Below is my own writing with references. During the completion of this task I was searching for the information everywhere especially in journals, but there wasn't any which discuss specifically on this issue. So I jas wanted to have it here in case if any medical imaging student or anyone make a search on this topic in google, yahoo etc, thus, at least they can have it here.
This article is just focusing on why for CXR there is a need of high kV usage and its justification. And also inserted together a simple comparison between CXR and AXR as for AXR needs moderate kV. Thus, no related pathology and deep anatomy being discussed here. Considering that I got 9/10 marks for this article, so I am confident to share it with the world for reference, for the sake of sharing the knowledge, as sharing is caring.
For non-medical imaging related person, you can read if you want and ask me for any part which is not understandable. But I think you will get bored to read this. Huu..here it goes, wassalam
The Usage of High Kilovoltage(kV) In Chest Imaging (CXR) and comparison with Abdominal Imaging (AXR) using Conventional Imaging Machine.
Chest x-ray (CXR) is the most common radiographic procedure. It is the upper part of the trunk or known as thorax, situated between neck and abdomen. For imaging purposes, it was divided into three parts which are the bony thorax, the respiratory system and the mediastinum.
The bony system refers to the bones which provide a protective framework for thoracic part which involved with breathing and blood circulation, those parts of the chest which consist the lungs and organs contained in mediastinum. The part of the skeletal system which related to this is the sternum, two clavicles, two scapulae, twelve pair of ribs and twelve thoracic vertebras.
For respiratory system, it was where the exchange of gaseous substances between the airs we gasp and the bloodstream. The four main parts are the pharynx, trachea, bronchi and lungs. One of the important structures is the diaphragm. Mediastinum is the medial portion of the thoracic cavity between the lungs; it is including the thymus gland, heart with great vessels, trachea and esophagus.
Image produced from CXR should be able to visualize lungs and mediastinal structures. Besides that, for general PA CXR, outlines of the mid and upper vertebrae and posterior ribs through the heart and other mediastinal structures should be at least seen on the radiographs.
THE USE OF HIGH KV TECHNIQUE
The primary controlling factor for contrast in film-based imaging is kilovoltage (kV). KV controlled the quantity and quality of the x-ray beam. The higher the kV the greater the energy and the more uniformly the x-ray beam penetrates the various mass densities of all tissues. When kV is increased, the speed and energy of the electrons applied across the x-ray tube will increase.
High kV is used when different tissue types are being imaged (bone, soft tissue, etc). And so, the general requirement for CXR is high kV. This requires high kV around 110 to 125. For kV more than 70, the interactions between tissues are predominantly Compton Scattering and image formation relies on the primary beam being scattered away from the image plane through the body. The main argument here for high kV technique in CXR is the reduction of the bone structures that are superimposed on lung structures.
High kV depends on different electron densities between tissues. Compton scatter influence the image formation while the subject contrast depends on electron density (between tissues) not the atomic number, which initiate the reduction of soft tissue differences. Thus, bone appears more transparent allowing features behind to be observed. This property being applied in CXR as the rib cage will appear more transparent.
Therefore, higher kV produced less variation in attenuation (differential absorption) resulting in lower contrast. Adequate contrast is necessitated in order to demonstrate the many different shades of gray needed to visualize the different tissue composition in chest. Low contrast is required due to the different substances composed by the three division of the chest which has been stated earlier. The compositions of the chest determine its radiographic appearance, thus imaging parameter have to be adjusted to adapt with these features.
High kV technique gives fast exposure times which freeze motion (movement unsharpness) due to either patient movement (during arrested inspiration) or cardiac motion. Besides that, in the standard textbook on diagnosis of diseases of the chest, Fraser et al state that two main advantages of the high kV technique for CXR are better penetration of the mediastinum and reduction of the visibility of the ribs relative to the lung structures. Easy penetration of the mediastinal regions is the valuable diagnostic property of high kV CXR.
If CXR done with lower kV, the contrast of the rib will be high because bone has a relatively high effective number. The high contrast rib shadows which presence lowers the conspicuity of the soft tissue structures, hence, the visibility of those structures will be impaired.
The use of high kV which is above 100 comes with a requirement to use grids. It is necessary to use high ratio grids in CXR to reduce the amount of forward high energy scatter reaching the film. But, for all rules, there is always be an exception. Some mobile chests taken with equipment that is limited to 80 to 90 kV.
WHEN RADIATION INTERACT WITH MATTER
As an x-ray beam passes through a patient, the beam is attenuated. Attenuation is the result when x-rays interact with matter. It is the reduction in the total number of x-ray photons remaining in the beam after passing through a given thickness of material.
Attenuation will be increased with the increase in the thickness of the body part being radiographed. As the incident beam passes through patient, it is significantly being altered. Another factor which will affect attenuation is the atomic number of the region of interest (ROI). Higher atomic number materials attenuate greater percentage of the beam. This is because photons may interact with the presence of greater number of electrons. Next will be the density which will give impact to the attenuation. Density refers to the tightness of a substance atoms being packed together. The denser the tissue, the lesser beam could penetrate through it.
As what we have here for CXR, the best to be compared are among these three substances which are air, bone and soft tissue, to give the deeper view why the need of high kV occurs. Air naturally present in lungs for arrested inhalation radiograph. The chest has high physical contrast primarily because of the air within the lungs. It has the lowest density among all although it has slightly higher atomic number than soft tissue. Air absorbs fewer photons when they pass through it. Hence, more photons reach the image receptor producing greater image receptor exposure. On the radiograph, air is virtually transparent and will give a dense black shadow, which is equivalent to 100% film blackening for all photon energies.
Usually the skeletal system could easily be seen on the radiograph due to the calcium content in bone as calcium has higher atomic number among all the substances in human body. For high kV technique, the greater tissue density leads the bone to have greater rate of radiation absorption. It means, fewer photons reach the image receptor and construct less image receptor exposure. That is why bones appear whiter on radiograph. Also the high kV needed to penetrate the bones.
Last are the soft tissue structures such as in mediastinum, where they have effective atomic number and tissue densities. Their density varies and slightly higher than air, thus, they absorbed x-ray photons slightly more than air and still produced more image receptor exposure even though it is slightly fewer than what produced by air.
THE KV USE FOR ABDOMINAL X-RAY
What consist in abdomen mainly are the abdominal organ system, urinary system and abdominal cavity. Those are all various abdominal structures which are generally could be classified under soft tissues while for CXR, there are various types of different tissues structures to be imaged and need low contrast produced by high kV exposure.
In order to demonstrate these various tissue types, the kV being used for abdominal x-ray (AXR) is lower than CXR which is in the medium kV exposure, around 70 to 80. The moderate contrast produced by the medium kV should be able to image the kidney outlines, lumbar vertebrae transverse process, psoas muscle and lower liver margin in average-sized patient which having about the same tissue density and subject contrast.
Medium kV will also generate good contrast between adjacent soft tissues in abdomen. In comparison to the CXR, abdomen contain no superimposed ribs which needs to reduced its visibility, thus AXR does not require Compton Scattering which could only be created by a high kV exposure.
KV controls the energy or the penetrating power of the x-ray beam while contrast allows the anatomic detail on a radiograph to be visualized. Therefore, optimum radiographic contrast and kV exposure for the ROI is important as kV exposure will affects the contrast created, by producing less or more variation in attenuation.
Selection of the appropriate kV is a balance between optimal image contrast and lowest possible patient dose. A general rule states that the each radiographic examination should use highest kV and lowest mAs which will yield sufficient diagnostic information.
I, Nurhasanah Mazalan (0715402) hereby declare that this work is entirely my own. Any copying made for knowledge purposes is allowed but if possible please state this blog and my name as reference. It is just a way to appreciate others' hard work. Thank you.