Comparative study of FAST gradient echo MRI sequences: Phantom study

Development and characterization of a dynamic lesion phantom for the quantitative evaluation of dynamic contrast-enhanced MRI

PURPOSE

To develop a dynamic lesion phantom that is capable of producing physiological kinetic curves representative of those seen in human dynamic contrast-enhanced MRI (DCE-MRI) data. The objective of this phantom is to provide a platform for the quantitative comparison of DCE-MRI protocols to aid in the standardization and optimization of breast DCE-MRI.

METHODS

The dynamic lesion consists of a hollow, plastic mold with inlet and outlet tubes to allow flow of a contrast agent solution through the lesion over time. Border shape of the lesion can be controlled using the lesion mold production method. The configuration of the inlet and outlet tubes was determined using fluid transfer simulations. The total fluid flow rate was determined using x-ray images of the lesion for four different flow rates (0.25, 0.5, 1.0, and 1.5 ml/s) to evaluate the resultant kinetic curve shape and homogeneity of the contrast agent distribution in the dynamic lesion. High spatial and temporal resolution x-ray measurements were used to estimate the true kinetic curve behavior in the dynamic lesion for benign and malignant example curves. DCE-MRI example data were acquired of the dynamic phantom using a clinical protocol.

RESULTS

The optimal inlet and outlet tube configuration for the lesion molds was two inlet molds separated by 30° and a single outlet tube directly between the two inlet tubes. X-ray measurements indicated that 1.0 ml/s was an appropriate total fluid flow rate and provided truth for comparison with MRI data of kinetic curves representative of benign and malignant lesions. DCE-MRI data demonstrated the ability of the phantom to produce realistic kinetic curves.

CONCLUSIONS

The authors have constructed a dynamic lesion phantom, demonstrated its ability to produce physiological kinetic curves, and provided estimations of its true kinetic curve behavior. This lesion phantom provides a tool for the quantitative evaluation of DCE-MRI protocols, which may lead to improved discrimination of breast cancer lesions.

[Magnetic resonance imaging of angiogenesis in tumors]

Tumor angiogenesis induces the proliferation of immature blood vessels that are both heterogeneous and leaky. These characteristics can be demonstrated by measuring the perfusion parameters with MRI. Perfusion MRI is usually performed with in T1-weighted dynamic imaging after bolus injection of an exogenous contrast agent such as gadolinium chelate. The perfusion parameters are obtained by semi-quantitative or quantitative analysis of the enhancement curves in the tumor and the arterial input. Perfusion can also be assessed without injecting a contrast agent using arterial spin labeling techniques, diffusion MRI, or BOLD (blood oxygen level dependent) MRI. However, these latter methods are limited by a low signal-to-noise ratio and problems with quantification. The main indication for perfusion MRI is the assessment of antiangiogenic and antivascular treatments. New possibilities for demonstrating angiogenic blood vessels are being opened by molecular imaging.

Semiquantitative assessment of first-pass renal perfusion at 1.5 T: comparison of 2D saturation recovery sequences with and without parallel imaging

OBJECTIVE

The purpose of this study was to assess the feasibility and reliability of measurements performed with true fast imaging with steady-state free precession (FISP) and turbo fast low-angle shot (FLASH) sequences with parallel imaging compared with those obtained with turbo FLASH sequences without parallel imaging in first-pass renal perfusion MRI.

SUBJECTS AND METHODS

The subjects in this prospective study were 15 healthy men who volunteered to undergo MRI for acquisition of renal perfusion measurements. Imaging was performed at 1.5 T with the following three techniques after administration of gadobenate dimeglumine at 4 mL/s: saturation recovery (SR) turbo FLASH sequences without parallel imaging, SR turbo FLASH sequences with parallel imaging, and SR true FISP sequences. The spatial resolution was 2.3 x 2.6 x 8 mm with a temporal resolution of four slices per second (turbo FLASH without parallel imaging and true FISP) or six slices per second (turbo FLASH with parallel imaging). The semiquantitative perfusion parameters mean transit time and maximal upslope were determined. Signal-to-noise ratio (SNR), delta ratio, and time to maximal signal intensity also were determined. Image quality was rated in consensus.

RESULTS

Image quality was best for turbo FLASH sequences without parallel imaging compared with true FISP and turbo FLASH sequences with parallel imaging. True FISP sequences yielded the highest baseline SNR (26.7) but the lowest delta ratio (3.2). Turbo FLASH sequences without and with parallel imaging had significantly lower SNRs (9.6 and 9.3) and significantly higher delta ratios (5.1 and 5.0). The first-pass perfusion parameters mean transit time and time to maximal signal intensity were independent of the technique used.

CONCLUSION

It seems that at 1.5 T, turbo FLASH sequences without parallel imaging are the best approach to renal first-pass perfusion imaging.

Core/shell quantum dots with high relaxivity and photoluminescence for multimodality imaging

A series of core/shell CdSe/Zn1-xMnxS nanoparticles were synthesized for use in dual-mode optical and magnetic resonance (MR) imaging techniques. Mn2+ content was in the range of 0.6-6.2% and varies with the thickness of the shell or amount of Mn2+ introduced to the reaction. These materials showed high quantum yield (QY), reaching 60% in organic solvent. Water-soluble nanoparticles were obtained by capping the core/shell particles with amphiphilic polymer, and the QY values in water reached 21%. These materials also demonstrated high relaxivity with r1 values in the range of 11-18 mM-1 s-1 (at room temperature, 7 T). Both optical and MR imaging were performed on nanoparticles in aqueous solution and applied to cells in culture. The results showed that the QY and manganese concentration in the particles was sufficient to produce contrast for both modalities at relatively low concentrations of nanoparticles.

Size-controlled synthesis of dextran sulfate coated iron oxide nanoparticles for magnetic resonance imaging

In the generation of nanoparticles for biological applications, the control over synthetic parameters influencing the particles' physicochemical properties are of great interest due to the strong influence of particle size and surface properties on cellular uptake and biodistribution. We have synthesized dextran sulfate coated particles and systematically evaluated synthetic parameters that may influence the properties of these nanoparticles as potential magnetic resonance (MR) contrast agents. The amount of base, polysaccharide content, ratio of iron salts, and reaction time were optimized to yield approximately 30 nm particles as determined by dynamic light scattering with good MR properties (r(1) = 14.46 mM(-1) s(-1) and r(2) = 72.55 mM(-1) s(-1)) and in good yield (50%). Particle sizes and relaxivities are compared with clinically available dextran coated particles and the resulting physical properties of the dextran sulfate coated particles show these particles could be used as potential MR contrast agents for cardiovascular imaging.

In vitro demonstration using 19F magnetic resonance to augment molecular imaging with paramagnetic perfluorocarbon nanoparticles at 1.5 Tesla

OBJECTIVES

This study explored the use of F spectroscopy and imaging with targeted perfluorocarbon nanoparticles for the simultaneous identification of multiple bio-signatures at 1.5 T.

MATERIALS AND METHODS

Two nanoparticle emulsions with perfluoro-15-crown-5-ether (CE) or perfluorooctylbromide (PFOB) cores were targeted in vitro to fibrin clot phantoms (n=12) in 4 progressive ratios using biotin-avidin interactions. The CE nanoparticles incorporated gadolinium. Fluorine images were acquired using steady-state gradient-echo techniques; spectra using volume-selective and nonselective sampling.

RESULTS

On conventional T1-weighted imaging, clots with CE nanoparticles enhanced as expected, with intensity decreasing monotonically with CE concentration. All clots were visualized using wide bandwidth fluorine imaging, while restricted bandwidth excitation permitted independent imaging of CE or PFOB nanoparticles. Furthermore, F imaging and spectroscopy allowed visual and quantitative confirmation of relative perfluorocarbon nanoparticle distributions.

CONCLUSIONS

F MRI/S molecular imaging of perfluorocarbon nanoparticles in vitro suggests that noninvasive phenotypic characterization of pathologic bio-signatures is feasible at clinical field strengths.