Equipment Requirements and Specifications for Radiography Unit

Practicing Radiography in a Diverse Society

A new mobile X-ray system is required for your hospital to use in the Special Care Baby Unit. Write an equipment specification for your ideal unit, explaining and justifying your requirements.

Purpose of imaging Neonates

With rising numbers of premature infants in special care baby units (SCBU), the number of radiographic imaging requests for complex medical conditions has increased (Yu 2010). My aim is to write a complete specification of the ideal x-ray mobile machine, around the requirements of a SCBU, evaluating; radiation dose, image quality, size of equipment and materials used. 

Specification

(Healthcare.siemens.com 2019),(Coremedicalimaging.com 2019).

Size of Mobile Machine:

The mobile machine needs to be compact due to the limited space in a SCBU. This has influenced the dimensions I have chosen for my x-ray machine, with a forefront of manoeuvrability. To assist the manoeuvrability, 4 independent wheels allow 4kmh speed. This allows radiographers to overcome floor gradients with less manual force, similar to the Canon IME which states that, 2 independent drive motors allows the unit to turn within its own radius for excellent manoeuvrability (Systems 2019).

An incubator is used to maintain environmental conditions suitable for a neonate. Therefore, there must be consideration of the extendable arm length which is why I have chosen the length of 200 cm and a rotation ability of 320°. The rotational ability has been adapted from the GE AMX-4+ that only has a 270° arm manoeuvrability, to better suit the needs of a SCBU unit (Rkymtnrad.com 2019).

Neonates have a developing immune system, which increases their vulnerability to infections (Basha Surendran and Pichichero, 2014). It has been shown that less than half of the near-patient surfaces are regularly cleaned (Siegel et al. 2007) thus paving the way to hospital acquired infection (McBryde et al. 2004). To prevent the spread of microbes/pathogens the mobile x-ray machine will contain an anti- microbial coating. Another addition the mobile machine will have, is similar to the Siemens mobilett mira max, which has giraffe design to create a relaxing environment (Healthcare.siemens.com 2019)

Radiation Dose:

Radiographic exposure of neonates attracts interest because of their greater cell proliferation rate and the increased opportunity for expression of delayed cancer effects (Khong et al. 2013). Due of their size, more body tissues may be irradiated than larger children or adults (Faghihi et al. 2011). The risk of cancer induction per unit of dose is believed to be 2-3 times higher than that of the average population (Smans et al. 2008). It is concluded that infants, especially premature, are more sensitive to radiation than adults. Neonates may also receive a higher radiation dose than necessary if exposure factor settings are not adjusted for their smaller body size. The amount of entrance surface dose (ESD) recommended by the European Commission of Radiography is 0.08 mSv for a chest x-ray (Chawla et al. 2009).

Considering the information above, the specification for the mobile unit will have adjustable; Kilo-voltage (KV), milliampere- seconds (mAs) and distance.

Kilo-voltage, governs the penetrating power of the photons. At lower kilo-voltage, there is a greater difference in attenuation by structures of different density than at higher kilo-voltage. Therefore, at lower kilo-voltage, there is a greater subject contrast. This can be used to advantage when examining areas of low subject contrast such as the abdomen.

MAs governs the number of x-rays reaching the detector. While, distance affects the number of x-rays reaching the detector, using the principal of the inverse square law  (Stokell 2019).

The adjustable exposure factors and distance will control the amount of radiation dose received by the neonate. This feature can be seen on the Samsung GM85 which has “weight-dependent imaging ,enables paediatric patients to avoid unnecessary x-ray exposure with precise dose management”.

Techniques used to improve image-quality can result in increased patient dose, such as the use of an anti-scatter grid. The anti-scatter grid improves image contrast by preferentially removing scattered X-rays from the X-ray beam before it reaches the image receptor. However, the anti-scatter grid also removes some primary X-rays from the beam, and the mAs must be increased to maintain a constant image noise level when an anti-scatter grid is used in digital radiography (Fritz and Jones 2013).

Existing guidelines offer conflicting advice for the use of anti-scatter grids. These include recommendations for the use of an anti-scatter grid for body parts thicker than 10 cm, or when using tube potentials greater than 60 kVp (Carlton R 2012) and for patients older than 6 months (European Commission1996).

The results of (Fritz and Jones 2013) indicate that for a given patient thickness, the scatter-to-primary ratio is strongly dependent on X-ray field of view (FOV) and partially dependent on kVp. This reinforces the importance of tight collimation of the X-ray field to the anatomy of interest, alongside reducing patient dose.

The decision not to use an anti-scatter grid is based upon the consideration of an uncooperative neonate. The collimation must be broad so that the anatomy of interest is not missed. Also, as the patient is the primary source of scatter, the scatter-to-primary ratio is not appreciably increased if the collimated X-ray FOV extends beyond the anatomy of the paediatric (Fritz and Jones 2013).

Light beam Diaphragm:

Light beam diaphragms provide a visual indication of the radiation field size delivered to a patient, serving 2 purposes: allowing collimation of the x-ray field size ensuring that only the required anatomy is irradiated, and aids correct radiographic centering (Carter 2007).

The use of collimation to improve image contrast is well established, extending to include the relationship among: field size, scatter, and radiation dose (Jeffery 1997)

Collimation is one of the most effective ways to reduce radiation dose to the patient. However, if collimation is used to control image quality and radiation dose, it would be assumed that the practitioner has precise control over the radiation field size. For this to happen, the light beam diaphragm and lead shutters must be perfectly aligned and operate in synchronization (Horner 1994). In the United Kingdom, light beam diaphragms are a requirement of the Ionising Radiations Regulations (Hse.gov.uk 2019)

Geometric and Movement un-sharpness:

The quality of a radiographic image is very important and has to be of an adequate standard for diagnosis: the better the radiographic image, the more anatomy and pathology can be seen. Therefore, the image has to be of good quality so anatomy is visible and the sharpness of the image should be high.

To eliminate motion un-sharpness of the neonate, supporting features will be stored within the mobile x-ray machine so they are easy to find and use, such as sandbags, supporting pads or tape to help stabilize the neonate (Dendy and Heaton, 2011), and also lead aprons. This will enable nurses on SCBU to safely immobilise the neonate, reducing movement un-sharpness. This means there will be reduced dose for the neonate and nurse (Edison et al. 2017).

Additionally, a shorter exposure time can be used (0.001-10s), so a better radiographic image with less motion un-sharpness and correct radiographic positioning and anatomy is acquired. 

As the distance between the detector and mobile x-ray tube shortens, x-rays disperse out. In comparison, when the distance is increased there is a decrease in image sharpness (Alice 2014). To maximise image sharpness the arm of the x-ray tube can extend to 200 cm, to allow the use of a standard radiographic distance (Willis 2009).

Focal spot size:

The relationship between focal spot size and geometric unsharpness is established.

An increase in focal spot size results in a larger penumbra about the region of interest when other factors such as focus object and object receptor distances remain constant, as demonstrated by the graph below (Gorham and Brennan 2010)

This research has influenced the size of both the fine (0.7 mm) and broad (1.3 mm) focus. When compared to the leading mobile competitors GE who have O.75mm focal spot size and Philips (2014), a smaller focal spot size has been used.

Factors such as x-ray tube loading, tube current and motion blur must be considered when choosing a focal spot size. The results of (Poletti and McLean’s 2011) study suggests that, the optimum size may be smaller than is commonly in use. This has influenced the size of the fine focus (0.7mm).

Due to the mobile x-ray machine using a DR system shorter exposure times and reduced x-ray tube loadings are possible. Consequently, it is possible to use smaller focal spots for many projections. This conclusion is also supported by the experimental observation that there is significant improvement of spatial resolution at the anode side of the x-ray field, due to the smaller projected length of the focal spot  (Katz and Nickoloff, 1992).

Although, a small focal spot might improve image quality, their excessive use could shorten tube life, as the heat generated from the bremsstrahlung interaction is dissipated over a small area. This might increase the heat loading of the tube and potentially shorten tube life (Bushberg et al. 2003), (Dowsett Kenny and Johnston, 2006). The mobile x-ray machine has been modified to use a small focal spot size, with a heat storage capability of 3000J.

X- ray tube life:

The length of the x-ray tube life is related to the thermal loading on the tube.

Long exposure times, high milli-ampere seconds, and high kilo-voltage can generate excessive heat in the x-ray tube, causing localized surface melting and pitting of the anode. More tungsten is vaporized, and the filament becomes thinner, increasing the filament’s potential to break.

(Bushong 2012) found that, a wide range of kilo-voltage and milliampere seconds can be used to produce images of acceptable quality. This suggests that a lower milliampere but shorter exposure time, or lower kilo-voltage value could be used to potentially extend x-ray tube life.

Taking into consideration that radiation dose to the neonate is directly proportional to the square of kV, while other factors like mA and film focus distance (FFD) play a role and a complex relation to patient dose (Goel 2019). The range of exposure factors available are:

Materials used in the mobile machine:

A rotating anode has been used to increase the anode function, as it allows even distribution of heat dissipation. Many materials have a high specific heat capacity i.e. Tungsten has a K edge at 60 keV, while Rhodium has a lower K edge but more penetrating power. When considering the neonate, I have chosen Molybdenum due to its higher heat capacity, allowing lower exposure factors to be used (Hacking 2018).

The filter used on the mobile device is 1.8mm aluminium with additional 0.1mm copper. Filtering is the removal of low energy x-rays from the beam spectrum, which would not contribute to image quality but would add to patient dose and scatter (Goel and Bell 2019). The filtration used, has an additional 1.3mm compared to the (Acbar.org 2019) mobile x-ray device. (Perks et al. 2013) showed added filtration to the paediatric examinations would reduce entrance dose by 36%, when 1.8mm aluminium filtration is used.

(Trauernicht et al. 2015) found that 0.1mm copper filter can significantly lower the radiation dose while maintaining diagnostic image quality at high kV exposures.

Filtration is necessary, as the risks associated with ionizing radiation are higher in paediatric due to their cells rapidly dividing, making them prone to DNA damage from radiation (Ron 2002).

A graph to demonstrate the use of Aluminium filtration to reduce the intensity of the beam (Pd.chem.ucl.ac.uk 2019)

The material chosen for the flat panel detector is a caesium iodide crystal. Similar to the “aerodr flat panel detector”.

The digital detector material was chosen due to its quantum efficiency (DQE) for high-quality images and dose efficiency (Coremedicalimaging.com 2019).

The use of an indirect digital detector allows a quicker examination to be undertaken unlike a CR cassette, as the image is acquired digitally and does not need processing, important for a SCBU unit that can potentially be very busy. The size of the detector( 24cm x 30 cm x 1.6 cm ) was influenced by siemens MAX mini detector. This act as a compromise to fit within the neonates incubator but also allow the anatomy for a chest and abdomen to fit on the detector for a single exposure (Siemens 2018)

Above this, the battery for the detector unlike many leading manufactures who use acid-battery is lithium. This produces up to 50 exposures on a single charge, and has 11% faster charging time in comparison to an acid-battery. Similar to the RadPRO® Mobile machine.  The lithium battery will have the operational time of 9 hrs for 200 images, essential when a SCBU unit will require a 24/7 imaging service (Coremedicalimaging.com 2019).

In conclusion, the mobile machine will have a variety of features which make it ideal for a high intensity special care baby unit, with added focus on the radiation dose emitted to the neonates, size and materials used.  These features are based on pre-existing evidence and aspects of a mobile machine, which are currently manufactured but have been adapted for use in a SCBU.

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