Since the introduction of PET / CT in the market in June 2001, PET / CT has developed by leaps and bounds. NBC News reported that the birth of PET / CT has made humans take a revolutionary step towards fighting diseases. The results of a study at the University of Zurich in Sweden show that the diagnostic accuracy of PET / CT is 20% higher than that of PET. Professor Lason (the earliest use of PET / CT in the world) believes that PET / CT is of great help in the diagnosis of head and neck tumors, staging and diagnosis of lung cancer, intestinal cancer, prostate cancer and lymph node metastasis of other pelvic tumors. PET / CT has gradually become an important means of early diagnosis of cancer.
1. Main technical parameters of PET / CT
The main design parameters of PET / CT are divided into CT and PET. The main technical parameters are:
(1) CT part:
(1) Number of detector rows, rotation speed, collection time
These parameters mainly determine the speed of spatial positioning. If the hospital has more examinations for heart diseases, higher requirements should be put on these parameters. Generally, more than 16 rows of CTs are needed, with a rotation speed of 0.4s, and a collection of 100cm takes about 1.5s. The faster the speed, the better the artifacts caused by the patient's breathing and other movements during the scanning process.
(2) Tube heat capacity, tube current
These parameters mainly determine the CT scanning power and the inspection site. The larger the parameter, the larger the adaptation range. It has certain advantages for the diagnosis of some deeper sites, and the image quality is relatively good.
(3) Layer thickness, horizontal scanning field, aperture
These parameters are also important parameters for CT. The smaller the layer thickness, the finer the imaging, which is generally 0.6 to 10 mm. The horizontal scanning field is generally 45-50 cm, and the aperture is 70 cm.
(2) PET part:
(1) Crystal material and size
The crystal part is the core of PET. PET mainly relies on crystals to receive positrons. Just like the lens of a camera, PET crystals are the "eyes" of PET. There are three kinds of crystal materials, namely BGO, LSO and GSO. They have their own advantages for collecting 511MeVγ photon information. BGO has a large atomic number and density, which makes it have a good ability to intercept γ photons, effectively increasing its sensitivity. Its main disadvantage is the long afterglow time, which is not conducive to 3D acquisition. But its cost is relatively low, so it is widely used in 2D-based PET equipment. LSO only has a persistence time of about 40ns, high light output and only 15 times lower sensitivity than BGO, making it a fast crystal that is very suitable for 3D acquisition. But LSO also has obvious shortcomings, its light output is not proportional to the energy, and the light output of different batches of crystals may vary greatly. In addition, there is about 26% of the long half-life isotope 176Lu in LSO, which causes the background count to increase by about 10000cps. It has little effect on routine clinical examination, but it will significantly increase the noise of single-photon transmission scanning (such as attenuation correction with 137Cs). GSO is a strong competitor of LSO. Although its photon interception ability is relatively poor and its light output is relatively low, its energy resolution is much higher than that of BGO and LSO. This makes its anti-scattering ability strong, coupled with short afterglow time, is also very suitable for 3D acquisition.
The design of the crystal size is a compromise between spatial resolution and sensitivity. The smaller the crystal cut, the better the spatial resolution during detection, but the sensitivity will decrease accordingly. At present, the ring size of PET crystal is about 4 ~ 6.45mm, the axial direction is about 4 ~ 8mm, and the corresponding maximum spatial resolution is about 4 ~ 6mm. The number of crystal rings is also an important indicator. For the same axial scanning field, the more the number of rings, the smaller the crystal size and the better the axial spatial resolution.
(2) Scanning speed, acquisition mode, attenuation correction
These parameters are very important in PET. One of the important reasons that trouble the development of PET is that the scanning speed is too slow, and it takes several hours to perform a whole body scan. The acquisition mode is mainly divided into 2D and 3D. Using CT instead of the conventional transmission scan for attenuation correction is the main aspect of saving time, and the 3D acquisition of the PET part is also an important aspect. Through 3D collection, the collection time of each bed can generally be shortened to 2 ~ 3min (2D collection is about 5 ~ 6min), among which LSO crystal has the most obvious advantage in this respect. In addition, when using BGO crystal, if the noise equivalent count rate (NECR) can be increased as much as possible, the acquisition time can also be shortened appropriately and the scanning speed can be increased.
(3) Spatial resolution, horizontal scanning field, aperture
These parameters determine the scanned image quality. The spatial resolution is generally 4 to 6 mm; the horizontal scanning field is 55 to 60 cm; the aperture is 59 to 70 cm. The aperture of CT is 70cm, while the aperture of special PET is generally designed around 60cm. The PET / CT design increases the PET aperture to 70CM, which can reduce the claustrophobic feeling and facilitate radiotherapy positioning. Increasing the aperture generally does not move the crystal spacing. The increased space mainly comes from the space where the attenuation correction source and the barrier are placed, so it does not reduce its spatial resolution.
2. Introduction of PET / CT products on the market
In 1998, the first functional and anatomical integrated imaging instrument SPECT / CT was put into clinical use; in 1999, nuclear instrument manufacturers began to develop PET / CT systems, and in 2000 the first PET / CT began clinical application in the United States. At present, there are three main manufacturers that provide commercial PET / CT, namely: Biograph and Reveal series produced by SIEMENS and CTI; GE company provides Discovery LS and Discovery ST series: Philips ’Gemini PET / CT . Its characteristics are as follows:
(1) Biograph and Reveal series produced by SIEMENS and CTI
The PET probe adopts a full-loop probe. The detectors include ECATHR + PET based on bismuth germanate (BGO) crystal and ECATAccel PET based on Lutetium silicate (LSO) crystal. Both of them are not provided with septum, they are collected in 3D mode completely, and 68Ge attenuation correction source is no longer used, and all CT images are used for attenuation correction. CT and PET are connected in series on the same frame, the width and height are 228cm and 188cm respectively, and the total length is 158cm, but through the front and rear concave design, the actual length of the tunnel can be reduced to 110cm. Increasing the scanning field aperture from 60cm to 70cm is conducive to radiotherapy positioning examination table entry and reduces claustrophobia. The range of one PET and CT simultaneous scan is 145cm, and then increased to about 2m. In addition, the control software has been combined into a system, which is more convenient for the display, measurement and analysis of fused images. This new PET / CT is called BioGraph by Siemens and Reveal by CTI. In December 2002, Siemens launched Biograph SensaTIon16 (CTI's corresponding trade name Reveal XVI). It is a combination of its 16 rows of high-performance CT and Accel type PET based on LSO crystal. This design pays more attention to improving the scanning speed of PET, so that most patients (patients with weight <81.7kg) can complete the examination within 13 minutes. After that, they reduced the LSO crystal to 4 × 4 × 20mm, thereby improving the spatial resolution of the image. This improved equipment was launched in December 2003, Siemens called Biograph LSOHR, CTI called Reveal HI REZ.
(2) GE provides Discovery LS and Discovery ST series
When GE first launched the Discovery LS series PET / CT at the end of 2000, it also simply combined its existing Advance NX i-type PET and Lightspeed Plus Hilite multi-row CT.The PETs are all BGO crystals with retractable barriers. (So ​​you can choose 2D or 3D acquisition mode), and you can choose to use the standard PET attenuation correction source (68Ge). The CT aperture is 70cm, and PET is 59cm. The smaller PET aperture limits its application in radiotherapy positioning. At the end of 2002, GE launched its redesigned PET / CT series, named Discovery ST. This series is more focused on improving its 3D acquisition capabilities. The new design changes the crystal size (62 × 62mm), increases the number of crystal rings (24 rings), reduces the diameter of the detector ring (88cm), shortens the depth of the barrier, reduces the time window, and increases the PET aperture to 70cm. However, BGO crystals are still used, and 2D and 3D acquisition options are reserved. The CT can be selected in 2, 4, 8, or 16 rows according to different needs.
(3) Philips' Gemini PET / CT
Philips' PET / CT is called Gemini. It uses Allegro type PET based on GSO crystal and is equipped with MX8000D dual-row CT. The CT part can be upgraded to 16 rows. The CT and PET are split, there is a gap in the middle, and the rear part can be moved a certain distance as needed. This open design effectively reduces the patient's claustrophobia in the pores, and is beneficial to perform some operations close to the patient during the examination, such as biopsy under CT guidance. The aperture of CT is 70cm, and the aperture of PET is 63cm. PET has no barriers, it is completely 3D acquisition. Attenuation correction can choose either CT image or 137Cs point source. The design of the PET detection part is different from the general crystal design.It uses proprietary Pixelar technology.The back of the crystal is connected to the closely arranged hexagonal photomultiplier tubes through continuous light guide materials.There are 7 photomultiplier tubes for each event Its position and energy make the photomultiplier tube more effective. In addition, its image reconstruction uses 3D reconstruction technology, although it requires a longer processing time, but more effectively use the collected information to reduce image artifacts.
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