Computed Tomography

1. Computed Tomography Historical Perspective

2. Meaning • Tomography – from the Greek word tomos meaning section • Early images lacked enough detail and clarity to be useful • CT had to wait until the discovery of higher order mathematics and computers • CT now overcomes limitations by using image computer reconstruction from projections to produce sharp, clear cross-sectional images

3. Conventional Tomography • Radiograph obtained with a moving x-ray tube • Results in an image with superimposed tissues (CT does not) • Tissues are blurred above and below the focal plane – no improvement in spatial resolution

4. Image Reconstruction from Projections • 1917 – Radon proved that a 2d or 3d model could be produced by collecting a large number of projections from different projections • This method is used in a variety of applications including astronomy and electron microscopy • Cormack developed reconstruction by back projection in the 1950’s and 1960’s

5. Projections • In CT radiation passed through each cross-section in a specific way and is projected onto a detector that sends signals to a computer for processing. After processing a clear, sharp, digital image is produced.

6. Technical Definition • Herman (1980) – Image reconstruction from projections is the process of producing an image of a two dimensional distribution from estimates of its line integrals long a finite number of lines of known locations.

7. Image Reconstruction in Medicine • Hounsfield (1967) – applied reconstruction techniques to produce the worlds first useful CT scanner for imaging the brain. • Hounsfield’s studies resulted in emission CT (nuclear medicine) and transmission CT (computed tomography). • Hounsfields first machine used a gamma source • Output too low • Source too large • Image reconstruction is also used in Ultrasound and MRI

8. Evolution of Terms • Hounsfield first coined the term computerized transverse axial scanning. • Other terms include: Computerized transverse axial tomography, computerized transverse axial tomography, computer-assisted tomography, computerized axial tomography, computerized transaxial transmission reconstructive tomography.

9. Terms • The term Computed Tomography was established by the Radiological Society of North America in their major journal Radiology • Additionally the American Journal of Roentgenology accepted this term. • The term Computed Tomography is considered the correct term

10. Process • Data acquisition • Image reconstruction • Image display, manipulation, storage, recording, and communication

11. Data Acquisition • Data acquisition – refers to the collection of x-ray transmission measurements from the patient • Patient -> detector = transmission values/attenuation values • Initial scanners took an inordinate amount of time to complete one slice

12. Image Reconstruction • Transmission measurements are sent to a computer • The computer uses mathematical techniques to reconstruct the CT image in a finite number of steps called reconstruction algorithms • Hounsfield developed an algorithm called the algebraic reconstruction technique

13. Image Reconstruction • A variety of computers are integral to the reconstruction process • Computer equipment includes array processor, minicomputer, and microprocessors

14. Image Display, Manipulation, Storage, Recording, and Communications • After reconstruction the images can be displayed, recorded, and analyzed • Typically images are displayed on a cathode ray tube. • Monitors allow a variety of individuals to view and manipulate the images

15. Manipulation • Many computer packages allow images to be manipulated after the scan is complete (post-processing) • Images can be reconstructed in a variety of planes, can be colored, and 3d models may be created.

16. Storage • Images can be recorded and stored on a variety of archive media • Archive media include radiographic film, mag tapes, optical disks, and cd-rom

17. Communications • CT scanners can be connected to a wide array of devices: • Laser printers, diagnostic workstations, display monitors, and computers outside the hospital. • Many different types of CT systems and equipment can communicate through a standard protocol called Digital Imaging and Communication in Medicine(dicom)

18. Brief History of CT • 1895 – Roentgen discovers x-rays • 1917 – Radon develops reconstruction mathematics • 1963 – Cormack formulates x-ray absorption in tissue • 1972 – Hounsfield demonstrates CT • 1974 – Convolution and back projection

19. Brief History of CT • 1975 – First whole body CT • 1976 – Somatom scanner operating on fan beam principle, 5s scan with instant image reconstruction • 1978 – Topogram • 1979 – Hounsfield and Cormack Nobel Prize • 1981 – 512 squared matrix • 1983 – High frequency generators & EBCT

20. Brief history of CT • 1984 – Opti 155 CT tube with 1.75 MHU, 70cm gantry opening, +/- 25 degree tilt • 1986 – Osteo CT (BMD), xenon CT (measures cerebral blood flow) • 1987 – Continuous rotation flying focal spot CT tube • 1989 – Spiral CT • 1991 – Multislice CT introduced

21. Brief history of CT • 1991 – intuitive mouse-driven Windows interface • 1992 – Integrated CT angiography • 1994 – Routine sub second spiral CT • 1996 – Spiral “4” everything; neuro and high-resolution spiral • 1997 – multi detector arrays; volume scans

22. Nobel Laureate Godfrey Hounsfield Associated PressMonday, August 23, 2004; Page B05 Godfrey Hounsfield, 84, who developed the first practical CAT scan machine and shared a Nobel Prize in 1979 for inventing CAT scan technology, died Aug. 12 at a hospital in Kingston upon Thames, England. The cause of death was not reported.

23. Nobel Laureate Godfrey Hounsfield • The Nobel committee described Mr. Hounsfield, who worked at EMI laboratories' medical research division, as "the central figure in computer-assisted tomography." The device uses X-rays to scan from different angles and a computer to assemble the images into a cross section.

24. EMI • Electric and Musical Industries • Not only did EMI employ Hounsfield and market the first UK “CAT” Scanner, they also signed contracts with

25. The Beatles

26. Pink Floyd

27. Queen

28. Emission vs. Transmission • Emission CT involves nuclear medicine and Gamma-ray emission from the patient • Computed Tomography utilizes x-ray transmission through a patient

29. Limitations of CT • Spatial resolution • Relatively high patient dose • Z-axis reformation • Distinct artifacts

30. Advantages of CT • Better contrast resolution • No superimposition of tissues • Less scatter radiation • 3D imaging • Bone mineral assay