Remote sensing of high energy particle current generated by megavoltage x-rays

In x-ray radiography and computed tomography (CT), absorbed dose is deposited in a radiation detector array in the form of charge carriers and collected. While these modalities are the standard for clinical imaging during the radiation therapy process, they require the use of bulk materials and adequate operating voltages. These constraints leave space for an imaging/dosimetry niche favoring low profile, low power, and non-invasive modalities.

The conversion of therapeutic radiation to absorbed dose begins with the generation of high energy electrons. If utilized correctly, the high energy particle currents (HEC) offer a unique prospect for a novel form of imaging and dosimetry. In this paper, we establish the theoretical and experimental framework behind the sensing of HEC by measuring currents in various homogeneous and heterogeneous phantoms and comparing the measured signals to both one-dimensional particle transport and Monte Carlo (MC) based simulations.

The experimental setup for HEC sensing consists of pairs of complementary electrodes placed upstream and downstream of the object or phantom in question. When irradiated with 6MV x-rays, two signals, s₁, and s₂, were collected with zero external bias. These signals are coupled to each other due to the distribution of HEC inside the phantom. Both homogeneous (water) and heterogeneous (water and bone) phantoms were irradiated, and the measured signals were reviewed against simulations (MCNP6, CEPXS).

The measured signals s₁ and s₂ (as a function of water equivalent thickness [WET]) for homogeneous phantoms matched the trends established by the corresponding radiation transport simulations; indicating that these signals convey information about the distribution of HEC inside the phantoms. Based on these findings, new signal metrics, α and β, were formalized and used to quantify the scanning of heterogeneous phantoms.

In this work, we demonstrated that information about the internal composition of an object can be obtained through HEC sensing. Specifically, the distribution of HEC inside of an object resulting from x-ray irradiation was measured using a simple system of planar electrodes and agreed well with radiation transport simulations. HEC sensing has the potential to be a disruptive method of imaging with its low power, low profile, and non-invasive nature.