Cerebral Small Resistance Artery Structure and Cerebral Blood Flow in Normotensive Subjects andHypertensive Patients Investigated with Perfusion MRImaging

PURPOSE It has been demonstrated previously that, in essential hypertensive patients, subcutaneous small resistance artery structural alterations, as indicated by an increased media to lumen ratio (M/L), may predict coronary and forearm flow reserve. In essential hypertension also human cerebral small arteries present a clear increase in M/L. The purpose of the study is to investigate the relationship between cerebral blood flow (CBF) and cerebral small resistance artery structure. MATERIALS & METHODS Ten subjects were included in the present study, five hypertensive patients and five normotensive control subjects. All subjects underwent a neurosurgical intervention. A small portion of morphologically normal cerebral tissue was excised and rapidly put in chilled physiologic saline solution. Cerebral small resistance arteries were dissected and mounted on an isometric myograph, and the M/L was measured. Before neurosurgical intervention patients underwent dynamic susceptibility-weighted contrast (DSC)-enhanced MR imaging (MRI) with a single-shot gradient-echo EPI sequence (TR/TE 1.4 sec/30 msec, slice thickness, 5 mm; field of view, 230 mm; acquisition matrix, 128x128) and bolus injection of Gd-DTPA at a rate of 4 ml/sec. Maps of regional cerebral blood flow (CBF), regional cerebral blood volume (CBV) and of the mean transit time (MTT) were calculated with the commercial software NordicICE (version 2.3.9, NordicImagingLab AS, Bergen, Norway). Cerebral blood volume and CBF maps were corrected for contrast agent leakage. Round-shaped ROIs were manually placed in the lenticular nucleus, thalami, fronto-temporo-occipital gray matter, frontal and temporal white matter and the mean values of CBF (ml/100g/min) and CBV (ml/100g) were determined in both intact and affected hemispheres. RESULTS Cerebral blood flow values were reduced in different areas of the brain in hypertensive patients compared with normotensive subjects. However, these differences reached statistical significance only in the thalamus. No difference between groups was observed for CBV (Table 1). A statistically significant inverse correlation was observed between M/L of cerebral arteries and CBF in the cortical gray matter (r=-0.65, p<0.05), lenticular nucleus (r=-0.74, p<0.01), thalamus (r=-0.71, p<0.01 and subcortical white matter (r=-0.60, p<0.05), while correlation with CBV in the different areas were not statistically significant. Microvessel density in the brain was not significantly correlated with CBF or CBV in any area. Table 1. Regional CBF and CBV as evaluated by MRI in the study population. Normotensive Hypertensive subjects (n=5) patients (n=5) CBF cortical gray matter (m;l/1--g/min) 72.9±5.19 66.4±6.74 CBF lenticular nucleus (ml/100g/min) 72.8±5.58 64.8±7.95 CBF thalamus (ml/100g/min) 73.1±6.33 59.2±9.55* CBF subcortical white matter (ml/100g/min) 22.9±3.29 21.9±4.1 CBV cortical gray matter (ml/100g) 5.48±0.42 5.93±0.58 CBV lenticular nucleus (ml/100g) 5.12±0.82 5.53±0.87 CBV thalamus (ml/100g) 4.99±0.79 5.53±0.3 CBV subcortical white matter (ml/100g) 1.69±0.21 1.81±0.11 *=p<0.05 vs normotensive subjects CONCLUSION The present study has shown a direct relationship between cerebral blood flow and cerebral small resistance artery structure. Our results indicate that microvascular structure might play a major role in controlling CBF, and this might help to explain the relevant role of structural alterations of small resistance arteries in predicting cerebrovascular events.