Conventional and Computed Tomography

Introduction Is a radiographic technique that employs motion to show anatomical structures lying in the a plane tissue while blurring or eliminating the detail in images of structures above and below the plane of the interest

Principle The principle is based on synchronous movement of 2 or 3 elements in a tomographic system Tomographic units synchronize the movements of the x-ray tube and the image receptor in opposite directions around stationary fulcrum (pivot point) during the exposure The fulcrum area is sharp

Principle The longer the blurring the less opportunity to create a sharp image The greater distance to the fulcrum the greater the blurring The further the object is from the fulcrum the greater difference between its projected motion of the image receptor and the motion of the image receptor This cause image more sharp

Tomographic Quality The tomographic amplitude is the total distance the tube travels The tomographic amplitude is equal or greater to the exposure amplitude Exposure amplitude is the total distance the tube travels during the exposure Blur is the smearing that results in the loss of nearly all recorded detail of objects outside the focal plane

Tomographic Quality Increased blurring causes decreased density It is effected by tomographic amplitude, distance from the fulcrum, distance from the image receptor, and orientation of tube motion Distance from the fulcrum has a direct relationship to blur width Distance from the image receptor has a direct relationship to blur width

Tomographic Quality Orientation of tube motion has a direct relationship to blur width Fulcrum controls the section level, and it may be fixed and patient can move up and down to change section level Focal plane is the section Section thickness is the width of the focal and its control by exposure angle

Tomographic Quality Exposure angle inversely proportional to section thickness The tomographic images exhibit less contrast than static image Section Interval is the distance between fulcrum levels

Types of Motion Linear it moving along a straight line The major quality problem is SID and OID Curvilinear reduce magnification and maintain SID and OID Circular Elliptical Figure eight Trispiral Hypocyclodial

Exposure Factors Time it must match the time of the x-ray tube to complete Complex tomographic motion often require 3-6 seconds mA usually used below 100 to set long time 30% more mAs is needed for wider angle tomography Zonography tomographs usually require the same mAs as static radiograph

Exposure Factors kV 15% rule is a critical tool to determine the adjustment 5% change in kVp is required to produce visible density Zonography is a narrow angle tomography exposure amplitudes less than 10o

Computed Tomography

Scanner Generations 1- First generation 2- Second generation 3- Third generation 4- Fourth generation

Gantry Is the movable frame of the CT unit It contains the x-ray tube and the detectors It maintain the alignment of the tube and the detectors and contains the equipment necessary to perform the scanning movement The aperture for the patient is 50-58 cm Do not force the obese patient into the aperture

Gantry Most gantrys can be angled up to 30o Table angulation can be sometimes be used in place of gantry angulation Positioning lights are usually mounted on the gantry intense white halogen lights and low power laser lights There are often 3 positioning lights for accurate sagittal, coronal, and transverse centering

X-ray Tubes CT images produce massive amounts of heat in the x-ray tube Focal spots size use 0.6 mm and 1.2 mm 0.6 mm use a pulsed beam to reduce the heat load Modern CT pulsed scanner tubes operate at 120 kVp, 1-5 msec pulses X-ray tube produce 0.5-5 million heat unit Liquid and air-cooled tube housing design have been developed

X-ray Tubes The radiation beam is double collimated to assist in eliminating scatter information Slight misalignment can create ring artifact image The anode is aligned with their long axis perpendicular to the scanner plane, this to prevent heel effect Collimation controls voxel length Can be vary between 1mm to 13 mm usually control by the software program The dimension width determine by section thickness or voxel length

Detectors CT detectors should have: (a) high capture efficiency i.e. how well the detectors receive the photon from the patient and that can be control by detectors size and the distance between detectors (b) high absorption efficiency i.e. how well the detectors convert incoming photons, it determine by the material used as well the size and thickness of the detector

Detectors (c) high conversion efficiency i.e. how well the detector convert the absorb photon to analog or digital signal CT detectors should have also high stability, fast response time, and wide dynamic range which is the ratio between the largest signal to the smallest that can be measured Typical modern scanners are capable of dynamic range of 1,000,000:1

Computer Is design to control the data acquisition, process and display, and storage The computer should be in enclosed room with controlled temperature and humidity CT console provide the radiographer access to the software program that data acquisition, controls data processing and display and storage functions

Computer A system program is used to start up the CT unit, this program turns on and perform quality assurance, and record various problems The CT console operate from the menu simply uses a keyboard, light pen

Data Acquisition Controls the tube and detector collimation (pixel size), matrix size, gantry angle, table top entrance, section increment movement, kVp, mA, scan speed

Display Console Controls the digital image production process, that compile the image and display parameters, such as window width and level

Exposure Factor Most CT performed at 120 kVp Time is not a factor as it must be controlled by scanning program mA should be setup Dual energy scanning units required usually 80 kVp and 140 kVp

Artifacts Motion Metal or Star Beam Hardening Partial Volume Effect Ring artifacts