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.
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.
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|Avdievich, Nikolai I; Pan, Jullie W; Hetherington, Hoby P (2013) Resonant inductive decoupling (RID) for transceiver arrays to compensate for both reactive and resistive components of the mutual impedance. NMR Biomed 26:1547-54|
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