Free-breathing 3D diffusion MRI for high-resolution hepatic metastasis characterization in small animals

Respiratory Motion Mitigation and Repeatability of Two Diffusion-Weighted MRI Methods Applied to a Murine Model of Spontaneous Pancreatic Cancer

Respiratory motion and increased susceptibility effects at high magnetic fields pose challenges for quantitative diffusion-weighted MRI (DWI) of a mouse abdomen on preclinical MRI systems. We demonstrate the first application of radial k-space-sampled (RAD) DWI of a mouse abdomen using a genetically engineered mouse model of pancreatic ductal adenocarcinoma (PDAC) on a 4.7 T preclinical scanner equipped with moderate gradient capability. RAD DWI was compared with the echo-planar imaging (EPI)-based DWI method with similar voxel volumes and acquisition times over a wide range of b-values (0.64, 535, 1071, 1478, and 2141 mm²/s). The repeatability metrics are assessed in a rigorous test-retest study (n = 10 for each DWI protocol). The four-shot EPI DWI protocol leads to higher signal-to-noise ratio (SNR) in diffusion-weighted images with persisting ghosting artifacts, whereas the RAD DWI protocol produces relatively artifact-free images over all b-values examined. Despite different degrees of motion mitigation, both RAD DWI and EPI DWI allow parametric maps of apparent diffusion coefficients (ADC) to be produced, and the ADC of the PDAC tumor estimated by the two methods are 1.3 ± 0.24 and 1.5 ± 0.28 × 10^-3 mm²/s, respectively (p = 0.075, n = 10), and those of a water phantom are 3.2 ± 0.29 and 2.8 ± 0.15 × 10^-3 mm²/s, respectively (p = 0.001, n = 10). Bland-Altman plots and probability density function reveal good repeatability for both protocols, whose repeatability metrics do not differ significantly. In conclusion, RAD DWI enables a more effective respiratory motion mitigation but lower SNR, while the performance of EPI DWI is expected to improve with more advanced gradient hardware.

Radial MP2RAGE sequence for rapid 3D T1 mapping of mouse abdomen: application to hepatic metastases

OBJECTIVES

The T₁ longitudinal recovery time is regarded as a biomarker of cancer treatment efficiency. In this scope, the Magnetization Prepared 2 RApid Gradient Echo (MP2RAGE) sequence relevantly complies with fast 3D T₁ mapping. Nevertheless, with its Cartesian encoding scheme, it is very sensitive to respiratory motion. Consequently, a radial encoding scheme was implemented for the detection and T₁ measurement of hepatic metastases in mice at 7T.

METHODS

A 3D radial encoding scheme was developed using a golden angle distribution for the k-space trajectories. As in that case, each projection contributes to the image contrast, the signal equations had to be modified. Phantoms containing increasing gadoteridol concentrations were used to determine the accuracy of the sequence in vitro. Healthy mice were repetitively scanned to assess the reproducibility of the T₁ values. The growth of hepatic metastases was monitored. Undersampling robustness was also evaluated.

RESULTS

The accuracy of the T₁ values obtained with the radial MP2RAGE sequence was > 90% compared to the Inversion-Recovery sequence. The motion robustness of this new sequence also enabled repeatable T₁ measurements on abdominal organs. Hepatic metastases of less than 1-mm diameter were easily detected and T₁ heterogeneities within the metastasis and between the metastases within the same animal were measured. With a twofold acceleration factor using undersampling, high-quality 3D T₁ abdominal maps were achieved in 9 min.

CONCLUSIONS

The radial MP2RAGE sequence could be used for fast 3D T₁ mapping, to detect and characterize metastases in regions subjected to respiratory motion.

KEY POINTS

• The Cartesian encoding of the MP2RAGE sequence was modified to a radial encoding. The modified sequence enabled accurate T ₁ measurements on phantoms and on abdominal organs of mice. • Hepatic metastases were easily detected due to high contrast. Heterogeneity in T ₁ was measured within the metastases and between each metastasis within the same animal. • As implementation of this sequence does not require specific hardware, we expect that it could be readily available for clinical practice in humans.

A novel mouse model of human prostate cancer to study intraprostatic tumor growth and the development of lymph node metastases

BACKGROUND

In this study, we aimed to establish a versatile in vivo model of prostate cancer, which adequately mimics intraprostatic tumor growth, and the natural routes of metastatic spread. In addition, we analyzed the capability of high-resolution ultrasonography (hrUS), in vivo micro-CT (μCT), and 9.4T MRI to monitor tumor growth and the development of lymph node metastases.

METHODS

A total of 5 × 10⁵ VCaP cells or 5 × 10⁵ cells of LuCaP136- or LuCaP147 spheroids were injected into the prostate of male CB17-SCID mice (n = 8 for each cell type). During 12 weeks of follow-up, orthotopic tumor growth, and metastatic spread were monitored by repetitive serum-PSA measurements and imaging studies including hrUS, μCT, and 9.4T MRI. At autopsy, primary tumors and metastases were harvested and examined by histology and immunohistochemistry (CK5, CK8, AMACR, AR, Ki67, ERG, and PSA). From imaging results and PSA-measurements, tumor volume doubling time, tumor-specific growth rate, and PSA-density were calculated.

RESULTS

All 24 mice developed orthotopic tumors. The tumor growth could be reliably monitored by a combination of hrUS, μCT, MRI, and serum-PSA measurements. In most animals, lymph node metastases could be detected after 12 weeks, which could also be well visualized by hrUS, and MRI. Immunohistochemistry showed positive signals for CK8, AMACR, and AR in all xenograft types. CK5 was negative in VCaP- and focally positive in LuCaP136- and LuCaP147-xenografts. ERG was positive in VCaP- and negative in LuCaP136- and LuCaP147-xenografts. Tumor volume doubling times and tumor-specific growth rates were 21.2 days and 3.9 %/day for VCaP-, 27.6 days and 3.1 %/day for LuCaP136- and 16.2 days and 4.5 %/day for LuCaP147-xenografts, respectively. PSA-densities were 433.9 ng/mL per milliliter tumor for VCaP-, 6.5 ng/mL per milliliter tumor for LuCaP136-, and 11.2 ng/mL per milliliter tumor for LuCaP147-xenografts.

CONCLUSIONS

By using different monolayer and 3D spheroid cell cultures in an orthotopic xenograft model, we established an innovative, versatile in vivo model of prostate cancer, which enables the study of both intraprostatic tumor growth as well as metastatic spread to regional lymph nodes. HrUS and MRI are feasible tools to monitor tumor growth and the development of lymph node metastases while these cannot be visualized by μCT.

State of the art in vivo imaging techniques for laboratory animals

In recent decades, imaging devices have become indispensable tools in the basic sciences, in preclinical research and in modern drug development. The rapidly evolving high-resolution in vivo imaging technologies provide a unique opportunity for studying biological processes of living organisms in real time on a molecular level. State of the art small-animal imaging modalities provide non-invasive images rich in quantitative anatomical and functional information, which renders longitudinal studies possible allowing precise monitoring of disease progression and response to therapy in models of different diseases. The number of animals in a scientific investigation can be substantially reduced using imaging techniques, which is in full compliance with the ethical endeavours for the 3R (reduction, refinement, replacement) policies formulated by Russell and Burch; furthermore, biological variability can be alleviated, as each animal serves as its own control. The most suitable and commonly used imaging modalities for in vivo small-animal imaging are optical imaging (OI), ultrasonography (US), computed tomography (CT), magnetic resonance imaging (MRI), and finally the methods of nuclear medicine: positron emission tomography (PET) and single photon emission computed tomography (SPECT).

Use of non-invasive imaging to monitor response to aflibercept treatment in murine models of colorectal cancer liver metastases

The liver is the most frequent metastatic site in colorectal cancer (CRC), and relevant orthotopic in vivo models are needed to study the efficacy of anticancer drugs in the metastatic setting. A challenge when utilizing such models is monitoring tumor growth during the experiments. In this study, experimental liver metastases were established in nude mice by splenic injection of the CRC cell lines HT29 and HCT116, and the mice were treated with the antiangiogenic drug aflibercept. Tumor growth was monitored using magnetic resonance imaging (MRI) and bioluminescence imaging (BLI). Aflibercept treatment was well tolerated and resulted in increased animal survival in HCT116, but not in HT29, while inhibited tumor growth was observed in both models. Treatment efficacy was monitored with high precision using MRI, while BLI detected small-volume disease with high sensitivity, but was less accurate in end-stage disease. Apparent diffusion coefficient (ADC) values obtained by diffusion weighted MRI (DW-MRI) were highly predictive of treatment response, with increased ADC corresponding well with areas of necrosis observed by histological evaluation of aflibercept-treated xenografts. The results showed that the efficacy of the antiangiogenic drug aflibercept varied between the two models, possibly reflecting unique growth patterns in the liver that may be representative of human disease. Non-invasive imaging, especially MRI and DW-MRI, can be used to effectively monitor tumor growth and treatment response in orthotopic liver metastasis models.