2 What do these words mean? IsotopeRadioactivityGeiger Muller TubeAtomic massHalf lifeBecquerel
3 Learning objectivesState qualitatively the differential absorption of X-rays by air, fat, other soft tissues and bone and the appearance of X-rays on film after passing through these media.• Explain techniques for improving quality of X-ray images; use of a grid, narrow beam, filtration.Describe how the use of image intensifying screens reduces dose rates.
5 Why are they useful? Fractures Breast cancer Lung cancer Dental Cardiac unitsAirportsTherapeutic
6 How do they work? X-rays can be used because of the differential absorption (attenuation)of the x-rays by different body tissues(such as bone and muscle). Toobtain an image, a uniform beamof x-rays is transmitted throughthe area of interest.Bone absorbs most of the x-raysand as such, will appear white onan x-ray film; soft-tissue (such asmuscle) only absorbs some ofthe x-rays and appears grey
7 How are x-rays produced X-rays are produced when a beam of energy electrons hits a metal target.When electrons collide they lose KE some of this energy is converted to x-rays but most is converted to heat
8 X ray beamcopper anodefilamentheavy metal targetelectron beamE.H.Tvacuum
9 X-ray attenuation Medium Attenuation Appearance on film Air Negligible BlackFatSmallDark greyOther soft tissuesGreyBoneHighWhite
10 Improving image quality The radiographer aims to produce:Sharp images by increasing resolution and reducing blurGood contrast by distinguishing clearly between different body materials.They can do this by use of a grid, narrow beam, filtration.
11 Use of a gridA grid of lead strips reduces the blurring of images caused by scattered radiation. If two crossed grids are used at right angles to each other, almost completely eliminate unwanted scatter. To prevent the grid lines appearing on the image the grid is slowly moved across the film during exposure.
13 Narrow beamThe cross section of the x-ray beam can be controlled by the diaphragm, this is usually a pair of lead sheets at right angles to each other. Another type of beam definer is the cone. A narrow bean is preferred as random scatter increases with beam size and blurs the image
15 FiltrationFilters reduce unwanted low-energy scatter. This increases the sharpness of the image and the contrast
16 Intensifying screensAbout 97% of the x-rays falling on a photographic film pass straight through without any interaction. The film alone is not a very sensitive method for detecting x-rays.
17 X-rays from patientCassette front (plastic)Front intensifying screenDouble sided film~ 12mmRear intensifying screenFelt paddingCassette back metal
18 X-rays from the patient X-ray absorbed in the fluorescent screenVisible lightFront fluorescent screenDouble sided filmRear fluorescent screenFar fewer a-rays pass straight throughDirect film blackening by x-raysIndirect film blackening by light
19 Modern screens can intensify the image by up to 250 times Modern screens can intensify the image by up to 250 times. The increased image intensity then gives shorter exposure times and reduced radiation doses to the patient. However some detail is lost by diffusion of fluorescent light. Typical resolution is mm, whilst direct exposure film can give better than 0.1mm