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High fields magnetic resonance imaging (MRI) experiments on humans have been historically limited by the so called “penetration effect” of B1 and by the power deposition in living tissues. The first effect refers to the non-homogeneous value of B1 field inside the sample (important when the wavelength of the r.f. field approaches the dimension of the sample i.e. when the Larmor frequency increase above 10–20 MHz) and the second refers to the increase in the power deposition in tissues when the Larmor frequency increases. Both phenomena are less important in animals, because of the smaller dimensions of animal bodies and the less stringent safety requirements. As a result, animal instruments were developed at high fields earlier compared with human ones. Today the great majority of imagers designed for animal studies operate at fields of 4.7 T or higher. The main advantages in high fields stand in higher signal to noise ratio (and consequent increase in space resolution or decrease in acquisition time) and higher frequency separation between metabolite peaks in in vivo spectroscopy. Disadvantages are in the higher cost of magnets and electronics, in shortening of T2 relaxation time, paralleled by a lengthening in T1 relaxation time, and in greater importance of susceptibility and chemical shift artefacts. Recent developments in applications of MRI (and magnetic resonance spectroscopy, MRS) in preclinical studies, as for example functional magnetic resonance imaging (fMRI), microscopy, diffusion-weighted (DW) spectroscopy and molecular imaging, pose increasing requirements to technical aspects of MRI instruments (increased signal-to-noise ratio (SNR), space resolution and chemical shift) and consequently push toward higher magnetic fields. In this paper the above mentioned developments are reviewed and discussed.

Purpose To investigate the additional role of MRI contrast enhancement (CE) in the primary tumor and the FDG uptake at PET in the lymph-node metastases. Materials and methods A model of colorectal cancer induced by orthotopic HT-29 cells microinjection, producing pelvic lymph node metastases, was assessed using CE-MRI and FDG-PET. Histology and GLUT-1 immunohistochemistry were performed on primary tumors and iliac lymph nodes. Results Primary tumors were characterized by low FDG-uptake but high CE-MRI, particularly at tumor periphery. Undetectable FDG-uptake characterized the metastatic lymph-nodes. Histology revealed large stromal bundles at tumor periphery and a dense network of stromal fibers and neoplastic cells in the inner portion of the tumors. Both primary tumors and positive lymph nodes showed poor GLUT-1 staining. Conclusion Our data support the complementary role of MRI-CE and FDG PET in some types of carcinomas characterized by abundant cancer-associated stroma and poor FDG avidity consequent to poor GLUT-1 transported. In these tumors FDG-PET alone may be not completely adequate to obtain an adequate tumor radiotherapy planning, and a combination with dual CE-MRI is strongly recommended.

OBJECTIVES: To compare DCE-MRI experiments performed using a standard small-molecular (Gd-DTPA) and an albumin-binding (MS-325) contrast agent in two carcinoma models with different stromal content. MATERIALS AND METHODS: DU-145 or BXPC-3 cancer cells were subcutaneously injected into nude mice. DCE-MRI was performed by a bolus injection of Gd-DTPA or MS-325 about 2 weeks after inoculation. For quantitative analysis a volume of interest was manually drawn over each tumor. To address the heterogeneous enhancement, each tumor volume was then divided into the 20% most-enhancing and the remaining 80% least-enhancing fractions. Mean tumor enhancement was calculated over these selected tumor volumes and compared between tumor groups and contrast agents. Maps of differential enhancement, peak enhancement and time-to-peak were used for visual evaluation. CD31 and VEGF immunohistochemistry were performed in excised tumors. RESULTS: In the 80% least-enhancing volume, at late time points of the dynamic scan, the mean enhancement elicited by MS-325 was higher in BXPC-3 than in DU-145 tumors. In the 20% most-enhancing volume, using either contrast agents, significant difference between the two tumors types were observed only early, while at later time points of the dynamic scan the difference were obscured by the faster washout observed in the BXPC-3 tumors. Enhancement maps confirmed that BXPC-3 tumors were characterized by marked washout rate using either contrast agent, particularly in the higher enhancing peripheral rim. With MS-325 this washout pattern appeared to be specific to the BXPC-3 carcinomas, since it was not observed in the DU-145 tumors. Finally, in both tumor types, MS-325 produced significantly higher enhancement than Gd-DTPA in the late phase of the dynamic scan. Ex vivo analysis confirmed the marked presence of aberrant infiltrative stroma in BXPC-3 tumors, in which tumor vessels were embedded. In all tumors the central portion was less viable and less infiltrated by stromal tissue then the peripheral areas. CONCLUSIONS: Contrast distribution proved to be related to stromal content, which presumably produced the higher enhancement and faster washout observed in the BXPC-3 tumors. In particular, 'early' contrast-enhanced MRI, appeared as the most sensitive technique to detect the tumor portions characterized by a high stromal content, i.e. the peripheral rim of the BXPC-3 tumors. Since the same tumor models were recently investigated using FDG-PET imaging, showing inverse relationship between FDG uptake and stromal content, contrast-enhanced MRI and FDG-PET could provide complementary and comprehensive sensitivity in the assessment of carcinomas.