Detection and Localization of Proteinuria by Dynamic Contrast-Enhanced Magnetic Resonance Imaging Using MS325

Four-dimensional MRI of renal function in the developing mouse

The major roles of filtration, metabolism and high blood flow make the kidney highly vulnerable to drug-induced toxicity and other renal injuries. A method to follow kidney function is essential for the early screening of toxicity and malformations. In this study, we acquired high spatiotemporal resolution (four dimensional) datasets of normal mice to follow changes in kidney structure and function during development. The data were acquired with dynamic contrast-enhanced MRI (via keyhole imaging) and a cryogenic surface coil, allowing us to obtain a full three-dimensional image (isotropic resolution, 125 microns) every 7.7 s over a 50-min scan. This time course permitted the demonstration of both contrast enhancement and clearance. Functional changes were measured over a 17-week course (at 3, 5, 7, 9, 13 and 17 weeks). The time dimension of the MRI dataset was processed to produce unique image contrasts to segment the four regions of the kidney: cortex (CO), outer stripe (OS) of the outer medulla (OM), inner stripe (IS) of the OM and inner medulla (IM). Local volumes, time-to-peak (TTP) values and decay constants (DC) were measured in each renal region. These metrics increased significantly with age, with the exception of DC values in the IS and OS. These data will serve as a foundation for studies of normal renal physiology and future studies of renal diseases that require early detection and intervention.

Prediction and assessment of responses to renal artery revascularization with dynamic contrast-enhanced magnetic resonance imaging: a pilot study

The aim of this study was to assess the potential of dynamic contrast-enhanced (DCE) MRI to predict and evaluate functional outcomes after renal artery revascularization for renal artery stenosis (RAS). The single-kidney glomerular filtration rate (SK-GFR) was measured in 15 patients with atherosclerotic RAS with DCE-MRI and radioisotopes at baseline and 4 mo after revascularization. DCE-MRI also produced measurements of blood flow, blood volume, extraction fraction, tubular transit time, and functional volume. Stented kidneys (n = 22) were divided into three response groups on the basis of the changes in radioisotope SK-GFR: improved (n = 5), stable (n = 13), and deteriorated (n = 4). A good agreement was found between SK-GFR values from DCE-MRI and radioisotopes (correlation coefficient: 0.91). Before intervention, kidneys that improved had lower extraction fraction, higher blood volume, longer tubular transit time, and lower SK-GFR. After intervention, improved kidneys had increased functional volume, and deteriorated kidneys had reduced functional volume and extraction fraction. Revascularization improved blood flow and blood volume in all groups. This pilot study led to the hypothesis that well-vascularized kidneys with reduced extraction fractions are most likely to benefit from revascularization. More generally, DCE-MRI has the potential to replace radioisotope measurement of SK-GFR and may improve patient management by providing additional information on tissue perfusion.

In vivo detection of PARACEST agents with relaxation correction

Several pulse sequences have been used to detect paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents in animals to quantify the uptake over time following a bolus injection. The observed signal change is a combination of relaxation effects and PARACEST contrast. The purpose of the current study was to isolate the PARACEST effect from the changes in bulk water relaxation induced by the PARACEST agent in vivo for the fast low-angle shot pulse sequence. A fast low-angle shot–based pulse sequence was used to acquire continuous images on a 9.4-T MRI of phantoms and the kidneys of mice following PARACEST agent (Tm³⁺-DOTAM-Gly-Lys) injection. A WALTZ-16 pulse was applied before every second image to generate on-resonance paramagnetic chemical exchange effects. Signal intensity changes of up to 50% were observed in the mouse kidney in the control images (without a WALTZ-16 preparation pulse) due to altered bulk water relaxation induced by the PARACEST agent. Despite these changes, a clear on-resonance paramagnetic chemical exchange effect of 4-7% was also observed. A four-pool exchange model was used to describe image signal intensity. This study demonstrates that in vivo on-resonance paramagnetic chemical exchange effect contrast can be isolated from tissue relaxation time constant changes induced by a PARACEST agent that dominate the signal change. Magn Reson Med 63:1184–1192, 2010. © 2010 Wiley-Liss, Inc.

Technical aspects of MR perfusion

The most common methods for measuring perfusion with MRI are arterial spin labelling (ASL), dynamic susceptibility contrast (DSC-MRI), and T(1)-weighted dynamic contrast enhancement (DCE-MRI). This review focuses on the latter approach, which is by far the most common in the body and produces measures of capillary permeability as well. The aim is to present a concise but complete overview of the technical issues involved in DCE-MRI data acquisition and analysis. For details the reader is referred to the references. The presentation of the topic is essentially generic and focuses on technical aspects that are common to all DCE-MRI measurements. For organ-specific problems and illustrations, we refer to the other papers in this issue. In Section 1 "Theory" the basic quantities are defined, and the physical mechanisms are presented that provide a relation between the hemodynamic parameters and the DCE-MRI signal. Section 2 "Data acquisition" discusses the issues involved in the design of an optimal measurement protocol. Section 3 "Data analysis" summarizes the steps that need to be taken to determine the hemodynamic parameters from the measured data.

In vivo performance of a liposomal vascular contrast agent for CT and MR-based image guidance applications

PURPOSE

This study evaluated the in vivo performance of a liposome formulation that co-encapsulates iohexol and gadoteridol as a multimodal contrast agent for computed tomography (CT) and magnetic resonance (MR)-based image guidance applications.

MATERIALS AND METHODS

The pharmacokinetics and biodistribution studies were conducted in Balb-C mice using high performance liquid chromatography (HPLC) and inductively coupled plasma atomic emission spectrometry (ICP-AES) to detect iohexol and gadoteridol concentrations. The imaging efficacy of this liposome system was assessed in New Zealand White rabbits using a clinical CT and a clinical 1.5 Tesla MR scanner.

RESULTS

The vascular half-lives of the liposome encapsulated iohexol and gadoteridol in mice were found to be 18.4 +/- 2.4 and 18.1 +/- 5.1 h. When administered at the same dose the distribution (alpha phase) half-lives for the free contrast agents were 12.3 +/- 0.5 min (iohexol) and 7.6 +/- 0.9 min (gadoteridol); while, the elimination (beta phase) half-lives were 3.0 +/- 0.9 h for free iohexol and 3.0 +/- 1.3 h for free gadoteridol. The CT and MR signal increases were measured and correlated with the concentrations of iohexol and gadoteridol detected in plasma samples.

CONCLUSION

The long in vivo circulation lifetime and simultaneous CT and MR signal enhancement provided by this liposome system make it a promising agent for image guidance applications.

Contrast agents for functional and cellular MRI of the kidney

Low-molecular-weight gadolinium (Gd) chelates are glomerular tracers but their role in evaluation of renal function with magnetic resonance (MR) imaging is still marginal. Because of their small size, they diffuse freely into the interstitium and the relationship between measured signal intensity and concentration is complex. New categories of contrast agents, such as large Gd-chelates or iron oxide particules, with different pharmacokinetic and magnetic properties have been developed. These large molecules could be useful for both functional (quantification of perfusion, quantification of glomerular filtration rate, estimation of tubular function) and cellular imaging (intrarenal phagocytosis in inflammatory renal diseases). Continuous development of new contrast agents remains worthwhile to get the best adequacy between the physiological phenomenon of interest and the pharmacokinetic of the agent.

Macromolecular MRI contrast agents for imaging tumor angiogenesis

Angiogenesis has long been accepted as a vital process in the growth and metastasis of tumors. As a result it is the target of several novel anti-cancer medications. Consequently, there is an urgent clinical need to develop accurate, non-invasive imaging techniques to improve the characterization of tumor angiogenesis and the monitoring of the response to anti-angiogenic therapy. Macromolecular MR contrast media (MMCM) offer this diagnostic potential by preferentially exploiting the inherent hyperpermeable nature of new tumor vessels compared with normal vessels. Over the last 10-15 years many classes of MMCM have been developed. When evaluated with dynamic contrast enhanced (DCE) MRI, a number of MMCM have demonstrated in vivo imaging properties that correlate with ex vivo histological features of angiogenesis. The enhancement patterns with some MMCM have been reported to correlate with tumor grade, as well as show response to anti-angiogenic and anti-vascular drugs. Future applications of MMCM include targeted angiogenesis imaging and drug delivery of anti-cancer 'payloads'. Herein we discuss the best known MMCMs along with their advantages and disadvantages.