Nuclear medicine imaging is a dynamic imaging technique that enables researchers to analyze the physiological and pathological processes, especially in the brain. When imaging awake and unconstrained mice, the free movement of their heads can cause motion artifacts in the nuclear medicine images. These artifacts reduce image resolution, decrease the concentration of the tracer in the region of interest, and affect the quantification of the standard uptake value and the estimation of tracer kinetic model parameters. Therefore, the elimination of head motion artifacts is crucial for improving the quality of brain positron emission tomography (PET) images. In recent years, some researchers have been using markers attached to the mice's heads to track their movement. However, attaching markers to the mice's heads may cause discomfort and anxiety. In addition, the freedom of movement of the head during imaging can lead to relative sliding or detachment of the markers, resulting in incorrect motion estimation.

In this study, we design a mouse head motion tracking system based on the you only live once (YOLO) v5 algorithm. This system can accurately monitor the position and posture of the mice's heads in real time, providing precise motion information for motion correction in nuclear medicine images. In contrast to traditional motion tracking systems, this system does not require markers attached to the mice's heads, effectively addressing the limitations of previous tracking methods. The proposed motion tracking system consists of three main stages, namely feature point recognition and positioning, three-dimensional reconstruction of feature points, and calculation of the rotation and translation parameters. First, the YOLO v5 algorithm automatically identifies and locates existing feature points on the mice's heads to obtain the pixel coordinates of each feature point. Then, using the parallax effect and triangulation principles, we reconstruct the three-dimensional coordinates of the feature points in the world coordinate system. Finally, we calculate the Euler angles of the mice's heads using the symmetry of the feature points and utilize inter-frame pose differential methods to compute the translation and rotation parameters of the head pose change between adjacent frames.

To verify the performance of the designed motion tracking system, we place a mouse phantom in a stationary position and measure the changes in its head position and posture angles using the designed system. The experimental results show that for the X, Y, and Z axes, the root-mean-square errors of the translational degrees of freedom are 0.04, 0.19, and 0.03 mm, whereas the root-mean-square errors of the rotational degrees of freedom are 0.58°, 0.34°, and 2.03°. We use the MATLAB function to obtain the histogram statistics of the detected translation and rotation parameters, all of which conform to a normal distribution.

The results indicate that the designed motion tracking system can accurately monitor the movement of the mice's heads during nuclear medicine imaging. The detected parameters of six degrees of freedom conform to a normal distribution, further confirming the reliability of the system. Moreover, this system does not rely on markers, effectively avoiding the risk of marker detachment. The motion data obtained through this system can be used to compensate for and correct motion artifacts of the mice's heads in nuclear medicine imaging, thereby enhancing the quality of nuclear medicine images.

Gamma-ray detection using a nuclear radiation locator is critical for monitoring, locating, and processing radioactive sources. In recent years, gamma cameras based on coded aperture imaging techniques have been extensively utilized to identify and monitor radioactive sources. However, these detectors have limitations in terms of the imaging field. To accurately determine the specific location of radioactive sources, constant adjustment of the detection angle is required, which is often time-consuming. To expand the detection field, multiple coded aperture cameras can be used simultaneously, but this approach increases cost and equipment complexity. Some researchers have attempted to combine Compton and coded aperture imaging techniques. While the Compton camera can extend the field-of-view (FOV) to 4p, this method is complicated, costly, and limited to detecting high-energy rays. As a result, the combination of these two techniques proves inadequate when searching for low-energy sources. In this work, we proposed a system and method for locating radioactive sources with a large FOV based on combining a coded aperture with pinholes. This method addresses the limited FOV issue encountered in the aforementioned system.

The coded aperture component of the system uses a modified uniformly redundant array as the uniform redundant array mask. The base mode class is 11, with a unit size of 3.3 mm?.3 mm, leading to a total size of 69.3 mm?9.3 mm. The mask thickness is 9 mm, and tungsten is used as the material. The detector section includes a 26?6 NaI (Tl) array, where each crystal pixel has dimensions of 1.45 mm?.45 mm?.00 mm. A crystal gap of 0.2 mm exists between each pixel, and the distance between the center of the coded aperture and the position-sensitive sensor is 77.5 mm.For the pinhole part of the system, a tapered pinhole with a center size of 4 mm is used. The pinhole is embedded in a shield with equally large pinholes on all four sides. For performance assessment of the system, Monte Carlo simulation experiments were performed with GATE software. A large FOV radioactive source location system is constructed, and simulation data are produced. MATLAB is employed to process the simulation data, compute the system transmission matrix using the Sidden algorithm, and conduct reconstruction using the maximum likelihood expectation maximization method. The projection and reconstruction results of the point sources at various positions are compared and analyzed. Thus, this work shows a comprehensive analysis and assessment of the developed system for locating radioactive sources with a large FOV using a combination of coded aperture and pinhole imaging techniques.

The results indicate that the full coding and semipseudo-film FOV of the coded aperture camera are 19.33?and 70.80? respectively, and the added pinhole extends the FOV of the system to 123.40? The developed system attains an angular resolution of 2.95?within the coded aperture FOV and 6.30?within the extended pinhole FOV, effectively imaging a 10 mCi radioactive source at a distance of 3 m.

The developed wide FOV radiation source location system and method effectively address the limited imaging field of the coded aperture camera.