Computed Tomography BOOk: physical processes of medical imaging

cT scan

1. Computed Tomography Book: Physical Processes of Medical Images, Perry Sprawls; Chapter: 23; Page: 327 M. R. Talukder Department of Electrical and Electronic Engineering University of Rajshahi

2. Computed Tomography http://www.stabroeknews.com/images/2009/08/20090830ctscan.jpg http://upload.wikimedia.org/wikipedia/commons/archive/d/da/20060904231838!Head_CT_scan.jpg http:// www.capitalhealth.org/subpage.cfm?ref =36

3. 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. Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine .

4. Computed Tomography - Introduction Computed Tomography , CT for short (also referred to as CAT, for Computed Axial Tomography), utilizes X-ray technology and sophisticated computers to create images of cross-sectional “slices” through the body. CT exams and CAT scanning provide a quick overview of pathologies and enable rapid analysis and treatment plans. Tomography is a term that refers to the ability to view an anatomic section or slice through the body. Anatomic cross sections are most commonly refers to transverse axial tomography. The CT scanner was developed by Godfrey Hounsfield in the very late 1960s. This x-ray based system created projection information of x-ray beams passed through the object from many points across the object and from many angles (projections). CT produces cross-sectional images and also has the ability to differentiate tissue densities, which creates an improvement in contrast resolution.

5. The x-ray tube in a CT scanner is designed to produce a fan shaped beam of x-rays that is approximately as wide as your body. Tissue attenuation is measured over a large region from one position of the x-ray tube The x-ray tube on a CT scanner is more heavy duty than tubes used for standard film imaging since the unit rotates. Opposite the patient is an array of detectors that measure the intensity of the x-ray beam at points laterally across the patients body. Modern CT scanners use solid state detectors that have very high efficiency at the low energy of x-rays produced by CT scanners. Solid state detectors are made of a variety of materials that create a semiconductor junction similar to a transistor. Ultrafast ceramic detectors use rare earth elements such as silicon, germanium, cadmium, yttrium or gadolinium, which create a semiconducting p-n junction. Ceramic solid-detectors are very fast, can be extremely stable, and are produced to form an array of very small, efficient detectors that can cover a large area. Computed Tomography - Introduction

6. Computed Tomography - The basics http:// www.endocrinesurgery.ucla.edu/images/adm_tst_ct_scan.jpg http://www.themesotheliomalibrary.com/ct-scan.html The x-rays are produced in a part of the ring and the ring is able to rotate around the patient. The target ring contains an array of detectors and is internally cooled so the to reduce electronic noise and to cool the anode. The patient is put into the system using a precise high speed couch . শয্যা শয্যা

7. Modern CT scanner Gantry aperture (  72cm) Microphone Sagittal laser alignment light Sagital laser alignment light x-ray exposure indicator light Gantry control panels Emergency stop buttons External laser alignment Patient couch ECG gating monitor

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

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

10. 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 dense material Less transmitted x-rays More transmitted x-rays

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

12. Conventional X-Ray 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 x-ray photons

13. X-ray System The x-ray beam must have: 1. Appropriate shape, 2. Intensity distribution, 3 Ability to be rotated around patent’s body. Tube & Gantry: x-ray tube is mounted on circular gantry assembly & rotate around patent’s body. Collimation: determines physical size & shape of the x-ray beam. 1 set determines angular span of the beam & another for thickness. Filtration: (1) Beam hardening increases photon energy, (2) Compensation requires for uniform beam thickness. Power supply: constant potential type is generally used.

14. Detectors In CT system, receptor is an array of small detectors & absorbs radiation it intercepts. It depends on the type of gantry used. Construction: made of solid scintillation crystals that convert x-ray energy into light. Light is then converted into electrical signal either by photo-diode or photo-multiplier Sensitivity Profile: uniformly sensitive to all radiation coming from body but insensitive to other. Detector Configuration: it is different in different scanners. It is designated as 1 st , 2 nd , 3 rd , 4 th etc generation depending on configuration developed.

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

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

17. Slice / Cut 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

18. TYPES OF DATA MEASUREMENT DATA: data that arises from detectors. it needs to be preprocessed to eliminate artifacts RAW DATA: it’s the result of scan data being pre-processed CONVOLVED DATA : 1. filtered back projection is the algorithm used by modern ct; 2. it requires filtering and then back projection; 3. raw data is filtered using mathematical filter or kernel (convolution) it removes blur; 4. convolution can only be applied to raw data IMAGE DATA: convolved data that have been back projected into the image matrix to create ct images displayed on the monitor

19. Algorithms applicable to CT ITERATIVE METHODS : simultaneous iterative reconstruction technique iterative least squares technique algebraic reconstruction technique ANALYTIC METHODS: Fourier reconstruction filtered back projection BACK PROJECTION : also called summation method or linear superposition method

20. http://www.themesotheliomalibrary.com/ct-sca.html Computed Tomography - The basics of image formation The x-ray tube and detectors rotate around the patient and the couch moves into the machine. This produces a helical sweep pattern around the patient. The patient opening is about 70cm in diameter. The data acquired by the detectors with each slice is electronically stored and are mathematically manipulated to compute a cross sectional slice of the body. Three dimensional information can be obtained by comparing slices taken at different points along the body. Or the commuter can create a 3D image by stacking together slices. As the detector rotates around many cross sectional images are taken and after one complete orbit the couch moves forward incrementally.

21. CT Image Formation: Production CT differs by formation of image. Operator adjust a number of imaging factors: 1. Slice selection to be imaged, 2. Rotation of x-ray beam, 3. Penetration measurement, 4. Scanning time: 1-15 sec depending on scanning variables, 5. Total penetration: No. of views x No. of rays, 6. CT image quality improved by longer scanning time.

22. Computed Tomography - The basics of image formation Here the x-ray tube and detector array makes many sweeps past the patient. The x-ray tube and detector array is capable of rotating around the axis of the patient.

23. Computed Tomography - The basics of image formation Pixel – picture element – a 2D square shade of gray. Voxel – volume element – a 3D volume of gray. This is a result of a computer averaging of the attenuation coefficients across a small volume of material. This gives depth information. Each voxel is about 1mm on a side and is as thick as 2 – 10mm depending on the depth of the scanning x-ray beam.

24. Scanning Scanning process is the 1 st step in CT image formation. It consists of rotating x-ray beam around patents’ body & making measurements of penetration through the body from various directions. 1 scan results in hundred of thousands of individual measurements. Rays: Ray is a portion of beam. Rate of absorption is obtained by linear atten . coef . Atten coef depends on types of tissue. Views: consists of a collection of rays that share a common point. Measurement & Samples: Total no. of data in 1 scan= no. of views x no. of rays per view = 5,00,000.

25. Computed Tomography - The basics of image formation The images are reconstructed by a method called back projection , or tracingbackwards along the x-rays forward path to reconstruct the image and calculating the absorption due to a localized region. This a mathematically tedious process, but is handled easily with computers. The detectors see the forward projected x-rays and measure the intensity given that the x-ray intensity without the body present is known. The intensity N i written as sum of attenuation coefficients along a given x-ray path. This generates a shade of gray and a number associated with this shade. Then the detector changes angles and the process repeats.

26. CT Image Formation: Reconstruction CT image production is known as image reconstruction. 1. Reconstruction is a math process, 2. It converts scan data into digital image, 3. Image is structured in array of individual picture element (pixel), 4. Each pixel is represented by numerical value or CT no., 5. Each pixel value is related to density of tissue in volume element ( voxel ).

27. CT Image Formation: Conversion The final stage is conversion of digital image into video by DAC. The relationship between pixel CT no. values and the shades of gray/brightness. The displayed image is determined by window levels selected by the operator. Brightness & contrast can be adjusted by manipulation of lower & upper window levels.

28. Image Reconstruction CT image reconstruction is the process of transforming x-ray penetration measurement into digital image of the body section. It is a mathematical process performed by computer. Image Format: (1) matrix size, (2) FOV, (3) slice thickness. CT No.:

29. CT Image Quality Before initiating a scan, the operator must adjust values for a relatively large number of imaging factors:

30. Computed Tomography - Image Quality Contrast Resolution – The ability to differentiate between different tissue densities in the image High Contrast - Ability to see small objects and details that have high density difference compared with background. - These have very high density differences from one another. - Ability to see a small, dense lesion in lung tissue and to see objects where bone and soft tissue are adjacent Low Contrast - Ability to visualize objects that have very little difference in density from one another. - Better when there is very low noise and for visualizing soft-tissue lesions within the liver. - Low contrast scans can differentiate gray matter from white matter in the brain.

31. Artifacts can degrade image quality and affect the perceptibility of detail. Includes Streaks – due to patient motion, metal, noise, mechanical failure. Rings and bands – due to bad detector channels. Shading - can occur due to incomplete projections. Computed Tomography - Imaging artifacts Streaks Rings and bands Shading

32. Computed Tomography - Advantages & Disadvantages Advantages: Desired image detail is obtained Fast image rendering Filters may sharpen or smooth reconstructed images Raw data may be reconstructed post-acquisition with a variety of filters Disadvantages Multiple reconstructions may be required if significant detail is required from areas of the study that contain bone and soft tissue Need for quality detectors and computer software X-ray exposure