Vessel size imaging in humans

Abstract

The relation of contrast-enhanced transverse relaxation rates R 2 * and R 2 provides in vivo mapping of the mean caliber of cerebral vessels. This technique is referred to as vessel size imaging (VSI) The microvasculature, which includes the smallest and intermediate vessels, cannot be imaged directly with current MRI techniques. MRI examinations yield averaged microvasculature-related characteristics of perfusion or pharmacokinetics. Recently, attempts have been made to measure the mean vessel caliber in vivo (1, 2) in animal models. The feasibility of such measurements is granted by the difference in the contributions of small vessels to the transverse relaxation rates of the free induction decay, R* 2 , and that of the spin echo (SE), R 2 , (3-7) (Fig. 1). The method is based on measuring the changes in both rates, ⌬R* 2 and ⌬R 2 , after administration of a paramagnetic contrast agent. The average vessel size in a tissue can be estimated from ⌬R* 2 and ⌬R 2 according to theoretical predictions provided either by Monte Carlo simulations (2, 3) or by analytical modeling (4,8,9)..A quantitative assessment of the microvasculature might have diagnostic impact on the differential diagnosis of intracranial tumors. In a recent study (5) a correlation was found between glioma grade and regional cerebral blood volume (rCBV) measured with a gradient echo (GE) technique, while SE data showed no significant correlation. This result was qualitatively explained by the higher sensitivity of R* 2 to large vessels, which are inherent to malignant tumor tissues. Similarly, it was reported (6, 7) that the difference in regional blood volume assessed with GE and SE techniques increases for high-grade brain tumors.In the present study, quantitative imaging of the mean vessel caliber was performed in brain tumor patients undergoing routine MRI examinations. Previous experiments were performed in animals (2) using an intravascular contrast agent (USPIO) and monitoring its blood concentration invasively. The present method has been developed to meet the conditions of human studies in a clinical environment, in particular, the transient character of the contrast enhancement during the first bolus passage. The analysis presented and computer simulations help to understand the restrictions of the method and its shortcomings, which are caused by the complex relationship between the microvascular morphology and the measured NMR signal. Four illustrative cases of brain tumors are discussed in the context of the model developed.

Vessel Size Imaging Reveals Pathological Changes of Microvessel Density and Size in Acute Ischemia

“…Despite the different techniques, the vessel radii of GM and WM in our healthy volunteers are very similar to their venous vessel radii. The vessel sizes of the normal-appearing tissue in tumor patients studied by Hsu et al (2009) and Kiselev et al (2005) are almost twice as large as our results in healthy brain tissue. Besides the possible vascular compensation in the normal-appearing tissue around the tumor, the difference in results is mainly because we normalized the global CBV to 3%, which has been found to be a better estimation to the real blood volume fraction than the 6% used by their groups.…”

“…Moreover, linear fitting avoids the accidental errors that often occur in picking a single signal drop. The method in this study is based on the tissue model introduced by Kiselev et al (2005), which results in a systematic overestimation of vessel sizes. The VSI we measured in the thalamus, where no large vessels present, is very similar to the value of 7.2 mm proposed by the microscale tissue model (Kiselev et al, 2005).…”

“…The method in this study is based on the tissue model introduced by Kiselev et al (2005), which results in a systematic overestimation of vessel sizes. The VSI we measured in the thalamus, where no large vessels present, is very similar to the value of 7.2 mm proposed by the microscale tissue model (Kiselev et al, 2005). Jochimsen et al (2010) assessed venous VSI in healthy human brains with the administration of carbogen by measuring the induced blood oxygenation level-dependent effect in a 7T scanner.…”

“…However, it does not record the dynamic bolus passage and only allows VSI qualification by additionally performing a blood test to define the contrast concentration. The later dynamic implementation was reported in a tumor study, in which the postprocessing method could be further improved (Kiselev et al, 2005).…”

Histogram analysis of the microvasculature of intracerebral human and murine glioma xenografts

“…This will be possible if the neovasculature of gliomas can be accurately described. The vessel size index (VSI) MRI technique allows to investigate brain tumor microvasculature using high molecular superparamagnetic contrast agents in rats (5) and gadolinium chelates in humans (6). The mean vessel caliber estimation relies on the simultaneous measurement of DR 2 * and DR 2 after administration of contrast agent.…”