Emergence of digital technology has brought innovations in the field of medicine. Amongst them is the emergence of mini C-arm. C-arm technology is based on the use of digital electronics and provides benefits that were not available in the 1970s and 1980s. Forces behind the modern mini C-arm technology are not different from those in any other field1. Some of the main features of mini C-arm are digital imaging technology, easily portable, image accessible at multiple locations at the same time, images can be transmitted over long distances without being distorted and better image quality.
In this article the emphasis will be on the features of the most advanced mini C-arm and its benefits over conventional and earlier versions of C-arm/mini C-arm. This study aims to review the mechanism, functioning, advantages, disadvantages and recent advances in mini C-arm technology2.
C-arm/mini C-arm mechanism
The mini C-arm mechanism is based on the fluoroscopic imaging. X-rays cause ionisation of atoms and can alter the DNA. The ionisation of atoms causes the movement of free electrons and that is termed the current. The fluoroscopy is performed by using the current value of around 3-6mA and it requires a voltage value of 80-125kVp. The production of X-ray is proportional to the current flowing through the X-ray tube; however, sensitivity is increased by increasing the voltage. A fluoroscope consists of various components; Figure 1 shows a typical fluoroscope and its components:
The X-ray generator allows the adjustments of voltages and currents through the X-ray tube; the function of the X-ray tube is to convert the electrical energy into an X-ray beam. The collimator determines the X-ray beam by using the blades; whereas patient table and pad provide support to the body of the patient and minimise the attenuation of X-ray. The image intensifier converts X-rays and enhances the brightness of the image. There are different sized diameter intensifiers based on the devices e.g. conventional and mini C-arm etc2.
A fluoroscopic unit operation is based on the automatic brightness model (ABC). In the ABC model the brightness of the image is monitored by the image intensifier. If there is not enough brightness the ABC enhances kVp which increases X-ray penetration through the body or tissue of the patient2.
X-rays are electromagnetic radiations and when they strike a tissue they either completely penetrate or are absorbed completely or partially with scatter. A complete penetration results in an image, complete absorption means no image. Partial absorption will result in an image with scattered radiation being potentially harmful to the theatre staff. To overcome such issues the C-arm technology was introduced2,3,4.
A C-arm unit can be manoeuvred in different ways based on whether the whole unit or only the C-arm part of the unit is moved, see Figure 2.
Figure 2: C-arm unit2
Some of the advancements in the C-arm technology are discussed in the next section.
Advantages and disadvantages
Fluoroscopic imaging systems such as mini C-arm and conventional C-arm both use X-rays to provide images, but there are many advantages offered by the mini C-arm compared with the conventional C-arm. Mini C-arm is smaller in size which gives better workplace access, mini C-arm is surgeon operated, it is easy to use and without the need for a radiographer, which will reduce delays, cost of surgery, and demand on the radiology department5,6,7.
It has been found that the use of the machine that allows surgeon control of the foot pedal can significantly reduce the screening time and scattered radiation dose7. The main benefit of mini C-arm over the conventional C-arm is to reduce the dose of scattered radiation to patient, surgeon and theatre personnel. And it can be explained by its small detector area, lower laser power, tight beam collimation, surgeon control of screening and the new technique of pulse fluoroscopy. Other benefits of mini C-arm are better manoeuvrability, precision and also immediate printout facility3,5,6,7,8,9.
There are some disadvantages as well, because of small low power X-ray generator and small field of view; the image quality of conventional C-arm is better than that of mini C-arm6. The Mini C-arm is mainly suitable for extremity surgery unlike the conventional C-arm that can be used for both axial and extremity surgery.
Uses Of mini C-arm
Mini C-arm is used intra-operatively in a wide range of procedures in foot, ankle and hand surgery. Mini C-arm can be used to identify fracture dislocations, stress views,
intra-articular injections, lesser toe fusions, bunion surgery and fracture fixations5,10,11.
In addition, mini C-arm has proved very useful for extremity imaging in outpatient setting11.
Recent advances in C-arm technology
A comparative study of some of the major brands of the mini C-arm technology is presented here. The models studied are:
Orthoscan mobile DI
Orthoscan HD with flat detector
Orthoscan FD pulse12
Table 1 describes the common features of the four models of mini C-arm. Table 2 shows the comparisons between FD pulse (most advanced) and earlier models.
A great advantage offered by the FD pulse model is the use of pulse fluoroscopy. This can be understood better by comparing the plot of dose (Y-axis) versus time (X-axis) for continuous versus pulsed fluoroscopy.
In Figure 3 the dose is the area under the curve of the graph. When compared with Figure 4 it can be seen that total dose is reduced greatly by the pulsed nature of the stream12,13.
Figure 5 shows the dose reduction comparison with operating mode of all four models. It can be seen that there is 82.5% reduction in dose with FD pulse mode compared with older models12,13.
Finally, the FD pulse model uses larger square shaped detectors. In the earlier models of mini C-arm, the technology employed for detector was antiquated rotating technology which was based on rectangular shaped detector. The square shaped detector in most advanced mini C-arm version has eliminated the need of antiquated rotating technology. As it is shown in Figure 6 that a squared shape detector has less spacing between the pixels which means a much better quality of image13.
Mini C-arm is a mobile fluoroscope with less radiation exposure to the surgeon, patient and theatre personnel. It is used for intra-operative imaging of a host of procedures and its simplicity of use, low cost and compact nature make it popular for routine use. Studies have shown that mini C-arm should be used in preference to the conventional C-arm for extremity surgery, mainly hand and foot surgery5,6.
Over the past several years mobile fluoroscopy has contributed greatly towards making the procedures less invasive and less expensive. Radiation exposure in orthopaedic procedures with standard C-arm is found to be in the range of 1200 to 4000 mrem/min. In contrast, a dose range of 120-400 mrem/min has been reported with the use of mini C-arm6. The international commission on radiology protection recommends annual whole body dose limit of 2 rem or 2000 mrem6. The radiation exposure with mini C-arm is less than that with the conventional C-arm. Radiation exposure should be kept to absolute minimum to the patient and the surgeon, and ionizing Radiation Regulation (2000) requires that the radiation exposure be clinically justified, performed by a trained professional and kept to as minimal exposure as possible and this is termed ALARA (As low as reasonably achievable) and delivered by trained medical personnel, because a lack of thorough knowledge of radiation physics may lead to imprudent images and may lead to increased exposure3,5,6. Even with mini C-arm it is possible to cause considerable radiation exposure if it is used in an injudicious manner. Further, the dose can be reduced by dose reducing recommendations8.
Optimum positioning of C-arm and extremity prior to imaging is essential to prevent the wasted exposure and it requires training and practice5. Image storage facility of C-arm should be used to avoid unnecessary exposures. The radiation dose can be further decreased by placing the rim directly on the image intensifier and also positioning theatre personnel beyond 15cm point of focus from image intensifier.
X-rays, just like any electromagnetic radiation, follows the inverse square law in that the intensity is inversely proportional to the square of the distance to the source3,5,6. Every attempt should be made to keep as far away from X-ray source/generator as possible (30cm minimum requirement by British law)5, and the intensifier as close as practically possible to the part being examined. Furthermore, the radiation is maximum at source and minimum behind the intensifier of the mini C-arm. Accordingly, positioning the limb such that the source is as far as possible and the intensifier as close as possible to the part being examined will reduce the direct exposure to the patient and scattered exposure to the surgeon. The other question that is frequently asked is the use of the protective lead gowns while using the mini C-arm. Though normally the surgeon is not exposed to direct radiation with mini C-arm but when the path between the radiation generator and image intensifier while the C-arm is active is crossed, the surgeon will be exposed to direct radiation. On the other hand, the patient and the surgical team are subjected to minimal radiation exposure beyond the direct path of X-ray beam. Therefore all the protective measures should be taken as per recommendations, even with mini C-arm. All the theatre personnel within a 2m zone should wear an apron with lead equivalence of 0.5mm. However with the mini C-arm 0.25mm lead equivalence is sufficient and the patient can also be protected from radiation by using a lead sheet over the abdomen5,7,8.
Recent advances in mini C-arm technology have seen significant improvements in radiation exposure due to enhancements in various aspects such as over rotation, flat detector area and FD pulse fluoroscopy12,13.
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