Automated Exposure Control and Anatomically Programmed Radiographic Devices in Radiology - Term Paper Example

AEC and APR Devices in Radiology Student’s Name/ID Course/Professor’s Name Date Contents AEC and APR Devices in Radiology 1 Student’s Name/ID 1 Course/Professor’s Name 1 Date 1 Contents 2 Introduction 2 The Ionization Chamber Sensor AEC 3 The Solid-State Device Sensor AEC 4 The Fluorescent Screen Phototimer 5 Clinical Operation of AECs 5 Advantages and Limitations with AEC usage 6 Anatomically Programmed Radiography Device 7 Conclusion 9 Introduction Within a year of its discovery in 1895, X-rays have been incorporated in medicine to aid in diagnosis of many conditions. Today Medicine is arguably completely dependant on this section of the electromagnetic spectrum for not only aiding in the diagnosis, but also for helping in treatment of many lethal diseases like Cancer. It has been widely known that X-rays are a double edged sword since the death of Clarence Madison Dally. The aim for Radiographical specialists is to provide techniques and devices that can be used to achieve the best results for their function consistently, while giving the patient the least amount of radiation that is necessary for the situation. Modern X-Ray machines have been designed to achieve this aim with the incorporation of two devices. The first is called Automated Exposure Control (AEC). The second is named Anatomically Programmed Radiographic Device (APR-Device). Aim and Basic Design of an AEC The Aim of an AEC device is to aid the technician in limiting the maximum exposure of a patient to X-rays (Long, Frank & Ehrlich, 2012). The theory behind the device involves a sensor to be placed on the radiographic cassette, or digital X-Ray reader. This sensor will use the radiation that reaches is to generate a current. The magnitude of this current directly relates to the magnitude of exposure. When the current registered reaches a level that has been set by the technician, the machine automatically shuts off the exposure (Bushberg, Seibert, Leidholt & Boone, 2011). It must be emphasized that the machine registers Maximum exposure, thus the technician still has full control over the exposure via a manual lever until harmful levels are reached. Although the overall system of an AEC has remained relatively unchanged since its conception, the way radiation is measured has evolved. Today, three kinds of sensors exist that are used to measure the amount of radiation a patient has received. The Ionization Chamber Sensor AEC The first sensor is composed of a series of thin ionization chambers that are placed on the surface of the X-Ray receiver. The ionization chambers function similar to a pocket Dosimeter. It consists of a clear chamber with a relatively inert gas trapped within. The periphery of the chamber is composed of frame of extremely thin copper wiring, which is kept positively charged via a battery or a generator. The exact arrangement of the wiring differs for sensors used for different regions of the body. The air inside the chambers is ionized as radiation passes through and releases electrons. These electrons are then taken up by the positively charged metal wires. These electrons will continue to flow down the wire and into the positive terminal of the battery or generator. This induced current is what is measured by the AEC and converted into a digital reading of the amount of radiation the patient has been exposed to (Bushberg, Seibert, Leidholt & Boone, 2011) (Carrol, 2011). The aforementioned induced current continues to flow until it reaches a capacitor, where it is stored. When the stored current reaches the maximum level, as set by the technician, it is released by a Thyrotron device, and flows into an electromagnet. The Thyrotron is a small device that regulates how much current a capacitor can hold before discharging. It can be set manually by the technician. One the current flows into the electromagnet, it activates and pulls open a switch which terminates the entire circuit of the machine, shutting down the emission of radiation. The failsafe to the entire mechanism involves two switches that must be held closed for the X-Ray machine to work. One is manually operated by the technician while the second is controlled by the AEC (Bushberg, Seibert, Leidholt & Boone, 2011) (Carrol, 2011). The Solid-State Device Sensor AEC The second sensor which is used is a solid state device. This device can detect radiation in one of many ways. It can be composed of a compound that emits visible light when exposed to radiation, and sensors within the device measure the amount of visible light emitted. The device may also be composed of a compound or metal that generates current when exposed to radiation itself. Modern solid state devices use a combination of these methods or use an amplification method to measure the exposure even more accurately. Solid state devices are usually placed behind the X-ray cassette due to their high radio-opacity. Some recent advancement in technology have produced solid state devices that are small enough to be placed in front while still being clear of the patient. The system of the AEC that uses solid state sensors works in the same principle way, though it has to be calibrated using a different protocol specifically designed for the sensor (Carrol, 2011). The Fluorescent Screen Phototimer An older yet still widely used AEC exists that must be mentioned. This AEC uses a Phototimer as a sensor. The Phototimer consists of a Fluorescent screen placed behind the cassette. It absorbs the radiation that passes through the Cassette and gives of fluorescent light. This is absorbed by a Photocathode, which is in the shape of a tube, and a current is generated (Currey Dowdey & Murrey, 1990) (Bushberg, Seibert, Leidholt & Boone, 2011). This kind of sensor is still in use in some places of America and the third world, although it has been replaced by ionization chamber AEC’s in most of the west and the more developed countries in Asia and the Far East. Clinical Operation of AECs Although the aim of an AEC device is to safely regulate the amount of radiation a patient is exposed to, it needs a significant amount of input from the technician before it can do so. When an X-ray machine is first set up, the technician has to use it on a selection of anatomical models made up of radio-absorbent materials for each region of the body the X-ray machine is used for. The exact amount of exposure is then calculated via a series of experimentation and extrapolation. The technician must then keep a chart in which he will write down the best settings used for each type of radiograph needed. However, the anatomical models mostly used simulate the average individual in an ideal X-ray imaging setting. Clinical settings hardly consist of these ideal situations and the technician needs to have the appropriate expertise to accommodate for such situations (Carrol, 2011) (Hertrich, 2005). Two situations that occur commonly in a clinical setting for which the technician needs to adjust for are imaging a large patient, or one on her side, and a patient who is thin or small. For larger patients, it takes the radiation a longer time to travel through and reach the sensor. This means that it will take the sensor a longer time to generate enough current to shut down the system. This will result in the larger patient being exposed to more radiation than what may be ideal. The same occurs when a patient is lying on their side. In smaller or thinner patients, the patient does not have enough mass to absorb radiation and the AEC may shut down prematurely, producing an image of poor quality. Previously, the technician had to rely on their instincts and quick calculations to manually account for these situations. Modern X-Ray machines have AEC’s that come with a density control. This density control allows the technician to alter the AEC’s limits by set amounts, usually increments of 25%, without having to recalibrate the entire AEC. Without this system the technician would either need to calibrate the entire machine using an endless amount of models on a frequent basis, or rely on their own instincts without the AEC, which would defeat the entire purpose of having such a device. Advantages and Limitations with AEC usage The AEC device has many advantages over a completely manually operated machine. The first advantage is that it limits the exposure f radiation to the patient. This is very useful in CT-scans and interventional radiological procedures like Angiographies, where a patient must be exposed to high amounts of radiation and the danger of exceeding normal levels is very real (Mulkens, Bellink, Baeyaert & Ghyson, 2005). The second advantage these devices provide is that of consistency of image quality. Over and under exposure and penetration of a radiograph was a common problem when the technician was recently acquainted with the machine or simply had poor skill, but the AEC can fix this problem by removing the highly variable human factor. This is very useful when taking radiographs which require very short bursts of exposure, which would require a technician to be very attentive and have fast reflexes. Other advantages include a lower film and processing cost and larger emission tube life, both have to do with the fact that the AEC enables the technician to take the image with the least amount of radiation need, thus reducing the resources used. The overall efficiency of the department and the hospital increases as doctors do not need to repeat X-rays if they were of poor quality (Rossi, Harnisch & Hendee 1982) (Mulkens, Bellink, Baeyaert & Ghyson, 2005) (Singh, Kalra & Thrall, 2011). The limitations of an AEC are that if at anytime the items that lie behind the AEC, like the Cassette holder or digital receiver, are changed, the technician will need to go through the entire process of experimentation and extrapolation to ensure that the settings he uses are ideal. The same holds true if the AEC sensor is renewed or the capacitor-Thyrotron system is changed. Another disadvantage is that radiological technicians have a tendency to become overly dependant on the AEC, which is a problem when the AEC is changed and they can not cope with the new system. Finally, the older Phototimer AEC systems had a tendency to shut down prematurely, despite the density control, if a patient could not cover the fluorescent screen enough and unhindered radiation reached it. Anatomically Programmed Radiography Device As mentioned above, a technician must keep a comprehensive chart of all the settings he has calculated. These need to be on hand at all times so they can be referenced to ensure an image of the best quality. This task has been simplified and made easier for a technician by modern technology with the introduction of sophisticated computers within an X-ray machine called the Anatomically Programmed Radiography device. The APR device is a computerized control that is programmable so that the settings and exposure factors listed in the technique chart can be made available on the screen of the X-ray machine. Instead of repeatedly referencing the technique chart again and again, the technician can simply select the desired settings from the X-ray machine. A well versed technician can even save the settings used on the machine for commonly found situations, which will reduce the amount of adjustment that the technician needs o make each time. The technician can even save personalized data for a patient that stays in a hospital and requires frequent X-rays over their stay (Long, Frank & Ehrlich, 2012) (Anatomical Programming Techniques, 2011) (U.S Food and Drug Assosiation, 2012.). Advantages and Limitations of APR Although the system provides an interactive interface to make the technician’s job easier, it must first be programmed by the technician. Given the amount of data that must be input, and the amount of factors that change exposure to a patient, it is easy for a technician to program to many settings and later get confused so as to which is the best. Also, since a technician must program the system when it is initially introduced, it is highly personalized to the technician. A different technician may not be able to comprehend another’s work and will have to reprogram and recalibrate the entire system. However, these grievances can be minimized when the system is in the hands of a well trained and intelligent technician (Anatomical Programming Techniques, 2011). Conclusion AEC systems aid in providing a good quality image every time the machine is used. APR devices can give a technician a selectable start point, so that he does not have to set the machine each time and instead, can devote time to adjust the machine for the differences found in each patient and each radiographic session. Working together, they allow the technician to set a minimum and maximum level of exposure that is personalized for patients of every body type. This almost completely takes away the exposure control from the technician at the time of active radiation emission. Hence it ensures that the patient receives the least amount of radiation required for the job, which is beneficial for the patient, the department and the hospital itself. Works Cited Long, B. W., Frank, E. D., & Ehrlich, R. A. (2012). Radiography essentials for limited practice. (4 ed.). Elsevier Science Health Science Division. Curry, T. S., Dowdey, J. E., & Murrey, R. C. J. (1990). Christensen ́s physics of diagnostic radiology 4 ed. (pp. 57-59). Lippincott Williams & Wilkins. Bushberg, J. T., Seibert, A., Leidholt, E. M., & Boone, J. M. (2011). The essential physics of medical imaging. (3rd ed., pp. 251-255). Lippincott Williams & Wilkins. Pettersson, H., & Allison, D. J. (1998). The encyclopaedia of medical imaging. (Vol. 3 pt1, p. 39). Taylor & Francis. Hertrich, P. H. (2005). Practical radiography. (1 ed., pp. 192-200). John Wiley & Sons. Brozino, J. D. (2000). The biomedical engineering handbook.. (1 ed., Vol. 1, p. 61.31). Springer. Carrol, Q. B. (2011). Radiography in a digital age. (pp. 156-159). Charles C. Thomas Ltd. Rossi, R. P., Harnisch, B. D., & Hendee, W. R. (1982). Evaluation of an automatic exposure control device for mobile radiography. Radiology., 823-827. Retrieved from http://radiology.rsna.org/content/145/3/823.full.pdf Mulkens, T. H., Bellink, B., Baeyaert, M., & Ghyson, D. (2005). Use of an automatic exposure control mechanism for dose optimization in multi–detector row ct examinations: Clinical evaluation. Radiology., (237), 213-223. Retrieved from http://radiology.rsna.org/content/237/1/213.full Singh, S., Kalra, M. K., & Thrall, J. H. (2011). Automatic exposure control in ct: Applications and limitations. The Medical Physics Consult., 447-449. Retrieved from http://doseoptimization.jacr.org/Content/PDF/Singh-Automatic.pdf Anatomical programming and techniques. (2011). Retrieved from http://www.leswilkins.com/anatomic.htm U.S. Food and Drug Assosiasion, Radiation Emission Products. Automatic exposure control (aec) performance testing – annual physics survey and mammography equipment evaluation. Retrieved from website: http://www.fda.gov/Radiation-EmittingProducts/MammographyQualityStandardsActandProgram/Guidance/PolicyGuidanceHelpSystem/ucm050621.htm Read More