Multiplexed Multiband MR at 7T: Studies of mild Traumatic Brain Injury

描述（由申请人提供）：尽管自20世纪90年代末以来7T系统已经可用，但由于传统头部线圈的传输性能（即仅发射的大体积线圈与仅接收的相控阵），7T脑成像的进展已经放缓。在7T时，大型单驱动传输头体积线圈的均匀性较差（40-50%），效率低，信噪比有限。在这些线圈中使用仅接收相控阵可以显著提高信噪比并实现并行接收，但不能提高发射性能。这些限制可以通过使用提供独立传输和接收的收发器阵列来解决。使用并行传输和/或射频振荡的收发器阵列提供了更好的均匀性、空间定制的激励、梯度独立的外部体积抑制和降低的SAR。迄今为止，这些收发器阵列中的线圈数量被限制在独立发射信道的数量（通常为8个），并且这些线圈的小尺寸需要保持最佳的信噪比。为了解决这些限制，我们将：i)通过开发利用射频多路复用驱动多行收发器阵列并减少功率沉积的新脉冲序列方法，消除对多行收发器阵列相等数量的发射和接收通道的需求；ii)通过开发多波段采集MRSI采集，提高多平面MRSI数据采集效率，降低SAR。为了评估所开发的方法，我们将研究由爆炸暴露引起的轻度创伤性脑损伤（mTBI）退伍军人。已经很清楚的是，暴露于爆炸损伤的mTBI的退伍军人表现出延迟的神经功能缺损，通常没有清晰的影像学相关。在缺乏客观确证的影像学证据的情况下，认知评估的不良表现可归因于受试者努力不足，使诊断、管理、康复复杂化，并对所报告的残疾的有效性提出质疑。我们最近的研究表明，在患有爆炸相关mTBI的退伍军人中，海马体中出现了显著的代谢改变，这与努力和认知表现的评估有关。利用所开发的方法所提供的增强的空间覆盖范围，我们将评估暴露于爆炸相关mTBI的退伍军人的认知和神经缺陷的存在和严重程度与功能相关脑区域的代谢异常/损伤相关的假设。

DESCRIPTION (provided by applicant): Although 7T systems have been available since the late 1990s, progress for brain imaging at 7T has been slowed by the transmit performance of conventional head coils (i.e. a large transmit only volume coil with a receive only phased array). At 7T large single drive transmit head volume coils suffer from poor homogeneity (40-50%) and low efficiency and limited SNR. The use of receive only phased arrays within these coils significantly enhances SNR and enables parallel reception but does not improve the transmit performance. These limitations can be addressed by the use of transceiver arrays which provide both independent transmission and reception. Transceiver arrays using parallel transmission and/or RF shimming offer improved homogeneity, spatially tailored excitation, gradient independent outer volume suppression and reduced SAR. To date, the number of coils in these transceiver arrays has been limited to the number of independent transmit channels (typically 8) and the small size of these coils required to maintain optimal SNR. To address these limitations we will: i) eliminate the need for equal numbers of transmit and receive channels for the multiple row transceiver arrays by developing new pulse sequence methods which utilize RF multiplexing to drive multi-row transceiver arrays and reduce power deposition; ii) enhance the efficiency of multi-plane MRSI data collection and reduce SAR by developing multi-band acquisition MRSI acquisitions. To evaluate the methods developed we will study veterans with mild traumatic brain injury (mTBI) arising from blast exposure. It has become clear that veterans exposed to mTBI from blast injury display delayed neurological deficits, often without clear imaging correlates. In the absence of objective confirmatory imaging evidence, poor performance on cognitive evaluations can be attributed to poor subject effort, complicating diagnosis, management, rehabilitation and raising questions as to validity of the reported disability. Our recent work has demonstrated that in veterans with blast related mTBI, significant metabolic alterations are seen in the hippocampi which correlate with assessments of effort and cognitive performance. Using the enhanced spatial coverage afforded by the methods developed we will evaluate the hypothesis that in veterans exposed to blast related mTBI, the presence and severity of cognitive and neurologic deficits are correlated with metabolic abnormalities/impairments in the functionally linked brain regions. PUBLIC HEALTH RELEVANCE: With the recent wars in Afghanistan and Iraq, mTBI due to blast exposure has reached dramatic proportions with up to 19% of all returning veterans having a history of blast related mTBI with estimates of 20 to 50% still experiencing some form of dysfunction one year post injury. For blast related mTBI, the clinical assessment of mTBI largely relies on accuracy of self-reporting and compliance/effort during cognitive testing. Currently, performance below a threshold level on specific tests is equated to poor effort levels, which are often attributed to poor motivation or "malingering" (e.g. disability claims. A finding of poor effrt in turn raises significant concerns as to the validity of all cognitive evaluations and self- repored claims of neurologic deficits, leaving few options for objective assessment. With effort failure rates of 17 to 58% in veterans referred for TBI, methods which can objectively identify the presence of injury and anatomically link it with a specific cognitive domain can have a significant impact in prioritizing treatment and clinical management. The goal of this project is to develop advanced imaging methods which can improve the accuracy, speed and comprehensiveness of MRSI studies of the human brain to evaluate mTBI due to blast exposure. Although this project focuses on mTBI due to blast exposure, the metabolic changes seen (decreased NAA/choline ratios), are consistent with that seen in civilian mTBI and sports related concussions. Thus the findings and approaches demonstrated in blast related mTBI studies should also be applicable to the broader TBI field in general.