Differences In Grid And Air Gap Technique

2.1 Introduction

In this chapter, a literature reappraisal was carried out so that equal information about the differences in grid and air-gap technique could be gathered with accent on why these techniques are of import in apparent skiagraphy of the sidelong hip. These two techniques will be analysed to explicate better the acquisition of the image. Both techniques will be compared and their advantages and disadvantages discussed. This was done by agencies of skiagraphy books and diaries. Where, possible primary beginnings of information were chosen. However, original surveies could non ever be obtained and secondary beginnings had to be considered. The usage of the cyberspace was besides of import as it served as a beginning for and entree to relevant articles. Related literature was chiefly sourced utilizing the online information bases of EBSCOA® , CINHALA® and PubmedA® every bit good as the Institute of Health Care ‘s library and installations.

2.2 Image quality

Image quality refers to the ability to see the anatomical construction under survey with preciseness and which makes it possible to place and descry any abnormalcies ( Bushong, 2008 ) . The quality of the image depends on several physical and physiological factors and this makes it difficult to mensurate. Image quality ;

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“ Is defined in radiological nomenclature as the relationship between the constructions of a trial sample to be irradiated with X raies and the parametric quantities of its visual image ” ( Hertrich, 2005, pg.244 )

Harmonizing to Bushong ( 2008 ) , the most of import factors that improve or degrade image quality are contrast declaration, spacial declaration, noise and artifacts. Image quality can non be measured in a precise manner since the quality of radiogram is difficult to specify ( Bushong, 2008 ) .

In digital skiagraphy ( DR ) the image quality depends on a figure of features that can alter the screening experience. One of these factors is frequence, which is a step of the entire sum of contrast within the image. This characteristic depends on the natural information ( x-ray energy ) that the imagination sensor absorbs. The frequence of the image is represented by different Greies scale degrees that show the denseness of a peculiar portion of the anatomical construction. This is how contrast is affected. A high contrast image has high frequence due to the sum of x-ray photons absorbed ( Carlton & A ; Adler, 2006 ) .

Image quality is besides subjective and depends on the spectator ( Sherer et al. , 2006 ) . Different people may necessitate to change the quality of the image by increasing or cut downing the contrast or by altering the acuteness of the image depending on their single ocular abilities ( Dendy & A ; Heaton, 2003 ) . Dendy and Heaton ( 2003 ) argue that image quality besides depends on the show system and the manner the image is produced. The writers further argue that room lighting besides affects image quality and might besides decrease image quality.

2.2.1 Image contrast

Image contrast refers to the difference in densenesss between next anatomical constructions. The sum of contrast produced on an image depends on the structural feature of the anatomical portion of the organic structure every bit good as the features of the x-ray beam when it penetrates the patient ‘s organic structure. Contrast depends on the fadings within the patient ‘s organic structure due to different densenesss in assorted parts of the organic structure. The higher the difference in densenesss, the higher is the contrast ( Sherer et al. , 2006 ) . However, little alterations in densenesss of constructions would non be detected on a high contrast image since high contrast does non hold a great adequate exposure latitude to give several sunglassess of Grey in the image ( Bushong, 2008 ) . This means that holding high contrast in an image does non needfully intend that it is optimum for every radiographic scrutiny. Having low contrast means that better contrast declaration is produced and this gives the spectator the ability to distinguish between anatomical constructions that have similar densenesss. This is why contrast is a really important factor in image quality ( Oakley, 2003 ) . Scattered radiation affects image contrast and the features of the receptor and show system. The anatomical item and contrast of little anatomical constructions may besides be reduced due to image blurring ( Carlton & A ; Adler, 2006 ) .

2.2.2 Noise

Noise affects the image ‘s contrast declaration and the item seen in the image. Like audio noise and picture noise, radiographic noise is caused by weak signals in countries of the image ( Oakley, 2003 ) . The lower the noise the better the contrast declaration and so image quality is better. Harmonizing to Bushong ( 2008 ) , there are four chief constituents that affect radiographic noise. These are coarseness, construction mottle, quantum mottle and spread radiation. Graininess and construction mottle can non be controlled by the radiographer since they are dependent on the image receptor. However, the radiographer can utilize several techniques and exposure factors to better image quality and cut down the noise every bit much as possible depending on the topic under scrutiny. Penetration energy of the x-ray photons ( kilovolt ) can be increased in topics that are corpulent and that are holding thick countries of their organic structures irradiated. Quantum mottle is besides a really of import feature in specifying noise. Bushong ( 2008 ) explains that quantum mottle depends on the sums of X raies that are exposed and absorbed by the image sensor. When few X raies react with the receptor the attendant image will look mottled. However, when more X raies are absorbed by the sensor, the image will look smooth. Noise can be calculated by mensurating the signal-to noise ratio of the image ( Bushong, 2008 ) . If non adequate x-ray photons reach the sensor, the image is said to be under-exposed, ensuing in a low signal/noise ratio ratio. However, a high signal/noise ratio ratio is achieved if the right radiographic technique is used with the right exposure factors ( Bushong, 2008 ) .

2.2.3 Spatial Resolution

Spatial declaration is a term used in imaging that refers to the declaration of a radiogram. Having a high declaration means that more item can be seen and detected on the image. Spatial declaration is a really of import public presentation index in skiagraphy. Quality control apparitions are used to look into the spacial declaration and contrast of an imagination system. Spatial declaration relies on spacial frequence and this measure could be calculated by seeing the figure of line braces per millimeter ( Lp/mm ) . These line braces are dark and white lines that are used to measure the declaration of an image. Detail is really of import in skiagraphy since lineations of tissues, variety meats and specific pathologies need to be crisp and elaborate. High spacial declaration is besides of import when measuring for elusive breaks like scaphoid breaks which could easy be missed if the radiogram is non crisp plenty ( Bushong, 2008 ) .

2.3 Scatter Radiation

When x-ray irradiation brushs matter, some photons pass unimpeded making the image receptor whereas other photons are wholly absorbed since the energy of the primary x-ray beam is deposited within the atoms consisting the tissue. This soaking up interaction of x-ray photons with affair is known as the ‘photoelectric consequence ‘ ( Fauber, et Al, 2009 ) . This photoelectric consequence is dependent on the affair and its consequence decreases quickly with increasing photon energy ( Dendy & A ; Heaton 2003 ) . Scatter radiation is made up of photons that are non absorbed but alternatively lose their energy during interactions with the atoms doing up the tissue ( Fauber, et Al, 2009 ) . This dispersing consequence is known as the ‘Compton consequence ‘ ( Carlton and Adler, 2006 ) . This happens when the incoming photon interacts with affair and loses energy. This will do the photon alteration way and it may go forth the anatomic portion to interact with the image receptor ( Fauber et Al, 2009 ) . Scattered low- photon energy reduces the contrast on the concluding radiogram and is besides extremely risky for patients and staff due to its changed way and low energy from the primary beam ( Dendy & A ; Heaton, 2003 ) .

2.4 Scatter Decrease

As explained above, scatter radiation is produced during a Compton interaction in which a primary photon interacts with an atom of the patient ‘s organic structure and loses its velocity and way. Scatter is produced chiefly in the patient due to the variable fading and densenesss of the different variety meats in the organic structure and this could be controlled by utilizing anti-scatter techniques and the right exposure factors. Consequently the radiographer should utilize the adequate technique and exposure factors to cut down the radiation produced within the patient ‘s organic structure. Carlton and Adler ( 2006 ) argue that when the energy of the primary beam is increased there is a higher opportunity for the photons to undergo the Compton interaction. This means that the higher the energy given to the photons ( kilovolt ) the more likely it is that there is Compton interaction with the organic structure ‘s atoms, hence making more scatter radiation and a diminution in radiographic contrast ( Bushong, 2008 ; Carlton & A ; Adler, 2006 ) . However, Shah, Hassan & A ; Newman ( 1996 ) think otherwise. In their survey they stressed the effectivity of anti-scatter techniques on image contrast and concluded that the influence of kilovolt on spread production is little. The writers farther stated that the betterment in contrast that occurs when the kilovolt is lowered is normally due to an increased topic contrast since less spread reaches the movie. Carlton and Adler ( 2006 ) besides gave importance to the size of the country of organic structure being irradiated. They suggested that by diminishing the country of irradiation every bit good as using compaction, spread radiation making the sensor could be significantly reduced. Using this technique Shah, Hassan and Newman ( 1996 ) noted a lessening in the dose country merchandise ( DAP ) when diminishing the country of irradiation, hence take downing the hazard of increasing patient dosage.

2.5 Anti-scatter techniques

Anti-scatter techniques are radiographic techniques that make usage of devices or applications such as grids and air spreads so that spread radiation is absorbed or deviated from making the image sensor. These anti-scatter techniques help in cut downing patient dosage every bit good as bettering the quality of the radiographic image. The two chief techniques relevant with this survey are explained and analysed in the undermentioned sub-sections.

2.5.1 Grid Technique

Grids are used in skiagraphy to protect the image sensor from spread radiation. Scatter radiation degrades the quality of the image and may take to loss of anatomical item and information ( Sherer et al. , 2006 ) . Anti-scatter grids are made up of parallel wireless opaque strips with a low-attenuation stuff interspacing the strips ( Sherer et al. , 2006 ) . The most normally used interspaced stuffs are aluminium and C fiber ( Court & A ; Yamazaki, 2004 ) . The chief map of these anti-scatter grids is contrast betterment. Harmonizing to Carlton & A ; Adler ( 2006 ) , the most effectual manner to see how good a grid is executing is by mensurating the contrast betterment factor. The contrast betterment factor measures the ability of a grid to better contrast. This factor is affected by the volume of tissue irradiated and by the kilovolt. If the sum of spread radiation additions, the contrast of the image will be reduced, hence cut downing the contrast betterment factor. This is calculated utilizing the undermentioned expression:

K= Radiographic Contrast with the grid / Radiographic contrast without the grid

( Carlton & A ; Adler, 2006, pg.263 )

The higher the contrast betterment factor the higher is the contrast betterment. However, Court and Yamazaki ( 2004 ) argue that since contrast can be digitally altered in digital skiagraphy, it is best to cipher the signal/noise ratio ratio ( SNR ) of the image. This is particularly utile in instances where there is low object contrast.

The interspaced stuff dividing the lead grid lines is besides really of import in supervising the functionality of a grid. In the survey performed by Court and Yamazaki ( 2004 ) it was concluded that aluminum has a higher atomic figure than C fiber and so it absorbs more low energy spread radiation. However, aluminum besides absorbs some of the primary photons hence increasing patient dosage. Alternatively, C fiber absorbs less primary radiation than aluminum ( Court & A ; Yamazaki, 2004 ) . Grid ratio is besides an of import factor to see in bettering image quality particularly image contrast. The grid ratio is obtained by spliting the tallness of the strips by the strip separation. As the grid ratio affects the rate of spread to that making the sensor it is instrumental in bettering image contrast ( Dendy & A ; Heaton, 2003 ) .

There are chiefly two types of grids, additive grids ( parallel grids ) and focused additive grids ( Fauber, 2009 ) . Both types have their ain advantages and disadvantages. Parallel grids are made up of additive lead strips with low-density stuff interspacing them and are parallel to each other. This assortment of grids green goodss grid cut-off at sidelong borders since they do non co-occur with the oblique divergencies of the beam ( Dendy & A ; Heaton, 2003 ; Fauber, 2009 ) . It is besides indispensable that these grids are positioned right, perpendicular to the cardinal beam of the primary beam. If this is done falsely, there will be grid cut-off and the lead strips will absorb a batch of the primary beam which will demo up on the image ( Dendy & A ; Heaton, 2003 ) . This will ensue in image impairment and in the patient having an excess dosage of radiation when reiterating the exposure. The focussed grids, nevertheless, are designed in such a manner that it allows the lead strips to be bit by bit angled traveling off from the cardinal axis. Although these grids are designed to extinguish the cut-off on the sidelong sides, they still have to be used at a specific focal point to image distance ( FID ) depending on the type of grid being used ( Dendy & A ; Heaton, 2003 ) .

Although grids are used to better image contrast and cut down spread making the sensor, this is at the disbursal of a high radiation dosage to the patient. This happens because the ma has to be increased when utilizing the grid. This is necessary in order to counterbalance the primary beam photons absorbed by the grid ( Carlton & A ; Adler, 2006 ) .

2.5.2 Air-gap Technique

The air spread technique is an alternate technique used to cut down the sum of spread making the sensor. By using an air-gap technique between the patient and the image sensor, the energy of the scattered photons decreases particularly in the first 10s of centimeters due to the big divergency of the beam ( Ball & A ; Price, 1995 ) . The primary radiation is non affected or reduced, since at this phase the primary beam is about parallel to the sensor ( Ball & A ; Price, 1995 ) . When the air-gap technique is used, the object to image distance ( OID ) is increased, which may bring forth some magnification ( Sherer et al. , 2006 ) .

Anti-scatter techniques are of import in cut downing low energy radiation making the sensors. However, the primary beam should non be deflected or disrupted so that the image acquisition and image quality is non affected ( Fauber, 2009 ) .When the grid technique is employed, the grid lines are unable to know apart between the primary radiation and the scattered radiation and so this could take to grid cut-off and ‘grid lines ‘ may look on the image ( Maynard, 1981 ) . Maynard ( 1981 ) argues that with the usage of an air-gap the image quality and diagnostic quality of many projections improves. A survey by Karoll et Al. ( 1985 ) analysed the patient dosage when the air-gap was employed compared to when the grid was used. In this survey the air spread was employed in a digital minus scrutiny. Karoll et Al. ( 1985 ) reported that by utilizing the air spread technique the ma could be lowered without losing spacial declaration. The consequences of this survey were singular as the air spread technique allowed 25 % to 88 % decrease on the ma without increasing the kilovolt or the clip of exposure ( Karoll et al. , 1985 ) . This meant that patient dosage was reduced since the ma was lowered and so the patient was irradiated less. Although this survey is 25 old ages old, it is still valid since in direct digital skiagraphy, windowing has given the radiographer the possibility to cut down the exposure factors to a certain bound while still obtaining a good diagnostic image. This means that patient dosage could be lowered.

Both grid and air-gap technique were studied and compared to measure patient dosage by Kottomasu and Kulms ( 1997 ) . The writers concluded that the air-gap improves musculoskeletal digital imagination without an addition in skin entryway dosage. Harmonizing to Kottomasu and Kulms ( 1997 ) , this happened since the scattered photons had less energy one time diverged by the patient ; they were deflected and did non hold plenty energy to make the image sensor ( Kottomasu & A ; Kulms, 1997 ) . Barall ( 2004 ) besides suggested that when using the air-gap technique the radiographer should use reverse square jurisprudence by increasing the SID and using tighter collimation. This will guarantee the highest lessening in patient dosage possible ( Barall, 2004 ) . The addition in SID could enable a better usage of the air spread while cut downing magnification by maintaining the beginning to object distant ( OID ) invariable. In relation to the horizontal beam sidelong hip projection, there is a decrease in dosage and a good diagnostic end point image when compared with the grid technique ( Barall, 2004 ) . Trimble ( 2000 ) concluded that imaging the thoracic spinal column without a grid was possible in kids and grownups of little size. In this survey a important dose decrease was noted and hence on this footing, imaging the hip laterally utilizing a horizontal beam and using the air-gap technique alternatively of the grid may besides ensue in a decrease of patient dosage as opposed to utilizing the grid technique.

2.6 Digital skiagraphy.

Radiography has been revolutionised and developed throughout the old ages from screen movie ( SF ) radiography a high quality digital system has evolved ( Oakley, 2003 ) . With the debut of digital imaging systems, image quality features have improved. The procedure of image formation in DR is similar as in SF. The image is foremost generated, so processed, archived and presented. Alternatively of movies, DR uses sensors which when exposed to x-ray radiation absorb this irradiated energy which is so transformed into electrical charges, recorded, digitized and configured into different Grey graduated tables ( Dendy & A ; Heaton, 2003 ) .

The Grey graduated tables presented on the produced image represent the sum of x-ray photons absorbed by the sensor. A large advantage in digital skiagraphy is image use post-processing. While sing the image, the radiographer can whizz in or out, alter the greyscale every bit good as usage measurement tools. Another great advantage of DR over SF is that images can be stored safely and archived. This solves the job of movies being lost and enables future mention of the images ( Carlton & A ; Adler, 2006 ) .

There are two types of digital imaging systems: computed skiagraphy ( CR ) and direct digital skiagraphy ( DR ) . In computed skiagraphy imaging home bases incorporating photostimuable crystals are used, which absorb the x-ray energy and shop it temporarily ( Korner et al. , 2007 ) . Processing involves scanning the detective bed pel by pel utilizing a high energy laser-beam of a specific wave-length. Since the exposed photon energies are merely stored temporarily in the detective bed, the read-out procedure should get down instantly after exposure. This is chiefly because the sum of energy stored in these crystals decreases over clip. Although this is a large measure from screen-film ( SF ) , spacial declaration in CR may diminish if viewing proctors are non of the appropriate declaration ( Korner et al. , 2007 ) .

Direct digital radiographic systems use a photoconductor straight change overing x-ray photons to electrical charges, one time the photons are absorbed. The most common stuff used as a photoconductor in industry is formless Se. This stuff has a high intrinsic spacial declaration. However, the stuff of the sensor does non impact the pel size, matrix and spacial declaration of the sensor ( Dendy & A ; Heaton, 2003 ) . These are affected by entering and read out devices used. Therefore image processing in DR is every bit of import as in SF and CR. In DR image processing is used chiefly to better the image quality by taking proficient artifacts, optimizing the contrast and cut downing the noise ( Dendy & A ; Heaton, 2003 ) .

2.7 Radiation Dose

The passage from SF to DR has besides changed the radiation dosage that the patient gets from an x-ray exposure. Radiation dosage is the sum of radiation absorbed by the patient due to a radiation exposure ( Carlton & A ; Adler, 2006 ) . In SF radiography the dynamic scope of the receptor ( movie ) is comparatively low and so it merely detects specific exposures that lie within its parametric quantities. However, in DR the digital receptor can observe a broad scope of exposures. This means that a somewhat underexposed or overexposed image is acceptable since image quality can be altered utilizing windowing. Therefore in DR the radiation dosage could be kept comparatively low when compared to SF while still bring forthing a good diagnostic image. This could besides work the other manner when patients are overexposed to radiation due to the broad dynamic scope of the receptors. The ALARA construct is based on the theory that there is no “ safe ” dosage of radiation utilizing any sort of irradiation or radioactive stuff ( The Ionising Radiation [ Medical Exposure ] Regulations, 2000/2007 & A ; The Medical Exposure Directive 97/43/Euratom ) . In this manner person ‘s internal and external exposure to radiation is kept to a lower limit. This rule does non merely address radiation used in medical specialty but besides societal, proficient and economic considerations of usage of radiation. This rule besides takes into consideration the clip of exposure of radiation, filtration, and appropriate stuffs selected to understate radiation lodging on surfaces. This besides ensures the safe disposal of stuffs incorporating radiation such as acerate leafs used in atomic medical specialty ( The Ionising Radiation [ Medical Exposure ] Regulations, 2000/2007, & A ; The Medical Exposure Directive 97/43/Euratom ) .

The usage of ionizing radiation should be monitored and used carefully to guarantee as low a dose exposure as is moderately accomplishable to the patient while at the same clip bring forthing an image of high diagnostic quality.

2.8 Relative Literature

The latest literature reviewed in relation to this thesis was that of Flinthman ( 2006 ) who assessed 35 horizontal beam sidelong hip radiogram for image quality. Nineteen of the instances were performed utilizing the air-gap technique whereas sixteen utilizing the grid technique. Several radiotherapists and radiographers were asked to measure the images. It was found that the air-gap was of higher image quality than the grid technique ( Flinthman, 2006 ) . In Flinthman ‘s survey several individuals were asked to measure an uneven figure of instances that were meant to be compared sing the technique used to obtain the radiogram. Harmonizing to Flinthman ( 2006 ) it is more of import to hold a little group of people measuring the radiogram. This is because the consequences could be more specific and more dependable ( Flinthman, 2006 ) . A restriction of this survey is that Flinthman ( 2006 ) did non utilize the same topics in both techniques to accomplish his consequences and so it is harder to achieve valid and conclusive consequences that could be applied in a clinical scene.

A similar survey comparing the grid and air-gap technique was conducted by Persliden and Carlsson ( 1997 ) . Persliden and Carlsson ( 1997 ) studied scatter decrease utilizing the air-gap and the grid technique. This survey investigated the consequence of the air-gap technique over the imaging home base and demonstrated the positional fluctuation of scattered radiation ( Persliden & A ; Carlsson, 1997 ) . The writers concluded that by utilizing the air-gap technique, the patient irradiation was lowered. Persliden and Carlsson ( 1997 ) argued that even field size and patient thickness greatly affected the usage of the air-gap.

Equally good as Persliden and Carlsson ( 1997 ) , Trimble ( 2000 ) looked and assessed image quality of sidelong thoracic spine radiogram and thorax radiogram. These scrutinies were both done utilizing the grid technique and the air-gap technique. Trimble ( 2000 ) found it of import to hold a big sample of topics while maintaining the specializers measuring the images little. Trimble ‘s survey resulted in the air-gap being better for high image quality than the grid.

Similiar to this survey, Goulding ‘s survey ( 2006 ) who looked at image quality in sidelong hip skiagraphy when utilizing both grid and air spread technique. The radiogram were obtained from the accident and exigency section Goulding ( 2006 ) worked in, where radiographers performed sidelong hip shoot through scrutinies utilizing their preferable air spread or grid technique. Goulding ( 2006 ) took a sample from the recorded scrutinies of both techniques. The research worker excluded scrutinies with an exposure of 100 ma or more every bit good as any duplicate patient Numberss due to re-assessment every bit good as those scrutinies that used both air spread and grid technique in the same scrutiny as this signified a really big patient. Goulding ( 2006 ) compared the sampled gird and air spread radiogram after describing radiographers evaluated five countries on each radiogram, chosen by the research worker. The radiographers had to hit each country from one to five where 1 is hapless and five is optimal. It resulted that the air spread technique had improved image quality more than the grid technique. A restriction of this survey was, nevertheless, that the patients used to prove for both techniques were non the same, and so this could hold meant that the consequences were non wholly dependable since patient size and exposure factors were non changeless but varied depending on each scrutiny.

2.9 Decision

The literature reviewed in this chapter has explored moreover the functions of the air-gap and grid technique in imaging. It has besides analysed the consequence of spread radiation and ways to cut down this in order to better radiographic image quality while restricting the radiation dosage to the patient every bit much as possible. Several surveies were reviewed and analysed and will assist to better this experimental research. Some surveies that are similar to this survey were reviewed and discussed.

In the following chapter, a description of the research design used in this survey will be presented.

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