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Detection threshold of single SPIO-labeled cells with FIESTA
Chris Heyn1, Chris V Bowen, Brian K Rutt
1Imaging Research Laboratories, Robarts Research Institute, London, Canada. cheyn@imaging.robarts.ca
Abstract:
MRI of superparamagnetic iron oxide (SPIO)-labeled cells has become a valuable tool for studying the in vivo trafficking of transplanted cells. Cellular detection with MRI is generally considered to be orders of magnitude less sensitive than other techniques, such as positron emission tomography (PET), single photon emission-computed tomography (SPECT), or optical fluorescence microscopy. However, an analytic description of the detection threshold for single SPIO-labeled cells and the parameters that govern detection has not been adequately provided. In the present work, the detection threshold for single SPIO-labeled cells and the effect of resolution and SNR were studied for a balanced steady-state free precession (SSFP) sequence (3D-FIESTA). Based on the results from both theoretical and experimental analyses, an expression that predicts the minimum detectable mass of SPIO (m(c)) required to detect a single cell against a uniform signal background was derived: m(c) = 5v/(K(fsl) x SNR), where v is the voxel volume, SNR is the image signal-to-noise ratio, and K(fsl) is an empirical constant measured to be 6.2 +/- 0.5 x 10(-5) microl/pgFe. Using this expression, it was shown that the sensitivity of MRI is not very different from that of PET, requiring femtomole quantities of SPIO iron for detection under typical micro-imaging conditions (100 microm isotropic resolution, SNR = 60). The results of this work will aid in the design of cellular imaging experiments by defining the lower limit of SPIO labeling required for single cell detection at any given resolution and SNR.
Insights
Magnetic resonance imaging (MRI) can detect single SPIO-labeled cells, rivaling PET sensitivity. This study defines the detection threshold for SPIO cell imaging, aiding experimental design.
Area of Science:
- Biomedical Imaging
- Cellular Tracking
- Nanotechnology
Background:
- Superparamagnetic iron oxide (SPIO) nanoparticles enhance MRI contrast for tracking transplanted cells in vivo.
- Current MRI sensitivity for cellular detection is perceived as lower than PET, SPECT, or optical methods.
- An analytical description of the detection threshold for single SPIO-labeled cells is lacking.
Purpose of the Study:
- To determine the detection threshold for single SPIO-labeled cells using MRI.
- To investigate the influence of resolution and signal-to-noise ratio (SNR) on detection.
- To derive an expression predicting the minimum detectable SPIO mass for single-cell detection.
Main Methods:
- Utilized a balanced steady-state free precession (SSFP) sequence (3D-FIESTA).
- Conducted theoretical and experimental analyses.
- Derived an equation for minimum detectable SPIO mass (m(c)) based on voxel volume (v), SNR, and an empirical constant (K(fsl)).
Main Results:
- Derived the expression: m(c) = 5v/(K(fsl) x SNR).
- Established K(fsl) as 6.2 +/- 0.5 x 10(-5) microl/pgFe.
- Demonstrated MRI sensitivity comparable to PET under typical micro-imaging conditions (100 microm resolution, SNR=60), requiring femtomole quantities of SPIO iron.
Conclusions:
- MRI sensitivity for SPIO-labeled cells is higher than previously assumed and comparable to PET.
- The derived expression provides a quantitative basis for designing cellular MRI experiments.
- Defines the minimum SPIO labeling required for single-cell detection at specific resolutions and SNR levels.
