The use of different technical factors is required to contribute to the image quality. These are; selection of exposure factor, focal spot selection, the scatter management and the beam geometry involved. It is important to use the correct technique and consider all other factors including patient anatomical characteristics, patient orientation and also radiation safety. All the above will allow the image produced to be used for accurate diagnostic purposes. The difference between acceptable and a good clinical image is also highlighted and the implications discussed.
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An x-ray (radiograph) is a medical test that helps physicians diagnose and treat medical conditions. Chest X-ray examination is one of the most requested imaging examination in clinical practice. When performing chest examination, it is important to use the correct radiographic technique to include all the necessary anatomic structures. The views required will depend on the clinical indication and the mobility of the patient (Armstrong, 1999).
To produce optimum chest x-ray, the radiation beam energy is influenced by three factors. Hay, (2000) outlines these factors as; time(s), the duration of the exposure; the rate at which x-rays are produced (mA); and the energy of the photons in the beam (kV). According to (Bontrager, 2005), the factors controlling density are mA and the exposure time, (mAs). This determines the quantity of radiation in the beam.
On a chest radiograph images of the heart shadow, lungs, rib bones, airways, blood vessels and the bones of the spine are visualised. This is performed to evaluate the lungs, heart and the chest wall.
A chest x-ray is typically the first imaging test used to help diagnose symptoms such as:
- Shortness of breath.
- A bad or persistent cough.
- Chest pain or injury.
Physicians use the examination to help diagnose or monitor treatment for conditions such as:
- Heart failure and other heart problems.
- Lung cancer
- Other chest-related pathologies.
Different parts of the body absorb the x-rays in varying degrees (Hay, 2000). The ribs and spine will absorb much of the radiation as they are dense bones which are radio-opaque, due to the calcium having a higher atomic number. The soft tissue, such as organs, muscle and fat allow more of the x-rays to pass through them as they are radiolucent. Bone appears white or light grey on the x-ray. Soft tissue shows up in different shades of grey. Air, has less mass causes the lungs to absorb little radiation, which appear black or dark on the image.
Typically, two views of the chest are taken, one from the back which is Posterior-anterior (PA) (fig.1), with exposure factors of 110-125kVp and 1.5-3mAs. The other view is taken from the side of the body. This is the Lateral view (fig.2). The exposures are 125kV and 6mAs. These exposure factors vary from different radiography departments. According to Bontrager, (2010) the radiographer will position the patient for the chest x-ray for the PA by getting the patient erect with their anterior aspect against the image recording plate.
The above selected exposure will provide sufficient density creating an optimum image. Centring the central ray, collimating accurately and restriction of the primary beam coverage are very important as this reduces radiation exposure to radiosensitive region of the neck area and exposure to the abdominal area below the diaphragm. This can produce scatter and secondary radiation to radiosensitive reproductive organs.
The patient is asked to stand still and a breathing technique is used for lung markings, and to move the ribs overlying the sternum. While the x-ray picture is taken, the patient has to hold his/her breath to avoid movement unsharpness or reduce blurring of the image. In addition to this, the lateral view is taken only when requested by a radiologist for further investigation. The other technique, anterior-posterior (AP) is done, where the posterior aspect of the patient is against the imaging plate. This has disadvantages to the image and patients. It magnifies the heart shadow and radiates the heart. This is because the heart and other radio-sensitive organs such as breast tissue are closer to the x-ray tube compared to PA positioning.
Bariatric patients will need a grid because the exposure factors will have to be increased. This is used to avoid scatter and increase the quality of the image. A grid is used to 'trap low energy photons and prevent them from reaching the film' (Chalmers 1998), however some of the primary beam is also absorbed so the exposure needs to be increased.
Movement unsharpness occurs when the patient is unable to stay still; this causes the image to become blurry. To avoid this 'a higher mA setting can be selected, and a lower exposure time,' (Shephard 2003), which means that the x-ray can be taken as quick as possible therefore not causing any movement unsharpness. 'Increased kV creates the overall darkness of the radiograph,' (Shephard 2003) and the 'contrast in the bone and the surrounding parts decrease,' (Van der Plaats 1999) therefore all appear a dark shade of grey. Due to 'patient safety and the ALARP principle which states that the exposure factors selected should be as low as reasonably practical and the IRMER 2006 (Ionising radiation medical exposures regulation) protocol. Therefore the kV cannot be increased too much. (Hobbs 2007) 'Raising kV increases the amount of scatter which is not image forming, they just blacken the film more,' (Van der Plaats 1999) and decreases the image quality.
To reduce the geometric magnification and distortion of structures within the chest x-ray, the film focus distance (FFD) is generally always at 180cm. The object image receptor alignment is important in controlling distortion of an image, 'the greater the distance between the object and the film, the greater the magnification.' (Whitley 2006) Therefore if possible, the patient should be positioned as close to the film as possible.' (Bontrager 2005) If this is not possible, such as in trolley case patients, the FFD should be increased to compensate for the increased OFD (object focal distance), and thus reduce the magnification. An increased FFD would result in a lower beam intensity due to the inverse square law, which states that 'doubling the distance between the point and the source of radiation, the intensity of the radiation beam is reduced to one-quarter'(Graham 2003), to attain this the kV is increased. (Bontrager 2005) To ensure no rotation or magnification of the thorax, 'the legs and feet of the patient must be slightly apart for stability, and this aids no rotation of the clavicles.' (Bontrager 2005) The centring points of the image are vital as this ensures all of the area of interest is included. 'The centring point for a chest is the T7 region of the vertebra prominens.' (Graham 2005) On a good PA chest projection the 'scapula should be away from the lung field and the clavicles should be equidistance with no rotation and no overlapping of the soft tissue margins.'
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Geometric unsharpness in the image can be caused due to X-ray being emitted from an area; the focal spot size, rather than from a point. Regions at the edges of an object will be formed in which the X-ray intensity will be gradually increasing, causing unsharpness. These regions are called penumbra. (Figure 3) The size of the penumbra is dependent on the focal spot size and the ratio FOD (Focal object distance): FFD. The penumbra causes blurriness on the edges of the x-ray. The focal spot size is identified 'as the area on the anode that is bombarded by electrons.' (Farr 2006)
'A large focal spot causes geometric unsharpness, and increases magnification of the image.' (Farr 2006) To avoid unsharpness and magnification, 'a smaller focal spot size is used to clarify pathology' (Bryan 1998) a fine focal spot is used for hand x-rays and thoracic spines. Conversely sometimes a 'broad focal spot is used where a high radiation output is required from the tube.' (Graham 2005).
In addition it is important to ensure all artefacts are removed from the patient in the area of interest for any x-ray, like jewellery, underwired bra, as any metal will show up appearing white on the film due to the very high atomic number metal has, and could consequently obscure pathologies.
Contrast sensitivity determines what tissues and body structures are visible in a radiograph.
The total attenuation by soft tissue is determined by both photoelectric and Compton interactions.
Increased penetration through an object decreases contrast. Increased penetration through the total body generally decreases the radiation dose to the patient.
The x-ray attenuation and total body penetration changes with photon energy. The penetration through soft tissue can be changed by changing the kV which changes the spectrum.
The spectrum of an x-ray beam is determined by: the anode material, the KV value, and the filtration. The chest has high physical contrast primarily because of the air within the lungs.
Good chest radiographs are produced by using a high kV value and a heavily filtered beam with a spectrum that gives good penetration.
In conclusion from the information, it is evident that many factors contribute to creating different appearances on the chest x-ray. The optimum way to achieve the best appearances on the radiograph without any distortions would be to carry out the views suggested with the correct exposure factors, and positioning of the patient including correct breathing instructions. In addition the x-ray tube must be set correctly to avoid magnification and distortion. However the optimum radiograph is not always achievable due to different patient conditions so radio-graphically the technique should be adapted as much as possible to ensure an optimum diagnostic quality x-ray is obtained. Computed Tomography is another modality that can be used for chest x-rays as it produces a more detailed image.
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