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Radiographic Imaging. The X-ray examination this assignment will critique is that of the Chest; chosen because it appears to be the most common procedure performed within X-ray departments.

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Radiographic imaging

Part A

The X-ray examination this assignment will critique is that of the Chest; chosen because it appears to be the most common procedure performed within  X-ray departments. There are two main projections for completing chest X-rays; the first and most common is the AP/PA projection which can be performed with a patient standing (erect technique) or in the supine position; for example, if confined to bed.  The second projection is the lateral grid.  

It is important to check why the x-ray has been request to ensure appropriate positioning for diagnostic imaging.  Darovic (1998:2) points out this significance; “fluid in the pleural space (pleural effusion) will fill in or "blunt" the costophrenic angle if the patient is upright when the X-ray is taken. If the patient is supine, pleural fluid is evenly distributed along the posterior pleural space and can't be detected”.

The exposure factors for both of these techniques are as follows:

Position

kV

mAs

AP/PA

125

1.00

Lateral grid

117

12.5

A kV value of 125, for the AP/PA projection, is used because the image produced requires a low contrast for a quality diagnostic image.  The x-ray beam becomes more energetic with higher voltage settings; thus the more energetic the “less effect the different levels of tissue density will have in attenuating the beam” (http://www.e-radiography.net).  A chest x-ray has extreme tissues types to consider; ranging from high density bone to very low density air within the lungs.  Fraser et al. cited by Venema, Straten and Heeten (2005:336) states that   “the two main advantages of using a high kV technique for Chest X-rays are better penetration of the mediastinum and reduction of the visibility of the ribs relative to the lung structures” (See Images 1 &2 in Appendix 1).   A high kV is used for the lateral grid technique for the same reasons as above; however, the slightly lower value of 117kV, compared to the 125kV used for AP/PA projection, is required because there is less tissue penetration necessary during a lateral view.image00.png

If a Chest X-ray was completed, using a low kV setting, then the contrast of the ribs would be high because bone has a relatively high effective number. This would result in an image in which the ribs impair the visibility of the softer tissue structure.

Fast exposure times are required in order to prevent / minimise any ‘blurring’ of the image due to movement through respiration. For example; the heart should appear as a still, sharp outline.  The patient is exposed to lower radiation with fast exposure hence adhering to ALARP principles.

1.0 mAs for AP/PA chest imaging ensures correct exposure factors. Too low mAs setting will result in underexposure, the image being too pale.  Overexposure, produces images too dark to be diagnostic and can result from mAs setting being too high.

The lateral grid has a higher mAs setting of 12.5.  A grid is used because a lateral chest is fairly thick; it’s required to reduce scattered radiation by limiting field exposure.  Using the grid improves image sharpness when imaging the thicker lateral chest; however the grid will stop some of the photons. In order to get the correct exposure the mAs therefore need to be increased.  

Part B

This section will discuss a digital x-ray system; the Siemens Polydoros LX 50.  

Digital radiography is comprised of the following components; a digital image receptor, digital image processing unit, image management system, image and data storage devices, an interface to patient information system, a communication network and a display device with viewer operated controls. (see Figure 2 in Appendix 1)image01.png

After the radiographer has set the exposure factors and exposed the patient, the X-ray beam will pass through the patient and then be intercepted by the digital receptor. On receiving the x-ray photons which are absorbed, the receptor converts the energy produced by this process into an electrical signal.  The electrical signal is in the form of analogue data which is converted into a digital format equating to one pixel.  (see Figure 1 in Appendix 1). (Each digital pixel value will be stored)  The image is processed into a digital format, that is, a matrix of pixels each with a numerical value.  The information data is sent as a digital signal via a fibre optic link to the imagining processing unit where it is viewed and if necessary can be manipulated to enhance quality; for example altering its contrast or using the zoom feature to aid diagnostic interpretation.  The image is stored via the information management system.  The image is linked to the patient information system and can be communicated via the hospital network system which can be viewed only on numerous display controls; access to which is password protected to ensure compliance with confidentiality policy and legislative requirements of the Data Protection Act. (see Figure 2 in Appendix 1)

(http://www.sprawls.org/resources/DIGRAD/module.htm#9)

.  

image02.png

References

Darovic G O (1998) ;  Shades of gray: Understanding chest X-rays;  Nursing, Jul 1998

Venema H W (2005);  Digital Radiography of the Chest: Reassessment of the High-Voltage Technique?  Radiology April 2005

(http://www.e-radiography.net).  

(http://www.sprawls.org/resources/DIGRAD/module.htm#9)

Bibliography

McEnteeM  F,    Brennan  P C,   O. Connor G (2003)  

The effect of X-ray tube potential on the image quality of PA chest radiographs when using digital image acquisition devices

Radiography (2004) 10, 287-292

(http://www.sprawls.org/resources/DIGRAD/module.htm#9)

image03.png

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