Digital Image Processing

Transcription

1 Digital Image Processing SPECIAL TOPICS CT IMAGES Hamid R. Rabiee Fall 2015

2 What is an image? 2 Are images only about visual concepts? We ve already seen that there are other kinds of image. In this lecture we will consider a special type of medical imaging

3 Medical Imaging 3 Medical imaging is the technique and process of creating visual representations of the interior of a body It seeks to reveal internal structures hidden by the skin and bones Medical imaging also establishes a database of normal anatomy and physiology to make it possible to identify abnormalities

4 Medical Imaging Variants 4 X-ray MRI Ultrasound Radionuclide Optical laser Thermography

5 X-Ray 5 X-radiation (composed of X-rays) is a form of electromagnetic radiation A wavelength ranging from 0.01 to 10 nanometers frequencies in the range 30 petahertz to 30 exahertz ( Hz to Hz) Energies in the range 100 ev to 100 kev X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays

6 X-ray computed tomography (X-ray CT) 6 It makes use of computer-processed combinations of many X-ray images taken from different angles Produces cross-sectional (tomographic) images (virtual 'slices') of specific areas of a scanned object Allows the user to see inside the object without cutting X-ray slice data is generated using an X-ray source that rotates around the object

7 X-ray CT (Cont d) 7 A visual representation of the raw data obtained is called a sinogram In terms of mathematics, the raw data acquired by the scanner consists of multiple "projections" of the object being scanned These projections are effectively the Radon transformation of the structure of the object Reconstruction, essentially involves solving the inverse Radon transformation Solution: The technique of filtered back projection

8 Filtered Back Projection 8 Let's assume that we have a finite number of projections of an object which contains radioactive sources The projections of these sources at 45 degree intervals are represented on the sides of an octagon Now simply run the projections back through the image to obtain a rough approximation to the original The projections will interact contstructively in regions that correspond to the emittive sources in the original image A problem that is immediately apparent is the blurring (star-like artifacts) that occur in other parts of the reconstructed image A high-pass filter could be used to eliminate blurring The combination of back projection and ramp filtering is known as filtered back projection.

9 Filtered Back Projection Example 9

10 Other CT s uses 10 Contrast differentiation Three-dimensional reconstruction Multiplanar reconstruction 3D rendering techniques

11 Multiplanar reconstruction 11 A volume is built by stacking the axial slices The software then cuts slices through the volume in a different plane (usually orthogonal) A special projection method, such as maximum-intensity projection (MIP) or minimum-intensity projection (mip/minip), can be used to build the reconstructed slices

12 3D rendering techniques 12 Surface rendering A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone) A three-dimensional model can be constructed using edge detection image processing algorithms See this link: otron_microtomography_of_atelopus_franciscus_head_- _pone s003.ogv

13 3D rendering techniques (Cont d) 13 Volume rendering Surface rendering is limited in that it will display only surfaces that meet a threshold density In volume rendering, transparency, colors and shading are used to allow a better representation of the volume to be shown in a single image For example, the bones of the pelvis could be displayed as semitransparent So that, even at an oblique angle, one part of the image does not conceal another

14 Volume rendering Example 14

15 3D rendering techniques (Cont d) 15 Image segmentation (We will discuss this later if you are interested!) Where different structures have similar radiodensity, it can become impossible to separate them simply by adjusting volume rendering parameters The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image

16 Image quality 16 Artifacts Dose vs. image quality

17 Artifacts 17 Streak artifact: Streaks are often seen around materials that block most X-rays, such as metal or bone Numerous factors contribute to these streaks: undersampling, photon starvation, motion, beam hardening, and Compton scatter

18 Artifacts (Cont d) 18 Partial volume effect: This appears as "blurring" of edges It is due to the scanner being unable to differentiate between a small amount of high-density material (e.g., bone) and a larger amount of lower density (e.g., cartilage) The reconstruction assumes that the X-ray attenuation within each voxel is homogenous This is most commonly seen in the z-direction, due to the conventional use of highly anisotropic voxels, which have a much lower out-of-plane resolution, than in-plane resolution This can be partially overcome by scanning using thinner slices, or an isotropic acquisition on a modern scanner

19 Artifacts (Cont d) 19 Ring artifact: Probably the most common mechanical artifact, the image of one or many "rings" appears within an image They are usually caused by the variations in the response from individual elements in a two dimensional X-ray detector due to defect or miscalibration "Rings" are suppressed by a transformation to polar space, where they become linear stripes

20 Artifacts (Cont d) 20 Noise: This appears as grain on the image and is caused by a low signal to noise ratio This occurs more commonly when a thin slice thickness is used It can also occur when the power supplied to the X-ray tube is insufficient to penetrate the anatomy

21 Artifacts (Cont d) 21 Motion artifact: This is seen as blurring and/or streaking, which is caused by movement of the object being imaged Motion blurring might be reduced using a new technique called IFT (incompressible flow tomography) Windmill: Streaking appearances can occur when the detectors intersect the reconstruction plane This can be reduced with filters or a reduction in pitch.

22 Artifacts (Cont d) 22 Beam hardening: This can give a "cupped appearance It occurs when there is more attenuation along a path passing through the center of an object, than a path that grazes the edge This is easily corrected by filtration and software

23 Dose vs. image quality 23 An important issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality: New software technology can significantly reduce the required radiation dose Individualize the examination and adjust the radiation dose to the body type and body organ examined Prior to every CT examination, evaluate the appropriateness of the exam whether it is motivated or if another type of examination is more suitable

24 End of Lecture 17 Thank You!