High Field MR in Patients with TBI

The potential advantages of high-field MR in routine clinical practice are increased spatial contrast, and spectral and temporal resolution, which are greater with the more advanced techniques. In principle, the performance of a 3.0 T MR imager should be double that of a 1.5 T machine [23]. However, artefacts and technical limitations do not allow this to be achieved in practice. A greater signal/noise ratio (SNR), the main advantage of high-field imaging, can be reduced by local magnetic field variations caused by magnetic field gradient inhomogeneities and will therefore no longer be double as predicted by theory [23-26].

Fig. 13.1. SE T1: comparison between images acquired in the same patient at 3.0 T (Allegra, Siemens) (a left) and 1.5 T (Vision, Siemens) (b right). The thin extra-axial right frontal blood collection, indicating a subacute subdural haematoma, is appreciated in both sequences

The reduction in T1 differences among tissue types induces a contrast loss between white and grey matter [27, 28] and can impair detection of lesions like contusion and intraparenchymal haematoma, which in the acute stage are difficult to appreciate also at 1.5 T. The MPRAGE sequence has been introduced to address this problem. Volumetric sequences such as MPRAGE have already been performed at 1.5 T to quantify white and grey matter volume in TBI patients with a view to documenting potential correlations between clinical state and atrophy of specific brain lesions (Fig. 13.1) [22].

As regards T2-weighted images, the greater spatial y

Fig. 13.2. GE T2*: comparison between images acquired in the same patient at 3.0 T (Allegra, Siemens) (a, c left) and 1.5 T (Vision, Siemens) (b, d right). Bilateral frontal subcortical (a, b) and left temporal and mesencephalic (c,d) small focal DAIlesions are more numerous and better visualized at 3.0 T

Fig. 13.2. GE T2*: comparison between images acquired in the same patient at 3.0 T (Allegra, Siemens) (a, c left) and 1.5 T (Vision, Siemens) (b, d right). Bilateral frontal subcortical (a, b) and left temporal and mesencephalic (c,d) small focal DAIlesions are more numerous and better visualized at 3.0 T

resolution, resulting in better anatomical detail and contrast due to the greater SNR, allows thicker slices and broader matrices to be used and small lesions such as contusions or DAI to be appreciated. A further advantage of high-field T2-weighted sequences is their ability to depict very small deposits of deoxyhaemoglo-bin or methaemoglobin as markedly hypointense areas by virtue of magnetic susceptibility effects, which are amplified by the high field [29].

High-field GE T2* sequences are more sensitive to the magnetic susceptibility of haemoglobin degradation products and are thus valuable for detecting DAI. A correlation between number of blood-containing lesions, DAI lesions identified on T2*-weighted sequences and GCS score was confirmed by Scheid et al. [15], whereas no relationship was documented between focal changes appreciable in T2*-weighted sequences and Glasgow Outcome Scale (GOS) scores [30], suggesting that lesion load may not predict prognosis in these patients (Fig. 13.2).

EPI sequences are also more sensitive at 3.0 T than 1.5 T, to the detriment of image geometry. On EPI images distortion is approximately 30% greater at 3.0 T than 1.5 T, but can be reduced to about 1 % using SSFSE [31].

Accurate study of intracranial vessels, which is especially useful in patients with TBI and clinically suspected vessel dissection, carotid-cavernous fistula, or dural venous sinus thrombosis, benefits from high-field imaging [32, 33].

Advanced High-Field Techniques in TBI

It is well established that standard MR sequences underestimate traumatic brain injury [6,15, 34-38]. New MR techniques such as DTI and MRS have been applied to TBI to gain insights into the mechanisms underlying the patient's clinical condition, predict its evolution and thus prognosis, and optimize therapeutic strategies [39-45].

Huisman et al. [42] demonstrated the superior performance of DWI in identifying DAI compared with standard (T2-, T2*-weighted, FLAIR) sequences, but did not address its clinical relevance and thus the potential correlations with prognosis.

DTI is an extension ofDWI and provides two essential types of information: a quantitative estimate of an-isotropy and its spatial orientation. Tractography uses these microscopic data to track macroscopic axon fibres and is currently the sole method capable of noninvasive in vivo imaging of these structures [24-26]. In DAI, the white matter fibres are typically interrupted; measuring white matter anisotropy with DTI allows the tissue damage to be quantified (Fig. 13.3).

Based on these considerations, Huisman et al. [43] went on to demonstrate that DTI is capable of detecting structural white matter changes, and that these changes correlate with clinical parameters such as GCS score in the acute phase and at discharge. The advantages of applying DTI to patients with head trauma are:

An ability to gain information on the microscopic brain damage responsible, alone or in association with the macroscopic damage, for the patient's clinical state

• An ability to document and quantify the brain plasticity phenomena underpinning clinical recovery, which may be enhanced using pharmacological and rehabilitation therapies

• The possibility of assessing the therapeutic response even in those patients on whom clinical studies do not provide adequate information; moreover, the quantitative data offered by DTI are not influenced by potential CNS side effects of drug therapy or invasive procedures (e.g. intubation)

• The possibility of using DTI data to predict outcome in trauma patients, since neither standard MR findings nor clinical parameters, such as GCS scores, can predict their future clinical condition

The technical characteristics of high-field MR have enhanced the quality of DTI data, particularly spatial resolution, the spatial deformation induced by magnetic field inhomogeneities and image SNR, thus improving the accuracy of nerve fibre tracking.

The advantage of using 3.0 T MRS consists of an enhancement of the chemical shift with better separation of the metabolite peaks and high SNR [46,47]. MRS can document a reduction in the N-acetylaspartate (NAA) peak, a neuronal and axonal marker, in ostensibly normal white matter free of macroscopic focal changes weeks or months after the trauma, and the extent of this reduction significantly correlates with scores on prognostic measures like GOS and Disability Rating Scale (DRS) [44, 48]. Concomitant elevation of the levels of other metabolites, such as myo-inositol (Ins) and choline (Cho), is to be ascribed to glial proliferation or an inflammatory process [49, 50]. Studies of white matter devoid of focal changes have demonstrated that the levels of these metabolites can eventually revert to normal [50] as well as a progressive reduction in NAA associated with increased Cho [49] or decreased Ins peaks [44]. Shutter et al. [45] studied a group of trauma patients in the subacute phase who underwent MRS within a week of the trauma to detect metabolic changes that could predict clinical outcome. Elevation of the levels of metabolites such as glutamate/glutamine (Glx) and Cho in apparently normal white and grey matter was found to be highly predictive of long-term adverse outcome with an accuracy of 89 % (Fig. 13.4).

Functional MR imaging (fMRI) currently has a small role in studying trauma patients. It has been proposed for use as a prognostic tool in TBI patients in a coma using medium-field MR. Visual, auditory and so-

Fig. 13.3. Tractography at 3.0 T (Allegra, Siemens): visualization of motor areas and pyramidal fibres in an extensive right frontal contusion. The right primary motor area and pyramidal fibres are depicted less clearly than in the contralateral area

Fig. 13.4. Magnetic resonance spectroscopy with TE = 135 ms. The left spectrum was obtained from a volume of interest located in the right corona radiata, showing reduced NAA/Cr ratio reflecting neuronal loss. The right spectrum obtained from a VOI positioned contralateral to the lesion, is normal

Fig. 13.4. Magnetic resonance spectroscopy with TE = 135 ms. The left spectrum was obtained from a volume of interest located in the right corona radiata, showing reduced NAA/Cr ratio reflecting neuronal loss. The right spectrum obtained from a VOI positioned contralateral to the lesion, is normal

Magnetic Resonance Spectroscopy Tbi

matosensory stimulation during fMRI induced an increment in the haemodynamic response that predicted subsequent recovery, which was confirmed 3 months from trauma by clinical and electrophysiological examination [51]. fMRI has also been proposed to investigate the mechanisms of motor recovery in trauma patients [52, 53]. The effects of trauma on brain tissue have recently been assessed with fMRI, transcranial Doppler and transcranial magnetic stimulation in a TBI patient without tissue damage on conventional MR. This multimodal study demonstrated a dissociation between haemodynamic and functional response [54].

High-field MR, one of whose advantages is a high sensitivity to the BOLD signal, could find applications in the study of the phases of clinical recovery after TBI, as suggested in the literature [52, 53]. In particular, the degree of activation of specific brain areas at fMRI could provide an index of haemodynamic and functional activity in ostensibly healthy tissue in patients with DAI. Longitudinal variations of this index in conjunction with structural parameters and clinical examination could thus assume a prognostic value (Fig. 13.5).

In conclusion, high-field MR has the potential to enhance the accuracy of morphostructural, metabolic

Fig. 13.5. fMRI at 3.0 T (Allegra, Siemens) in a patient with TBI exhibiting an extensive right frontal cortico-subcortical contusion, top: activation of left primary and supplementary sensorimotor areas during a motor task (finger tapping, right hand); bottom: bilateral activation of primary and supplementary motor areas during the motor task (finger tapping, left hand). Note a low activation of the right primary sensorimotor area and recruitment of the left primary and supplementary sensorimotor areas in the motor task of the clinically affected left hand

Fig. 13.5. fMRI at 3.0 T (Allegra, Siemens) in a patient with TBI exhibiting an extensive right frontal cortico-subcortical contusion, top: activation of left primary and supplementary sensorimotor areas during a motor task (finger tapping, right hand); bottom: bilateral activation of primary and supplementary motor areas during the motor task (finger tapping, left hand). Note a low activation of the right primary sensorimotor area and recruitment of the left primary and supplementary sensorimotor areas in the motor task of the clinically affected left hand and functional studies of the brain parenchyma in trauma patients, advancing the knowledge of the relationships between changes in nerve fibre ultrastructure and clinical symptoms and allowing the quantification of the brain plasticity phenomena that follow spontaneous clinical recovery or administration of pharmacological and rehabilitation therapies.

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