brief introduction

To explain MLC, SLC must be mentioned. MLC and SLC belong to two different types of NAND flash memory, which can be used as storage media for MP3 players, removable storage disks and other products. The full name of SLC is single-level cell, that is, single-level cell flash memory, while the full name of MLC is multi-level cell, that is, multi-level cell flash memory. The difference between them is that each cell of SLC can only store one bit of data, while each cell of MLC can store two bits of data, and the data density of MLC is twice that of SLC.

From the explanation of ranking, of course, MLC has its advantages because of its high density, and MLC also has great advantages in cost. It is understood that many chip manufacturers began to shift from SLC process to MLC process. In August 2006, Samsung officially switched from SLC to MLC. 10 in June, Samsung began mass production of MLC flash memory chips. Samsung's chip number is K9G, the chip at the beginning of K9L is MLC chip, and the chip at the beginning of HYUU is MLC chip.

trait

SLC is characterized by high cost, small capacity and high speed, while MLC is characterized by large capacity and low cost, but slow speed. Each cell of MLC is 2 bits, which is twice that of SLC. However, due to the large amount of data stored in each MLC memory cell and the relatively complex structure, the probability of errors will increase, and the errors need to be corrected, resulting in its performance greatly lagging behind the SLC flash memory with simple structure. In addition, the advantage of SLC flash memory is that the number of copies is as high as 100000 times, which is 10 times higher than MLC flash memory. In addition, in order to ensure the life of MLC, the control chip verified the intelligent wear-leveling algorithm, so that the write times of each memory cell can be shared equally, reaching 6,543.8+0,000 hours of time between failures (MTBF).

disadvantaged

However, although MLC has its own advantages, it can't hide its shortcomings.

1, poor reading and writing efficiency.

Compared with SLC flash memory, MLC has poor reading and writing performance. SLC flash memory can read and write about 65438+ million times repeatedly, while MLC can only read and write about 10000 times, and even some products can only read and write about 5000 times.

2. Slow reading and writing speed

Under the same conditions, the reading and writing speed of MLC is slower than that of SLC chip, and the speed of MLC chip is only about 2M.

3. High energy consumption

Under the same operating conditions, MLC consumes more energy than SLC, consuming about 15% current.

These reasons largely depend on the change of MLC system and need the support of new control chips. However, some products, such as MP3 and USB flash drive, still continue the old-fashioned design, and MLC will bring various problems, including data loss and slow transmission speed. In 2006, a large number of SD cards were recalled because MLC chips were used instead of new main control chips, which had a great impact.

condition

With the mass production of MLC flash memory chips from Samsung and Toshiba, MLC chips are more and more widely used. Due to the increase of storage density of new MLC chips, the requirements for the main control chip are getting higher and higher. Digital devices that read and write frequently, such as digital players and flash drives, also increase the error probability of MLC flash memory. For applications such as video and audio, control chip and ECC verification mechanism are needed. At present, some main control chips have passed the pure software verification, which invisibly increases the burden of the main control chips. There are also some hosts that combine hardware 4bitECC verification with software verification, which reduces the burden on the host, but this only reduces the probability of errors to a certain extent. The disadvantages of MLC, such as the limitation of chip write times and transmission speed, cannot be overcome.

MLC is better than SLC in architecture. Many manufacturers have done a lot of optimization and development on MLC, and the future may be a mainstream direction, and the technology is not very mature. From the cost point of view, MLC is cheaper than SLC chip, so many manufacturers choose MLC chip in the original architecture, but without control chip and ECC check, which causes many problems and makes many people in the industry exclaim that MLC is a "black core". Therefore, when you buy digital products such as MP3 and USB flash drive, you should not only look at the price, but also consider it at multiple levels.

MLC technology began to heat up. It should be said that Toshiba launched the first MLC architecture NAND Flash in February 2003. At that time, Samsung Electronics, as the leading enterprise of NAND Flash, was very disdainful of this architecture and still insisted on promoting SLC architecture. In the following year, in April, 2004, Toshiba successively launched 4-bit and 8-8Gbit NAND Flash with MLC technology, which obviously added strength to NAND Flash, which has been known for its capacity. Samsung Electronics has been advocating SLC architecture for a long time, claiming that SLC is superior to MLC, but the ISSCC papers on MLC technology published by the company in 2004 and 2005 initially show that its views have changed. Samsung has not provided any marketing materials about MLC flash memory on its website, but it has developed a 4gb MLC NAND flash memory. The die area of this product is 156mm2, which is larger than Toshiba's 90nm MLC NAND flash memory by 18mm2. The competition between the two mainstream NAND flash memory manufacturers in MLC architecture officially began. In addition to Samsung and Toshiba, IM Technology, which owns Intel MLC technology, hopes to surpass its competitors in technology and MLC and has the momentum of latecomers. The competition of MLC technology is in full swing. Multi-line center. Beijing Metro has added a special line center called MLC between ACC (Ticket Clearing Center) and LC (Line Center). The functions and functions of MLC are as follows:

Suppose there is a rail transit ticketing sorting center ACC and n rail transit lines LC in a certain area. These N lines are managed by M different rail transit operators, among which M

It is a mechanical moving part used to generate conformal radiation field, commonly known as multi-leaf grating and multi-leaf aperture, and is widely used in medical field.

concept

English name: multi-leaf collimator (MLC for short)

In 2000, IEC60976 revised IEC976 and IEC977, mainly adding the content of multi-leaf collimator.

type

According to multi-leaf collimator's sports mode, there are two kinds of multi-leaf collimator: manual and electric. The role of the latter is far greater than that of the former, and it is the main form; Manual multi-leaf collimator is to adjust the profile of radiation field by manually driving each blade; The electric multi-leaf collimator is controlled by a computer, which drives each blade to move independently, so as to achieve the purpose of dynamically or statically forming a shooting field.

Multi-leaf collimator usually needs to be used in conjunction with the secondary collimator of the radiation head. Therefore, according to the installation method of multi-leaf collimator, there are two types: external and internal.

Because for most target areas with different shapes and sizes, only a few blades are generally within the effective field of view, and the remaining blades outside the effective field of view should be combined in pairs to prevent radiation leakage. However, in order to avoid mechanical damage caused by the relative collision of paired blades, a gap is usually left. In this way, it is necessary to specify a minimum circumscribed rectangular field for the conventional therapeutic collimator of the accelerator, so that it can not only shield the intermittent leakage rays of each pair of incompletely closed blade end faces in the effective field, but also shield the tiny circumscribed rectangular field between adjacent blades, and transmit the corresponding control data to the corresponding control system, so as to realize the automatic setting and following of the minimum rectangular field and MLC effective field. This is unnecessary for people who have no leakage between pairs of blades in the conformal field.

design feature

Since the advent of multi-leaf collimator, the structural design of multi-leaf collimator has been improving and perfecting. In order to adapt to various functions and uses, countries all over the world have successively introduced various structural forms of multi-leaf collimator. Throughout the development history of multi-leaf collimator, it mainly focuses on improving consistency, reducing transmission penumbra, reducing leakage, adapting to dynamic and dynamic wedge plates and other advanced functions. For example, the number of leaves is from small to large, and the width of leaves is from large to small; The maximum irradiation field develops to both ends of the size as needed; The focusing mode ranges from no focusing to single focusing or double focusing; Adjacent blades are inserted into concave and convex by plane contact; The stroke of the opposite blade from the centerline to the centerline is from small to large. Coupled with the rapid development of hardware of independent drive mechanism, the functions of MLC system have greatly increased, and it has gradually developed in the direction of meeting the requirements of clinical application, reducing costs, facilitating processing, simple operation, high reliability and low fault.

The width of the blade directly determines the geometric fitness (coincidence) between the irregular radiation field composed of multi-leaf collimator and the shape of the planned target area (PTV). The thinner the blade, the better the fit, but the more difficult it is to process. The driving motor and other mechanisms become more and more complex, and the higher the cost. Therefore, a reasonable compromise must be made between consistency and cost.

The height of the blade must be able to reduce the original radiation and radiation intensity to less than 5%, that is, at least 4. 5 half-value thicknesses. Due to the need to maintain low-resistance relative dynamic motion between blades, there are often some leakage rays between blades, which will reduce the shielding effect of blades on the original rays. The height of the blade needs to be properly thickened, generally not less than 5cm thick tungsten alloy. If the leaked radiation dose is reduced below 2%, a tungsten alloy thickness of 7.5cm is usually required.

Two factors should be considered when designing the longitudinal section of the blade:

A. In order to ensure the minimum leakage dose between adjacent blades and when the opposite blades are closed, it is determined that most of the side surfaces of the blades are embedded with concave-convex grooves. The concave-convex groove can be machined in the middle of the blade height, but this structure requires high machining accuracy and great technical difficulty, and sometimes it is found that individual blades fail due to large motion resistance in use, so many manufacturers later adopted the step structure for the blades.

B the bottom surface and top surface of the blade must converge to the position of the X-ray target in the plane perpendicular to the moving direction, which determines that the cross section of the blade should be trapezoidal, that is, the width of the bottom surface should be greater than the width of the top surface, so that any blade is parallel to the ray radiated from the source (target) and passing through this surface. Through machining, all the blades move on the circumference with the radiation source as the center and the distance from the radiation source to the bottom of the blade as the radius, and a penumbra-free double focusing structure can be formed.

In order to reduce the influence of blade end face on penumbra, the design of blade end face is particularly important. There are usually two design types; Curved end face and upright end face. After the circular arc design is adopted, the original light can be tangent to the end face at any position where the blade moves in the direction perpendicular to the central axis of the light. The penumbra of the field can be enlarged by using the curved end face, and the penumbra will change with the position of the blade leaving the central axis of the beam. However, if the curvature radius of the end face is reasonably selected, the tangential length between the light and the end face can be kept approximately constant during the whole linear motion of the blade, so that the penumbra of the shooting field of view can be kept basically constant and does not change with the position of the blade.

When the vertical end face design is adopted, the blade can move in two ways:

A. the blade moves along an arc trajectory centered on the X-ray source (target). At this time, no matter where it is, its end face is always tangent to the original light.

B. If the blade moves along a straight track perpendicular to the central axis of the beam, the blade must be rotated by a small angle after reaching the designated position, so that its upright end face is tangent to the divergence of the original light. Because there are many blades, this corner design technology is difficult.

(1) non-focus structure

Early MLC was mainly used for small lesions in the head and body, mostly with unfocused leaf translation structure. This kind of blade has the same thickness from top to bottom, left and right, and all blades move in translation. The field of view formed by the top and bottom of the blade is the same size and shape, and the penetrating penumbra cannot be eliminated. For Xiaoye, because the opening angle of the beam is very small, it has little influence; However, for Xiao Ye, it will cause a bigger semi-shadow which is unacceptable in clinic.

(2) Single focusing structure

This structure makes all blades move on the circumference with the radiation source as the center and the distance from the radiation source to the bottom of the blade as the radius, so that the end face of the blade is always parallel to the ray beam, and the penumbra in the direction of blade movement is eliminated. However, in the direction perpendicular to the blade movement, there is still a penumbra, because the upper and lower widths of the blade are equal. Before 1996, the large MLC(40cm×40cm as the maximum field of view) produced for body was mostly of this single focus structure.

(3) Double focusing structure

It is necessary to adopt a dual-focus structure for MLC installed on an unfocused secondary collimation therapy machine. Double focus structure is to process each blade of MLC with single focus structure into divergent shape with unequal width in the width direction, and the end face is trapezoidal, and the upward extension line of each end face should intersect with the radioactive source point. In other words, both sides and end faces of each blade must be parallel to its adjacent beam at any position. When installed on any therapeutic machine, this structure can eliminate penetrating penumbra. Of course, the focus design of penumbra elimination is related to the installation height of MLC, and many factors such as circuit connection, counterweight, structural space, drive control and so on should be considered. Because of the complex structure and high design requirements of the accelerator head, it will cause many difficulties to transform the accelerator head that has been used. Therefore, except for the small and medium-sized MLC, most MLC with double focus and blade structure in multi-leaf collimator are produced by accelerator manufacturers at home and abroad.

(4) Leakage-proof structure

Clinical application requires each blade to move independently and flexibly with low friction. Adjacent blades should not be squeezed too tightly, and sticking too loosely will easily lead to radiation leakage. In order to solve this contradiction, each blade can be processed into a groove on one side and a tenon on the other, so that two adjacent blades can obtain a good anti-leakage effect by overlapping the groove and tenon, and taking advantage of the characteristic that light can only travel in a straight line. This combination of groove and tenon is neither too tight nor too deep. The MLC of some companies is composed of 40 pairs of tungsten alloys, the blade thickness is 7.5cm, the projected width on the isocentric plane is 1. 1cm, and the overlapping thickness of tenons of adjacent blades on the isocentric plane is only 0. 1cm, so the projected center distance between adjacent blades is 1.0cm, which can ensure the leakage rate.

(5) Design of crossing the center line

With the further development of MLC application, the application technologies such as dynamic nonlinear wedge field, dynamic intensity modulation and reverse design for different shapes and complex dose distribution fields are more and more advanced, which often require pairs of blades to move in the same direction from the farthest end to the other end at different speeds. Therefore, the line stroke of blade movement is a necessary condition to realize high-performance conformal strong irradiation method, and it has become one of the important indexes to measure the function of modern MLC. The centerline stroke of the blade is required to be as large as possible, generally not less than 12cm.

control point

In order to make each blade reach the accurate position at any time, each manufacturer adopts different blade control methods, but they must include three contents:

A. monitoring blade position

Include using mechanical limit switches to monitor the switch states of blades, optical camera systems, linear encoders, etc.

B. Blade control logic

Include control switch state, blade position, blade moving speed and blade dose compensation.

C. blade moving mechanism

Use digital or analog mode to control the blade in place.

(1) Monitor blade position

In order to ensure that the blades are in place safely and reliably, the position of the blades must be monitored regularly. For the switch collimator, the mechanical limit switch is used to monitor the switch (on/off) state of the blade. Another common method is to use a high-precision linear potentiometer as a linear encoder, which has good linearity and accuracy. However, due to too many wires and large space, once the potentiometer has problems, it is difficult to find them in the compact MLC, so it is necessary to use a potentiometer with high reliability and high quality. Another monitoring method is to use an optical camera: a beam splitter is added to the original field illumination system of the accelerator therapeutic head, and the light reflected from the upper end face of MLC is reflected to the position receiver of MLC through the beam splitter. The common receiver is CCD camera, which converts the video signal into digital signal and sends it to the image processor in MLC controller, so that the blade position of MLC can be monitored. The advantages of this optical camera system are: real-time display of MLC leaf position, less wiring, high spatial resolution and good position linearity. However, CCD camera is not resistant to radiation and needs to be replaced frequently.

(2) Control of blade position

The determination and control of blade position is the premise to realize MLC function. The position of the blade should be consistent with the boundary of the field it is intended to form. The blade position recorded or displayed by linear coding potentiometer or optical camera system should be equivalent to the size of light field and must also be the size of actual light field. For the double-focus MLC with vertical end face, because its end face is always parallel to the light diffusion, the calibration method of its field is the same as the conventional method. But for MLC blades with circular end faces, the situation becomes complicated, because the light indicates the position of the tangent point of the end face, not the position where the original light intensity is weakened by 50%. Fortunately, it is proved by calculation and practice that the maximum difference between light field and light field does not exceed 1mm within the used field of view. On some MLC, by shortening the distance from the light source to the center of 1cm(SAD=99cm), the indicating range of the light field can be slightly expanded to conform to the light field, but at this time, the light field of the lower Ye Zhun collimator will be slightly larger than the light field. In order to solve this problem, a pair of thin aluminum extinction devices are attached to the upper end face of the lower Ye Zhun collimator, which are consistent with each other.

The above method solves the problem that the light and radiation fields are inconsistent at the isocenter level, but there are still errors in the irradiation of non-nominal source skin distance. Therefore, in some designs, the size of the radiation field and the corresponding position of MLC blades are listed in the table and stored in the MLC control microcomputer. As long as the size of the prescription radiation field is well guided, the position where the blade should move can be obtained. The blade movement control logic can also control the speed of blade movement and the collision between the opposite blade and the adjacent blade according to the treatment needs (such as whether to adjust the intensity).

(3) Blade drive mechanism

For switching MLC, piston pneumatic control is usually adopted, which can make the blades enter the open and close state quickly; For the non-switching standard MLC, it is generally driven by a micro motor, and the rotary motion of the motor is converted into the linear motion of the blade through the lead screw. The moving speed of the blade can be designed to be about 0.2-50 mm/s, and the common speed is 1-2 cm/s.

(4) proofreading of blade position

The proofreading of blade position is an important measure to ensure that the blade is in place accurately. It corresponds the pixel signal from CCD camera or the voltage signal from linear potentiometer to the position of the blade one by one, and repeats it regularly. MLC self-calibration systems produced by different companies are also different. In some MLC systems, a narrow and long infrared beam perpendicular to the moving direction of MLC is preset. When the MLC is driven, the blade automatically passes over it. After the infrared width of the blade is intercepted, it is compared with the coded information of the blade position, and then calibrated according to the pre-listed geometric relationship calculation formula, and stored in the corresponding table of MLC control computer. In some MLC, four fixed reference reflectors are preset in the treatment head to form a fixed reference field frame. When proofreading, only a set of preset default fields need to be irradiated by film, and MLC fields are calibrated by film method.

(5) Automatic tracking of therapeutic collimator or standby collimator.

The automatic tracking of the therapeutic or standby collimator is to shield the leaking rays between the opposite blade and the adjacent blade. In addition to standby collimators, some use standard accelerator treatment collimators to follow. The position of the next collimator should be controlled by the coded signal of the current position of the corresponding MLC leaf.

use

The main purpose of the development of multi-leaf collimator is to realize conformal radiotherapy. However, it has many potential functions because of its excellent mechanical structure and flexible and diverse precise movements under the control of computer automation.

The simplest example is to replace the traditional solid stop. Actually, in radiation therapy, In particular, some large fields and some postoperative radiotherapy only need a few fixed fields with apertures. For example, large-area cloak field, hoe field, face-neck combined field, character field, electronic one-way fixed field of superficial tumor, cross field in standardized radiotherapy, three-field angle irradiation and so on. This static fixed field irradiation has no requirement for dynamic control of stopping, and can be easily completed by manual MLC and MLC with precise position control function.

No matter whether it is the positioning film or the body surface marker field obtained by simulation positioning, as long as the shape and isocenter (or coordinate origin) of the field are traced in proportion, and the three-dimensional treatment planning system used in conjunction with MLC is input by digitizer or scanner, the system can soon drive each leaf of MLC with the edited data file to form the required conformal stop field shape.