Erythrocyte velocity and fluorescein transit time in the cerebral microcirculation of macroglobulinemic mice: differential effect of a hyperviscosity syndrome on the passage of erythrocytes and plasma

Selective internal radiation therapy: quantifying distal penetration and distribution of resin and glass microspheres in a surrogate arterial model

PURPOSE

To experimentally investigate the effects of microsphere density and diameter on distal penetration.

MATERIALS AND METHODS

A surrogate hepatic arterial system was developed to replicate the hemodynamics (pressures, flow rates, pulsatile flow characteristics) and anatomic geometry (vessel diameters) proximal and distal to the microsphere injection point. A planar tumor model, placed distal to the injection point, allowed visualization of deposited microspheres. Bland resin and glass microspheres, with physical characteristics approximating the characteristics of commercially available products, were injected into the surrogate system. Microsphere type, injection rate, systemic flow rate, and tumor model inclination were varied among tests (glass, n = 7; resin, n = 6) with replicates for 2 conditions. After injection, 254 micrographs were obtained at previously defined locations throughout the tumor model to document microsphere distributions. Average microsphere distributions and mass measurements of microspheres collected at the tumor outlet were analyzed to quantify distal penetration for each case.

RESULTS

Across all test conditions, average penetration depths of resin microspheres were higher compared with glass microspheres (45.1 cm ± 11.8 vs 22.3 cm ± 9.9). The analysis of variance indicated that the observed difference between microsphere type (glass vs resin) was significant (P = .005, df = 1,2). The observed distance means did not differ significantly across flow rate or inclination angle.

CONCLUSIONS

Penetration depths of resin microspheres were significantly higher than penetration depths of glass microspheres in the surrogate hepatic arterial system.

In vivo X-ray digital subtraction and CT angiography of the murine cerebrovasculature using an intra-arterial route of contrast injection

BACKGROUND AND PURPOSE

Investigation of the anatomy, patency, and blood flow of arterial and venous vessels in small animal models of cerebral ischemia, venous thrombosis, or vasospasm is of major interest. However, due to their small caliber, in vivo examination of these vessels is technically challenging. Using micro-CT, we compared the feasibility of in vivo DSA and CTA of the murine cerebrovasculature using an intra-arterial route of contrast administration.

MATERIALS AND METHODS

The ECA was catheterized in 5 C57BL/6J mice. During intra-arterial injection of an iodized contrast agent (30 μL/1 sec), DSA of the intra- and extracranial vessels was performed in mice breathing room air and repeated in hypoxic/hypercapnic mice. Micro-CTA was performed within 20 seconds of intra-arterial contrast injection (220 μL/20 sec). Image quality of both methods was compared. Radiation dose measurements were performed with thermoluminescence dosimeters.

RESULTS

Both methods provided high-resolution images of the murine cerebrovasculature, with the smallest identifiable vessel calibers of ≤ 50 μm. Due to its high temporal resolution of 30 fps, DSA allowed identification of anastomoses between the ICA and ECA by detection of retrograde flow within the superficial temporal artery. Micro-CTA during intra-arterial contrast injection resulted in a reduced injection volume and a higher contrast-to-noise ratio (19.0 ± 1.0) compared with DSA (10.0 ± 1.8) or micro-CTA when using an intravenous injection route (1.3 ± 0.4).

CONCLUSIONS

DSA of the murine cerebrovasculature is feasible using micro-CT and allows precise and repeated measurements of the vessel caliber, and changes of the vessel caliber, while providing relevant information on blood flow in vivo.

Temporal dynamics of the BOLD fMRI impulse response

Using computer simulations and high-resolution fMRI experiments in humans (n=6) and rats (n=8), we investigated to what extent BOLD fMRI temporal resolution is limited by dispersion in the venous vasculature. For this purpose, time-to-peak (TTP) and full-width at half-maximum (FWHM) of the BOLD impulse response (IR) function were determined. In fMRI experiments, a binary m-sequence probe method was used to obtain high-sensitivity model-free single-pixel estimates of IR. Simulations of postcapillary flow suggested that flow-related dispersion leads to a TTP and FWHM increase, which can amount to several seconds in larger pial veins. fMRI experiments showed substantial spatial variation in IR timing within human visual cortex, together with a correlation between TTP and FWHM. Averaged across the activated regions and across subjects, TTP and FWHM were 4.51+/-0.52 and 4.04+/-0.42 s, respectively. In regions of interest (ROI) weighted toward the larger venous structures, TTP and FWHM increased to 5.07+/-0.64 and 4.32+/-0.48 s, respectively. In rat somatosensory cortex, TTP and FWHM were substantially shorter than in humans (2.73+/-0.60 and 2.28+/-0.63 s, respectively). These results are consistent with a substantial macrovascular dispersive contribution to BOLD IR in humans, and furthermore suggest that neurovascular coupling is a relatively rapid process, with a resolution below 2.3 s FWHM.

Carbon dioxide exchange across the walls of arterioles: implication for the location of the medullary chemoreceptors

The location of the medullary chemoreceptors is not conclusively established. The original experiments, which were believed to suggest a shallow surface location in the ventrolateral medulla, have been questioned because substances, particularly CO2, applied on the surface of the medulla could diffuse into small arterioles. Because the whole tissue blood flow is supplied by surface arterioles, they could transport substances from the surface into the tissue to the respiratory centers. We studied simple transport equations describing movement of CO2 in arterioles bathed by rapidly flowing cerebrospinal fluid (CSF) and arterioles in tissue perfused by capillaries. Substantial exchange of CO2 could occur across the arteriole wall for all expected sizes of vessels when the partial pressure of CO2 at the outside wall was determined by CSF. When an arteriole is surrounded by tissue, only vessels with inside diameters (ID) less than or equal to 50 micron will exchange substantial amounts of CO2 but the smallest arterioles may be nearly in equilibrium with the tissue. The CO2 gradient in tissue around the arteriole will extend approximately 1 mm. Our simple theoretical description of CO2 transport in arterioles predicts substantial exchange in precapillary vessels. CO2 picked up by the smallest surface arterioles when the medulla is perfused at a high rate with CSF will not stay in the blood past the putative depth of the chemoreceptors. In arterioles greater than 30 micron, however, the CO2 could be carried to the respiratory centers.

Velocity-diameter relationships of the microcirculation

An analytical expression for the relationship between velocity (and blood flow) and diameter for vessels in the microcirculatory range up to 900 mum diameter is presented. Both the arterial and venous systems are considered. The analytical expression for these relationships has been derived from computer analysis of data obtained from individual measurements reported in the literature. Computer analysis shows that blood velocity is a nearly linear function of diameter of blood vessels for the arteriolar tree. The relationship for blood flow as a function of diameter is Q = 108d3.101 for arteries, arterioles and capillaries in the diameter range 5 to 900 mum and for the venous system in the range 5 to 115 mum is Q = 334d2.388 where Q is in mum3 per second and d is in mum. Comparison of the results of this study for the arterial system to the Hagen-Poiseuille Law reveals close agreement, if correction is made for geometric factors of blood vessels and for the variation of viscosity with diameter for blood.

Can plasma skimming of inconstancy of regional hematocrit introduce serious errors in regional cerebral blood flow measurements or their interpretation?

Recent reports indicate that in cases of elevated plasma or blood viscosity, red cell velocity through the cerebral microcirculation is unimpaired while plasma transit is retarded. One explanation may be that plasma skimming is increased, with increased shunting of plasma into longer, less direct flow paths, while an increasing proportion of red cells take shorter paths. Such inhomogeneities of flow on a microvascular level can be directly demonstrated in cases of endothelial damage or anoxia, and may produce an altered regional hematocrit.

Measurements of regional cerebral blood flow are dependent upon regional hematocrit because this affects the tissue:blood partition coefficient (λ) of diffusible indicators. Hence changes in regional HCT may produce errors in the calculation of regional blood flow unless the value of λ. is corrected to reflect altered HCT. Moreover, no matter what kind of indicator is used, our functional interpretation of regional blood flow measurements is dependent upon our assumption that the ‘blood’ in the region maintains a constant capacity to carry nutrients like oxygen. Thus the lack of a constant relationship between plasma flow and red cell flow may produce errors either in the measurement or interpretation of regional flow measurements. These errors will increase in importance as blood flow is measured in smaller and smaller volumes of brain.