Contrast enhanced susceptibility weighted imaging (CE-SWI) of the mouse brain using ultrasmall superparamagnetic ironoxide particles (USPIO)

Cerebrovascular MRI in the mouse without an exogenous contrast agent

PURPOSE

To assess the effect of a variety of anesthetic regimes on T 2 ∗ -weighted MRI of the mouse brain and to determine the optimal regimes to perform T 2 ∗ -weighted MRI of the mouse cerebrovasculature without a contrast agent.

METHODS

Twenty mice were imaged with a 3D T 2 ∗ -weighted sequence under isoflurane, dexmedetomidine, or ketamine-xylazine anesthesia with a fraction of inspired oxygen varied between 10% and 95% + 5% CO2 . Some mice were also imaged after an injection of an iron oxide contrast agent as a positive control. For every regime, whole brain vessel conspicuity was visually assessed and the apparent vessel density in the cortex was quantified and compared.

RESULTS

The commonly used isoflurane anesthetic leads to poor vessel conspicuity for fraction of inspired oxygen higher or equal to 21%. Dexmedetomidine and ketamine-xylazine enable the visualization of a significantly larger portion of the vasculature for the same breathing gas. Under isoflurane anesthesia, the fraction of inspired oxygen must be lowered to between 10% and 14% to obtain similar vessel conspicuity. Initial results on automatic segmentation of veins and arteries using the iron oxide positive control are also reported.

CONCLUSION

T 2 ∗ -weighted MRI in combination with an appropriate anesthetic regime can be used to visualize the mouse cerebrovasculature without a contrast agent. The differences observed between regimes are most likely caused by blood-oxygen level dependent effects, highlighting the important impact of the anesthetic regimes on cerebral blood oxygenation of the mouse brain at rest.

The capability of detecting small vessels beyond the conventional MRI sensitivity using iron-based contrast agent enhanced susceptibility weighted imaging

Imaging brain microvasculature is important in cerebrovascular diseases. However, there is still a lack of non-invasive, non-radiation, and whole-body imaging techniques to investigate them. The aim of this study is to develop an ultra-small superparamagnetic iron oxide (USPIO) enhanced susceptibility weighted imaging (SWI) method for imaging micro-vasculature in both animal (~10 μm in rat) and human brain. We hypothesized that the USPIO-SWI technique could improve the detection sensitivity of the diameter of small subpixel vessels 10-fold compared with conventional MRI methods. Computer simulations were first performed with a double-cylinder digital model to investigate the theoretical basis for this hypothesis. The theoretical results were verified using in vitro phantom studies and in vivo rat MRI studies (n = 6) with corresponding ex vivo histological examinations. Additionally, in vivo human studies (n = 3) were carried out to demonstrate the translational power of the USPIO-SWI method. By directly comparing the small vessel diameters of an in vivo rat using USPIO-SWI with the small vessel diameters of the corresponding histological slide using laser scanning confocal microscopy, 13.3-fold and 19.9-fold increases in SWI apparent diameter were obtained with 5.6 mg Fe/kg and 16.8 mg Fe/kg ferumoxytol, respectively. The USPIO-SWI method exhibited its excellent ability to detect small vessels down to about 10 μm diameter in rat brain. The in vivo human study unveiled hidden arterioles and venules and demonstrated its potential in clinical practice. Theoretical modeling simulations and in vitro phantom studies also confirmed a more than 10-fold increase in the USPIO-SWI apparent diameter compared with the actual small vessel diameter size. It is feasible to use SWI blooming effects induced by USPIO to detect small vessels (down to 10 μm in diameter for rat brain), well beyond the spatial resolution limit of conventional MRI methods. The USPIO-SWI method demonstrates higher potential in cerebrovascular disease investigations.

Contrast-enhanced susceptibility weighted imaging with ultrasmall superparamagnetic iron oxide improves the detection of tumor vascularity in a hepatocellular carcinoma nude mouse model

PURPOSE

To evaluate the effectiveness of contrast-enhanced susceptibility-weighted imaging with ultrasmall superparamagnetic iron oxide (USPIO-enhanced SWI) in the assessment of intratumoral vascularity in hepatocellular carcinoma (HCC).

MATERIALS AND METHODS

Orthotopic xenograft HCC nude mouse models were established first and magnetic resonance imaging (MRI) examinations were performed on a 1.5T MR scanner 28 days later. Three groups of mice, 10 in each, were imaged using unenhanced and USPIO-enhanced SWI at doses of 4, 8, and 12 mg Fe/kg. Intratumoral susceptibility signal intensity (ITSS) was scored. ITSS-to-tumor contrast-to-noise ratio (ITSST-CNR) was measured. These measurements were compared between unenhanced and USPIO-enhanced SWI at each dose and differences in the measurements between different dose groups were estimated. Correlation between ITSS and tumor microvessel density (MVD) was analyzed.

RESULTS

Compared with unenhanced SWI, significantly higher ITSS was identified on USPIO-enhanced SWI at doses of 8 mg Fe/kg (Z = -2.000, P = 0.046) and 12 mg Fe/kg (Z = -2.333, P = 0.020). Significantly higher ITSST-CNR was found on USPIO-enhanced SWI than that on unenhanced SWI (P < 0.05). Significantly higher ITSST-CNR at a dose of 8 mg Fe/kg was observed than that at 4 mg Fe/kg (Z = -3.326, P = 0.001). Positive correlation between ITSS on USPIO-enhanced SWI at a dose of 8 mg Fe/kg and tumor MVD was demonstrated (r = 0.817, P = 0.004).

CONCLUSION

USPIO-enhanced SWI at a dose of 8 mg Fe/kg greatly improves the detection of intratumoral vascularity in a xenograft HCC model. J. Magn. Reson. Imaging 2016;44:288-295.

MRI in rodent models of brain disorders

Magnetic resonance imaging (MRI) is a well-established tool in clinical practice and research on human neurological disorders. Translational MRI research utilizing rodent models of central nervous system (CNS) diseases is becoming popular with the increased availability of dedicated small animal MRI systems. Projects utilizing this technology typically fall into one of two categories: 1) true "pre-clinical" studies involving the use of MRI as a noninvasive disease monitoring tool which serves as a biomarker for selected aspects of the disease and 2) studies investigating the pathomechanism of known human MRI findings in CNS disease models. Most small animal MRI systems operate at 4.7-11.7 Tesla field strengths. Although the higher field strength clearly results in a higher signal-to-noise ratio, which enables higher resolution acquisition, a variety of artifacts and limitations related to the specific absorption rate represent significant challenges in these experiments. In addition to standard T1-, T2-, and T2*-weighted MRI methods, all of the currently available advanced MRI techniques have been utilized in experimental animals, including diffusion, perfusion, and susceptibility weighted imaging, functional magnetic resonance imaging, chemical shift imaging, heteronuclear imaging, and (1)H or (31)P MR spectroscopy. Selected MRI techniques are also exclusively utilized in experimental research, including manganese-enhanced MRI, and cell-specific/molecular imaging techniques utilizing negative contrast materials. In this review, we describe technical and practical aspects of small animal MRI and provide examples of different MRI techniques in anatomical imaging and tract tracing as well as several models of neurological disorders, including inflammatory, neurodegenerative, vascular, and traumatic brain and spinal cord injury models, and neoplastic diseases.

Susceptibility-weighted imaging: technical aspects and clinical applications, part 1

Susceptibility-weighted imaging (SWI) is a new neuroimaging technique, which uses tissue magnetic susceptibility differences to generate a unique contrast, different from that of spin density, T1, T2, and T2*. In this review (the first of 2 parts), we present the technical background for SWI. We discuss the concept of gradient-echo images and how we can measure local changes in susceptibility. Armed with this material, we introduce the steps required to transform the original magnitude and phase images into SWI data. The use of SWI filtered phase as a means to visualize and potentially quantify iron in the brain is presented. Advice for the correct interpretation of SWI data is discussed, and a set of recommended sequence parameters for different field strengths is given.