Magnification error in radiographs of cervical spine in lateral projection

Uvod: Rentgensko slikanje vratne hrbtenice je ena izmed osnovnih diagnostičnih preiskav. Pri slikanju vratne hrbtenice stransko moramo za dosego optimalnega in diagnostično uporabnega radiograma, upoštevati anatomske posebnosti predela slikanja in fizikalne lastnosti rentgenskega snopa. Namen: Želeli smo ugotoviti, kako oddaljenost vratnih vretenc od slikovnega sprejemnika in oddaljenost gorišča od slikovnega sprejemnika vplivata na velikost vratnih vretenc na rentgenogramu pri slikanju vratne hrbtenice v stranski projekciji. Eden izmed namenov je raziskati, kako se spreminja vstopna kožna doza pri različni razdalji objekta od slikovnega sprejemnika in spreminjanju razdalje gorišče–slikovni sprejemnik. Metode dela: V teoretičnem delu smo podatke pridobili z metodo preučevanja literature. Podatke za praktičen del zaključnega dela, smo pridobili s pomočjo meritev na fantomu glave in vratu ter trupa v radiološkem laboratoriju Zdravstvene fakultete Univerze v Ljubljani. Da bi meritve lahko povezali s praktičnim slikanjem vratne hrbtenice v stranski projekciji, smo med sošolci izbrale fanta s širokimi rameni in dekle z ozkimi rameni. Postavili smo ju ob stativ, kot pri stranski projekciji vratne hrbtenice. Izmerili smo razdaljo od sedmega vratnega vretenca do stativa. Na podlagi pridobljenih meritev smo za 1 cm spreminjali razdaljo med objektom in slikovnim sprejemnikom. Pri slikanju smo spreminjali dva parametra: razdaljo objekt–slikovni sprejemnik (ROS) in razdaljo gorišče– slikovni sprejemnik (RGS). Razdaljo objekt–slikovni sprejemnik smo spreminjali za 1 cm, med oddaljenostma ROS 24 in 39 cm. Vsako spremembo ROS smo slikali na treh razdaljah gorišče–slikovni sprejemnik, in sicer na 115 cm, 150 cm in 180 cm. Pri vseh razdaljah ROS in RGS smo na rentgenogramu izmerili velikost telesa petega vratnega vretenca ter izračunali faktor povečave. Ob vsaki vrednosti ROS in RGS smo izračunali vstopno kožno dozo (VKD). Rezultati: Pri povečanju razdalje ROS iz 24 na 39 cm se je vretence pri RGS 115 cm povečalo za 23 % (teoretično izračunan faktor povečave je 24,9 %), pri RGS 150 in 180 cm pa za 17 % (16,1 %) in 11 % (12,3 %). VKD z zmanjševanjem ROS in z večanjem RGS, pada. Pri zmanjšanju razdalje RGS iz 150 na 115 cm, pri ROS med 24 in 29 cm, se VKD poveča za 26 %, pri ROS med 30 in 34 cm, se poveča za 31 %, pri ROS med 35 in 39 cm se VKD poveča za 35 %. Pri zvečanju razdalje RGS iz 150 na 180 cm, pri ROS 24 do 29 cm, se VKD zmanjša za 8,5 %, pri ROS med 30 in 34 cm, se zmanjša za 11,6 %, pri ROS od 35 do 39 cm pa se VKD zmanjša za 12,5 %. Razprava in zaključek: Na podlagi zbranih rezultatov smo ugotovile, da obe razdalji (RGS in ROS) vplivata na velikost vretenca na rentgenogramu. S povečanjem razdalje ROS se velikost vretenc na rentgenogramu poveča, s povečanjem razdalje RGS pa se velikost vretenc na rentgenski sliki zmanjša. Če razdalje ROS povežemo z indeksom telesne mase (ITM), lahko sklepamo, da je povečava vretenc na rentgenogramu pri pacientih z večjim ITM večja, kot pri pacientih z manjšim ITM. Ugotovile pa smo tudi, da se VKD zmanjšuje z večanjem razdalje RGS in hkrati narašča s povečevanjem razdalje ROS. Meritve so pokazale, da je priporočljivo slikanje vratne hrbtenice stransko na razdalji RGS 150 cm ustrezno iz dveh razlogov. Z večjo razdaljo RGS vplivamo na manjšo povečavo objekta na rentgenogramu in hkrati prihranimo vstopno kožno dozo pacientu.Introduction: One of the main diagnostical tests is the radiography of the cervical spine. During the lateral radiography of the cervical spine it is essential to consider the anatomic and physical characteristics of the X-ray beam. That way it is assured to obtain a proper use of the radiographic image. Purpose: The main purpose was to determine how the distance between the cervical spine and the image receptor on one hand and the distance between the source and the image receptor on the other hand affects the image size of the cervical vertebrae. The later was examined at the lateral radiography of the cervical spine. Further it was important to understand how the entrance skin dose varies when the distance between the object to image receptor and the distance source to image receptor changes. Methods: The theoretical part was obtained by the analysis of the readings. The practical part was carried out on the phantom of head, neck and trunk at the radiological laboratory of the Faculty of Health Sciences (University of Ljubljana). Measurements were compared to those on humans, which were obtained by the lateral radiography. Two parameters were varied: object to image receptor distance (OID) and source to image receptor distance (SID). The object was first 24 cm apart from the image receptor but the OID was then varied for 1 cm until the object was finally 39 cm apart from the receptor. At each OID there were three measurements taken at the SID 115 cm, 150 cm and 180 cm. After each set of measurements the image size of the fifth cervical vertebra was measured. The magnification error and entrance skin dose were also taken into account. Results: When increasing the OID from 24 to 39 cm the image size of the vertebra increased by 23 % (theoretically calculated magnification error is 24,9 %) at SID 115 cm. At SID 150 cm it increased by 17 % (theoretically 16,1 %) and at SID 180 cm by 11% (theoretically 12,3 %). ESD decreases if OID decreases or SID increases. When SID is decreased from 150 to 115 cm at OID between 24 and 29 cm the entrance skin dose increases by 26 %. As the OID is increased further the ESD is even higher. Similarly, the EDS decreases by the increase of SID. For instance, when SID is increased from 150 to 180 cm at OID from 24 to 29 cm the ESD decreases by 8,5 %. Discussion and conclusion: According to the results both OID and SID significantly affect the image size of the cervical vertebra. The image size of the vertebrae increases by the increase of the OID, while it decreases by the increase of the SID. If the OID is linked to the body mass index (BMI) it is clear that the image size of the vertebrae of patients with a higher BMI is greater than of patients with a lower BMI value. A similar effect occurs with the entrance skin dose (ESD) which significantly decreases by the increase of the SID, while at the same time it increases by the increase of the OID. Results have shown that it is recommended that the lateral radiography of the cervical spine is carried out at a SID of 150 cm. By doing so it is assured that a proper image size is obtained and at the same time the input skin dose is not harmful to the patient