced off‐resonance artifacts was obtained using the proposed filters. The background signal in the 3D MR venography data was well suppressed. Venous vasculature in the peripheral regions of the brain was well depicted and the adverse effect of noise in air in the maximum‐intensity projection display of the 3D SWI data was well suppressed.

Conclusion:

Image‐domain high‐pass filtering with second‐order phase difference provides an alternative display of 3D SWI data with enhanced visibility of the venous vasculature and effective suppression of artifacts. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Time-resolved 3D MR angiography of the abdomen with a real-time system.

This study reports on the implementation of a real-time MR acquisition, reconstruction, and display system on standard hardware for 3D contrast-enhanced MR abdominal angiography. The system allows for dynamic imaging of the contrast passage with good spatial resolution in a single breathhold. It is capable of synchronously acquiring and processing multiple coil data. Bolus arrival can be detected confidently from the display of MIP images from sagittal 3D slabs with moderate spatial resolution. Upon detection of the contrast agent, the patient is asked to hold their breath and a novel 3D acquisition is started for a coronal volume. Temporal imaging within the breathhold is achieved through applying a modified time-resolved imaging of contrast kinetics (TRICKS) technique with elliptical centric view ordering. The technique displays contrast passage from the arterial phase through enhancement of the hepatic venous system. It also provides the ability to quantify motion of the diaphragm over a long breathhold.     

MR angiography using spin-lock flow tagging.

A method for MR angiography using an RF labeling technique is suggested. The method utilizes a slice-selective spin-lock pulse sequence for tagging the spins of inflowing blood. The pulse sequence begins with a spatially selective 90 degrees (x) RF pulse, followed by a nonselective composite locking pulse of 135 degrees (y) - n[360 degrees (y)]-135 degrees (y) and by a 90 degrees (-x) pulse. A spoiler gradient is then applied. A rapid imaging stage, which yields a T(1)rho-weighted signal from the tagged spins, completes the sequence. Untagged spins are thoroughly dephased and consequently suppressed in the image. Thus, contrast is obtained without an injection of a contrast material or image subtraction. Furthermore, the flow of the tagged bolus can be visualized. The sequence was implemented on phantoms and on human volunteers using a 1.5T scanner. The results indicate the feasibility of the suggested sequence.     

Scoutless stepping-table peripheral contrast-enhanced MR angiography

PURPOSE: To evaluate the feasibility of a scoutless method, termed EZ-STEP, for stepping-table peripheral contrast-enhanced magnetic resonance angiography (CE-MRA). MATERIALS AND METHODS: This scoutless method involves the use of a stepping-table, fast 3D MRA acquisition that incorporates spatially nonselective radiofrequency (RF) pulses for excitation to reduce the repetition time (TR). The sequence was tested in a phantom. The EZ-STEP protocol was optimized in four healthy volunteers and used in 15 subjects. The image quality was scored in a blinded fashion and compared with conventional MRA in eight patients. RESULTS: The acquisition speed of the EZ-STEP sequence was approximately 30% faster in the phantom study compared to the conventional MRA sequence. The total examination time for EZ-STEP was 6 minutes, compared to an average of 23 minutes for conventional MRA. The average image quality scores for EZ-STEP and conventional MRA for stations 1-3 were 3.50 vs. 3.06 (P = 0.087), 3.53 vs. 3.00 (P = 0.033), and 2.97 vs. 2.50 (P = 0.090), respectively. CONCLUSION: EZ-STEP is a more efficient method than the conventional approach for stepping-table peripheral CE-MRA, and provides comparable or better image quality. This method shortens the examination time substantially and eliminates the risk of failing to image a vessel because of improper positioning of the scan volume.     

Visualization of the lenticulostriate artery with flow‐sensitive black‐blood acquisition in comparison with time‐of‐flight MR angiography

Purpose

To evaluate the capability of flow‐sensitive black blood (FSBB) acquisition to visualize the lenticulostriate artery (LSA) in comparison with time‐of‐flight (TOF) angiography.

Materials and Methods

Twenty‐one healthy subjects (13 males and 8 females, 19–44 years old) were enrolled in this study after obtaining written informed consent. Magnetic resonance imaging (MRI) examinations were performed with FSBB and TOF to visualize the LSA using a 1.5T MRI unit. In FSBB acquisition a motion probing gradient of b = 4 sec/mm² was applied to dephase blood flow. Images were reconstructed into coronal sections and were evaluated in terms of number, length, and image quality at origins and distal areas of visualized LSA branches with a four‐point scale.

Results

In all, 145 LSA branches were visualized with FSBB and 66 branches with TOF. There was no LSA visualized only with TOF. In all evaluated terms, FSBB was significantly better than TOF.

Conclusion

We could better visualize the LSA with FSBB than with TOF, both quantitatively and qualitatively. FSBB is a promising method, although it remains to be evaluated in clinical cases. J. Magn. Reson. Imaging 2009;29:65–69. © 2008 Wiley‐Liss, Inc.     

Non‐contrast enhanced MR angiography: Established techniques

Until recently, time‐of‐flight (TOF) and phase contrast (PC) were the only non‐contrast MR angiography (NC‐MRA) techniques practically used in clinical. In the decade, NC‐MRA have been gained a revival of an interest among the MR researchers and scientists, in part because of safety concerns related to the possible link between gadolinium‐based contrast agents and nephrogenic systemic fibrosis (NSF). This article introduces other established NC‐MRA techniques, such as ECG‐gated partial Fourier fast spin echo (FSE) and balanced steady‐state free precession (bSSFP), both with and without arterial spin labeling. Then, the article focuses on two main applications: peripheral run‐off and renal MRA. Recently, both applications have achieved remarkable advancements and have become a viable clinical option as an alternative to contrast‐enhanced (CE)‐MRA. In addition, developments on the horizon including whole body MRA applications and further advancement at 3 Tesla are discussed. J. Magn. Reson. Imaging 2012 © 2011 Wiley Periodicals, Inc.     

Coarctation of the aorta: comparison of aortic dimensions between conventional MR imaging, 3D MR angiography,and conventional angiography

Magnetic resonance angiography is increasingly used as a non-invasive method in the evaluation of coarctation of the aorta. The aim of this study was to compare aortic dimensions calculated by MR angiography and those obtained by more conventional MR sequences and conventional angiography. Twenty-six consecutive patients with coarctation underwent three-dimensional MR angiography. Two independent observers retrospectively evaluated three aortic segments, site of coarctation, presence of aneurysm and existence of collateral circulation. Three aortic segments were also compared with those obtained on classical MR sequences and conventional angiography. The MR angiography was successfully performed in all showing 1 aneurysm and collateral circulation in 8 patients. Almost perfect intraobserver ( r(2)>0.91) and excellent interobserver ( r(2)>0.80) reliabilities were obtained for each aortic segment no matter which MR sequence was employed. Similarly, mainly excellent ( r(2)>0.80) concordance analysis was observed between MR angiography measurements and those calculated by either spin-echo/gradient-echo sequences or conventional angiography. This study demonstrates that MR angiography is a fast, accurate and reproducible method in the evaluation of coarctation of the aorta. It provides excellent anatomic information and reliably detects collateral vessels. Magnetic resonance angiography could probably replace the conventional angiography and will provide an additional diagnostic value in combination with turbo spin-echo sequence.     

Three-dimensional low dose gadolinium-enhanced peripheral MR venography

Our initial experience with low dose contrast-enhanced (LCE) peripheral MR venography (MRV) is presented. Five subjects were studied using three-dimensional (3D) fast imaging with a steady-state precession (FISP) sequence. A dose of 60 ml of gadopentetate dimeglumine diluted 1:20 was used. A tourniquet was applied during lower extremity MR venography. The venous anatomy was well depicted with the 3D LCE technique in all subjects. Compared to the two-dimensional (2D) time-of-flight (TOF) technique, acquisition time of 3D LCE MRV was much shorter, images looked sharper, and more veins could be seen. It is not affected by in-plane saturation and can be performed repeatedly because of the low dose of contrast. This technique holds promise for the detection of venous thrombosis and other disorders.     

Time-resolved contrast-enhanced 3D MR angiography

An MR angiographic technique, referred to as 3D TRICKS (3D time-resolved imaging of contrast kinetics) has been developed. This technique combines and extends to 3D imaging several previously published elements. These elements include an increased sampling rate for lower spatial frequencies, temporal interpolation of k-space views, and zero-filling in the slice-encoding dimension. When appropriately combined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2-6 s. Acquiring a temporal series of images offers advantages over the current contrast-enhanced 3D MRA techniques in that it i) increases the likelihood that an arterial-only 3D image set will be obtained, ii) permits the passage of the contrast agent to be observed, and iii) allows temporal-processing techniques to be applied to yield additional information, or improve image quality.     

Fast whole‐body magnetic resonance angiography in mice

High‐throughput magnetic resonance imaging (MRI) tools are required for the longitudinal investigation of vascular diseases in mouse models. Angiographic data from various anatomic regions may be needed in a single experiment. This study involves a three‐dimensional (3D) time‐of‐flight (TOF) magnetic resonance angiography (MRA) method using sequential acquisitions of four data sets corresponding to the head, the thorax, the abdomen, and the hind limbs of a mouse. After repositioning the animal, each anatomic region was acquired in 2 min, and the TOF effect was provided by the spatial selectivity of the radio frequency (RF) resonator. No slab selection was needed and whole‐body MRA was performed in a total experiment time of 10 min. The voxel size was equal to or greater than 131 × 195 × 188 μm³. To suppress the signal arising from stationary tissues, both inversion recovery and interspersed saturation, used as magnetization preparations, were compared from a theoretical and an experimental perspective. The arterial tree (carotid, aortic, iliac, renal, and smaller arteries) was well visualized by this method, both in control healthy mice and in mice with common carotid artery ligation. The potential interest of this method for evaluating arterial diseases is discussed. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Dual‐echo arteriovenography imaging with 7T MRI

Purpose:

To implement a dual‐echo sequence MRI technique at 7T for simultaneous acquisition of time‐of‐flight (TOF) MR angiogram (MRA) and blood oxygenation level‐dependent (BOLD) MR venogram (MRV) in a single MR acquisition and to compare the image qualities with those acquired at 3T.

Materials and Methods:

We implemented a dual‐echo sequence with an echo‐specific k‐space reordering scheme to uncouple the scan parameter requirements for MRA and MRV at 7T. The MRA and MRV vascular contrast was enhanced by maximally separating the k‐space center regions acquired for the MRA and MRV and by adjusting and applying scan parameters compatible between the MRA and MRV. The same imaging sequence was implemented at 3T. Four normal subjects were imaged at both 3T and 7T. MRA and MRV at 7T were reconstructed both with and without phase‐mask filtering and were compared quantitatively and qualitatively with those at 3T with phase‐mask filtering.

Results:

The depiction of small cortical arteries and veins on MRA and MRV at 7T was substantially better than that at 3T, due to about twice higher contrast‐to‐noise ratio (CNR) for both arteries (164 ±57 vs. 77 ± 26) and veins (72 ± 8 vs. 36 ± 6). Even without use of the phase‐masking filtering, the venous contrast at 7T (65 ± 7) was higher than that with the filtering at 3T (36 ± 6).

Conclusion:

The dual‐echo arteriovenography technique we implemented at 7T allows the improved visualization of small vessels in both the MRA and MRV because of the greatly increased signal‐to‐noise ratio (SNR) and susceptibility contrast, compared to 3T. J. Magn. Reson. Imaging 2010;31:255–261. © 2009 Wiley‐Liss, Inc.     

New approach to 3D time-resolved angiography.

TRICKS is an acquisition and reconstruction method capable of generating 3D time-resolved angiograms. Arguably, the main problem with TRICKS is the way it handles the outer regions of the k-space matrix, leading to artifacts at the edges of blood vessels. An alternative to the data- processing stage of TRICKS, designed to better represent edges and small vessels, is presented here. A weakness of the new approach is an increased sensitivity to motion compared to TRICKS. Since this method can use the same data as TRICKS, a hybrid reconstruction method could conceivably be developed where the advantages of both approaches are combined. Magn Reson Med 47:1022-1025, 2002.     

A modified Fourier-based phase unwrapping algorithm with an application to MRI venography

PURPOSE: To present a single-step deterministic procedure for unwrapping MRI phase maps. MATERIALS AND METHODS: Using an algorithm previously developed for optical applications, Laplacian operators were applied in the Fourier space of the MRI phase map. The original Fourier-based phase unwrapping algorithm was modified so that demodulation accomplished the required signal symmetrization in Fourier space. To evaluate the method's performance in the presence of thermal noise, a set of wrapped phase maps were simulated at different levels of noise in k-space, and the response of the algorithm at different levels of signal-to-noise ratio (SNR) was evaluated for stability. To demonstrate its utility in MRI, the algorithm was applied to the wrapped phase maps of susceptibility-weighted imaging (SWI) studies, which were then used to generate venograms. RESULTS: In simulated phase wrapping, the algorithm correctly reproduced the original phase for a wide range of phase gradients and noise. The procedure was fast and produced useful maps of venous structures in SWI images. CONCLUSION: A fast and stable single-step deterministic method for unwrapping MRI phase maps is available for such applications as SWI and mapping of static magnetic field inhomogeneity.     

Hybrid of opposite‐contrast MR angiography (HOP‐MRA) combining time‐of‐flight and flow‐sensitive black‐blood contrasts

For the purpose of visualizing low‐flow as well as high‐flow blood vessels without using contrast agents, we propose a new technique called a hybrid of opposite‐contrast MR angiography (HOP‐MRA). HOP‐MRA is a combination of standard time‐of‐flight (TOF) using a full first‐order velocity‐compensation for white‐blood (WB) and flow‐sensitive black‐blood (FSBB) techniques, which use motion‐probing gradients to introduce intravoxel flow dephasing. A dual‐echo three‐dimensional gradient echo sequence was used to reduce both imaging time and misregistration. HOP‐MRA images were obtained using a simple‐weighted subtraction (SWS) or a frequency‐weighted subtraction (FWS) applying different spatial filtering for WB and BB images. We then assessed the relationships among the contrast‐to‐noise ratios (CNR) of the blood‐to‐background signals for those three images. In both volunteer and clinical brain studies, low‐flow vessels were well visualized and the background signal was well suppressed by HOP‐MRA compared with standard TOF‐ or BB‐MRA. The FWS was better than the SWS when whole‐maximum intensity projection was performed on a larger volume including with different types of tissue. The proposed HOP‐MRA was proven to visualize low‐flow to high‐flow vessels and, therefore, demonstrates excellent potential to become a clinically useful technique, especially for visualizing collateral vessels which is difficult with standard TOF‐MRA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

MR imaging and MR angiography in popliteal artery entrapment syndrome

Popliteal artery entrapment (PAE) syndrome is an uncommon congenital anomaly seen in young adults causing ischemic symptoms in the lower extremities. It is the result of various types of anomalous relationships between the popliteal artery and the neighboring muscular structures. The purpose of this study was to define the role of MR imaging combined with MR angiography in the diagnosis of PAE cases. Four cases with segmental occlusion and medial displacement of popliteal artery in digital subtraction angiography (DSA) examinations were diagnosed as PAE syndrome by MR imaging and MR angiography. The DSA and MRA images are compared. All of the cases showed various degrees of abnormal intercondylar insertion of the medial head of the gastrocnemius muscle. The MR images showed detailed anatomy of the region revealing the cause of the arterial entrapment. Subclassification of the cases were done and fat tissue filling the normal localization of the muscle was evaluated. The DSA and MRA images demonstrated the length and localization of the occluded segment and collateral vascular developments equally. It is concluded that angiographic evaluation alone in PAE syndrome might result in overlooking the underlying cause of the arterial occlusion, which in turn leads to unsuccessful therapy procedures such as balloon angioplasty. Magnetic resonance imaging combined with MR angiography demonstrates both the vascular anatomy and the variations in the muscular structures in the popliteal fossa successfully, and this combination seems to be the most effective way of evaluating young adults with ischemic symptoms suggesting PAE syndrome. Received 7 April 1997; Revision received 15 July 1997; Accepted 13 November 1997