Maria Ljungberg

Longitudinal study of the diffusion tensor imaging properties of the corpus callosum in acute and chronic diffuse axonal injury

Background: Magnetic resonance diffusion tensor imaging (MR-DTI) is used increasingly to detect diffuse axonal injury (DAI) after traumatic brain injury (TBI). Primary objective: The primary objective was to investigate the changes in the diffusion properties of the corpus callosum in the acute phase and 6 months after TBI and to examine the correlation between DTI parameters and clinical outcome. Research design: Longitudinal prospective study. Methods and procedures: MR-DTI was performed in eight patients with suspected DAI within 11 days and at 6 months post-injury. Six controls were also examined. Fractional anisotropy (FA), trace and parallel and perpendicular diffusivity of the corpus callosum were analysed. The main outcome was the extended Glasgow Outcome Scale score, assessed at 6 months. Main outcomes and results: A significant reduction in FA in the corpus callosum was seen in the acute phase in patients compared with the healthy controls. There was no significant change in the parallel or perpendicular eigenvalues or trace. At 6 months, a significant reduction in FA and a significant increase in trace and perpendicular eigenvalues were noticed compared with controls. Conclusions: The diffusion properties of the corpus callosum correlated with clinical outcome in this longitudinal investigation.

Tumour size measurement in a mouse model using high resolution MRI

BackgroundAnimal models are frequently used to assess new treatment methods in cancer research. MRI offers a non-invasive in vivo monitoring of tumour tissue and thus allows longitudinal measurements of treatment effects, without the need for large cohorts of animals. Tumour size is an important biomarker of the disease development, but to our knowledge, MRI based size measurements have not yet been verified for small tumours (10−2–10−1u2009g). The aim of this study was to assess the accuracy of MRI based tumour size measurements of small tumours on mice.Methods2D and 3D T2-weighted RARE images of tumour bearing mice were acquired in vivo using a 7u2009T dedicated animal MR system. For the 3D images the acquired image resolution was varied. The images were exported to a PC workstation where the tumour mass was determined assuming a density of 1u2009g/cm3, using an in-house developed tool for segmentation and delineation. The resulting data were compared to the weight of the resected tumours after sacrifice of the animal using regression analysis.ResultsStrong correlations were demonstrated between MRI- and necropsy determined masses. In general, 3D acquisition was not a prerequisite for high accuracy. However, it was slightly more accurate than 2D when small (<0.2u2009g) tumours were assessed for inter- and intraobserver variation. In 3D images, the voxel sizes could be increased from 1603u2009μm3 to 2403u2009μm3 without affecting the results significantly, thus reducing acquisition time substantially.Conclusions2D MRI was sufficient for accurate tumour size measurement, except for small tumours (<0.2u2009g) where 3D acquisition was necessary to reduce interobserver variation. Acquisition times between 15 and 50 minutes, depending on tumour size, were sufficient for accurate tumour volume measurement. Hence, it is possible to include further MR investigations of the tumour, such as tissue perfusion, diffusion or metabolic composition in the same MR session.

Visualizing Meyer's loop: A comparison of deterministic and probabilistic tractography

BACKGROUNDnDiffusion tensor tractography of the anterior extent of the optic radiation - Meyers loop - prior to temporal lobe resection (TLR) may reduce the risk for postoperative visual field defect. Currently there is no standardized way to perform tractography.nnnOBJECTIVEnTo visualize Meyers loop using deterministic (DTG) and probabilistic tractography (PTG) at different probability levels, with the primary aim to explore possible differences between methods, and the secondary aim to explore anatomical accuracy.nnnMETHODSnTwenty-three diffusion tensor imaging exams (11 controls and 7 TLR-patients, pre- and post-surgical) were analyzed using DTG and PTG thresholded at probability levels 0.2%, 0.5%, 1%, 5% and 10%. The distance from the tip of the temporal lobe to the anterior limit of Meyers loop (TP-ML) was measured in 46 optic radiations. Differences in TP-ML between the methods were compared. Results of the control group were compared to dissection studies and to a histological atlas.nnnRESULTSnFor controls and patients together, there were statistically significant differences (p<0.01) for TP-ML between all methods thresholded at PTG ≤1% compared to all methods thresholded at PTG ≥5% and DTG. There were no statistically significant differences between PTG 0.2%, 0.5% and 1% or between PTG 5%, 10% and DTG. For the control group, PTG ≤1% showed a closer match to dissection studies and PTG 1% showed the best match to histological tracings of Meyers loop.nnnCONCLUSIONSnChoice of tractography method affected the visualized location of Meyers loop significantly in a heterogeneous, clinically relevant study group. For the controls, PTG at probability levels ≤1% was a closer match to dissection studies. To determine the anterior extent of Meyers loop, PTG is superior to DTG and the probability level of PTG matters.

Long-Term Follow-up of a Patient with Traumatic Brain Injury Using Diffusion Tensor Imaging

This case report describes a patient who sustained severe head trauma with diffuse axonal injury (DAI). Examination with magnetic resonance diffusion tensor imaging (MR-DTI), 6 days post-injury, showed a severe reduction in fractional anisotropy (FA) in the rostral pons containing the corticospinal tract, which correlated to the patients severe hemiparesis. By 18 months post-accident, the patient had recovered completely and conventional MRI showed no pathology. However, although her FA values in the rostral pons had increased, they were still not normalized. It seems that a complete normalization of the FA values is not required to achieve clinical recovery, and that MR-DTI seems to be more sensitive to DAI compared to conventional MRI.

Effects of k-space filtering and image interpolation on image fidelity in 1H MRSI

2D MRSI suffers from the effect of the spatial response function due to the truncation of the sampling of k-space. Filtering of the k-space data-set is often used to suppress the side lobes caused by the effects of the SRF, where the sampled data-set is multiplied with a weighting function before inverse FT. Commonly used filters in MRSI are the cosine, Hanning and Hamming filters. The data-set is often interpolated into a larger image matrix size for analysis, where Fourier interpolation (FoI) and cubic spline interpolation (CSpI) are two common methods. In this work, the effects of k-space filtering in practical usage was examined, and the image representations of the object for the two interpolation methods were compared. This study showed that application of filtering improves the image representation of the structures in the object and should be used in MRSI. FoI correctly visualizes the information inherent in the data-set, while the features of the object were dependent on the position of the object relative the original matrix in the CSpI interpolated images. FoI should therefore be used for quantitative evaluation of MRSI images.

Lateralisation with magnetic resonance spectroscopic imaging in temporal lobe epilepsy: an evaluation of visual and region-of-interest analysis of metabolite concentration images

Abstract We carried out spectroscopic imaging (MRSI) on nine consecutive patients with temporal lobe epilepsy being assessed for epilepsy surgery, and nine neurologically healthy, age-matched volunteers. A volume of interest (VOI) was angled along the temporal horns on axial and sagittal images, and symmetrically over the temporal lobes on coronal images. Images showing the concentrations of N-acetylaspartate (NAA) and of choline-containing compounds plus creatine and phosphocreatine (Cho + Cr) were used for lateralisation. We compared assessment by visual inspection and by signal analysis from regions of interest (ROI) in different positions, where side-to-side differences in NAA/(Cho + Cr) ratio were used for lateralisation. The NAA/(Cho + Cr) ratio from the different ROI was also compared with that in the brain stem to assess if the latter could be used as an internal reference, e. g., for identification of bilateral changes. The metabolite concentration images were found useful for lateralisation of temporal lobe abnormalities related to epilepsy. Visual analysis can, with high accuracy, be used routinely. ROI analysis is useful for quantifying changes, giving more quantitative information about spatial distribution and the degree of signal loss. There was a large variation in NAA/(Cho + Cr) values in both patients and volunteers. The brain stem may be used as a reference for identification of bilateral changes.

Extended ISIS sequences insensitive to T1 smearing

Image selected in vivo spectroscopy ( ISIS ) is a volume selection method often used for in vivo 31P MRS, since it is suitable for measurements of substances with short T2. However, ISIS can suffer from significant signal contributions caused by T1 smearing from regions outside the VOI. A computer model was developed to simulate this contamination. The simulation results for the ISIS experiment order implemented in our MR system (ISIS‐0) were in agreement with results obtained from phantom measurements. A new extended ISIS experiment order ( E‐ISIS ) was developed, consisting of four “optimal” ISIS experiment orders (ISIS‐1 to ISIS‐4) performed consecutively with dummy ISIS experiments in between. The simulation results show that contamination due to T1 smearing is, effectively, eliminated with E‐ISIS and is significantly lower than for ISIS‐0 and ISIS‐1. E‐ISIS offers increased accuracy for quantitative and qualitative determination of substances studied using in vivo MRS. Hence, E‐ISIS can be valuable for both clinical and research applications. Magn Reson Med 44:546–555, 2000.

Signal profile measurements of single- and double-volume acquisitions with image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy

The volume-selection performance was studied for single- and double-volume-of-interest (VOI) acquisition with the volume-selection method image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy. High-resolution signal profiles were measured using a phantom simulating a brain. Inside the phantom there was a small, remotely controlled, movable signal source filled with ortho-phosphoric acid. Signal profiles of the VOI were measured in three perpendicular directions for 1VOI (single VOI) and 2VOI (double VOI) acquisition. The measured signal profiles for both acquisitions were very similar, but they showed a discrepancy with regard to the intended VOI (iVOI). The transition regions were on average 3.8 mm and the average full width at half maximum of the signal profile was 30 mm for an iVOI size of 30*30*30 (mm3). No displacement was observed in the signal profiles. To avoid overlapping signal profiles, the minimum separation between two iVOIs was found to be 10 mm in our magnetic resonance (MR) system. A substantial negative signal contribution from regions outside the iVOI was measured in the y-direction for 1VOI acquisition and one of the two VOIs in 2VOI acquisition. The other VOI in 2VOI acquisition exhibited only minor contamination. The measurements presented underline the importance of detailed knowledge on the volume selection performance in in vivo magnetic resonance spectroscopy.

31P MRS evaluation of fatigue in anterior tibial muscle in postpoliomyelitis patients and healthy volunteers

Changes in concentration of high energy phosphates and pH were studied during rest, exercise and subsequent recovery in the anterior tibial muscle of 10 patients with late effects of poliomyelitis and 10 age‐ and sex‐matched healthy volunteers using 31P MRS. The exercise was dynamic and isometric, and the force levels were individually adapted to each subject and stepwise increased. In general, there were no differences in metabolite changes between the groups, except for lower Pi and Pi/PCr for the volunteers during the recovery phase, also reflected by shorter recovery half‐time for Pi. The interindividual variation was much higher for the patient group. Some of the patients showed deviating results probably because of differences in muscle fibre type.

Performance of 2D 1H spectroscopic imaging of the brain: some practical considerations regarding the measurement procedure

This paper deals with some of the practical considerations in the planning and performance of chemical shift imaging (MRSI or CSI) of the brain. It contains some aspects of 1) the imaging procedure (MRI), i.e., suggestions of an imaging protocol useful for the spectroscopic planning, 2) the planning of the spectroscopic volume, i.e., size and position, 3) evaluation and judgment of the preparation results, and 4) evaluation of the MRSI images. The paper also contains suggestions of developmental work and quality assessment to be done before patient studies are begun. Examples are given for MRSI studies of temporal lobe epilepsy. Several of the aspects described are obvious for the experienced spectroscopist but may be useful in the initiation of MRSI. The goal of this paper was to share our experiences of how to achieve high quality MRSI, experiences that we would had been grateful for in our prelude of MRSI experiments.