A basic principle of PET imaging
PET is the abbreviation of Positron Emission Tomography. Its clinical imaging process is: the emission of positron radionuclides (such as F-18, etc.) labeled to participate in human tissue blood flow or metabolic process of compounds, labeled with positron compounds radionuclides injected into the body of the examinee. The subject is allowed to undergo PET imaging within the effective field of view of the PET. The positrons emitted by the radionuclide move about 1 mm through the body before combining with the negative electrons in the tissue to undergo annihilation radiation. Two gamma photons of equal energy (511 keV) and in opposite directions
are produced. Due to the two photons in the body of the path is different, the time to reach the two detectors also have a certain difference, if in the specified time window (generally 0-15 us), the probe system detects two mutually 180 degrees (Shi 0.25 degrees) of photons. That is, a conformity event, the detector will be sent out a time pulse, pulse processor will be changed into a square wave pulse, in line with the circuit for its data classification, sent to the workstation for image reconstruction. The image of each part of the human body in cross section, coronal section and sagittal section is obtained.
The main components of PET system include frame, ring detector, compliance circuit, examination bed and workstation. The detector system is the main part of the entire positron emission imaging system, which uses a block detector structure that facilitates the elimination of scatter and improves the count rate. Many block structures form a ring, and then dozens of rings form the whole detector. Each block structure consists of approximately 36 small bismuth germanate (BGO) crystals followed by 2 pairs (4) of photomultiplier tubes (PMTs) (see Fig. 1). The BGO crystals convert the high-energy photons into visible light; the PMTs convert the optical signals into electrical signals, which are then converted into time-pulsed signals, and the temporal coupling of the signals of each probe is verified by an interlaminar line of conformity, which eliminates interference from other sources. Excluding the interference of rays from other sources, the position of positron is given by the operation, and the computer uses scattering, incidental conformity signal correction and photon time-of-flight calculation to complete the reconstruction of the image. The reconstructed image increases the overall resolution of PET to about 2 mm.
PET adopts the conformal detection technique for electronic collimation correction, which greatly reduces the random conformal events and background, and the electronic collimator has very high sensitivity (without the influence of lead shielding) and resolution. In addition．The size of the BGO crystal is positively correlated with the sensitivity. PET probe with block structure. Capable of 2D or 3D acquisition. 2D acquisition involves spacing lead or tungsten plates between rings to minimize the effect of scattering on image quality 2D image reconstruction conforms to the counts within only a few adjacent rings (generally 2-3 rings), which has a high resolution and a low count rate; 3D data acquisition is different. The elimination of the interval between rings and the compliance calculation in all rings significantly improves the count rate, but the scattering is serious, the image resolution is also lower, and a large number of data operations are required for data reconstruction. Another important difference between the two acquisition methods is the difference in sensitivity, which is highest in the center of the field of view for 3D acquisition.
Second, the working principle of multilayer spiral CT
CT's basic principle is the image reconstruction, according to the human body's various tissues (including normal and abnormal tissues) on the X-ray absorption of unequal characteristics of a selected level of the human body is divided into a number of cubic blocks (also known as the voxel) X-rays pass through the voxel, the measured density or gray value is called the pixel. x-ray beam through the selected level, the detector receives the X-ray beam along the direction of the voxel. Detector received along the X-ray beam direction of the voxel absorption of X-rays after the sum of the attenuation value, for the known value, the formation of the total amount of each voxel X-ray attenuation value for the unknown value, when the X-ray source and detector around the human body to do the arc or circumference of the relative motion. An iterative method
is used to find the X-ray attenuation value for each voxel and to reconstruct the image to obtain a black-and-white image of the different densities of tissue at that level.
Spiral CT breaks through the design of traditional CT, using the sliding ring technology, the power supply cable and some signal wires are connected to different metal rings in the fixed frame of the moving X-ray tube and detector sliding brushes and the metal ring conduction. Ball tube and detector is not limited by the length of the cable, along the long axis of the human body continuous and uniform rotation, the scanning bed synchronized with uniform progress (traditional CT scanning bed in the scanning of stationary), the scanning trajectory is a spiral forward, can be quickly and uninterruptedly to complete the volume scanning.
Multilayer spiral CT is characterized by a multi-layer arrangement of detectors. It is the best combination of high speed and high spatial resolution. The wide detector of multilayer spiral CT is made of high-efficiency solid rare earth ceramic material. Each unit is only 0.5, 1 or 1.25 mm thick, up to 5 mm thick Thin-layer scanning detector photoelectric conversion efficiency of up to 99% can continuously receive X-ray signals. The afterglow is extremely short and stable. Multi-slice spiral CT can complete a large range of volumetric scanning at high speed, good image quality, fast imaging speed, with high longitudinal resolution and good time resolution. This greatly broadens the range of CT applications
Compared with single-slice spiral CT. The same volume of data acquisition, the scanning time is greatly reduced, in the case of no increase in the dose of X-rays, every 15 S or so can be scanned a part of the chest; 5S can be completed within the layer thickness of 3 mm of the entire thorax scanning; the use of a larger pitch P value, a time to hold the breath of 20 S, you can complete the scanning of the body; the same layer thickness, the same time, scanning range increased by 4 times. The coverage per unit time of the scan is significantly improved, the radiation dose received by the patient is significantly reduced, the service life of the x-ray bulb is significantly prolonged, and at the same time, the amount of contrast is saved, the low contrast resolution and spatial resolution are improved, and the noise, artifacts, and sclerosis are significantly reduced. In addition, the width of the X-ray cone beam can be automatically adjusted according to different layer thicknesses, and the collimated X-ray beams are focused on the corresponding number of detectors The detectors are connected to four Data Acquisition Systems (DAS) via electronic switches. The detectors are connected to four data acquisition systems (DAS) by electronic switches. Each DAS is capable of acquiring a complete set of images independently, depending on how the DAS are matched to the detectors. The electronic switching allows selective acquisition of 1-, 2-, or 4-layer images, with the thickness of each layer being freely selectable (0.5, 1.0, 1.25 mm, or 5, 10 mm). The acquired data can be used for both conventional image display and post-processing at the workstation for 3D stereo reconstruction, multilayer reconstruction, and organ surface reconstruction, and can be displayed in real time or near-real time. In addition, the data can be displayed in real time or near real time. Different angles of rotation, different colors of marking, so that the image is more three-dimensional sense more intuitive and realistic. Simulation of endoscopy, three-dimensional CT angiography technology is also more mature and faster.
Three, PET-CT image fusion
PET and CT two different imaging principles of the equipment with the combination of the same machine, is not a simple addition of its functions. But on this basis for image fusion, the fused image has both fine anatomical structure and a wealth of physiological and biochemical information. The fused image has both fine anatomical structure and rich physiological and biochemical function information, which can provide the basis for determining and finding the precise location of tumors and other foci for quantitative and qualitative diagnosis. And can be used to X-ray nuclear medicine image attenuation correction.
The core of PET-CT is fusion, image fusion refers to the same or different imaging modalities of the image after a certain transformation process to make their spatial location and spatial coordinates to match, the image fusion processing system to make use of their respective imaging modalities of the two images of the spatial alignment and combination of the registration of the image data synthesized into a single image. The image data is registered and synthesized into a single image. PET-CT co-location fusion (also called hardware fusion, non-image alignment) has the same positioning coordinate system, so the patient does not have to change the position during scanning to perform PET-CT co-location acquisition, which avoids the errors caused by patient displacement. After acquisition, the two images do not need to be aligned, converted and calibrated, and the computer image fusion software can conveniently carry out
2D and 3D fusion accurately, and the fused images simultaneously show the human anatomy and metabolic activity of the organs, which greatly simplifies the technical difficulties of the whole image fusion process, avoids the complex labeling methods and a large number of calculations after the acquisition, and solves the problem of temporal and spatial alignment to a certain extent. It also solves the problem of temporal and spatial alignment to a certain extent, and greatly improves the reliability of the images.
PET in the imaging process due to the Compton effect, scattering, accidental compliance with the event, dead time and other attenuation factors, the collected data and the actual situation is not consistent with the distortion of the image quality, effective measures must be used to correct, in order to obtain a more realistic medical imaging. Isotope correction to obtain the penetrating image system resolution is generally 12 mm, while the X-ray method of penetrating image system resolution of about 1 mm The amount of information in the image is much larger than the isotope method. Attenuation correction of PET with CT images results in a much sharper PET image with significantly better image quality than isotope correction of the penetration source (see Fig. 2), with more than a 25% increase in resolution, a 30% increase in correction efficiency, and ease of operation. The fusion of the corrected PET images with CT images provides more information on the relationship between anatomical structures and physiological functions, which is of great clinical significance for the positioning of tumor patients for surgery and radiation therapy.