Two strong opposing magnets produce a field gradient with a millimeter-scale field free point (FFP) at the isocenter where SPIO nanoparticles are not magnetically saturated. Magnetic particles are fully saturated elsewhere. The FFP can be moved across the sample using additional magnetic fields or mechanically in a scanning trajectory to produce an image.
Overview of the x-space MPI imaging process. (a) Let us consider a one-dimensional phantom and rapid movement of the FFP across the sample. (b) As the FFP passes over iron oxide nanoparticles in the sample, the total magnetic moment, M(t), changes in a non-linear manner. (c) An inductive receive coil detects the time-varying magnetic moment as a voltage, s(t). The received signal is then converted into a native x-space MPI image using a two-step process of velocity compensation and gridding to the instantaneous position of the FFP.
The MPI process can be generalized into two- and three-dimensions, but with a point spread function, h(x), that changes orientation depending on the FFP velocity vector, .
Hardware overview. (a) 3D MPI scanner with 2 cm × 2 cm × 4 cm FOV. The excitation coil generates a 30 mT peak-to-peak oscillating magnetic field at 19 kHz. The NdFeB magnets generate a gradient of G z = 6.0 T/m down the imaging bore. (b) Photograph of the small-scale x-space MPI scanner. The free bore before addition of the transmit and receive coils is 8.4 cm. The scanner is potted in epoxy to eliminate vibration.
Analog signal chain.
Two-dimensional pulse sequence used in the Berkeley x-space scanner. (a) Rapid movement in z of ±2.5 mm occurs at 19 kHz through the use of a resonant transmit coil, moving the FFP at approximately 200 m/s in the z axis. The FFP displacement is proportional to the current in the transmit coil. This is a schematic representation as the actual movement as over 12 × 103 cycles occur during the scanning period. (b) The sample is mechanically translated down the bore in the z axis in steps of 2.5 mm per scan for a 50% partial FOV overlap. (c) The sample is mechanically rastered during the scan across the FOV in the xaxis. (d) The full 2D pulse sequence in real space. For a 3D scan, we mechanically step the sample in the y axis in a similar manner to the stepping in the z axis.
(Top) Magnitude and phase of signal following 8th order Butterworth analog filtering in the receive chain. (Bottom) Recovered phase following inverse filtering. The stop-band also benefits with improved rejection.
Measured signal showing phase corrected signal from a single scan across a point source in z and y. (Top) The amplitude changes slowly as we scan 1.5 cm in y. (Bottom) Time-slice near y = 0 showing the raw signal as we rapidly scan ±2.5 mm in z. Total scan time of 650 ms.
(Top) Experimental data showing 40 overlapped partial FOV scans of a 400 μm wide Resovist point source phantom without baseline correction. (Middle) Experimental data with baseline correction. (Bottom) The assembled image recovers the linearity across the full FOV.
Comparison between theoretical and measured collinear component of the PSF. The measured FWHM is 1.6 mm along the imager bore and 7.4 mm transverse to the imager bore. Total imaging time of 28 s not including robot movement. The resolution of the PSF can be improved by increasing the main gradient field strength.
Line scan of a linear resolution phantom with point sources separated by 1 mm, 2 mm, and 3 mm. As can be seen, the 1 mm spaced samples are not resolvable as the spacing between them is less than the native resolution of the system (FWHM ≈ 1.6 mm).
X-space MPI generates a native MPI image. Further processing of the native image can be done with standard image processing techniques.23 (a) Acrylic phantom with 300 μm ID tubing containing undiluted Resovist tracer. (b) Native x-space MPI image. (c) Native MPI image deconvolved using Wiener deconvolution. Total imaging time of 28 s, not including robot movement.
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