Combined time-resolved and high-spatial-resolution 3D MRA using an extended adaptive acquisition

PURPOSE: To combine the benefits of time-resolved dynamic imaging and single elliptical centric acquisitions in a reasonable scan time. MATERIALS AND METHODS: A time series of images with moderate spatial resolution was acquired using the 3D Time-Resolved Imaging of Contrast KineticS (3D TRICKS) technique with elliptical centric encoding during contrast arrival. Following venous opacification, a complete large centrically encoded k-space volume was acquired. The high-spatial-frequency portions of this volume were combined with a 3D TRICKS time frame to form a high-resolution image. An additional single image is formed by suppressing background and signal averaging all acquired data, including post-venous low-spatial-frequency data. For this image, 2D temporal correlation analysis is used to suppress low-spatial-frequency vein contributions. Arrival time and spatial correlations are used to suppress background. RESULTS: The 3D TRICKS time frame may be selected to ensure a combined high-resolution image that has optimal central k-space sampling for any vascular region. The single image formed by signal averaging all acquired data has increased contrast-to-noise (CNR) and signal-to-noise (SNR) ratios. CONCLUSION: The advantages of time-resolved and high-spatial-resolution imaging were combined using an extended dual-phase acquisition. Some SNR and CNR gain was achieved by signal averaging. This process is facilitated by background and vein suppression.     

APPLICATION OF MAGNETIC RESONANCE ANGIOGRAPHY IN PORTAL HYPERTENSIVE SURGERY

Objective To compare magnetic resonance angiography （MRA） with traditional angiography and doppler ultrasonography （DUS） in the assessment of portal venous anatomy and its hemodynamics. Methods Three dimensional dynamic contrast-enhanced （ 3 D-DCE） MRA and two dimensional phase-contrast （2D-PC） MR were used for the study of portal venous system in the patients with portal hypertension and those without liver cirrhosis. The comparison of the portal blood flow （PBF） measured with 2D-PC MR and DUS was made. Results The portal vein, splenic vein, superior mesenteric vein, cephalic collateral veins and anastomotic stoma of surgical shunting were clearly displayed in 3D-DCE MRA. There was no significant difference between PBF measured with 2D-PC MR and DUS. Conclusion The results of present study indicate that the anatomical imaging of the portal venous system can be dearly revealed in MRA and the PBF can be accurately measured with 2D-PC MR. It is believed that MRA is a useful tool in the management of the patients with portal hypertension.     

Accelerated coronary MRA by simultaneous acquisition of multiple 3D stacks

The implementation and first in vivo results of a novel coronary magnetic resonance angiography (MRA) protocol allowing simultaneous acquisition of multiple geometrically independent 3D imaging stacks are presented. Each imaging stack is acquired in a separate cardiac phase using an individual magnetization preparation and navigator-based gating and prospective motion correction. Each stack covers one of the main coronary vessels. Thus, an improvement of scan efficiency was achieved, which was used in this study to reduce total scan time at standard image quality. Experiments performed in healthy volunteers and in patients using a two-stack approach yielded a total scan time reduction of 50% with an image quality equivalent to standard single-stack coronary MRA.     

4D CE MRA技术在血管外科中的应用

目的:探讨4DCE-MRA技术在血管外科中的应用价值。方法:对21例颈部动脉、锁骨下动脉及主动脉病变患者,使用syngo TWIST技术提高3D CE-MRA的时间分辨率,实现4D CE-MRA,观察血管和血流动力学状况,将诊断结果和手术结果对照。结果:21例中均可观察到2～4期的纯动脉影像,避免了静脉污染,有利于判断血流动力学改变,诊断结果和手术结果完全相符。结论:4D CE-MRA的应用对进一步诊断疾病和指导外科手术有着重要的意义,在血管外科有着更广泛的应用前景。     

Three-Dimensional Contrast-Enhanced Magnetic Resonance Angiography of Carotid Artery after Stenting

PURPOSE: This study presents 3-dimensional, contrast-enhanced, magnetic resonance (MR) angiographic appearances of the carotid artery after stenting. METHODS: The authors reviewed contrast-enhanced MR angiograms of 18 carotid arteries in 13 patients. These arteries were not stenotic or occluded in the stented segment, as shown on carotid angio-grams (11 patients) or carotid Doppler images (2 patients) obtained later. RESULTS: Five patients had 1 stent in each of their bilateral carotid arteries. One patient with 1 stent in 1 artery underwent the examination twice with different MR coils. Different artifact patterns were found in 19 carotid-stent examinations. Type 1a involved short segments of false narrowing or false focal occlusion at both ends of the stent (n = 12); the carotid artery in the stented segment between the ends was normal. Types 2a and 2b occurred along the stented segment of the artery between the 2 ends of the stent. Type 2a was milder, with segmental faint signal intensity or luminal stenosis in the entire length of the stented segment (n = 5). Type 2b was total loss of luminal signal intensity over the whole length of the stented segment (n = 2). Last, type 1b was a band of hyperintensity at the ends of type 2a or 2b artifacts. Types 1a and 1b were due to susceptibility effect, and types 2a and 2b were from radio-frequency shielding effect. Both of these effects were identified in MR angiogram of nitinol stents and stainless steel stents. CONCLUSION: Awareness of these artifacts is essential to prevent misdiagnosis.     

MRA studies of arterial stenosis: Improvements by diastolic acquisition

Cardiac-phase-specific data acquisition is used to reduce signal loss in MR Angiography resulting from disturbed flow. RF pulses are delivered continuously throughout the cardiac cycle, but incrementation of phase-encoding gradients and data storage are enabled only during the chosen part of the cycle. Studies in a stenotic pulsatile flow phantom demonstrate that poststenotic signal loss is primarily determined by the mean flow velocity, and is not appreciably affected by acceleration or deceleration of the mean flow rate. The signal loss is least in diastole. In vivo studies in patients with carotid artery disease show that data acquisition in diastole reduces the apparent degree and extent of carotid bifurcation stenosis and provides a crisper definition of the vascular lumen. The additional time required for cardiac-phase-specific acquisition can be reduced by gating only the lower-order phase-encoding lines while retaining acceptable image quality.     

Combined use of the intravascular blood-pool agent, gadomer, and carbon dioxide: a novel type of double-contrast magnetic resonance angiography (MRA)

PURPOSE: To evaluate the combined use of carbon dioxide (CO(2)) and a gadolinium-based blood-pool agent for magnetic resonance angiography (MRA). MATERIALS AND METHODS: After an initial intravenous injection of the blood-pool agent Gadomer (Schering AG, Germany), repeated transcatheter CO(2) injections were performed in the aorta and the renal arteries of two fully-anesthetized pigs. Real-time images were acquired using a true fast imaging with steady-state precession (FISP) sequence. RESULTS: During the CO(2) injections, the Gadomer-enhanced blood was totally replaced, resulting in an immediate, temporary, total signal loss in the vessel lumen. Susceptibility artifacts during the injections or catheter manipulations rarely occurred. CONCLUSION: Due to T1-shortening, the circulating blood-pool agent prevents flow artifacts during catheter manipulations because the steady-state is reached much earlier. Therefore, this double-contrast MRA method improves catheter conspicuity and might be helpful for guiding and controlling intravascular procedures during interventional MRI.     

Non-gated fetal MRI of umbilical blood flow in an acardiac twin

Currently, the standard method of diagnosis of twin reversed arterial perfusion (TRAP) sequence is ultrasound imaging. The use of MRI for flow visualization may be a useful adjunct to US imaging for assessing the presence of retrograde blood flow in the acardiac fetus and/or umbilical artery. The technical challenge in fetal MRI flow imaging, however, is that fetal electrocardiogram (ECG) monitoring required for flow imaging is currently unavailable in the MRI scanner. A non-gated MRI flow imaging technique that requires no ECG monitoring was developed using the t-test to detect blood flow in 20 slices of phase-contrast MRI images randomly scanned at the same location over multiple cardiac cycles. A feasibility study was performed in a 24-week acardiac twin that showed no umbilical flow sonographically. Non-gated MRI flow images clearly indicated the presence of blood flow in the umbilical artery to the acardiac twin; however, there was no blood flow beyond the abdomen. This study leads us to conjecture that non-gated MRI flow imaging is sensitive in detecting low-range blood flow velocity and can be an adjunct to Doppler US imaging.     

Magnetic resonance imaging in neonatal encephalopathy

Magnetic resonance imaging may provide invaluable information in the term born neonate with encephalopathy. However, both hardware and sequences may need adaptation from normal adult protocols. Sedation is often required to obtain good quality imaging, but anaesthesia is not necessary in this population. The perinatal history may predict the pattern of brain lesions, which, in turn, may be used to predict the neurodevelopmental outcome. Image interpretation is not easy and requires a full clinical history in addition to experience of both normal and abnormal neonatal brain appearances. Lesions evolve rapidly, and perinatally acquired leasions are at the most obvious 1-2 weeks from delivery. Early imaging in the first few days from presentation should always include diffusion-weighted sequences to identify early ischaemic change. Advanced techniques such as venography, angiography and perfusion-weighted imaging may be useful in certain situations, and serial imaging may help differentiate perinatal-acquired lesions from other pathologies.     

Stroke after varicella vaccination

Two children presented with acute hemiparesis 5 days and 3 weeks following varicella vaccination. Both showed unilateral infarction of the basal ganglia and internal capsule, a distribution consistent with varicella angiopathy. Both children had small patent foramen ovale (PFO), and one child also had severe iron-deficiency anemia, which may have predisposed the patient to this adverse effect.