Quantitative T2* imaging of metastatic human breast cancer to brain in the nude rat at 3 T

Accelerated 3D multi-echo spin-echo sequence with a subspace constrained reconstruction for whole mouse brain T 2 mapping

PURPOSE

To accelerate whole-brain quantitativeT 2 $$ {\mathrm{T}}_2 $$ mapping in preclinical imaging setting.

METHODS

A three-dimensional (3D) multi-echo spin echo sequence was highly undersampled with a variable density Poisson distribution to reduce the acquisition time. Advanced iterative reconstruction based on linear subspace constraints was employed to recover high-quality raw images. Different subspaces, generated using exponential or extended-phase graph (EPG) simulations or from low-resolution calibration images, were compared. The subspace dimension was investigated in terms ofT 2 $$ {\mathrm{T}}_2 $$ precision. The method was validated on a phantom containing a wide range ofT 2 $$ {\mathrm{T}}_2 $$ and was then applied to monitor metastasis growth in the mouse brain at 4.7T. Image quality andT 2 $$ {\mathrm{T}}_2 $$ estimation were assessed for 3 acceleration factors (6/8/10).

RESULTS

The EPG-based dictionary gave robust estimations of a large range ofT 2 $$ {\mathrm{T}}_2 $$ . A subspace dimension of 6 was the best compromise betweenT 2 $$ {\mathrm{T}}_2 $$ precision and image quality. Combining the subspace constrained reconstruction with a highly undersampled dataset enabled the acquisition of whole-brainT 2 $$ {\mathrm{T}}_2 $$ maps, the detection and the monitoring of metastasis growth of less than 500μ m 3 $$ \mu {\mathrm{m}}^3 $$ .

CONCLUSION

Subspace-based reconstruction is suitable for 3DT 2 $$ {\mathrm{T}}_2 $$ mapping. This method can be used to reach an acceleration factor up to 8, corresponding to an acquisition time of 25 min for an isotropic 3D acquisition of 156μ $$ \mu $$ m on the mouse brain, used here for monitoring metastases growth.

Imaging angiogenesis, inflammation, and metastasis in the tumor microenvironment with magnetic resonance imaging

With the development of new imaging techniques, the potential for probing the molecular, cellular, and structural components of the tumor microenvironment in situ has increased dramatically. A multitude of imaging modalities have been successfully employed to probe different aspects of the tumor microenvironment, including expression of molecules, cell motion, cellularity, vessel permeability, vascular perfusion, metabolic and physiological changes, apoptosis, and inflammation. This chapter focuses on the most recent advances in magnetic resonance imaging methods, which offer a number of advantages over other methodologies, including high spatial resolution and the use of nonionizing radiation, as well as the use of such methods in the context of primary and secondary brain tumors. It also highlights how they can be used to assess the molecular and cellular changes in the tumor microenvironment in response to therapy.

High-field small animal magnetic resonance oncology studies

This review focuses on the applications of high magnetic field magnetic resonance imaging (MRI) and spectroscopy (MRS) to cancer studies in small animals. High-field MRI can provide information about tumor physiology, the microenvironment, metabolism, vascularity and cellularity. Such studies are invaluable for understanding tumor growth and proliferation, response to treatment and drug development. The MR techniques reviewed here include (1)H, (31)P, chemical exchange saturation transfer imaging and hyperpolarized (13)C MRS as well as diffusion-weighted, blood oxygen level dependent contrast imaging and dynamic contrast-enhanced MRI. These methods have been proven effective in animal studies and are highly relevant to human clinical studies.

Preclinical molecular imaging of the translocator protein (TSPO) in a metastases model based on breast cancer xenografts propagated in the murine brain

Previous studies have demonstrated the feasibility of translocator protein (TSPO) imaging to visualize and quantify human breast adenocarcinoma (MDA-MB-231) cells in vivo using a TSPO-targeted near-infrared (NIR) probe (NIR-conPK11195). This study aimed to extend the use of the TSPO-targeted probe to a more biologically relevant and clinically important tumor microenvironment as well as to assess our ability to longitudinally detect the presence and progression of breast cancer cells in the brain. The in vivo biodistribution and accumulation of NIR-conPK11195 and free (unconjugated) NIR dye were quantitatively evaluated in intracranial MDA-MB-231-bearing mice and non-tumor-bearing control mice longitudinally once a week from two to five weeks post-inoculation. The in vivo time-activity curves illustrate distinct clearance profiles for NIR-conPK11195 and free NIR dye, resulting in preferential accumulation of the TSPO-targeted probe in the intracranial tumor bearing hemisphere (TBH) with significant tumor contrast over normal muscle tissue (p < 0.005 at five weeks; p < 0.01 at four weeks). In addition, the TSPO-labeled TBHs demonstrated significant contrast over the TBHs of mice injected with free NIR dye (p < 0.001 at four and five weeks) as well as over the TSPO-labeled non-tumor-bearing hemispheres (NTBHs) of control mice (p < 0.005 at four and five weeks). Overall, TSPO-targeted molecular imaging appears useful for visualizing and quantifying breast cancer xenografts propagated in the murine brain and may assist in preclinical detection, diagnosis and monitoring of metastatic disease as well as drug discovery. Furthermore, these results indicate it should be possible to perform TSPO-imaging of breast cancer cells in the brain using radiolabeled TSPO-targeted agents, particularly in light of the fact that [11C]-labeled TSPO probes such as [11C]-PK 11195 have been successfully used to image gliomas in the clinic.

Longitudinal evaluation of MPIO-labeled stem cell biodistribution in glioblastoma using high resolution and contrast-enhanced MR imaging at 14.1 tesla

To optimize the development of stem cell (SC)-based therapies for the treatment of glioblastoma (GBM), we compared the pathotropism of 2 SC sources, human mesenchymal stem cells (hMSCs) and fetal neural stem cells (fNSCs), toward 2 orthotopic GBM models, circumscribed U87vIII and highly infiltrative GBM26. High resolution and contrast-enhanced (CE) magnetic resonance imaging (MRI) were performed at 14.1 Tesla to longitudinally monitor the in vivo location of hMSCs and fNSCs labeled with the same amount of micron-size particles of iron oxide (MPIO). To assess pathotropism, SCs were injected in the contralateral hemisphere of U87vIII tumor-bearing mice. Both MPIO-labeled SC types exhibited tropism to tumors, first localizing at the tumor edges, then in the tumor masses. MPIO-labeled hMSCs and fNSCs were also injected intratumorally in mice with U87vIII or GBM26 tumors to assess their biodistribution. Both SC types distributed throughout the tumor in both GBM models. Of interest, in the U87vIII model, areas of hyposignal colocalized first with the enhancing regions (ie, regions of high vascular permeability), consistent with SC tropism to vascular endothelial growth factor. In the GBM26 model, no rim of hyposignal was observed, consistent with the infiltrative nature of this tumor. Quantitative analysis of the index of dispersion confirmed that both MPIO-labeled SC types longitudinally distribute inside the tumor masses after intratumoral injection. Histological studies confirmed the MRI results. In summary, our results indicate that hMSCs and fNSCs exhibit similar properties regarding tumor tropism and intratumoral dissemination, highlighting the potential of these 2 SC sources as adequate candidates for SC-based therapies.

Targeting classical but not neurogenic inflammation reduces peritumoral oedema in secondary brain tumours

Dexamethasone, the standard treatment for peritumoral brain oedema, inhibits classical inflammation. Neurogenic inflammation, which acts via substance P (SP), has been implicated in vasogenic oedema in animal models of CNS injury. SP is elevated within and outside CNS tumours. This study investigated the efficacy of NK1 receptor antagonists, which block SP, compared with dexamethasone treatment, in a rat model of tumorigenesis. Dexamethasone reverted normal brain water content and reduced Evans blue and albumin extravasation, while NK1 antagonists did not ameliorate oedema formation. We conclude that classical inflammation rather than neurogenic inflammation drives peritumoral oedema in this brain tumour model.

Phase contrast MRI is an early marker of micrometastatic breast cancer development in the rat brain

The early growth of micrometastatic breast cancer in the brain often occurs through vessel co-option and is independent of angiogenesis. Remodeling of the existing vasculature is an important step in the evolution of co-opting micrometastases into angiogenesis-dependent solid tumor masses. The purpose of this study was to determine whether phase contrast MRI, an intrinsic source of contrast exquisitely sensitive to the magnetic susceptibility properties of deoxygenated hemoglobin, could detect vascular changes occurring independent of angiogenesis in a rat model of breast cancer metastases to the brain. Twelve nude rats were administered 10(6) MDA-MB-231BRL 'brain-seeking' breast cancer cells through intracardiac injection. Serial, multiparametric MRI of the brain was performed weekly until metastatic disease was detected. The results demonstrated that images of the signal phase (area under the receiver operating characteristic curve, 0.97) were more sensitive than T(2)* gradient echo magnitude images (area under the receiver operating characteristic curve, 0.73) to metastatic brain lesions. The difference between the two techniques was probably the result of the confounding effects of edema on the magnitude of the signal. A region of interest analysis revealed that vascular abnormalities detected with phase contrast MRI preceded tumor permeability measured with contrast-enhanced MRI by 1-2  weeks. Tumor size was correlated with permeability (R(2)= 0.23, p < 0.01), but phase contrast was independent of tumor size (R(2)= 0.03). Histopathologic analysis demonstrated that capillary endothelial cells co-opted by tumor cells were significantly enlarged, but less dense, relative to the normal brain vasculature. Although co-opted vessels were vascular endothelial growth factor-negative, vessels within larger tumor masses were vascular endothelial growth factor-positive. In conclusion, phase contrast MRI is believed to be sensitive to vascular remodeling in co-opting brain tumor metastases independent of sprouting angiogenesis, and may therefore aid in preclinical studies of angiogenic-independent tumors or in the monitoring of continued tumor growth following anti-angiogenic therapy. Published 2011. This article is a US Government work and is in the public domain in the USA.

Imaging and quantification of metastatic melanoma cells in lymph nodes with a ferritin MR reporter in living mice

Cellular MRI with a reporter gene offers the opportunity to track small numbers of tumor cells and to study metastatic processes in their earliest developmental stages in the target organs of interest. This study demonstrates the feasibility of using the MR reporter ferritin for the noninvasive imaging and quantification of metastatic melanoma cells in the lymph nodes (LNs) of living mice. A B16F10 murine melanoma cell line expressing human ferritin heavy chain (hFTH) and green fluorescent protein (GFP) was constructed to allow the detection of cells by MRI and fluorescence imaging. Stable overexpression of hFTH and GFP in B16F10 murine melanoma cells was feasible and showed no cellular toxicity. In addition, hFTH cells were detectable by 9.4-T MRI in  vitro and in  vivo, yielding significant changes in T(2)* relative to control cells. In BALB/c nude mice, the presence of hFTH- and GFP-expressing metastatic melanoma cells in deep-seated axillary LNs was demonstrated as areas of low T(2)* on MRI, but the same LNs were not visible by fluorescence imaging because the light was unable to penetrate the tissue. Furthermore, the metastatic volume of each LN, which was assessed by cumulative histogram analysis of the T(2)* MRI data, correlated well with tumor burden, which was determined by histology (r = -0.8773, p = 0.0001). This study is the first to use MRI and an MR reporter gene for both the visualization and quantification of metastatic cancer cells in LNs.

Differential microstructure and physiology of brain and bone metastases in a rat breast cancer model by diffusion and dynamic contrast enhanced MRI

Pharmacological approaches to treat breast cancer metastases in the brain have been met with limited success. In part, the impermeability of the blood brain barrier (BBB) has hindered delivery of chemotherapeutic agents to metastatic tumors in the brain. BBB-permeable chemotherapeutic drugs are being developed, and noninvasively assessing the efficacy of these agents will be important in both preclinical and clinical settings. In this regard, dynamic contrast enhanced (DCE) and diffusion weighted imaging (DWI) are magnetic resonance imaging (MRI) techniques to monitor tumor vascular permeability and cellularity, respectively. In a rat model of metastatic breast cancer, we demonstrate that brain and bone metastases develop with distinct physiological characteristics as measured with MRI. Specifically, brain metastases have limited permeability of the BBB as assessed with DCE and an increased apparent diffusion coefficient (ADC) measured with DWI compared to the surrounding brain. Microscopically, brain metastases were highly infiltrative, grew through vessel co-option, and caused extensive edema and injury to the surrounding neurons and their dendrites. By comparison, metastases situated in the leptomenengies or in the bone had high vascular permeability and significantly lower ADC values suggestive of hypercellularity. On histological examination, tumors in the bone and leptomenengies were solid masses with distinct tumor margins. The different characteristics of these tissue sites highlight the influence of the microenvironment on metastatic tumor growth. In light of these results, the suitability of DWI and DCE to evaluate the response of chemotherapeutic and anti-angiogenic agents used to treat co-opted brain metastases, respectively, remains a formidable challenge.

The biology of brain metastases-translation to new therapies

Brain metastases are a serious obstacle in the treatment of patients with solid tumors and contribute to the morbidity and mortality of these cancers. It is speculated that the frequency of brain metastasis is increasing for several reasons, including improved systemic therapy and survival, and detection of metastases in asymptomatic patients. The lack of preclinical models that recapitulate the clinical setting and the exclusion of patients with brain metastases from most clinical trials have slowed progress. Molecular factors contributing to brain metastases are being elucidated, such as genes involved in cell adhesion, extravasation, metabolism, and cellular signaling. Furthermore, the role of the unique brain microenvironment is beginning to be explored. Although the presence and function of the blood-brain barrier in metastatic tumors is still poorly understood, it is likely that some tumor cells are protected from therapeutics by the blood-tumor barrier, creating a sanctuary site. This Review discusses what is known about the biology of brain metastases, what preclinical models are available to study the disease, and which novel therapeutic strategies are being studied in patients.