Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany.
Invest Radiol. 2012 Aug;47(8):445-50. doi: 10.1097/RLI.0b013e31824ef21f.
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.
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.
In the considered parameter range for excitation flip angle α of 15° to 35° and TR of 20 ms to 35 ms, sufficient saturation flip angles (αSAT) were 30° to 50°.
This work shows that by lowering the flip angle αSAT, saturation pulses can be applied in high-resolution clinical TOF protocols using a TR as short as 20 ms. An αSAT of α + 15° 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° 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.
在时间飞跃（TOF）磁共振血管造影（MRA）中，血管的可视性非常受益于增加的磁场强度。然而，在 7T 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）限制内，重复时间（TR）需要延长，从而防止在可接受的临床采集时间内获得高分辨率 MRA 数据集。在这项工作中，通过改变翻转角和持续时间来修改饱和脉冲，以满足 SAR 约束并最小化总测量时间。
为了缓解 SAR 限制，可变率选择性激发（VERSE）算法被用于激发和饱和射频脉冲。通过这种方式，饱和脉冲（在每个 TR 执行）在高分辨率 TOF MRA 协议中变得可行，但仍然显著延长了总测量时间。在这项工作中，进一步修改了饱和脉冲的翻转角和持续时间，以满足 SAR 约束并最小化总测量时间。
在考虑的激发翻转角α的参数范围内为 15°至 35°和 TR 为 20ms 至 35ms，足够的饱和翻转角（αSAT）为 30°至 50°。
这项工作表明，通过降低翻转角αSAT，可以在使用最短 TR 为 20ms 的高分辨率临床 TOF 协议中应用饱和脉冲。在通常在 7T 下使用的参数范围内，αSAT 为α+15°即可满足 TOF MRA 协议中静脉系统的抑制要求。代替标准的 90°饱和脉冲，仅需要一半的翻转角（甚至更少），这大大改善了 SAR 限制，并能够在可接受的成像时间内获得高分辨率。
