Barbro Vikhoff-Baaz

Method to correct for the effects of limited spatial resolution in phase-contrast flow MRI measurements

Phase‐contrast (PC) magnetic resonance imaging (MRI) flow measurements suffer from the effect of the point spread function (PSF) due to the limited sampling of k‐space. The PSF, which in this case is a sinc function, deforms the flow profile and forms a ringing pattern around the vessel. In this work, an empirical method is presented that corrects for errors due to the deformation of the flow profile. The ringing pattern is used to obtain a well‐defined vessel segmentation, which after correction provides more accurate vessel radius and volume flow rate (VFR). The correction method was developed from phantom measurements at constant flow and applied on phantom measurements at moderately pulsatile flow. After correction, the error of the estimated tube radius and the VFR was less than 10% and 5%, respectively. Corresponding errors without correction overestimated the radius by 60% and the VFR by 35%. Preliminary results indicate that the method is also valid in vivo. The variation in the estimated radius and VFR for different spatial resolution decreased when the method was applied. The presented method gives a more accurate estimation of the radius and VFR in vessels of the size of a few pixels without prior knowledge about the true vessel radius. Magn Reson Med 48:883–889, 2002.

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.

Quantitative phase-contrast flow MRI measurements in the presence of a second vessel closely positioned to the examined vessel

To examine the influence of the truncated sampling of k‐space data on the accuracy of phase‐contrast (PC) flow quantifications in the presence of nearby vessels.

A new set of fiducial markers for MRI, CT and SPET alignment.

A set of fiducial markers for use in the alignment of MR, CT and SPET images is presented. The marker system consists of individually produced imprints of the external auditory meatus, a pair of plastic spectacle frames and three fiducial markers. The markers are filled with a suitable contrast agent, fixed into the imprints and screwed onto the spectacle frame just before examination. The reproducibility in positioning the marker system on the patient was evaluated by repeated MRI examinations of a volunteer. The fiducial markers could be repositioned with an accuracy of less than

Contrast agent influences MRI phase-contrast flow measurements in small vessels

PT 1.6 mm in the left-right and anterio-posterior directions and

31P MR spectroscopy to evaluate the efficacy of hepatic artery embolization in the treatment of neuroendocrine liver metastases.

PT 1.9 mm in the cranio-caudal direction (95% confidence interval). The marker system is inexpensive and easy to produce, taking less than 20 min to produce on the ward. Only a few minutes are required to fix the system prior to examination. It is comfortable for the patient, even during long examinations. In cases lacking visible intrinsic structures and external contours, fiducial markers provide a good means of alignment for image fusion. The marker system described here for the alignment of SPET, CT and MR images can be used on a routine basis without the invasive fixation of fiducial markers.