Rationale and Objectives
The aims of this study were to evaluate the morphologic characteristics and growth pattern of hepatic tumors in H- ras 12V transgenic (TG) mice using a micro–magnetic resonance (MR) system and to assess the usefulness of gadolinium ethoxybenzyl diethylenetriamine penta-acetic acid (Gd-EOB-DTPA) enhancement for the detection of hepatic tumors in these mice.
Materials and Methods
Hepatocellular carcinoma lines were established to allow insertion of the H- ras 12V transgene under the control of the albumin enhancer/promoter. Seven H- ras 12V TG mice and four wild-type mice were included in this study. The mice underwent various MR imaging examinations, including T1-weighted imaging (repetition time, 300 ms; echo time, 11 ms), Gd-EOB-DTPA-enhanced T1-weighted imaging (dose, 0.025 mmol/kg), and T2-weighted imaging (repetition time, 3500 ms; echo time, 36 ms), with a 4.7-T MR scanner, at 4, 6, 8, and 9 months of age. All mice were euthanized after the final MR imaging procedure, except for one TG mouse and two wild-type mice that were euthanized after MR imaging procedures at 4 months of age. For imaging analysis, the tumor characteristics in each MR sequence, including tumor size, number, and signal intensity (SI), were recorded, and the contrast-to-noise ratio and contrast enhancement ratio were calculated to quantify the SI of the tumor. The MR images were correlated with the findings of histopathologic examinations.
Results
No tumors were detected in the four wild-type mice. In the six TG mice, a total of 67 tumors were found in histopathologic specimens obtained at 9 months of age. Of the 67 tumors, 62 were detected on Gd-EOB-DTPA-enhanced T1-weighted images with fat saturation. The majority of hepatic tumors showed high SI on T1-weighted images without fat saturation. The SI diminished on T1-weighted images with fat saturation. The tumor contrast-to-noise ratio for Gd-EOB-DTPA-enhanced T1-weighted imaging was significantly better than that for the other sequences. The tumors were histopathologically confirmed as hepatocellular adenomas ( n = 32) and well-differentiated hepatocellular carcinomas ( n = 35).
Conclusions
Micro-MR imaging can reveal the characteristics of hepatic tumors in a live murine model. Gd-EOB-DTPA-enhanced T1-weighted imaging is helpful in the detection of hepatic tumors in H- ras 12V TG mice.
Hepatocellular carcinoma (HCC) is a common malignancy in humans . Mutations of the ras gene have been detected in many human tumors. This gene encodes a small, signal-transducing guanosine triphosphatase that regulates the signaling pathways that control cell growth, differentiation, and survival . Although ras mutations are rare in human liver tumors , receptor-mediated hyperactivation of ras -dependent signal transduction pathways frequently occurs in human hepatocarcinogenesis . The three types of ras include N- ras , K- ras , and H- ras . Among these, murine HCCs express H- ras , which is activated in 70% of these cases; this finding suggests that H- ras plays a key role in hepatocarcinogenesis . Recently, H- ras transgenic (TG) mice were generated by using an H- ras 12V construct with a mouse albumin enhancer/promoter . The H- ras 12V TG mouse has 2 main advantages: (1) liver-specific expression and (2) long-term survival because of low expression levels of H- ras .
In vivo imaging technologies provide a unique opportunity for noninvasive and quantitative molecular-level analyses of diseases and for repeated and noninvasive monitoring of disease progression and/or response to treatment. The currently used small-animal model imaging technologies include magnetic resonance (MR), radionuclide imaging, computed tomography, ultrasonography, and optical imaging . Among these modalities, micro-MR imaging affords good microscopic resolution (micrometer-range resolution) and provides tissue-specific information when used with tissue-specific contrast agents . Tissue-specific liver MR imaging is useful for detecting and characterizing tumors. Hepatocyte-specific contrast agents are preferred over reticuloendothelial cell–specific contrast agents because the former offer superior tumor detection and lesion discrimination in murine hepatic tumors . In addition, the differences in the enhancement shown by hepatocyte-specific contrast agents can indicate additional tumor differentiation .
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Materials and methods
Generation of H- ras 12V TG Mice
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Animals
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MR Imaging
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Imaging Analysis and Statistical Analysis
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CNR=(SIofthetumor−SIoftheliver)/SIofbackmuscle, CNR
=
(
SI
of
the
tumor
−
SI
of
the
liver
)
/
SI
of
back
muscle
,
and
contrastenhancementratio=(SIonenhancedT1−weightedimaging−SIonnonenhancedT1−weightedimagingwithfatsaturation)/SIonnonenhancedT1−weightedimagingwithfatsaturation. contrast
enhancement
ratio
=
(
SI
on
enhanced
T
1
-
weighted
imaging
-
SI
on
nonenhanced
T
1
-
weighted
imaging
with
fat
saturation
)
/
SI
on
nonenhanced
T
1
-
weighted
imaging
with
fat
saturation
.
On an enhanced image, a CNR value < 0 indicates negative contrast enhancement, and a CNR value > 0 indicates positive contrast enhancement.
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Histopathology
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Results
MR Imaging
Number and Sizes of Tumors Depicted on MR Images
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Table 1
Mean Sizes (mm) of Hepatic Tumors in H- ras 12V Transgenic Mice on Magnetic Resonance Images
Age (months) Mouse 4 6 8 9 1 1.20 1.80 ± 0.81 5.45 ± 3.28 6.81 ± 3.89 2 0.60 2.10 ± 1.32 4.43 ± 3.02 4.78 ± 2.45 3 — 2.10 4.45 ± 2.55 4.80 ± 3.44 4 1.10 2.13 ± 1.27 4.44 ± 1.95 4.28 ± 2.79 5 1.20 ± 0.17 2.43 ± 1.17 4.10 ± 2.16 4.36 ± 2.71 6 — 1.60 ± 0.36 3.58 ± 2.43 3.76 ± 2.96 Mean 1.08 ± 0.26 2.11 ± 1.02 4.44 ± 2.48 4.79 ± 3.06
Data are expressed as mean ± SD.
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Morphologic Characteristics Depicted on MR Images
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Morphologic Changes Depicted on MR Images
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Histopathology-MR Correlation
Number and Sizes of Tumors
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Table 2
Detectability of Hepatic Tumors in H- ras 12V Transgenic Mice on Magnetic Resonance Images
Number of Tumors (Detectability) Tumor Size (mm) Histopathology T1WIs without Fat Saturation T2WIs with Fat Saturation Gd-EOD-DTPA–Enhanced T1WIs >2 50 49 (98.0%) 45 (90.0%) 50 (100%) 0.4–2 17 3 (17.6%) 1 (5.9%) 12 (70.6%) Total 67 52 (77.6%) 46 (68.7%) 62 (92.5%)
Gd-EOB-DTPA, gadolinium ethoxybenzyl diethylenetriamine penta-acetic acid; T1WI, T1-weighted image; T2WI, T2-weighted image.
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Morphological Characteristics of Tumors
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Discussion
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Conclusions
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