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This assignment will be considering the advances in angiography equipment. In order to assess the advances with the newest angiography equipment I will consider two main aspects. These aspects are the dose saving features and the ergonomic characteristics available. In order to understand why these aspects are important, I will explain the effects of radiation on matter i.e. the human body, and what ergonomics is and how it effects the radiographer or radiologist using the equipment and the work flow of an angiography room. I will also explain what angiography is and what sort of procedures are undertake to allow a gauged understanding or equipment necessities. To conclude I will highlight any further advancements which are or could be expected in the future and which angiographic system is currently the best available.
Angiography is vascular medical imaging technique done within the radiology department. Angiography is based on the principle of injecting contrast media (x-ray dye) into a vein to map out specific sections of the circulatory system. Once a contrast media is injected, the patient is the screened (x-rayed to produce a live run of images) allowing the radiographer or radiologist to follow the vasculature of the patients anatomy to identify problems or to complete specific therapeutic procedures. Some common procedure which will be undertaken in an angiography suit are:
Angiogram - Vascular mapping
Angioplasty - Inserting a guide wire and a catheter balloon into a vein to remove or widen an arteriole stenosis or blockage by inflating the balloon.
Vascular stent insertion - A stent is introduced into a vein using a guide wire and catheter. The stent is manoeuvred to the correct position and deployed to allow a vessel to remain open and fully functioning (Carver and Carver, 2006).
Embolization - A technique exercise to stem off the blood flow to a specific vascular area or organ. Normally done pre surgery to prevent mass blood loss in theatre (Carver and Carver, 2006).
Those are just a few of the more common procedures undertaken with angiographic equipment, the one thing most angiography procedures have in common is the length of time taken to complete them. Time implicates a lot of problems but also highlights the need for dose saving techniques and good ergonomic design. Angiography is continually developing with regards to software. One of the greatest advancements in software was digital subtraction angiography (DSA), this is where part of an image can be digitally subtracted. An example of DSA would be to remove bone and soft tissue from an image leaving only vascular networks in place, this is done by taking a pre-contrast image (mask), then injecting contrast and taking another image. The software would then overlay the mask image on to the post contrast image and remove all similarities, hence leaving the contrast filled blood vessels on the resultant image.
Angiographic equipment works on the principle of issuing the patient with ionising radiation to image them. When the patient is being irradiated they are absorbing dose. This occurs due to the energy of the x-ray beam being attenuated by the patient's body tissues. The greater the quantity of energy attenuated by the patient the greater their absorbed dose. Absorbed dose can be defined as the energy absorbed by a medium per unit mass (Cember and Johnson, 2009).
As the human body is made up of many different chemicals with varying atomic structures attenuation is not uniform throughout, however when x-rays interact with matter there are two different types of attenuation which may occur (Ball et al, 2008). This processes may take place in different quantities dependent on the make-up of the particular area being irradiated. The two type of attenuation processes which occurs within the medical imaging range are:
Photoelectric absorption - The photoelectric effect occurs when there is a direct interaction between an x-ray photon and an electron within an atom of the attenuating medium. The x-ray photon usually possesses a similar ionisation energy as that of the electron it interacts with. When the photon collides with the electron all of its energy is transmitted to the electron. As a result the photon is completely absorbed (no longer exists) and the electron is expelled from the atom. The atom has then become ionised and to compensate an electron from an outer electron shell moves down to fill the resultant void. The movement of this electron from an outer shell produces an emission of characteristic radiation. This emission energy is dependent on which electron shell (energy level) the electron has moved down from. The emission energy is normally absorber by the surrounding material producing heat, increasing the temperature of the medium. The original electron which was expelled from the atom moves through the medium continually colliding and interacting with other atoms until its energy is spent. This normally results in more heat and the electron is seized by another atom (Ball et al, 2008).
Compton scatter - Compton scatter acts on a similar principle to the photoelectric effect. A photon will undergo a collision with an electron of the attenuating medium. The main difference between Compton scatter and the photoelectric effect is the level of energy possessed by the photon, photons which contribute to Compton scatter retain a large amount of energy. When a photon collides with an electron it imparts energy to it which is equal to that of the ionisation energy of the atom. This causes the electron to be expelled from the atom, the remaining energy of the photon continues on a different course determined by the deflection of the collision. The photon of energy left may go on to ionise more atoms until its energy is spent. Any electrons which are expelled during this process will be collected by other atoms (Cember and Johnson, 2009).
Ionisation of atoms within a cell correlates directly with that cell becoming ionised. As atoms become ionised an electron is released due to the breakdown of covalent bonds. It is the alteration to these atoms and the release of an electron (free radicals) which can cause damage within the cell structure. Cells are constructed in a very complex pattern, however the vital part of a cell is its Deoxyribonucleic acid (DNA). The DNA contained within a cell is responsible for its functionality which is reliant upon protein synthesis (Cwikel et al, 2010). Catastrophic damage can arise as a result of radiation damage to the chemical bonds in DNA. By damaging the DNA it may alter the cells ability to regenerate. DNA damage can occur as a result of:
Direct effect - When photons of energy interact directly with the DNA's structure. (Khan, 2010).
Indirect effect - Indirect effect is when photons interact with the water content within a cell. The majority of cells within the human body consists of 70-80 percent water. Water is highly reactive to radiation meaning it ionises easily. The production of free radicals can then damage or alter the structure of DNA (Ball et al, 2008).
Illness to an individual or resultant hereditary conditions can occur as a result of either of the above effects. If a cell's DNA within the gonads is damaged the structural template used in reproduction can be altered. This new genetic sequence can the be passed on to a future generation causing mutations such as leukaemia (Graham et al, 2007).
The stochastic and non-stochastic effects must also be considered when looking at the effects of ionising radiation:
Stochastic effects - Related to chance, the likelihood of damage occurring is proportional to the level of radiation employed (Als-Nielsen & McMorrow, 2011).
Non-stochastic effects - Related to cumulative dose. If the cumulative dose surpasses the threshold dose biological damage will be noted, conversely if the threshold dose is not surpassed no damage will occur (Als-Nielsen & McMorrow, 2011).
The word ergonomics is derived from Greek origins, ergon meaning work and nomics meaning natural laws. Ergonomics which is also known as human factors is a scientific speciality with the interaction and understanding of how humans interact with fundamental systems and elements in their environment (Helande, 2006). The field of ergonomics makes uses of statistical data, enhanced design methods and theoretical principles to produce equipment to optimise well-being and match human physical and cognitive abilities. The field of ergonomics can be broken down into three main sections; Physical ergonomics, Cognitive ergonomics and Organisational ergonomics (Bush, 2012).
Physical ergonomics is with regard to human biomechanical and physiological characteristics, and how they interact when subjected to physical activity. The main concerns of this field are posture, repetitive movements, induced musculoskeletal disorders and layout of equipment to maintain optimum user health (Helande, 2006).
Cognitive ergonomics is based on mental processes, these include memory, perception, motor response and reasoning. Cognitive ergonomics is used to design equipment interfaces and programmes to reduce the mental strain on these outlined areas. By increasing computer human interactions decreases can be seen in mental workload, time length of decision making, and stress which conversely increases performance (Bush 2012).
Organisational ergonomics is a field which stemmed from cognitive ergonomics. The main principles of this field are communication and organisation of processes. This is the designing of software allowing communication between equipment and easy prioritisation of tasks, as a result work flow is often increases due to ease of use (Bush, 2012).
There are currently three main setup designs of fluoroscopic equipment available. The first is an under couch design, this is where the x-ray tube is located underneath the table with the image intensifier (II) above the table. If a patient was lying supine on the table the direction of the x-ray beam would pass through the patient posteroanteriorly. The x-ray tube and II have a fixed centring accompanied with a variable field of view. With regards to movement the x-ray tube and II can move up and down the table, the II's distance from the x-ray tube can be increased or decreased and the motorised table can move up, down, in and out. This design of fluoroscopic equipment is unable to produce oblique images without physically positioning the patient. One other consideration to note with this design is the area of scatter produced. The main contributor to scatter is the underside of the table, as x-rays interact with the table the scatter produced is below operator waist level where radiosensitive organs such as the gonads are found (Carter et al, 1994).
Over couch designs share many similarities with the under couch design as far as movement and centring is concerned. However the obvious difference between the two is the x-ray tube and II set up. The x-ray tube on an over couch design is located above the table top and the II is located underneath. The main scatter production by this design is the x-ray interaction with the patient, the area of the operator susceptible is above the waist where radiosensitive organs such as the thyroid and eyes are located (Oppelet, 2005).
The third design is the C-arm. This device is called the C-arm because the x-ray tube and II are linked in a C shape. The C-arm is can be either floor or ceiling mounted dependent on room structure. As the C-arm is mounted independently of the table its range of movements are greatly increased, as the C-arm is isocentric in design it is capable of all the movements of the previous two designs with the addition of oblique positions and the option to choose where the x-ray tube is in relation to the table top i.e. above or below. However due to the C-arms ability to invert and move in every plane the area of scatter coincides with the x-ray tube position on exposure. Some manufacturers also offer a slightly different setup called a duel header setup, this is where a room is fitted with two C-arms allowing the operator the ability to produce an anteroposterior and a lateral image at the same time as they work perpendicular to each other (Abrams, 2006).
Dose Saving Features of Angiography equipment
The addition of filters can help reduce skin dose to a patient. This is not a new concept as filters used to be manually added into the x-ray tube head. The principle of filtration is the addition of a low attenuating material placed between the radiation source and the patient. This filter attenuates the lower energy x-ray photons, the filter is most commonly made from copper and can reduce skin does by more than 70%. The newer equipment however has the ability to add extra filtration, this is automatically set dependent on the weight of the patient of the C-arm angulation (Ball et al, 2008).
Collimation is an important factor for reducing patient dose. The way collimators are used has altered from past to present designs. The older designs had interchangeable collimates which needed to be physical inserted or moved inside of the x-ray tube head. The main problem associated with this is it was difficult to alter the collimation during a procedure. Modern equipment has motorised collimators built into the x-ray tube head. The collimator can be controlled to move remotely at any time. Collimator position is also accompanied by a virtual outline on the imaging monitor, allowing the collimators to be positioned without screening the patient (Shastri, 2008).
Audible alarms for screening time are a feature found on both old and newer fluoroscopy designs. The timer is set to trigger after screening time has exceeded five minutes. This is to remind the operator of the dose the patient is receiving. An advancement to this is the addition of a real time visual reminder of screening time and patient dose on the imaging display monitor (Carter et al, 1994).
The control panel is a key utility to the system operator as it allows them to move the equipment remotely, adjust collimation and manipulate the exposure factors as well as selecting the pulse variation of screening. The newer designs of control panel allow the user to move them to anywhere in the examination room, this reduces the dose to the operator due to the principle of the inverse square law (Oppelet, 2005), (Ball et al, 2008).
The film frame rate or pulsed screening program is an important dose reduction element. With continuous screening the patient will receive 100% of the dose which is the equivalent to screening at 30 frames per second (P/s). With the required training operators can become accustomed to view screening runs at a lower frame rate, for example a frame rate of 15P/s reduces patient dose by 45 - 50%. On the most modern equipment can produce screening runs at a frame rate of 3P/s reducing the dose by up to 90%. Pulsed film rate began to make an appearance in fluoroscopy during the 1970's it was viable as a result of the modified IIs. The newest advances in pulsed film rate is accompanied but the introduction of digital flat panel detectors (FPD) (oppelet, 2005), (Shastri, 2008).
Digital flat panel detectors are the latest advancement in image detection for fluoroscopy. Their introduction occurred from the year 2000 to present, and they are replacing II's in fluoroscopy setups especially for more detailed angiography. The advantages of FPDs are mainly related to increased sensitivity. As the FPD is more sensitive to x-rays, lower dose procedures can be undertaken without compromising image quality. The image quality is not effected due to the enhanced contrast and spatial resolution accompanied with uniform detection across the detector. The increased visualisation reduces the number of images required i.e. the number of exposures that are needed, increasing quantum efficiency (patient dose) (Bushberg et al, 2002). As the IIs input fluorescence screen is convex this can cause slight distortion to the image after the x-ray photons have been converted to light, this is not an issue for FPDs. The other main advantage of FDPs is the retaining (unbinned) of pixels, as all the pixels are available in the image the ability to zoom on a FPDs image is more detailed than that of an IIs (Abrams et al, 2006).
On older equipment the only way to increase the picture size was to magnify (MAG) up the image. This was achieved by altering the focal spot distance within the II to allow a smaller section of an image to cover all of the output phosphor resulting in a a magnified image. The main problem associated with this is the loss of contrast and brightness of the image, during a procedure the image must remain a uniform appearance. In order to maintain uniformity the must be an increase in exposure leading to increased patient dose (Carter et al, 1994). Newer equipment have a built in algorithm to zoom in on an image. This is done by enlarging pixels of information over the required area, this has no impact on the image with regards to contrast or brightness but can reduce sharpness. Even though sharpness is compromised it is not below diagnostic quality and does not cause an increase to patient dose (Shastri, 2008).
Lead has been used in both past and present angiography designs as it is a vital part of reducing dose to both the patient and he operator. Lead tube housing is a feature pioneered by William Herbert Rollins between 1896-1904, lead tube housing prevents the escape of scatter from the x-ray source. The addition of lead rubber curtains is also noted in past equipment designs (Assmus, 1992). The newer radiation safety mechanism is the moveable lead glass screen. This screen has lead equivalent glass, it can be altered in height and moved anywhere in the examination room to provide optimum protection (Oppelet, 2005).
The major difference of ergonomic design from past to present is based around motorised equipment movements and an auto detect feature. The control panels of modern equipment allows the user to operate and move the equipment in any desired way. The tube or C-arm, table and monitors can all be configured for remote use. An auto detect or memory programme is often found on modern equipment, this allows the user to move equipment to a pre-set position at the touch of a button. The auto detect feature always synchronises the tube and detector so any angulation applied to the tube is matched by the detector. The use of such programmes reduces manual handling and mental stresses to which the user is subjected.
Most tables used in fluoroscopy are floating top design with a rise and fall function. The table is fitted with magnetic locks, when these are disengaged the table is in a free flow mode and can be manipulated in all directions, the table is designed to produce minimal friction allowing heavy patients to be moved with ease. The rise and fall function is utilised when positioning a patient to help avoid back injuries.
Another tool to be considered is the control panel. In modern equipment there control panel and exposure pedal can be moved anywhere in the examination room independently of one another. The design of the control panel itself however is the major factor aiding cognitive ergonomics, common controls to find are:
kVp and mAs
Level of pulsed screening or constant screen
MAG or zoom depending on the age of the equipment
Tube or C-arm movement controls
Displays found on the control panel are:
kVp and mAs values
These displays and buttonas are designed to allow the user to easily control the equipment and be aware of paient dose at all times.
There have been many advanced within the fluoroscopic/angiography area in both dose saving features and ergonomic design. The current day manufacturers all try to encompass as many practical enhancements as possible whilst continually trying to make head way with further developments. One of the newest features Siemens include in their angiography equipment is the ability to conduct low slice CT scans by rotating the C-arm through 360 degrees. Looking at available equipment on the market and based on the dose saving and ergonomic factors the best angiography equipment available is the Siemens Artis zee biplane system (Siemens, 2010).
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(Carver and Carver, 2006)
(Cember and Johnson, 2009).
(Ball et al, 2008).
(Cwikel et al, 2010)
(Graham et al, 2007).
(Als-Nielsen & McMorrow, 2011).
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