Computed tomography (CT) scanning

14 Computed tomography (CT) scanning

Definition of computed tomography scanning

Is the process of producing a cross-sectional image of the body by using a collimated beam of radiation that rotates round the patient. Some of the radiation is absorbed and scattered by the body and some is transmitted through the patient and is collected by a number of detectors which are linked to photomultipliers. A signal is then sent to a computer which calculates the amount of radiation absorbed by the patient and reconstructs an image which can then be viewed on a television monitor

Terminology

Dynamic CT	When a number of scans are performed in rapid succession, e.g. to demonstrate blood flow
Enhanced CT	The use of a contrast agent to improve the appearance of vessels or organs that are similar in density to the surrounding tissues
Field of View	The part of the scanned plane which may be included in the final image
Helical	Spiral
Image Acquisition	The collection of data in order to produce an image
Image Format	The process of storing an image, on computer disk, magnetic tape, film or on the World Wide Web
Image Manipulation	To digitally change the appearance of the acquired image in order to improve it
Image Reconstruction	The process of generating an image from raw data or a set of unprocessed measurements
Isotropic	Having the same properties in all directions, e.g. density
Matrix	The columns and rows that form a digital image
Mean Window Level	The average range of pixel values in an image
Noise	Anything that distracts from the information required on an image
Nutating Detector Ring	When the detectors vibrate in such a way as to keep the detectors nearest the tube out of the way of the X-ray beam
Pitch	The table movement during one complete rotation of 360° divided by the column width (or slice thickness)
Pitch	Example If the table moves 10 mm during one complete rotation and a beam width of 5 mm was used, the pitch value = 2
Pixel	A two dimensional ‘picture cell’ or ‘dot’ that makes up the image on a digital display screen
Profile	Line of data
Slice	A section through the patient which is recorded when the X-ray tube and detector make one complete rotation
Slice Interval	The distance between reconstructed slices
Spatial Resolution	The smallest part of an image that can be seen
Translate	Movement in a horizontal direction
Voxel	A three dimensional pixel

WindowThe range of colour (or grey) scale values displayed on a digital imageWindow WidthThe range of pixel values displayed in the digital image

Hardware

Gantry	A circular device for holding the detectors
X-ray Tube	A method of producing X-rays which are collimated so that they are aligned to a specific number of detectors
Detectors	• A solid state device containing caesium iodide crystals which collects the amount of radiation transmitted through the patient • Or UFCTM (Ultrafast Ceramic) LSO (Lutetium Oxyorthosilicate) crystal technology
Photomultipliers	• A device for increasing the number of electrons • The photons from the detector hit a cathode which produces electrons which bombard plates and produce more electrons • Used in earlier scanners – replaced by photodiodes
Photodiode	• Simply, a light controlled variable resistance • When dark there is very little current flow • But when the PN junction is exposed to light, current flows in direct proportion to the quantity of light it is exposed to • Solid state devices coated with a fluorescent rare earth phosphor, smaller, more stable and more sensitive and a third of the size of photomultipliers
Housing	• A ‘doughnut’ shaped structure which contains the X-ray tube and the detectors • With a patient opening (aperture) in the order of 70 cm diameter
Movement	• Originally high tension cables wound round a drum • Disadvantages that the cables had to be ‘unwound’ after each cycle • Replaced with slip ring and brushes • Gives a constant electrical supply • Therefore allows continual movement during rotations
Table	For the patient to lie on and can move forward at a predetermined distance or at a constant speed to enable the next ‘slice’ to be taken
Operator Console	Where the operator can determine the settings for the scan
Display Station	For the viewing, analysis, networking and storage of the final image