The CT surrogate tube technology has been playing a role in hospital imaging diagnosis for many years. With the continuous development of CT technology, its clinical role is also constantly improving. Based on the technical development process of X-ray CT, it can be divided into several stages: non-slip ring CT, slip ring CT, spiral CT, and multi-slice spiral CT. Soon, X-ray beam CTS with flat panel detectors will also be available. However, in terms of clinical functions, the role played by each new technology is different. Some are merely accumulations in quantity, while others lead to a qualitative leap. And we can view the technological development of CT from the perspective of application performance.
(1) Performance request-oriented technology development
For hospitals, the main requirements for the performance of CT applications are: examination speed, image quality, and analytical processing capabilities. These three aspects are the basic guarantees for clinical performance. Of course, the scalability of the software and the issue of X-ray dosage will also be taken into consideration. The inspection speed includes: scanning speed, the speed of image reconstruction, and the speed at which the user operates the machine. The three factors jointly affect and examine the patient's circulation volume. Among them, the improvement in scanning speed is particularly crucial. The improvement in scanning speed has brought about significant changes to CT. Earlier CT scans took tens of seconds to scan one layer of image, and the image quality of the chest and abdomen affected by respiratory movement could not be guaranteed.
To solve this problem, the slip ring technology was developed, which also brought about the second qualitative progress in CT clinical technology. The X-ray tube ball and detector of the slip ring CT can rotate in the same direction all the time, which significantly improves the scanning speed. This enables holding one's breath to scan one layer of image, ensuring image quality. It also significantly enhances the diagnostic ability of CT in the chest and abdomen, as well as the examination speed.
However, it is difficult to accurately determine the false negatives and false positives (missed scans or repeated scans of lesions) caused by the positional changes of organs during each breath holding when using the slip ring CT substitute tube. To solve this problem, it is necessary to constantly increase the scanning speed. However, the continuous step method of the examination bed poses a significant obstacle to the improvement of the scanning speed. The spiral acquisition (volumetric acquisition) technology has solved this difficult problem and brought about the second leap in CT clinical technology at the same time. Spiral CT acquires overall volumetric data within the scanning range while the scanning frame rotates at a constant speed and the examination bed moves continuously at a constant speed. It can reconstruct cross-sectional images at any position within the scanning range. This has greatly improved the continuous acquisition capacity and speed of CT. With this technology, holding one's breath at a time can scan one area, eliminating false negatives and false positives caused by respiratory movements. This not only provides a technical foundation for ensuring image quality and diagnostic accuracy, but also gives rise to clinically significant analytical treatment methods such as CTA (CT angiography), CEV (Simulated Endoscopy), and three-dimensional reconstruction. Nowadays, spiral CT can achieve scanning times of one second or less.
The next objective is to counteract the impact of heartbeats. For this purpose, multi-row detector technology and gating technology have found reasonable solutions. This advancement represents the third qualitative leap in CT clinical technology. It is impossible to complete the heart scan in one cardiac cycle. However, gating technology enables us to selectively remove the data from the systolic period of the heart when collecting data, so that we can select the data of the same time and phase within each cardiac cycle to reconstruct the image. Multi-slice CT with four or more layers (including four layers), after injecting contrast agents and combining with gating technology, can display coronary arteries and their branches. Compared with previous coronary artery examination methods, this examination is a less invasive method. This examination has the ability to display coronary artery branches and the diagnostic value of lesions. After being compared and evaluated with coronary angiography using catheters and comprehensive clinical diagnosis, it once entered the stage of clinical application. The acceleration of scanning speed has also continuously improved the temporal resolution of enhanced scanning.
To reduce X-ray exposure, automatic milliampere control technology was once applied to CT. In the past, the automatic control method was to calculate and determine the current of each layer of the scanning image based on the scanning positioning image. However, due to the difficulty in differentiating the patient's breath-holding position when using the scanning positioning image and the cross-sectional image, coupled with the significant difference in body thickness between the frontal and lateral positions of the human body, this method is not easy to maintain the stability of image quality. The new automatic milliampere control technology, during the scanning process, adjusts the tube current in real time through the automatic response function, achieving accurate automatic control of the X-ray quantity.
The continuous acceleration of the reconstruction time has improved the inspection speed and enabled the examiner to view the scanned images earlier during the enhanced scanning, so as to decide whether to stop the scanning of other phases. The ideal reconstruction speed is simultaneous reconstruction, that is, the scanning time is the same as the reconstruction time, ensuring that the examiner can immediately see all the images when the scanning is completed.
The time spent operating the machine is related to the sensitivity and convenience of the software, which is of great significance for improving the patient flow rate. The image quality has been constantly improving through years of technological development. The limit of its spatial resolution is the human eye's resolution ability - 0.2mm.
The improvement in analytical processing capabilities is mainly reflected in the increase in processing speed and image display methods. During the period of rapid development of information technology, the scalability of machine software is an indispensable request for users.
The existing multi-slice spiral CT substitute tubes have evolved from single-slice to 64-slice detectors, continuously improving the scanning speed, image quality and analytical processing capabilities. However, due to the conical shape of the X-rays emitted by the X-ray tube ball, if the detector is too wide, the image near the edge will tend from the ideal cross-section to a funnel shape. This problem limits the width of the detector and also hinders another leap in scanning speed.
To address this issue, some manufacturers are currently discussing cone-beam CT for flat panel detectors. The continuous progress of technology will bring us one surprise after another.
(2) Clinical demand-oriented CT market
The technology of CT is advancing at an astonishing pace. We can roughly classify the existing CTS on the market into three categories based on clinical needs:
(1) CT that meets the requirements of general clinical examinations. Such as ordinary scanning slip ring CT.
(2) A CT that is fast and has high image quality, meeting the requirements for all examinations except cardiovascular ones. Such as spiral CT with less than 4 rows and a scanning time of 1 second or less. In terms of checking the patient circulation volume, there is no significant difference between single-slice and 4-slice CT (taking 50 people examined daily and each person scanned 30 layers as an example, the scanning time of a 1-second single-slice CT only takes 25 minutes). Even a faster scanning speed would only save more than 25 minutes a day.
(3) Not only is it required that the examination speed be fast and the image quality be high, but also that the CT for cardiovascular examination, that is, multi-slice CT with 4 or more (including 4 slices), should have gating technology.
What kind of CT to purchase is not only related to the hospital's level, but also to its purchasing capacity and clinical needs. Hospitals should purchase equipment based on their own research, teaching and clinical needs. For the majority of hospitals in our country, spiral CT ranging from 1-second single-slice to sub-second 4-layer multi-slice can meet clinical needs. For large hospitals that require more multi-slice CT scans and have research and teaching tasks, due to the extremely large number of patients undergoing CT examinations, a single multi-slice spiral CT cannot meet the demand. They often also need a spiral CT with less than four slices at the same time. Therefore, the demand for single-slice 1-second to sub-second 4-layer multi-slice spiral CTS in the Chinese market will be large in the coming years. Of course, some hospitals will also consider ensuring that their technology does not fall behind within the service life. However, the "not falling behind" here should be compared with the development of the hospital, rather than with the technological development of CT. We believe that as long as the equipment can meet the clinical needs of the hospital throughout its normal service life, it is an advanced device.
Nowadays, the competition among domestic hospitals is also extremely fierce, leading to a sense of comparison in purchasing equipment. The charging standards for CT substitute tube examinations formulated by various central governments will also guide the standards for hospitals to purchase equipment. Some hospitals find it difficult to obtain funds from their superiors at one time. Therefore, within the allowable range of funds, they should try to purchase equipment as much as possible. Due to numerous reasons, some hospitals have purchased equipment that exceeds their actual needs, resulting in some advanced equipment not fully exerting its performance and causing a certain level of resource waste. These phenomena of unreasonable resource allocation will be improved with the reform of the country's system and the continuous strengthening of people's economic awareness.