Radiology, Image Production and Evaluation

Article Author:
Justyn Nakashima
Article Editor:
Hieu Duong
Updated:
4/5/2020 11:00:28 PM
PubMed Link:
Radiology, Image Production and Evaluation

Definition/Introduction

The production of X-ray images is a complex process that uses electromagnetic radiation. X-rays are high-frequency energy waves that penetrate through the body or the target organ and are either absorbed, reflected off, or traversed through the body. The X-ray tube, which produces the X-ray, is composed of a cathode and an anode. The cathode is a tungsten filament, which is heated during the process by electricity and ultimately produces electrons that travel through the tube to the target (anode). Once the high-speed electrons hit the target, producing X-rays in the form of photons.[1]

X-rays are primarily regulated by the kVp, mA setting, and exposure time. Image production can be individualized by manipulating these three factors to produce the best image.[2] The kVp controls the energy and penetration quality of the X-ray, which is important when considering the patient's size, age, and amount of movement. The quality of the X-ray beam becomes enhanced by increasing the kVp. Contrast is also controlled by kVp by regulating the absorption of the X-ray beam. The mA setting controls the number of electrons emitted by monitoring the heating of the filament and thus controls the quantity of X-rays produced at the anode. Exposure time determines the number of X-rays. A longer exposure time increases the amount of X-rays. Other factors that control x-ray quality or production include filtration, collimation, distance from the object or source, motion, and anatomy/pathology of the target organ or body.[3]

Once the patient is positioned in the right spot with the anatomic target in place, X-rays pass through the body, and remnant rays strike the image receptor located behind the patient. The image receptor may be a charged electron device used in digital radiography, a photosensitive phosphor plate used in computed radiography, or a conventional film screen. In conventional radiography, X-rays pass through the body and hit a film screen receptor, which is a transparent film composed of a silver bromide crystal emulsion spread between polyester base sheets. Once it strikes the film, the crystals fluoresce and produce a latent image. A latent image needs further processing for it to be visible. Although it is invisible to the naked eye, it is now more sensitive to chemicals used to produce the image. After the use of these chemicals called developers, a process of fixation, washing, and drying takes place to create an image. In computed radiography, X-rays strike a photosensitive phosphor plate, and the resulting electrons in the phosphor particles are placed in a high energy state, producing a latent image. The latent image is processed and scanned via lasers to develop a visible image. Digital radiography uses different chemical elements to interact with X-rays. The latent image is produced in the form of electric signals, which is directly transferred to a computer to create an image in real-time. Digital radiography is the latest advancement in image production and is currently preferable to other modalities due to its efficiency and improved quality.[1][2]

There are several things to consider when evaluating an image. The contrast of an image gets produced by the difference in tissue density displayed on a radiograph, and it can thus be manipulated to distinguish an area of interest. Contrast is dependent on the film density, film processing, curve of the film, and the use of intensifying screens. The density is the amount of light transmitted through the film and measures the degree of film darkening. Distortion of the image can be dependent on the size, shape, and position of the target structure within the body. Identification markers serve to differentiate the location, type of image, position, and orientation of the image. These markers are necessary to identify and analyze the anatomical structures correctly. Awareness of these factors is essential when evaluating radiographic images.[2]

Issues of Concern

As described in the previous section, many factors affect the quality of the image. Although these factors are controllable, mistakes can be made that can ultimately reduce the quality and evaluation of the image. The main factors that contribute to the quality of the image are contrast, blur, noise, artifacts, and distortion.[2] Contrast refers to the difference in surrounding densities on a radiographic image. Without contrast, objects within the body will not be visible, and differentiation between organs and adjacent tissues would be difficult. The degree of contrast required to visualize a specific object in the body is called image contrast sensitivity, and it depends on the imaging system and method utilized to obtain the image. Imaging systems that have a low contrast sensitivity will detect only objects with high object contrast while systems with a high contrast sensitivity visualize low contrast objects. Using the proper imaging method is essential when looking for certain abnormalities or pathology in the body. For example, A CT image is preferable in evaluating soft tissue masses due to its higher contrast sensitivity than conventional radiography.[4] Another factor that may disturb the image quality is the amount of blur. The visibility of small detail in an image can be limited as a result of the blurring effect. The blurring effect restricts the user's ability to look for minor pathology and can result in missing a significant diagnosis.[5] Image noise, another component that affects image quality, gives images a grainy appearance, and causes random variations of image brightness. Factors that can contribute to image noise include a variation of photon concentration, thermal activity within an electronic device, fluorescent lights, or sensitivity of the image receptor. Noise can be controlled but not completely prevented. Objects with low contrast are typically most affected.[6] Artifacts are image features that are not truly present but appear due to the abnormality of the imaging modality. While artifacts do not affect the visibility of an object, they can be misinterpreted as anatomical structures or block important structures within the body. Artifacts can result from foreign materials, lead markers, loose filters in the X-ray tube, patient movements, or improper handling and processing of film.[7] Distortion of an image is any misrepresentation or inaccurate impression of the target structure. Size distortion is the magnification of the anatomical structure and is usually caused by an increased object to image receptor distance or decreased distance between the source of radiation and image receptor. This phenomenon causes the object to look larger than it is in reality. Shape distortion refers to elongation or shortening of the target object. This appearance results from improper angulation of the image receptor or axis or by technical and/or structural errors of the X-ray tube.[8] These factors should be avoided if possible, to minimize mistakes in diagnosing a patient.

Conventional radiography, fluoroscopy, and CT scans all use ionizing radiation to produce images, which is another issue of concern. Due to the rise in the use of CT scans and x-rays, it is important to note the harmful effects caused by radiation. Ionizing radiation causes detrimental biological effects on a cellular level either by indirectly producing free radicals to interrupt cellular metabolism or directly damaging the DNA of cells in the body. Cells are considered damaged when the level of cell damage exceeds the cell's ability to repair itself. There is an elevated risk of cancer development as a result of DNA mutations that occur with ionizing radiation. Studies of radiation-induced cancer from Hiroshima and Nagasaki have shown that there can be a 10 to 20 year latent period from the time an individual gets exposed to radiation to the time of cancer development.[9] Although the dose of radiation exposure from medical imaging tests is minuscule compared to the radiation from atomic bombs, excessive or unnecessary usage of these image modalities should be limited. All healthcare providers should be cautious and understand the risks associated with radiation.[10][11][12][13]

Clinical Significance

Medical imaging is among the most valuable technological advancements in the field of medicine and has resulted in many improvements in the diagnosis and treatment of numerous medical conditions. Image production and evaluation have allowed healthcare professionals to efficiently recognize and assess emergent cases and has been a crucial tool in the improvement of public health. Examples include the use of mammography in breast cancer screening, radiography in the assessment of fractures or identifying pneumonia, ultrasound image guidance in treating tumors, and the use of multiple imaging modalities during procedures such as joint injections and in the insertion of stents, catheters, and other medical devices.[14] Imaging aids in diagnosis and reduces the risk of unnecessary medical or surgical interventions. Medical professionals need to understand the risks and benefits of imaging and learn how to utilize imaging modalities properly.[15] Understanding the mechanism behind image production and knowledge of the different components that affect image accuracy is crucial in optimizing image quality and providing the best care for patients.[13][14]

Nursing, Allied Health, and Interprofessional Team Interventions

Understanding image production techniques and knowledge of image evaluation are crucial in proposing the appropriate diagnosis and treatment.[16] Balancing the risks and benefits must always be done before the use of imaging tests that utilize ionizing radiation. The risk of developing cancer from imaging radiation depends on the patient's age, sex, target organ, and radiation dose.[3] There are certain populations, including pediatric patients and pregnant women that are at higher risk for adverse effects of imaging. These factors merit consideration by medical professionals in their approach to imaging as well as communicated amongst the medical team to ensure patient safety and ultimately improve outcomes. The use of imaging procedures should only be done when necessary, with the appropriate dose and imaging modality, and if benefits outweigh the risks. The ALARA principle "as low as reasonably achievable" is currently being used to emphasize safety and the importance of using the lowest dose possible to limit the exposure of ionizing radiation. Healthcare professionals should also be trained in radiation safety to protect themselves and to avoid excessive exposure. [12][13][15] [Level 1 & 3]


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