Transcript Document
X Rays Medical Physics Notes Ideal X-Ray Examination • a film that showed sufficient contrast between the features that the doctor wanted to examine • while putting the patient at minimal risk from the ionizing effect of the radiation. The Beam • An X-ray tube does not produce a monochromatic beam • It produces a spectrum of X-ray energies limited at the high energy end by the accelerating voltage applied. • With characteristic peaks relating to the material of the target Attenuation (reduction of the beam strength) • Attenuation occurs as the Xrays pass through matter. • This attenuation is exponential. – Let Io = Intensity of the incident beam – I = Intensity of emerging beam – x = the thickness of the material the beam travels through m = linear attenuation coefficient Syllabus Extract • Exponential attenuation Linear coefficient m, mass attenuation coefficient mm and half-value thickness x • Remember r is density!! • You did something similar with gamma ray absorption Half Thickness I = Ioe-mx I/Io = e-mx natural log of (I/Io) = -mx If the intensity halves I = 0.5 Io ln 0.5 = -mx ln 0.5 = -ln2 x = ln2/m Half Thickness • This is similar to half life – the thickness of material that the rays must pass through in order for the intensity of the beam to be cut in half. • This varies with density of material – the denser the material the smaller the half thickness. Half Thickness • Ensure you can calculate half thicknesses as well as find them off graphs. • In a similar way you can find out the thickness needed to reduce penetration to a tenth etc. ( just put I = 0.1 Io into the equation). • Values for the mass attenuation coefficient mm can be changed into m by using the equation mm = m / r Where r is the density of the material Attenuation mechanisms • The lower energy rays are more likely to be attenuated by the body than the high energy ones. • Attenuation occurs as the radiation passes through the body of the patient by two principal mechanisms: – photoelectric absorption and – Compton scattering. Syllabus Extract • Differential tissue absorption of X- rays (excluding details of the absorption processes) • Note that you do NOT need details of the attenuation processes! • But you do need to know the names! Attenuation • The lower energy rays are more likely to be attenuated by the body than the high energy ones. Attenuation occurs as the radiation passes through the body of the patient by two principal mechanisms: photoelectric absorption and Compton scattering. Attenuation Processes _ Detail NOT required • Photoelectric absorption occurs when a photon of energy is absorbed by an orbital electron and this electron is then promoted to a higher energy level (more outer orbit) or leaves the influence of the nucleus completely (ionization). Attenuation Processes – Detail NOT required • A.H. Compton discovered that if he bombarded graphite with monochromatic X-rays, the scattered X-rays had lower energies (longer wavelengths) than the undeflected ones: the greater the deflection the bigger the energy loss. Attenuation Processes – Detail NOT required • The bombarding X-ray photon has a lot of energy - the force binding the electron to the atom is insignificant compared to the force exerted by the photon on impact. • When the photon 'bounces off' the electron, the electron recoils and thereby picks up some of the photon's energy. This is called Compton Scattering. Attenuation Processes – need to know! • Photoelectric absorption is the dominant mechanism for low energy X-ray photons (used in soft tissue) whereas Compton Scattering becomes more significant for higher energy photons (bone). Contrast in Breast Tissue • Low energy photon energies produce a better contrast between media of similar density (as in mammography) but overall absorption is greater. • This means that a higher anode current (resulting in a more intense beam) has to be used the lower the accelerating potential employed across the tube. Contrast in Chest XRay • In a chest X-ray the densities of tissue to be investigated is diverse (bone/lung/heart) and ‘harder’ X-rays can be employed. • These still give the contrast required in the image but absorption is reduced by using high energy rays and filtering out the lower energy ones (soft X-rays) produced by the tube. Contrast in Chest XRay • This can be done using an aluminium filter. Suitable energy for a chest Xray would be 60-100 keV depending upon the exact nature of the detail required to make the diagnosis. Collective Dose • When calculating the collective dose to the population the average dose received per person is multiplied by the number of persons. Average Dose • To calculate the average dose received because of X-ray examinations the number of each type of investigation would be found and then the typical dose given for each procedure would become the multiplying factor. Typical Doses X-ray examinations of • limbs, joints and teeth involve a typical effective dose of about 0.01 mSv whereas a • chest CT scan involves 8.0 mSv. and a • barium enema 7.2 mSv. Hence one CT scan is equivalent in dose to about 800 knee X-rays!! Typical Doses • This is why although many more low dose X-rays are carried out, they do not contribute very much to the population dose. • The much lower number of major scans make a significant contribution to population dose because they individually are equivalent to a vast number of low dose investigations. Effective Dose • The effective dose of each procedure varies because dose depends on: – X-ray intensity, – energy and – application time. Real Time Investigations • A real time investigation such as Barium meal involves the patient being bathed in X-rays as the doctor watches an image on a TV monitor. • The dose is minimized by pulse application and image freezing but necessarily involves a much bigger dose than a simple ‘snapshot’ method as used in a chest X-ray. Real Time Investigations • The dose varies in its effect on tissue too as this is dependent upon the quantity of cell division taking place and summed absorption of layers of tissue.