Biomedical subjects

Mark J Hamamura

Publications and source records attributed to Mark J Hamamura.

4 recordsLinked to original sources

PubMed2006

Contrast and spatial resolution in MREIT using low amplitude current.

Magnetic resonance-electrical impedance tomography employs low amplitude currents injected or induced inside an object. The additional magnetic field due to these currents results in a phase in the MR images. In this study, a modified fast spin-echo sequence was used to measure this magnetic field, which is obtained by scaling the MR phase image. A finite element method with first order triangular elements was used for the solution of the forward problem. An iterated sensitivity matrix-based algorithm was developed for the inverse problem. The resulting ill-conditioned matrix equation was regularized using the Tikhonov method and solved using a conjugate gradient solver. The spatial and contrast resolution of the technique was tested using agarose gel phantoms. A circular phantom with 7 cm diameter and 1 cm thickness is used in the phantom experiments. The amplitude of the injected current was 1 mA. 3, 5 and 8 mm diameter insulators and high conductor objects are used for the spatial resolution study and an average full-width half-maximum value of 4.7 mm is achieved for the 3 mm insulator case. For the contrast analysis, the conductivity of a 15 mm object is varied between 44% and 500% with respect to the background and results are compared to the ideal reconstruction.

Algorithms↗

PubMed2006

Measurement of ion diffusion using magnetic resonance electrical impedance tomography.

In magnetic resonance electrical impedance tomography (MREIT), currents are applied to an object, the resulting magnetic flux density measured using MRI and the conductivity distribution reconstructed using these MRI data. In this study, we assess the ability of MREIT to monitor changes in the conductivity distribution of an agarose gel phantom, using injected current pulses of 900 microA. The phantom initially contained a distinct region of high sodium chloride concentration which diffused into the background over time. MREIT data were collected over a 12 h span, and conductivity images were reconstructed using the iterative sensitivity matrix method with Tikhonov regularization. The results indicate that MREIT was able to monitor the changing conductivity and concentration distributions resulting from the diffusion of ions within the agarose gel phantom.

Algorithms↗

PubMed2006

In vivo MRI electrical impedance tomography (MREIT) of tumors.

A significant increase in electrical conductivity of neoplasticities compared to healthy tissues and benign formations has been reported in several studies. We previously reported preliminary results with MR based Electrical Impedance Tomography (MREIT) on several phantoms and a single animal. In the presented study, we applied the technique on ten tumor-bearing rats and collected MREIT images to investigate the potential of MREIT for characterizing malignant tumors. Results show that the tumors had significantly higher mean conductivity compared to the mean of conductivity in the rest of the body. Although heterogeneity of conductivity was observed in the tumor, the mean was still higher than the background.

Algorithms↗

PubMed2004

Increased blood clotting, microvascular density, and inflammation in eotaxin-secreting tumors implanted into mice.

An important theme that is emerging in cancer research is the interaction between tumor cells and the host stroma. Because many types of human cancer are infiltrated by eosinophils that are believed to mediate an anti-tumor cytotoxic effect, we developed and studied a transfected B16 murine melanoma cell line that secretes high levels (510 pg/ml/100,000 cells/day) of eotaxin, a chemokine that recruits and activates primarily eosinophils. Here we report that there was increased inflammation (eosinophils, mast cells, mononuclear cells), blood clotting, and microvascular density within the tumors produced by subcutaneous implants of eotaxin-secreting tumor cells in 10 C57BL/6 compared to tumors produced by wild-type tumor cells. The extensive blood clotting in the eotaxin-transfected tumors was associated with significantly decreased blood flow to the tumors as measured by magnetic resonance imaging [(mean maximum signal enhancement of eotaxin-secreting tumors, 147 +/- 57 (n = 7) compared to 202 +/- 36 signal enhancement units (n = 8) for the wild-type melanoma cells; P = 0.04 by two-tailed, unpaired t-test]. Surprisingly, there was no significant difference between the growth rates or mean masses of the eotaxin-secreting tumors (750 +/- 280 mg, n = 10) and the wild-type tumors (780 +/- 290, n = 10) after 20 days of growth in vivo, despite the significantly slower growth rate in vitro of the eotaxin-secreting tumor cells. We conclude that eotaxin and the resultant tumor-infiltrating inflammatory cells are not likely to mediate a significant anti-tumor effect in vivo. Instead, elevated eotaxin is associated with increased inflammation, microvascular density, and blood clotting. Thus, eotaxin and eosinophils may play a more complex role in modulating the growth of tumors than the simple, anti-tumor cytotoxic effect that has been previously proposed.

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