Computed Tomography Basic principles V G Wimalasena Principal

Computed Tomography Basic principles V. G. Wimalasena Principal School of Radiography

Introduction n n Computed tomography (CT) is a medical imaging method employing tomography. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write). A large series of two-dimensional X-ray images (slices) of the inside of an object are taken around a single axis of rotation. Digital geometry processing is used to generate threedimensional images of the object from those slices.

History The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967. and it was publicly announced in 1972. n Allan Mc. Leod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine. n

Prototype CT scanner

Historic EMI Scanner

Modern CT scanner

Label 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. gantry aperture (720 mm diameter) microphone sagittal laser alignment light patient guide lights x-ray exposure indicator light emergency stop buttons gantry control panels external laser alignment lights patient couch ECG gating monitor

CT Gantry –Internal structure

Label 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. x-ray tube filters, collimator, and reference detector internal projector x-ray tube heat exchanger (oil cooler) high voltage generator (0 -75 k. V) direct drive gantry motor rotation control unit data acquisition system (DAS) detectors slip rings

Understanding Basic factors n n Absorption : -stopping of x-rays with transfer of energy Scatter: - deflection of xrays Incident Intensity : - No. of x-ray photons falling on an object Transmitted Intensity: - Incident x. No. of photons passing ray beam through Scattered x-rays Transmitted X-ray beam

Attenuation The reduction of the beam intensity on passing through the material due to absorption plus scatter The degree of attenuation is obtained by measuring and comparing the incident and transmitted intensities More dense material Less transmitted x-rays More transmitted x-rays Less dense material

Applications of X-ray attenuation & detection Conventional X-ray (Radiography) n Conventional Tomography n Computed Tomography n

Conventional X-Ray n n n Conventional x-ray produces a compression of a volume to a plane The detector is the Silver halide crystal on a x-ray film The degree of blackening represents the total attenuation through the path of xray photons

n n The higher the attenuation the lesser is the blackness The structure which results more attenuation or more transmission predominates in the image

Conventional Tomography n n n The source and detector moves Produces Images of coronal or sagittal sections (cuts) of areas of interest Eliminates the superimposition of structures above and below

CT Scan n CT scan produces axial sections/cuts /Slices The CT image is recorded through a SCAN. n Scan? n A scan is made up of multiple X-Ray attenuation measurements around an objects periphery n X-ray tube Detector

Slice / Cut n n n The cross sectional portion of the body which is scanned for the production of CT image is called a slice. The slice has width and therefore volume. The width is determined by the width of the x-ray beam To be continued …. CTComplementary 2