these are straight from som1's notes from phys resources.
they seem to explain the process very well
• Soft tissue anatomy is not very clear with standard x-ray radiography. Clarity is markedly increased by using x-rays and computers together in computed axial tomography. The word tomography comes from the Greek tomos = slice, and graph = picture
• Computerised axial tomography is a non invasive technique that uses x-rays to produce images of various internal parts of the body such as the head, heart and abdomen.
• To produce a CAT image, the patient lies on a table that is able to pass through a circular scanning machine called a gantry (the table lies on the axis of the gantry hence the axial in CAT)
• X-rays from the gantry are fired at the organ being scanned and pass through the tissue. The degree of absorption – attenuation – depends on the tissue type that it passes through. Detectors measure the amount of x-rays that pass through. The gantry rotates around the patient and x-rays are continuously fired from different angles. The data is analysed by a computer using an algorithm to produce a cross sectional ‘slice’ on a video screen. The slice is collimated (focused) to a thickness between 1mm and 10mm using lead shutters in the front of the x-ray tube and detectors
• a series of slices can be made to build a picture of an entire organ or even the whole body
• Tomographic pictures are made up of the small square or ‘pixel’ each which can be given a grey scale value ranging from 1 (black) to 256 (white). CAT scans are capable of producing high resolution anatomical images
• CAT scans are used in an ever increasing range of diagnostic applications including the detection of cancerous tumours, infections, bone fractures and even blood clots. Contrast solutions such as iodine and barium sulphate can be added to improve contrast between the scanned organ and surrounding tissue
