Date Published: January 31, 2017
Publisher: Public Library of Science
Author(s): Stefan Hindel, Anika Söhner, Marc Maaß, Wolfgang Sauerwein, Dorothe Möllmann, Hideo Andreas Baba, Martin Kramer, Lutz Lüdemann, Wolfgang Rudolf Bauer.
The purpose of this study was to assess the accuracy of fractional blood volume (vb) estimates in low-perfused and low-vascularized tissue using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). The results of different MRI methods were compared with histology to evaluate the accuracy of these methods under clinical conditions. vb was estimated by DCE-MRI using a 3D gradient echo sequence with k-space undersampling in five muscle groups in the hind leg of 9 female pigs. Two gadolinium-based contrast agents (CA) were used: a rapidly extravasating, extracellular, gadolinium-based, low-molecular-weight contrast agent (LMCA, gadoterate meglumine) and an extracellular, gadolinium-based, albumin-binding, slowly extravasating blood pool contrast agent (BPCA, gadofosveset trisodium). LMCA data were evaluated using the extended Tofts model (ETM) and the two-compartment exchange model (2CXM). The images acquired with administration of the BPCA were used to evaluate the accuracy of vb estimation with a bolus deconvolution technique (BD) and a method we call equilibrium MRI (EqMRI). The latter calculates the ratio of the magnitude of the relaxation rate change in the tissue curve at an approximate equilibrium state to the height of the same area of the arterial input function (AIF). Immunohistochemical staining with isolectin was used to label endothelium. A light microscope was used to estimate the fractional vascular area by relating the vascular region to the total tissue region (immunohistochemical vessel staining, IHVS). In addition, the percentage fraction of vascular volume was determined by multiplying the microvascular density (MVD) with the average estimated capillary lumen, π(d2)2, where d = 8μm is the assumed capillary diameter (microvascular density estimation, MVDE). Except for ETM values, highly significant correlations were found between most of the MRI methods investigated. In the cranial thigh, for example, the vb medians (interquartile range, IQRs) of IHVS, MVDE, BD, EqMRI, 2CXM and ETM were vb = 0.7(0.3)%, 1.1(0.4)%, 1.1(0.4)%, 1.4(0.3)%, 1.2(1.8)% and 0.1(0.2)%, respectively. Variances, expressed by the difference between third and first quartiles (IQR) were highest for the 2CXM for all muscle groups. High correlations between the values in four muscle groups—medial, cranial, lateral thigh and lower leg – estimated with MRI and histology were found between BD and EqMRI, MVDE and 2CXM and IHVS and ETM. Except for the ETM, no significant differences between the vb medians of all MRI methods were revealed with the Wilcoxon rank sum test. The same holds for all muscle regions using the 2CXM and MVDE. Except for cranial thigh muscle, no significant difference was found between EqMRI and MVDE. And except for the cranial thigh and the lower leg muscle, there was also no significant difference between the vb medians of BD and MVDE. Overall, there was good vb agreement between histology and the BPCA MRI methods and the 2CXM LMCA approach with the exception of the ETM method. Although LMCA models have the advantage of providing excellent curve fits and can in principle determine more physiological parameters than BPCA methods, they yield more inaccurate results.
Capillaries form a microvascular network that supplies the tissue with nutrients. Changes in microcirculation can be used as a diagnostic indicator of abnormal lesions and to optimize the treatment of disease . For example, the fractional tissue blood volume, vb, which is defined as the volume fraction inside the capillary bed , represents one of the most valuable parameters for noninvasive tumor grading and can be estimated using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) [3, 4].
Isolectin staining of the medial thigh muscle labeling endothelium enabled determination of the vascular area (Fig 1). A clear separation of the different compartments is apparent. However, owing to collapse of blood vessels due to the disappearance of blood pressure, this method most likely underestimates the in vivo blood volume. The percentage of the segmented vascular area relative to the total muscle area was compared to the in vivo blood volume determined by MRI, see Table 1. Table 1 and Fig 5 present the vb values obtained with the two histological (IHVS and MVDE) and with four different MRI techniques (BD, EqMRI, ETM, and 2CXM) in different muscle groups  of the pigs’ hind legs.
In the present study, we found good agreement between the results obtained with MRI and histology and between the results obtained using the different methods and models with the exception of IHVS histology and the ETM. For all techniques and methods, consistently different vb values were found in the different muscle areas investigated. The ETM yielded unrealistic low blood volumes of less than one percent.
Our results show good agreement between the median values obtained by histology and the BPCA MRI methods and the 2CXM LMCA approach. However, the ETM yielded results that were completely different from the histological vb values. Using BPCA methods for measuring blood volume has the crucial advantage of simple implementation, very short acquisition times, and a stable and accurate measurement. While LMCA models provide excellent curve fits and can in principle determine more physiological parameters than BPCA methods, they yield more inaccurate individual results.