The ultra-compact gamma camera

The smallest and lightest gamma camera is based on a single MiniPIX TPX3 device equipped with a 2 mm thick CdTe sensor. It makes images of the distribution of gamma sources in an environment with a very broad observation angle (in fact in all directions – even backward). Except for the image, it provides the gamma spectra of observed isotopes enabling their identification (see the second article here).

The camera itself is very compact (2 x 1.5 x 8 cm) and lightweight (30 g). It has to be connected to the PC during the measurement via a micro USB port. The standalone version is planned for the future (other versions depending on demand e.g. Bluetooth). Several cameras can be used together to cover larger space or providing 3D information.

The small camera size and low power consumption allow for camera integration to small autonomous vehicles or drones.

The MiniPIX TPX3 detector is position, energy, and time-sensitive: For each ionizing particle (e.g. X-ray photon) it digitally registers its position, energy, time of arrival, and track shape – basically all information you can want. The detector spatial resolution is 55 micrometers.

The sensitivity to penetrating gamma rays is provided by the thick (2 mm) and heavy (CdTe) sensor implemented within this miniaturized device.

The camera distinguishes other sources of radiation as well: radon in the air, cosmic rays on board the plane, X-rays generated by electrostatic discharges …

New gamma camera principle: Compton in single layer

Since the traditional gamma camera principle requires large and heavy collimators we decided to use excellent properties of the MiniPIX TPX3 detector and implement an alternative principle developed in ADVACAM: the Compton camera within a single detector layer. This principle is explained in the following animation.

For experts (and those who browsed Compton scattering):

The primary gamma-ray with energy E0 hits the electron inside of the MiniPIX sensor losing energy E1 and being scattered to angle theta. The scattered gamma-ray continues flying through the sensor till it is absorbed delivering energy E2. The scattering angle theta is calculated from the two energies E1 and E2. The energy conservation law holds for such a case: E0=E1+E2.

The MiniPIX TPX3 measures energies (E1 and E2) for both interactions, their lateral positions (x1, y1, and x2, y2), and the time of incidence (t1 and t2) with nanosecond precision. The third coordinate (z1 and z2) is unknown for both interactions.

For the reconstruction of the scattering “cone” (see picture above) we need to know at least the “depth distance” dz=z2-z1 of both interactions. To get this information we use the incidence time measurement (t1 and t2). Since the gamma rays move at the speed of light the time difference dt=t2-t1 should be practically zero. But in our case, we measure it ranging from -100 to 100 ns. The reason is the slow drift speed of ionization charges created by both interactions inside of the CdTe sensor. The charge corresponding to E2 is say collected by the pixelated electrode first and then with a certain delay dt the charge E1 comes. This delay dt can be converted to the “depth distance” dz as the drift speed in the CdTe is constant. The precision of “depth distance” determination within a 2 mm thick CdTe sensor is about 40 micrometers.

Example: Three flasks with Iodine-131 solution in room

The three flasks of Iodine-131 isotope were placed in the empty room. The I-131 isotope is often used in methods of nuclear medicine for imaging and treatment (e.g. thyroid) or as an industrial radioactive tracer.

The activity of I-131 in each flask is tens of MBq. The distance between the flasks and the detector was 3.5 meters. The measurement time is few hours here but the sources were recognized much earlier (minutes). The full details on ranges and sensitivity will be published this July at International Workshop on Radiation Imaging Detectors (IWORID 2019).

Traditional gamma camera principle

The traditional gamma cameras are based on the ancient principle of camera obscura or pinhole camera as illustrated in the following picture. The camera is basically a light-tight room or box with a small hole in its front wall. The light from an object in front of such a camera passes through the hole and creates the image on its back wall where the light-sensitive imaging medium such as photographic film can be installed.

This method works for gamma rays as well: Placing gamma sensitive imager or detector to the room back wall and having camera walls made gamma-ray tight. Since the gamma rays are very penetrating it is not that easy to make it gamma-ray tight. Depending on gamma-ray energy it has to be made heavy (lead, tungsten) and thick. A further complication is that the imaging hole(s) in the thick wall does not allow the broad field of view. The last and most important disadvantage is low sensitivity: The probability that gamma-ray would hit the hole is very low as it can fly in any direction. There are solutions of camera obscura with more than a single hole (coded aperture, MURA mask) offering sensitivity improved by an order of magnitude.

The gamma camera based on Compton scattering does not need heavy collimators and its sensitive area is much larger compared to the area of the pinhole. The field-of-view of the Compton camera is in principle unlimited.

The sensitivity of our MiniPIX TPX3 camera is about 50 times better than the pinhole camera of the same angular resolution.

MiniPIX TPX3 compton gamma camera exploitation fields

Safety: Localization of gamma sources in the environment, manipulation with waste, fire brigades, traffic accidents, border controls … stationary installations, mobile cameras, combination with robots, vehicles, drones …

Medical applications: Diagnostics in nuclear medicine (SPECT, scintigraphy), monitoring of treatment in nuclear medicine (isotopic radiotherapy, brachytherapy, ion therapy …).

Industrial applications: Imaging with industrial radioactive tracers, radioactive waste monitoring and logistics, nuclear power … 

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