MEDICAL IMAGING 2nd Part Computed Tomography

Transcription

1 MEDICAL IMAGING 2nd Part Computed Tomography

2 Introduction 2 In the last 30 years X-ray Computed Tomography development produced a great change in the role of diagnostic imaging in medicine. In convetional projection radiograpgy the image represents the 2D projection of a 3D structure. To obtain information on the real structure the number of projections have to be increased A tomogram is an image of a slice or a plane within the body The basics of CT is to take a series of conventional cross-sectional x-rays while the patient is rotated slightly around an axis between each exposure. A series of projection data is obtained, thes data are used to reconstruct cross-sectional images.

3 Introduction : The austrian mathematician Radon develops a way to reconstruct the density distribution of an object if the line integrals (for every direction) are available : The physician Cormack develops a lot of theoretical work on x- rays creating the basis of CT scanning (without knowing Radon work) 1971: The first CT system has been created by Godfrey N. Hounsfield, he received the Nobel prize in 1979 (together with Cromack) 1972: The first clinical use of a CT system in London, the acquisition time of a couple of tomogram was 5 minutes

4 Introduction : The first II Generation CT system total-body scanner has been realised in the U.S., the acquisition time for 1 tomogram was 1 minute : III and IV Generations CT with acquisition time for 1 tomogram lower than 10 secs 1983: First electron beam CT, very expensive (scarce diffusion) 1989: First helical CT, very low acquisition time (less than 1 sec) and able to explore a large body volume 1992: Dual-slice CT scanner, able to acquire a large number of layers in a very low time 2000: Multislice CT scanner, multiple arrays of detectors; continuous development.

5 Computed Tomography Generations First-generation scanner Second-generation scanner 5 Single source collimated in a pencil beam and single detector Linear scan Arbitrary number of rays and angular projections can be measured, scattered radiation is mostly undetected Partial fan-beam and detectors array. Linear scan Able to complete a full scan in less time than I Generation scanners

6 Computed Tomography Generations Third-generation scanner Fourth-generation scanner 6 Fan-beam and detectos array. Source and detectors rotate in synchrony Dramatic decrease of scan time Fan-beam and large ring of stationary detectos array. Single rotating source Collimators cannot be used in detectors (they must receive energy from a mobile source) high detection efficiency and scattering can cause problems

7 Computed Tomography Generations 7 Main characteristics of the first four CT scanners generations I II III IV Scan Time (sec) Number of detectors (for each layer) Movement Rotation + Transl Rotation + Transl. Rotation Rotation X-ray tube rotation Data amount (each tomogram) 57.6 kb 57.6 kb 1 MB 2-42 MB Matrix 80x80 80x80 256x x512

8 Computed Tomography Generations Fifth-generation scanner: electron beam computed tomography (EBCT); no moving parts, a electronically steered electron beam is used, this beam hit one of the four tugsten anode strip encircling the patient. This produces x-rays that are collimated in a fan-beam and revealed by a stationary ring of detectors. A full set of data are acquired in 50 milliseconds. Very expensive Sixth-generation scanner: helical CT addresses the need for rapid volumetric data acquisition. Conventional arrangement of source and detctors (III and IV generations) which can continuously rotate. During tube rotation patient table is slightly moved through the sourcedetectors plane. The image acquisition is not sequential (as it is for previous CT generations) but volumetric 8 A) sequential scanning B) helical scanning

9 Computed Tomography Generations Seventh-generation scanner: multislice CT scanners, a thick fan-beam is used and multiple detectors array are used to collect the x-rays. This technology is in continuous development, recently (and 256) multiple detectors array has been developed. 9 With the 256-slice system the entire heart can be covered in a single rotation Helical and multislice CT has made the requirements for new development in data processing techniques even more critical Future development: multislice scanners able to interact with a cone-beam Due to the large volume that can be explored it will be possible to stop using mobile patient table, this will decrease the complexity in image reconstruction

10 CT Instrumentation 10 Most of the commercial medical CT scanners use just one x-ray source. X-ray tubes usually wear out in less than one year and need frequent status control and calibration. X-rays generated by the tube require collimation and filtration. Typically fan-beam geometry (having degress of fan angle) is obtained through two pieces of lead that form a slit between them. Slit width is adjustable by the operator in order to obtain different fanbeam thickness, typically between 0.5 and 10 mm for single slice systems and 20 to 30 mm for multislice CT systems. Fan-angle Thickness mm X-rays fan-beam

11 CT Instrumentation 11 Different types of CT detectors: Solid state detectors contain a scintillation crystal able to interact with x-rays and produce photoelectrons that are captured by a photo-diode and converted to electric current Xenon gas detectors long thin tubes containing compressed xenon gas, when ionized the gas generates a current between anode and cathode. Less efficient than solid state detectors but highly directional Slip ring solves the mechanical problem of continuous electrical contact with rotating gantry; the gantry holds the x-ray tube (that needs high voltage to work properly) and the detectors that generate hundreds of signals to be transmitted out

12 Data Acquisition If a thin beam of monoenergetic x-rays, having an intensity of, propagates for a distance d through an omogenic material, the intensity of the attenuated beam is: I 0 12 I I e d 0 I0 I Being the linear attenuation coefficient is a function of material atomic number and electronic density d If the beam propagates through different omogenic materials the intensity of the attenuated beam is: d i ( n) d i I I e I e n I d x n I

13 Data Acquisition 13 If the material has a continuous variable attenuation coefficient the intensity of the attenuated beam is: I I e 0 0 D x dx Given a measurement of and knowledge of, the projection measurement is defined as: p I 0 I0 I ln( ) I 0 D dx Therefore the basic measurement of a CT scanner is a line integral of the linear attenuation coefficient x I 0 x x D I

14 Data Acquisition A CT scanner has to be calibrated in order to obtain the reference intensity I 0 Multiple projections (along parallel lines) create a so-called attenuation profile 14 CT detectors are able to measure the line integrals of bidimensional distribution of linear attenuation coefficients Through a suitable number of attenuation profiles it is possible to identify the bidimensional distribution of attenuation coefficients and it is possible to reconstruct the bidimensional image. Attenuation profile