Experimental validation of a high-resolution diffuse optical imaging modality: photomagnetic imaging

Abstract

We present experimental results that validate our imaging technique termed photomagnetic imaging (PMI). PMI illuminates the medium under investigation with a near-infrared light and measures the induced temperature increase using magnetic resonance imaging. A multiphysics solver combining light and heat propagation is used to model spatiotemporal distribution of temperature increase. Furthermore, a dedicated PMI reconstruction algorithm has been developed to reveal high-resolution optical absorption maps from temperature measurements. Being able to perform measurements at any point within the medium, PMI overcomes the limitations of conventional diffuse optical imaging. We present experimental results obtained on agarose phantoms mimicking biological tissue with inclusions having either different sizes or absorption contrasts, located at various depths. The reconstructed images show that PMI can successfully resolve these inclusions with high resolution and recover their absorption coefficient with high-quantitative accuracy. Even a 1-mm inclusion located 6-mm deep is recovered successfully and its absorption coefficient is underestimated by only 32%. The improved PMI system presented here successfully operates under the maximum skin exposure limits defined by the American National Standards Institute, which opens up the exciting possibility of its future clinical use for diagnostic purposes.

Fig. 1

(a) A schematic of PMI setup showing the phantom and the optical instrumentation inside the MRI bore. (b) The picture of the PMI interface sitting on the MRI bed. It consists of a specially designed RF coil with four windows for illumination and four ports that hold the collimation optics.

Fig. 2

(a) Timeline of PMI data acquisition showing the laser status, sample temperature at any particular point and the MRT acquisition for different cycles. (b) An illustration for the mesh of the cross section of the phantom and the collimated laser illumination beams directed onto the sample from four sides.

Fig. 3

(a) The phantom cross section showing the inclusion size and position. The laser used to heat the phantom from its top side is represented by the red arrow S. The temperature maps (b) measured using MRT and (c) simulated using the forward solver. (d) The profiles taken along the $y$-axis on the measured and simulated temperature maps.

Fig. 4

Temperature maps simulated when: (a) one, (b) two, and (c) four lasers are used. The temperature profiles along the (d) $y$-axis and (e) $x$-axis. The noise level of the MRT sequence corresponds to 0.1°C (green highlighted). With only one laser illumination from the top of the phantom, the signals measured at the bottom half of the phantom is less than the noise level (red). When two-port illuminations are utilized, the measurements at the center of the phantom suffer from low SNR in the direction of illumination and mostly under the noise level in the orthogonal direction (blue). By increasing the illumination ports to four, this problem is solved and the SNR is increased in the overall phantom (green).

Fig. 5

(a) Axial MRI of the cylindrical phantom. The size and position of the inclusions are shown using black circles since no absorption contrast can be seen on the MRI image. All inclusions have the same contrast set to four times more absorbent than the background. (b) The temperature map at the end of the heating cycle. (c) The PMI absorption reconstructed map. (d) The absorption (red) and the temperature (blue) profiles across the three inclusions along the red arrow shown on (a).

Fig. 6

(a) Axial MRI of the cylindrical phantom. The size and position of the inclusions are shown in black circles since no absorption contrast can be seen on the MRI image. All inclusions have the same contrast set to be four times more absorbent than the background. (b) The PMI absorption reconstructed map. (c) The absorption profiles across the four inclusions along the red lines shown on (a).

Fig. 7

(a) Axial MRI of the cylindrical phantom. The size and position of the inclusions are delimited with the black circles since no absorption contrast can be seen on the MRI image. The inclusions are 2, 4, 6, and 8 times more absorbent than the background. (b) The PMI absorption reconstructed map. (c) A graphic showing the linear proportional relationship between the reconstructed and the real absorption coefficient.