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造影剂增强磁共振血管造影在腹部、主动脉及周围血管疾病诊断中的新进展

造影剂增强磁共振血管造影在腹部、主动脉及周围血管疾病诊断中的新进展

 

Advances in abdominal, aortic and peripheral contrast-enhanced MR angiography

 

基本原理(Basic Principles

 

应用钆 (gadolinium, Gd) 造影剂的增强三维血管造影术(Three-dimensional Contrast-enhanced Magnetic Resonance Angiography3D CE MRA)现已经成为无创性显示血管病变的有效方法,它利用了顺磁性造影剂能缩短纵向弛豫时间(T1)的效应,使血管与周围组织提供了强烈的对比,很好的显示了血管。

实际上,不论采用何种采集平面,CE MRA 都可以使用较大的激发角度最大限度地抑制背景信号,同时也保留流入层面及流经层面血流的信号强度。钆造影剂可增强血管内信号,使采用3D脉冲序列采集成为现实,这样可以提供比2D脉冲序列高得多的空间分辨率。

采用短TRRepetition Time)技术还可通过几个途径提高CE MRA的图像质量。短TR除允许快速数据采集外,还可以减少由于患者移动带来的影响并可消除呼吸伪影;短扫描时间,能获得更加紧密的造影剂液团,由此就可以增加强化噪声比(CNR),减少造影剂的用量。扫描时间的缩短也可以通过减少相位编码的数量而获得。

因此,MRA钆造影剂不仅可以增加血管的信号强度和信号对比度,而且还可以提高MRA的成像速度。由于消除了层内饱和,CE MRA可以在与感兴趣血管平行的层面采集数据,这样就能够用较少的层面包含了较大的血管扫描范围并减少扫描时间。由于采用了3D采集,所有数据都可以在造影剂首次通过期间获得,这样就减少了再循环的稀释效应,并最大强度减少静脉强化。应用短TR能在一定的成像时间内增加血管造影的成像范围并提高其分辨率。

Contrast-enhanced, three-dimensional (3D) MR angiography (CE MRA) using gadolinium has become a powerful tool for noninvasive arterial imaging. It exploits the T1-shortening effects introduced by paramagnetic contrast material in vessels, thereby providing high contrast between vessels and surrounding tissues. Because T1-shortening contrast agents eliminate in-plane saturation, acquisition planes can be chosen for optimal efficiency.

   With CE MRA, larger flip angles can be used to maximize background suppression while maintaining signal intensity from both in-plane and through-plane flow, regardless of the acquisition plane. The strong intravascular signal enhancement of gadolinium also permits the use of 3D pulse sequences that provide higher spacial revolution than can be achieved with two-dimensional (2D) techniques.

   A short TR (Repetition Time) can be used to improve image quality in CE MRA in several ways. It allows rapid image acquisition, reducing the effects of patient motion and eliminating respiratory artifacts. With short scan times, a tighter bolus of contrast can be delivered, so that the contrast-to-noise ratio (CNR) is increased and the total dose of required contrast is reduced. Scan times can also be reduced by reducing the number of phase-encoding steps.

   Gadolinium contrast agents, therefore, improve both vessel signal and contrast and the speed of MR angiography. By eliminating in-plane saturation, CE MRA can be acquired in the plane of the vessel of interest, with fewer slices covering large vascular territories and reduced scan time. Using rapid 3D acquisitions, all data can be acquired during the first pass of the contrast agent, thereby reducing the effects of dilution owing to recirculation and minimizing venous enhancement. Breath-hold techniques eliminate respiratory artifacts and reduce the effects of motion. With short TR sequences, the angiographic coverage or resolution can be increased for a given imaging time.

 




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