多模态分子影像设备
分子医学影像技术于1999年首次提出，应用影像学的方法对活体状态下的生物过程进行细胞和分子水平的定性和定量研究，在分子水平上对生物体生理、病理的变化进行实时、动态、在体、无创的成像技术。它是研究靶向性、特异性分子探针及治疗药物的关键、核心技术，是当今生物医学工程领域最先进的成像技术。
多模态分子医学影像通过靶向不同生物分子的探针组，利用不同的影像技术获得活体动物内分子层次生理和病理的动态变化信息，进行影像学显示来支撑肿瘤等重大疾病的研究，是医学影像未来发展的。
课题组以多模态分子医学影像为目标，致力于研发小动物多模态分子影像设备，将正电子发射断层成像技术（PET）、单光子发射断层成像技术（SPECT）、荧光分子层析成像（FMT）与计算机断层扫描（CT）四种成像模态在同一系统中进行整机集成和同机融合。在该系统基础上，进行应用于新型肿瘤放射性分子探针和治疗药物的分子影像学在体评价研究。通过这些典型应用示范研究，一方面及时取得反馈信息并对设备进行完善，另一方面为这些研究提供科学依据，推动生命科学领域病理机制探索和新型药物研发的发展。
图 1 多模态分子影像设备
利用配准模型实现多模态融合配准与校正参数的测定，完成多模态融合成像的几何参数校正,本课题组在国际上首次实现了同机融合PET、SPECT、CT以及FMI这四大主流分子医学影像成像技术的四模态分子医学影像系统，实现生物体结构和功能信息的多角度成像，实现定性、定位和定量分析功能，并进行了初步的生命科学应用探索。对肿瘤-炎症双疾病小鼠动物模型进行多模态成像，利用多种分子探针成功区分了不同部位的肿瘤与炎症，灵敏度和特异性均较各单独模态有显著性提升。
图 2 动物实验研究
1. 计算机断层成像技术
计算机断层成像（computed tomography, CT）根据不同密度物体对X射线吸收不同的原理，使用X射线对物体某一范围进行扫描，取得信息，经计算机处理后获得重建的图像，获得的图像为物体的横断解剖图。CT成像具有分辨率高（临床CT：0.5-1.0 mm，小动物CT：0.05-0.2 mm）、成像速度快、设备简单等特点，在人体成像与小动物成像中均得到了广泛应用。
本课题组研制的CT系统，采用牛津低剂量钨靶球管射线源，CMOS快速平板X射线探测器，透明的半开放式动物床等，保证长时间的实验动物自然体位，保证动物福利及数据一致性。另外通过自主设计的模拟退火算法精确计算出系统的几何参数偏差，利用参数偏差值对系统进行偏差补偿，利用GPU对锥束CT的重建算法进行硬件加速，大大提高了系统成像分辨率和成像速度。
图 3 CT断层成像
2. 正电子发射计算机断层成像技术
正电子发射计算机断层扫描(PET)为全身提供三维的和功能运作的图像。PET技术是目前唯一的用解剖形态方式进行功能、代谢和受体显像的核医学成像技术，具有无创伤性的特点，是目前临床上用以诊断和指导治疗肿瘤最佳手段之一。小动物PET能加速一系列新技术的发展和新药物的研制。
目前，PET 成像系统通常包括固定的探测环机架、电机控制的动物床模块、电子信号处理模块和电源模块。本课题组使用高灵敏度的晶体探测器，优化电子信号处理模块的设计，仪器性能优越，具有3.3-4.15mm的轴向空间分辨率，2.55-3.2mm的径向空间分辨率，2.575-3.225mm切向空间分辨率。在能窗300-650keV符合时间窗12ns下最大敏感度为0.76%。能量分辨率14.0-30.5%。
图 4 PET系统（左）和相关动物实验（右）
单光子发射计算机断层摄影（SPECT）利用注入人体内的单光子放射性核素发出的γ射线，在计算机辅助下重建影像，构成断层影像。SPECT是一种由电子计算机断层与核医学示踪相结合的技术，通过从体外探测γ射线，精确的反映体内的生物功能信息。
单光子断层成像装置通常分为三个部分：机架、准直器、探测器。本课题组利用GATE模拟了基于平行孔准直器和LYSO闪烁晶体为核心的SPECT系统。根据模拟结果设计并搭建了小动物SPECT原理样机，开展了了应用研究。并将针孔准直器与碲锌镉半导体探测器相结合，进一步提高了小动物SPECT的能量、空间分辨率。
图 5
4. 荧光分子成像
荧光分子成像利用荧光探针对体内标定的蛋白进行成像，基于发光方式可分为自发荧光和激发荧光。20世纪90年代起，随着荧光蛋白在基因表达中的成功表达与复制，使得荧光成像在分子医学领域中开始了一个崭新的发展阶段，主要应用包括基因在细胞中的表达、标记蛋白的表达与作用过程、监测肿瘤发展与转移。随着研究的发展，二维荧光分子成像已经不能满足对深层组织分子影像的要求。20世纪90年代后期出现的荧光分子层析成像（FMT），是一种对生物组织光学特性参数进行成像的近红外光学散射断层成像技术。
目前，荧光分子层析成像技术在深层组织三维成像方面的研究得到了世界范围内的广泛关注。本课题组全新研发的荧光分子层析成像系统可实现自由空间非接触式360°荧光激发与采集。在本系统中，卤钨灯用于激发光源，科研级高灵敏度互补金属氧化物半导体（sCMOS）用于光子探测器。本系统采用线光源照明方式，可在十分钟内完成实验动物周身36个角度的数据采集。同时，采用透明的半开放式动物床，可以保证长时间的实验动物自然体位，保证动物福利及数据一致性。
BALB/c 雄性裸鼠动物实验验证了荧光成系统的成像优势，在动物深度麻醉状态下，从尾部处插入含有8μM浓度ICG的细管，microCBCT与FMT同时成像，为荧光断层重建提供轮廓和解剖信息。
图 6
Multi-modality molecular imaging system
Molecular imaging (MI) traces the biologic processes at the molecular level in the intact living tissues with high resolution and specificity, sensing diseases at their very early stages. Since it was proposed in the 1990s, molecular imaging has become a key tool in developing targeted molecular probes and medication. Up to now, it is the most advanced imaging technology in biomedical research and clinical imaging.
Multi-modality molecular imaging method utilizes label targets to obtain dynamic information of physiological and pathological within the molecular level. It can give imaging support to study of severe disease such as cancer, directing the development of future medical imaging.
Our research group is dedicated to developing an integrated quad-modality system that included three molecular imaging methods (PET, SPECT, and fluorescence molecular imaging [FMI]) and an anatomic imaging modality (CT). Based on this system, we have studied radioactive target probes and drugs and with the help of imaging technology, we also study various biologic processes in the same animal using multiple molecular tracers, which will give support to pharmaceutical and medical research.
Figure 1 Multi-modality molecular imaging system
Based on the registration phantom, geometric calibration was done for parameter correction and image fusion. This is the first time in the worldwide to combine the mainstream modality of PET, SPECT and FMI in a single system，which acquired the functional and metabolic information and gave support to qualitative and quantitative analysis of the biological process. We introduced a xenograft tumor and chronic inflammation a nude mouse for multi-modality imaging. With the help of multi-probe, we succeeded in identifying the location of tumor’s and inflammation. resulting with improved specificity and sensitivity.
Figure 2 Animal experiment
1. Computed Tomography
Computed Tomography (CT) is a technique that relies on differential levels of X-ray attenuation by tissues within the body to produce images reflecting anatomy. Some advantages of CT include its fast acquisition time, high spatial resolution (preclinical = 0.05–0.2 mm, clinical = 0.5–1.0 mm), cost-effectiveness, availability, clinical utility, and relative simplicity.
Our research group developed a novel CT system with full angle projection. In this system, a low dose X-ray tube is applied as the X-ray source, while a CMOS flat detector is used as a detection device. We also employ a novel transparent animal bed, which is suitable to hold the animal for long time experiments.
We use the simulated annealing algorithm combined with nonlinear least square method to get system geometry parameters, use GPU hardware to accelerate CT reconstruction algorithm, as a result, the spatial resolution and imaging speed was improved.
Figure 3 CT reconstruction images
2. Positron emission tomography
Positron emission tomography (PET) is a nuclear medicine imaging technology that produces a three-dimensional functional image on the body. The PET imaging system detects two back-to-back 511keV gamma photons produced simultaneously following positron–electron annihilation by a positron-emitting radionuclide (tracer), which is introduced into the body conjugated to biologically active molecule. Advances in PET imaging have accelerated recently due to the development of a number new technologies and the use of small animal models in the basic and pre-clinical sciences.
The PET imaging system consists of a stationary detector gantry, motor-controlled animal bed module, electronics modules, and power supply modules.
The reconstructed axial spatial FWHM resolution varies from 3.3 mm to 4.15 mm; the reconstructed transverse spatial FWHM resolution in radial and tangential directions varies from 2.55 mm to 3.2 mm, and 2.575 mm to 3.225 mm, respectively. The maximum sensitivity was 0.76% at the coincidence window of 12 ns and energy window of 300 – 650 keV. The energy resolution across all crystals in the PET imaging system ranged from 14.0 to 30.5% with a mean of 21.0%.
Figure 4 The PET system (left) and animal experiments: Tumor imaging of a 25 g C57BL/6 mouse with an implanted tumor in its right leg and left leg separately. The transverse, coronal and sagittal images of the mouse are illustrated. The tumor (arrow) is 12 mm and 3mm in diameter with liquefaction necrosis in the internal parts, which was consistent with the PET images.
3. Single photon emission computed tomography
Single photon emission computed tomography (SPECT) uses radioactive tracers and a scanner to record data that a computer constructs into two- or three-dimensional images. A small amount of a radioactive drug is injected into a vein and a scanner is used to make detailed images of areas inside the body where the radioactive material is taken up by the cells. SPECT can give information about blood flow, bone remodeling, tumor growth, or other biological processes.
The SPECT system mainly including three parts: Acquisition Geometry, Collimation Systems and Detector
Our lab use GATE (GEANT 4 Application for Tomographic Emission) to simulate LYSO-based parallel hole Micro-SPECT for validation and detector optimization. We designed and developed a small animal SPECT system based on simulated result. Our lab combined CZT detectors with pinhole collimators significantly improves energy and spatial resolution of Micro-SPECT.
Figure 5
4. Fluorescence molecular imaging
Fluorescence molecular imaging (FMI) gains increasing interests in deep tissue imaging. Here we report a novel FMT system setup with full angel projections. In this system, a tungsten-halogen lamp is applied as illumination, while a scientific complementary metal oxide semiconductor (sCMOS) is used as a detecting device. With a unique line-pattern illumination and a high sensitivity sCMOS, our FMT system can complete data acquisition over 36 perspective angles along the animal within 10 minutes. We also employ a novel transparent animal bed, which is suitable to hold the animal for long time experiments. Both phantom and in vivo animal experiments have been studied, and our results demonstrate this FMT system has a great potential for small animal study. In addition, our design allows this FMT system to be easily applied in either stand-alone fluorescent systems or combined with other molecular imaging methods.
Our FMT systems, it has several advantages: The whole equipment is divided into four different modules, and each one is independent; The motion of gantry, and the transparent animal holder keep the sample in a natural and steady posture during imaging process; The excitation with a line pattern is able to illuminate the whole body of the small animal and therefore resulting in a faster scanning rate; A low-cost tungsten-halogen lamp with appropriate filters could provide multi-spectra images for different fluorescent probes, meeting the needs of optical molecular imaging in the lab.
In the animal study, a BALB/c male nude mouse, 6-8 week old, weighing 20-25g, was used for animal study. The mouse was firstly given overdose of anesthesia of chloral hydrate, and then the quartz tube with 8μM ICG solution was inserted into the intestine from the anus. 36-angle images were acquired and a microCBCT provided profiles in the reconstruction. In the abdominal region, fluorescence could penetrate from the central cavity. The CT/FMT fused image showed that, although the QE is 40% for ICG, this FMT based on a sCMOS and a tungsten-halogen lamp could acquire precise reconstruction results in the animal study under current conditions.
Figure 6