x-ray Flashcards
What is the biological damage possible with X-rays?
X-rays are ionising, they can produce electron pairs in tissue. Damage to tissue results from direct interaction with DNA. Indirect ionisation of water molecules leads to formation of radicals.
What are the properties of X-ray waves
They are part of the electromagnetic spectrum. They have short wavelength, high frequency and high energy. They interact with matter in three ways: penetration, absorption or scatter. Energy is inversely proportional to wavelength. A Photon is a discrete bundle of electromagnetic radiation.
Describe the classifications of biological effects
Stochastic effects - probability of damage increases with dose, eg cancer, genetic effects and there is no known lower threshold.
Deterministic effects - Due to high radiation exposure, damage only over a threshold dose. Severity increases with dose, eg cataract. Concern in radiation therapy.
Describe Radiation protection
The goal is to obtain maximal diagnostic information with minimal exposure of the patient, radiology personnel and the general public. ALARA: as low as reasonably achievable. Higher risk groups should not be involved. Justification - there must be medical indication. Must be dose monitoring for personnel and dose limits. There must be controlled and regulated areas with ionising radiation.
What is the difference between effective dose and absorbed dose?
The absorbed dose is the amount of energy transferred by radiation per mass. The effective dose is a measure of radiation and organ system damage in humans: quality factor for different radiations.
What is the limited effective dose to the whole body?
For doses which are not received as a patient - dose level below which no harmful effect to people would be expected. For employees over 18 - 20mSv / year, for general public 1mSv/year. Pregnant women - employer must be notified in writing, maximum of 1msV for the remainder of pregnancy.
What are the UK specific veterinary radiation protection regulations?
Manual restraint only permissible if there is good clinical reason - if it cannot be kept still by other means. This means that all small animals should be at least sedated and restrained with sandbags except in critical illness where sedation would deteriorate patient condition or where time delay is critical.
What is personnel Dose monitoring?
Every person who could get more than 1mSv per year in an occupation needs to wear a dosimeter. Either a film badge or thermoluminescent. Need to be worn during work, UNDER radio protective clothing, need to be analyzed quaterly.
Describe the Production of X-rays
X-rays are produced through conversion of Kinetic energy of accelerated electrons into electromagnetic radiation. Production, acceleration and deceleration of electrons takes place within the X-ray tube. The source of electrons is the cathode. Acceleration takes place due to the potential difference and deceleration takes place at the anode. KvP: High potential difference leads to acceleration of the electrons in a vacuum towards the anode.
What is the Basis of the X-ray image formation?
Attenuation:decrease in intensity of an x-ray beam as it passes through matter. Absorption- photon removed from beam. Scatter- photon changes direction. Transmission: X-ray passes through matter without interaction. X-rays are differentially absorbed by different tissues. Attenuation of the beam by the different tissues of the patient reduces its intensity and creates the image ‘shadowgram’.
Describe the Photoelectric Effect
Photoelectric effect - complete absorption of x-ray photon. The photoelectron is removed from the shell. Ionization - electron from higher shell falls into space, characteristic radiation given off. This is the predominant interaction in low kVP range radiation. It is proportional to cubed atomic number of matter and proportional to thickness and density. Therefore provides image contrast when low kVp can be used.
Describe the Compton Effect
Compton effect - incoming photon ejects free outer shell electron from the tissue atom. The photon is scattered. The scattered photon has lower energy but may produce more ionizations, fog the film and is a radiation safety hazard. The ejected orbital electron is absorbed in the patient.This is the predominant interaction in high kVp range. It is only directly proportional to thickness and density and is independent of atomic number. results in low contrast image - high penetration at kVp, scatter, does not increase density differential of tissues.
What is the Latent Image of Xray?
Electons in silver bromide are released by light energy. silver atom formation at the sensitivity speck - latent image. The latent image reflects the pattern of the part being radiographed. It is viewed through processing. The developer provides electrons to exposed crystals.
What is the developer?
The latent image centre catalyses the reaction which reduces the remaining silver ion into grain of metallic silver.
What is the purpose of the Fixer?
To prevent further development, and to remove undeveloped silver bromide from film.
What is the purpose of the wash?
Removes fixer chemicals that would otherwise discolour the film over time.
What is the use of a Grid?
Used to absorb scatter radiation to improve radiographic contrast. Recommended when body part thickness is >10cm.
What are the advantages and disadvantages of computed radiography?
filmless, no chemistry, no dark room. the image is captured in cassettes containing phosphor storage layer. The cassette is put into laser film reader. The computer generates digital image in DICOM format. The image is sent and stored in server with PACS. It is time efficient, robust and gives good quality image. There is a lower radiation dose possible and it is cost effective. However the laser reader is sensitive to dust, and maintenance is required.
