Hvt

RADIOGRAPHIC SCIENCE (UZY201C1) Module Assessment: Component B Assignment Introduction The filtration of an X-ray beam is undertaken in both radiotherapy and diagnostic imaging.

Filter materials such as aluminium and copper are

utilised in radiotherapy equipment (ie orthovoltage) and diagnostic X-ray tubes and primarily improve the quality of the X-ray beam, but reduces its intensity. This assignment is segmented into two parts: Part One: The evaluation of an experiment to determine the Half Value Thickness (HVT) of an X-ray beam for a given material (Aluminium) Half Value Thickness may be defined as: “The half value thickness is the thickness of a specified material which will reduce the number of X-rays (Intensity) in a beam by one half.” Using either WebCT or a CD-ROM (obtainable from the library), observe either the video footage and/or image stills of an experiment undertaken in room 1F01, where the intensity of

a heterogeneous X-ray beam was

measured using a dose meter (ionisation chamber) and various thicknesses of an aluminium filter material. You should carefully observe the experiment and relate to the physical principles of the interaction of X-ray photons with matter. 1

A summary of the experiment:

Experiment to determine Half Value Thickness (HVT) of an X-ray beam. Materials • • • •

X-ray source Radcheck ionisation chamber Aluminium sheets Sticky tape

Method The ionisation chamber was positioned approximately 100cm from the X-ray source. The light beam diaphragm (LBD) was collimated to the sensitive area of the Radcheck. The chamber is switched on and reset to zero before each exposure. An exposure of 80k kVp and 200mAs was set on the control panel.

All

personnel were behind the screen before an exposure was undertaken. The initial dose reading on the ionisation chamber was recorded (Table one) and the meter reset. Using sticky tape a 0.5mm thick sheet of aluminium was attached beneath the LBD. The exposure was repeated and dose reading was recorded. Additional sheets of aluminium were added to the first sheet, building up the thickness in 0.5mm increments to an eventual total of 5mm. The exposures were repeated and dose readings recorded for each increment of aluminium added to the bottom of the LBD. (Table one). Calculate the relevant transmission of the X-ray beam, assuming an initial 100% transmission with no aluminium. Using Excel software, plot an appropriate graph and calculate the value of HVT of aluminium for the X-ray beam utilised in this experiment. 2

Table One Thickness Al (mm) 0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

Recorded Dose (mGy) 0,903 0,750 0,631 0,553 0,893 0,452 0,374 0,332 0,311 0,279 0,252

Your graph should be in an exponential format and utilise the mathematical formula relating to radiation intensity and thickness of an attenuating material (ie Aluminium). You should also indicate the actual half value thickness figure either on your graph or body of assignment.

I = Ioe –

I = Intensity Io = Original Intensity e= Exponential Constant = A quantity which is related to the attenuating properties of the medium through which the X-ray beam has passed t = Thickness of attenuator material (Aluminium)

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Part Two Assess the HVT experiment and describe the theoretical concepts of this experiment. In addition, you should also attempt to discuss the methodology of this experiment and how it may have been improved for accuracy / consistency. Analyse the results in table one and your graph (part one) and describe the findings in relation to the theoretical concepts of the attenuation of an X-ray beam. Discuss the physical applications of filtration and Half Value Thickness in clinical practice in relation to either radiotherapy or diagnostic imaging equipment. Your discussion and conclusion should relate to both academic and clinical experience.

Questions you may like to answer: •

Was the methodology for this experiment appropriate?

•

Was the initial transmission value of the X-ray beam really 100%?

•

Were all factors constant throughout the experiment?

•

How may the experiment be improved for accuracy / consistency?

•

Why are different material used for filtration?

•

What are the fundamental reasons for using filters in clinical practice?

•

Where are the filters positioned inside X-ray equipment?

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What effect does scatter have on the results and can this be reduced/ eliminated?

Points to consider:

Interaction of X-ray photons with matter Remember: either be: •

photons of X-rays which are directed towards a body can

Transmitted (no interaction so pass through unchanged) 4

•

Attenuated (either absorbed of scattered) but in effect removed from the primary beam.

The main interaction process of X-ray photons with matter are photoelectric absorption; pair production; (in both cases the X-ray photons are totally absorbed) and Compton scatter (partial absorption as some of the energy is absorbed; and scatter as the photon is deflected from its original path) The process of attenuation of the X-ray photon is a random event but attenuation of the beam increases with: • •

increasing thickness of material increasing atomic number or density

It also decreases with an increase in photon energy. Before completing and submitting this assignment read your Faculty & School handbook for guidelines on assignments & assessment. Marking Criteria See page 19-25 on assessment & appendices 4&6 in the Faculty Undergraduate Student Handbook. This assessment will use the marking grid for Level 1. The main weighting for the marks awarded will be on the cognitive descriptors page 50 i.e., knowledge & understanding; Analysis; Synthesis and evaluation. Other factors taken into consideration will be in key transferable skills on: Problem solving and communication & presentation p51. Word Count: 2000 Words References & useful literature: Ball J. & Moore A., (1997) Essential Physics for Radiographers, London, Blackwell Scientific Publications Bushong S.C. (1997) Radiologic Science for Technologists, 6th Edition, London, Mosby. 5

Farr R.F. & Allisy-Roberts P.J. (1997) Physics for Medical Imaging, London, Saunders Submission details You should attempt to submit two copies of your assignment, in order to obtain a returned marked copy. Assignment submission date: 29th January 2002 (16:30PM)

The module team will discuss the general principles and specific details of the experiment. However, your interpretation, discussion and conclusion of the results must be performed on an individual basis.

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