Brain choline concentration. Early quantitative marker of ischemia and infarct expansion?

OBJECTIVE

Better prediction of tissue prognosis in acute stroke might improve treatment decisions. We hypothesized that there are metabolic ischemic disturbances measurable noninvasively by proton magnetic resonance spectroscopy ((1)H MRS) that occur earlier than any structural changes visible on diffusion-tensor imaging (DTI), which may therefore serve for territorial identification of tissue at risk.

METHODS

We performed multivoxel (1)H MRS plus DTI within a maximum of 26 hours, and DTI at 3-7 days, after ischemic stroke. We compared choline, lactate, N-acetylaspartate, and creatine concentrations in normal-appearing voxels that became infarcted (infarct expansion) with normal-appearing voxels around the infarct that remained "healthy" (nonexpansion) on follow-up DTI. Each infarct expansion voxel was additionally classified as either complete infarct expansion (infarcted tissue on follow-up DTI covered > or =50% of the voxel) or partial infarct expansion (<50% of voxel).

RESULTS

In 31 patients (NIH Stroke Scale score 0-28), there were 108 infarct nonexpansion voxels and 113 infarct expansion voxels (of which 80 were complete expansion and 33 partial expansion voxels). Brain choline concentration increased for each change in expansion category from nonexpansion, via partial expansion to complete expansion (2,423, 3,843, 4,158 IU; p < 0.05). Changes in lactate, N-acetylaspartate, and creatine concentrations in expansion category were insignificant although for lactate there was a tendency to such association.

CONCLUSIONS

Choline concentration measurable with (1)H MRS was elevated in peri-ischemic normal-appearing brain that became infarcted by 3-7 days. The degree of elevation was associated with the amount of infarct expansion. (1)H MRS might identify DTI-normal-appearing tissue at risk of conversion to infarction in early stroke.

Effects of dexamethasone on cerebral perfusion and water diffusion in patients with high-grade glioma

BACKGROUND AND PURPOSE

The mechanisms by which the glucocorticoid dexamethasone produces its therapeutic action in patients with intracranial tumors still remain unclear. The purpose of this study was to investigate whether dexamethasone affects cerebral perfusion and water molecule diffusion by using quantitative dynamic susceptibility contrast perfusion MR imaging (DSC-MR imaging) and diffusion tensor MR imaging (DT-MR imaging).

METHODS

Ten consecutive patients with glioblastoma multiforme underwent DSC-MR imaging and DT-MR imaging before and 48-72 hours after dexamethasone treatment (16 mg/day). Cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), and water mean diffusivity (<D>) were measured for enhancing tumor, nonenhancing peritumoral edematous brain, and normal-appearing contralateral white matter before and after steroid therapy. The percentage change in CBF, CBV, MTT, and <D> for the 3 tissue types was calculated for each patient, a mean value obtained for the population, and the statistical significance determined by using a paired-samples Student t test.

RESULTS

After dexamethasone treatment, there was no significant change in tumor CBF, CBV, or MTT. Edematous brain CBV and MTT were also unchanged. There was, however, an increase in edematous brain CBF (11.6%; P = .05). <D> was reduced in both enhancing tumor (-5.8%; P = .001) and edematous brain (-6.0%; P < .001). There was no significant change in CBF, CBV, MTT, or <D> for normal-appearing contralateral white matter after treatment.

CONCLUSION

These data suggest that dexamethasone does not significantly affect tumor blood flow but may, by reducing peritumoral water content and local tissue pressure, subtly increase perfusion in the edematous brain.

Do acute diffusion- and perfusion-weighted MRI lesions identify final infarct volume in ischemic stroke?

BACKGROUND AND PURPOSE

An acute mismatch on diffusion-weighted MRI (DWI) and perfusion-weighted MRI (PWI) may represent the "tissue-at-risk." It is unclear which "semiquantitative" perfusion parameter most closely identifies final infarct volume.

METHODS

Acute stroke patients underwent DWI and PWI (dynamic-susceptibility contrast imaging) on admission (baseline), and T2-weighted imaging (T2WI) at 1 or 3 months after stroke. "Semiquantitative" mean transit time (MTTsq=first moment of concentration/time curve), cerebral blood volume (CBVsq=area under concentration/time curve), and cerebral blood flow (CBFsq=CBVsq/MTTsq) were calculated. DWI and PWI lesions were measured at baseline and final infarct volume on T2WI acquired > or =1 month after stroke. Baseline DWI, CBFsq, and MTTsq lesion volumes were compared with final T2WI lesion volume.

RESULTS

Among 46 patients, baseline DWI and CBFsq lesions were not significantly different from final T2WI lesion volume, but baseline MTTsq lesions were significantly larger. The correlation with final T2WI lesion volume was strongest for DWI (Spearman rank correlation coefficient rho=0.68), intermediate for CBFsq (rho=0.55), and weakest for MTTsq (rho=0.49) baseline lesion volumes. Neither DWI/CBFsq nor DWI/MTTsq mismatch predicted lesion growth; lesion growth was equally common in those with and without mismatch.

CONCLUSIONS

Of the 2 PWI parameters, CBFsq lesions most closely identifies, and MTTsq overestimates, final T2WI lesion volume. "DWI/PWI mismatch" does not identify lesion growth. Patients without "DWI/PWI mismatch" are equally likely to have lesion growth as those with mismatch and should not be excluded from acute stroke treatment.

Temporal evolution of water diffusion parameters is different in grey and white matter in human ischaemic stroke

OBJECTIVES

Our purpose was to investigate whether differences exist in the values and temporal evolution of mean diffusivity (<D>) and fractional anisotropy (FA) of grey and white matter after human ischaemic stroke.

METHODS

Thirty two patients with lesions affecting both grey and white matter underwent serial diffusion tensor magnetic resonance imaging (DT-MRI) within 24 hours, and at 4-7 days, 10-14 days, 1 month, and 3 months after stroke. Multiple small circular regions of interest (ROI) were placed in the grey and white matter within the lesion and in the contralateral hemisphere. Values of <D>[grey], <D>[white], FA[grey] and FA[white] were measured in these ROI at each time point and the ratios of ischaemic to normal contralateral values (<D>R and FAR) calculated.

RESULTS

<D> and FA showed different patterns of evolution after stroke. After an initial decline, the rate of increase of <D>[grey] was faster than <D>[white] from 4-7 to 10-14 days. FA[white] decreased more rapidly than FA[grey] during the first week, thereafter for both tissue types the FA decreased gradually. However, FA[white] was still higher than FA[grey] at three months indicating that some organised axonal structure remained. This effect was more marked in some patients than in others. <D>R[grey] was significantly higher than <D>R[white] within 24 hours and at 10-14 days (p<0.05), and FAR[white] was significantly more reduced than FAR[grey] at all time points (p<0.001).

CONCLUSIONS

The values and temporal evolution of <D> and FA are different for grey and white matter after human ischaemic stroke. The observation that there is patient-to-patient variability in the degree of white matter structure remaining within the infarct at three months may have implications for predicting patient outcome.

Local haemodynamics of arterial bypass graft anastomoses

One of the main causes of failure of expanded polytetrafluoroethylene (PTFE) bypass grafts used in the lower limbs is the development of myointimal hyperplasia (MIH). Clinical studies show that higher patency rates can be obtained with the use of an autologous vein cuff (the Miller cuff) interposed between the graft and artery. The reasons for the improved performance are still unclear, but preliminary studies suggest that the change in local haemodynamics due to the cuff geometry may be the significant factor rather than the presence of autologous material. If this is the case, then PTFE grafts can be produced with an integral cuff, i.e. a precuffed graft, with similar haemodynamic patterns to that of the Miller cuff. In this paper, two different types of precuffed graft are presented and their flow patterns are compared with those recorded in the Miller cuff and the conventional end-to-side anastomosis. The haemodynamic studies were carried out using optically clear silicone rubber models under simulated in vivo pulsatile flow conditions. Flow structures similar to those observed in the Miller cuff were seen in the precuffed grafts.