Time-of-Flight Magnetic Resonance Angiography at 7 T Using Venous Saturation Pulses With Reduced Flip Angles
Time-of-Flight Magnetic Resonance Angiography at 7 T Using Venous Saturation Pulses With Reduced Flip Angles
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DOI：
10.1097/rli.0b013e31824ef21f
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发表时间：
2012-08-01
影响因子：
6.7
通讯作者：
Maderwald, Stefan
中科院分区：
文献类型：
--
作者：
Johst, Soeren;Wrede, Karsten H.;Maderwald, Stefan
目的：在飞行时间（TOF）磁共振血管造影（MRA）中血管的可见性非常受益于增加的磁场强度。然而，在7 T时，通常不可能应用额外的饱和脉冲来抑制静脉系统;为了保持在规定的比吸收率（SAR）限值内，需要延长重复时间（TR），从而防止在临床可接受的采集时间内采集高分辨率MRA数据集。在这项工作中，饱和脉冲的翻转角和持续时间进行修改，以满足SAR的限制，并尽量减少总measurementtime.Materials和方法：为了改善SAR的限制，可变速率选择性激励（VERSE）算法用于激励和饱和射频脉冲。以这种方式，饱和脉冲（每TR执行一次）变得适用于高分辨率TOF MRA协议，但仍然显著延长了总测量时间。结果：在所考虑的参数范围内，当激励翻转角α为15 ° ~ 35 °，TR为20 ms ~ 35 ms时，饱和脉冲的翻转角和持续时间可以满足SAR的要求，并使测量时间最短（α（SAT））为30度至50度。这项工作表明，通过降低翻转角α（SAT），饱和脉冲可以应用于使用短至20 ms的TR的高分辨率临床TOF协议中。在TOF MRA协议中，15度足以在7 T正常使用的参数范围内抑制静脉系统。代替标准的90度饱和脉冲，仅需要翻转角的一半（或甚至更小），从而大大改善了SAR约束并使得能够在可接受的成像时间内获取高分辨率。
Objectives: The visibility of the vasculature in time-of-flight (TOF) magnetic resonance angiography (MRA) highly profits from increased magnetic field strengths. However, the application of additional saturation pulses for suppression of the venous system is often not possible at 7 T; to remain within the regulatory specific absorption rate (SAR) limits, the repetition time (TR) needs to be prolonged, preventing the acquisition of high-resolution MRA data sets within clinically acceptable acquisition times. In this work, saturation pulses were modified regarding flip angle and duration to meet SAR constraints and minimize total measurement time.Materials and Methods: To ameliorate SAR restrictions, the variable-rate selective excitation (VERSE) algorithm was used for both excitation and saturation radio frequency pulses. In this way, saturation pulses (executed every TR) become applicable in high-resolution TOF MRA protocols but still lengthen total measurement time notably. In this work, saturation pulses were further modified in terms of flip angle and duration to meet SAR constraints and minimize total measurement time.Results: In the considered parameter range for excitation flip angle alpha of 15 degrees to 35 degrees and TR of 20 ms to 35 ms, sufficient saturation flip angles (alpha(SAT)) were 30 degrees to 50 degrees.Conclusions: This work shows that by lowering the flip angle alpha(SAT), saturation pulses can be applied in high-resolution clinical TOF protocols using a TR as short as 20 ms. An alpha(SAT) of alpha + 15 degrees is sufficient for suppression of the venous system in TOF MRA protocols in the parameter range normally used at 7 T. Instead of the standard 90 degrees saturation pulse, only half the flip angle (or even less) is necessary, substantially ameliorating SAR constraints and enabling acquisition of high resolution in acceptable imaging time.