Changes in DWI and MRS associated with white matter hyperintensities in elderly subjects

OBJECTIVE

To assess normal-appearing white matter (NAWM) characteristics by magnetic resonance spectroscopy (MRS) and diffusion-weighted imaging (DWI) in elderly subjects.

METHODS

The authors studied 60 volunteers (mean age 72.6 years; SD 4.7; range 64 to 84 years) without signs of neurologic illness. They used DWI and spectroscopic imaging to investigate whether there were changes in the NAWM that related to the presence of white matter hyperintensities (WMH).

RESULTS

The authors found a correlation (p < 0.001) between the apparent diffusion coefficient in the NAWM and the total volume of WMH. The metabolite ratios N-acetylaspartate/creatine and N-acetylaspartate/choline of the NAWM also correlated significantly with total WMH volume. These correlations were independent of age.

CONCLUSIONS

Damage associated with WMH is detectable in NAWM.

Regional cerebral blood flow in Parkinson's disease with and without dementia

Tc99 HMPAO SPECT and T1 weighted 3D MRI scans were acquired in cognitively intact subjects with Parkinson's disease (PD) (n = 31), and in PD subjects with dementia (PDD) (n = 34), healthy controls (n = 37), those with Alzheimer's disease (AD) (n = 32), and those with dementia with Lewy bodies (DLB) (n = 15). We used SPM99 to look for regions which showed a reduction in perfusion on SPECT not related to associated structural brain changes assessed by a MRI scan. The precuneus and inferior lateral parietal regions showed a perfusion deficit in Parkinson's disease with dementia, similar to the pattern observed in DLB. In comparison, AD showed a perfusion deficit in the midline parietal region, in a more anterior and inferior location than in PDD, involving the posterior cingulate as well as the precuneus. The perfusion deficits in PDD are similar those in DLB, and in a location associated with visual processing, and may be associated with the visuospatial perception deficits which are present in persons with DLB and PDD.

A comprehensive review of proton magnetic resonance spectroscopy studies in dementia and Parkinson's disease

We reviewed the literature of proton magnetic resonance spectroscopy (MRS) in dementia and Parkinson's disease (PD) and quantitatively compared the reported values of the markers N-acetyl aspartate (NAA), choline, and myo-Inositol between control and disease groups. We analysed a total of 27 reports in dementia. Combining the quantitative data from these showed a relative decrease of 15% in NAA level in the temporal lobe tissue in patients with Alzheimer's disease (AD) compared with controls. The rest of the brain showed a seemingly uniform 10% decrease in NAA levels in AD compared with controls. myo-Inositol was raised by about 15%, again uniformly throughout the brain, but there was no evidence for changed levels of choline. We found 15 reports of MRS in PD, which show a small decrease (5%) in the NAA level in the lentiform nucleus compared with controls. In progressive supranuclear palsy (PSP), there is a greater decrease in NAA levels in the frontal region and the lentiform nucleus. This may aid in the diagnosis of PSP. Further research is needed to determine spectroscopic changes in other dementias, to monitor how markers change with disease progression and to establish clinical utility.

Measuring extraocular muscle volume using dynamic contours

The effect of medical treatment on extraocular muscle enlargement in thyroid associated ophthalmopathy (TAO) may be monitored by measuring the change in volume of the extraocular muscles on serial orbital MRI examinations. In theory, 3D image sets offer the opportunity to minimise errors due to poor repositioning and partial volume effects. This study describes an automated technique for estimating extraocular muscle volumes from 3D datasets. Operator input is minimal and the technique is robust. Verification of the technique on both simulated and real datasets is described. For simulated image sets, both automated segmentation and manual outlining produced estimates of volume which were on average 4% less than "true" volume. For real patient data, extraocular muscle volumes measured by the automated technique were 1.6% (SD 13%) less than volumes measured by manual outlining. Coefficient of variation for repeat outlining of the same image dataset for the automated technique was 1.0%, compared with 4% for manual outlining. The manual technique took an experienced operator approximately 20 min to perform, compared to 7 min for the automated technique. The automated method is therefore rapid, reproducible and at least as accurate as other available methods.

Evaluation of a technique for estimation of extraocular muscle volume using 2D MRI

Measurements of extraocular muscle volume may be useful as an outcome indicator in the assessment of treatment options in thyroid-associated ophthalmopathy. In this study, a technique for estimating extraocular muscle volume from two-dimensional T1 weighted MR images acquired in the coronal plane is described. For each image slice, the cross-sectional area of individual extraocular muscles was estimated using a combination of semi-automated outlining based on a local thresholding technique and manual outlining where thresholding was not possible. The technique was assessed using three sets of images: a computer-generated virtual image set with "extraocular muscles" of known volumes; a set of images from 19 biochemically and clinically euthyroid patients; and images from 7 patients with thyroid-associated ophthalmopathy. From the computer-generated images, the accuracy was determined as 7.6%. Intraobserver agreement was investigated using repeated MRI scans on a single subject and repeat outlining of muscles on five sets of images from different subjects. The mean difference between repeat measurements was 2% (SD 5.9%). We conclude that two-dimensional MR images can be used to determine changes in muscle volume of greater than 12% during treatment of thyroid-associated ophthalmopathy.

Quality assurance for MRI: practical experience

The aim of this study is to propose guidelines for quality assurance (QA) in MRI, based on a comprehensive assessment of QA parameters undertaken on a busy clinical MRI scanner over the course of 1 year. QA phantoms supplied by the scanner manufacturer were used together with the Eurospin MRI phantom set. Signal-to-noise ratio (SNR) and image uniformity were measured daily from spin echo images acquired using a quadrature send-receive head coil and from a gradient echo sequence using the Helmholtz body coil. The voltage of the transmit radiofrequency (RF) amplifier was noted. Monthly measurements of slice thickness, geometric distortion, slice position, image resolution and image ghosting were acquired using the head coil. In addition, SNR was measured monthly on a selection of commonly used coils. Apart from some drift of the RF amplifier voltage, all measurements were within acceptable limits and were stable over the course of 1 year. Satisfactory measurements of SNR were possible using the simple phantom supplied with the scanner. The SNR, geometric distortion and RF amplifier voltage are simple to determine and can be measured in less than 15 min by the scanner operator, using the scanner software. Weekly recording of these parameters is recommended for busy clinical MRI scanners, as this should allow deviations from acceptable limits to be identified early. Such in-house checks can usefully be compared with the less frequent estimations performed by the service engineer. Comprehensive QA routines are discussed for systems used for quantitative measurements.

A comparison of two methods for measuring the signal to noise ratio on MR images

The signal to noise ratio (SNR) is one of the important measures of the performance of a magnetic resonance imaging (MRI) system. The object of this study was to compare a single acquisition method, which estimates the noise from background pixels, with a dual acquisition method which estimates the noise from the subtraction of two sequentially acquired images. The dual acquisition method is more exact, but is slower to perform and requires image manipulation. A comparison between the two methods gave a good correlation, and a regression equation of SNRsingle = 1.1 + 0.94 SNRdual. The single acquisition method is therefore appropriate for use in a quality assurance programme, since it is quicker and simpler to perform and is a good indicator of the more exact measure.

Partial volume effects in MRI studies of multiple sclerosis

We have estimated the accuracy of volume measurements of multiple sclerosis (MS) lesions made using magnetic resonance imaging (MRI) for lesions of comparable diameter to the image slice thickness. We used a phantom containing objects of known volume and obtained images using a range of slice thicknesses. Measurements on the phantom were used to assess a theoretical model, which was then employed to investigate the effects of image dimensions and geometry upon volume measurement accuracy. We observed measured volume to be dependent upon slice thickness. Thin slices gave the most accurate estimate of volume. As slice thickness increased relative to object diameter, the error in the volume measurement increased (to as much as 100%), the volume measured being dependent on the position of the object relative to the slice center. Using a signal intensity threshold value of 50% to outline objects gave results closest to the actual volume. As expected, a lower threshold value tended to give higher volume estimates (up to 100% larger), as did a semi-automated local edge detection technique. For accurate volume measurement, the slice thickness should be no more than a fifth of anticipated object diameter. For typical MS lesions (7 mm in diameter), this implies using a 1.5-mm slice thickness. For serial studies, a repositioning error of 1 mm could lead to differences in the volume measurement of individual lesions of up to 12% between studies for lesions of typical MS size and 5-mm slice thickness. These results emphasize the need for accurate patient repositioning, relatively thin slices, for regular quality assurance checks to ensure that pixel size and slice position are correct and stable over time, and that lesion outlining is performed in a consistent fashion. We would recommend the use of a 3D sequence with 1 mm cubic voxels for accurate measurements of MS lesions.